Cobalt(II)/Nickel(II)-Centered Keggin-Type Heteropolymolybdates: Syntheses, Crystal Structures, Magnetic and Electrochemical Properties

New Keggin-type cobalt(II)/nickel(II)-centered heteropolymolybdates, (C3H5N2)6[CoIIMo12O40]·10H2O (1) and (NH4)3(C4H5N2O2)3[NiIIMo12O40] (2), were isolated and characterized by IR, UV–vis, single-crystal X-ray diffraction, thermogravimetric, magnetic, as well as electrochemical analyses. The polyanion in the two compounds displays the well-known α-Keggin structure, which is composed of four Mo3O13 units formed by edge-sharing octahedra. Four Mo3O13 units connect each other by vertices, and the Co2+ or Ni2+ is located in the center. Magnetic measurements show that the central Co2+ and Ni2+ are in high spin states (with S = 3/2 and S = 1, respectively) exhibiting paramagnetic behaviors. Cyclic voltammetric experiments for 1 represent a quasi-reversible one-electron redox Co3+/Co2+ couple and two four-electron reversible redox processes ascribed to Mo centers, while 2 only shows two four-electron redox processes attributed to Mo centers in pH = 0.5 H2SO4 solution.