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

Abstract

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 (E^r_1/2 + 791 mV) and two reversible fully solution phase diffusion controlled reduction reactions (E^r_1/2 −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.