The synthesis, structure and catalytic properties of the [MoO(μ-MoO)MoO] anion formed via two intermediate heptamolybdates [Co(en)][NaMoO]Cl·nHO and (HO)[Co(en)][MoO]Cl·9HO.

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)]Na[MoO(μ-MoO)MoO]·nHO (1). The new polyoxomolybdate anion 1 comprised three fragments, namely, two {MoO} units bridged by a {MoO} unit, which were interconnected by the terminal oxygen atoms of MoO 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)][NaMoO]Cl·nHO (2), comprising {MoO} units bridged by a sodium atom, and the heptamolybdate (HO)[Co(en)][MoO]Cl·9HO (3). An identical reaction procedure with [Co(CO)(en)] instead of [Co(en)] yielded the orthomolybdate [Co(CO)(en)][MoO]·9HO (4) and the hydrogen malonate [Co(CO)(en)]COH (5). The new polyoxomolybdate [Co(en)]Na[MoO(μ-MoO)MoO]·nHO 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(NH)][MoO]·8HO (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.