Transition metal quinone-thiosemicarbazone complexes 1: Evaluation of EPR covalency parameters and redox properties of pseudo-square-planar copper(II)-naphthoquinone thiosemicarbazones

EPR, optical and redox studies have been carried out on six copper(II) complexes of naphthoquinone-thiosemicarbazones having [CuLCl] stoichiometry to elucidate the electronic structure, nature of metal-ligand bonding and electrochemical features. EPR spectra, simulated with an axial spin-Hamiltonian, exhibit a four-line pattern with nitrogen super-hyperfine couplings originating from imine/hydrazinic nitrogen atoms. These planar complexes possess a significant amount of tetrahedral distortion leading to a pseudo-square-planar geometry as evidenced from EPR and optical properties. The evaluation of covalency parameters suggest that the unpaired electron, present in the d(x2-y2) orbital, spends about 42-45% of its time on the nitrogen donor site of the coordinated thiosemicarbazones, reflecting the pi-acceptor property of the sulfur center as well as the charge accumulating character of the quinone molecules. The presence of a strong pi-interaction leads to extensive delocalization thorough the entire chelate rings formed by the tridentate ligands. Moderate covalency is observed in sigma-bonding while in plane pi-bonding possess appreciable covalent character. IR spectral data indicate a tridentate ONS donor set for these ligands. A quasi-reversible Cu(II)/Cu(I) redox couple is observed at relatively higher potential (Delta E-p = -0.45 to -0.66 V) as a consequence of structural reorganizations, while these complexes exhibit lower redox potentials for the Cu(II)/Cu(III) oxidation process. There exists a linear relationship between the degree of tetrahedral distortion f(alpha) with spectroscopic and redox parameters. (C) 2005 Elsevier Ltd. All rights reserved.