Lability and Basicity of Bipyridine-Carboxylate-Phosphonate Ligand Accelerate Single-Site Water Oxidation by Ruthenium-Based Molecular Catalysts

A critical step in creating an artificial
photosynthesis system
for energy storage is designing catalysts that can thrive in an assembled
device. 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, L = 4-picoline or isoquinoline) catalysts
proceed through a single-site water nucleophilic attack pathway. The
pendant phosphonate base mediates O–O bond formation via intramolecular
atom-proton transfer with a calculated barrier of only 9.1 kcal/mol.
Additionally, the labile carboxylate group allows water to bind early
in the catalytic cycle, allowing intramolecular proton-coupled electron
transfer to lower the potentials for oxidation steps and catalysis.
That a single-site catalyst can be this fast lends credence to the
possibility that the oxygen evolving complex adopts a similar mechanism.