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 O–O bond formation
via the radical coupling pathway. Herein, we manipulate the equatorial
backbone ligand, change the doubly charged bda2– 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 (ΔG⧧) at T = 298.15 K was 20.2 ± 1.7 kcal mol–1. The electronic structures of a series of RuVO species were studied by density function theory
calculations, revealing that the spin density of ORuO of RuVO is largely dependent on the surrounding
ligands. Seven coordination configuration significantly enhances the
radical character of RuVO.