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

Abstract

For the RuIII/II couple in [(bpy)2ClOs(4,4‘-bpy)Ru(NH3)5]3+ (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 ΔE1/2 = 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, ΔG° for the mixed-valence equilibrium, [(bpy)2ClOsIII(4,4‘-bpy)RuII(NH3)5]4+ ⇌ [(bpy)2ClOsII(4,4‘-bpy)RuIII(NH3)5]4+, varies from +5.8 kcal/mol in nitromethane to −7.5 kcal/mol in dimethyl sulfoxide. It differs from ΔE1/2 by up to ∼20% even though it has sometimes been assumed in the literature that ΔE1/2 = −ΔG°. For [(bpy)2ClOs(pz)Ru(NH3)5]3+ (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]3+ and [(bpy)2ClOs(4,4‘-bpy)Ru(NH3)5]3+ for significant through-bridge electronic coupling in [(bpy)2ClOsIII(pz)RuII(NH3)5]4+, but not in [(bpy)2ClOsII(pz)RuIII(NH3)5]4+. The difference in behavior is caused by extensive H-bonding to the solvent at −RuIII(NH3)53+ in [(bpy)2ClOsII(pz)RuIII(NH3)5]4+. This mixes solvent character into dπ(RuIII) which decreases electronic coupling across the bridge.