Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

Abstract

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-Ti^IV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic ligand helps enforce a constant Ti^IV inner-sphere, as characterized by UV-visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state ¹H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by Ti^IV centers.