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)(4)N](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 [Ru(bpy)(3)](2) [S2Mo18O62] (solid) + PF6- (solution) [GRAPHICS] [Ru(bpy)(3)](2) [S2Mo18O62][PF6] (solid) (i) [Ru(bpy)(3)](2) [S2Mo18O62] (solid) + Bu4N+ (solution) [GRAPHICS] [Bu4N][Ru(bpy)(3)](2) [S2Mo18O62] (solid) (ii) [Bu4N][Ru(bpy)(3)](2) [S2Mo18O62] (solid) + Bu4N+(solution) [GRAPHICS] [Bu4N](2) [Ru(bpy)(3)](2) [S2Mo18O62] (solid) (iii) [Bu4N](2) [Ru(bpy)(3)](2) [S2Mo18O62] (solid) + Bu4N+ (solution) [GRAPHICS] [Bu4N](3) [Ru(bpy)(3)](2) [S2Mo18O62] (solid) (iv) of 0.2 mM [Ru(bpy)(3)](2) [S2Mo18O62] in DMF (0.1 M Bu4NPF6) reveals one reversible oxidation (E-1/2(r) + 791 mV) and two reversible fully solution phase diffusion controlled reduction reactions (E-1/2(r) -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.