Bonding properties of copper(II)-imidazole chromophores: structures and electronic properties of tetrakis imidazole copper(II) complexes. Molecular structures of Cu( N-methylimidazole) 4(ClO 4) 2 and [Cu( N-methylimidazole) 4(H 2O) 2]Cl 2(H 2O)

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The crystal and molecular structures of [Cu(NMIm) 4(ClO 4) 2] ({Bd1}) and [Cu(NMIm) 4(H 2O) 2]Cl 2·(H 2O) ({Bd2}) (NMIm = N-methylimidazole) have been determined from three-dimensional X-ray diffraction data. The structure of {Bd1} consists of discrete centrosymmetric [Cu(NMIm) 4(ClO 4) 2] units having a CuN 4 plane with a set of two trans N-methylimidazole ligands nearly perpendicular to the CuN 4 plane (dihedral angle 87.8°)) and another set approximately parallel (dihedral angle 4.4°), and two perchlorates on the long z-axis with CuO length of 2.718(13) Å. The structure of {Bd2} comprises two discrete structurally similar centrosymmetric [Cu(NMIm) 4(H 2O) 2] cations interconnected with Cl − ions and hydrate molecules by hydrogen bonds. The dihedral angles between the imidazole and the CuN 4 planes are 98.8° and 57.6° for one unit and 97.0° and 50.6° for another. Electronic and EPR spectra are reported for {Bd1} and {Bd2} and also for tetrakis complexes of imidazole, 4-methylimidazole, 4-phenylimidazole and benzimidazole. Analysis of the electronic spectra of {Bd1} and {Bd2} reveals that the former has D 2 h symmetry with non-degenerate d xz and d yz orbitals, and the latter has D 4 h symmetry with degenerate d xz and d yz . This is ascribed to the difference in donor abilities of the axial ClO 4 − and H 2O ligands. In solution, all tetrakis species exhibit different structures depending on the donor abilities of the solvent molecules.