Assembly of polyoxometalates and Ni-bpy cationic units into the molecular core–shell structures as bifunctional electrocatalysts

Abstract

Four polyoxometalate-based supramolecular assemblies were designed and synthesized under hydrothermal conditions: [Ni(2,2-bpy)₃]₅[PW₁₁NiO₃₉(H₂O)]₂·1.08H₂O (1), [Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)BW₁₂O₄₀] (2), {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)GeW₁₂O₄₀]}⁻ (3), and {[Ni(2,2-bpy)₃]_1.5[Ni(2,2-bpy)₂(H₂O)PW₁₂O₄₀]}²⁻ (4). These structures were characterized by IR, TG analysis, elemental analysis, X-ray powder diffraction and single-crystal X-ray diffraction. In compound 1, two mono-substituted [PW₁₁NiO₃₉(H₂O)]⁵⁻ anions were fused together via the H-bonding interactions forming a dimeric polyoxoanion [PW₁₁NiO₃₉(H₂O)]₂¹⁰⁻, which was surrounded by Ni-bpy cationic units into the molecular core–shell structures. Compounds 2–4 were composed of saturated polyoxoanions [BW₁₂O₄₀]⁵⁻, [GeW₁₂O₄₀]⁴⁻ and [PW₁₂O₄₀]³⁻, respectively. These saturated anions were surrounded by [Ni(2,2-bpy)₃]²⁺ and [Ni(2,2-bpy)₂(H₂O)]²⁺ cationic units forming isostructural core–shell organic–inorganic hybrid materials. These isostructural polyoxometalates with different heteroatoms and charges all formed the isostructural materials, indicating that their charges do not have a significant influence on the structure of supramolecular assemblies. A detail study showed that substituted polyoxometalate with the terminal H₂O molecule is essential for constructing the larger core–shell structure. An electrocatalytic study indicated that these four compounds are bifunctional electrocatalysts towards water oxidation and reduction of nitrite.