Metal–metal bonded pentamolybdate hybrids as electron storage materials

Abstract

Two electron-rich, metal–metal bonded pentamolybdate hybrids, 2D-[Mo^IV₃Mo^VI₂O₁₀Sr₂(H₂O)₅(C₆H₄O₇)₂py₃]·3.5H₂O (1) and 1D-[Mo^IV₃Mo^VI₂O₁₀Sr(H₂O)₃(C₆H₄O₇)₂py₃]·py·2[NH₂(CH₃)₂]·2H₂O (2, py = pyridine), were prepared by the partial solvothermal oxidation of [Mo^IV₃O₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄·8H₂O and citric acid in py/H₂O (for 1) or py/H₂O/DMF (for 2). Both 1 and 2 feature a triangularly metal–metal bonded incomplete cuboidal [Mo^IV₃O₄] unit. Redox-active 6e-[Mo^IV₃O₄] units can serve as an “electron sponge” to store/release six electrons reversibly via Δ-bond breakage and re-formation during charging/discharging processes. 1 and 2 further form 3D and 2D supramolecular structures, respectively, through slipped π–π stacking interactions between the pyridine ligands. Both the incorporated 6e-redox active [Mo^IV₃O₄] unit and the 3D/2D supramolecular conductive networks in hybrid-POM 1/2 remarkably enhance the electronic conductivity and reversible multi-electron redox ability with the structural integrity retained. Consequently, 1 and 2 exhibited high discharge specific capacities of 236.0 and 277.0 mA h g⁻¹ at 50 mA g⁻¹, respectively, and a good cycling performance at high current density (121.8 mA h g⁻¹, 2 A g⁻¹ for 2), providing a new way for improving POM-based electrode materials.