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, H2L=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 CO2 and [Ru(bda)(pic)(2)](+), which is a highly active WOC.