Grafted Ta–calixarenes: Tunable, selective catalysts for direct olefin epoxidation with aqueous hydrogen peroxide

Supported Ta(V) oxide catalysts were prepared by grafting calixarene–Ta(V) and TaX5 complexes on SiO2 at coverages less than 0.25 Ta nm−2. Thermogravimetric and elemental analyses and UV–visible spectroscopy indicate that catalysts consist of isolated, 1:1 Ta:ligand surface sites. Catalysts obtained by this one-pot procedure were studied in cyclohexene and cyclooctene epoxidation with H2O2. In sharp contrast with bare oxides, calixarene-containing catalysts had initial cyclohexene direct epoxidation turnover rates of 3.9 ± 0.1 × 10−2 s−1 unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3× more active than the corresponding TaCl5-based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus <65% selectivity for TaCl5-based catalysts or only ∼20% selectivity for grafted calixarene–Ti(IV) catalysts. Capping silanols with octanol reduced epoxide hydrolysis from >50% to <30%. These catalysts demonstrate the utility of ligand-protected, supported Ta catalysts for epoxidation with H2O2 and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways.