Aerobic Oxidations Catalyzed by Polyoxometalates

Abstract

Two major reaction modes have been perceived for the catalytic activity of polyoxometalates in oxidation reactions. In one case, the catalytic cycle has been described by the division of the reaction into two stages. First, the substrate is oxidized by consecutive electron and proton transfer by the polyoxometalate in the oxidized form to yield the product and the reduced polyoxometalate catalyst. The reduced polyoxometalate catalyst is then reoxidized, importantly by molecular oxygen to form water, in the second and possibly separate stage completing the catalytic cycle. The polyoxometalates often most effective in this reaction are the phosphovanadomolybdates of the Keggin structure, H3+xPVxMo12−xO40 (x = 1 − 6, especially x = 2). Now, new investigations of the reactivity of PV2Mo10O405− with aldehydes and quinones enables the differentiation between the reactivity of the five inseparable isomers of α-H5PV2Mo10O40 using 31P NMR and ESR spectroscopy, and UV-vis absorption-time profiles. The 1,11 isomer with vanadium in distal positions is the most abundant, although the isomers with vanadium in vicinal positions appeared to be the most kinetically viable. For example, alkane/aldehyde/O2 oxidizing systems were found to be quite effective and selective for oxidation of alkanes to ketones. Further studies of PV2Mo10O405− - quinone interactions has shown the formation of semiquinone intermediates. The later are active in the dehydrogenation of benzylic and allylic alcohols to aldehydes and can be used as models for the reactivity of PV2Mo10O405− on carbon supports. The second reaction type views the oxidation catalyzed by the polyoxometalate as an interaction with a primary oxidant. This interaction yields an activated catalyst intermediate eg a peroxo, hydroperoxo or oxo species which can be used to oxidize the organic substrate. In this mode, one can consider reaction at a transition metal substituted position within the polyoxometalate. Here the polyoxometalate acts as an “inorganic ligand” for transition metals such as cobalt, manganese, ruthenium, etc. In mechanistic scenarios for such reactions, the catalytically active site is a tetragonally (pyrimidal) oxo coordinated transition metal while the polyoxometalate as a whole functions as a ligand with a strong capacity for accepting electrons. In this last group of oxidation reactions the actual reaction mechanism certainly varies as a function of the transition metal and oxidant, but can be conceived to take place via a general intermediate “transition metal - oxidant” species. The ruthenium substituted “sandwich” type polyoxometalate, [WZnRuIII2(ZnW9O34)2]11−, has been shown to activate molecular oxygen in a dioxygenase type mechanism, and selectively catalyze thereby a) the hydroxylation of alkanes at the tertiary carbon position and b) the stereoselective epoxidation of alkenes. For comparison, catalytic oxidation of a novel ruthenium substituted polyoxometalate, [RuII(H2O)W17O55F6NaH2]9−, in similar reactions appears to occur by a metal catalyzed autooxidation.