Half-sandwich ruthenium complex with a very low overpotential and excellent activity for water oxidation under acidic conditions

Molecular catalysts are acknowledged for the ability to design reaction sites within well-defined structures to achieve high catalytic activities. However, in many cases, molecular catalysts undergo structural changes to some other form(s), which are finally the real catalysts. Here, we report two half-sandwich ruthenium complexes; [Ru([9]aneN3)(bpy)Cl]⁺ 3 and [Ru([9]aneN3)(pic)OH₂]²⁺ 4, based on the macrocyclic 1,4,7-triazacyclononane ([9]aneN3) ligand for water oxidation (WO). The two complexes have similar core but different ancillary ligands, which greatly affected their stability as well as activity for WO. Complex 3 retained high stability and excellent activity (turnover number [TON] = 1250) in chemical WO and first-order reaction kinetics with respect to [Ce^IV] with a calculated rate constant (k_cat) of 34.59 s⁻¹. Further, the complex demonstrated very low overpotential of ~210 mV in electrochemical WO. At an overpotential of only 400 mV, turnover frequency (TOF) of complex 3 was electrochemically estimated to be 131.2 s⁻¹. In contrast, complex 4 underwent picolinate ligand dissociation, as a deactivation pathway, to form the tri-aqua derivative. Density functional theory (DFT) calculations are used to explain the dissociation mechanism of picolinate ligand in complex 4, which happens through a stepwise dissociation mechanism.