The synthesis, structure and catalytic properties of the [Mo7O24(μ-Mo8O26)Mo7O24]16− anion formed via two intermediate heptamolybdates [Co(en)3]2[NaMo7O24]Cl·nH2O and (H3O)[Co(en)3]2[Mo7O24]Cl·9H2O

Abstract

Ageing a mixture of sodium molybdate, malonic acid, and tris(ethylenediamine)cobalt(III) chloride using different synthetic routes, namely, solution-based methods at room temperature or 110 °C, and a mechanochemically accelerated vapour-assisted method, yielded the polyoxomolybdate [Co(en)₃]₅Na[Mo₇O₂₄(μ-Mo₈O₂₆)Mo₇O₂₄]·nH₂O (1). The new polyoxomolybdate anion 1 comprised three fragments, namely, two {Mo₇O₂₄} units bridged by a {Mo₈O₂₆} unit, which were interconnected by the terminal oxygen atoms of MoO₆ octahedra and represent a unique structural motif not yet described in the structurally versatile chemistry of polyoxomolybdates (POMos). The ageing reaction was found to occur via a series of intermediates, two of which were isolated and identified as the heptamolybdate coordination polymer [Co(en)₃]₂[NaMo₇O₂₄]Cl·nH₂O (2), comprising {Mo₇O₂₄} units bridged by a sodium atom, and the heptamolybdate (H₃O)[Co(en)₃]₂[Mo₇O₂₄]Cl·9H₂O (3). An identical reaction procedure with [Co(C₂O₄)(en)₂]⁺ instead of [Co(en)₃]³⁺ yielded the orthomolybdate [Co(C₂O₄)(en)₂]₂[MoO₄]·9H₂O (4) and the hydrogen malonate [Co(C₂O₄)(en)₂]C₃O₄H₃ (5). The new polyoxomolybdate [Co(en)₃]₅Na[Mo₇O₂₄(μ-Mo₈O₂₆)Mo₇O₂₄]·nH₂O was also examined as a catalyst for the epoxidation of cyclooctene, and was superior to both heptamolybdate and octamolybdate catalysts over 24 h. The heptamolybdate [Co(NH₃)₆]₂[Mo₇O₂₄]·8H₂O (6) was isolated as the only reaction product of sodium molybdate and hexaamminecobalt(III) nitrate in the presence of malonic acid using solution-based methods.