Solvent-Induced Electron Transfer and Delocalization in Mixed-Valence Complexes. Electrochemistry

For the RuIII/II couple in [(bpy)2ClOs(4,4′-bpy)Ru(NH3)5] (bpy ) 2,2′-bipyridine; 4,4′-bpy ) 4,4′bipyridine), E1/2 varies linearly with the donor number (DN) of the solvent with a slope of -26 ( 3 mV/DN unit ranging from nitromethane to dimethyl sulfoxide. For the OsIII/II couple, the variation is -3 ( 1 mV/DN unit. Plots of ∆E1/2 ) E1/2(2) E1/2(1) vs DN (E1/2 is the half wave potential for the first or second wave by cyclic voltammetry) undergo a change in slope at DN ∼ 14 where there is a change in oxidation states in the mixed-valence form from OsIII-RuII to OsII-RuIII. By extrapolation of these data, ∆G° for the mixed-valence equilibrium, [(bpy)2ClOs(4,4′-bpy)Ru(NH3)5] h [(bpy)2ClOs(4,4′-bpy)Ru(NH3)5], varies from +5.8 kcal/mol in nitromethane to -7.5 kcal/mol in dimethyl sulfoxide. It differs from ∆E1/2 by up to ∼20% even though it has sometimes been assumed in the literature that ∆E1/2 ) -∆G°. For [(bpy)2ClOs(pz)Ru(NH3)5] (pz ) pyrazine) both OsIII/II and RuIII/II couples are significantly solvent dependent for solvents of DN < 24. In these solvents oxidation states in the mixed-valence form are OsIII-RuII. The slopes of E1/2 vs DN plots are -21 ( 4 mV/DN unit (RuIII/II) and -8 ( 4 mV/DN unit (OsIII/II). At DN > 24 the oxidation states switch to OsII-RuIII and the solvent dependence reverts to being largely in RuIII/II. There is evidence in the electrochemical data, in comparisons between [(bpy)2ClOs(pz)Ru(NH3)5] and [(bpy)2ClOs(4,4′-bpy)Ru(NH3)5] for significant through-bridge electronic coupling in [(bpy)2ClOs(pz)Ru(NH3)5], but not in [(bpy)2ClOs(pz)Ru(NH3)5]. The difference in behavior is caused by extensive H-bonding to the solvent at -Ru(NH3)5 in [(bpy)2ClOs(pz)Ru(NH3)5]. This mixes solvent character into dπ(RuIII) which decreases electronic coupling across the bridge.