Syntheses, crystal structures and electronic properties of a series of copper(II) complexes with 3,5-halogen-substituted Schiff base ligands and their solutions

We have systematically investigated the structural features, electronic properties, thermally-induced structural phase transitions and absorption spectra depending on the solvent for ten Cu(II) complexes with 3,5-halogen-substituted Schiff base ligands. Structural characterization of two new complexes, bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosalicydenaminato-kappa N-2,O)copper(II), reveals that they afford a compressed tetrahedral trans-[CuN2O2] coordination geometry with trans-N-Cu-N = 159.4(2)degrees and trans-O-Cu-O = 151.7(3)degrees for the 1-phenylethyl complex and trans-N-Cu-N = 157.9(3)degrees and trans-O-Cu-O = 151.0(3)degrees for the 2-butyl one. All the complexes exhibit a structural phase transition by heating in the solid state regardless of their structures at room temperature. The absorption spectra of a series of ten complexes exhibit a slight shift of the d-d band at 1600020 000 cm(-1) and remarkable shift of the pi-pi* band at 24 000-28 000 cm-1, which suggests that the dipole moment of the solvents presumably affects the conformation of the pi-conjugated moieties of the ligands rather than the coordination environment. We have also attempted 'photochromic solute-induced solvatochromism' by a system of bis(N-R-1-phenylethyl-3,5-dichlorosalicydenaminato-kappa N-2,O)copper(II) and photochromic 4-hydroxyazobenzene in chloroform solution. We successfully observed a change of the d-d and pi-pi* bands of the complex in the absorption spectra caused by cis-trans photoisomerization of 4-hydroxyazobenzene. (c) 2005 Elsevier Ltd. All rights reserved.