Unraveling the Role of Pendant Anionic Substituents in Tuning the Redox Properties of Ruthenium-Based Water Oxidation...

Water oxidation catalysts based on ruthenium polypyridyl complexes provide highly tunable platforms for investigating how ligand structure influences redox behavior and catalytic performance. In this work, we employed density functional theory (DFT) calculations to examine the effect of pendant anionic substituents on the redox properties of a series of ruthenium complexes containing terpyridine and substituted phenanthroline ligands of the form [Ru(terpy)(phenX)], where X = -SO3-, -COO-, and -PO3iPr-. To quantify how through-space noncovalent interactions between these substituents and the ruthenium center affect redox properties, we first analyzed the RuII/III oxidation in a series of MeCN-bound surrogate complexes, in which the coordinating acetonitrile spatially separates the anionic groups and the Ru center and eliminates complications associated with proton transfer and further oxidation beyond RuIII. The results show that introducing pendant anionic substituents consistently lowers the RuII/III oxidation potential for the MeCN-bound complexes, indicating that these groups effectively modulate redox potentials through long-range electrostatic interactions, as further confirmed by energy decomposition analysis and electrostatic potential calculations. We then extended the computational investigation to the corresponding aqua complexes, focusing on the proton-coupled oxidation of [RuII(terpy)(phenX)(H2O)] to form [RuIII(terpy)(phenX)(OH)], as well as the RuIV/V oxidation in the corresponding oxo-bound complexes. In contrast to the MeCN-bound surrogate complexes, the substituent-dependent trends are not fully preserved, as the oxidation energetics are influenced by additional factors such as proton transfer and structural rearrangement across oxidation states. These findings provide molecular-level insights into how second-sphere anionic substituents influence the redox properties of ruthenium-based water oxidation catalysts, demonstrating both the effectiveness and limitations of electrostatic tuning strategies.