Chemical and Photochemical Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a Negatively Charged Tridentate Ligand

Two mononuclear ruthenium complexes [RuL(pic)3] (1) and [RuL(bpy)(pic)] (2) (H2L=2,6‐pyridinedicarboxylic acid, pic=4‐picoline, bpy=2,2′‐bipyridine) have been synthesized and fully characterized. Both complexes could promote water oxidation chemically and photochemically. Compared with other known ruthenium‐based water oxidation catalysts using [Ce(NH4)2(NO3)6] (CeIV) as the oxidant in solution at pH 1.0, complex 1 is one of the most active catalysts yet reported with an initial rate of 0.23 turnover s−1. Under acidic conditions, the equatorial 4‐picoline in complex 1 dissociates first. In addition, ligand exchange in 1 occurs when the RuIII state is reached. Based on the above observations and MS measurements of the intermediates during water oxidation by 1 using CeIV as oxidant, [RuL(pic)2(H2O)]+ is proposed as the real water oxidation catalyst. Efficient water oxidation: A mononuclear RuII complex (see scheme) undergoes ligand exchange to give the related aqua complex, which catalyzes water oxidation with a high turnover frequency (initially 0.23 turnover s−1) by using CeIV as the oxidant in solution at pH 1.0. Photochemical water oxidation was achieved by using a three‐component system containing this catalyst, a Ru‐based sensitizer, and a sacrificial electron acceptor.