Bonding properties of Cu(II)-imidazole chromophores: spectroscopic and electrochemical properties of monosubstituted imidazole copper(II) complexes. Molecular structure of [Cu(4-methylimidazole) 4(ClO 4) 2

Abstract

The synthesis, electronic, vibrational and EPR spectra, and cyclic voltammetry are reported for methylimidazoles and N-acetylhistamine copper(II) complexes. The crystal and molecular structure of [Cu(4-methylimidazole) 4(ClO 4) 2] has been determined using three-dimensional X-ray diffraction data. The complex crystallizes in the monoclinic space group C2 /c with a=15.742(5), b=10.052(3), c=16.654(5) Å, β=111.81(2)°, and Z=4. Least-squares refinement of the structure gave a final R factor of 0.0454 and R w=0.0450 for 1284 independent reflections. The structure consists of discrete centrosymmetric [Cu(4-methylimidazole) 4(CIO 4) 2] units interconnected by hydrogen bonds, having a CuN 4 plane with CuN distances of 1.993(4) and 2.007(5) Å and two perchlorates on the long z axis with Cu-O of 2.659(4) Å. The dihedral angles between the imidazole and the CuN 4 planes are 79.3 and 105.6°. The properties of the copper(II)-imidazole bonds are analyzed with reference to the electronic structures of the tetrakis(monosubstituted imidazole)-copper(II) complexes having effective local symmetry of D 4h or D 2h . The Cu(II) ion can accommodate at most four imidazole ligands for N-acetylhistamine and 2-methylimidazole, but upto six for 4-methylimidazole, N-methylimidazole and imidazole. In acetonitrile solution, the reduction potentials for the tetragonal copper(II) complexes containing four imidazole ligands parallel the p K a values of the imidazole ligands. This may be ascribed to the enhanced stabilities of Cu(I) complexes by the σ-donor abilities of the imidazole ligands.