{Co(H2O)2[C6(COO)6]}n 4n— — Ein neuartiges kettenförmiges Polyanion in Co3[C6(COO)6]·18H2O (Tricobalt(II)mellitat Octadecahydrat) / {Co(H2O)2[C6(COO)6]}n 4n— – A Novel Chain-Like Polyanion in Co3[C6(COO)6]·18 H2O (Tricobalt(II) Mellitate Octadeca Hydrate)

Abstract

Red orthorhombic single crystals of Co3[C6(COO)6]·18 H2O were grown in aqueous silica gel (space group Pbca, a = 852.7(2), b = 2015.6(3), c = 1712.3(3) pm, 269 parameters, 2675 reflections, Rg = 0.0217). The Co2+ ions are coordinated octahedrally by water molecules and oxygen atoms of carboxylate groups. Co(1) is bound to five water molecules and one carboxylate oxygen atom, Co(2) is situated on a crystallographic center of symmetry and coordinated by four carboxylate oxygen atoms and two water molecules. Folded chains extending parallel to [100] made up by Co(2) and mellitate hexaanions with composition {Co(H2O)2[C6(COO)6]}n 4n— are the main feature of the crystal structure. The excess negative charge is compensated for by Co(1) cations bound to these chains via carboxylate groups related by a center of symmetry. Hydrogen bonds connect neighbouring chains primarily with carboxylate oxygen atoms which are not bound to Co2+ as the proton acceptors. Endothermic loss of water was observed to commence at 120 °C, followed (in air) by a further exothermic two step decomposition at 370 and 450 °C to yield Co3O4 as a crystalline residue. With (CH3)2SO a topotactical ligand exchange occurs without destruction of the single crystals causing the colour of the compound to change to deep-blue.