Voltammetry of [R4N]4[M18O54(SO3)2] and [Ru(bpy)3]2[M18O54(SO3)2] (M = Mo, W) as microcrystals adhered to a glassy carbon electrode surface in contact with ionic liquid media

Abstract

Based on studies with microcrystals of [Ru(bpy)₃]₂[α-W₁₈O₅₄(SO₃)₂] adhered to an electrode surface in contact with the ionic liquids [BMIM][PF₆] and [BMIM][BF₄], it has been found that a series of well defined monoelectronic reduction processes associated with the tungsten-oxo cage of the [α-W₁₈O₅₄(SO₃)₂]⁴⁻ species are observed at much less negative potentials than found in conventional organic solvents. Furthermore, since the potentials of the [Ru(bpy)₃]²⁺ 2,2′-bipyridyl ligand based reduction processes are not strongly medium dependent, ionic liquid investigations allowed the characterisation of the [Ru(bpy)₃]₂[α-W₁₈O₅₄(SO₃)₂] complex without extensive overlap of the ligand based reductions from the [Ru(bpy)₃]²⁺ cation and the [W₁₈O₅₄(SO₃)₂]⁴⁻ anions. In the case of the Mo analogues, [α and β-Mo₁₈O₅₄(SO₃)₂]⁴⁻, the much higher reactivity of the reduced forms precluded access to well defined monoelectronic reversible steps at negative potentials in all media examined, even though reduction is far easier and hence availability of extensively reduced species should be simpler, in a thermodynamic sense.