Oxidovanadium(V) Anchored to Silanol-Functionalized Polyoxotungstates: Molecular Models for Single-Site Silica-Supported Vanadium Catalysts

The metalation of two different types of silanol-decorated polyoxotungstates, [XW9O34–x(tBuSiOH)3]3– (X = P, x = 0; X = Sb, x = 1) and [PW10O36(tBuSiOH)2]3–, by Cl3VO or (iPrO)3VO has been achieved. The characterization of the resulting oxidovanadium­(V) complexes [XW9O34–x(tBuSiO)3VO]3– (X = P, 3; X = Sb, 3′) and [PW10O36(tBuSiO)2VO­(iPrO)]3– (4) has been fully detailed, including X-ray analysis. These compounds are present in monomeric forms and therefore represent original molecular models for tris-grafted and bis-grafted isolated (VO)3+ species dispersed onto silica. Their ability as precatalysts for the epoxidation of cyclic olefins and allylic alcohols with tert-butyl hydroperoxide (TBHP) has been studied. The results suggested that a confined tris-grafted species, such as 3 or 3′, does not act as an efficient catalyst whereas a more labile bis-grafted species, such as 4, does. To gain a better understanding, we have assessed their suitability for ligand exchange with alcohols and TBHP and performed reaction progress kinetic analysis by monitoring the epoxidation of 3-methyl-2-buten-1-ol by 1H and 51V nuclear magnetic resonance.