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.