Electronic Modification of the RuII(tpy)(bpy)(OH2)2+ Scaffold: Effects on Catalytic Water Oxidation

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as Ru(tpy)(L)(OH(2))(2+) L = 2,2'-bipyridine (bpy), 1; 4,4'-dimethoxy-2,2'-bipyridine (bpy-OMe), 2; 4,4'-dicarboxy-2,2'-bipyridine (bpy-CO(2)H), 3; tpy = 2,2';6'',2''-terpyridine is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the Ru(V)(tpy)(L)O(3+) (Ru(V)=O(3+)) motif; the relative positions of each of these redox couples reflects the nature of the electron-donating or withdrawing character of the substituents on the bpy ligands. The first two (proton-coupled) electron-transfer reaction steps (k(1) and k(2)) were determined by stopped-flow spectroscopic techniques to be faster for 3 than 1 and 2. The addition of one (or more) equivalents of the terminal electron-acceptor, (NH(4))(2)Ce(NO(3))(6) (CAN), to the Ru(IV)(tpy)(L)O(2+) (Ru(IV)=O(2+)) forms of each of the catalysts, however, leads to divergent reaction pathways. The addition of 1 eq of CAN to the Ru(IV)=O(2+) form of 2 generates Ru(V)=O(3+) (k(3) = 3.7 M(-1) s(-1)), which, in turn, undergoes slow O-O bond formation with the substrate (k(O-O) = 3 × 10(-5) s(-1)). The minimal (or negligible) thermodynamic driving force for the reaction between the Ru(IV)=O(2+) form of 1 or 3 and 1 eq of CAN results in slow reactivity, but the rate-determining step is assigned as the liberation of dioxygen from the Ru(IV)-OO(2+) level under catalytic conditions for each complex. Complex 2, however, passes through the Ru(V)-OO(3+) level prior to the rapid loss of dioxygen. Evidence for a competing reaction pathway is provided for 3, where the Ru(V)=O(3+) and Ru(III)-OH(2+) redox levels can be generated by disproportionation of the Ru(IV)=O(2+) form of the catalyst (k(d) = 1.2 M(-1) s(-1)). An auxiliary reaction pathway involving the abstraction of an O-atom from CAN is also implicated during catalysis. The variability of reactivity for 1-3, including the position of the RDS and potential for O-atom transfer from the terminal oxidant, is confirmed to be intimately sensitive to electron density at the metal site through extensive kinetic and isotopic labeling experiments. This study outlines the need to strike a balance between the reactivity of the Ru═O(z) unit and the accessibility of higher redox levels in pursuit of robust and reactive water oxidation catalysts.