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 {N-4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N-4} ligands provide a constrained equatorial coordination with one large N-Ru-N angle, and additional non-coordinating N atoms in case of 2. Their redox properties have been investigated (spectro)electrochemically, and their potential to serve as water oxidation catalysts has been probed using cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged and exhibits 80 % efficiency in the Ce-IV-driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07x10(-2) s(-1) (at 100 equiv. CAN). The initial rate of O-2 evolution exhibits 1(st) order dependence on catalyst concentration, suggesting a water nucleophilic attack mechanism. Repeated addition of CAN and control experiments show that high ionic strength conditions (both NO3- and Ce-III) significantly decrease the TOF.