Bonding properties of Cu(II)-imidazole chromophores: spectroscopic and electrochemical properties of monosubstituted imidazole copper(II) complexes. Molecular structure of [Cu(4-methylimidazole)4(ClO4)2]
摘要:
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(ClO4)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) angstrom, beta = 111.81(2)-degrees, 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(C]O4)2] units interconnected by hydrogen bonds, having a CuN4 plane with Cu-N distances of 1.993(4) and 2.007(5) angstrom and two perchlorates on the long z axis with Cu-0 of 2.659(4) angstrom. The dihedral angles between the imidazole and the CuN4 planes are 79.3 and 105.6-degrees. 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 D4h or D2h. 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 pK(a) values of the imidazole ligands. This may be ascribed to the enhanced stabilities of Cu(I) complexes by the sigma-donor abilities of the imidazole ligands.