Monomeric and Dimeric Cobalt(III) Polyoxomolybdates: Crystal Structures and Cyclic Voltammetry of Na3 [H6CoMo6O24].8H2O and K6 [H4Co2Mo10O38].7H2O

Abstract

The crystal structures of Na₃ [H₆CoMo₆O₂₄].8H₂O and K₆ [H₄Co₂Mo₁₀O₃₈].7H₂O have been determined by X-ray diffraction. The monomer, Na₃ [H₆CoMo₆O₂₄].8H₂O, is triclinic, space group P^-1, a 6·451(1), b 10·866(2), c 10·922(2) Å, α 109·20(1), β 106·90(1), γ 95·43(1)°, V 676·3(2) ų, Z 1, and the structure was solved to an R₁ value of 0·0243 (wR₂ 0·0784) for 3761 independent observed reflections. The anion exhibits the well known Anderson structure with six octahedral MoO₆ edge-sharing units surrounding the central ‘CoO₆’ octahedron, with all metals in a common plane. The dimer, K₆ [H₄Co₂Mo₁₀O₃₈].7H₂O, is monoclinic, space group P2₁/c, a 11·795(5), b 11·626(2), c 29·731(13) Å, β 95·33(2)°, V 4059(3) ų, Z 4, and the structure was solved to an R₁ value of 0·0215 (wR₂ 0·1040) for 6546 independent observed reflections. The anion can be derived from the monomeric hexamolybdocobaltate(III) ion by removing one ‘MoO₅’ unit (ignoring the hydrogen atoms) from each of two monomer anions, turning one 180° around a CoO₆ octahedral diagonal, and joining them to create two CoO₆ octahedra sharing an edge. Cyclic voltammetry shows that both anions are irreversibly reduced at low pH values (4·0–4·5), likely as a result of chemical reactions following the initial reduction steps. At higher pH values (4·5–5·4), a change in speciation occurs in both cases, most likely the result of the formation of less highly protonated species, which also display irreversible electrochemical behaviour.