Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a 2,2′-Bipyridine-6,6′-dicarboxylate (bda) Ligand: How Ligand Environment Influences the Catalytic Behavior

A new
water oxidation catalyst [RuIII(bda)­(mmi)­(OH2)]­(CF3SO3) (2, H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid;
mmi = 1,3-dimethylimidazolium-2-ylidene) containing an axial N-heterocyclic
carbene ligand and one aqua ligand was synthesized and fully characterized.
The kinetics of catalytic water oxidation by 2 were measured
using stopped-flow technique, and key intermediates in the catalytic
cycle were probed by density functional theory calculations. While
analogous Ru-bda water oxidation catalysts [Ru­(bda)­L2]
(L = pyridyl ligands) are supposed to catalyze water oxidation through
a bimolecular coupling pathway, our study points out that 2, surprisingly, undergoes a single-site water nucleophilic attack
(acid–base) pathway. The diversion of catalytic mechanisms
is mainly ascribed to the different ligand environments, from nonaqua
ligands to an aqua ligand. Findings in this work provide some critical
proof for our previous hypothesis about how alternation of ancillary
ligands of water oxidation catalysts influences their catalytic efficiency.