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

Recently, our group has developed a Ru-based water oxidation catalyst (WOC) with pendant sulfonate (<b>1,</b> Ru(4'-X-terpyridine)(phenanthroline-SO₃)OTf (X = H, <b>1a</b>) 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 <b>(1b)</b> makes potentials of Ru^II/Ru^III redox catalysis more negative, whereas when X = NO₂ <b>(1c)</b> and CF₃ <b>(1d)</b>, potentials are more positive. For <b>1b,</b> full conversion of the sacrificial oxidant Ce^IV occurred in 0.4 h (7 h for <b>1a</b>), 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 <b>1a</b> at [cat]₀ = 20 μM. Under electrocatalytic conditions, water oxidation by <b>1b</b> is three times faster than that by the parent catalyst <b>1a</b> at close to the same potential. Extensive computations have identified differences in the initial PCET steps of the water oxidation by catalysts <b>1a, 1b,</b> and <b>1d</b>, and demonstrated the increased probability of the O₂ formation via the oxide relay pathway in the order <b>1b</b>< <b>1a</b> < <b>1d</b>.