Peroxo-Zr/Hf-Containing Wells-Dawson Polyoxometalates for H2O2-based Homogeneous, Heterogeneous, and Biphasic Oxidation Catalysis

Abstract

Four peroxo-Zr and Hf containing polyanions, [M₂(O₂)₂(X₂W₁₇O₆₁)₂]¹⁶⁻ (X=P, M=Zr^IV (ZrP), Hf^IV (HfP); X=As, M=Zr^IV (ZrAs), Hf^IV (HfAs) were synthesized and structurally characterized. The structures consist of two monolacunary Wells-Dawson ions which are connected head-on via two side-on peroxo bridges. The four new polyanions are solution-stable as shown by ³¹P NMR, Raman and FT-IR spectroscopic studies. Comparative studies on alcohol oxidation with a solvent- and phase-transfer catalyst-free biphasic oxidative system with homogeneous and supported-catalyst were performed and several noteworthy observations in reactivity and selectivity were observed, such as the highly selective ring-opening oxidation of cyclohexanol to adipic acid. Tetra-n-butylammonium salts of ZrP and HfP were also synthesized to study the catalytic activity of the compounds in the homogenous H₂O₂-based oxidation of olefins, α,β-unsaturated carbonyl compounds, and thioethers in organic solvent.