Coordination-Adaptive Molecular Platform for Catalytic Water Oxidation

Enzymes utilize adaptive frameworks to accommodate diverse structural and electronic requirements of intermediates, enhancing the catalysis. This Review outlines efforts to construct adaptive artificial molecular water oxidation catalysts, mainly based on ruthenium complexes, in a biomimetic manner. We initiated by discussing the importance of the coordinating ability of anionic ligands in the low oxidation state, in the aspect of water coordination feasibility, active species generation, and charge accumulation prevention via proton-coupled electron transfer (PCET). We then illustrated the introduction of additional ligands in the oxidized state, which helps to increase the electron density of the catalytic site, thereby lowering the overpotential. Beyond the first coordination sphere, we then discussed the effort of adjusting the identities of pendant base groups and introducing controlled microenvironments close to the catalytic sites, in order to regulate the key proton transfer. In regard to mechanistic understanding, the isolation of catalytic intermediates is also discussed, with particular emphasis on intermediates during the transition between 6- and 7-coordinate states. Applying the adaptive molecular platform to other metals, we conclude by prospecting the design principles of nonprecious metal-based water oxidation catalysts.