Ligand Geometry Directs O-O Bond-Formation Pathway In Ruthenium-Based Water Oxidation Catalyst

Four-electron oxidation of water to molecular dioxygen is one of the key steps that needs to be fully solved to be able to develop new sustainable energy conversion schemes based on water splitting by sunlight. In addition, this reaction is also one of the crucial steps that occur in the oxygen evolving center of photosystem II (OEC-PSII) in green plants and algae. Therefore the understanding of how water oxidation occurs at a molecular level is nowadays one of the most important challenges faced by the scientific community, and by extension to our society, which needs an urgent energy solution. The initial development of the water oxidation catalysis field was based on dinuclear ruthenium complexes bearing polypyridyl-type ligands. Recently this family of catalysts has been extended to mononuclear ruthenium complexes as well as to other firstand third-row transition-metal complexes. Despite these important developments, the mechanistic description of the different pathways under which these reactions take place, together with the full spectroscopic characterization of reaction intermediates, is still a formidable challenge. We and others have contributed significantly towards this endeavor based on kinetics and mainly UV/Vis spectroscopy. In addition O-labeling experiments can be a tremendously informative mechanistic tool, given the fact that one of the critical steps in water oxidation is the formation of O O bonds, as has been put forward in previous mechanistic examples. Herein we present the synthesis and spectroscopic and electrochemical characterization of new water oxidation catalyst, 2, which contains the dinucleating bpp ligand and the facial tpym ligand (see Figure 1 for a crystal structure and Table 1 for ligand structures and compound numbering). Further we also report a mechanistic description of the water