Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

In this work, we describe the synthesis, structural characterization, and redox properties of two new Ru complexes containing the dianionic potentially pentadentate [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate (tda2–) ligand that coordinates Ru at the equatorial plane and with additional pyridine or dmso acting as monondentate ligand in the axial positions: [RuII(tda-κ-N3O)­(py)­(dmso)], 1II and [RuIII(tda-κ-N3O2)­(py)­(H2O)ax]+, 2III(H2O)+. Complex 1II has been characterized by single-crystal XRD in the solid state and in solution by NMR spectroscopy. The redox properties of 1II and 2III(H2O)+ have been thoroughly investigated by means of cyclic voltammetry and differential pulse voltammetry. Complex 2II(H2O) displays poor catalytic activity with regard to the oxidation of water to dioxygen, and its properties have been analyzed on the basis of foot of the wave analysis and catalytic Tafel plots. The activity of 2II(H2O) has been compared with related water oxidation catalysts (WOCs) previously described in the literature. Despite its moderate activity, 2II(H2O) constitutes the cornerstone that has triggered the rationalization of the different factors that govern overpotentials as well as efficiencies in molecular WOCs. The present work uncovers the interplay between different parameters, namely, coordination number, number of anionic groups bonded to the first-coordination sphere of the metal center, water oxidation catalysis overpotential, pKa and hydrogen bonding, and the performance of a given WOC. It thus establishes the basic principles for the design of efficient WOCs operating at low overpotentials.