Aqueous solution as a playground of {MoO4}, {Mo4O12}, {Mo8O26}, {Mo8V5O40} and {V7Mo2O27} species in the presence of carboxylic acids and [Co(C2O4)(NH3)4]+ or [Co(en)3]3+ cations

Abstract

Nine tetraammineoxalatocobalt(III) salts built of different oxomolybdate anions, i.e. {β-Mo₈O₂₆}⁴⁻ (in 1, 2, and 6), {Na₂Mo₈O₂₉(H₂O)₄}_n^4m− (in 3), {[β-Mo₈O₂₆(H₂O)₂][γ-Mo₈O₂₆]}⁸⁻ (in 4), {γ-Mo₈O₂₆(H₂O)₂}⁴⁻ (in 5, 7), {Mo₄O₁₂(C₄H₄O₄)}_n^4m− (in 8), and {γ-Mo₈O₂₆}_n^4m− (in 9) were isolated as the product(s) of the reaction of sodium molybdate and [Co(C₂O₄)(NH₃)₄]NO₃·H₂O in the presence of acetic or succinic acid. Octamolybdate isomers, namely distinct β- and γ-octamolybdates along with complex cobalt(III) cations and water molecules are the building blocks of three-dimensional supramolecular frameworks (in 1, 2, and 4–7). In some cases, octamolybdate subunits are interconnected by sodium cations or a pair of shared oxygen atoms to form infinite anionic chains (in 3 and 9), or by coordinated succinate anions to assemble into a two-dimensional MOF (in 8). When [Co(CO₃)(NH₃)₄]NO₃·H₂O was used as a precursor, neutral compounds [Mo₅Co₂O₁₇(HCO₃)(H₄C₄O₄H)(NH₃)₇] (10) and [Mo₇Co₂O₁₈(NH₃)₇] (11) were obtained. Furthermore, two molybdovanadates of proposed formula [Co(C₂O₄)(NH₃)₄]₂[H₂Mo₈V₅O₄₀Na₂(H₂O)₈] (12) and unique Na₃[Co(en)₃[V₇Mo₂O₂₇] (13) were obtained in the reactions of sodium molybdate, ammonium vanadate and [Co(C₂O₄)(NH₃)₄]NO₃·H₂O or [Co(en)₃]Cl₃ in the presence of acetic or succinic acid, respectively. The products 1–9 were characterized in the solid state via single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric and elemental analyses.