PROTON-COUPLED ELECTRON-TRANSFER IN HIGH-VALENT OXOMANGANESE DIMERS - ROLE OF THE ANCILLARY LIGANDS

The aqueous electrochemistry of selected binuclear mixed-valence oxo-bridged manganese clusters has been investigated. For [Mn(III)Mn(IV)(O)2(bpy)4]3+ (1, bpy = 2,2'-bipyridyl), it was observed that there is a substantial isotope effect (k(H)/k(D) = 4.3) for its electrochemical reduction, consistent with the proton-coupled electron transfer observed previously. Studies on [Mn(III)Mn(IV)-(O)2(edda)2]- (2, edda = ethylenediamine-N, N'-diacetate) show that the E1/2 values are also pH-dependent in a manner consistent with a one-proton/one-electron (slope of 0.07 +/- 0.016 V/pH, R = 0.99) mechanism, as with 1. For [Mn(III)Mn(IV)(O)2(bispicen)2]3+ (3, bispicen = N,N'-bis(2-methylpyridyl)ethane-1, 2-diamine), similar measurements indicate an EC mechanism where the electron transfer and protonation are kinetically decoupled. The current response as a function of pH was measured to determine a pK(a) approximately 8.35 for [Mn(III)2(O)(OH)(bispicen)2]3+. Comparison of the electrochemical properties of 1-3 suggests that the proton-coupled mechanism is facilitated by low steric demand of the ancillary ligands and high basicity of the oxo bridges.