Synthesis and characterization of [Ru(NC^NHCO)(bpy)L]⁺ complexes and their reactivity towards water oxidation

While the role of negatively charged axial ligands (e.g. Cl^-, Br^- and I^-) in tuning the reactivity and stability of molecular Ru water oxidation catalysts was well-documented, less was explored when axial ligands are neutral molecules. In this work, we reported the synthesis and full characterization of [Ru(NC^NHCO)(bpy)L]⁺ complexes (L = DMSO, 1; L = 4-picoline, 2), where L is a neutral axial ligand. The effects of these axial ligands on the stability and reactivity of the catalysts were thoroughly investigated. Both complexes were competent to oxidize water with turnover numbers up to 463 and 107, respectively. Complex 1 initiated the water oxidation involving a PCET process. It was found that complex 2 afforded its corresponding Ru-aqua complex owing to the carboxylate dissociation, while complex 1 exhibited considerable stability in acidic aqueous solutions. On the other hand, in the presence of an oxidant, for complex 1 DMSO/H₂O exchange occurred, facilitating water oxidation. In contrast, the Ru-O bond cleavage of complex 2 was inhibited, leading to an inferior catalytic performance.