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 (Lbut) and bis(1,4,7-triazacyclononan-1-yl)-m-xylene (Lmx) have been synthesized: [Cu2LbutBr4] (1), [Cu2Lbut(imidazole)2Br2](ClO4)2 (2), [Cu2Lmx(μ-OH)(imidazole)2](ClO4)3 (3), [Cu2Lbut(imidazole)4](ClO4)4 · H2O (4), [Cu2Lmx(imidazole)4](ClO4)4 (5), [Ni2 Lbut(H2O)6](ClO4)4 · 2H2O (6), [Ni2Lbut(imidazole)6](ClO4)4 · 2H2O (7) and [Ni2Lmx (imidazole)4(H2O)2](ClO4)4 · 3H2O (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.