Oxovanadium(IV)-sulfite compounds: Synthesis and structural and physical studies

Reaction of VIVOCl2 in strongly acidic aqueous solution with either (NH4)2SO3 or Na2SO3 and Bu4NBr at ~70°C in the pH range 2.5-4.5 gives the clusters (NH4)2{[V4 IV(μ 4-O)2(μ 3-OH)2](VIVO)2(μ 3-SO3)4O4(H2O)2} and (n-Bu4N)2{[V4 IV(μ 4-O)2(μ 3-OH)2](VIVO)2(μ 3-SO3)4O4(H2O)2}, respectively. Reaction of NH4VVO3 with (NH4)2SO3 resulted in the isolation of the first compound. When the latter reaction is carried out in the presence of MgO, compound (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O was isolated instead. Compound (n-Bu4N)2{[V4 IV(μ 4-O)2(μ 3-OH)2](VIVO)2(μ 3-SO3)4O4(H2O)2} and (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O were characterized by X-ray structure analysis. The crystal structure of species (n-Bu4N)2{[V4 IV(μ 4-O)2(μ 3-OH)2](VIVO)2(μ 3-SO3)4O4(H2O)2} revealed a unprecedented hexanuclear cluster consisting of a cubane core [M4(μ 4-O)2(μ 3-OH)2] connected to two other metal atoms through the core oxo-groups and four μ 3-SO3 bridges. Compound (NH4)[VIVO(SO3)1.5H2O]∞.2.5H2O represents a rare example of an open-framework species prepared under mild conditions. Cyclic voltammetric examination of compound (n-Bu4N)2{[V4 IV(μ 4-O)2(μ 3-OH)2](VIVO)2(μ 3-SO3)4O4(H2O)2} revealed a redox process which was assigned to the oxidation of one core of vanadium(IV) to vanadium(V).