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

Abstract

The polynucleating oxygen donor ligands, [(SbAr)(4)O(2)(PhPO(3)H)(4)(PhPO(3))(4)] 1 and [(SbAr)(2)O(HO(3)P(t)Bu)(6)] 2, based on condensation of p-chlorophenylstibonic acid (ArSbO(3)H(2)) 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, [Co(2)(SbAr)(4)O(4)(O(3)PPh)(4)(OMe)(4)py(2)] 3, is formed, with four Sb(ii) and two Co(ii) centres bridged by μ(3)-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, [Co(2)(SbAr)(4)O(4)(O(3)PPh)(4)(OMe)(4)(C(4)H(4)N(2))](n)11, is formed. A similar 1D-polymer forms with 4,4'-bipyridyl-ethylene 12 in place of pyrazine. With Et(3)N/pyridine as base in MeCN a tetranuclear cobalt cage, [Co(4)(SbAr)(5)O(9)(O(3)PPh)(6)(py)(4)] 13, with five Sb(ii) centres forming a "bowtie" and bridging to four Co(ii) centres by phosphonates and μ(3)-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, [Co(2)(SbAr)(2)(O(3)P(t)Bu)(3)O(2)(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.