Imidazole derivatives of binuclear copper(II) and nickel(II) complexes incorporating bis(1,4,7-triazacyclononan-1-yl) ligands

A series of new binuclear copper (II) and nickel (II) complexes of the macrocyclic ligands bis(1,4,7-triazacyclononan-1-yl)butane (L-but) and bis(1,4,7-triazacyclononan-1-yl)-m-xylene (L-mx) have been synthesized: [(Cu2LBr4)-Br-but] (1), [Cu2Lbut(imidazole)(2)Br-2](ClO4)(2) (2), [Cu2Lmx(mu-OH)(imidazole)(2)](ClO4)(3) (3), [CU2Lbut(imidazole)(4)](ClO4)(4) center dot H2O (4), [Cu2Lmx(imidazole)(4)](ClO4)(4) (5), [Ni-2 L-but(H2O)(6)](ClO4)(4) center dot 2H(2)O (6), [Ni2Lbut(imidazole)(6)](ClO4)(4) center dot 2H(2)O (7) and [Ni2Lmx (imidazole)(4)(H2O)(2)](ClO4)(4) center dot 3H(2)O (8). Complexes 1, 2, 7 and 8 have been characterized by single crystal X-ray studies. In each of the complexes, the two tridentate 1,4,7-triazacyclononane rings of the ligand facially coordinate to separate metal centres. The distorted square-pyramidal coordination sphere of the copper (II) centres is completed by bromide anions in the case of 1 and/or monodentate imidazole ligands in complexes 2, 4 and 5. Complex 3 has been formulated as a monohydroxo-bridged complex featuring two terminal imidazole ligands. Complexes 6-8 feature distorted octahedral nickel (II) centres with water and/or monodentate imidazole ligands occupying the remaining coordination sites. Within the crystal structures, the ligands adopt trans conformations, with the two metal binding compartments widely separated, perhaps as a consequence of electrostatic repulsion between the cationic metal centres. The imidazole-bearing complexes may be viewed as simple models for the coordinative interaction of the binuclear complexes of bis (tacn) ligands with protein molecules bearing multiple surface-exposed histidine residues. (c) 2005 Elsevier B.V. All rights reserved.