Transition metal quinone–thiosemicarbazone complexes 2: Magnetism, ESR and redox behavior of iron (II), iron (III), cobalt (II) and copper (II) complexes of 2-thiosemicarbazido-1,4-naphthoquinone

Abstract

Reactions of 2-thiosemicarbazido-1,4-naphthoquinone (NQTSC) with iron (II), iron (III), cobalt (II) and copper (II) chloride yield different complexes depending on the synthetic conditions. Under acidic medium, thionato coordination is favored with the formation of cationic complexes (A-1 to A-4) while neutral (B-1, B-3 and B-4) and cationic (B-2) complexes are isolated with thiolato binding under basic conditions. IR spectral data indicates that NQTSC acts as both a neutral and a mono-anionic tridentate ligand, bonding through thione, thiol or both as observed in the case of iron (III) and cobalt (III) complexes. The magnetic and Mössbauer measurements of iron (II) complexes reveal the presence of an unusual ( S = 1) spin state possessing small zero-field splitting (ZFS). Reaction of NQTSC with iron (III) chloride results in the stabilization of the spin-triplet state ( S = 3/2) with negligible contribution from ZFS components while cobalt complexes exhibit different magnetic properties displaying diamagnetic (A-3) cobalt (III) and low-spin cobalt (II) states (B-3). Evaluation of ESR covalency parameters for A-4 and B-4 indicates considerable delocalization of the unpaired electron present in the d x 2 - y 2 ground state of the tetragonally distorted octahedral geometry over the entire chelate rings formed by the NQTSC ligands. Electrochemical behavior of the complexes exhibit mostly metal-centered redox changes with the iron (III) complexes having a reversible Fe(III)/Fe(II) couple at relatively positive potentials while the copper (II) compounds display quasi-reversible Cu(II)/Cu(I) as well as Cu(III)/Cu(II) redox processes. Thiolato binding probably exerts a pronounced effect on the oxidation processes while thionato coordination seems to influence reduction steps in these complexes.