Activation of Molecular Oxygen

Molecular oxygen is kinetically quite stable towards reaction at room temperature because of its triplet ground state and strong oxygen-oxygen bond.  On the other hand the thermodynamic tendency for the reaction of O2 is combustion, that is to form carbon dioxide (CO2) and water (H2O). Thus, hydrocarbons typically react with O2 via a complex free-radical pathway termed autooxidation.  These reactions are usually not selective and often have little synthetic utility.  In order to overcome this limitation, in our group we are developing catalysts that catalyze reactions by new reaction pathways. Catalysis is used both to lower the activation energy of the reaction and to change the chemoselectivity of the reaction.

Electron Transfer-Oxygen Transfer Pathways Catalyzed by Polyoxometalates for Hydrocarbon Functionalization.  The general methodology for aerobic oxidation reactions by such mechanisms involves activation of a substrate, typically a hydrocarbon, by electron transfer. In earlier work described in the 1990’s this electron transfer led to a dehydrogenated product while concomitantly the reduced catalyst was reoxidized by O2. More recently we were able to show that the electron transfer oxidation of the hydrocarbon substrate can be coupled to an oxygen transfer from the polyoxometalate catalyst by a mechanism we now term an electron transfer-oxygen transfer (ET-OT) mechanism. This is a unique mechanism in liquid phase oxidation Some salient features of these reactions are that (a) electron transfer or reduction of the catalyst is needed for subsequent oxygen transfer and (b) the catalyst can be recycled by O2. Such concepts were extended to novel reactions such as oxidation of primary alcohols by C-C bond cleavage rather than by the very prevalent C-H bond activation, the aerobic ortho selective hydroxylation of nitrobenzene, and the aerobic oxidation of sulfides, which may have important commercial potential for the “complete” removal of sulfur compounds from fuel. Just recently we have carried out extensive computational research to support out experimental finding.

The atypical aerobic oxidation of primary alcohols through carbon-carbon bond cleavage catalyzed the H5PV2Mo10O40 polyoxometalate

Activation of Molecular Oxygen at Transition Metal Centers and Intramolecular Activation of Molecular Oxygen by a Dioxygenase Pathway. An important potential method for the use of O2 as oxidant is via its coordination to a transition metal, in our case, substituted within a polyoxometalate framework. Further reaction with another metal center can lead to oxygen-oxygen bond cleavage, formation of a higher valent metal-oxo species that is a viable oxygen transfer agent. The net result is a “dioxygenase” type activation of O2. Such non-porphyrin activation of O2 has been demonstrated by us using a ruthenium-substituted polyoxometalate. The first step in oxygen activation by a metal is its coordination to a metal that in nature is often iron. Surprisingly, except for iron porphyrins, intermediates formed by coordination of O2 to iron compound have not been previously isolated. Just recently we have been able to isolate such Fe-hydroperoxo intermediate and determine their somewhat unexpected structure by X-ray diffraction. The use of electron energy loss spectroscopy proved to be a powerful tool in the analysis of these intermediates. Further, we have shown that the reaction of O2 with a ruthenium coordination compound with proximal selenium sites to yield the molecular oxygen cleaved product containing two Ru-O-Se moieties. Therefore, the intramolecular oxygen cleavage event in a dioxygenase mechanism has been observed via isolation of the product and structural analysis. The research thus provides a new paradigm for bi-centered catalysis where in this case a transition metal (Ru) acts in concert with a neighboring chalogen atom (Se) to active O2.

Pathway for the intramolecular activation of O2; isolation of the cleaved intermediate and oxygen transfer reactions.

Intermediates in Oxidation of Hydrocarbons and Water. High valent oxo species are key intermediates in the oxidation of hydrocarbons and also of water. In our group we are interested in identifying such new intermediates, which can help us in understanding the mechanism of oxidation reactions. In the past we have isolated an unique manganese(V)-oxo intermediate within a polyoxometalate framework. More recently we have captured a high-valent cobalt based oxo dimer that is active for carbon-hydrogen bond activation and water oxidation.

A dimeric cobalt substituted polyoxometalate as a reactive intermediate for C-H bond activation and water oxidation