SIGNIFICANT ANTIFERROMAGNETIC COUPLING IN A TEREPHTHALATE (TA)-BRIDGED MANGANESE(II) COMPOUND - PREPARATION, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF THE CHAIN [MN-2(BIPY)(4)(TA)][CLO4](2) (BIPY=2,2'-BIPYRIDINE)

A new manganese(II) compound [Mn-2(bipy)(4)(ta)][ClO4](2) 1 (bipy = 2,2'-bipyridine, 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 (1) over bar, with a = 9.516(2), b = 11.613(3), c = 11.615(3) Angstrom, alpha = 70.92(2), beta = 80.16(2), gamma = 76.64(2)degrees and Z = 1. The structure consists of cationic terephthalate-bridged [Mn-2(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) Angstrom, 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 -0.04 cm(-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.