Water Oxidation Initiated by In Situ Dimerization of the Molecular Ru(pdc) Catalyst

The mononuclear ruthenium complex [Ru­(pdc)­L3] (H2pdc = 2,6-pyridinedicarboxylic acid, L = N-heterocycles such as 4-picoline) has previously shown promising catalytic efficiency toward water oxidation, both in homogeneous solutions and anchored on electrode surfaces. However, the detailed water oxidation mechanism catalyzed by this type of complex has remained unclear. In order to deepen understanding of this type of catalyst, in the present study, [Ru­(pdc)­(py)3] (py = pyridine) has been synthesized, and the detailed catalytic mechanism has been studied by electrochemistry, UV–vis, NMR, MS, and X-ray crystallography. Interestingly, it was found that once having reached the RuIV state, this complex promptly formed a stable ruthenium dimer [RuIII(pdc)­(py)2-O-RuIV(pdc)­(py)2]+. Further investigations suggested that the present dimer, after one pyridine ligand exchange with water to form [RuIII(pdc)­(py)2-O-RuIV(pdc)­(py)­(H2O)]+, was the true active species to catalyze water oxidation in homogeneous solutions.