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

摘要

The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as [Ru(tpy)(L)(OH₂)]2+ [L = 2,2′-bipyridine (bpy), 1; 4,4′-dimethoxy-2,2′-bipyridine (bpy-OMe), 2; 4,4′-dicarboxy-2,2′-bipyridine (bpy-CO₂H), 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 [RuV(tpy)(L)O]3+ ([RuV=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 (k₁ and k₂) 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, (NH₄)₂[Ce(NO₃)₆] (CAN), to the [RuIV(tpy)(L)O]2+ ([RuIV=O]2+) forms of each of the catalysts, however, leads to divergent reaction pathways. The addition of 1 eq of CAN to the [RuIV=O]2+ form of 2 generates [RuV=O]3+ (k₃ = 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 [RuIV=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 [RuIV−OO]2+ level under catalytic conditions for each complex. Complex 2, however, passes through the [RuV−OO]3+ level prior to the rapid loss of dioxygen. Evidence for a competing reaction pathway is provided for 3, where the [RuV=O]3+ and [RuIII−OH]2+ redox levels can be generated by disproportionation of the [RuIV=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.