The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway.

Many Ru water oxidation catalysts have been documented in the literature. However, only a few can catalyze the O-O bond formation via the radical coupling pathway, while most go through the water nucleophilic attack pathway. Understanding the electronic effect on the reaction pathway is of importance in design of active water oxidation catalysts. The Ru-bda (bda = 2,2'-bipyridine-6,6'-dicarboxylate) catalyst is one example that catalyzes the 0-0 bond formation via the radical coupling pathway. Herein, we manipulate the equatorial backbone ligand, change the doubly charged bda(2-) ligand to a singly charged tpc- (2,2':6',2 ''-terpyridine-6-carboxylate) ligand, and study the structure activity relationship. Surprisingly, kinetics measurements revealed that the resulting Ru-tpc catalyst catalyzes water oxidation via the water nucleophilic attack pathway, which is different from the Ru-bda catalyst. The O-O bond formation Gibbs free energy of activation (AGO) at T = 298.15 K was 20.2 +/- 1.7 kcal mol(-1). The electronic structures of a series of Ru-v=O species were studied by density function theory calculations, revealing that the spin density of O-Ru=O of Ru-v=O is largely dependent on the surrounding ligands. Seven coordination configuration significantly enhances the radical character of Ru-v=O.