X-ray crystal structural and spectral studies of copper(II) and nickel(II) complexes of two asymmetric bis(thiosemicarbazone) ligands and the investigation of relationship between the N(4)-substituent and the electrochemical behavior

Abstract

Asymmetric, potentially tetradentate ligands (H2L5, H2L6·2MeOH) are formed by subsequent condensation a 4-chlorophenyl thiosemicarbazide and 4-phenyl thiosemicarbazide for H2L5, 4-chlorophenyl thiosemicarbazide and 4-bromophenyl thiosemicarbazide for H2L6·2MeOH on dialdehyde (1). The mixed bis(thiosemicarbazones) and their copper(II) and nickel(II) complexes have been characterized with 1HNMR, IR, electronic spectra and X-ray crystallography. Both bis(thiosemicarbazones) form square-planar N2S2 complexes with nickel(II) and copper(II). Electrochemical investigation showed that all Cu(II) and Ni(II) complexes were reducible. The differences observed in the electrochemistry between symmetric and asymmetric complexes cannot be explained only by considering the electronic and steric effects of the substituents, it seems that the arrangement of energy levels of molecular orbitals has been changed in converting a symmetric into an asymmetric structure. The redox potential of asymmetric complexes has been shifted toward more positive potentials compared to the symmetric compounds.