Publications
2024
-
(2024) Journal of the American Chemical Society. 146, 40, p. 27871-27885 Abstract
Electrocatalytic CO2 reduction (e-CO2RR) to CO is replete with challenges including the need to carry out e-CO2RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO2 to CO with a very high faradaic efficiency albeit at −2.5 V versus Fc/Fc+. It is now demonstrated that introducing a nonredox metal Lewis acid, preferably GaIII, as a binding site for CO2 in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCuIIFeIIIGaIII(H2O)3W9O37]8-, leads to bimodal activity optimal both at −2.5 and −1.5 V versus Fc/Fc+; reactivity at −1.5 V being at an overpotential of ∼150 mV. These results were observed by cyclic voltammetry and quantitative controlled potential electrolysis where high faradaic efficiency and chemoselectivity were obtained at −2.5 and −1.5 V. A reaction with 13CO2 revealed that CO2 disproportionation did not occur at −1.5 V. EPR spectroscopy showed reduction, first of CuII to CuI and FeIII to FeII and then reduction of a tungsten atom (WVI to WV) in the polyoxometalate framework. IR spectroscopy showed that CO2 binds to [SiCuIIFeIIIGaIII(H2O)3W9O37]8- before reduction. In situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with pulsed potential modulated excitation revealed different observable intermediate species at −2.5 and −1.5 V. DFT calculations explained the CV, the formation of possible activated CO2 species at both −2.5 and −1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO2 binding steps, the active site for reduction, and the role of protons in facilitating the reactions.
2023
-
(2023) Archaeometry. 65, 6, p. 1353-1373 Abstract
Tel Megiddo is a major type-site for the Middle Bronze in the Levant. During this period a new burial custom appeared in which the dead were interred beneath occupied houses. This tradition is well attested in Area K at the site. Area H, near the palatial complex, revealed a monumental masonry-constructed chamber tomb. This article presents the results of residue analysis conducted on ceramic vessels placed in these tombs as burial offerings. Hundreds of intramural Middle Bronze tombs with ceramic offerings were excavated in the Levant, but this is the first time the content of the vessels is revealed.
-
(2023) Journal of the American Chemical Society. 145, 36, p. 19912-19924 Abstract
The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.
2022
-
(2022) Archaeometry. p. 18- Abstract
Organic residue analysis was conducted on various vessels from burials at Tel Yehud, Israel. The analyses led to new reliable evidence for the presence of opioid alkaloids and their decomposition products. This research revitalizes a decades-old discussion on the presence and function of the opium trade across a cultural region of utmost significance in the Ancient Near East and the use and role of Base-Ring juglets during the Late Bronze Age IIA (14th century bce). Furthermore, it was found that opium storage was not limited to Base-Ring juglets. Opium was possibly diluted into storage jars and juglets, signifying the importance of opium utilization at a larger scale during this period.
-
(2022) ACS Catalysis. 12, 7, p. 4149-4155 Abstract
The two-electron oxidation of water (2e-WOR) has been studied in the past as a possible method for the alternative preparation of hydrogen peroxide. Often, fluorinated tin oxide (FTO) is used as an anode and FTO itself was found also to be active for 2e-WOR. Because one use of H2O2 is as an oxygen donor for BaeyerVilliger oxidation of ketones catalyzed by tin compounds and materials, presently we were interested in studying the use of in situ formed H2O2 for these reactions. First, the formation of H2O2 was verified in an acetonitrile/water solvent in a 2e-WOR reaction, which is more efficient than a comparable reaction in water in terms of the H2O2 concentration attained and faradaic efficiency at comparable potentials, that is, ∼3 V vs SHE. Second, initial studies on oxygenation of reactive substrates such as sulfides showed normalized reaction rates (NRRs) for two-electron oxidation reactions that were about 3 times higher than the NRR for H2O2 formation, indicating the formation of an active oxygen-donating or oxidizing species on the electrode surface prior to the formation and release of H2O2 into solution. Third, the BaeyerVilliger oxygenation of 2-adamantanone at 2.1 V versus SHE in acetonitrile/water showed both the formation of the expected lactone product and hydroxylation at both tertiary and secondary CH bonds. Hydroxylation is most easily explained by the presence of hydroxyl radical species as supported by the formation of a spin adduct and its identification by electron paramagnetic resonance. However, the potential used, 2.1 V versus SHE, is an underpotential for the formation of a solvated hydroxyl radical in solution, thereby leading to the conclusion that surface-bound hydroxyl species, OH*, are those that are reactive for the apparent one-electron water oxygenation reaction. Fourth, it was shown that although H2O2 can be thermally activated on FTO as a catalyst to a minor degree, electrochemical activation is by far more significant, leading to the use of FTO as an electrochemical catalyst for activation of H2O2 for the BaeyerVilliger oxygenation and also alkene epoxidation.
-
(2022) PLoS ONE. 17, 3, e0266085. Abstract
The article presents results of residue analysis, based on Gas Chromatograph Mass Spectrometer (GC-MS) measurements, conducted on 13 ceramic storage jars unearthed in the Babylonian destruction layer (586 BCE) in Jerusalem. Five of the jars bear rosette stamp impressions on their handles, indicating that their content was related to the kingdom of Judah's royal economy. The identification of the original contents remains is significant for the understanding of many aspects related to the nutrition, economy and international trade in the ancient Levant. The study shed light on the contents of the jars and the destruction process of the buildings in which they were found. The jars were used alternatively for storing wine and olive oil. The wine was flavored with vanilla. These results attest to the wine consumption habits of the Judahite elite and echo Jerusalem's involvement in the trans-regional South Arabian trade of spices and other lucrative commodities on the eve of its destruction by Nebuchadnezzar.
-
(2022) Angewandte Chemie. 134, 5, e202112915. Abstract
Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidationCO2 reduction reaction and are of interest in the context of CO2 abatement and carbon-neutral solar fuels. Bioinspired by the active-site composition of the CODHs, polyoxometalates triply substituted with first-row transition metals were modularly synthesized. The polyanions, in short, {SiM3W9} and {SiM2MW9}, M, M, M=CuII, NiII, FeIII are shown to be electrocatalysts for reversible CO oxidationCO2 reduction. A catalytic Tafel plot showed that {SiCu3W9} was the most reactive for CO2 reduction, and electrolysis reactions yielded significant amounts of CO with 98% faradaic efficiency. In contrast, FeNi compounds such as {SiFeNi2W9} preferably catalyzed the oxidation of CO to CO2 similar to what is observed for the [NiFe]-CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO2 electrocatalytic reduction.
-
(2022) Angewandte Chemie (International ed.). 61, 5, e202112915. Abstract
Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidationCO2 reduction reaction and are of interest in the context of CO2 abatement and carbonneutral solar fuels. Bioinspired by the activesite composition of the CODHs, polyoxometalates triply substituted with firstrow transition metals were modularly synthesized. The polyanions, in short, {SiM3W9} and {SiM2MW9}, M, M, M=CuII, NiII, FeIII are shown to be electrocatalysts for reversible CO oxidationCO2 reduction. A catalytic Tafel plot showed that {SiCu3W9} was the most reactive for CO2 reduction, and electrolysis reactions yielded significant amounts of CO with 98% faradaic efficiency. In contrast, FeNi compounds such as {SiFeNi2W9} preferably catalyzed the oxidation of CO to CO2 similar to what is observed for the [NiFe]CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO2 electrocatalytic reduction.A modular synthesis of trimetallosubstituted polyanions enables the preparation of designer catalysts. Here this approach was used to prepare electrocatalysts for the selective reduction of CO2 to CO using a tricoppersubstituted polyanion. In contrast, FeNisubstituted polyanions preferably catalyzed the oxidation of CO to CO2, showing a reactivity profile reminiscent of the reactivity of FeNibased carbon monoxide dehydrogenase enzymes.
2021
-
Photoelectrochemical Reduction of Carbon Dioxide with a Copper Graphitic Carbon Nitride Photocathode(2021) Chemistry : a European journal. 27, 54, p. 13513-13517 Abstract
Research on the photoreduction of CO2 often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO2 reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of coppersubstituted graphitic carbon nitride (Cu−CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical twoelectron reduction of CO2 to CO as a major product (>95%) and formic acid (
-
(2021) Journal of archaeological science, reports. 36, 102904. Abstract
The article presents and discusses the results of Residue Analysis performed on 27 pottery vessels, placed as offerings in burials dating to the Iron I period (ca. 1050900 BCE) at the site Ḥorvat Tevet (Israel). The results show that heated beeswax was used during the burial ceremonies and placed in variety of vessels. These results shed new light on burial practices of South Levantine rural communities. They also contribute to the growing body of evidence regarding bee-product economy in the Southern Levant during the beginning of the Iron IIA.
-
(2021) ACS Catalysis. 11, 3, p. 1495-1504 Abstract
The mechanism of the photochemical reduction of carbon dioxide to carbon monoxide coupled to hydrocarbon dehydrogenation catalyzed by a three-component hybrid construct has been studied by combining density functional theory and complete active space self-consistent field/CASPT2 calculations. The electron and proton coupled transfer mechanism is summarized as follows. Graphitic carbon nitride (g-CN), as the photosensitive semiconductor, absorbs visible blue light and transfers electrons to polyoxometalate (POM), a process that is driven by the relative energy of the lowest unoccupied molecular orbital levels of the two species. Then, the hydrocarbon substrate (cyclohexene or cyclohexadiene) is dehydrogenated by the effect of holes formed during the photoexcitation of g-CN, which leads to the transfer of electrons to the oxidized g-CN and protons to the reduced POM. Under red light irradiation, the reduced POM transfers two electrons to the bimetallic rhenium [Re(I)] CO2 reduction catalyst {[Re1(bipyNNH2)(CO)3Cl-Re2(bipy)(CO)3Cl], bipy = 2,2-bipyridine}. The activated catalyst reduces CO2 to CO, while the proton-rich POM transfers two protons to generate the reduction co-product, H2O. Acting as an electron/proton switch, POM promotes hydrocarbon dehydrogenation by increasing the electron/hole separation upon photoactivation of g-CN and facilitates the transfer of electrons and protons to the Re catalyst for CO2 reduction. The first electron transfer to the Re complex is to the bipyNNH2 ligand. The unpaired electron is then transferred to the Re 5dz2 orbital, activating the coordination of CO2. The combination of a strong interaction between the nearly degenerate d-π∗ orbitals of Re and the bipyNNH2 ligand, the low reduction potential of -0.85 V, and an extremely low free energy barrier of +2.4 kcal mol-1 for CO2 ligation makes the bimetallic Re compound one of the best catalysts for transforming CO2 to CO.
-
(2021) ACS Catalysis. 11, p. 10494-10501 Abstract
The selective catalytic oxidative monofunctionalization of gaseous alkanes found in natural gas and commodity chemicals such as benzene and cyclohexane is an important objective in the field of carbon-hydrogen bond activation. Past research has demonstrated the possibility of stoichiometric oxyesterification of such substrates using lead(IV) trifluoroacetate (PbIV(TFA)4) as oxidant, which is driven by the high 2-electron redox potential of lead(IV). However, this redox potential then precludes reoxidation of lead(II) by a convenient oxidant such as O2, nullifying an effective catalytic cycle. In order to utilize renewable energy resources as alternatives to high-temperature thermocatalysis, we demonstrate the room-temperature electrocatalytic oxyesterification of alkanes and benzene with PbIV(TFA)4 as catalysts. At 1.67 V versus SHE, alkanes and benzene yielded the corresponding trifluoroacetate esters at room temperature; typically, good yields and high faradaic efficiencies were observed. High intrinsic turnover frequencies were obtained, for example, of >1000 min-1 for the oxyesterification of ethane at 30 bar. An analysis of the possible mechanistic pathways based on previously investigated stochiometric reactions, cyclic voltammetry measurements, kinetic isotope effects, and model compounds led to the conclusion that catalysis involves lead-mediated proton-coupled electron transfer of alkanes at and to the anode, followed by reductive elimination through an SN2 reaction to yield the alkyl-TFA products. Similarly, lead-mediated electron transfer from benzene at and to the anode leads to phenyl-TFA. Cyclic voltammetry also shows the viability of in situ reoxidation of Pb(II) species. The synthesis results obtained as well as the mechanistic insight are important advances towards the realization of selective alkane and arene oxidation reactions.
2020
-
(2020) Chemical Science. 11, 42, p. 11584-11591 Abstract
In the past the formyloxyl radical, HC(O)O, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O is formed in the anodic electrochemical oxidation of formic acid/lithium formate. Using a [CoIIIW12O40]5- polyanion catalyst, this led to the formation of phenyl formate from benzene. Here, we present our studies into the reactivity of electrochemically in situ generated HC(O)O with organic substrates. Reactions with benzene and a selection of substituted derivatives showed that HC(O)O is mildly electrophilic according to both experimentally and computationally derived Hammett linear free energy relationships. The reactions of HC(O)O with terminal alkenes significantly favor anti-Markovnikov oxidations yielding the corresponding aldehyde as the major product as well as further oxidation products. Analysis of plausible reaction pathways using 1-hexene as a representative substrate favored the likelihood of hydrogen abstraction from the allylic C-H bond forming a hexallyl radical followed by strongly preferred further attack of a second HC(O)O radical at the C1 position. Further oxidation products are surmised to be mostly a result of two consecutive addition reactions of HC(O)O to the CC double bond. An outer-sphere electron transfer between the formyloxyl radical donor and the [CoIIIW12O40]5- polyanion acceptor forming a donor-acceptor [D+-A-] complex is proposed to induce the observed anti-Markovnikov selectivity. Finally, the overall reactivity of HC(O)O towards hydrogen abstraction was evaluated using additional substrates. Alkanes were only slightly reactive, while the reactions of alkylarenes showed that aromatic substitution on the ring competes with C-H bond activation at the benzylic position. C-H bonds with bond dissociation energies (BDE) ≤ 85 kcal mol-1 are easily attacked by HC(O)O and reactivity appears to be significant for C-H bonds with a BDE of up to 90 kcal mol-1. In summary, this research identifies the reactivity of HC(O)O towards radical electrophilic substitution of arenes, anti-Markovnikov type oxidation of terminal alkenes, and indirectly defines the activity of HC(O)O towards C-H bond activation.
-
(2020) Inorganic Chemistry. 59, 17, p. 11945-11952 Abstract
Dissolution of the polyoxometalate (POM) cluster anion H5[PV2Mo10O40] (1; a mixture of positional isomers) in 50% aq H2SO4 dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of 1 (115 mM) in 50% (9.4 M) H2SO4. Data from 51V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H2SO4 in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO2+, ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for 1 in 1.0 M aq H2SO4. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO43-, which at 50% H2SO4 accounts for all the P(V) initially present in 1. Addition of (NH4)2SO4 leads to the formation of crystalline [NH4]6[Mo2O5(SO4)4] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo2O5]2+ core, conceptually derived by acid condensation of two MoO3 moieties. While 1 in 50% aq H2SO4 oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO2+ alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH4]6[Mo2O5(SO4)4], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO2+ alone. Finally, 31P and 51V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H2SO4
-
(2020) Journal of the American Chemical Society. 142, 34, p. 14504-14512 Abstract
Host-guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate type molybdenum oxide based porous, spherical clus-ters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here we demonstrate the synthetically controlled encapsula-tion of first row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate ani-ons, {Mo132PO4}. The resulting M2+x{Mo132PO4} host-guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encap-sulation of up to 10 equivalents showed little magnetic interaction between the encapsulated metals, indicating that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal was possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photo-initiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.
-
(2020) ACS Catalysis. 10, 7, p. 4227-4237 Abstract
The reductive activation of molecular oxygen catalyzed by iron-based enzymes toward its use as an oxygen donor is paradigmatic for oxygen transfer reactions in nature. Mechanistic studies on these enzymes and related biomimetic coordination compounds designed to form reactive intermediates, almost invariably using various "shunt" pathways, have shown that high-valent Fe(V)=O and the formally isoelectronic Fe(IV)=O porphyrin cation radical intermediates are often thought to be the active species in alkane and arene hydroxylation and alkene epoxidation reactions. Although this four decade long research effort has yielded a massive amount of spectroscopic data, reactivity studies, and a detailed, but still incomplete, mechanistic understanding, the actual reductive activation of molecular oxygen coupled with efficient catalytic transformations has rarely been experimentally studied. Recently, we found that a completely inorganic iron-tungsten oxide capsule with a keplerate structure, noted as {Fe30W72}, is an effective electrocatalyst for the cathodic activation of molecular oxygen in water leading to the oxidation of light alkanes and alkenes. The present report deals with extensive reactivity studies of these {Fe30W72} electrocatalytic reactions showing (1) arene hydroxylation including kinetic isotope effects and migration of the ipso substituent to the adjacent carbon atom ("NIH shift"); (2) a high kinetic isotope effect for alkyl C.H bond activation; (3) dealkylation of alkylamines and alkylsulfides; (4) desaturation reactions; (5) retention of stereochemistry in cis-alkene epoxidation; and (6) unusual regioselectivity in the oxidation of cyclic and acyclic ketones, alcohols, and carboxylic acids where reactivity is not correlated to the bond disassociation energy; the regioselectivity obtained is attributable to polar effects and/or entropic contributions. Collectively these results also support the conclusion that the active intermediate species formed in the catalytic cycle is consistent with a compound I type oxidant. The activity of {Fe30W72} in cathodic aerobic oxidation reactions shows it to be an inorganic functional analogue of iron-based monooxygenases.
2019
-
(2019) Chemistry of Materials. 31, 19, p. 8106-8111 Abstract
The 3d transition-metal oxides are a class of promising catalysts for the oxygen evolution reaction (OER) in alkaline media. Manganese-based spinel oxides usually do not show impressive OER performance, and their activity is generally lower than the counterparts containing cobalt. The OER activity and the electronic structure of spinel oxides ZnMn2O4, ZnCo2O4, and the cobalt-substituted ZnMn2O4 were investigated. It has been found that compared to ZnCo2O4, the lower activity of ZnMn2O4 arises from the limitation of spin-selective electron transfer and orbital symmetry restrictions between adsorption of the oxygen species onto the conduction band and the electron transfer from the conduction band to the valence band. The substitution of cobalt into ZnCo2O4 alleviates the spin incompatibility and enables the increase in OER activity.
-
(2019) ACS Catalysis. 9, 9, p. 8819-8824 Abstract
The aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis offers the potential for the introduction of additional concepts for such reactions. Visible light photoactive semiconductors such as bismuth oxyhalides (BiOX, X = Cl and Br) used in this research typically oxidize organic compounds through photocatalyzed formation of strongly oxidizing holes. The reactive oxygen species formed react with organic compounds in one-electron processes, leading to radical intermediates and nonselective oxidation. Such oxidation reactions generally lead to total oxidation. Here, impregnation of BiOX with a polyoxometalate, H-5 PV2Mo10O40, as a strong electron acceptor changed the reactivity of BiOX, leading to Mars-van Krevelen-type reactivity, that is, photoactivated oxygen donation from BiOX to the organic substrate followed by reoxidation by O-2 and catalysis. This conclusion was supported by mechanistic studies involving isotope labeling studies. In this way, ethane was selectively oxidized to acetaldehyde in a flow reactor with a turnover number (24 h) of 415.
-
(2019) Journal of Archaeological Science: Reports. 25, p. 77-84 Abstract
Four small ceramic juglets that had been used as containers for offerings in an elite Middle Bronze Age III (ca. 1650-1550 BCE) masonry tomb uncovered at Tel Megiddo in the Jezreel Valley, Israel were tested using organic residue analysis. Notably, residues of vanillin, 4-hydroxybenzaldehyde, and acetonvanillone were identified in three of the four juglets examined. These are the major fragrance and flavour components of natural vanilla extract. To date, it has been commonly accepted that vanilla was domesticated in the New World and subsequently spread to other parts of the globe. Our research first ruled out all possibility of sample contamination and then conducted a post-organic residue analysis investigation of various species within the plant kingdom from which these principle compounds could have been exploited. The results shed new light on the first known exploitation of vanilla in an Old World context, including local uses, the significance and employment in mortuary practices, and possible implications for understanding trade networks in the ancient Near East during the second millennium BCE.
-
(2019) Dalton Transactions. 48, 19, p. 6396-6407 Abstract
Commonly, iron(ii) and copper(i) complexes bind dioxygen (O-2) and then activate O-2 through a reductive reaction pathway. There is, however, significant interest in low temperature oxygenation with O-2 without the use of a sacrificial reductant. Here, earth-abundant metal complexes (Fe-II, Co-II, Ni-II and Cu-II) coordinated by two different tetra-dentate mono-carbon bridged bis-phenanthroline ligands, (1,10-Phen)(2)-2,2-CR1R2, where R-1 = n-butyl and R-2 = n-butyl or H were synthesized. The structures all showed the expected metal complexation in the equatorial plane by the bridged bis-phenanthroline ligands. For R-1 = n-butyl; R-2 = H, where the ligand has a tertiary carbon bridging group, fast intramolecular oxygenation occurred at the pseudobenzylic position. Depending on the transition metal the main products formed were oxygen bridged dimers of the metal complexes (Co and Fe) or metal complexes with a carbonyl moiety at the bridging pseudobenzylic position as a result of C-R-1 bond cleavage (Ni and Cu). The different product assemblages are explained by different reaction pathways that are metal specific. For quaternary carbon bridged ligands, R-1 = R-2 = n-butyl, the complexes catalytically activated C-H bonds of cyclohexene under catalytic conditions, showing higher effective turnover numbers at low catalyst loading. The reactivity observed is commensurate with a room temperature autooxidation reaction although the initiation of the free radical reaction is metal specific. In general labelling studies with O-18(2), UV-vis and EPR spectroscopy as well as cyclic voltammetry measurements led to a conclusion that the reaction pathways involved both C-H bond activation and O-2 activation.
-
(2019) European Journal of Inorganic Chemistry. 2019, 3-4, p. 482-485 Abstract
Polyfluoroxometalates (PFOMs) that have a quasi Wells-Dawson structure and have low valent transition metal substitution at the so-called "belt" position, alpha(1)-[H2F6NaM(H2O)W17O55](q-), can reversibly interchange between dimeric and monomeric structures. The dimers have two unique M-mu O-W bridges between two PFOM units. The dimerization occurs through dehydration and was studied as a function of temperature using the visible spectrum that is sensitive to the wavelength and extinction coefficient of the d-d transition. The calculated thermodynamic parameters of the dimerization reaction were iteratively fitted using a function derived from the equilibrium constant and the Beer-Lambert law. Such reversible dimerization reactions have not been observed for similar alpha(1)-[P2M(H2O)W17O61](q-) structures, thus fluorine atoms in an axial position to the transition metal are apparently critical for dimerization by reducing the bond strength of the transition metal-aqua bond trans to the fluorine atom.
2018
-
(2018) Journal of the American Chemical Society. 140, 39, p. 12451-12456 Abstract
The electrochemical reduction of CO2 has been extensively investigated in recent years, with the expectation that a detailed mechanistic understanding could achieve the goal of finding a stable catalyst with high turnover frequencies and low reduction potentials. In the catalytic cycle of the carbon dioxide hydrogenase enzyme, it has been suggested that the reduced metal center reacts with CO2 to form a carboxylate intermediate that is stabilized by hydrogen bonding using a histidine moiety in the second coordination sphere. Using the well-known fac-Re(I)bipyridine(CO)(3)Cl complex as a starting point, the bipyridine ligand was modified in the second coordination sphere with a thiourea tether that is known to form hydrogen bonds with carbonyl moieties. The resulting Re(I) catalyst was an excellent electrocatalyst for the selective reduction of CO2 to CO, with a turnover frequency of 3040 s(-1). The binding of CO2 to the thiourea tether was observable by H-1 NMR, and NOE experiments showed that the hydrogen atoms of the thiourea group were labile. Further experiments indicated that the thiourea moiety is also a local proton source and addition of an external proton source actually inhibits catalysis. The absence of a kinetic isotope effect was explained through DFT calculations that showed that the proton invariably jumps to the nearest CO2 oxygen atom to form a metal-carboxylic acid without going through any minimum or transition state. EPR and NMR spectroscopies were used to identify the various reduced intermediates. Thus, the thiourea tether in the second coordination sphere can bind CO2, stabilize carboxylic acid reaction intermediates, and directly act as a local proton source, leading to a significantly more active catalyst.
-
(2018) Angewandte Chemie - International Edition. 57, 19, p. 5403-5407 Abstract
The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [(CoW12O40)-W-III](5-) as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H2OArOH+H-2. Non-optimized reaction conditions showed a Faradaic efficiency of 75% and selective formation of the mono-oxidized product in a 35% yield. Decomposition of formic acid into CO2 and H-2 is a side-reaction.
-
(2018) ACS Catalysis. 8, 4, p. 3232-3236 Abstract
The selective oxidation of light hydrocarbons and their valorization with only dioxygen (O-2) are important transformations toward development of efficient chemical processes. Monooxygenase enzymes can catalyze selective aerobic reactions under reducing and protic conditions. The translation of such enzymatic pathways to the practical electrocatalytic oxidation of light, gaseous hydrocarbons, using O-2 as sole oxidant is now reported. An iron tungsten oxide inorganic molecular catalyst with a capsular structure {Fe30W72} stabilized inside by sulfate/bisulfate anions provides a protic environment where three iron atoms are located at each of the pores of the capsule leading to a unique and potent active site for the oxidation reactions. Under mild electrochemical conditions, 1.8 V, in water at room temperature, using O-2 from air, we demonstrate the low-pressure (1-2 bar) hydroxylation of alkanes, notably ethane to acetic acid, and the ozone like cleavage of the carbon carbon double bonds of alkenes. Typical turnover frequencies were 300-400 min(-1). Initial mechanistic studies support a reaction through a very active iron-oxo species.
-
(2018) Physical Chemistry Chemical Physics. 20, 11, p. 7579-7587 Abstract
The reactivity of the H5PV2Mo10O40 polyoxometalate and its analogues as an electron transfer and electron transfer-oxygen transfer oxidant has been extensively studied in the past and has been shown to be useful in many transformations. One of the hallmarks of this oxidant is the possibility of its re-oxidation with molecular oxygen, thus enabling aerobic catalytic cycles. Although the re-oxidation reaction was known, the kinetics and mechanism of this reaction have not been studied in any detail. Experimentally, we show that both the one-and two-electron reduced polyoxometalate are reactive with O-2, the two-electron one more so. The reactions are first-order in the polyoxometalate and O-2. Solvents also have a considerable effect, protic solvents being preferred over aprotic ones. H5PV2Mo10O40 was reduced either by an electron transfer reaction (H-2) or an electron transfer-oxygen transfer reaction (Ph3P). Similar rate constants and activation parameters were observed for both. DFT calculations carried out on the re-oxidation reactions strongly suggest an inner-sphere process. The process involves first the formation of a coordinatively unsaturated site (CUS) and subsequently the binding of O-2 to form superoxo and then peroxo eta(2)-O-2 adducts. Most interestingly, although vanadium is the reactive redox centre as well as a necessary component for the oxidative activity of H5PV2Mo10O40, and a CUS can be formed at both Mo and V sites, O-2 coordination occurs mostly at the Mo CUSs, preferably those where the vanadium centers are distal to each other.
-
(2018) Chemical Reviews. 118, 5, p. 2680-2717 Abstract
In this review article, we consider the use of molecular oxygen in reactions mediated by polyoxometalates. Polyoxometalates are anionic metal oxide clusters of a variety of structures that are soluble in liquid phases and therefore amenable to homogeneous catalytic transformations. Often, they are active for electron transfer oxidations of a myriad of substrates and upon reduction can be reoxidized by molecular oxygen. For example, the phosphovanadomolybdate, H5PV2Mo10O40, can oxidize Pd(0) thereby enabling aerobic reactions catalyzed by Pd and H5PV2Mo10O40. In a similar vein, polyoxometalates can stabilize metal nanoparticles, leading to additional transformations. Furthermore, electron transfer oxidation of other substrates such as halides and sulfur containing compounds is possible. More uniquely, H5PV2Mo10O40 and its analogues can mediate electron transfer-oxygen transfer reactions where oxygen atoms are transferred from the polyoxometalate to the substrate. This unique property has enabled correspondingly unique transformations involving carbon carbon, carbon hydrogen, and carbon metal bond activation. The pathway for the reoxidation of vanadomolybdates with O-2 appears to be an inner-sphere reaction, but the oxidation of one-electron reduced polyoxotungstates has been shown through intensive research to be an outer-sphere reaction. Beyond electron transfer and electron transfer oxygen transfer aerobic transformations, there a few examples of apparent dioxygenase activity where both oxygen atoms are donated to a substrate.
-
(2018) European Journal of Inorganic Chemistry. 6, p. 791-794 Abstract
The coordination of CO2 to Lewis bases is well-known; however, its binding to Lewis acids is much less common. The ligation of CO2 to a zinc-substituted Wells-Dawson type polyoxometalate yielded [(n-C8H17)(4)N](8)[(2)-P2W17O61Zn(CO2)], which was studied by variable temperature H-1 NMR, C-13 NMR, and P-31 NMR spectroscopy from room temperature to 201 K. The spectra were analyzed in consideration of predicted end-on coordination, POM-Zn-O-C-O, and side-on coordination where in addition an adjacent basic oxygen atom in the polyoxometalate binds the carbon atom of CO2. After adding 2,4,6-trimethypyridine into the solution, a plausible adduct, POM-CO2-2,4,6-trimethypyridine, was observed.
-
(2018) Chemistry-A European Journal. 24, 2, p. 369-+ Abstract
High-order elementary reactions in homogeneous solutions involving more than two molecules are statistically improbable and very slow to proceed. They are not generally considered in classical transition-state or collision theories. Yet, rather selective, high-yield product formation is common in self-assembly processes that require many reaction steps. On the basis of recent observations of crystallization as well as reactions in dense phases, it is shown that self-assembly can occur by preorganization of reactants in a noncovalent supramolecular assembly, whereby directing forces can lead to an apparent one-step transformation of multiple reactants. A simple and general kinetic model for multiple reactant transformation in a dense phase that can account for many-bodied transformations was developed. Furthermore, the self-assembly of polyfluoroxometalate anion [H2F6NaW18O56](7-) from simple tungstate Na2WO2F4 was demonstrated by using 2D F-19-F-19 NOESY, 2D F-19-F-19 COSY NMR spectroscopy, a new 2D F-19{W-183} NMR technique, as well as ESI-MS and diffusion NMR spectroscopy, and the crucial involvement of a supramolecular assembly was found. The deterministic kinetic reaction model explains the reaction in a dense phase and supports the suggested self-assembly mechanism. Reactions in dense phases may be of general importance in understanding other self-assembly reactions.
2017
-
(2017) Israel Journal of Chemistry. 57, 10-11, p. 990-998 Abstract
The iron(II) triflate complex (1) of 1,2-bis(2,2'-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30% H2O2 to an acetonitrile solution of 1 yielded 2, a green compound with lambda(max)=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (delta) of 0.35 mm/s and a quadrupole splitting (Delta E-Q) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm(-1) that downshifted to 832, 540 and 441 cm-(1) when 1 was treated with (H2O2)-O-18. All the spectroscopic data support the initial formation of a (m-hydroxo)(mu-1,2-peroxo) diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0 degrees C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C-H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C-H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, k(H)/k(D), of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (m-hydroxo)(mu-1,2-peroxo) diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.
-
(2017) Nature Chemistry. 9, 4, p. 369-373 Abstract
How molecules in solution form crystal nuclei, which then grow into large crystals, is a poorly understood phenomenon. The classical mechanism of homogeneous crystal nucleation proceeds via the spontaneous random aggregation of species from liquid or solution. However, a non-classical mechanism suggests the formation of an amorphous dense phase that reorders to form stable crystal nuclei. So far it has remained an experimental challenge to observe the formation of crystal nuclei from five to thirty molecules. Here, using polyoxometallates, we show that the formation of small crystal nuclei is observable by cryogenic transmission electron microscopy. We observe both classical and non-classical nucleation processes, depending on the identity of the cation present. The experiments verify theoretical studies that suggest non-classical nucleation is the lower of the two energy pathways. The arrangement in just a seven-molecule proto-crystal matches the order found by X-ray diffraction of a single bulk crystal, which demonstrates that the same structure was formed in each case.
-
(2017) ACS Catalysis. 7, 4, p. 2725-2729 Abstract
The valorization of alkanes is possible via carbon carbon coupling reactions. A series of dialkyl cobalt complexes RRCH2)(2)Co-III(bpy)(2)]ClO4 (R = H, Me, Et, and Ph) were reacted with the H5PV2Mo10O40 polyoxometalate as a catalyst, leading to a selective oxidative carbon carbon bond coupling reaction. The reaction is initiated by electron transfer from [(RCH2)(2)Co-III(bpy)(2)](+) to (H5PV2Mo10O40)-Mo-V to yield an intermediate [(RCH2)(2)Co-IV(bpy)(2)](2+)-(H5PVVMo10O40)-V-IV-Mo-V, as identified by a combination of EPR and X-ray photoelectron spectroscopy experiments. The reaction is catalytic with O-2 as terminal oxidant representing an aerobic C-C bond coupling reaction.
-
(2017) Advances in Inorganic Chemistry. p. 67-90 Abstract
The unique ability of H5PV2Mo10O40 to catalyze electron transferoxygen transfer oxidations via activation of sulfides, and carbonhydrogen, carboncarbon, and carbon metal bonds, is discussed in the context of sustainable transformations of arenes, the deconstruction of carbohydrates to synthesis gas as a liquid fuel precursor and the prospects for hydroxylation of alkanes. Emphasis is placed on the mechanistic aspects of these reactions including isolation and or observation of intermediates, and the importance of solvents on the oxidation potential of H5PV2Mo10O40. Descriptions possible structures of reactive intermediates as deduced from EPR spectroscopy and DFT calculations are also included.
-
(2017) Chemistry-A European Journal. 23, 1, p. 92-95 Abstract
The photochemical reduction of CO2 to CO requires two electrons and two protons that, in the past, have been derived from sacrificial amine donors that are also non-innocent in the catalytic cycle. Towards the realization of a water-splitting reaction as the source of electrons and protons for CO2 reduction, we have found that a reduced acidic polyoxometalate, (H5PW2W10O40)-W-V, is a photoactive electron and proton donor with visible light through excitation of the intervalence charge-transfer band. Upon linking the polyoxometalate to a dirhenium molecular catalyst, a cascade of transformations occurs where the polyoxometalate is electrochemically reduced at a relatively low negative potential of 1.3 V versus Ag/AgNO3 and visible light, a 60 W tungsten lamp, or a red LED is used to transfer electrons from the polyoxometalate to the dirhenium catalyst active for the selective reduction of CO2 to CO.
2016
-
(2016) ACS Sustainable Chemistry and Engineering. 4, 10, p. 5799-5803 Abstract
Based on the knowledge that o-aminomethylphenylboronic acids reversibly bind to carbohydrates, relevant water-soluble derivatives of the former were prepared by appending hydrophilic tethers. In this way the phenylboronic acid derivatives were used to hydrolytically dissolve, i.e. depolymerize cellulose in water at nearly neutral pH values. Some of these hydrophilic tethers consisted of moieties that were surmised to be able to promote hydrolysis of the glycosidic bonds such as a carboxylic acid, phosphonic acid as weak Bronsted acids, or an imidazole functionality as a nucelophilic catalyst; water-soluble polyethylene glycol and polyethyenelimine appendages were also used. The results show that at around 120 °C efficient hydrolysis of cellulose to form water-soluble oligosaccharides could be attained in a period of 24 h. Importantly preimpregnation of a morpholine substituted o-aminomethylphenylboronic acid led to the very selective formation of glucose.
-
(2016) ACS Catalysis. 6, 10, p. 6422-6428 Abstract
The photoreduction mechanism of carbon dioxide to carbon monoxide by the Re-organic hybrid polyoxometalates (POMs) {NaH[PW12O40]3-ReIL(CO)3DMA}na (L = 15-crown-5 phenanthroline, DMA = N,N-dimethylacetamide) has been investigated by means of DFT and TD-DFT calculations. The reaction mechanism can be divided into several steps, including (i) photoexcitation and charge transfer, (ii) DMA release, (iii) CO2 addition, (iv) protonation, and (v) CO release and regeneration of the catalyst. The charge transfer (CT) states, POM to Re complex, are efficiently induced by metal-centered (MC) excitations occurring on the reduced POM. Once one electron is transferred to the organometallic unit from the excited POM, the Re is able to bind and activate the CO2 substrate. Subsequent steps that involve protonation of CO2 and CO release are favorable thermodynamically and are induced by a second electron transfer from the POM to the Re complex. In this reaction, the POM acts as photosensitizer, electron reservoir, and electron donor.
-
(2016) Chemistry-A European Journal. 22, 43, p. 15231-15236 Abstract
The use of confined space to modulate chemical reactivity and to sequester organic compounds spans significant disciplines in chemistry and biology. Here, the inclusion and assembly of arenes into a water-soluble porous metal oxide nanocapsule [{(MoVI)MoV5O21(H2O)6}12{MoV2O4(CH3COO)}30]42−(Mo132) is reported. The uptake of benzene, halobenzenes, alkylbenzenes, phenols, and other derivatives was studied by NMR, where it was possible to follow the encapsulation process from the outside of the capsule through its pores and then into the interior. The importance of size or shape of the arenes, and various intermolecular bond interactions contributed by the benzene substituent on the encapsulation process was studied, showing the importance of ππ stacking and CHπ interactions. Furthermore, by using NOESY, ROESY, and HOESY NMR techniques it was possible to understand the interaction of the encapsulated arenes and the acetate linkers or ligands that line the interior of the Mo132capsule.
-
(2016) ACS Catalysis. 6, 10, p. 6403-6407 Abstract
The activation of very strong C-H bonds, such as those found in benzene, is important also in the quest for new routes for its functionalization. Using the H5PV2Mo10O40 polyoxometalate as an electron transfer oxidant in >50% aqueous H2SO4 as solvent, the formation of a benzene radicaloid species at RT as probed by visible spectroscopy and by EPR spectroscopy recorded at X-band and W-band, including ELDOR-detected NMR, was verified. The viability of the ET oxidation of benzene is supported by DFT calculations, showing the reaction to be exergonic under these conditions. Furthermore, we show that in the presence of O2, very selective hydroxylation to phenol took place.
-
(2016) Dalton Transactions. 45, 37, p. 14534-14537 Abstract
First row transition metal substituted polyfluorooxmetalates with quasi Wells-Dawson structures and a nitro terminal ligand, [NaH2M(NO2)W17F6O55]q-, were used as catalysts for the aerobic epoxidation of cyclic alkenes. The Cu(NO2) analog combined the best traits of conversion and selectivity. Some C-C bond cleavage was also observed and cis isomers reacted preferentially without stereochemical inversion indicating an oxygen atom to double bond concerted reaction.
-
(2016) Physical chemistry chemical physics : PCCP. 18, 32, p. 22487-22493 Abstract
The electrochemical properties of Keggin type polyoxometalates Qn[XW12O40] (X = P, Si, B; Q = n-tetraoctylammonium and n-trioctylmethylammonium) in organic solvents were investigated in order to understand the interrelation between the redox potentials, solvents and ion pairing. A logarithmic correlation between the dielectric constant of the solvent (ϵ ranged from 4.8 to 46.6) and the reduction potential of the [PW12O4]3-/[PW12O4]4- couple was found. This reduction potential increased significantly when the surface charge of the polyoxometalate went from 3- to 5-. The investigation of the ion pairing properties by diffusion NMR revealed the presence of intimate ion pairs in less polar solvents (e.g. dichloromethane) and less compact ion pairs in more polar solvents (e.g. DMSO). Using a V atom within the polyoxometalate an NMR experiment showed that a n-trioctylmethyl ammonium cation bonded to the polyoxometalate anion more intimately than a n-tetraoctyl ammonium cation.
-
(2016) European Journal of Inorganic Chemistry. 2016, 17, p. 2757-2763 Abstract
Various ruthenium(II) complexes with proximal oxophilic phenylselenium groups of the general formula [RuIILALB]X2{LA= LB= 6,6-bis[(4-methoxyphenyl)selanyl]-2,2-bipyridine; 6,6-bis[(nitrophenyl)selanyl]-2,2-bipyridine; 3,6-bis(phenylselanyl)dipyrido[3,2-a:2,3-c]phenazine; LA= 6,6-bis(phenylselanyl)-2,2-bipyridine, LB= terpyridine} were prepared. The substitution patterns of these compounds were designed to have different electron-withdrawing/-donating properties or different binding motifs in comparison to the previously reported compound with LA= LB= 6,6-bis(phenylselanyl)-2,2-bipyridine. The research objective was to evaluate the potential of these compounds to activate ground-state molecular oxygen to form higher-valent RuOSe bonds by cleavage of the OO bond of O2. All of the compounds prepared indeed activated O2to form RuOSe moieties, as observable by UV/Vis spectroscopy, mass spectrometry, or X-ray crystallography.
2015
-
(2015) Journal of Organometallic Chemistry. 793, p. 134-138 18919. Abstract
The reactivity of various vanadium-substituted polyoxometalates with different structures and oxidation potentials was compared for the electron transfer-oxygen transfer oxidation of anthracene and dihydroanthracene as model substrates. Electron transfer initiates the C-H bond activation. Analysis of reactivity via an outer sphere electron transfer model demonstrates that in addition to the oxidation potential, the polyoxometalate structure and charge are important parameters in the electron transfer reaction. A further comparative study for electron transfer and oxygen transfer reactivity showed that the eventual formation of the oxygenated product, although initiated by electron transfer was dependent on other factors.
-
(2015) Journal of the American Chemical Society. 137, 27, p. 8738-8748 Abstract
Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH2(Mn-L)W17F6O55]q-, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O2, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K+, Rb+, and Cs+) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li+ and Na+ did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O2 formation is fastest in the presence of Cs+ and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O2 formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O2 formation.
-
(2015) ACS Catalysis. 5, 6, p. 3336-3341 Abstract
The epoxidation of alkenes with peroxides by W-VI, Mo-VI, V-V, and Ti-IV compounds is well established, and it is well accepted that the active intermediate peroxo species are electrophilic toward nucleophilic substrates. Polyoxotungstates, for example, those of the sandwich structure, [WZn(TML)(2)(ZnW9O34)(2)](q-) in which TM = transition metal and L = H2O, have in the past been found to be excellent epoxidation catalysts. It has now been found that substituting the Lewis basic Bi-III into the terminal position of the sandwich polyoxometalate structure to yield [Zn2Bi2III(ZnW9O34)(2)](14-) leads to an apparent umpolung of the peroxo species and formation of a nucleophilic peroxo intermediate. There are two lines of evidence that support the formation of a reactive nucleophilic peroxo intermediate: (1) More electrophilic sulfoxides are more reactive than more nucleophilic sulfides, and (2) nonfunctionalized aliphatic alkenes and dienes showed ene type reactivity rather than epoxidation pointing toward dark formation of singlet oxygen from the nucleophilic intermediate peroxo species. Allylic alcohols reacted much faster than alkenes but showed chemoselectivity toward C-H bond activation of the alcohol and formation of aldehydes or ketones rather than epoxidation. This explained via alkoxide formation at the Bi-III center followed by oxidative beta-elimination.
-
(2015) Journal of the American Chemical Society. 137, 18, p. 5916-5922 Abstract
The synthesis of benzaldehyde derivatives by oxygenation of methylarenes is of significant conceptual and practical interest because these compounds are important chemical intermediates whose synthesis is still carried out by nonsustainable methods with very low atom economy and formation of copious amounts of waste. Now an oxygenation reaction with a 100% theoretical atom economy using a polyoxometalate oxygen donor has been found. The product yield is typically above 95% with no "overoxidation" to benzoic acids; H2 is released by electrolysis, enabling additional reaction cycles. An electrocatalytic cycle is also feasible. This reaction is possible through the use of an aqueous sulfuric acid solvent, in an aqueous biphasic reaction mode that also allows simple catalyst recycling and recovery. The solvent plays a key role in the reaction mechanism by protonating the polyoxometalate thereby enabling the activation of the methylarenes by an electron transfer process. After additional proton transfer and oxygen transfer steps, benzylic alcohols are formed that further react by an electron transfer-proton transfer sequence forming benzaldehyde derivatives. (Chemical Equation Presented).
-
(2015) Organic & Biomolecular Chemistry. 13, 15, p. 4374-4394 Abstract
The investigation of new oxidative transformations for the synthesis of carbon-heteroatom and heteroatom-heteroatom bonds is of fundamental importance in the synthesis of numerous bioactive molecules and fine chemicals. In this context, NaIO4, an exciting reagent, has attracted increasing attention enabling the development of these unprecedented oxidative transformations that are difficult to achieve otherwise. Thus, NaIO4 has been successfully explored as a versatile oxidant for a variety of fundamental organic transformations such as C-H activation, oxidative functionalization of alkenes and other interesting oxidative transformations and its application in the synthesis of bioactive natural products. This review summarizes recent developments in this area with NaIO4 as a versatile oxidant and brings out many challenges that still remain elusive for the future.
-
(2015) Inorganic Chemistry. 54, 2, p. 628-634 Abstract
The crystal packing and secondary structure of H5PV2Mo12O40 was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H5PV2Mo12O40 becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H5PV2Mo12O40 to crystalline MoO3 and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO3. Importantly, H5PV2Mo12O40 can be easily recovered by dissolution in water at 80 °C.
2014
-
(2014) Chemical Communications. 50, 87, p. 13247-13249 Abstract
The H5PV2Mo10O40 polyoxometalate catalysed the electron transfer oxidation of sulphite to yield a sulphite radical, SO3- that upon addition of O2 yielded a peroxosulphate species efficient for the H5PV2Mo10O40 catalysed epoxidation of alkenes. The acidic polyoxometalate further catalysed hydrolysis of the epoxide to give vicinal diols in high yields.
-
(2014) Journal of the American Chemical Society. 136, 31, p. 10941-10948 Abstract
A new aerobic carbon-carbon bond cleavage reaction of linear di-substituted alkenes, to yield the corresponding aldehydes/ketones in high selectivity under mild reaction conditions, is described using copper(II)-substituted polyoxometalates, such as {α2-Cu(L)P2W 17O61}8- or {[(Cu(L)]2WZn(ZnW 9O34)2}12-, as catalysts, where L = NO2. A biorenewable-based substrate, methyl oleate, gave methyl 8-formyloctanoate and nonanal in >90% yield. Interestingly, cylcoalkenes yield the corresponding epoxides as products. These catalysts either can be prepared by pretreatment of the aqua-coordinated polyoxometalates (L = H 2O) with NO2 or are formed in situ when the reactions are carried with nitroalkanes (for example, nitroethane) as solvents or cosolvents. Nitroethane was shown to release NO2 under reaction conditions. 31P NMR shows that the Cu-NO2-substituted polyoxometalates act as oxygen donors to the C-C double bond, yielding a Cu-NO product that is reoxidized to Cu-NO2 under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {α2-Co(NO2)P2W17O 61}8- as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C-C bond cleavage involving two molecules of {α2-Co(NO2)P2W 17O61}8- and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by 31P NMR.
-
-
(2014) Nature Communications. 5, 4621. Abstract
Terrestrial plants contain ∼70% hemicellulose and cellulose that are a significant renewable bioresource with potential as an alternative to petroleum feedstock for carbon-based fuels. The efficient and selective deconstruction of carbohydrates to their basic components, carbon monoxide and hydrogen, so called synthesis gas, is an important key step towards the realization of this potential, because the formation of liquid hydrocarbon fuels from synthesis gas are known technologies. Here we show that by using a polyoxometalate as an electron transfer-oxygen transfer catalyst, carbon monoxide is formed by cleavage of all the carbon-carbon bonds through dehydration of initially formed formic acid. In this oxidation-reduction reaction, the hydrogen atoms are stored on the polyoxometalate as protons and electrons, and can be electrochemically released from the polyoxometalate as hydrogen. Together, synthesis gas is formed. In a hydrogen economy scenario, this method can also be used to convert carbon monoxide to hydrogen.
-
(2014) Journal of Cluster Science. 25, 3, p. 687-693 Abstract
The tri-rhenium(VI) capped Wells-Dawson polyoxometalate, [Re3P2W15O62]6- with quaternary ammonium cations was synthesized by reacting the trivacant lacunary species, [P2W15O56]12- with [ReOCl3(PPh3)2] in an organic solvent. Elemental analysis by thermogravimetry and inductively coupled plasma mass spectroctrometry confirmed the substitution of three rhenium atoms, single-crystal X-ray diffraction as well as infra red spectroscopy showed the complete Wells-Dawson structure. The presence of all three rhenium atoms in one cap is indicated by 31P nuclear magnetic resonance spectroscopy and the electron spin resonance spectrum shows that the tri-rhenium(V) species with three unpaired electrons is low spin, S = 1/2.
-
(2014) Journal of Archaeological Science. 42, 1, p. 509-518 Abstract
Biologically produced compounds preserved in ancient ceramics can provide invaluable information on the vessel contents. Analysis and interpretation of these so-called archaeological "residues" is therefore important for understanding and reconstructing aspects of social and cultural behaviors of ancient societies. Based on the reaction of unsaturated compounds with iodine, we developed and apply two new methods. The first is a simple and relatively rapid method for assessing the amounts of unsaturated compounds in archaeological ceramics using X-ray fluorescence (XRF). We show that this method is a reliable indicator for assessing the general preservation state of the organic material and therefore a potential pre-screening method for identifying ceramic samples suitable for organic residue analysis. The second complementary approach, based on the same iodine reaction, makes it possible to map the unsaturated molecules on ceramic surfaces at a scale that enables to correlate organic matter distribution and the underlying mineral grains using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) detector. With this method we show that the extent of adsorption of lipids onto ceramic surfaces varies with the surface properties of the different minerals in the ancient ceramic, with calcium containing minerals showing the highest affinity for compounds with unsaturated bonds. The ceramic substrate therefore influences the types of organic compounds bound and hence preserved in the ceramic. Fundamental information obtained using this method is essential for better interpreting molecular assemblages extracted from archaeological ceramics.
-
(2014) Inorganic Chemistry. 53, 3, p. 1779-1787 Abstract
High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbonhydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-mu-oxo polyoxometalate compound [(alpha(2)-P2W17O61Co)(2)O](14-), [(POMCo)(2)O], has now been isolated and characterized by the oxidation of a monomeric [alpha(2)-P2W17O61CoII(H2O)](8), [(POMCoH2O)-H-II], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a CoO bond length of similar to 1.77 A. In aqueous solution [(POMCo)(2)O] was identified by P-31 NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo)(2)O](2)O] is an active compound for the oxidation of H2O to O-2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)(2)O](2)O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.
2013
-
(2013) Journal of the American Chemical Society. 135, 51, p. 19304-19310 Abstract
The polyoxometalate H5PV2Mo10O 40 mediates the insertion of an oxygen atom from H5PV 2Mo10O40 into the tin-carbon bond of n-Bu 4Sn through its activation by electron transfer to yield 1-butanol and (n-Bu3Sn)2O. The reaction is initiated by electron transfer from n-Bu4Sn to H5PVV2Mo10O40 to yield the ion pair n-Bu 4Sn+-H5PVIVV VMo10O40. The H5PV IVVVMo10O40 moiety was identified by UV-vis and EPR. DFT calculations show that n-Bu4Sn +-H5PVIVVVMo 10O40 is relatively unstable and forms more stable Bu + and Bu3Sn+ cations coordinated to the polyoxometalate, which were also identified by ESI-MS. Products are released at higher temperatures. In the presence of molecular oxygen as the terminal oxidant the reaction is catalytic.
-
(2013) ACS Catalysis. 3, 9, p. 1915-1918 Abstract
Alkylated polyethylenimine or homochiral polyethylenimine derivatives, or both, were used to stabilize palladium nanoparticles dispersed in water. Such constructs have hydrophobic regions that enabled the aqueous biphasic hydrogenation of imines formed in situ by the reaction of ketones and chiral primary amines. In the presence of MgCl2, good yields of the direct amination product were obtained with high diastereoselectivity. In the presence of a homochiral polyethylenimine, enantiospecific direct amination was observed via kinetic resolution, ostensibly as a result of preferred access of one enantiomer to the achiral reaction center.
-
(2013) European Journal of Inorganic Chemistry. 10-11, p. 1649-1653 Abstract
Crystallization of the {HW9O33} isopolyanion in the presence of MII(DMSO)4Cl2 (where M = Ru, Os) and Na and K cations yielded one-dimensional chains, {K[HW9O 33M2(C2H6SO)6] 6-}n. The {HW9O33} isopolyanion is capped by MII(DMSO)3 moieties, which are, in turn, linked by potassium cations. The chains for the RuII-based compound show a prismatic coordination around the K+ linker and {HW9O 33} moieties that are eclipsed relative to each other along the chain, while the OsII-based compound has an octahedral coordination around the K+ linker and {HW9O33} moieties that are staggered relative to each other along the chain. The three-dimensional arrangement of the two compounds is quite different. For the RuII compound, channels are obtained by the arrangement of six {K[HW 9O33Ru2(C2H6SO) 6]6-}n chains along the c axis. The channel is stabilized by the binding of sodium cations, which interlock the chains through coordination of different oxygen atoms and leads to a tight staggered arrangement between the chains of {K[HW9O33Ru 2(C2H6SO)6]6-} n. The channels have an approximate diameter of 8 Å and are occupied by layers of six sodium cations that form chair-like hexagons with Na-Na interatomic distances of 2.807 Å. There are five water molecules that separate the layers of the Na+ cations. For the osmium compound, two different types of channels, defined by the arrangement three {K[HW 9O33Os2(C2H6SO) 6]6-}n chains, are formed. The channels are formed through interchain bonding through a bridge formed by connecting sodium atoms to the terminal atoms of the {HW9O33} unit. Here, the {K[HW9O33Os2(C2H 6SO)6]6-}n chains are eclipsed relative to one another. Water molecules occupy one channel, while the other channel is occupied by K+ cations hexacoordinated to aqua ligands. The approximate diameter of the channels is 5.5 Å.
-
Cinnamaldehyde in early iron age phoenician flasks raises the possibility of levantine trade with South East Asia(2013) Mediterranean Archaeology & Archaeometry. 13, 2, p. 1-19 Abstract
Small ceramic flasks with thick walls and narrow openings were produced in Phoenicia. These flasks were common in Phoenicia, the southern Levant and Cyprus in the early Iron Age, namely in the 11th-mid-9th centuries BCE. Their shape, size, decoration and find-contexts suggest that they contained some precious materials and were part of a commercial network operating in these regions. We analyzed the lipid contents of 27 such containers from 5 archaeological sites in Israel using gas chromatography coupled with mass spectrometry (GC-MS). The organic extractions of 10 of these flasks contained cinnamaldehyde (C9H8O), a major component of cinnamon. In antiquity the cinnamon tree grew only in South and South East Asia. As cinnamaldehyde is found in small quantities in some modern potential contaminants, possible contamination of the small flasks with this compound was carefully assessed. Significantly, two recently excavated small flasks that were not handled directly contained relatively high concentrations of cinnamaldehyde. Other vessel types from the same archaeological sites and in some cases the same contexts did not contain cinnamaldehyde. Thus it is unlikely that the presence of cinnamaldehyde in the flasks results from contamination. This finding raises the intriguing possibility of long distance trade in the early Iron Age, assuming that the extracted cinnamaldehyde is indeed derived from the bark of the cinnamon tree. This is consistent with other suggestions that trade from South/South East Asia to the West took place at such an early date.
-
(2013) Chemical Communications. 49, 17, p. 1720-1722 Abstract
Trifluoroethanol was oxidized with O2 to trifluoroethyl trifluoracetate (>98% selectivity) using PtII(dppz)Cl2 as a catalyst in the presence of H2SO4; PtII(phen)Cl2 was inactive. Kinetic isotope effects suggest the CH bond activation as the rate determining step and DFT calculations showed different frontier orbitals for PtII(dppz) and PtII(phen)-based catalysts.
2012
-
(2012) ACS Catalysis. 2, 12, p. 2531-2536 Abstract
Asymmetric homogeneous catalysis is well established, but the design of new chiral catalysts and the optimization of an existing catalytic system are time- and manpower-exhausting processes. Desirable, heterogeneous, chiral analogues to facilitate catalyst recovery are relatively rare or function poorly. Here we further develop our strategy for facile adaptation of known reactions involving homogeneous achiral catalysts to those involving heterogeneous asymmetric catalysts by use of a homochiral solid scaffold and show the generality of this concept, from hydrolysis and oxidation reactions to hydrogenations. In this case, an inexpensive "off-the shelf" achiral hydrogenation catalyst, RuIICl2(R2PCH2CH2NH) 2, R = Ph, i-Pr, or t-Bu, embedded within a homochiral matrix is an enantioselective, recyclable heterogeneous catalyst for ketone hydrogenation. The amorphous matrix consists of tripodal poly(phenylglycine) capped with phosphonate moieties and cross-linked with titanium oxide. Hydrogenation of acetophenone derivatives can proceed with high enantioselectivity (up to 95% ee), and catalyst recycling by filtration is very effective.
-
(2012) Journal of the American Chemical Society. 134, 51, p. 20669-20680 Abstract
Metal oxides as a rule oxidize and oxygenate substrates via the Mars-van Krevelen mechanism. A well-defined α-Keggin polyoxometalate, H 5PV2Mo10O40, can be viewed as an analogue of discrete structure that reacts via the Mars-van Krevelen mechanism both in solution and in the gas phase. Guided by previous experimental observations, we have studied the key intermediates on the reaction pathways of its reduction by various compounds using high-level DFT calculations. These redox reactions of polyoxometalates require protons, and thus such complexes were explicitly considered. First, the energetics of outer-sphere proton and electron transfer as well as coupled proton and electron transfer were calculated for seven substrates. This was followed by identification of possible key intermediates on the subsequent reaction pathways that feature displacement of the metal atom from the Keggin structure and coordinatively unsaturated sites on the H5PV2Mo10O40 surface. Such metal defects are favored at vanadium sites. For strong reducing agents the initial outer-sphere electron transfer, alone or possibly coupled with proton transfer, facilitates formation of metal defects. Subsequent coordination allows for formation of reactive ensembles on the catalyst surface, for which the selective oxygen-transfer step becomes feasible. Weak reducing agents do not facilitate defect formation by outer-sphere electron and/or proton transfers, and thus formation of metal defect structures prior to the substrate activation is suggested as an initial step. Calculated geometries and energies of metal defect structures support experimentally observed intermediates and demonstrate the complex nature of the Mars-van Krevelen mechanism.
-
(2012) ChemPlusChem. 77, 11, p. 977-981 Abstract
Chiral cross-linked polyethylenimines were used to intercalate MnIII salen catalysts, thereby inducing a chiral environment upon an achiral metal complex. The synzyme, dispersed in water, catalyzes the aqueous biphasic asymmetric epoxidation of styrene derivatives (see structure; L=Cl, OAc, R=isopropyl, phenyl). In the presence of the chiral catalyst there is a significant synergistic effect that increases the enantioselectivity of epoxidation.
-
(2012) Fundamentals of Materials for Energy and Environmental Sustainability. cahen D. & S. Ginley D.(eds.). p. 536-548 Abstract
Focus The chemical industry is intimately linked with the realities of transformation of fossil fuels to useful compounds and materials, energy consumption, and environmental sustainability. The questions that concern us are the following: how can one minimize energy consumption in the chemical industry and reduce waste formation in chemical reactions, and can we transform this industry from one based primarily on petroleum to one that utilizes renewable feedstocks? In this chapter we will see how this \u201cgreening\u201d of this major industrial and energy sector will require the use of catalysis and development of new catalysts. Synopsis Thomas Jefferson stated that \u201cthe earth belongs in usufruct to the living.\u201d This premise is the basis of present recognition that, if the natural capacity of planet Earth to deal with pollution and waste is exceeded, then our lifestyle will become unsustainable. In this context, it is difficult to imagine life in the twenty-first century without accounting for the role the chemical industry plays in our modern society. For example, in the transportation sector, the most obvious aspect is the production of efficient fuels from petroleum, but one also should consider catalytic converters that reduce toxic emissions, and engineering polymers and plastics that reduce vehicle weight and therefore reduce fuel consumption. In daily consumer life we use chemicals in products such as paints, DVDs, synthetic carpets, refrigerants, packaging, inks and toners, liquid-crystal displays, and synthetic fibers. Pesticides and fertilizers are needed in order to increase agricultural productivity and pharmaceuticals to keep us healthy. In this chapter we will try to understand the aspects involved in the sustainability of the chemical industry. We will describe how to measure and control environmental performance and then define what we mean by green processing in the chemical industry, specifying green process metrics, introducing key concepts such as atom economy, E-factors, and effective mass yields. With these concepts together with the evaluation raw material costs, waste treatment, and unit processes needed for production of a chemical one can appreciate the \u201cgreenness\u201d of a chemical process. The second part of this chapter will describe the role catalysis has in chemical transformations and how catalysis is used to reduce energy consumption and waste formation, together leading to increased sustainability. Examples will be given for a spectrum of applications ranging over oil refining, ammonia production, manufacture of important chemical intermediates and materials, and use of catalysis for the production of commodity chemicals. Finally, we will discuss the role catalysis may play in the replacement of fossil-fuel feedstocks with renewable ones, and how catalysis may contribute to our search for solar fuels.
-
(2012) Journal Of Molecular Catalysis A-Chemical. 356, p. 152-157 Abstract
An antimony(V) containing α-Keggin type acidic polyoxometalate, H 4PSbMo 11O 40, was prepared by reacting NaMoO 4, H 3PO 4 and Sb 2O 3 in the presence of aqua regia to appraise its reactivity compared to the well known vanadate analog, H 4PVMo 11O 40. Characterization was by X-ray diffraction, MALDI-TOF MS, IR, UV-vis and 31P NMR spectroscopy. Catalytic redox reactions, such as oxidative dehydrogenation using O 2 and N 2O as terminal oxidants were studied and showed very different reactivity of H 4PSbMo 11O 40 versus H 4PVMo 11O 40. It was found by DFT calculations that in contrast to analogous H 4PVMo 11O 40 where vanadium centered catalysis is observed, in H 4PSbMo 11O 40 catalysis is molybdenum and not antimony centered.
2011
-
(2011) Inorganic Chemistry. 50, 22, p. 11273-11275 Abstract
The catalytic photocleavage of CS2 to S8 and a (CxSy)n polymer with visible light using a dinuclear ruthenium(II) compound with a bipyridine units for photoactivity and a vicinal tertiary amine binding site for CS2 activation was studied. The catalyst was characterized by X-ray diffraction, 1H NMR, and 13C NMR, ESI-MS and elemental analysis. CS2 photocleavage was significant (240 turnovers, 20 h) to yield isolable S8 and a (CxSy)n polymer. A mononuclear catalyst or one without an amine binding site showed significantly less activity. XPS of the (CxSy)n polymer showed a carbon/sulfur ratio ∼1.5-1.6 indicating that in part both C-S bonds of CS2 had been cleaved. Catalyst was also included within the polymer. The absence of peaks in the 1H NMR verified the (CxSy)n nature of the polymer, while 13C NMR and IR indicated that the polymer had multiple types of C-S and C-C bonds.
-
(2011) Journal of Physical Chemistry A. 115, 18, p. 4811-4826 Abstract
Protonated phosphovanadomolybdates of the Keggin structure, H 3+xPVxMo12-xO40 where x = 0, 1, 2, and derivatives with surface defects formed by loss of constitutional water were studied using high-level DFT calculations toward determination of the most stable species and possible active forms in oxidation catalysis in both the gas phase and in polar solutions. The calculations demonstrate that protonation at bridging positions is energetically much more favorable than protonation of terminal oxygen atoms. The preferential protonation site is determined by the stability of the metal-oxygen bond rather than the negative charge on the oxygen atom. In H3PMo12O40, maximum distances between protons at bridging oxygen atoms are energetically favored. In contrast, for H4PVMo11O40 and H5PV 2Mo10O40 protons prefer nucleophilic sites adjacent to vanadium atoms. Up to three protons are bound to the nucleophilic sites around the same vanadium atom in the stable isomeric forms of H 5PV2Mo10O40 that result in strong destabilization of oxo-vanadium(V) bonding to the Keggin unit. Such behavior arises from the different nature of the Mo-O and V-O bonds that can be traced to the different sizes of the valence d orbitals of the metals. Coordination of two protons at the same site yields water and an oxygen defect as a result of its dissociation. The energetic cost for the formation of surface defects decreases in the order: Ot ≫ Oc ≲ Oe and is lower for the sites adjacent to vanadium atoms. Vanadium atoms near defects also have a significant contribution to the LUMO. Thus, vanadium-substituted polyoxometalates with defects near and, especially, between vanadium atoms present a plausible active form of polyoxometalates in oxidation reactions.
-
(2011) European Journal of Inorganic Chemistry. 11, p. 1792-1796 Abstract
Copper(I) complexes with perfluorinated tails, (CuIL 2AcN)BF4 and (CuIL3AcN)BF 4, where L2 = 6,6-bis(perfluorohexyl)-2,2: 6,2-terpyridine and L3 = 6,6-bis(2,2,3,3,4,4,5, 5,6,6,7,7,7-tridecafluoroheptyl)-2,2-bipyridine and AcN = acetonitrile, were synthesized and their rather unique crystal structures were elucidated. (CuIL2AcN)BF4 crystallizes in a low-symmetry space group, P$\bar {1}$, with a very large unit cell that required synchrotron radiation to locate all atoms. The molecular structure of (CuIL2AcN)BF4 has an asymmetric tetrahedrally distorted square-planar coordination sphere around CuI. Eight similar but distinct molecular structures were found. The crystal packing of (Cu IL2AcN)(BF4) has a layered structure with a clear segregation between the terpyridine moieties and the fluorous chains. The terpyridine units are arranged in a "back to back" fashion and tilted 20-25° relative to the interlayer plane, and the layered structure is staggered with offsets of approximately 4 Å between layers. The molecular structure of (CuIL3AcN)(BF4) features a Y-shaped trigonal planar coordination sphere around copper(I). Comparison of the structure with analogous CuI(neocuproine)AcN showed that the fluorous tails with the CH2CH2 spacers basically have no effect on the bond lengths and angles to Cu1. The crystal structure of (CuIL3AcN)(BF4) shows a "herring bone" conformation with an approximately 96° angle in the CubipyAcN units. The "herring bone" conformation leads to an accordion-type fluorous layer. Copper(I) complexes of bipyridine (Y-shaped trigonal planar) and terpyridine (tetrahedrally distorted square-planar) with fluorous tails showed crystal packing with segregation of the aromatic units and the fluorous tails. Layered structures that are tilted in two different dimensions as well as "herring bone" structures with accordion-like fluorous layers were observed.
-
(2011) ChemSusChem. 4, 3, p. 346-348 Abstract
Gone in 7200 Seconds: Heteroaromatic thiophene derivatives, refractory to hydrodesulfurization, are removed from hydrocarbons by oxidative polymerization over H5PV2Mo10O40/SiO2 as a recyclable, heterogeneous catalyst. The levels of sulfur-containing compounds reach sub-ppm to nondetectable levels, as analyzed by gas chromatography with a flame photometric detector.
-
(2011) Chemical Communications. 47, 9, p. 2535-2537 Abstract
An insoluble, porous, amorphous, homochiral material based on a polypeptide titaniumphosphonate scaffold with an encapsulated achiral MnIIIsalen was prepared and characterized. Consecutive epoxidation and hydration of styrene and its derivatives by aqueous hypochlorite in THF showed the highly enantioselective (>99%) formation of styrene diol derivatives.
-
(2011) Journal of the American Chemical Society. 133, 2, p. 188-190 Abstract
A phenanthroline ligand decorated at the 5,6-position with a 15-crown-5 ether was used to prepare a metalorganic-polyoxometalate hybrid complex Re I(L)(CO)3CH3CN-MHPW12O40 (L = 15-crown-5-phenanthroline, M = Na+, H3O+). X-ray diffraction, 1H and 13C NMR, ESI-MS, IR, and elemental analysis were used to characterize this complex. In the presence of Pt/C, the polyoxometalate moiety in ReI(L)(CO)3CH 3CN-MHPW12O40 can oxidize H2 to two protons and two electrons which in the presence of visible light can catalyze the photoreduction of CO2 to CO with H2 as the reducing agent instead of the universally used amines as sacrificial reducing agents. An EPR spectrum of a stable intermediate species under reaction conditions shows characteristics of a PWVWVI11O40 and a Re0 species with a tentative assignment of the intermediate as Re0(L)(CO)3(S)-MH3PWVW VI11O40.
2010
-
-
(2010) Chemistry-A European Journal. 16, 33, p. 10014-10020 Abstract
An in-depth spectroscopic EPR investigation of a key intermediate, formally notated as [PVIVVVMo10O40] 6- and formed in known electron-transfer and electrontransfer/oxygen- transfer reactions catalyzed by H5PV2Mo10O 40, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W-band electron-electron double resonance (ELDOR)-detected NMR and two-dimensional (2D) hyperfine sub-level correlation (HYSCORE) measurements, which resolved 95Mo and 17O hyperfine interactions, and electron-nuclear double resonance (ENDOR), which gave the weak 51V and 31P interactions. In this way, two paramagnetic species related to [PVIVVVMo10O40]6- were identified. The first species (30-35%) has a vanadyl (VO2+)-like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO2+ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PVVMo10O39] 8--[VIVO2+]. This species originates from the parent H5PV2Mo10O40 in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electrontransfer/oxygen-transfer reaction oxidation reactions. In the second (70-65 %) species, the VIV remains embedded within the polyoxometalate framework and originates from reduction of distal H5PV2Mo10O40 isomers to yield an intact cluster, [PVIVVVMo10O 40]6-.
-
(2010) Advanced Synthesis & Catalysis. 352, 13, p. 2159-2165 Abstract
A porous, homochiral titanium-phosphonate material based on a tripodal peptide scaffold was used as a heterogeneous reaction medium for the enantioselective hydration (>99%) of styrene oxide. This titanium-phosphonate material, which was shown to contain confined chiral spaces, was prepared by polymerization of L-leucine onto a tris(2-aminoethyl)amine initiator, followed by capping with phosphonate groups and completed by non-aqueous condensation with titanium isopropoxide. Circular dichroism confirmed that the peptide tethers yielded a secondary structure. X-ray powder diffraction and transmission electron microscopy supported by a semi-empirical model showed the likely formation of a porous, lamellar material that was quantified by nitrogen adsorption.
-
(2010) Journal of the American Chemical Society. 132, 33, p. 11446-11448 Abstract
The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H5PV2Mo10O40 and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H 5PV2Mo10O40 to yield a cation radical-reduced polyoxometalate ion pair, R2+·, H5PVIVVVMo10O40, that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe +· and H5PVIVVVMo 10O40) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for VIV). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO2+ moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET-OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt2)x(solv)5-x]2+ cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90%) but also some disulfide (10%) via carbon-sulfide bond cleavage.
-
(2010) Dalton Transactions. 39, 31, p. 7266-7275 Abstract
A ditopic 1,2-bis(2,2-bipyridyl-6-yl)ethyne ligand, L, has been synthesized for the first time by consecutive Suzuki and Sonogashira coupling reactions either in a one- or two-step synthesis. Coordination of L with some first-row transition metals, Fe, Mn and Co showed a very rich structural diversity that can be obtained with this ligand. Reaction of L with Mn II(OAc)2 yielded a dimanganese(ii) complex, [Mn 2L(μ-OAc)3]PF6, (1) where the two somewhat inequivalent trigonal-bipyramidal Mn atoms separated by 3.381 Å are bridged by L and three acetate moieties. A similar reaction of L with Mn III(OAc)3 yielded a very different dimanganese complex [Mn2L(OH)(OAc)2(DMF)2]PF 6·DMF (2) where L is a E-1,2-bis(2,2-bipyridyl- 6-yl)ethene fragment that was formed in situ. The L ligand bridges between the two Mn centers, despite its trans configuration, which leads to a very strained ethene bridging moiety. The Mn atoms are also bridged by two acetate ligands and a hydroxy group that bridges between the Mn atoms and the ethene fragment; DMF completes the octahedral coordination around each Mn atom which are separated by 3.351 Å. A comproportionation reaction of L with MnII(OAc)2 and n-Bu4NMnO4 yielded a tetramanganese compound, [Mn4(μ3-O)2(OAc) 4(H2O)2L2](PF6) 2·2CH3CN (3). Compound 3 has a dimer of dimers structure of the tetranuclear Mn core that consists of binuclear [Mn 2O(OAc)2L]+ fragment and a PF6 anion. BVS calculations indicate that 3 is a mixed-valent 2MnII plus 2MnIII compound where two [MnII2O(OAc) 2L]+ fragments are held together by MnIII-O inter-fragment linkers which have a distorted octahedral geometry. The Mn atoms in the [Mn2O(OAc)2L]+ fragments have a capped square-pyramid configuration where an aqua ligand is capped on one of the faces. Although the aqua ligand is well within a bonding distance to a carbon atom of the proximal ethyne bridge, there does not appear to be an oxygen-carbon bond formation, rather the ligand is constrained in this position, as deduced by the observation that the bond lengths and angles of the ligand are essentially the same as those for the free ligand, L. Reaction of L with perchlorate or triflate salts of Fe(ii), Mn(ii) and Co(ii) in dry acetonitrile yielded binuclear triple helicate structures (2:3 metal to L ratios) [Fe2L 3](CF3SO3)4·CH3CN (4), [Mn2L3](ClO4)4·1. 7CH3CN·1.65EtOEt (5) and [Co2L3] (ClO4)4·2CH3CN·2EtOEt (6) where each M(ii) center with a slightly distorted octahedral geometry is bridged by three of the ditopic ligands. The M-M distances varied; 5.961 Å (Mn), 6.233 Å (Co) 6.331 Å (Fe). Reaction of L with Co(ClO 4)2·6H2O in wet acetonitrile yielded a dicobalto(iii) compound, [Co2L3(O) 2](ClO4)2·H2O (7), with two types of L fragments; one bridging between the two Co centers and two non-bridging ligands, each bonded to a Co atom via one bipyridyl group where the other is non-bonding. The octahedral coordination sphere around each Co atom is completed by the formation of a cobalt-carbon bond from the two carbon atoms of the ethene moiety of the bridging ligand and by a hydroxy moiety that is also bonded to the ethene group of the non-bridging ligand. Reaction of L with Co(ClO4)2·6H2O in dry acetonitrile in the presence of Et3N yielded the tetracobalto(ii) complex {[Co 2L4(OH)4](ClO4)4} 2 (8) with a unique twisted square configuration of cobalt ions with Co-Co distances of 3.938 to 4.131 Å. In addition to the L bridging ligand the Co atoms are linked by hydroxy moieties. Some preliminary catalytic studies showed that the Mn compounds 1 and 2 were active (high yield within 3 min) for alkene epoxidation with peracetic acid and hydrogen peroxide dismutation (catalase activity).
-
(2010) Inorganic Chemistry. 49, 8, p. 3594-3601 Abstract
In this Forum Article, we discuss the use of dioxygen (O2) in oxidations catalyzed by polyoxometalates. One- and two-electron-transfer oxidation of organic substrates is catalyzed by H5PV 2Mo10O40 and often occurs via an outer-sphere mechanism. The reduced polyoxometalate is reoxidized in a separate step by O2 with the formation of water. H5PV2Mo 10O40 also catalyzes electron transfer-oxygen transfer reactions. Here, in contrast to the paradigm that high-valent oxo species are often stronger oxygen-transfer species than lower-valent species, the opposite occurs. Thus, oxygen transfer from the catalyst is preceded by electron transfer from the organic substrate. The monooxygenase-type reduction of O2 with polyoxometalates is also discussed based on the formation of a stable iron(III) hydroperoxide compound that may have implications for the oxidation of other lower-valent polyoxometalates such as vanadium(IV)- and ruthenium(II)-substituted polyoxometalates. Finally, the formation of hybrid compounds through the attachment of electron-accepting polyoxometalates to coordination compounds can modify the reactivity of the latter by making higher-valent oxidation states more accessible.
-
(2010) Advanced Synthesis & Catalysis. 352, 2-3, p. 293-298 Abstract
A hexadecyltrimethylammonium salt of a "sandwich" type polyoxometalate has been used as a ligand to attach a palladium(II) center. This Pd-POM compound was an active catalyst for the fast aerobic oxidation of alcohols. The unique property of this catalyst is its significant preference for the oxidation of primary versus secondary aliphatic alcohols. Since no kinetic isotope effect was observed for the dehydrogenation step, this may be the result of the intrinsically higher probability for oxidation of primary alcohols attenuated by steric factors as borne out by the higher reactivity of 1-octanol versus 2-ethyl-1-hexanol. The reaction is highly selective to aldehyde with little formation of carboxylic acid; autooxidation is inhibited. No base is required to activate the alcohol. The fast reactions appear to be related to the electron-acceptor nature of the polyoxometalate ligand that may also facilitate alcohol dehydrogenation in the absence of base.
-
(2010) Chemistry-A European Journal. 16, 4, p. 1356-1364 Abstract
A polyoxometalate of the Keggin structure substituted with Ru(III), (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] in which (6)Q=(C(6)H(13))(4)N(+), catalyzed the photoreduction of CO(2) to CO with tertiary amines, preferentially Et(3)N, as reducing agents. A study of the coordination of CO(2) to (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] showed that 1) upon addition of CO(2) the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.100, and g(z)=1.935), and 3) the (13)C NMR spectrum had a broadened peak of bound CO(2) at 105.78 ppm (Delta(1/2)=122 Hz). It was concluded that CO(2) coordinates to the Ru(III) active site in both the presence and absence of Et(3)N to yield (6)Q(5)[Ru(III)(CO(2))SiW(11)O(39)]. Electrochemical measurements showed the reduction of Ru(III) to Ru(II) in (6)Q(5)[Ru(III)-(CO(2))SiW(11)O(39)] at -0.31 V versus SCE, but no such reduction was observed for (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/PC1-DF level of theory showed that CO(2) is preferably coordinated in a side-on manner to Ru(III) in the polyoxometalate through formation of a Ru-O bond, further stabilized by the interaction of the electrophilic carbon atom of CO(2) to an oxygen atom of the polyoxometalate. The end-on CO(2) bonding to Ru(III) is energetically less favorable but CO(2) is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp(2) hybridization state. Formation of a O(2)C-NMe(3) zwitterion, in turn, causes bending of CO(2) and enhances the carbon sp(2) hybridization. The synergetic effect of these two interactions stabilizes both Ru-O and C-N interactions and probably determines the promotional effect of an amine on the activation of CO(2) by [Ru(III)(H(2)O)SiW(11)O(39)](5-). Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO(2) and Et(3)N. A mechanistic pathway for photoreduc
-
(2010) Journal of the American Chemical Society. 132, 2, p. 517-523 Abstract
A ruthenium(II) bipyridine complex with proximal phenylselenium tethers, [Ru](H2O)2, reacted intramolecularly with O2 in a protic slightly acidic solvent, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), to yield an O-O bond cleaved product, [Ru](O)2, with formation of two Ru-O-Se moieties. This stable compound was isolated, and its structure was determined by X-ray diffraction. The identification of the compound in solution was confirmed by ESI-MS and the 1H NMR with the associated Curie plot that showed that [Ru](O)2 was paramagnetic. The magnetic susceptibility was 2.8 μB by Evan's method suggesting a ground state triplet or biradical. DFT calculations, however, predicted a ground state singlet and an oxidized Se atom. Further it was shown that [Ru](O)2 is a potent oxygen transfer species of both O2-derived atoms to triphenylphosphine and a nucleophilic alkene such as 2,3-dimethyl-2-butene in both HFIP and acetonitrile. UV-vis spectroscopy combined with the measured stoichiometry of PPh3:O2 = ̃2 in a catalytic oxidation of PPh3 suggests a dioxygenase type activation of O 2 with structural identification of the O-O bond cleavage reaction step, formation of [Ru](O)2 as an intermediate, and the proof that [Ru](O)2 is a donor of both oxygen atoms.
2009
-
(2009) Inorganica Chimica Acta. 362, 13, p. 4760-4766 Abstract
Reaction of nickel (II) perchlorate with the ligand N,N-bis-(3,5-dipiperidin-1-yl-[2,4,6]triazin-1-yl)-pyridin-2-ylmethyl-ethane-1,2-diamine yields an octahedral Ni(II) high-spin complex 1 ([C40H56N14Ni(H2O)(CH3OH)](ClO4)2(CH3OH)2) with moderate zero-field splitting (ZFS) axial distortion parameter D/kB = 5.37 K. The ligand contributes a N4 donor set; the remaining two coordinating positions are occupied by coordinating solvents molecules. Exchange of the coordinating solvents molecules in complex 1 to thiocyanate moieties leads to formation of complex 2 ([C40H56N14Ni(NCS)2](CHCl)3) with an extended parameter D/kB = 8.80 K. The analysis of the structural and magnetic properties of complexes 1 and 2 led to the design of dinuclear complex 3 ([C40H56N14NiN3]2(ClO4)2(CH3OH)2), where two azido groups were utilized as bridging ligands. The double azido bridges in complex 3 cross each other to form a rarely observed non-coplanar (N3)2 structure. The magnetic behavior of complex 3 reveals ferromagnetic coupling interactions characterized by J/kB = 23.25 K, D1/kB = 7.90 K, D2/kB = 0.54 K.
-
(2009) Inorganica Chimica Acta. 362, 13, p. 4713-4720 Abstract
Manganese(II) complexes, Mn2L13(ClO4)4, MnL1(H2O)2(ClO4)2, MnL2(H2O)2(ClO4)2, and {(μ-Cl)MnL2(PF6)}2 based on N,N-bis(2-pyridinylmethylene) ethanediamine (L1) and N,N-bis(2-pyridinylmethylene) propanediamine (L2) ligands have been prepared and characterized. The single crystal X-ray diffraction analysis of Mn2L23(ClO4)4 shows that each of the two Mn(II) ion centers with a Mn-Mn distance of 7.15 Å are coordinated by one ligand while a common third ligand bridges the metal centers. Solid-state magnetic susceptibility measurements as well as DFT calculations confirm that each of the manganese centers is high-spin S = 5/2. The electronic structure obtained shows no orbital overlap between the Mn(II) centers indicating that the observed weak antiferromagentism is a result of through space interactions between the two Mn(II) centers. Under different reaction conditions, L1 and Mn(II) yielded a one-dimensional polymer, MnL1(H2O)2(ClO4)2. Ligand L2 when reacted with manganese(II) perchlorate gives contrarily to L1 mononuclear MnL2(H2O)2(ClO4)2 complex. The analysis of the structural properties of the MnL2(H2O)2(ClO4)2 lead to the design of dinuclear complex {(μ-Cl)MnL2(PF6)} where two chlorine atoms were utilized as bridging moieties. This complex has a rhomboidal Mn2Cl2 core with a Mn-Mn distance of 3.726 Å. At room temperature {(μ-Cl)MnL2(PF6)} is ferromagnetic with observed μeff = 4.04 μB per Mn(II) ion. With cooling, μeff grows reaching 4.81 μB per Mn(II) ion at 8 K, and then undergoes ferromagnetic-to-antiferromagnetic phase transition.
-
(2009) Inorganic Chemistry. 48, 16, p. 7947-7952 Abstract
The H5PV2Mo10O40 polyoxometalate and Pd/AI2O3 were used as co-catalysts under anaerobic conditions for the activation and oxidation of CO to CO 2 by an electron transfer-oxygen transfer mechanism. Upon anaerobic reduction of H5PV2Mo10O40 with CO in the presence of Pd(0) two paramagnetic species were observed and characterized by continuous wave electron paramagnetic resonance (CW-EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopic measurements. Major species I (65-70%) is assigned to a species resembling a vanadyl cation that is supported on the polyoxometalate and showed a bonding interaction with 13CO. Minor species II (30-35%) is attributed to a reduced species where the vanadium(IV) atom is incorporated in the polyoxometalate framework but slightly distanced from the phosphate core. Under aerobic conditions, CO/O2, a nucleophilic oxidant was formed as elucidated by oxidation of thianthrene oxide as a probe substrate. Oxidation reactions performed on terminal alkenes such as 1-octene yielded a complicated mixture of products that was, however, clearly a result of alkene epoxidation followed by subsequent reactions of the intermediate epoxide. The significant competing reaction was a hydro-carbonylation reaction that yielded a ∼1:1 mixture of linear/branched carboxylic acids.
-
(2009) Journal of Archaeological Science. 36, 3, p. 629-636 Abstract
Cornets are cone-shaped ceramic vessels, characteristic of the Chalcolithic period (ca. 4700-3700 BC) in Israel and Jordan. Their contents and use are unknown. Gas chromatography with flame ionization and mass-selective detection, showed that extracts of cornets from five different sites with different related activities (domestic, habitation cave and a cultic complex) all contain the same assemblage of mainly n-alkanes adsorbed within their walls. This assemblage differs from those found in other types of ceramic vessels from the same sites, as well as from the residues found within the associated sediments. The assemblage of odd and even-numbered n-alkanes found in the cornets is almost identical to that found in the residues of beeswax heated on modern ceramic fragments, as well as in a beehive from the Iron Age IIA strata at Tel Rehov, Israel. Thus the cornets are most likely to have contained beeswax. The presence of beeswax in the cornets contributes to our understanding of the Chalcolithic period; a time when secondary products such as milk, olive oil and wine are thought to have come into use.
-
(2009) Journal of the American Chemical Society. 131, 1, p. 4-+ Abstract
A1, 10-phenanthroline ligand decorated at the 5,6-position by a 15-crown-5 ether moiety was prepared. Ligation of PT at the nitrogen atom positions followed by complexation at the crown ether group of a redox active H(5)PV(2)Mo(10)O(40) polyoxometalate yielded a hybrid metallorganic-polyoxometalate complex, Pd(II)(15-crown-5-phen)Cl(2)-H(5)PV(2)Mo(10)O(40). This complex was characterized by IR, UV-vis, ESI-MS, and NMR spectroscopy and elemental analysis that all confirmed the hybrid nature of the complex. Pd(II)(15-crown-5-phen)Cl(2)-H(5)PV(2)Mo(10)O(40) was used as a catalyst for the Wacker type oxidation of 1-alkenes to yield the corresponding methylketones in essentially quantitative yields using nitrous oxide as the terminal oxidant.
2008
-
(2008) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 47, 51, p. 9908-9912 Abstract
(Figure presented) Head over EELS: Reaction of O2 with a hexa-iron(II)-substituted polyoxometalate in water yields a polyoxometalate with "end-on" hydroperoxo groups, {FeIII-O2H}, at the terminal positions. The hydroperoxo moiety, stabilized by hydrogen bonding with water, is unusual for its long O-O bond and nearly linear Fe-O-O bond angle. Electron energy-loss spectroscopy (EELS) is used to determine the oxidation state of iron.
-
(2008) Journal of Molecular Structure. 891, 1-3, p. 491-497 Abstract
Both closed and open framework structures were designed for copper complexes with N,N-bis-pyridin-2-ylmethyl-ethane-1,2-diamine (2-bpen)-based ligands. The design included substitution at the bridging aliphatic nitrogen atoms by reaction with cyanuric chloride to yield the 3,5-dichloro-2,4,6-triazine derivative. The chloride atoms on the triziane rings were further substituted by either electron donating amines, or an electron withdrawing thiomethyl moiety. The substitution of bridging nitrogen atoms by more electron donating aminated 2,4,6-triazines led to the formation of copper(II) complexes with closed square pyramidal architecture. On the other hand, substitution of bridging nitrogen atoms by the more electron withdrawing 2,4,6-triazines with thioether groups led to the formation of square planar or tetrahedral copper complexes with an open framework architecture whose specific structure and oxidation state depended on the anion.
-
(2008) Journal of the American Chemical Society. 130, 44, p. 14474-14476 Abstract
Primary alcohols such as 1-butanol were oxidized by the H5PV2Mo10O40 polyoxometalate in an atypical manner. Instead of C-H bond activation leading to the formation of butanal and butanoic acid, C-C bond cleavage took place leading to the formation of propanal and formaldehyde as initial products. The latter reacted with the excess 1-butanol present to yield butylformate and butylpropanate in additional oxidative transformations. Kinetic studies including measurement of kinetic isotope effects, labeling studies with 18O labeled H5PV2Mo10O40, and observation of a prerate determining step intermediate by 13C NMR leads to the formulation of a reaction mechanism based on electron transfer from the substrate to the polyoxometalate and oxygen transfer from the reduced polyoxometalate to the organic substrate. It was also shown that vicinal diols such as 1,2-ethanediol apparently react by a similar reaction mechanism.
-
(2008) Antiquity. 82, 317, p. 629-639 Abstract
Beehives were discovered in a densley built area in the Iron Age city of Rehov (tenth-ninth century BC). They consisted of hollow clay cylinders, each with a little hole at one end (for the bee) and a removable lid at the other (for the bee keeper). These beehives, the earliestfound in the Near East, were identified by analogy with examples pictured on Egyptian tombs and in use by traditional peoples. The suggested identification was confirmed by chemical analysis.
-
(2008) Journal of the American Chemical Society. 130, 36, p. 11876-11877 Abstract
It has been found that in apolar reaction media the nitrosonium cation (NO+) activated alkenes under mild conditions toward electrophilic substitution of arene substrates to yield the alkylated arene with Markovnikov orientation. In the absence of arenes the alkenes react with themselves to yield a mixture of dimeric alkenes. The nitrosonium cation can be dissolved in the reaction medium by using the tetrakis-(bis-(3,5-trifluromethyl)phenyl) borate anion, where upon the reactions occur effectively at 30 °C. Alternatively an insoluble, heterogeneous catalyst was prepared so as to yield a NO+ cation with a polyoxometalate (PW12O403-) anion. This catalyst was generally more effective and selective toward a broader range of substrates at 70 °C.
-
(2008) Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis. p. 415-428 Abstract
This chapter reviews work on epoxidations with aqueous dihydrogen peroxide catalyzed by tungstate and polytungstate catalysts. The activity of the sandwich polyoxometalate (sandwich POM) in cyclooctene epoxidation is compared to other W-based catalyst systems under ceteris paribus conditions. Catalyst systems based on H2WO4 display the highest activity of the 11 W-catalyst systems tested. Replacing H2WO4 with Na2WO4 in these systems results in a strong decrease of catalyst activity, and this points to the importance of catalyst acidity. An experimental screening of various carboxylic acids cocatalysts has generated a novel W-based epoxidation system. The sandwich POM is among the most efficient of the nonacidic W-based catalyst systems. Examples where the pH neutral nature of the sandwich POM is used advantageously in the preparation of acid-sensitive epoxides are given. Allylic alcohols are epoxidized efficiently with very high reactor yields and very low sandwich POM catalyst loadings. Allylic alcohol epoxidation also proceeds very efficiently with the vanadium containing catalyst and an organic hydroperoxide instead of H2O2 as the terminal oxidant. The diastereoselectivity of the epoxidation of chiral allylic alcohols is rationalized via 1,2- and/ or 1,3-allylic strain effects. Fine-tuning of catalyst activity and selectivity is achieved by varying the relative amount and nature of phase-transfer cocatalysts. Attention is paid to practical aspects that are of relevance for large-scale, industrial use of the sandwich POM catalyst, such as catalyst preparation, handling, and recycling. For the latter, an efficient method based on nanofiltration employing an a-alumina-supported g-alumina membrane has been developed.
2007
-
(2007) Apidologie. 38, 5, p. 453-461 Abstract
Beeswax is composed of fatty acids, odd numbered n-alkanes and wax esters. Focusing on the most stable components of beeswax, namely the n-alkanes, we have found by gas chromatography and gas chromatography-mass spectrometry analyses of combs from twelve colonies from Israel and Jordan that as beeswax ages and darkens its n-alkane composition changes. The amount of even numbered n-alkanes ( C-22-C-32) is significantly higher in darker colored beeswax as compared to light colored beeswax. We attribute this in part to the accumulation of cuticular residues found in the darker colored comb cells. Cuticular residues are known to contain C-23-C-32 odd and even numbered n-alkanes.
-
(2007) Inorganic Chemistry. 46, 14, p. 5798-5804 Abstract
The reaction of a quaternary ammonium salt of the tin chloride-substituted polyoxometalate, [PSn(Cl)W11O39]4-, with a variety of n-nucleophiles including primary, secondary, and tertiary amines and a tertiary phosphine, yielded tin-centered Lewis acid-base adducts, [PSn(Cl)W11O39]4--n-nucleophile; with more nucleophilic secondary amines such as diisopropylamine, apparently some [PSnN[CH(CH3)2]2W11O 39]4- was formed as a minor product. The compounds were identified by 1H, 119Sn, 15N, 31P, and 183W NMR, ESI-MS, and elemental analyses. The key connectivity of the Sn-Cl center with the amine was clarified by the observation of 3J Sn-H couplings (Sn from the polyoxometalate cluster and H from the amine moiety) in a 2D 119Sn-1H heteronuclear multiple-bond correlation NMR experiment. This new, rather simple synthetic method was also utilized for preparing amino acid-polyoxometalate hybrid compounds.
-
(2007) Advanced Synthesis & Catalysis. 349, 10, p. 1624-1628 Abstract
A polyoxometalate with two alkyl thiol appendages, Q4[SiW 11O40(SiCH2CH2CH2SH) 2] (Q = ammonium salt) stabilized the formation of palladium nanoparticles. This tethered polyoxometalate-palladium binary catalyst was effective for the aerobic oxydehydrogenation of vinylcyclohexene and vinylcyclohexane to styrene as the major product via activation of the allylic, tertiary carbon-hydrogen bond.
-
(2007) Journal of Physical Chemistry C. 111, 21, p. 7711-7719 Abstract
The geometries and electronic structures of the five isomers of [PV 2Mo10O40]5- and its mono- and direduced species were theoretically investigated by means of B3LYP calculations, with an attempt to understand their role as oxidative catalysts in electron-transfer-initiated processes. The calculations reveal the following features: (a) Either in the gas phase or in a solvent, the reduction of the [PV2Mo10O40]5- produces [PV 2Mo10O40]6,7- ions in which the excess electrons are localized on the vanadium atoms in the corresponding δ orbitals. By contrast, the direduced [PV2Mo 10O40]7- isomers where the one electron is localized on molybdenum are high in energy. Consequently, whereas the five isomers of [PV2Mo10O40]5- have roughly a statistical distribution, in the direduced species, the 1,4-[PV 2Mo10O40]7- isomer becomes more stable than others. (b) The gas-phase reduction of the parent [PMo 12O40]3- anion is exceedingly more favorable (by ca. 250 kcal/mol) than the reduction of [PV2Mo10O 40]5-. By contrast, contribution of the solvent exerts a strong leveling effect and makes the reduction potentials of the two species very similar, (c) Since the reduction of [PV2Mo10O 40O40]5- concentrates high negative charge near the O=VO4 moiety, this negative charge accumulation will act as an attractor that binds the organic radical cation (cation), which causes a subsequent proton transfer to an oxo ligand of the vanadium. As such, in terms of an electron-transfer catalytic effect, the presence of vanadium in [PV 2Mo10O40]5- creates function for the catalyst, (d) The computations reveal that [PV2Mo10O 40]7- is a diradicaloid that possesses two virtually degenerate states, with singlet and triplet spins. Thus, we may expect two-state reactivity (TSR) in the catalytic reactions of [PV2Mo 10O40]5-. Some predictions are made based on these features.
-
(2007) Journal Of Molecular Catalysis A-Chemical. 262, 1-2, p. 109-113 Abstract
Polyoxometalates of the ε-Keggin structure [Mo12VO39(μ2-OH)10H2{XII(H2O)3}4] (X = Ni, Co, Mn and Cu), have been synthesized. While the nickel substituted compound was known, the cobalt, manganese and copper analogs are new. These ε-Keggin polyoxometalates are insoluble and, except for the nickel analog, could not be obtained in crystalline form. The IR spectra indicate that for cobalt and manganese substitution the compounds are isostructural to the nickel derivative. The ε-Keggin polyoxometalates were used as heterogeneous catalysts for the aerobic oxidation of aldehydes to carboxylic acids. The selectivity of the reaction was highly dependent on the identity of the aldehyde. Aliphatic linear aldehydes, e.g. octanal, and benzaldehyde react quantitatively and selectively to the respective carboxylic acids. 2-Methyl-undecanal reacted quite selectively yielding as minor product 2-undecanone by decarboxylation. 2-Phenylpropanal showed more significant β-methyl cleavage. Substrates with a variety of functional moieties such as myrtenal and 5-norbornene-2-carboxaldehyde yielded considerable amounts of formates via a Dakin type reaction and allylic oxidation products.
-
(2007) Chemical Communications. 38, p. 3957-3959 Abstract
Together with a strongly oxidizing polyoxometalate, H5PV2-Mo10O40, Pt-II(N-(2,6-diisopropylphenyl)pyrazin-2-ylmethanimine)Cl-2 forms a combined catalyst that was active in the tandem pinacol coupling-rearrangement of aryl aldehydes to give mostly the corresponding diarylacetaldehyde in high yields using molecular hydrogen as the reducing agent.
-
2006
-
(2006) Journal of the American Chemical Society. 128, 49, p. 15697-15700 Abstract
A cross-linked polyethyleneimine polymer containing the [ZnWZn 2(H2O)2(ZnW9O34) 2]12- polyoxometalate was prepared from branched polyethyleneimine (Mw = 600), the polyoxometalate, and a n-octylamine-epichlorohydrin cross-linking reagent. This catalytic assembly was active for the selective oxidation of 2-alkanols to 2-alkanones with aqueous H2O2 with reactions presumably occurring at a hydrophobic domain. Most importantly, the catalyst showed distinctive lipophiloselectivity, that is selectivity as a function of the lipophilic nature of a reaction substrate. The lipophiloselectivity was proportional to the relative partition coefficient (1-octanol/water) of the substrates.
-
(2006) Journal of the American Chemical Society. 128, 48, p. 15451-15460 Abstract
A manganese(III)-substituted polyoxometalate, [α2-P 2MnIII(L)W17O61]7- (P2W17MIII), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F 5Phl(TFAc)2) as a monooxygen donor. Pink P 2W17MnIII turns green upon addition of F 5Phl(TFAc)2. The 19F NMR spectrum of F 5Phl(TFAc)2 with excess P2W17Mn III at -50°C showed the formation of an intermediate attributed to P2W17MnIII-F5-Phl(TFAc) 2 that disappeared upon warming. The 31P NMR spectra of P2W17MnIII with excess F5Phl(TFAc) 2 at -50 and -20°C showed a pair of narrow peaks attributed to a diamagnetic, singlet manganese(V)-oxo species, P2W 17MnV=O. An additional broad peak at -10.6 ppm was attributed to both the P2W17-MnIII-F 5Phl(TFAc)2 complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P 2W17MnV=O were modeled by DFT calculations using the analogous Keggin compound, [PMnV=OW11O 39]4-. Calculations with a pure functional, UBLYP, showed singlet and triplet ground states of similar energy. Further calculations using both the UBLYP and UB3LYP functionals for epoxidation and hydroxylation of propene showed lowest lying triplet transition states for both transformations, while singlet and quintet transition states were of higher energy. The calculations especially after corrections for the solvent effect indicate that [PMnV=OW11O39]4- should be highly reactive, even more reactive than analogous MnV=O porphyrin species. Kinetic measurements of the reaction of P2W17Mn V=O with 1-octene indicated, however, that P2W 17MnV=O was less reactive than a MnV=O porphyrin. The experimental enthalpy of activation confirmed that the energy barrier for epoxidation is low, but the highly negative entropy of activation leads to a high free energy of activation. This result originates in our view from the strong solvation of the highly charged polyoxometalate by the polar solvent used and adventitious water. The higher negative charge of the polyoxometalate in the transition versus ground state leads to electrostriction of the solvent molecules and to a loss of degrees of freedom, resulting in a highly negative entropy of activation and slower reactions.
-
(2006) Organic Letters. 8, 24, p. 5445-5448 Abstract
Palladium nanoparticles were prepared, stabilized, and dispersed in water by alkylated branched polyethyleneimine. The palladium nanoparticles were effective aqueous biphasic catalysts for the chemoselective hydrogenation of alkenes with preferential reduction of less hindered double bonds, such as reduction of 3-methylcyclohexene in the presence of 1-methylcyclohexene and 1-octene in the presence of 2-methyl-2-heptene.
-
(2006) Inorganic Chemistry. 45, 21, p. 8655-8663 Abstract
This work uses density functional calculations to design a new high-valent Fe(V)=O catalyst [Mo5O18Fe=O]3-, which is based on the Lindqvist polyoxometalate (Mo6O192-). Because the parent species is stable to oxidative conditions, one may assume that the newly proposed iron-oxo species will be stable, too. The calculated Mössbauer spectroscopic data may be helpful toward an eventual identification of the species. The calculations of C-H hydroxylation and C=C epoxidation of propene show that, if made, [Mo5O18Fe=O] 3- should be a potent oxidant that will be subject to strong solvent effect. Moreover, the Lindqvist catalyst leads to an intriguing result; the reaction that starts along an epoxidation pathway with C=C activation ends with a C-H hydroxylation product (46) due to rearrangement on the catalyst. The origins of this result are analyzed in terms of the structure of the catalyst and the electronic requirements for conversion of an epoxidation intermediate to a hydroxylation product. Thus, if made, the [Mo 5O18Fe=O]3 will be a selective C-H hydroxylation reagent.
-
(2006) Green Chemistry. 8, 8, p. 679-681 Abstract
Na12[WZn3(H2O)2[ZnW9O34)2], easily self assembled from Na2WO4 and Zn(NO3)2, catalyses the in situ oxidation of ammonia with hydrogen peroxide to hydroxylamine, which further reacts with ketones and aromatic aldehydes in an aqueous biphasic medium without organic solvent to yield oximes as valuable intermediates.
-
(2006) Journal of Cluster Science. 17, 2, p. 235-243 Abstract
Organic-inorganic hybrid compounds were prepared by the reaction of a tin chloride-substituted polyoxometalate, [PSn(Cl)W11O39](4-) with tris(2-aminoethyl)amine, and poly(propylene)imine (DAR-Am) tetramnine and octaamine dendrimers. Translational diffusion coefficients of the hybrid compounds were measured in DMSO-d(6) by the stimulated echo diffusion (STE) NMR technique. Molecular radii were derived from the diffusion coefficients by the Stokes-Einstein equation and appeared to be incorrect because of fast exchange on the NMR time scale of the counter cation in the solution, which led to an averaging of the NMR signal and high diffusion coefficients. An effective hydrodynamic diameter of the [PSn(Cl)W11O39](4-)-polypropylenimine octaamine hybrid adduct was measured in a light scattering experiment.
-
(2006) Chemistry of Materials. 18, 12, p. 2781-2783 Abstract
The synthesis of a new vanadium phosphonate material (TPPhA-V), prepared by the non-hydrolytic condensation of a vanadium(V) alkoxide with an arylphosphonic acid, is presented. Transmission electron microscopy (TEM) of TPPhA-V showed the specimen to be granular with small highly contrasting dots situated within the a more poorly contrasting substance, while the scanning electron microscopy (SEM) examination revealed formation of spongelike amorphous material as the only type of the specimen morphology. Room-temperature electron spin resonance measurement of the TPPhA-V isotropic broad singlet resonance line indicated strong interaction between neighboring vanadium (IV) atoms. The test of the catalytic properties of TPPhA-V showed that the oxidation generally proceeded smoothly with the formation of corresponding benzylic aldehydes at high conversion and selectivity.
-
(2006) Inorganica Chimica Acta. 359, 9, p. 3072-3078 Abstract
An EPR spectrum of as synthesized [G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437-441], orange colored, H5PV2Mo10O40 polyoxometalate showed the presence of a reduced vanadium(IV) addenda atom. Surprisingly, further 31P ENDOR (electron-nuclear double resonance) measurements indicated the absence of a phosphorous heteroatom leading to the suggestion that H5VVVIVMo11O40 exists as a previously unrecognized impurity in the typically synthesized H5PV2Mo10O40 compound. H5/4PVVO4VIV/VMo11O36 was then synthesized in low yield (0.8 mol%) by omitting the addition of phosphate in a typical H5PV2Mo10O40 preparation. The molecular formulation and structure was supported by X-ray crystallography, infrared and mass spectrometry. Further use of EPR/ENDOR/ESEEM (electron-spin echo envelope modulation) allowed the formulation of [VVVIVMo11O40]5- as [VVO4VIVMo11O36]5-. Accordingly, the polyoxometalate has a multiscripts(VO, 4, mml:none(), mml:none(), 3 -) heteroatom core with 11 molybdenum addenda and one VO2+ moiety at the polyoxometalate surface. The redox potential and the catalytic activity of the new vanadomolybdate polyoxometalate compound were essentially identical to the often-studied H5PV2Mo10O40 polyoxometalate isomeric mixture.
-
(2006) Journal Of Molecular Catalysis A-Chemical. 251, 1-2, p. 185-193 Abstract
This paper reviews our work on oxidations with aqueous hydrogen peroxide catalyzed by inorganic catalysts devoid of organic ligands. In the first part of the review, the use of the [WZn3(ZnW9O34)2] 12- "sandwich" polyoxometalate as a multi-purpose oxidation catalyst is described. Attention is paid to practical aspects that are of relevance for large-scale, industrial use of this catalyst, such as catalyst preparation, handling, and recycling. Its activity in cyclooctene epoxidation compared to other W-based catalyst systems has been determined under ceteris paribus conditions. The second part of the review deals with homogeneous and heterogeneous inorganic catalysts for "dark" singlet oxygenation, i.e., singlet oxygenation by catalytic disproportionation of hydrogen peroxide into singlet oxygen and water. The industrial advantages of "dark" singlet oxygenation compared to conventional photo-oxidation are described. It is pointed out that the reaction medium and nature of the catalyst strongly influence the selectivity and scope of the method.
-
(2006) Journal of the American Chemical Society. 128, 15, p. 4968-4969 Abstract
Micelle directed polyoxometalate nanoparticles were synthesized by depositing H3+xPVxMo12-xO40 (x = 0, 2) by precipitation on micelles prepared from cesium dodecyl sulfate. The cryo-TEM image showed particles of about ∼10 nm roughly consistent with the particle size computed from an idealized model. HRTEM coupled with EELS imaging to map the distribution of the elements also supported the formation of micelle directed polyoxometalate nanoparticles. In the aerobic oxidation of various sulfides to sulfoxides and sulfones, the clustered polyoxometalate assemblies supported on hydrophilic silica showed significantly higher catalytic activity versus that of nonclustered assemblies.
-
(2006) Chemistry-A European Journal. 12, 13, p. 3507-3514 Abstract
An organic-inorganic hybrid material. TPPhA-Ti, was constructed by non-hydrolytic condensation of a dendritic tetrakis-1,3,5,7-(4- phosphonatophenyl)adamantane precursor and titanium(iv) isopropoxide. One preparative pathway yielded insoluble materials with a Ti/P ratio of ∼1 which was confirmed by a combination of FT-IR, TGA, and EDS measurements. N 2 sorption experiments showed that TPPhA-Ti is a porous solid (micro pores ∼13 Å; mesopores ∼38 Å) with a high surface area, ∼550 m2g-1. The structure and morphology of the TPPhA-Ti as investigated by transmission and scanning electron microscopy showed a layered-type material. Additional X-ray diffraction data suggest a paracrystalline material; an optimization of possible molecular arrangements of TPPhA-Ti was simulated that was in agreement with the experimental data. A second preparative pathway yielded a Ti oxide-phosphonate with a Ti/P ratio of ∼3.4. Both TEM and SEM revealed that hollow nanospheres were formed with diameters of ∼180- 300 nm.
-
(2006) Chemical Communications. 24, p. 2529-2538 Abstract
The history of aerobic catalytic oxidation mediated by a subclass of polyoxometalates, the phosphovanadomolybdates of the Keggin structure, [PV xMo12-xO40](3+x)-, is described. In the earlier research it was shown that phosphovanadomolybdates catalyze oxydehydrogenation reactions through an electron-transfer oxidation of a substrate by the polyoxometalate that is then reoxidized by oxygen. These aerobic oxidations are selective and synthetically useful in various transformations, notably diene aromatization, phenol dimerization and alcohol oxidation. Oxygen transfer from the polyoxometalate to arenes and alkylarenes was also discussed as a homogeneous analog of a Mars-van Krevelen oxidation. "Second generation" catalysts include binary complexes of the polyoxometalate and a organometallic compound useful, for example, for methane oxidation and nanoparticles stabilized by polyoxometalates effective for aerobic alkene epoxidation.
-
(2006) Chemical Communications. 21, p. 2230-2232 Abstract
Selective aerobic oxidation of benzylic alcohols and of activated aromatic hydrocarbons occurs in supercritical CO2 as reaction medium using H5PV2Mo10O40 as a quasi-heterogeneous catalyst without further additives or co-solvents; efficient recycling is possible and no metal leaching is detectable in the product stream.
2005
-
(2005) Journal of the American Chemical Society. 127, 50, p. 17712-17718 Abstract
This study uses density functional theory (DFT) calculations to explore the reactivity of the putative high-valent iron-oxo reagent of the iron-substituted polyoxometalate (POM-FeO4-), derived from the Keggin species, PW12O403-. It is shown that POM-FeO 4- is in principle capable of C-H hydroxylation and C=C epoxidation and that it should be a powerful oxidant, even more so than the Compound I species of cytochrome P450. The calculations indicate that in a solvent, the barriers, and especially those for epoxidation, become sufficiently small that one may expect an extremely fast reaction. An experimental investigation (by R.N. and A.M.K.) shows, however, that the formation of POM-FeO4- using the oxygen donor, F5PhI-O, leads to a persistent adduct, POM-FeO-I-PhF54-, which does not decompose to POM-FeO 4- + F5Ph-I at the working temperature and exhibits sluggish reactivity, in accord with previous experimental results (Hill, C. L.; Brown, R. B., Jr. J. Am. Chem. Soc. 1986, 108, 536 and Mansuy, D.; Bartoli, J.-F.; Battioni, P.; Lyon, D. K.; Finke, R. G. J. Am. Chem. Soc. 1991, 113, 7222). Subsequent calculations indeed reveal that the gas-phase binding energy of F5PhI to POM-FeO4- is high (ca. 20 kcal/mol) compared to the corresponding binding energy of propene (ca. 2-3 kcal/mol). As such, the POM-FeO-I-PhF54- complex is expected to be persistent toward the displacement of F5PhI by a substrate like propene, leading thereby to sluggish oxidative reactivity. According to theory, overcoming this technical difficulty may turn out to be very rewarding. The question is, can POM-FeO4- be made?
-
(2005) Synlett. 16, p. 2525-2527 G25505ST. Abstract
A new method for the synthesis of vicinal diols from alkenes has been developed. Reaction of molecular iodine in the presence of a polyoxometalate as oxidation catalyst under aerobic conditions in acetic acid solvent leads to the oxidative iodoacetoxylation of an alkene, i.e. formation of a 1,2-iodoacetate. Further in situ substitution of the iodide by water yields the 1,2-diol monoacetate with a predominantly (ca. 4.5:1) cis-configuration. Further esterification under the reaction's acidic conditions leads also to the cis-diacetate. The method may be valuable for the synthesis of cis-vicinal diols without use of toxic osmium catalysts.
-
(2005) Organic Letters. 7, 22, p. 5039-5042 Abstract
(Chemical Equation Presented) [cis-Ru(II)(dmp)2(H 2O)2]2+ (dmp = 2,9-dimethylphenanthroline) was found to be a selective oxidation catalyst using hydrogen peroxide as oxidant. Thus, primary alkenes were very efficiently oxidized via direct carbon-carbon bond cleavage to the corresponding aldehydes as an alternative to ozonolysis. Secondary alkenes were much less reactive, leading to regioselective oxidation of substrates such as 4-vinylcyclohexene and 7-methyl-1,6-octadiene at the terminal position. Primary allylic alcohols were chemoselectively oxidized to the corresponding allylic aldehydes, e.g., geraniol to citral.
-
(2005) Chemical Communications. 36, p. 4595-4597 Abstract
Ag and Ru nanoparticles stabilized by H5PV2MO 10O40, prepared by a sequence of redox reactions and supported on α-alumina, were effective catalysts for the direct aerobic epoxidation of alkenes in the liquid phase.
-
(2005) Journal of the American Chemical Society. 127, 28, p. 9988-9989 Abstract
Nitrobenzene was regioselectively oxidized to 2-nitrophenol with oxygen in a reaction catalyzed by the H5PV2Mo10O40 polyoxometalate. The reaction was first order in oxygen and catalyst. 15N NMR showed the interaction between nitrobenzene and the polyoxometalate. Use of labeled 18O2, H218O, a competitive kinetic isotope experiment, and use of phenyl-tert-butylnitrone as a spin-trap and identification by EPR provided evidence for formation of a radical intermediate involving a selective intramolecular interaction at the ortho position due to formation of a H5PV2Mo10O40−nitrobenzene complex.
-
(2005) Inorganic Chemistry. 44, 14, p. 4900-4902 Abstract
A phenanthroline ligand has been covalently modified at the 2 and 9 positions by an aminophenylhexamolybdate substituent. The 1H NMR spectrum indicated a strong electron-withdrawing effect of the hexamolybdate (Mo 6O192-) moiety on the phenanthroline ligand. UV-vis and cyclic voltammetry showed extended conjugation of the hybrid phenanthroline-polyoxometalate compound and the possibility of easy oxidation of the extended phenanathroline ligand. Further EPR experiments provided strong evidence for an intramolecular charge-transfer process with the formation of a phenanthroline cation radical and a reduced hexamolybdate.
-
(2005) Topics in Catalysis. 34, p. 93-99 Abstract
A basic premise behind the use of polyoxometalates in oxidation chemistry is the fact that polyoxometalates are oxidatively stable. This, a priori, leads to the conclusion that for practical purposes polyoxometalates would have distinct advantages over widely investigated organometallic compounds that are vulnerable to decomposition due to oxidation of the ligand bound to the metal center. Since polyoxometalate synthesis is normally carried out in water by mixing the stoichiometrically required amounts of monomeric metal salts and adjusting the pH to a specific acidic value many structure types are accessible by variation of the reaction stoichiometry, replacement of one or more addenda atoms with other transition or main group metals, and pH control. The structural variety available has enabled the use of polyoxometalates as catalysts for oxidation of hydrocarbons and functionalized organic substrates (alcohols, amines, sulfides, etc.) with a wide range of oxygen donors ranging from molecular oxygen, hydrogen peroxide, nitrous oxide, ozone, alkyl hydroperoxides, periodate, sulfoxide and others. The wide purview of oxidation reactions is enabled because the structural variety leads to oxidation through a number of different mechanistic motifs.From a synthetic organic point of view, the most applicable uses of polyoxometalates as catalysts involve the \u201cgreen\u201d oxygen donors hydrogen peroxide and molecular oxygen. Since practical applications are in hand in this area, practical considerations concerned with catalyst recycle and/or recovery and the elimination of environmentally problematic solvents are also coming to the forefront. In this paper, we will present some of our activities in the area of \u201ccatalyst engineering\u201d for catalytic synthetic applications by polyoxometalates including: (a) catalytic mesoporous solids from organic-polyoxometalate hybrid materials, (b) fluorous phase polyoxometalates with and without fluorous solvents and (c) the use of aqueous biphasic media for oxidation with hydrogen peroxide.
-
(2005) Topics in Catalysis. 32, 3-4, p. 185-196 Abstract
Two new paradigms for separating the homogeneous catalyst from the substrate and products of oxidation were reviewed. The first example demonstrated the functionalization in water of hydrophobic substrates, i.e., hydrocarbons, with methane monooxygenase biomimetic complexes embedded in a derivatized surface silica system using tert-butyl hydroperoxide/O2 as the oxidants. For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼ 3:1:2. The balance between polyethylene oxide and polypropylene oxide, tethered on the silica surface, was crucial for maximizing the catalytic activity. The mechanism for the silica-based catalytic assembly occurred via the Haber-Weiss process. The second precatalyst separation paradigm, the use of the fluorous solvents, which was predicted on solubilizing the precatalyst in a fluorocarbon solution, allowed the functionalization of alkanes and alkenes, while selective oxidation of alcohols to aldehydes was also possible; both precatalyst and product were in separate solvent phases.
-
(2005) Chemical Communications. 9, p. 1206-1208 Abstract
A "sandwich" type polyoxometalate catalyst ([MeN-(n-C 8H17)3]12[WZn3(ZnW 9O34)2]) was very efficiently recycled by nanofiltration with almost quantitative retention, using an α-alumina supported mesoporous γ-alumina membrane.
-
(2005) Advanced Synthesis & Catalysis. 347, 1, p. 39-44 Abstract
First and second-generation rigid dendrimers based on polyphenylated tetrahedral adamantane cores with four or sixteen peripheral phosphonate moieties, PD1 and PD2, respectively, were synthesized and characterized. Further reaction of the dendritic phosphonates with tungstic acid in the presence of hydrogen peroxide led to the stepwise in situ formation of mono- and dinuclear phosphoperoxotungstates. These assemblies were effective catalysts for the epoxidation of alkenes in an aqueous acetonitrile solvent.
2004
-
(2004) Advanced Synthesis & Catalysis. 346, 12, p. 1445-1448 Abstract
Nine- and 27-armed dendrimers with a peroxophosphotungstate core were synthesized by an ionic-bonding approach and used as air-stable, recoverable catalysts for oxidation reactions using hydrogen peroxide.
-
(2004) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 43, 42, p. 5661-5665 Abstract
The structure and reactivity of high-valent FeVO and Mn VIO oxidation catalysts containing a polyoxometalate [PW 11O39]7- lacunary ligand (2) have been investigated by a computational study. Calculations have demonstrated there is an intriguing analogy between these species and the active species (1) of the enzyme cytochrome P450 (see scheme).
-
(2004) Journal of the American Chemical Society. 126, 38, p. 11762-11763 Abstract
Combinations of alkylated polyethylenimine and polyoxometalates yield water-soluble synzymes with hydrophobic regions that allowed the aqueous biphasic selective oxidation of very hydrophobic, water-insoluble substrates with hydrogen peroxide. With the alkylated PEI/{PO4[WO(O2)2]4}3- highly effective C-C bond cleavage of alkenes to aldheydes was observed. The synzymes have both tertiary and quaternary amine centers as shown by a combination of 15N-1H HMBC and XPS measurements. The existence of hydrophobic regions was concluded from the measurement of contact angles and a hypsochromic shift of a fluorescent probe.
-
(2004) Journal of the American Chemical Society. 126, 33, p. 10236-10237 Abstract
We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.
-
(2004) Synlett. 9, p. 1575-1576 Abstract
A new high yield method for the synthesis of aromatic aldehydes has been developed. The procedure is based on an acid catalyzed hydroxymethylation of an arene substrate by paraformaldehyde with concurrent selective oxidation of the intermediate aromatic carbinol by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to the aldehyde product.
-
(2004) Journal of the American Chemical Society. 126, 20, p. 6356-6362 Abstract
Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H3+xPVxMo12-xO40 (x = 0-2). H 5PV2Mo10O40 was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant. The conversion to the aldehyde/ketone could be increased by the addition of water to the reaction mixture. As evidenced by IR and 15N NMR spectroscopy, initially the nitrate salt reacted with H5PV2Mo10O 40 to yield a NVO2+[H 4PV2Mo10O40] intermediate. In an electron-transfer reaction, the proposed NVO2+[H4PV2Mo10O40] complex reacts with the alkylarene substrate to yield a radical-cation-based donor-acceptor intermediate, NIVO2[H4PV 2Mo10O40]-ArCH2R+.. Concurrent proton transfer yields an alkylarene radical, ArCHR., and NO2. Alternatively, it is possible that the NVO 2+[H4PV2Mo10O 40] complex abstracts a hydrogen atom from alkylarene substrate to directly yield ArCHR. and NO2. The electron transfer-proton transfer and hydrogen abstraction scenarios are supported by the correlation of the reaction rate with the ionization potential and the bond dissociation energy at the benzylic positions of the alkylarene, respectively, the high kinetic isotope effect determined for substrates deuterated at the benzylic position, and the reaction order in the catalyst. Product selectivity in the oxidation of phenylcyclopropane tends to support the electron transfer-proton transfer pathway. The ArCHR. and NO2 radical species undergo heterocoupling to yield a benzylic nitrite, which undergoes hydrolysis or acetolysis and subsequent reactions to yield benzylic acetates and corresponding aldehydes or ketones as final products.
-
(2004) Organic Process Research & Development. 8, 3, p. 524-531 Abstract
Eleven W-based catalyst systems for alkene epoxidation with aqueous H 2O2 were compared under identical conditions and at equal level of 0.1 mol % W-atoms. Of these, those based on a combination of H 2WO4 and a methyltrioctylammonium phase transfer catalyst turned out to be most active in particular systems that contain a source of phosphate. Evidence is presented that under our conditions the actual epoxidizing species in H2WO4-based catalyst systems without phosphate source is mononuclear [WO(OH)(O2)2] - rather than binuclear [{WO(O2)2} 2O]2- that is usually thought to be active. For large-scale applications, however, the polyoxometalate Na12[WZn 3-(ZnW9O34)2] (NaZnPOM) in combination with a suitable phase transfer catalyst such as methyltrioctylammonium chloride is preferred over H2WO 4-based catalysts. This preference results from the fact that use of H2WO4 requires a catalyst activation step that is troublesome on a large scale, whereas epoxidations catalyzed by NaZnPOM start without induction period on addition of H2O2. Optimizations of epoxidations catalyzed by QCl/NaZnPOM or QCl/H 2WO4 have shown that the optimum Q/W ratio depends on the alkene that is epoxidized and differs from that expected from catalyst stoichiometry. An attractive feature of NaZnPOM from the viewpoint of industrial applicability is that epoxidations and other reactions with H2O 2 are efficiently catalyzed by a readily available aqueous solution of NaZnPOM prepared through self-assembly. A 1 mol scale example is provided of an epoxidation catalyzed by a combination of self-assembled NaZnPOM and Luviquat mono CP as a multifunctional cocatalyst with emulsifying, buffering, and phase-transferring properties.
-
(2004) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 43, 22, p. 2924-2928 Abstract
Regenerable and reusable dendritic catalysts have been synthesized by the ionic assembly of polyammonium dendrimers and polyoxometalate (POM) trianionic units. These dendrimers (see example depicted, in which the POM is [PO 4{WO(O2)2}4]3-) are found to catalyze, at ambient temperature, the quantitative epoxidation of olefins by H2O2 in water/ CDCl3 and the selective and quantitative oxidation of thioanisole to its sulfone.
-
(2004) Advanced Synthesis & Catalysis. 346, 2-3, p. 339-345 Abstract
A "sandwich" type polyoxometalate, Na12[(WZn 3(H2O)2][(ZnW9O34) 2], was used as an oxidation catalyst in aqueous biphasic reaction media to effect oxidation of alcohols, diols, pyridine derivatives, amines and aniline derivatives with hydrogen peroxide. The catalyst was shown by 183W NMR to be stable in aqueous solutions in the presence of H 2O2 and showed only minimal non-productive decomposition of the oxidant. Secondary alcohols were selectively oxidized to ketones, while primary alcohols tended to be oxidized to the corresponding carboxylic acids, although secondary alcohols were selectively oxidized in the presence of primary alcohols. Vicinal diols yielded carbon-carbon bond cleavage products in very high yields. Pyridine derivatives were oxidized to the respective TV-oxides, but strongly electron-withdrawing moieties inhibited the oxidation reaction. Primary amines were oxidized to the oximes, but significantly hydrolyzed in situ. Aniline derivatives were oxidized to the corresponding azoxy or nitro products depending on the substitution pattern in the aromatic ring. Catalyst recovery and recycle was demonstrated.
-
(2004) Journal of the American Chemical Society. 126, 3, p. 884-890 Abstract
Inorganic-organic hybrid mesoporous materials were prepared by cocrystallization of a "sandwich" type polyoxometalate, [ZnWZn 2(H2O)2(ZnW9O34) 2]12-, and branched tripodal organic polyammonium salts, tris[2-(trimethylammonium)ethyl]-1,3,5-benzenetricarboxylate or 1,3,5-tris[4-(N,N,N-trimethylammonium-ethylcarboxyl)phenyl]benzene trications. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed formation of three-dimensional perforated coral-shaped amorphous materials with the organic cations surrounding polyoxometalate anions. N 2 sorption analysis showed that the hybrid materials have a BET surface area of ∼30-50 m2 g-1 and an average pore diameter of 36 Å leading to the classification of these materials as mesoporous materials with moderate surface areas. These hybrid materials behaved as very effective and selective heterogeneous catalysts for the epoxidation of allylic alcohols and oxidation of secondary alcohols to ketones with hydrogen peroxide as oxidant. The activity and selectivity of the heterogeneous catalysts based on the hybrid materials was similar to those of homogeneous catalysts based on the same [ZnWZn2(H2O) 2(ZnW9O34)2]12- polyoxometalate.
2003
-
(2003) Journal of Organic Chemistry. 68, 24, p. 9510-9512 Abstract
Iodination of arenes was carried out by reacting 1 equiv of arene substrate with 0.5 equiv of iodine under an oxygen atmosphere with H 5PV2Mo10O40 as oxidation catalyst. The synthesis is an inherently waste-free method for the preparation of iodoarenes.
-
(2003) Chemical Communications. 2003, 21, p. 2690-2691 Abstract
A metal-organic-polyoxometalate hybrid compound with two functional centers consisting of a rhodium(I)bis(diphenylphospine) unit connected through two alkylene bridging groups to a lacunary Keggin type polyoxometalate was synthesized and used as an effective, recyclable hydrogenation catalyst in monophasic and aqueous biphasic reaction modes.
-
(2003) Organic Letters. 5, 20, p. 3547-3550 Abstract
[Matrix presented] Polyfluorinated quaternary ammonium cations, [CF 3(CF2)7(CH2)3] 3CH3N+ (RFN+), were synthesized and used as countercations for the [WZnM2(H 2O)2(ZnW9O34)2] 12- (M = Mn(II), Zn(II)) polyoxometalate. The (RFN +)12[WZnM2(H20)2 (ZnW9O34)2] compounds were fluorous biphasic catalysts for alcohol and alkenol oxidation, and alkene epoxidation with aqueous hydrogen peroxide. Reaction protocols with or without a fluorous solvent were tested. The catalytic activity and selectivity was affected by both the hydrophobicity of the solvent and the substrate.
-
(2003) Journal of the American Chemical Society. 125, 40, p. 12116-12117 Abstract
We have demonstrated that a polyfluorinated alcohol, 2,2,2-trifluoroethanol, solvent enables haloperoxidase type activity and the oxychlorination of arenes (benzene and its alkylated derivatives) without a metal catalyst. The polyfluorinated alcohol has a dual function; it catalyzes electrophilic chlorination of less reactive arenes by several orders of magnitude and oxidation of chloride at lower H+ concentrations. DFT calculations show that a complementary charge template in the transition state explains the catalysis of the electrophilic chlorination.
-
(2003) Journal of Organic Chemistry. 68, 21, p. 8222-8231 Abstract
The epoxidation of allylic alcohols is shown to be efficiently and selectively catalyzed by the oxidatively resistant sandwich-type polyoxometalates, POMs, namely [WZnM2(ZnW9O 34)2]q- [M = OV(IV), Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], with organic hydroperoxides as oxygen source. Conspicuous is the fact that the nature of the transition metal M in the central ring of polyoxometalate affects significantly the reactivity, chemoselectivity, regioselectivity, and stereoselectivity of the allylic alcohol epoxidation. For the first time, it is demonstrated that the oxovanadium(IV)-substituted POM, namely [ZnW(VO)2(ZnW 9O34)2]12-, is a highly chemoselective, regioselective, and also stereoselective catalyst for the clean epoxidation of allylic alcohols. A high enantioselectivity (er values up to 95:5) has been achieved with [ZnW(VO)2(ZnW9O 34)2]12- and the sterically demanding TADOOL-derived hydroperoxide TADOOH as regenerative chiral oxygen source. Thus, a POM-catalyzed asymmetric epoxidation of excellent catalytic efficiency (up to 42 000 TON) has been made available for the development of sustainable oxidation processes. The high reactivity and selectivity of this unprecedented oxygen-transfer process are mechanistically rationalized in terms of a peroxy-type vanadium(V) template.
-
(2003) Journal of Molecular Structure. 656, 1-3, p. 27-35 Abstract
Co-crystallization of a tri-ammonium cation with short and somewhat flexible 'arms', [N,N,N-tris[2-(dimethylamino)ethyl]-1,3,5-benzenetricarboxamide]3+, with a polyoxometalate trianion, PW12O403-, yielded an insoluble channeled or microporous structure. The polyoxometalate clusters are arranged in a layered and zig-zag fashion along the xy plane. Looking along the x-axis, channels of a dimension of ∼3.5 × ∼6.5 Å are observed. It was found that C-H⋯O bonds aided in determining the crystal packing by providing directionality to the anion-cation interaction. On the other hand the co-crystallization of a tetraammonium cation with an extended and rigid tetrahedral configuration, 1,3,5,7-tetrakis{4-[(E)-2(N-methylpyridinium-4-yl)vinyl]phenyl adamantane tetraiodide, with a polyoxometalate tetracation, SiW12O 404-, yielded a lamellar structure with alternating layers with spacing of 16.6 Å of the inorganic-organic hybrid material.
-
(2003) Inorganic Chemistry. 42, 11, p. 3677-3684 Abstract
Hybrid compounds with two functional centers consisting of a metallosalen moiety (M-salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen-polyoxometalate compounds (M-salen-POM) it was shown by the use of a combination of UV-vis, H-1 NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn-salen-POM, the metallosalen center is best described as a metal-salen cation radical species; that is, a localized "hole" is formed on the salen ligand. For the other M-salen-POM compounds, the metallosalen moiety can be described as a hybrid of a metal-salen cation radical species and an oxidized metal-salen species, that is, a delocalized "hole" is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor-polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers.
-
(2003) Inorganic Chemistry. 42, 10, p. 3331-3339 Abstract
A new heptamolybdate polyoxometalate structure containing ruthenium(II) or osmium(II) metal centers, [M(II)(DMSO)3Mo7O24]4- (M = Ru, Os), was synthesized by reaction between (NH4)6Mo7O24 and cis-M(DMSO)4Cl2. X-ray structure analysis revealed the complexes to contain a ruthenium/osmium center in a trigonal antiprismatic coordination mode bound to three DMSO moieties via the sulfur atom of DMSO and three oxygen atoms of the new heptamolybdate species. The heptamolybdate consists of seven condensed edge-sharing MoO6 octahedra with C2v symmetry. Three Mo atoms are in classic type II octahedra with a cis dioxo configuration. Two Mo atoms are also type-II-like, but one of the short Mo-O bonds is associated with bridging oxygen atoms rather than terminal oxygen atoms. Two molybdenum atoms are unique in that they are in a trigonally distorted octahedral configuration with three short Mo-O bonds and two intermediate-long M-O bonds and one long Mo-O bond. The [M(II)(DMSO)3Mo7O24]4- polyoxometalates were effective and in some cases highly selective catalysts for the aerobic oxidation of alcohols to ketones/aldehydes. The integrity of the polyoxometalate was apparently retained at high turnover numbers and throughout the reaction, and a variation of an oxometal type mechanism was proposed to explain the results.
-
(2003) Journal of the American Chemical Society. 125, 18, p. 5280-5281 Abstract
We have demonstrated that a simply prepared water-soluble polyoxometalate, Na12[WZnZn2(H2O)2(ZnW9O34)2], synthesized from readily available zinc and tungsten salts in the presence of nitric acid, is an effective catalyst for selective alcohol oxidation with hydrogen peroxide in biphasic (water-alcohol) reaction media. Experiments have shown that the "self-assembled" catalyst in its mother liquor was as active as the isolated catalyst. The aqueous catalyst solution is easily separated from the water-insoluble products and can be recycled without loss in activity or selectivity.
-
(2003) Journal of Organic Chemistry. 68, 7, p. 2903-2912 Abstract
Experimental observations show that direct olefin epoxidation by H2O2, which is extremely sluggish otherwise, occurs in fluorinated alcohol (RfOH) solutions under mild conditions requiring no additional catalysts. Theoretical calculations of ethene and propene epoxidation by H2O2 in the gas phase and in the presence of methanol and of two fluorinated alcohols, presented in this paper, show that the fluoro alcohol itself acts as a catalyst for the reaction by providing a template that stabilizes specifically the transition state (TS) of the reaction. Thus, much like an enzyme, the fluoro alcohol provides a complementary charge template that leads to the reduction of the barrier by 5-8 kcal mol-1. Additionally, the fluoro alcohol template keeps the departing OH and hydroxyalkenyl moieties in close proximity and, by polarizing them, facilitates the hydrogen migration from the latter to form water and the epoxide product. The reduced activation energy and structural confinement of the TS over the fluoro alcohol template render the epoxidation reaction observable under mild synthetic conditions.
-
(2003) Organic Letters. 5, 5, p. 725-728 Abstract
(Matrix presented) The oxovanadium(IV) sandwich-type POM catalyzes the chemo-, regio-, and stereoselective epoxidation of allylic alcohols by chiral hydroperoxides with very high catalytic efficiency (up to 42 000 TON), a potentially valuable oxidation for the development of sustainable processes. By using the sterically demanding, TADDOL-derived hydroperoxide TADOOH as the chiral oxygen source, enantiomeric ratios (er) of up to 95:5 have been achieved.
-
(2003) Journal of Organic Chemistry. 68, 5, p. 1721-1728 Abstract
Sandwich-type polyoxometalates (POMs), namely [VZnM2(ZnW9O34)2]q- [M = Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], are shown to catalyze selectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide under mild conditions (ca. 20 °C) in an aqueous/organic biphasic system. The transition metals M in the central ring of polyoxometalate do not affect the reactivity, chemoselectivity, or stereoselectivity of the allylic alcohol epoxidation by hydrogen peroxide. Similar selectivities, albeit in significantly lower product yields, are observed for the lacunary Keggin POM [PW11O39]7-, in which a peroxotungstate complex has been shown to be the active oxidizing species. All these features support a tungsten peroxo complex rather than a high-valent transition-metal oxo species operates as the key intermediate in the sandwich-type POM-catalyzed epoxidations. On capping of the hydroxy functionality through acetylation or methylation, no reactivity of these hydroxy-protected substrates [1a(Ac) and 1a(Me)] is observed by these POMs. A template is proposed to account for the marked enhancement of reactivity and selectivity, in which the allylic alcohol is ligated through metal - alcoholate bonding, and the H2O2 oxygen source is activated in the form of a peroxotungsten complex. 1,3-Allylic strain promotes a high preference for the threo diastereomer and 1,2-allylic strain a high preference for the erythro diastereomer, whereas tungsten - alcoholate bonding furnishes high regioselectivity for the epoxidation of the allylic double bond. The estimated dihedral angle α of 50 - 70° for the metal - alcoholate-bonded template of the POM/H2O2 system provides the best compromise between 1,2A and 1,3A strain during the oxygen transfer. In contrast to acyclic allylic alcohols 1, the M-POM-catalyzed oxidation of the cyclic allylic alcohols 4 by H2O2 gives significant amounts of enone.
-
Applications of polyoxometalates in homogeneous catalysis(2003) Polyoxometalated Molecular Science. 98, p. 327-349 Abstract
Keywords: MANGANESE-CONTAINING POLYOXOMETALATE; ENVIRONMENTALLY BENIGN TECHNOLOGY; HYDROGEN-PEROXIDE; MOLECULAR-OXYGEN; ALKENE OXIDATION; LIQUID-PHASE; MULTIIRON POLYOXOANIONS; HETEROPOLY COMPOUNDS; OLEFIN EPOXIDATION; SOLID-STATE
-
(2003) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 42, 1, p. 92-95 Abstract
A vanadium-substituted polyoxomolybdate ion ([PV2Mo10O40]5-, top right) can activate nitrous oxide to promote the highly selective catalytic oxidation of primary and secondary alcohols to aldehydes and ketones, and the oxidation of various alkylaromatic substrates (see scheme).
2002
-
(2002) Journal of Organic Chemistry. 67, 20, p. 7075-7079 Abstract
Benzylic, allylic, and aliphatic alcohols are oxidized to aldehydes and ketones in a reaction catalyzed by Keggin-type polyoxomolybdates, PVxMo(12-x)O40-(3+x) (x = 0, 2), with DMSO as a solvent. The oxidation of benzylic alcohols is quantitative within hours and selective, whereas that of allylic alcohols is less selective. Oxidation of aliphatic alcohols is slower but selective. Further mechanistic studies revealed that, for H3PMo12O40 as a catalyst and benzylic alcohols as substrates, the sulfoxide is in fact an oxygen donor in the reaction. Postulated reaction steps as determined from isotopelabeling experiments, kinetic isotope effects, and Hammett plots include (a) sulfoxide activation by complexation to the polyoxometalate and (b) oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. The mechanism is supported by the reaction kinetics.
-
(2002) Organic Letters. 4, 20, p. 3529-3532 Abstract
(matrix presented) Palladium 15-20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K5PPdW11O39 with H2. The nanoparticles were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent.
-
(2002) Advanced Synthesis & Catalysis. 344, 9, p. 1017-1021 Abstract
An Anderson-type polyoxometalate, [IVIIMo6O24]5-, has been used as a catalyst for the aerobic oxidation at 80°C of vicinal diols (glycols). This is the first report on the use of such a polyoxometalate as an oxidation catalyst. Reactivity and selectivity were dependent on the substrate. Thus, aryl-substituted diols yielded mostly the carbon-carbon bond cleavage products, while 1,2-cyclohexanediol yielded cyclohexanone-2-ol and 1,2-cyclohexanedione. Aliphatic diols were less reactive but yielded carbon-carbon bond cleavage products in the presence of additional acid. An abbreviated mechanistic study was carried out indicating that the polyoxometalate oxidizes the diol to the various products even under anaerobic conditions. The reduced polyoxometalates (heteropoly blues and heteropoly browns) formed in the oxidation of the diols are re-oxidized by the molecular oxygen.
-
(2002) Journal of the American Chemical Society. 124, 30, p. 8788-8789 Abstract
A manganese(III)-substituted polyoxometalate of the "sandwich" structure, [MnIII2ZnW(ZnW9O34)2]10-, catalyzed the highly selective (>99.9%) epoxidation of alkenes, such as 1-octene, 2-octene, and cyclohexene with nitrous oxide. Reactions occurred in homogeneous media at 150 °C under 1 atm N2O. The epoxidation had a linear reaction profile; turnover frequencies of 0.5-1.4 h-1 were measured. The reactions were also stereoselective; for example, cis-stilbene gave cis-stilbene oxide. From ESR spectroscopy, it was shown that a Mn(II) octahedral species is reversibly formed by reaction between the original Mn(III) polyoxometalate and N2O. Therefore, it would appear that a Mn(V)-oxo active species is not formed; it is possible that the activation of nitrous oxide was by its oxidation by the Mn(III) polyoxometalate.
-
(2002) Journal of the American Chemical Society. 124, 16, p. 4198-4199 Abstract
The polyoxomolydate of the Keggin structure, PMo12O403-, catalyzes, under anaerobic conditions, oxygen transfer from sulfoxides to alkylarenes such as xanthene and diphenylmethane to yield xanthen-9-one and benzophenone, respectively. With use of 17O and 18O labeled phenylmethylsulfoxide it was shown that the sulfoxide is complexed by the polyoxometalate and the oxygen is transferred from the sulfoxide to the alkylarene. There is a good correlation between the reaction rate and the heterolytic benzylic C-H bond energy indicating a hydride transfer reaction from the alkylarene to the polyoxometalate-sulfoxide complex. In the case of triphenylmethane the resulting carbocation reacts to yield 9-phenylfluorene as the major product. The reaction kinetics supports such a reaction pathway.
-
(2002) Synlett. 12, p. 2011-2014 Abstract
Oxidatively and solvolytically persistent sandwich-type polyoxometalates, namely [WZnM2(ZnW9O34)(2)](q-) [M = Mn(II), Ru(III), Fe(Ill), Zn(II)], catalyze chemoselectively, diastereoselectively and regioselectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide through a tungsten peroxo species, in which the allylic alcohol is coordinated as alcoholate (template effect).
-
(2002) Chemical Communications. 2, 8, p. 876-877 Abstract
The H5PV2Mo10O40 polyoxometalate in a polyethylene glycol solvent was effective for a series of aerobic oxidation reactions including oxydehydrogenation of alcohols and cyclic dienes, oxidation of sulfides and the Wacker reaction; the solvent-catalyst phase can be recovered and recycled.
-
(2002) New Journal of Chemistry. 26, 3, p. 272-274 Abstract
A palladium-substituted polyoxometalate having a Keggin structure, supported on γ-alumina or active carbon, was used as a catalyst precursor for catalytic hydrogenation. The catalyst system enabled fast hydrogenation of arenes at 30 bar H2 and 230°C. Most interesting was the finding that arenes could be selectively reduced in the presence of distal ketone groups under similar conditions, 30 bar H2 and 200°C. For example, 1-phenyl-2-propanone yielded 1-cyclohexyl-2-propanone with no reduction of the ketone moiety. Additionally, aromatic compounds with vicinal (conjugated) ketone moieties underwent complete hydrogenation to saturated hydrocarbons and catalytic McMurry coupling was observed for aliphatic aldehydes.
2001
-
(2001) Journal of Organic Chemistry. 66, 25, p. 8650-8653 Abstract
The discovery of new environmentally friendly methods for selective catalytic oxidation of alcohol substrates to aldehyde and ketones is an important goal in the development of modern methods for chemical synthesis. 1 In this context, of special significance is the use of intrinsically non-waste-producing oxidants such as molecular oxygen from air and hydrogen peroxide. It has been shown in the past that stable nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) is able to mediate the oxidation of primary alcohols to aldehydes with a variety of terminal oxidants. 2 Especially common is the use of hypochlorite, 3 but the use of electrocatalytic conditions, 4 peracetic acid together with a catalytic amount of bromide, 5m-chloroperbenzoic acid, 6 bromite, 7 persulfate, 8 and hydrogen peroxide together with hydrogen bromide and methylrhenium trioxide as catalyst have also been reported. 9 The use of these nitroxyl reaction systems has one or more disadvantages including a high price of oxidant, formation of considerable amounts of organic waste, and the use of halide (chloride and/or bromide)-containing oxidants which in turn form nondesirable halide-containing wastes. To obviate these disadvantages, molecular oxygen can also as be used as terminal oxidant in the presence of cupric ion catalysts, 10 a ruthenium catalyst, RuCl2(PPh3)3, 11 or enzymes with or without metal complexes.12 These methods have the advantage of being intrinsically waste-free synthetic systems as concerns the oxidant. However, they have other disadvantages. The copper-based method is largely ineffective for simple (nonallylic or nonbenzylic) aliphatic especially secondary alcohols. In addition, relatively high catalytic loads, usually at least 10 mol % each of TEMPO and cupric ion are needed. The ruthenium-based system overcomes the disadvantage of the lack of reactivity for simple aliphatic alcohols, but it requires an expensive noble metal and preferably an intrinsically oxidatively unstable triphenylphosphine ligand, which has to be present in order to obtain appreciable catalyst activity. The enzyme-based method affords only low to modest conversions after long reaction times, and the rate of these enzymatically catalyzed oxidation reactions cannot be increased by significantly increasing the temperature because of the thermal lability of the enzymes.
-
(2001) Synthetic Metals. 124, 1, p. 59-61 Abstract
The film preparation properties and amplified spontaneous emission (ASE) features of a conjugated polymer blended with a zirconium-organosilicon xerogel glassy matrix were studied. Dip and spin coating methods were used to perform polarization and washing steps for the conjugated polymer. Silicon wafer covered with a 7μm thick silica layer was used as a waveguide substrate. The characterization of the absorption and fluorescence spectra of the films signified the evidence of an enhanced gap between the two bands.
-
(2001) Journal of the American Chemical Society. 123, 35, p. 8531-8542 Abstract
The mechanism of aerobic oxidation of aromatic and alkyl aromatic compounds using anthracene and xanthene, respectively, as a model compound was investigated using a phosphovanadomolybdate polyoxometalate, H5PV2Mo10O40, as catalyst under mild, liquid-phase conditions: The polyoxometalate is a soluble analogue of insoluble mixed-metal oxides often used for high-temperature gas-phase heterogeneous oxidation which proceed by a Mars-van Krevelen mechanism. The general purpose of the present investigation was to prove that a Mars-van Krevelen mechanism is possible also in liquid-phase, homogeneous oxidation reactions. First, the oxygen transfer from H5PV2Mo10O40 to the hydrocarbons was studied using various techniques to show that commonly observed liquid-phase oxidation mechanisms, autoxidation, and oxidative nucleophilic substitution were not occurring in this case. Techniques used included (a) use of 18O-labeled molecular oxygen, polyoxometalate, and water; (b) carrying out reactions under anaerobic conditions; (c) performing the reaction with an alternative nucleophile (acetate) or under anhydrous conditions; and (d) determination of the reaction stoichiometry. All of the experiments pointed against autoxidation and oxidative nucleophilic substitution and toward a Mars-van Krevelen mechanism. Second, the mode of activation of the hydrocarbon was determined to be by electron transfer, as opposed to hydrogen atom transfer from the hydrocarbon to the polyoxometalate. Kinetic studies showed that an outer-sphere electron transfer was probable with formation of a donor-acceptor complex. Further studies enabled the isolation and observation of intermediates by ESR and NMR spectroscopy. For anthracene, the immediate result of electron transfer, that is formation of an anthracene radical cation and reduced polyoxometalate, was observed by ESR spectroscopy. The ESR spectrum, together with kinetics experiments, including kinetic isotope experiments and 1H NMR, support a Mars-van Krevelen mechanism in which the rate-determining step is the oxygen-transfer reaction between the polyoxometalate and the intermediate radical cation. Anthraquinone is the only observable reaction product. For xanthene, the radical cation could not be observed. Instead, the initial radical cation undergoes fast additional proton and electron transfer (or hydrogen atom transfer) to yield a stable benzylic cation observable by 1H NMR. Again, kinetics experiments support the notion of an oxygen-transfer rate-determining step between the xanthenyl cation and the polyoxometalate, with formation of xanthen-9-one as the only product. Schemes summarizing the proposed reaction mechanisms are presented.
-
(2001) Catalysis Letters. 72, 3-4, p. 241-243 Abstract
New metallosilicate catalysts were prepared by reacting a silanol capped dendrimer, Si[CH2CH2Si(CH3)2OH]4 with MCp2Cl2 (M = TiIV, MoVI, WVI and VV). The resulting Si[CH2CH2Si(CH3)2OMCp 2Cl]4 compounds were incorporated in a silica matrix by the sol-gel method. The catalytic activity of the metallosilicates after calcination revealed excellent activity and selectivity towards epoxidation of alkenes with tert-butylhydroperoxide. Maximum activity was observed with molybdenum-containing materials. Analysis of the catalytic activity revealed that the catalysts were truly heterogeneous.
-
(2001) Chemical Communications. 5, p. 487-488 Abstract
A silane functionalized by octafluoroacetophenone was polymerized by the sol-gel method to form an insoluble silicate with perfluoroketone pendants; the silicate was used as a heterogeneous catalyst for the activation of aqueous hydrogen peroxide and the oxidation of aromatic amines and alkenes.
-
(2001) Synthetic Metals. 119, 1-3, p. 143-144 Abstract
Composites of PPV in PVA, PVK and other polymeric matrixes were prepared by an in-situ polymerization of 1,4 phenylene dimethylene-bis-(tetramethylene sulfonium chloride) or □□□-dibromo-p-xylene. The monomer was converted directly to PPV in the polymeric matrix. This polymerization technique was successfully carried out by the use of a base, at room temperature, without requiring removal of oligomers and was completed in several minutes. EL and PL of the composites were measured and compared with PPV. A blue shift was found in the PL spectrum of the PPV-PVA composite compared to PPV. Other composites showed similar PL to that of PPV.
-
(2001) European Journal of Inorganic Chemistry. 3, p. 789-794 Abstract
An antimony-substituted polyoxomolybdate, [PSbIII(H2O) -Mo11O39]4-, of the α-Keggin structure, has been synthesized. Although the molecular structure is typical, we have found from the analysis of the X-ray diffraction data that the crystal is anisotropic with alignment of the molecular units so that the substituted antimony atoms are all directed along the polar c axis and on the same side of the molecular unit. UV/Vis, ESR and magnetic susceptibility data support a formulation of [PSbIII(H2O)Mo11O39]4- whereby there is partial electron delocalization with some formation (10-20%) of MoV. Oxidation of [PSbIII(H2O)Mo11O39]4- with ozone or bromine yields the oxidized species [PSbV(O)Mo11O39]4- and [PSbV(Br)Mo11O39]3-. In the anaerobic oxydehydrogenation of alcohols, [PSbV(O)Mo11O39]4- is more active than [PSbV(Br)Mo11O39]3-. Catalytic aerobic oxidation of benzylic alcohols to aldehydes was also possible with no over-oxidation and formation of carboxylic̈ acids.
-
2000
-
(2000) Angewandte Chemie (International ed.). 39, 22, p. 4088-4090 Abstract
Vanadium-containing polyoxometalates, such as PV2Mo10O40, are shown to act as oxygen-donating compounds and catalyze the oxygenation of hydrocarbons in a liquid homogeneous phase by a MarsvanKrevelen type mechanism [Eqs.(1) and (2)].
-
(2000) Angewandte Chemie - International Edition. 39, 22, p. 4090-4092 Abstract
A sterol biosynthetic intermediate from sea cucumbers is parkeol (1). By mutation of residues at two positions, the enzyme lanosterol synthase was changed enough to make parkeol as its major product, along with lanosterol (2) and lanost-24-ene-3β,9α-diol (3) as by-products.
-
(2000) Chemistry-A European Journal. 6, 20, p. 3722-3728 Abstract
A series of transition metal substituted polyfluorooxometalates (PFOM) [M(L)H2F6NaW17O55]q-, M = Zn2+, Co2+, Mn2+, Fe2+, Ru2+, Ni2+ and V5+ and L = H2O, O2-, of quasi-Wells -Dawson structure, was synthesized. In the series prepared, only the nickel-substituted polyfluorooxometalate was capable of catalytic activation of hydrogen peroxide in biphasic reaction media, the reaction leading mainly to the selective epoxidation of alkenes and alkenols. The manganese-, cobalt-, ruthenium-, iron-, vanadium-, and zinc-substituted polyfluorooxometalates were catalytically inactive, although, except for the zinc polyfluorooxometalate, very significant catalase activity was observed. Oxidation of thianthrene showed that sulfoxides were oxidized more easily than sulfides. Kinetic profiles of cyclooctene epoxidation showed that the reaction was zero order in both cyclooctene and hydrogen peroxide. Hydrogen peroxide was consumed at a rate 40% higher than the rate of epoxidation of cyclooctene. The reaction appears to proceed through an intermediate peroxo/hydroperoxo species that was observed in the IR spectrum. Atomic absorption, IR and 19F NMR spectroscopy indicated that the [Ni(H2O)-H2F6NaW17O 55]9- compound was stable under reaction conditions.
-
(2000) Organic Letters. 2, 18, p. 2861-2863 Abstract
The catalytic electrophilic activation of hydrogen peroxide with transition metal compounds toward reaction with nucleophiles is a matter of very significant research and practical interest. We have now found that use of perfluorinated alcoholic solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol in the absence of catalysts allowed electrophilic activation of hydrogen peroxide toward epoxidation of alkenes and the Baeyer−Villiger oxidation of ketones.
-
(2000) Catalysis Letters. 68, 1, p. 109-111 Abstract
Irradiation with visible light of a secondary alcohol, benzhydrol, in the presence of molecular oxygen within a titanium-substituted mesoporous molecular sieve, Ti-MCM-41, led to the formation of the α-hydroperoxoalcohol and then hydrogen peroxide. The peroxide species reacted in situ with alkenes and sulfides and led to the titanium center catalyzed selective formation of epoxides from alkenes and sulfoxides from sulfides. The cascade of reactions represents a new method for the photoactivation of oxygen in the presence of alcohols.
-
(2000) Inorganic Chemistry. 39, 16, p. 3455-3462 Abstract
For the first time, mixed-addenda vanadium-substituted polyfluorooxometalates, PFOMs, have been synthesized. Depending on the workup procedure used, two types of compounds were prepared. The first PFOM was a quasi Wells-Dawson type compound, [H2F6NaV(V)W17O56]8-, and the second a mixture of vanadium-substituted polyfluorooxometalates of the Keggin structure, XV(IV)W11F(n)O(40-n) (X = H2, V, W; n = 1-4). From the X-ray diffraction analysis, [H2F6NaV(V)W17O56]8- has an elliptic (egg) shape with a central sodium atom surrounded by six fluorine atoms in a trigonal prism coordination. One may differentiate between two types of addenda atoms to be found in belt and capped positions. According to 1H, 19F, and 51V NMR analysis, it is concluded that vanadium is isomorphically substituted in both the belt and capped position of [H2F6NaV(V)W17O56]8-. The mixture of vanadium-substituted PFOMs of the Keggin structure was shown, by HPLC and ESR, to contain at least two species of different charge and of a different vanadium environment. The [H2F6NaV(V)W17O56]8- PFOM was active for the catalytic aerobic oxidation of alkyl aromatic compounds in biphasic (water-catalyst and substrate) media. The reaction selectivity (autoxidation versus oxydehydrogenation) depended on the substrate and reaction conditions such as temperature and oxygen pressure. The selectivity to oxydehydrogenation was significantly higher compared to the prototypical cobalt acetate catalytic system.
-
(2000) Journal of Applied Physics. 88, 3, p. 1236-1243 Abstract
Improved diffraction efficiency was observed in holograms stored in disordered conjugated polymer/glass composites. The conjugated polymers used were alkoxy substituted poly(phenylenevinylne) analogs and the glass matrices were zirconia-organosilica xerogels. Investigation of the mechanism of hologram formation revealed evidence of a photochromic process consisting of light induced photo-oxidation (bleaching) of the embedded conjugated polymer resulting in the formation of an absorption grating and a phase grating. Investigation of the hologram formation revealed that the process was oxygen dependent. Oxygen removal increases hologram formation time by more than an order of magnitude and halves the total hologram efficiency. The oxygen dependence was also highly correlated with photobleaching of the samples and beam interaction of the writing beams. The chemical transformations upon photobleaching were shown by infrared and Raman spectroscopy to involve chain scission and oxidation of the polymer at the vinylic position of the conjugated polymer. Film preparation of the composites was optimized showing a tenfold improvement in the holographic properties compared to our previous results. The optimized treatment method allows for a high, >20%, diffraction efficiency, η, to be obtained for the 2.5-μm-thick polymer/glass films. Light sensitivity was compared for several polymer/glass composites and was correlated to the absorption curves and holographic diffraction efficiency showing that the new composites and film preparation techniques are promising for holographic materials sensitive in the blue and ultraviolet spectral regions. A method of information fixing by preventing oxygen entry to the composite film resulted in a fourfold increase of the erasure time. These findings suggest that holograms can be fixed for a long term by nonoxygen permeable coating, applied after hologram formation.
-
(2000) Journal of Ocular Pharmacology and Therapeutics. 16, 3, p. 217-230 Abstract
The purpose of this study was to characterize the intraocular pressure (IOP) lowering activity and possible mechanism of action of the synthetic, non-psychotropic cannabinoid dexanabinol (HU-211) [(+)(3S,4S), 7-hydroxy-Delta-6-tetrahydrocannabinol 1, 1 dimethylheptyl], following intravenous (i.v.) administration in the rabbit. IOP (pneumatonometry), aqueous humor inflow rate (fluorophotometry), blood pressure, and heart rate (computerized physiograph system connected to central ear artery cannula) were measured in unanesthetized albino rabbits. Intravenous administration of HU-211 resulted in a dose-related reduction in IOP; a maximal IOP reduction of 5.0 +/- 0.2 mmHg was observed 4 hr after a 0.5 mg/kg dose. No significant changes in blood pressure or heart rate were observed during the first hr following this dose of HU-211. Pupil diameter did not change significantly during the 5 hr following the 0.5 mg/kg i.v. dose. No significant change in the rate of aqueous humor inflow occurred during the 6 hr after a 0.5 mg/kg dose of HU-211, thereby implicating outflow changes as the major source of IOP reduction. IOP reduction by HU-211 following pre-treatment with the alpha(2) adrenergic antagonist, yohimbine (1 mg/kg, i.v.), was only 30% of that of HU-211 alone. IOP reduction following pretreatment with the alpha(2) agonist, clonidine (0.5 mg/kg i.v.), was twice as large as that of HU-211 alone. Pretreatment with the beta-adrenergic antagonist, propranolol (0.5 mg/kg i.v.), resulted in a 50% reduction in the IOP-lowering effect of HU-211. In summary, HU-211, administered i.v., is an effective IOP-lowering agent, devoid of any significant side effects (blood pressure, heart rate or pupil diameter, all of which have been reported previously for cannabinoids). Involvement of the adrenergic system is indicated in mediating the IOP-lowering effects of HU-211 that appear to reflect a change in fluid outflow from the eye.
-
(2000) Chemistry-A European Journal. 6, 5, p. 875-882 Abstract
Quinones have been considered as reactive compounds present on the surface of active carbon. Thus, the co-catalytic use of quinones combined with the phosphovanadomolybdate polyoxometalate, PV2Mo10O405-, has been studied as an analogue of the known PV2Mo10O405-/C catalyst in oxidative dehydrogenation reactions. From the synthetic point of view both biphasic the quinone (org)-Na5PV2Mo10O40-(aq) and monophasic quinone (org)-4Q5PV2Mo10O40-(org) [4Q = (nC4H9)4-N+] systems are effective for the selective oxidation of benzylic and allylic alcohols to their corresponding aldehydes. Kinetic measurements carried out on the model oxidative dehydrogenation of 4-methylbenzyl alcohol in the presence of p-chloranil, 4Q5PV2Mo10O40, and molecular oxygen showed that the reaction was non-elementary, although the 4-methylbenzyl alcohol oxydehydrogenation was the rate-determining step. ESR measurements showed the presence of the semiquinone of p-chloranil, probably as a complex with the polyoxometalate. This proposed complex was shown to be a more potent oxidant than p-chloranil. Thus, for the oxidation of 4-methoxytoluene the semiquinone complex was active, whereas p-chloranil alone was inactive. Beyond the importance of understanding quinone-phosphovanadomolybdate polyoxometalate-catalyzed reactions, insight gained from the formation of semiquinone active species can be applied for heterogeneous and aerobic oxidative transformations catalyzed by PV2Mo10O405- with carbon matrices as active supports.
1999
-
(1999) Catalysis Letters. 63, 3-4, p. 189-192 Abstract
The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM-41 and amino-modified MCM-41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity.
-
(1999) Angewandte Chemie - International Edition. 38, 22, p. 3331-3334 Abstract
Excellent deoxygenation of ketones and aldehydes is achieved with Keggin-type polyoxometalates in the presence of hydrogen (see Equation (1) for an example). The mixed addenda phosphovanadomolybdate [PV2Mo10O4]5- was found to be the best catalyst. X-ray diffraction and IR studies suggest that the polyoxometalates are structurally stable under the strongly reducing conditions.
-
(1999) Journal of molecular catalysis. A, Chemical. 146, 1-2, p. 291-298 Abstract
Use of the sol-gel technique has allowed the preparation of silicates with polyfunctionalized surfaces. In this way, silicates containing combinations of hydrophilic poly(ethylene oxide) (PEO), hydrophobic poly(propylene oxide) (PPO) and cationic quaternary ammonium (Q+) groups were prepared. These silicates coupled with polyoxometalates led to active catalytic assemblies. In one application, H5PV2Mo10O40 was complexed to PEO-SiO2 and used to catalyze the oxydehydrogenation of dihydroanthracene with improved activity and selectivity compared to the non-supported catalyst. Recycle of the assembly was possible. In a further use, [ZnWMn2/(II)(ZnW9O34)2]12-, bound by a Q+ moiety to the silicate surface modified also with polyethers formed an assembly catalytically active, with excellent recycle for the epoxidation of alkenes with aqueous 30% H2O2. A silicate with a hydrophobic (PPO)/hydrophilic (PEO) balance showed the highest activity due to the optimal contact of the apolar hydrocarbon and the aqueous oxidant. Dispersion of the silicate in a solvent depended much on the polyether tether. Hydrophilic PEO-SiO2 formed a homogeneous gel in water but was not dispersed in chloroform. For hydrophobic PPO-SiO2, the opposite was observed. Maximal dispersion together with ease of recovery was possible with intermediate 10% PEO, 10% PPO-SiO2.
-
(1999) Synthetic Metals. 102, 1-3, p. 1215-1216 Abstract
Poly(phenylenevinylene) (PPV) grafted with triethylenetetramine (TETA), PPV:TETA, unlike PPV, can interact with anionic sulfonated polymer dyes and can also coordinate metal ions, eg Eu, by chelation with the grafted TETA moiety. We have used these interactions and chelation to construct two types of self-assembled films using the layer-by-layer self-assembly technique. Structural characterization by means of specular X-ray reflectivity of heterostructures containing PPV:TETA:Eu showed that in self-assembled films containing metal ions there is a clear Bragg reflection (up to the third harmonic) indicating the formation of well modulated heterostructures. Photoluminescence (PL) measurements of heterostructures containing PPV:TETA and anionic polymer dyes showed evidence for energy transfer between the PPV:TETA and the polymer dyes.
-
(1999) Journal of Catalysis. 182, 1, p. 82-91 Abstract
The oxidation of alkanes with molecular oxygen using aldehydes as reducing agents (aldehydes are cooxidized) was studied using the α-H5PV2Mo10O40polyoxometalate as catalyst. Emphasis was placed on the initiation of the radical chain reaction by investigation of the aldehydepolyoxometalate interaction. Using31P NMR and ESR spectroscopy one could differentiate between the reactivity of the five inseparable isomers of α-H5PV2Mo10O40. Contrary to previous belief, the 1,11 isomer with vanadium in distal positions is the most abundant. The31P NMR and ESR spectra supported by UVvis absorptiontime profiles of the reduction of α-H5PV2Mo10O40indicated that isomers with vanadium in vicinal positions were most kinetically viable in the alkane oxidation. Addition of isobutyraldehyde to α-H5PV2Mo10O40gave in the51V NMR spectrum a new downfield peak attributed to the formation of an aldehydepolyoxometalate intermediate. The alkane/aldehyde/O2oxidizing system was found to be quite effective and selective for ketone formation. Reaction probes indicated that acyl peroxo radicals were the active oxidizing intermediates. Five pathways for its reaction were identified: chain propagation, alkane oxidation, decomposition to form oxygen, decomposition to acyl oxo radicals leading to CO2and ketone, and capture and inhibition by the polyoxometalate.
-
(1999) Journal of molecular catalysis. A, Chemical. 138, 2-3, p. 315-318 Abstract
New transition metal substituted silicate xerogels, ML(n) -SiO2, have been prepared by the sol-gel technique and have been used as heterogeneous catalysts for the side-chain oxidation of alkyl arches with anhydrous t- butylhydroperoxide. The cobalt substituted silicates were the most active and selective, for example, ethylbenzene was cleanly oxidized to acetophenone at 65% conversion and > 99% selectivity. The metal salt precursor used was a key factor in determining the activity of the metallosilicate. Diffuse reflectance UV-vis and IR measurements indicated that when using Co(OSV)2 as metal precursor, the acetate group was detached from the metal and Co(II) was tetrahedral and site substituted in the silicate network. Addition of the oxidant led to stabilization of a Co(III) oxidation state and/or retention of the tetrahedral configuration.
-
(1999) Synthetic Metals. 102, 1-3, p. 1178-1181 Abstract
We report here on the fabrication and optimization of composites of conjugated polymer in Sol-Gel and PVA matrices. FTIR spectroscopy as well as other observations show that efficient phase and absorption holograms can be created in the material upon illumination, via the photochromic effect. The main contribution to the absorption change is chain scission and photo-oxidation mechanism. Optimized holographic composites with diffraction efficiencies as high as 26% are shown. Beam interaction between the writing beams is observed for these composites, while the diffraction efficiencies was improved by an order of magnitude.
-
(1999) Inorganic Chemistry. 38, 15, p. 3575-3580 Abstract
The biomimetic, methane monooxygenase enzyme (MMO) precatalyst, [Fe 2O(ν1-H2O)(ν1-OAc)(TPA)2] 3+ (TPA) tris[(2-pyridyl)methyl]amine), 1, formed in situ at pH 4.2 from [Fe2O(μ-OAc)(TPA)2]3+, 2, was embedded in an amorphous silicate surface modified by a combination of hydrophilic poly(ethylene oxide) and hydrophobic poly(propylene oxide). The resulting catalytic assembly was found to be a biomimetic model for the MMO active site within a hydrophobic macroenvironment, allowing alkane functionalization with tert-butyl hydroperoxide (TBHP)/O2 in an aqueous reaction medium (pH 4.2). For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼3:1:2. The balance between poly- (ethylene oxide) and poly(propylene oxide), tethered on the silica surface, was crucial for maximizing the catalytic activity. The silica-based catalytic assembly showed reactivity somewhat higher in comparison to an aqueous micelle system utilizing the surfactant, cetyltrimethylammonium hydrogen sulfate at its critical micelle concentration, in which functionalization of cyclohexane with TBHP/O2 in the presence of 1 was also studied at pH 4.2 and was found to provide similar products: cyclohexanol, cyclohexanone, and cyclohexyl-tert-butyl peroxide, in a ratio of ∼2:3:1. Moreover, the mechanism for both the silica-based catalytic assembly and the aqueous micelle system was found to occur via the Haber-Weiss process, in which redox chemistry between 1 and TBHP provides both the t-BuO and t-BuOO radicals. The t-BuO radical initiates the C-H functionalization reaction to form the carbon radical, followed by O2 trapping, to provide cyclohexyl hydroperoxide, which produces the cyclohexanol and cyclohexanone in the presence of 1, whereas the coupling product emanates from t-BuOO and cyclohexyl radicals. A discussion concerning both approaches for alkane functionalization in water will be presented.
-
(1999) Journal of Materials Chemistry. 9, 7, p. 1387-1389 Abstract
Light emitting diode devices prepared from poly(phenylenevinylene) grafted with poly(ethylene imine) show blue shifted spectra (λ(max)≃475- 490nm) proportional to the ratio of the grafted to ungrafted segmers and significantly (up to sevenfold) improved lifetimes; further coordination of transition metals led to increased electroluminescence intensity.
1998
-
(1998) Journal of the American Chemical Society. 120, 46, p. 11969-11976 Abstract
The ruthenium-substituted 'sandwich' type polyoxometalate [WZnRum(III)2(XW9O34)2]11- where X = Zn(II) or Co(II) has been shown to be unique in its ability to catalyze the selective hydroxylation of adamantane at the tertiary carbon position with molecular oxygen as the oxygen donor. The hydroxylation features an adamantane:dioxygen stoichiometry of 2:1. Kinetic studies of the reaction show that the reaction is second order in the ruthenium-substituted polyoxometalate and zero order in adamantane. The reaction rate dependence on molecular oxygen is complicated. Highest reaction rates were observed at approximately 1 atm of dioxygen, but the rates decreased at lower (0.2 atm) and higher (3 atm) dioxygen pressures. Kinetic analysis as a function of temperature showed a low entropy of activation, indicating a highly ordered transition state. The reaction commenced only after an induction period related to formation of the 'active' species from the precursor polyoxometalate. Alternatively, the induction period could be eliminated by adding a reducing agent. The induction period was found to be a function of both the reaction temperature and oxidation pressure. Coordination studies carried out via UV-vis and IR spectroscopy indicate the formation of a ruthenium(IV) oxo or μ-peroxo ruthenium(III) dimeric species as the active oxygenation species. This formulation is supported by the ESR spectra observed upon addition of spin traps such as 2- methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline N-oxide to the reaction mixtures. In the oxidation of alkenes, catalytic and highly selective epoxidation may be observed. Especially informative was the catalytic epoxidation of trans-cyclooctene, which yielded a trans:cis-cyclooctene oxide ratio of 20:1, providing strong evidence of a nonradical oxidation pathway. On the basis of the reaction stoichiometry, spectroscopic evidence, reaction probes, and kinetic studies, a mechanism is proposed calling for a dioxygenase type activation of molecular oxygen via complexation to a ruthenium(II) species followed by formation of a ruthenium(IV)oxo species via a ruthenium(III) μ-peroxo intermediate.
-
(1998) Acta Polymerica. 49, 10-11, p. 642-651 Abstract
A review of recent research in Jerusalem concerning electroluminescence phenomena from conjugated polymers with emphasis on poly(arylenevinylene) polymers and copolymers as the active emitting polymers is presented. Electroluminescence (EL) is described in both simple spin-cast thin films and multilayers produced by a layer-by-layer deposition technique.
-
(1998) Polyhedron. 17, 20, p. 3557-3564 Abstract
The ruthenium substituted polyoxomolybdate of the Keggin structure, Q4PRuIII(H2O)Mo11O39 (Q=n-Bu4N), has been synthesized and characterized. The IR spectra show that this compound is isostructural with the known manganese and cobalt analogs. The cyclic voltammogram showed similar redox potentials and the UV-vis spectra showed similar energies for the d-d transitions compared to the corresponding tungstate, Q4PRuIII(H2O)W11O39. The catalytic activity of the molybdate versus tungstate in reactions with molecular oxygen was, however, significantly different. IR and 31P NMR evidence indicated that treatment of Q4PRuIII(H2O)Mo11O39 with oxygen showed no structural changes whereas, for Q4PRuIII(H2O)W11O39, a clear change was observed. This finding probably explains the lack of catalytic activity for the latter in the co-oxidation of cumene and 1-octene to cumyl alcohol and 1-octene oxide. For the molybdenum compound, this reaction took place by a kinetic balance of ruthenium metal-catalyzed autooxidation of cumene to cumene hydroperoxide and the molybdenum catalyzed oxygen transfer from cumene hydroperoxide to 1-octene to yield the products. High catalyst loading led to reaction inhibition whereas low loading and excess cumene led to increased autooxidation.
-
Oxidative dehydrogenation of 4-vinylcyclohexene to styrene catalyzed by PV2Mo10O5-40 heteropolyacids(1998) Applied catalysis. A, General. 172, 1, p. 67-72 Abstract
The gas-phase oxidative dehydrogenation of 4-vinylcyclohexene (VCH) to styrene in high selectivities was successfully carried out at moderate temperatures, 200-260°C, using a vanadium substituted polyoxometalate, PV2Mo10O5-40, supported on carbon as catalyst. The major co-product was ethylbenzene and only a small amount of over-oxidation to COx was observed. Maximum conversions and selectivity were obtained at a O2/VCH ratio of ∼1.9. The identity of the counter cation also affected the results with activity and selectivity decreasing in the following order H5∼(NH4)4K>Cs3H 2≫(NH4)5. Ethylbenzene and styrene are not formed by the same reaction pathway. For ethylbenzene formation, oxydehydrogenation is preceded by isomerization of the exocylic double bond to an endocyclic position, whereas for styrene formation there is no such isomerization. A mechanism is proposed whereby the active catalyst is a polyoxometalate - carbon support complex, which yields in the presence of oxygen quinone/hydroquinone or aroxy/phenol redox couples responsible for the oxydehydrogenation.
-
(1998) Physical review. B, Condensed matter. 57, 20, R12647(R). Abstract
We report on holographic storage with relatively high diffraction efficiencies in films ~2 mm to 20 mm thick! of conjugated polymer/glass and conjugated polymer/polymer composites with emphasis on the former. Anindex of refraction change Dn as high as 8.631023 was obtained. The grating formation was attributed primarily to the photochromic effect. However, the anomalous large two-beam coupling that was observed inthese materials cannot be explained by a photochromic mechanism. This, together with the small nonlinear electro-optic coefficient r eff50.42 pm/V may suggest that a weak photorefractive process also contributes tothe hologram formation in these materials. Holograms written on these conjugated polymer composite films are stable in the dark for ~at least! one year. @S0163-1829~98!50420-1#
-
(1998) Chemical Communications. 10, p. 1123-1124 Abstract
Macroporous, site isolated metal silicates are synthesized by a molecular route; the molybdenum silicate is especially active for the selective epoxidation of alkenes with tert-butyl hydroperoxide.
-
(1998) Journal of Applied Physics. 83, 2, p. 725-732 Abstract
Neutron and x-ray reflectivity studies of modulated heterostructurcs consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterosanctures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV). and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fining procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/ spacer interface. This roughness parameter, of the order of 12 ± 3 Å. was found to be smaller than the thickness of the D-PPV and the spacer layers, suggesting that the length over which interdigitation between neighboring polymer layers occurs is significantly smaller than the spacer layer. We demonstrate that the conversion to conjugated polymer by the heat treatment leads to ∼7% reduction of the repeat unit and the film thickness without significant changes of other structural properties; in fact, the interfacial roughness was somewhat improved. The fabrication of high quality modulated structures with controlled layer thickness and relatively small interfacial roughness may be a first step towards polymer-based multiquantum wells analogous to such devices in inorganic heterostructures.
-
(1998) Journal of Materials Chemistry. 8, 4, p. 919-924 Abstract
Poly(arylenevinylene) homopolymers and copolymers with polar hydroxy and carboxy moieties attached to the aromatic phenyl ring were prepared. The copolymers and the related homopolymers are copoly[5-(2-hydroxyethoxy)-2-methoxy-1,4-phenylenevinylene/1,4- phenylenevinylene], co(PHydroxyV-PV), and copoly(5-carboxymethoxy-2-methoxy-1,4-phenylenevinylene/1,4-phenylenevinylene), co(PCarboxyV-PV). For co(PHydroxyV-PV) the photoluminescence and electroluminescence spectra can be adjusted over a range of 100 nm as a function of the percentage of the hydroxy substituted phenyl ring. For co(PCarboxyV-PV) the observed spectral features are a function of the pH from which the final conjugated polymer was prepared. The optical density and photoluminescence of co(PCarboxyV-PV) films prepared from solutions at pH = 12 were significantly blue-shifted compared to polymers prepared from pH = 2 solutions at up to 30% substitution at the phenyl ring. The presence of the polar side chain was used to form self-assembled multilayer films, poly(ethyleneimine)/poly(styrenesulfonate)/co(PHydroxyV-PV), based on hydrogen bonding interactions rather than electrostatic forces. The luminescence spectra in the layered systems were blue-shifted compared to the spin coated films.
1997
-
(1997) Applied Physics Letters. 71, 23, p. 3344-3346 Abstract
We show that the degradation of a nonencapsulated polymer-based light-emitting diode (LED) is accompanied by the appearance of strong fluctuations,that is noise both in the radiance and in the film resistance. We demonstrate a correlation between the morphological changes which occur during the degradation process and the noise, suggesting that the sampling of noise during LED operation can be used as a very efficient tool to predict the approaching failure of LEDs in real-life applications. The morphological changes in LED degradation are essentially a two-stage process. First, there is formation of \u201cbubbles\u201d at the metalpolymer interface due to delamination of the polymer film from the metal surface. Second, carbonized areas in the form of \u201cblack spots\u201d are formed. Accumulation of carbonized areas leads to short and/or open circuits and final LED failure.
-
(1997) Journal of molecular catalysis. A, Chemical. 117, 1-3, p. 169-183 Abstract
The oxidation of organic substrates catalyzed by 'sandwich' type transition metal substituted polyoxometalates of the general formula, Na(x)M2Zn3W19O68, (M = Ru, Mn, Zn, Pd, Pt, Co, Fe, Rh) was examined in three different reaction media. The manganese analog was dissolved in a 1,2-dichloroethane phase using a lipophilic quaternary ammonium counter cation. Various organic substrates were oxidized with 30% aqueous H2O2. Alkenes reactivity increased as a function of the nucleophilicity of the double bond, but decreased as a function of steric crowding in the cyclohexene series. Alkenols with primary hydroxyl groups reacted chemo- and stereoselectively to form the corresponding epoxy alcohols. On the other hand, alkenols with secondary hydroxyl units did not react chemoselectively; both ketones and epoxy alcohols were formed. Diols were oxidized in most cases to ketols, except for 1,4-butanediol which yielded γ-butyrolactone. Secondary amines yielded hydroxyl amines except for piperidine which reacted with the solvent. A manganese containing catalyst supported on a functionalized silica particle was as active and selective as the organic solvent containing biphasic system for the oxidation of alkenes and alkenols. Reactions were also carried out by dissolving Na(x)M2Zn3W19O68 in aqueous solutions of 30% H2O2, 70% t-butylhydroperoxide or 0.02 M potassium persulfate in the absence of solvent. Hydrogen peroxide degraded all the TMSP compounds. One degradation product was an effective and chemo- and stereoselective catalyst for the epoxidation of primary alkenols. In alcohol oxidation only the ruthenium precursor was active. For oxidations with 70% t-butylhydroperoxide all compounds were stable but only the Na11Ru2Zn3W19O68 compound was active. Alcohols were oxidized selectively, however, alkenols yielded a mixture of products. With persulfate, some catalytic effects were observed in double bond oxidation.
-
(1997) Synthetic Metals. 85, 1-3, p. 1247-1248 Abstract
PPV pyridine based copolymers were studied. The copolymers were synthesized via the conventional precursor polymer route, conversion time dependence was investigated. PL emission was determined and depended on excitation wavelength.
-
(1997) Synthetic Metals. 85, 1-3, p. 1187-1190 Abstract
We describe a method to microfabricate a light emitting diode (LED) pixel array based on conjugated electroluminescent polymers sandwiched between ITO and aluminum. The method, based on direct photoablation using a 193 nm excimer laser, maintains intact the properties of the polymers. The technique as described here has already achieved array of 20 μm × 20 μm pixels with enhanced electroluminescence (EL) from pixels. The method can be extended to achieve nanometer sizes using near-field nanolithography. The microfabrication of the LED array requires also the patterning of the ITO and the aluminum electrode. For better performance of the device it is important to map the conductivity of the patterned electrodes. For that purpose we have used a novel mm-wave conductivity microscope which is capable to measure the local conductivity of the patterned film with a spatial resolution of ∼10-30 μm.
-
(1997) Synthetic Metals. 85, 1-3, p. 1269-1270 Abstract
We have studied the current versus voltage (I-V) and the luminescence versus voltage (EL-V) characteristics in PPV-based LED devices for different film thickness, d. For voltages below the EL threshold voltage, the I-V curves exhibit a linear regime (I∼V) at low voltages and quadratic (I∼V2) regime for higher voltages. The later is attributed to the space charge limited current mechanism assuming a single shallow trap level. The fit of the model to the experimental results is very good with a single fitting parameter, namely, the effective mobility. The extracted mobilities are compared to those measured independently by time-of-flight (TOF) photoconductivity. The SCLC model gives a reasonable explanation for the voltage threshold in EL.
-
(1997) Synthetic Metals. 85, 1-3, p. 1265-1266 Abstract
We present photoluminescence (PL) and electroluminescence (EL) studies of multilayer heterostructures containing alternate layers of conjugated copolymers and non-conjugated polymers (spacers). The self-assembled films are prepared either by electrostatic interactions between oppositely charged polyelectrolytes or using hydrogen bonding between molecules. We can tune both the PL and EL emission from blue to red region by: (a) using different copolymers with different emission properties; (b) controlling the thickness of the spacer layers in the assemblies; and to some degree (c) using different pH values of the polyelectrolyte solution
-
(1997) Journal of Catalysis. 166, 2, p. 206-217 CA971479. Abstract
TiO2, MoO3, and WO3 have been dispersed in amorphous silica using the low temperature sol-gel procedure for xerogel preparation. These simply prepared amorphous compounds are proposed as possible alternatives to metal-substituted crystalline molecular sieves in H2O2 oxidations. The metallosilicate compounds are catalytically active in the 30% aqueous H2O2 oxidation of alkenes and alcohols provided the metal oxide precursor in the xerogel synthesis is a metal-dichlorodialkoxy compound yielding MOx(Cl)-SiO2, and not the tetraalkoxy derivative yielding MOx-SiO2. Catalyst efficiency is increased by using low loading of metal oxide in the silica framework. Excess H2O2 reduces yield due to the detrimental effect of water, so more hydrophobic silicates with phenyl-silicon units increases catalyst efficiency. IR studies show that in the xerogels, the absorption at ∼950 cm-1 is mainly due to the Si-OH vibrations in (SiO)3Si-OH units and not (SiO)3Si-OM as has often been reported in studies of titanium-substituted zeolites. 29Si MAS NMR spectra, sensitive to second neighbor atoms, of catalytically active MOx(Cl)-SiO2 versus inactive MOx-SiO2 reveals that the former have larger Q3 peaks and therefore more (SiO)3Si-OM units, indicating higher molecular dispersion of the metal oxide in the xerogels. Diffuse reflectance UV-vis measurements indicate, however, that this molecular dispersion is not complete as absorptions attributable to polymeric forms of metal oxide are observable. ESR spectra of the metal oxide substituted silicates in the presence of hydrogen peroxide or in the reduced form are not useful in differentiating between active and inactive xerogel compounds. Atomic force microscopy imaging of the xerogels at ∼10 nm resolution shows that the xerogel has a basically smooth surface. Large cylindrical pits of 500-700 nm diameter and depth of 15-40 nm are also observable as imperfections in the xerogel. There is also formation of small silicate droplets on the surface with dimensions similar to that of the pits. The catalytic xerogels are microporous with an average pore diameter of 15 Å and a surface area of 750 m2/g.
-
(1997) Angewandte Chemie (International Edition in English). 36, 16, p. 1738-1740 Abstract
A new immobilization technique for homogeneous catalysts is represented by the title process. Polyethers that are covalently attached to silica surfaces (shown schematically on the right) act as solvent and/or ligands for the catalysts, in this case polyoxometalates (POM). Such systems allow, for example, the quantitative oxidation of cyclooctene to cyclooctene oxide. The catalyst can be recycled without loss of activity.
-
(1997) Synthetic Metals. 84, 1-3, p. 621-622 Abstract
We present transient and AC electroluminescence (EL) measurements on thin films of a newly synthesized copolymer of PPV where pyridine units were incorporated into the polymer backbone (co(PyPV)), and compare it to the EL of thin films of PPV prepared by the method of self-assembly and by spin casting. Transient EL under voltage pulses of co(PyPV) indicates that positive charge carriers have lower mobility than in PPV. Under pulses and AC square wave voltage we have observed a new type of prompt transient EL at the switch-off of the voltage. The transient EL effects are explained by the presence of non-bonding electrons in the polymer chain of co(PyPV). Co(PyPV) based LEDs have improved EL yield compared to PPV. Operations with sine AC voltage mode up to 10 kHz show better stability as compared to DC, without loss in efficiency.
-
(1997) Chemical Communications. 19, p. 1915-1916 Abstract
Methyltrioxorhenium has been supported on silica functionalized with polyether tethers; in the absence of an organic solvent, this catalytic assembly catalysed the epoxidation of alkenes with 30% aqueous H2O2 in high selectivity compared to the ring opening products observed in homogeneous media.
-
(1997) Synthetic Metals. 84, 1-3, p. 401-402 Abstract
Copolymers derived from PPV in which naphthalene and 2,3,5,6-tetrafluorobenzene were incorporated into the PPV backbone were synthesized. The dark DC conductivity, electroluminescence (EL) and photoluminescence (PL) of the copolymers are reported.
-
(1997) Supramolecular Science. 4, 1-2, p. 67-73 Abstract
The fabrication and characterization by means of photoluminescence (PL), UV-vis absorption, electro-luminescence (EL) and X-ray reflectivity of multilayer heterostructures consisting of alternate layers of conjugated and non-conjugated polymers are studied. The heterostructures are prepared by the layer-by-layer self-assembly technique, using two-types of polyelectrolytes. The thickness of bilayer or multilayer was controlled by changing the non-conjugated polymer layer. Fabrication of light emitting diodes (LEDs) which emit in the blue region is accomplished by using the assembly technique.
-
(1997) Nature. 388, 6640, p. 353-355 Abstract
The development of a procedure for the epoxidation of alkenes with molecular oxygen is an important industrial and synthetic goal. Non-radical activation of dioxygen by monooxygenase enzymes such as the iron-porphyrin- based cytochrome P-450 involves the insertion of one oxygen atom of O2 into the organic substrate while the other oxygen is reduced to water in the presence of an electron donor. Dioxygenase enzymes, on the other hand, catalyse insertion of both oxygen atoms without reducing agents. Oxidation of hydrocarbons with dioxygen catalysed by transition-metal compounds invariably proceeds by free-radical pathways rather than by dioxygenase-type reactions, and so do not allow the epoxidation of alkenes with molecular oxygen. The sterically hindered ruthenium tetramesityl porphyrin complex has been shown previously to activate dioxygen in a dioxygenase mode. We describe here the use of the polyoxometalate {[WZnRu2(OH)(H2O)](ZnW9O34)2}11- (ref. 2) as a catalyst for non-radical molecular oxygen activation and alkene epoxidation. The polyoxometalate can be considered to act as an inorganic dioxygenase catalyst. The advantages of using inorganic catalysts such as polyoxometalates, as opposed to organometallic complexes, is their well documented stability against decomposition by self-oxidation.
-
(1997) Progress in Inorganic Chemistry. Vol. 47. p. 317-370 Abstract
This chapter contains sections titled:IntroductionThe Polyoxometalate Reduction-Substrate Oxidation Reaction Mode: Oxygen as Primary OxidantThe Polyoxometalate Oxidation-Substrate Oxidation Reaction Mode: Peroxygen Compounds as Primary OxidantsTransition Metal Substituted PolyoxometalatesOther Uses of Polyoxometalates in CatalysisA Final Word and a View to the FutureCiting Literature
1996
-
(1996) Journal of molecular catalysis. A, Chemical. 114, 1-3, p. 169-180 Abstract
The rhodium containing polyoxometalate, [(WZnRh2/(III))(ZnW9O34)2]10 was prepared by reaction of [(WZn3)(ZnW9O34)2]12- with RhCl3. The X-ray diffraction showed this compound to be isostructural to other compounds of this series. This was confirmed by the IR spectra. The elementary analysis and cyclic voltammetry measurements indicated that the polyoxometalate was of high purity. Epoxidation of alkenes with 30% aqueous H2O2 in 1,2-dichloroethane/water biphasis systems using [(WZnRh2/(III))(ZnW9O34)2]10 as catalyst showed high overall activity as compared to other noble metal substituted polyoxometalates of this series. The reactions were highly selective to epoxidation of cyclohexene and similar to previously reported [(WZnRh2/(II))(ZnW9O34)2]12-, except that hydrogen peroxide yields were measurably higher in the rhodium case. The [(WZnRh2(III))(ZnW9O34)2]10 catalyst appears to be stable under turnovers conditions. Using IR spectroscopy as a probe to compare active polyoxometalates (Rh and Mn) versus an inactive one (Zn), showed that in the former case peroxo-tungstate intermediates could be identified whereas for the inactive compound no such intermediate was observed. A possible mechanism taking into account reactivity, steric constraints and IR spectra assuming structural integrity under catalytic turnover conditions was put forth.
-
(1996) Applied Physics Letters. 69, 24, p. 3650-3652 Abstract
We describe a method to microfabricate a light emitting diode array with pixels based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. This method, based on direct photoablation with the 193 nm emission of an excimer laser, maintains the properties of these unique polymers. The technique as described here has already achieved an array of 20 μm×20 μm pixels with enhanced electroluminescence (EL) from these pixels and possible spectral tuning of the EL by the application of varying external field. This method can be extended to achieve nanometer dimensionalities using near-field nanolithography.
-
(1996) Tetrahedron. 52, 26, p. 8781-8788 Abstract
Polyoxometalates can be used as catalysts for the activation of molecular oxygen and 30% aqueous hydrogen peroxide for the selective transformation of various organic substrates. In this paper results are presented for the oxidation of alkenes, dienes, alkenols, and sulfides using 30% aqueous H2O2 as oxidant and [WZnMn(II)2(ZnW9O34)2] 12-as catalyst. In certain but not all cases high reactivity, chemoselectivity and sterospecificity has been observed especially in the epoxidation of alkenols with primary hydroxyl units.
-
(1996) Journal of Applied Physics. 79, 6, p. 3082-3088 Abstract
We present photoluminescence (PL), UV absorption, electroluminescence and x-ray reflectivity studies of self-assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be "tuned" by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self-assembled ultrathin films containing thin (∼7 Å) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1ld where d is the thickness of the assembly, as expected for confined photogenerated electron-hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (∼40 Å) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on self-assembled multilayer films with improved efficiency and stability and with threshold voltage as low as 2.6 V could be fabricated.
-
(1996) Chemistry of Materials. 8, 2, p. 352-355 Abstract
A fluorinated poly(p-phenylenevinylene) analogue, co(TFPV-PV), has been prepared by copolymerization of fluorinated and nonfluorinated bissulfonium salt monomers. Conversion and yield decreased as a function of fluorine substitution. The UV - vis absorption was strongly shifted to the blue with increasing fluorine content. Complementary shifts in photoluminescence spectra were not observed, presumably because the fluorinated segments of the copolymer are very weakly luminescent relative to nonfluorinated segments. The photoluminescence spectra are therefore similar to those found for poly(p-phenylenevinylene) in the past. The co(TFPV-PV) compounds showed electroluminscence at an ac potential of ∼10 V (rms, 600 Hz) between ITO and Al electrodes. Maxima were shifted slightly to the blue with an increase in fluorine.
-
(1996) Chemical Communications. 23, p. 2643-2644 Abstract
Alkanes are oxidized selectively to ketones using V-MCM-41 with isobutyraldehyde/dioxygen as the preferred oxidant in terms of product selectivity and catalyst stability and recycle.
-
(1996) Advanced Materials. 8, 10, p. 833-837 Abstract
Composites of a sol-gel glass and a conjugated polymer have the potential to combine their unique properties: the high stability and ease of preparation of the sol-gel host and the optical fluorescence of the polymers. Two methods of preparation of such new composites are described, one of which successfully leads to composites with photoluminescence similar to that of homogeneous films of conjugated polymers. This result opens up new vistas in the fabrication of devices based on thin films of conjugated polymers using composite materials.
1995
-
(1995) Advanced Materials. 7, 10, p. 846-849 Abstract
Highqality selfassembled heterostructures for polymer LEDs have been fabricated from PPV and newly synthesized copolymers of PPV and naphthalene. It is shown that the layerbylayer selfassembly of multilayer structures has advantages with respect to control of the thickness and homogeneity of the polymer films compared to the conventional spin coating technique. The fabrication and characterization of the heterostructures are detailed and features of the electroluminescence and photoluminescence discussed.
-
(1995) Inorganic Chemistry. 34, 23, p. 5753-5760 Abstract
The polyoxometalates substituted with noble metals, Pd(II), Pt(II) and Ru(III), K12{[WZnPdII2(H2O)2](ZnW9O34)2}· 38H2O, K12{[WZnPtII2(H2O)2](ZnW9O34)2}·36H2O, and Na11{[WZnRuIII2(OH)(H2O)](ZnW9O34)2}·42H2O, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Na12-{[WZn3(H2O)2](ZnW9O34)2]}·46H2O. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW9O34 units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (S = 1/2) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% tert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H2O2. Kinetic profiles as well as UV-vis and IR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with tert-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with tert-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substituted-polyoxometalates.
-
(1995) Angewandte Chemie (International ed.). 34, 15, p. 1587-1589 Abstract
Highly selective aerobic hydroxylation of tertiary carbon centers in alkanes is catalyzed by the title polyoxometalate under mild reaction conditions [Eq. (a)]. This is of great significance, in particular in the search for ways to activate molecular oxygen in the oxidation of hydrocarbons. (Figure Presented.)
-
Tunable electroluminescence and photoluminescence in phenylenevinylenenaphthylenevinylene copolymers(1995) Advanced Materials. 7, 8, p. 742-745 Abstract
Color tuning of the electro and photoluminescence from naphthalenevinylenePPVbased lightemitting diodes between yellowgreen and red is reported. Band structure considerations are employed to explain that chemical selectivity and selective bandgap tuning are observed in some of the copolymers studied but not in others. It is also found that light emission can be tuned by changing the thickness of the copolymer film in the LED.
-
(1995) Journal of the American Chemical Society. 117, 18, p. 5066-5074 Abstract
The disubstituted manganese polyoxometalate, [WZnMn2(ZnW9O34)2]12-, has been used as a catalyst for the epoxidation of alkenes and the oxidation of secondary alcohols to ketones in biphasic (waterorganic) reaction media using hydrogen peroxide (30% aqueous H2O2) as the oxygen donor. At subambient temperatures, epoxidations are highly selective with little dismutation of hydrogen peroxide by homolysis or formation of side products by allylic oxidation. Thus, cyclohexene was oxidized to cyclohexene oxide at 2 °C with 99% selectivity. At low catalyst loadings and depending on the substrate and reaction temperature, hundreds to thousands of catalytic turnovers were obtained with only a 2-fold excess of hydrogen peroxide over substrate. Kinetic measurements on a model epoxidation of cyclooctene showed that the reaction is first order in cyclooctene, hydrogen peroxide, and [WZnMnII2(ZnW9O34)2]12- catalyst. Magnetic susceptibility measurements along with ESR and atomic absorption spectroscopy reveal that the manganese atom is in a terminal position coordinated by five bridging oxygen atoms and one labile aquo ligand which is disassociated upon transfer of the polyoxometalate anion into an organic phase. Comparison of the [WZnMnII2(ZnW9O34)2]12- anion with other mono-, tri-, or tetrasubstituted manganese polyoxometalates or catalysts having no manganese atoms showed that the catalytic activity of the former was unique. Kinetic evidence exhibited the absence of an induction period for catalyst activation or catalyst deactivation over a period of 12500 turnovers. IR spectra demonstrated that the original catalyst reacted with hydrogen peroxide to form a peroxo intermediate with a typical absorbance at 818 cm-1. After completion of the reaction, the original IR spectrum was measured again. ESR and atomic aborption spectroscopy also revealed that the [WZnMnII2(ZnW9O34)2]12- anion is solvolytically stable to aqueous hydrogen peroxide. Cyclic voltammetry, IR and UV-vis, and a comparative reaction with iodosobenzene as the oxygen donor seem to exclude a high valent manganeseoxo compound as the reactive intermediate. Rather, high reactivity is probably due to a tungstenperoxo intermediate somehow uniquely activated by an adjacent manganese atom.
-
(1995) Journal of Organic Chemistry. 60, 5, p. 1315-1318 Abstract
The synthesis of (2-bromoethyl)benzene by the anti-Markovnikov addition of gaseous hydrogen bromide to styrene has been found to be promoted by αpha;-bromo carbonyl compounds such as 2-bromo-2-methylpropanal. These compounds were found to catalyze the \u201cabnormal\u201d addition in a variety of solvents such as ethyl acetate, heptane, toluene and dioxane. High concentrations of 2-bromo-2-methylpropanal and hydrogen bromide and low concentrations of styrene favor formation of (2-bromoethyl)benzene. Using the free-radical catalyzed cyclization of 6-bromo-l-hexene as a probe we have found that the 2-bromo-2-methylpropanal does not in itself initiate a free-radical chain reaction by thermal formation of radicals. Instead, radicals may react with 2-bromo-2-methylpropanal to form relatively stable 2-methylpropanal radicals. The presence of such radicals increases the effective length or inhibits termination of the free-radical chain reaction (propagation) and in the case of hydrobromination of styrene raises the yield of (2-bromoethyl)benzene.
-
(1995) Applied catalysis. A, General. 122, 2, p. 85-97 Abstract
Vanadium silicate xerogels (V2O5{single bond}SiO2) were prepared by the sol-gel method by hydrolysis of vanadium and silicon alkoxides. The use of these xerogels as catalysts for oxidation of alkenes, alcohols and phenol was studied using 30% aqueous hydrogen peroxide as oxidant. It was found that the manner of xerogel preparation strongly influenced the catalytic activity of V2O5{single bond}SiO2. Alcohols were the preferred solvents for the reaction and did not leach vanadium oxide into solution. For alkenes, epoxidation was the dominant oxidation reaction the yield and selectivity depending on the nucleophilicity and oxidizability of the substrate. For alkenes of intermediate nucleophilicity allylic oxidation was significant. Secondary alcohols were oxidized in low to fair yields whereas primary alcohols were inert. Phenol was oxidized selectively to a 2:1 mixture of hydroquinone/catechol. UV-vis and electron spin resonance spectra of V2O5{single bond}SiO2 treated with hydrogen peroxide conclusively showed formation of vanadium-peroxo species with polarization of the O{single bond}O bond as seen by formation of vanadium (IV) electron spin resonance spectra leading to oxidation by both heterolytic and homolytic cleavage.
-
(1995) Journal of the Chemical Society, Chemical Communications. 22, p. 2277-2278 Abstract
An insoluble catalytic assembly consisting of a silicate xerogel covalently modified with phenyl groups and quaternary ammonium-polyoxometalate ion pairs was used to catalyse the oxidation of alkenes with 30% aqueous hydrogen peroxide in the absence of an organic solvent.
-
(1995) Journal of the Chemical Society, Chemical Communications. 2, p. 171-172 Abstract
In a one-pot reaction, transition metal substituted polyoxomolybdates are shown to be bifunctional catalysts for epoxidation of alkenes by via oxygen transfer from intermediate alkylhydroperoxides; the latter being formed by catalytic autoxidation with molecular oxygen.
1994
-
(1994) Journal of Organic Chemistry. 59, 25, p. 7577-7579 Abstract
Transition metal catalyzed oxidations carried out in liquidliquid biphasic reaction media using aqueous hydrogen peroxide as oxygen donor and quaternary ammonium cations are shown to be reactions in inverse emulsions and not typical phase transfer catalyzed reactions.
-
-
(1994) Photochemistry and Photobiology. 59, 3, p. 385-387 Abstract
A concept has been presented whereby Xray irradiation can be used to induce visible luminescence of chromophores such as porphyrins. The essential aspects of the idea have been demonstrated in a supramolecular assembly consisting of a waterdispersed polystyrene latex with an embedded scintillating compound, 2,5diphenyloxazole, and an externally attached hematoporphyrin. Xray excitation of the assembly yields an emission spectra typical for hematoporphyrin with fluorescent yields. of up to 3% with respect to 2,5diphenyloxazole emission. Significant quenching in the presence of dioxygen indicates that singlet oxygen is formed at atmospheric conditions.
-
(1994) Molecular Crystals and Liquid Crystals. 240, 1, p. 33-37 Abstract
The orientation of a porphyrin molecule in a thermotropic nematic liquid crystal can be controlled by molecular design. Thus, the porphyrin plane of tetraphenylporphyrin (TPP) is aligned parallel to the director of a liquid crystal whereas a porphyrin with orthogonal substitution of mesogenic 4-n-butoxybiphenyl appendages (MesogenP) is oriented in a perpendicular conformation. The alignment was determined by time resolved EPR spectroscopy of the triplet state of the free base porphyrin. Manganese TPP and MesogenP porphyrins then were used as catalysts for the epoxidation of alkenes using iodosobenze as oxidant. Reaction yields and regioselectivity for enlongated substrates such as cis-stilbene and 4-vinylbiphenyl were dependent on the alignment of the alkene and its carbon-carbon double bond relative to the director and metalloporphyrin catalyst.
1993
-
(1993) Journal of Molecular Catalysis. 84, 1, p. 93-108 Abstract
Keggin type mixed addenda heteropolyanions containing vanadium atoms such as PV2Mo10O5-40 are shown to be effective catalysts for the oxidation of alkylaromatic compounds to the respective acetates or alcohols and aldehydes or ketones using 30% hydrogen peroxide as oxidant. Use of spectroscopic techniques including UV-Vis, IR, vanadium-51 and phosphorus-31 NMR, and ESR shows that the H5PV2Mo10O40 compound is not degraded during the catalytic cycle. Oxidation proceeds by homolytic cleavage of H5PV2Mo10O40-peroxo intermediates. The resulting hydroperoxy and hydroxy radicals initiate the formation of benzyl radicals which lead to the product formation.
-
(1993) Journal of the Chemical Society, Chemical Communications. 12, p. 986-988 Abstract
Manganese porphyrins oriented both parallel and perpendicular to the director of a nematic liquid crystal have been used as catalysts for the epoxidation of aligned alkenes.
1992
-
(1992) Journal of the American Chemical Society. 114, 18, p. 7278-7286 Abstract
The aerobic oxidative dehydrogenation of α-terpinene to p-cymene catalyzed by the mixed-addenda heteropolyanion PV2Mo10O405- has been used to investigate the oxidation mechanism catalyzed by this class of compounds. The kinetics of the reaction show it to be zero order in α-terpinene, first order in dioxygen, and second order in the heteropolyanion catalyst. The kinetic results along with the use of UV-vis, ESR, 31P NMR, and IR spectroscopy have enabled the formulation of a reaction scheme involving the formation of a stable substrate-catalyst complex in the catalyst reduction (substrate oxidation) stage and a μ-peroxo catalyst intermediate in the catalyst reoxidation stage where dioxygen is reduced to water in a four-electron redox reaction.
-
(1992) Journal of the American Chemical Society. 114, 10, p. 3612-3618 Abstract
Molecular compatibility of a guest porphyrin in a thermotropic nematic liquid crystalline host phase was shown to control the orientation of the porphyrin chromophore. In this manner, the normal alignment of a porphyrin plane parallel to the director in a nematic phase was inverted to a perpendicular orientation by attachment of mesogenic type appendages orthogonal to a porphyrin ring on one side of the ring plane. This molecular design also leads to a head-to-tail ordering of the porphyrin even in isotropic phases. The conclusions reached are based on detection and analysis of time-resolved EPR spectra of photoexcited triplet states.
-
(1992) Tetrahedron Letters. 33, 13, p. 1795-1798 Abstract
The H5PV2Mo10O40 heteropolyanion has been found to catalyse the highly selective aerobic oxidation of dialkylphenols to diphenoquinones and the oxidation of 2, 3, 5-trimethylphenol to the 2, 3, 5-trimethyl-1, 4-benzoquinone. The rate is highly dependent on the oxidation potential of the substrate and is proceeds by electron transfer from the phenol substrate to the heteropolyanion catalyst.
1991
-
-
(1991) Chemical Physics Letters. 182, 3-4, p. 249-252 Abstract
The orientation of the porphyrin ring in nematic liquid-crystalline phases is shown to be controlled by modification of guest-host dynamics through molecular design. Thus, attachment of rod-like mesogenic residues or "arms" to α,α,α,α,-meso-tetrakis(o -aminophenyl)porphyrin yields the porphyrin, MesogenP, having molecular compatibility with standard liquid-crystalline compounds. Both the free-base and zinc porphyrin were photoexcited to the triplet state and detected by time-resolved EPR spectroscopy. The spectra show that the orientation, porphyrin ring parallel to the director, found for the simple tetraphenyl porphyrin, is inverted in the case of the MesogenP so that the porphyrin ring is perpendicular to the director in both frozen and fluid nematic phases.
-
(1991) Studies in Surface Science and Catalysis. p. 121-127 Abstract
Molecular oxygen was used as the primary oxidant in transition metal substituted heteropolyanion, SiM(H2O)W11O39q-(M = Mn3+, Cu2+, Co2+, Fe3+), catalysed oxidative transformations of olefins in the presence of reducing agents. Product selectivity depended on the reductant used; thus in the presence of sodium dithionite allylic oxidation products were formed, collodial platinum and hydrogen catalysed epoxidation, and the use of sodium borohydride yielded saturated alcohols with both Markownikoff and anti-Markownikoff orientations.
1990
-
(1990) Journal of the American Chemical Society. 112, 16, p. 6025-6031 Abstract
A ruthenium-substituted heteropolyanion SiRu(H20)Wl|0395· was synthesized and characterized. The hydrophobic quaternary ammonium salt of the heteropolyanion ((C6H|3)4N)5SiRuIII(H20)Wn039 was used as a catalyst for the oxidationof alkenes with Zert-butyl hydroperoxide, potassium persulfate, iodosobenzene, and sodium periodate as primary oxidants. Reactivity and selectivity were found to be dependent on the oxidant used; several different types of oxidation processes could be identified including allylic oxidation, epoxidation, and oxidative cleavage. Use of sodium periodate as oxidant enabled selective bond cleavage with aldehydes as the exclusive product. Different product selectivity and UV-vis and IR spectra of the ruthenium heteropoly compound in the presence of the various oxidants shows that unique mechanisms are operating in each case. A series of further experiments into the oxidation of styrene derivatives to benzaldehydes by sodium periodate including investigation of the reaction kinetics, substituent effects, and isotope incorporation enabled the formulation of a reaction mechanism. The reaction proceeds by interaction of the styrene with the catalyst forming a metallocyclooxetane which rearranges in the rate-determining step to a cyclic diester through two different transition states depending on the ring substituent. In the final step the cyclic diester decomposes yielding the cleavage products.
1989
-
(1989) Journal of the American Chemical Society. 111, 22, p. 8410-8413 Abstract
The vanadium(V)/vanadium(III) redox couple was investigated in an acidic organic medium by reduction of sodium metavanadate by hydrogen bromide with 1,2-dichloroethane as solvent in the presence of tetraglyme as a complexing agent. In the absence of dioxygen a vanadium(III)-tetraethylene glycol (teg) pentagonal-bipyramidal complex, [V(teg)(Br)2]+Br-, is formed. In the presence of dioxygen a catalytical cycle is initiated whereby vanadium(V) oxidizes hydrogen bromide to molecular bromine. The reduced vanadium(III) species thereby formed is reoxidized by dioxygen to vanadium(V) as confirmed by ESR and UV-vis of the organic phase.
-
(1989) Journal of Organic Chemistry. 54, 19, p. 4607-4610 Abstract
The mixed addenda heteropoly acid H6PMo10V2O40 dissolved in 1,2-dichloroethane with tetraglyme, forming the (tetraglyme)3-H6PMo10V2O40complex, catalyzes the aromatization of cyclic dienesat moderate temperatures in the presence of molecular oxygen. Dehydrogenations of exocyclic dienes such as limonene show that dehydrogenation is preceded by isomerization to their endocyclic isomers. Aromatization takes place by successive one-electron transfers and proton abstractions from the organic substrate to the heteropoly acid, the latter being reoxidized by dioxygen coupled with the formation of water.
-
(1989) Journal of the American Chemical Society. 111, 8, p. 2900-2909 Abstract
A membrane-spanning porphyrin has been synthesized by attaching four 3β-hydroxy-5-cholenic acid moieties to,β,α,β-meso-tetrakis(o-aminophenyl)porphyrin. The resulting steroidal porphyrin, H2ChP, and the corresponding metalloporphyrins, MChP, were shown by gel permeation chromatography,31P NMR, and differential scanning calorimetry to intercalate into vesicle bilayers. The steroidal porphyrin was found to be in a well-defined and highly ordered microenvironment within the bilayer. The anisotropic ESR spectra of Cu11(ChP) in orientated bilayer assemblies on Mylar film clearly indicated that the plane of the porphyrin ring was parallel to the plane defined by the bilayer-water interface. The porphyrin ring was also found to be in the middle of the bilayer with fluctuations of ±3-4 Å around the center. This was shown by use of tethered imidazole ligands of the general formula Im(CH2)nCOOH as molecular probes. Ligation to Co11(ChP) could be monitored by ESR as a function of the length of the tethered ligand and conclusively demonstrated that only ligands where n > 6 were able to coordinate to the metal. Iron(III) and manganese(III) steroidal porphyrins were then used as regioselective epoxidation and hydroxylation catalysts. Diolefinic sterols were epoxidized exclusively at the side chain. Epoxidation of polyunsaturated fatty acids was preferred by a ratio of 2/1 at the more hydrophobic terminus and it was found that by increasing the rigidity of the bilayer by the addition of cholesterol the selectivity could be raised to 9/1. Finally, it was shown that cholesterol could be selectively hydroxylated at the C25tertiary carbon.
-
(1989) Journal of the Chemical Society, Chemical Communications. 18, p. 1324-1325 Abstract
A ruthenium heteropolyanion, SiRu(H2O)W11O 395-, has been synthesized which catalyses the liquid phase oxidation of alkanes and alkenes with various primary oxidants including potassium persulphate, sodium periodate, t-butyl hydroperoxide, and iodosylbenzene; the activity and selectivity vary with the oxidant used.
-
(1989) Journal of the Chemical Society, Chemical Communications. 9, p. 547-548 Abstract
An oxonium cation complexed by a noncyclic polyether has been synthesized and structurally characterized; the oxonium cation is complexed assymetrically with short-ether bonds and the oxonium cation is found in the thermodynamically preferred pyramidal configuration.
1988
-
(1988) Journal of Organic Chemistry. 53, 16, p. 3891-3893 Abstract
The hydroxylation of simple alkanes and the selective C-25 hydroxylation of cholesterol have been achieved with a membrane-spanning Mn(III) porphyrin positioned in a synthetic bilayer assembly by appendedsteroidal substituents.
-
(1988) Journal of the Chemical Society, Chemical Communications. 19, p. 1285-1287 Abstract
Selective bromination of phenol and its derivatives and the bromination of ketones and alkenes have been achieved by oxybromination at ambient conditions catalysed by the mixed addenda heteropolyanion compound H5PMo 10V2O40, which is dissolved in nonpolar chlorohydrocarbon solvent by complexation with tetraglyme.
1987
1986
-
(1986) Journal of Molecular Catalysis. 35, 1, p. 131-136 Abstract
Potassium hydroxide impregnated on a γ-alumina support, KOH/Al2O3, was found to be an active catalyst for the base-catalyzed dehydrogenation of cyclic hydrocarbons such as cycloolefins, cycloparaffins and alkyl aromatics. Among reactions studied were the dehydrogenation of cyclohexene, pKa 36, and cyclohexane, pKa 45, to benzene, and ethylbenzene to styrene. The reactions proceeded with high conversions and selectivity at temperatures above 500 °C. Cracking reactions were suppressed and the catalyst was stable to deactivation over long time periods. Comparison of γ-Al2O3 with other common supports showed it was the most effective and selective.
-
(1986) Journal of Organic Chemistry. 51, 15, p. 2880-2883 Abstract
Toluene and methylbenzene ring substituted with electron-withdrawing substituents were oxidized to the corresponding carboxylic acids. The oxidation system is biphasic consisting of an organic substrate phase and an aqueous sodium hypochlorite phase with both ruthenium and quaternary ammonium salts acting as catalysts. Yields are essentially quantitative after 2 h at room temperature with a stoichiometric ratio hypochlorite/toluene of 4.5:1 provided the pH of the aqueous phase is between 8.0 and 10.5. Kinetic studies show the reaction to be of first order in the substrate, zero order in sodium hypochlorite, and combined first order in the catalysts. The reaction mechanism consists of a Ru04-catalyzed hydride abstraction and a phase-transfer-catalyzed proton-dependent step.
-
(1986) Tetrahedron. 42, 13, p. 3569-3574 Abstract
Bis-tertiary diols accelerate bis-eliminations of HX from 1,2-dihalides in the presence of an onium salt as phase transfer catalyst and potassium hydroxide as base. PEG 400 catalyzes the same elimination, but isomerizations from 1-alkyne to mixtures with the 2-alkyne and the 1,2-diene occur easily.
-
(1986) Journal of the Chemical Society, Perkin Transactions 2. 6, p. 823-826 Abstract
The known Hofmann degradation of quaternary ammonium salts under basic phase-transfer catalytic conditions has been studied. The base-catalysed isomerization of p-allylanisole to p-methoxy-β-methylstyrene was used as a kinetic probe to find experimentally the rate constant and activation energy of the Hofmann decomposition without isolating the quaternary ammonium basic salt R4N+B-(B-=base anion). Reactions performed at various temperatures showed that the higher the temperature the greater was the initial rate but the lower the final conversion in the isomerization reaction. The quaternary ammonium hydroxide was found to catalyse the isomerization and the Hofmann degradation more effectively than the corresponding alkoxide. This indicates that the former is a stronger base in the nonpolar aprotic solvents common in phase-transfer catalysis.
-
(1986) Synthesis. 1986, 7, p. 569-570 Abstract
The para selectivity of the Reimer-Tiemann reaction was increased by use of polyethylene glycol in the reaction medium. The polyethylene glycol complexes the phenolate substrate masking the ortho position of the benzene nucleus increasing para selectivity without affecting the total yield.
1985
-
(1985) Journal of Molecular Catalysis. 33, 2, p. 201-208 Abstract
The gas-phase base-catalyzed isomerization of allylbenzene has been used as a model reaction to evaluate the activity of polyethylene glycol in liquid-gas phase transfer catalysis by-use of pulse reaction kinetics. Polyethylene glycol-potassium carbonate complexes adsorb the allylbenzene and reaction occurs at the catalyst interface. The activity of the catalyst is determined by two factors: the equilibrium of adsorption of the organic substrate and the solvation of the carbonate anion. We have found that polyethylene glycol adsorbs poorly vis-à-vis high surface area γ-alumina, and that solvation of the carbonate anion by the polyethylene glycol also reduces its activity. Carbonate anions impregnated on γ-alumina are not solvated and therefore have high activity.
-
(1985) Industrial and Engineering Chemistry Product Research and Development. 24, 3, p. 390-393 Abstract
The use of quaternary ammonium salts (Quats) as phase-transfer catalysts is analyzed from a technological and economic point of view. The limiting price for Quats Is determined by establishing a qualitative relationship between technical parameters and economic factors. Specific examples show that the present price of Quats limits their commercial use to the area of high-priced commodities and specialty chemicals. Quats may be used in commodity manufacture provided they are produced on a larger scale. A simulation of such a large-scale production of a representative Quat (tetrabutylammonium bromide) has been made and its consequences are discussed.
-
(1985) Journal of Molecular Catalysis. 31, 1, p. 81-88 Abstract
Polyethylene glycol was found to undergo anionic 'catalyst poisoning' by lipophilic anions in a manner similar to that of quaternary ammonium salts, even though the interaction between alkali salts and the catalyst is generally considered to be influenced only by the cation. From reaction profiles which indicated catalyst poisoning, an exact kinetic equation describing the conversion as a function of time could be used to compute selectivity constants, Ks, by graphic iterations.
-
(1985) Journal of Molecular Catalysis. 29, 3, p. 291-297 Abstract
Aromatic aldehydes were oxidized to carboxylic acids in high yields and selectivity under mild conditions. The reactions were performed using aqueous sodium hypochlorite as oxidant in a liquid-liquid phase transfer catalytic system using quaternary ammonium salts as catalysts. The reaction was strongly pH-dependent, with maximum reaction rates at pH 911. Similarly, extraction of the hypochlorite ion was maximal at these pHs. The maxima are attributed to coextraction of hypochlorous acid together with the hypochlorite anion into the organic phase, the former significantly increasing the reaction rate.
-
(1985) Journal of Molecular Catalysis. 29, 3, p. 299-303 Abstract
Benzyl alcohol was oxidized by aqueous sodium hypochlorite in a phase transfer catalysis reaction catalyzed by quaternary ammonium salts. Two products were obtained, benzaldehyde and benzyl benzoate. The concentration of the organic phase and the pH of the aqueous phase affected the product distribution: a dilute organic phase and low pH favored benzaldehyde as a product, whereas a concentrated organic phase and high pH favored benzyl benzoate as a product.
-
(1985) Journal of the Chemical Society. Chemical communications. 9, p. 616-617 Abstract
The use of ultrasonic agitation in the base-catalysed autoxidation of alkylnitrobenzenes in polyethylene glycol phase-transfer systems changes reaction selectivity from the usual dimeric products to the carboxylic acid.
-
(1985) Israel Journal of Chemistry. 26, 3, p. 239-242 Abstract
The complexation of alkali metal salts by polyethylene glycol is strongly dependent on the counter anion of the salt. In general, potassium salts are more easily complexed. In fact, sodium salts at low free energy of formation are barely complexed at all. By conductometric and index of refraction measurements it was shown that anions capable of hydrogen bonding such as OH−, F−, HSO−4 and HCO−3 were ion paired to the complexed potassium cation and existed as relatively \u201cnaked\u201d anions. Other anions such as Cl−, Br−, I−, SCN−, NO−3, NO−2 AND HF−2 existed as disassociated anions with significant solvation shells. The use of polyethylene glycol as a phase transfer catalyst considering the results found is discussed.
1984
-
(1984) Industrial and Engineering Chemistry Process Design and Development. 23, 4, p. 654-659 Abstract
The conversion of diluted acetic acid to methyl acetate by methanol esterification in a chemorectificatlon column packed with an acidic organic polymer catalyst was studied. The process was tested both experimentally and theoretically by use of computer simulation, variables such as reflux ratio, feed plate, feed rate, etc., were studied and a comparison of results was made. In general the results were compatible and consistent. The ability to achieve high conversions and pure methyl acetate even with dilute acetic acid as reactant was shown. In addition, the kinetics of the heterogeneous esterification process was studied. The reaction was found to be second order in acetic acid, zero order In methanol, and partially diffusion controlled.
-
-
(1984) Journal of Organic Chemistry. 49, 19, p. 3448-3451 Abstract
The mechanism of base-catalyzed reactions with poly (ethylene glycol) (PEG) as phase-transfer catalysts was studied by using the isomerization of allylanisole as a model reaction. The reaction kinetics showed the reaction to be chemical reaction controlled. The reaction system was a three-phase system consisting of an organic solvent phase, a PEG-potassium hydroxide complex phase, and a basic aqueous phase. The reaction mechanism included diffusion of the substrate from the solvent to the complex phase reaction and back diffusion of the product. The concentration of the aqueous phase is also important. When the aqueous phase is unsaturated there is no reaction. Concentrations above saturation increase rate because the basic complex becomes more potent under anhydrous conditions. The chain length and chain end moiety of the PEG catalysts have significant influence on the reaction rate. In general, short chain catalysts were more effective per gram but not per mole of catalyst. Etherification of the terminal hydroxyl group reduced activity. When alkoxide species were used as bases the trends were reversed, long chain catalysts being more effective and etherification increasing activity.
1983
-
(1983) Tetrahedron. 39, 20, p. 3437-3440 Abstract
Mono and di-halobenzenes and alkoxide ions gave monoalkoxybenzenes, but no phenols, when catalyzed by high molecular weight poly(ethylene glycol)s as phase transfer catalysts. Bromobenzenes were more reactive than chlorobenzenes but gave more side products, and for alcohols the reactivity was primary > secondary > tertiary.