Structural evolution of the Ru-bms complex to the real water oxidation catalyst of Ru-bda: the bite angle matters

Abstract

Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in catalysis and mechanism. The electronic effect has always been considered as an essential factor for the catalyst properties while less attention has been focused on the bite angle effect on water oxidation catalysis. The Ru-bda ([Ru(bda)(pic)₂]; bda²⁻ = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline) catalyst is one of the most active WOCs and it has a largely distorted octahedral configuration with an O–Ru–O bite angle of 123°. Herein, we replaced the carboxylate (–COO⁻) groups of bda²⁻ with two methylenesulfonate (–CH₂SO₃⁻) groups and prepared a negatively charged ligand, bms²⁻ (2,2′-bipyridine-6,6′-dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)₂]. The O–Ru–O bite angle changed from 123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bdavia oxidative decomposition under Ce^IV-driven water oxidation conditions.