Publications

2006

1. Characterization of manganese(V)-oxo polyoxometalate intermediates and their properties in oxygen-transfer reactions

   Khenkin A. M., Kumar D., Shaik S. & Neumann R. (2006) Journal of the American Chemical Society. 128, 48, p. 15451-15460  Abstract

   A manganese(III)-substituted polyoxometalate, [α₂-P ₂Mn^III(L)W₁₇O₆₁]^7- (P₂W₁₇M^III), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F ₅Phl(TFAc)₂) as a monooxygen donor. Pink P ₂W₁₇Mn^III 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 P₂W₁₇Mn^III-F₅-Phl(TFAc) ₂ that disappeared upon warming. The ³¹P NMR spectra of P₂W₁₇Mn^III 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 ₁₇Mn^V=O. An additional broad peak at -10.6 ppm was attributed to both the P₂W₁₇-Mn^III-F ₅Phl(TFAc)₂ complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P ₂W₁₇Mn^V=O were modeled by DFT calculations using the analogous Keggin compound, [PMn^V=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 [PMn^V=OW₁₁O₃₉]^4- should be highly reactive, even more reactive than analogous Mn^V=O porphyrin species. Kinetic measurements of the reaction of P₂W₁₇Mn ^V=O with 1-octene indicated, however, that P₂W ₁₇Mn^V=O was less reactive than a Mn^V=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.
2. An example of lipophiloselectivity: The preferred oxidation, in water, of hydrophobic 2-alkanols catalyzed by a cross-linked polyethyleneimine- polyoxometalate catalyst assembly

   Haimov A. & Neumann R. (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 (M_w = 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.

3. Palladium nanoparticles stabilized by alkylated polyethyleneimine as aqueous biphasic catalysts for the chemoselective stereocontrolled hydrogenation of alkenes

   Vasylyev M. V., Maayan G., Hovav Y., Haimov A. & Neumann R. (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.

4. Catalysts for monooxygenations made from polyoxometalate: An iron(V)-oxo derivative of the Lindqvist anion

   Derat E., Kumar D., Neumann R. & Shaik S. (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 (Mo₆O₁₉^2-). 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.

5. Aqueous biphasic catalysis with polyoxometalates: Oximation of ketones and aldehydes with aqueous ammonia and hydrogen peroxide

   Sloboda-Rozner D. & Neumann R. (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.

6. Structural and EPR/ENDOR/ESEEM spectroscopic investigations of a vanadomolybdate Keggin-type polyoxometalate in organic solvent

   Khenkin A. M., Carl P., Baute D., Raitsimring A. M., Astashkin A. V., Shimon L. J. W., Goldfarb D. & Neumann R. (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 H₅V^VV^IVMo₁₁O₄₀ exists as a previously unrecognized impurity in the typically synthesized H₅PV₂Mo₁₀O₄₀ compound. H_5/4PV^VO₄V^IV/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 [V^VV^IVMo₁₁O₄₀]^5- as [V^VO₄V^IVMo₁₁O₃₆]^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.
7. Synthesis and characterization of polyoxometalate-polyamino dendritic hybrid compounds

   Vasylyev M. V., Gatard S., Bar-Nahum I., Konstantinovski L., Wachtel E. J. & Neumann R. (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 (DAB-Am) tetraamine and octaamine dendrimers. Translational diffusion coefficients of the hybrid compounds were measured in DMSO-d₆ 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)W ₁₁O₃₉]^4--polypropylenimine octaamine hybrid adduct was measured in a light scattering experiment.
8. Preparation, characterizaton, and catalytic aerobic oxidation by a vanadium phosphonate mesoporous material constructed from a dendritic tetraphosphonate

   Vasylyev M. & Neumann R. (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.

9. Inorganic compounds and materials as catalysts for oxidations with aqueous hydrogen peroxide

   Nardello V., Aubry J., De Vos V. D., Neumann R., Adam W., Zhang R., ten Elshof E. J., Witte P. & Alsters P. (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.
10. Micelle directed synthesis of polyoxometalate nanoparticles and their improved catalytic activity for the aerobic oxidation of sulfides

    Maayan G., Popovitz-Biro R. & Neumann R. (2006) Journal of the American Chemical Society. 128, 15, p. 4968-4969  Abstract

    Micelle directed polyoxometalate nanoparticles were synthesized by depositing H_3+xPV_xMo_12-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.

11. Titanium phosphonate porous materials constructed from dendritic tetraphosphonates

    Vasylyev M., Wachtel E., Popovitz-Biro R. & Neumann R. (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.

12. Selective aerobic oxidation in supercritical carbon dioxide catalyzed by the H₅PV₂Mo₁₀O₄₀ polyoxometalate

    Maayan G., Ganchegui B., Leitner W. & Neumann R. (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.
13. Molecular oxygen and oxidation catalysis by phosphovanadomolybdates

    Neumann R. & Khenkin A. (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 _xMo_12-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.