Ability of terephthalate (ta) to mediate exchange coupling in ta-bridged copper(II), nickel(II), cobalt(II) and manganese(II) dinuclear complexes

The exchange coupling between the unpaired electrons of divalent first-row transition-metal ions M-II (M = Cu, Ni, Co or Mn) bridged by the terephthalate (ta) ligand (intramolecular metal-metal separation ca. 10 Angstrom) has been systematically studied. The following complexes have been synthesized: [Cu-2(bipy)(4)(ta)][ClO4](2) 1, [Cu-2(terpy)(2)-(H2O)(2)(ta)][ClO4](2) 2' [Ni-2(bipy)(4)(ta)][ClO4](2) 3, [Co-2(bipy)(4)(ta)][ClO4](2) 4, [Mn-2(phen)(4)(H2O)(2)(ta)][ClO4](2) 5 and [Mn-2(phen)(4)(ta)][ClO4](2) 6 (bipy = 2,2'-bipyridine, terpy = 2,2':6',2''-terpyridine, phen = 1,10-phenanthroline). Complexes 1, 2, 4 and 5 have been characterized by single-crystal X-ray analysis. Complex 6 was obtained by thermal dehydration of 5 at 60 degrees C or under vacuum at room temperature. The structures have in common the presence of cationic terephthalate-bridged [M-2(ta)](2+) dinuclear units with bidentate (1, 3-6) and terdentate (2) blocking ligands and unco-ordinated perchlorate counter ions. A co-ordinated water molecule per metal ion is present in complexes 2 and 5. Variable-temperature magnetic susceptibility data for all the complexes have been measured over the range 2.0-298 K. In 1-5 only very weak antiferromagnetic coupling has been observed [J ca. -2.2 (1), -0.01 (2), -0.6 (3), -0.3 (4) and -0.065 cm(-1) (5), the Hamiltonian being (H) over cap = -J (S) over cap(A).(S) over cap(B) with S-A = S-B = 1/2 (1, 2), 1 (3), 3/2(4) and 5/2(5)]. In contrast to the lack of magnetic interaction detected for complex 5, a significant antiferromagnetic coupling (J = -1.6 cm(-1); maximum of susceptibility at 8.0 K) is observed in its dehydrated phase (6). The magnetostructural data and theoretical calculations demonstrate the low efficiency of terephthalate as a bridge to mediate exchange interactions between first-row transition-metal ions. The significant exchange coupling observed in 6 is due to the occurrence of a carboxylate bridge between the manganese(Ir) ions induced by a carboxylate-assisted loss of the co-ordinated water molecule.