Significant antiferromagnetic coupling in a terephthalate (ta)-bridged manganese(II) compound: preparation, crystal structure and magnetic properties of the chain [Mn2(bipy)4(ta)][ClO4]2(bipy = 2,2′-bipyridine)

Abstract

A new manganese(II) compound [Mn₂(bipy)₄(ta)][ClO₄]₂1(bipy = 2,2′-bipyridme, ta = terephthalate dianion) has been synthesised and its crystal structure determined by single-crystal X-ray diffraction. It crystallizes in the triclinic system, space group P, with a= 9.516(2), b= 11.613(3), c= 11.615(3)Å, α= 70.92(2), β= 80.16(2), γ= 76.64(2)° and Z= 1. The structure consists of canonic terephthalate-bridged [Mn₂(bipy)₄(ta)]²⁺ chains and non-co-ordinated perchlorate anions. Each terephthalate is bound to four manganese atoms through carboxylate oxygens with the sym-sym bridging mode, affording manganese(II) pairs which are linked by the terephthalate group to yield a one-dimensional chain running along the a axis. The intrachain Mn ⋯ Mn separations through the double carboxylate and terephthalate bridges are 4.643(1) and 9.637(2)Å, respectively The manganese(II) ion displays a severely distorted octahedral co-ordination being linked to four nitrogen atoms of two bipy ligands and to two oxygen atoms of two carboxylate groups from two terephthalate ligands. The magnetic susceptibility of 1 as a function of temperature (4.2–290 K) exhibits a sharp maximum at 6.0 K which is consistent with a significant antiferromagnetic coupling between the metal centres. This magnetic behaviour has been interpreted in terms of a dimer with a molecular field approximation, the values obtained for J, g and zJ′ being –1.35 cm^–1,1.97 and –0.04cm^–1, respectively. In the light of the structural data, it can be concluded that the double-carboxylate bridge between the manganese(II) pairs provides a more efficient exchange pathway than that of terephthalate and most likely is responsible for the observed J value.