Metal–metal bonded pentamolybdate hybrids as electron storage materials.

Two electron-rich, metal–metal bonded pentamolybdate hybrids, 2D-[Mo IV3 Mo VI2 O10Sr2(H2O)5(C6H4O7)2py3]·3.5H2O (1) and 1D-[Mo IV3 Mo VI2 O10Sr(H2O)3(C6H4O7)2py3]·py·2[NH2(CH3)2]·2H2O (2, py = pyridine), were prepared by the partial solvothermal oxidation of [Mo IV3 O2(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 IV3 O4] unit. Redox-active 6e-[Mo IV3 O4] 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 IV3 O4] 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. [ABSTRACT FROM AUTHOR]