Synthesis and structural and magnetic characterisation of cobalt(ii)–sodium phosphonate cage compounds

Abstract

The reaction of cobalt salts with phosphonic acids in the presence of 6-chloro-2-hydroxypyridine as a co-ligand, using sodium methoxide as a base, leads to a series of new polymetallic cobalt cages. Variation of the phosphonate present and the cobalt salt leads to {Co₆Na₈}, {Co₁₂}, {Co₁₃Na₆}, {Co₁₄Na₄} and {Co₁₅Na} cages, all of which have been characterized by X-ray crystallography. Using lithium methoxide produces a sixth cage with a {Co₆Li₉} core. The structures are, in general, extremely irregular with no structural motifs common to the six cages. Magnetic studies of these cages show a general decline in the product χ_mT with T, but for {Co₁₃Na₆}, {Co₁₅Na} and {Co₁₂} there are maxima at low temperature, which suggest non-diamagnetic ground states. Investigation of the dynamic behaviour of the magnetisation of these complexes shows that the {Co₁₃Na₆}, and possibly the {Co₁₂} cage, appear to display slow relaxation of magnetisation.