Electron-Donating para-Substituent (X) Enhances the Water Oxidation Activity of the Catalyst Ru(4′-X-terpyridine)(phenanthroline-SO3)+

Abstract

Recently, our group has developed a Ru-based water oxidation catalyst (WOC) with pendant sulfonate (1, Ru(4′-X-terpyridine)(phenanthroline-SO₃)OTf (X = H, 1a) that shows high activity under both sacrificial oxidant (CAN, Ce(NH₄)₂(NO₃)₆, Ce^IV) and electrocatalytic conditions, in both acidic and neutral media. Here, we demonstrate that the functionalization of the 4′-X-terpyridine ligand with an electron-donating substituent X = OEt (1b) makes potentials of Ru^II/Ru^III redox catalysis more negative, whereas when X = NO₂ (1c) and CF₃ (1d), potentials are more positive. For 1b, full conversion of the sacrificial oxidant Ce^IV occurred in 0.4 h (7 h for 1a), with an initial rate of 2.07 μmol O₂ s^–1 and a turnover frequency of 7.6 s^–1, which is 30-fold faster than that for 1a at [cat]₀ = 20 μM. Under electrocatalytic conditions, water oxidation by 1b is three times faster than that by the parent catalyst 1a at close to the same potential. Extensive computations have identified differences in the initial PCET steps of the water oxidation by catalysts 1a, 1b, and 1d, and demonstrated the increased probability of the O₂ formation via the oxide relay pathway in the order 1b< 1a < 1d.