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

Abstract

Abstract Two mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N4} 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 CeIV‐driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07×10−2 s−1 (at 100 equiv. CAN). The initial rate of O2 evolution exhibits 1st 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 CeIII) significantly decrease the TOF.

Two RuII complexes with equatorial {N4} ligands that enforce one wide N−Ru−N angle show very different performance in CeIV driven water oxidation. 1 undergoes oxidative degradation, whereas 2 featuring non‐coordinating N atoms is more rugged and exhibits good efficiency and high initial turnover frequency. The non‐symmetric structure of 2 has been established by 2D NMR spectroscopy using residual dipolar couplings.

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