Enhanced Catalysis Based on the Surface Environment of the Silica-Supported Metal Complex

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.

Anchored complexes of rhodium and iridium for the hydrogenation of alkynes and olefins with parahydrogen

“…3,[8][9][10][11][12][13][14] Among these, the catalysts comprising metal-ligand complexes immobilized on (or anchored to) a suitable porous solid support are appealing because their synthesis may be rather straightforward and also because immobilization of metal-ligand complexes known to be excellent homogeneous hydrogenation catalysts increases the chances of success. [15][16][17][18][19][20] One approach for the synthesis of immobilized catalysts is the use of modified solid porous supports functionalized with surface linkers that contain phosphino, amino or other suitable functional groups. 21 These groups can bind a metal complex, forming a covalent bond with the central atom of the complex directly, as well as replacing one or more ligands in the original complex.…”

“…3,8–14 Among these, the catalysts comprising metal–ligand complexes immobilized on (or anchored to) a suitable porous solid support are appealing because their synthesis may be rather straightforward and also because immobilization of metal–ligand complexes known to be excellent homogeneous hydrogenation catalysts increases the chances of success. 15–20…”

Small molecule binding to surface-supported single-site transition-metal reaction centres

“…Such high-fidelity on-surface coordination chemistry and related metal-organic networks offer a potential route to designing reactivity similar to solution-based molecular counterparts used in homogeneous catalysis. Indeed, the emerging field of single-site heterogeneous catalysis takes advantage of molecular design concepts from homogeneous catalysts to prepare relatively well-isolated and controlled metal-organic sites for reactivity that are anchored to surfaces, typically through solution-based chemistry on oxide surfaces [38][39][40][41] . This control borrowed from homogeneous catalysis is appealing, however, even with a relatively benign surface, changes in reactivity brought about by the surface environment 41,42 or participation of surface atoms 41,43 offer up complexity alongside opportunities for reaction design if the interaction with the surface is well understood.…”

The coordination chemistry of oxide and nanocarbon materials

“…What is surprising is the reductionist principles developed for small molecules in solution overlap so recognizably with design strategies to obtain well-defined sites on surfaces. The extent of the relationship remains to be developed, particularly in cases where material interfaces are specifically engineered to contain a ligand to coordinate to a transition metal, [187][188][189][190] but if past is prologue the insights provided by the molecular chemistry community will continue to impact the atomistic design of structurally defined sites on surfaces for a range of applications.…”