Intermediates in the Oxidative Degradation of a Ruthenium‐Bound 2,2′‐Bipyridyl–Phenoxy Ligand during Catalytic Water Oxidation

Water‐oxidation catalysts (WOCs) based on transition‐metal complexes often undergo oxidative degradation of the ligands, which leads to unidentifiable products. We report herein the use of a ruthenium(III) complex bearing a 2,2′‐bipyridyl (bpy)–bis(phenoxy) ligand, [Ru(L)(pic) 2 ] + [ 1 , H 2 L=6,6′‐bis(2‐hydroxyphenyl)‐2,2′‐bipyridine, pic=4‐picoline] as a WOC. The phenoxy‐type ligand in 1 is readily oxidized by Ce IV in aqueous acidic medium to give a ruthenium bpy–dicarboxylate complex ([Ru(bda)(pic) 2 ] + , H 2 bda=2,2′‐bipyridine‐6,6′‐dicarboxylic acid) via ruthenium bpy–phenoxybenzoquinone ( 1′ ) and ruthenium bpy–bis(benzoquinone) ( 1′′ ) intermediates. All four ruthenium species can oxidize water; however, 1 and 1′ are rapidly oxidized to 1′′ by Ce IV . On the other hand, 1′′ is robust enough that it is responsible for the initial phase of O 2 evolution before it is finally oxidized to CO 2 and [Ru(bda)(pic) 2 ] + , which is a highly active WOC.