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

Nicolas A. Grosso-Giordano, Christian Schroeder, Alexander Okrut, Andrew Solovyov, Christian Schottle, Walter Chassé, Nebojsa Marinkovic, Hubert Koller, Stacey I. Zones, Alexander Katz

Abstract

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-TiIV 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 TiIV 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 TiIV centers.