Enhanced Catalysis Based on the Surface Environment of the Silica-Supported Metal Complex
Ken Motokura, Siming Ding, Kei Usui, Yuanyuan Kong
Abstract
Silica-supported metal complex catalysts have been developed and used for organic transformations. The surface environment around the supported metal complex enhances the catalysis based on a unique surface effect. The design of the linker ligand structure induces the formation of a highly reactive, coordinatively unsaturated metal complex on the silica surface because of the isolated environment. In contrast to the site-isolation effect, the accumulated metal complexes and cocatalysts on the same surface facilitate the acceleration of the catalytic reaction by concerted catalysis. The immobilization of multiactive sites also promotes the tandem catalysis and development of complex products from simple molecules through successive reactions. Surface silanol species originating from the silica support also participate in the catalysis. The control of the immobilization density/location of metal complex/coimmobilized functionality/surface silanol is a key factor for the achievement of site-isolation/concerted catalysis. The direct interaction between the metal complex and coimmobilized functionality facilitates the formation of unique reactive species. The confinement effect of the pore structure of the support enhances the accumulation of active species in mesopores, which boosts the reaction rate, and slightly changes the ligand conformation, which increases the enantioselectivity. The direct support electronic effect is also one of the key factors affecting the surface organometallic chemistry (SOMC) and photooxidation of linker metal complexes. These acceleration effects were detected in both supported homogeneous catalysis and SOMC. Not only the local structure of the metal complex and its ligand but also the surface environment play the most important roles in enhancing the catalysis. In this Review, representative examples of silica-supported metal complexes whose catalysis is significantly enhanced by their surface long-range environment are summarized. The contributions of recent developments of spectroscopic techniques, including DNP-enhanced solid-state NMR and XAFS, which support the evaluation of such long-range interactions, are also discussed. The surface design of the silica-supported metal complex facilitates highly active, selective, and durable catalysis.