Hydrothermal Reactions of [CoIII(C2O4)(NH3)4]+ and Polyoxomolybdates: Depolymerization of Polyoxomolybdates and in Situ Reduction of Cobalt

The hexa-, hepta-, and octamolybdates as common precursors with dynamic configuration transformation in aqueous solution have been introduced into the hydrothermal reaction system containing the [Co-III(C2O4)(NH3)(4)](+) cation, leading to the isolation of four new compounds: [Co-III(C2O4)(NH3)(4)](3)[(NH3)(4)Co-III(mu-C2O4)Co-II(H2O)(4)][Mo7O24]center dot nH(2)O (n approximate to 9.25) (1), [Co-III(C2O4)(NH3)(4)](2)[MoO4]center dot H2O (2), [Co-III(C2O4)(NH3)(4)](3)(NH4)[Co-II(OH)(6)Mo6O18]center dot 6H(2)O (3), and (Co-III(C2O4)(NH3)(4)](2)[(CoC2O4)-C-II()(2)(H2O)(2)]center dot 4H(2)O as two polymorphic forms (4 and 4a). The choice of the starting polyoxomolybdate strongly influences the obtained products-while 2 was obtained in all reactions, 1 was isolated only in reactions with hexa- and octamolybdate, and 3 and 4 only with heptamolybdate as the molybdenum source. Three species present in the isolated products, the binuclear cation [(NH3)(4)Co-III(mu-C2O4)Co-II(H2O)(4)](3+) in 1, the Anderson-type anion [Co-II(OH)(6)Mo6O18](4-) in 3, and the [Co-II(C2O4)(2)(H2O)(2)](2-) anion in 4 and 4a, were formed by the intramolecular reduction of the [Co-III(C2O4)(NH3)(4)](+) precursor. The presence of Co(II) in these compounds was confirmed by electron spin resonance studies. The compounds were characterized by elemental and thermogravimetric analysis as well as infrared spectroscopy and single crystal X-ray diffraction analysis.