Single-site catalysts have an advantage over bimolecular catalysts because they remain effective when immobilized. Hybrid water oxidation catalysts described here, combining the features of single-site bis-phosphonate catalysts and fast bimolecular bis-carboxylate catalysts, have reached turnover frequencies over 100 s–1, faster than both related catalysts under identical conditions. The new [(bpHc)Ru(L)2] (bpH2cH = 2,2’-bipyridine-6-phosphonic acid-6’-carboxylic acid) catalysts proceed through a single-site water nucleophilic attack pathway. The pendant phosphonate base accelerates catalysis in two ways: (1) The typically high entropic penalty for O-O bond formation via concerted atom-proton transfer is lessened by the pre-positioned base, with a calculated barrier of only 9.1 kcal/mol. (2) Proton-coupled oxidation steps are accelerated by going through intramolecular proton-coupled electron transfer (i-PCET) before losing the proton to the bulk solution. Meanwhile, the labile carboxylate group allows water to bind early in the catalytic cycle at the ruthenium(III) state, providing more opportunities for proton-coupled oxidations and lowering the potential for catalysis relative to the bis-phosphonate predecessor. The advantages of this new single-site catalyst catalyst are highlighted by photochemical water oxidation and surface-bound electrochemistry.

Fast single-site water oxidation catalysis by ruthenium bipyridine-phosphonate-carboxylate complexes

254th National Meeting (2017)

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