Synthesis and electrochemistry of [Ru(2,2′-bpy)3]2[S2Mo18O62] at electrode–solvent (electrolyte) interfaces

Addition of [Ru(bpy)3][PF6]2 to [(C6H13)4N]4[S2Mo18O62] in CH3CN resulted in the formation of analytically pure [Ru(bpy)3]2[S2Mo18O62]. Insolubility in CH3CN and solubility in DMF allow solid state and solution phase electrochemical data to be compared. Voltammetric studies of [Ru(bpy)3]2[S2Mo18O62] adhered to a glassy carbon electrode surface and placed in contact with CH3CN (0.1 M Bu4NPF6) indicate diffusion of ions within the solid occurs rapidly in order to achieve charge neutralisation required for oxidation of the [Ru(bpy)3]2+ cation and reduction of the [S2Mo18O62]4− anion. Mass increases on the electrode surface (detected by the electrochemical quartz crystal microbalance method) accompany both oxidation and reduction processes, eqns. (i)–(iv). Cyclic voltammetry of 0.2 mM [Ru(bpy)3]2[S2Mo18O62] in DMF (0.1 M Bu4NPF6) reveals one reversible oxidation (Er1/2 + 791 mV) and two reversible fully solution phase diffusion controlled reduction reactions (Er1/2 −33 and −310 mV vs. Fc+/Fc), corresponding to the [Ru(bpy)3]2+/3+ and [S2Mo18O62]4−/5− and [S2Mo18O62]5−/6− charge transfer processes, respectively. With 1 mM [Ru(bpy)3]2[S2Mo18O62] in DMF (0.1 M Bu4NPF6) precipitation on the electrode surface occurs on scanning to potentials more negative than the initial reduction process, due to formation of [Bu4N]2[Ru(bpy)3]2[S2Mo18O62]. Enhanced levels of surface based processes are also observed in the voltammetry of [Ru(bpy)3]2[S2Mo18O62] in DMF (0.01 M Bu4NPF6), corresponding to precipitation of [Bu4N][Ru(bpy)3]2[S2Mo18O62] and [Ru(bpy)3]2[S2Mo18O62][PF6] on the electrode surface. Data imply that mixed cation salts are more soluble than those containing only [Ru(bpy)3]2+ in all redox levels.