Synthesis and structural and magnetic characterisation of cobalt(ii) complexes of mixed phosphonate-antimonate ligands

Abstract

The polynucleating oxygen donor ligands, [(SbAr)₄O₂(PhPO₃H)₄(PhPO₃)₄] 1 and [(SbAr)₂O(HO₃P^tBu)₆] 2, based on condensation of p-chlorophenylstibonic acid (ArSbO₃H₂) and phosphonic acids, were used to prepare polymetallic cobalt(II) complexes. Reaction of 1 with cobalt acetate under solvothermal conditions produces three different types of polymetallic cobalt complexes. With LiOMe/pyridine as base in MeOH a dinuclear cobalt cage, [Co₂(SbAr)₄O₄(O₃PPh)₄(OMe)₄py₂] 3, is formed, with four Sb(II) and two Co(II) centres bridged by μ₃-oxides and phosphonates. The pyridine in the structure can be replaced by 3-picoline 4, 4-picoline 5, quinoline 6, 1,2-diazole 7, 4-phenylpyridine 8, 4-ethyl pyridine 9 and methanol 10. Using pyrazine (pyz) instead of pyridine under the same conditions a polymer, [Co₂(SbAr)₄O₄(O₃PPh)₄(OMe)₄(C₄H₄N₂)]_n11, is formed. A similar 1D-polymer forms with 4,4′-bipyridyl-ethylene 12 in place of pyrazine. With Et₃N/pyridine as base in MeCN a tetranuclear cobalt cage, [Co₄(SbAr)₅O₉(O₃PPh)₆(py)₄] 13, with five Sb(II) centres forming a “bowtie” and bridging to four Co(II) centres by phosphonates and μ₃-oxides, is formed. The reaction of 2 with cobalt acetate using LiOMe/pyridine as base in methanol, under solvothermal conditions, produces a dinuclear Co(II) complex, [Co₂(SbAr)₂(O₃P^tBu)₃O₂(OMe)₂(py)₂] 14, with two Co^II and two Sb centres at the vertices of a distorted tetrahedron. Magnetic measurements on selected number of these cobalt cages are reported.