Ruthenium Complexes of Rigid, Dianionic, Tetradentate N‐Donor Ligands and their Potential as Catalysts for Water Oxidation

Two mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The provide a constrained equatorial coordination one large N−Ru−N angle, additional non-coordinating N atoms in case of 2. Their redox properties investigated (spectro)electrochemically, their potential to serve as water oxidation catalysts has probed cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged exhibits 80 % efficiency the CeIV-driven high initial turnover frequency (TOFi) 3.07×10−2 s−1 (at 100 equiv. CAN). rate O2 evolution 1st order dependence catalyst concentration, suggesting nucleophilic attack mechanism. Repeated addition CAN control experiments show that ionic strength conditions (both NO3− CeIII) significantly decrease TOF.