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 it s crystal structure determined by single-crystal X-ray diffraction. It crystallizes in the triclinic system, space group P (1) over bar, wit h 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 c onsists of cationic terephthalate-bridged [Mn-2(bipy)(4)(ta)](2+) chai ns and non-co-ordinated perchlorate anions. Each terephthalate is boun d 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 carbo xylate and terephthalate bridges are 4.643(1) and 9.637(2) Angstrom, r espectively. The manganese(II) ion displays a severely distorted octah edral co-ordination being linked to four nitrogen atoms of two bipy li gands and to two oxygen atoms of two carboxylate groups from two terep hthalate ligands. The magnetic susceptibility of 1 as a function of te mperature (4.2-290 K) exhibits a sharp maximum at 6.0 K which,is consi stent with a significant antiferromagnetic coupling between the metal centres. This magnetic behaviour has been interpreted in terms of a di mer 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 t he light of the structural data, it can be concluded that the double-c arboxylate bridge between the manganese(ii) pairs provides a more effi cient exchange pathway than that of terephthalate and most likely is r esponsible for the observed J value.