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 MII (M = Cu, Ni, Co or Mn) bridged by the terephthalate (ta) ligand (intramolecular metal–metal separation ca. 10 Å) has been systematically studied. The following complexes have been synthesized: [Cu2(bipy)4(ta)][ClO4]2 1, [Cu2(terpy)2(H2O)2(ta)] [ClO4]2 2, [Ni2(bipy)4(ta)][ClO4]2 3, [Co2(bipy)4(ta)][ClO4]2 4, [Mn2(phen)4(H2O)2(ta)][ClO 4]2 5 and [Mn2(phen)4(ta)][ClO4]2 6 (bipy = 2,2′-bipyridine, terpy = 2,2′[hair space]:[hair space]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 °C or under vacuum at room temperature. The structures have in common the presence of cationic terephthalate-bridged [M2(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 with combining macron]; = -J[S with combining macron] A·[S with combining macron]B with SA = SB = ½ (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(II) ions induced by a carboxylate-assisted loss of the co-ordinated water molecule.