Synthesis, spectroscopic and cyclic voltammetry studies of copper(II) complexes with open chain, cyclic and a new macrocyclic thiosemicarbazones

Abstract

New copper complexes have been prepared from benzilbisthiosemicarbazone (L 1H 6) and from the cyclic 6-methoxi-1,6-diphenyl-4-thio-3,4,5,6-tetrahydro-2,3,5-triazine (L 2H 2). The complexes were characterized by mass spectrometry, IR, electronic and electron paramagnetic resonance spectra. Complexes from L 1H 6 ( 1– 5) present 1:1 stoichiometry and show different characteristics with a variable grade of deprotonation in the ligand, depending on the salt used (chloride, nitrate or sulfate) and the presence of acid in the medium. A macrocyclic Schiff base, 3,4,9,10-tetraphenyl-1,2,5,6,8,11-hexaazacyclododeca-7,12-dithione-3,4,9,10-tetraene (L 3H 2), containing thiosemicarbazone moieties is readily prepared and characterized for the first time, with fairly good yield, from the mesocycle (L 2H 2) in methanol with copper chloride as template. Near quantitative synthesis of the precursor, L 2H 2, which is a potential chemotherapeutic agent against cancer, has been achieved by reacting benzil and thiosemicarbazide in the absence of metal salt and using a high dilution technique. Macrocyclic complexes 6 and 7 containing thiosemicarbazone moieties were obtained from L 2H 2 and copper nitrate and sulfate. Direct reaction between the copper salt and the macrocyclic thiosemicarbazone (L 3H 2) gave similar complexes but with a lower grade of purity. Macrocyclic complexes 6 and 7 present metal–ligand ratios of 1:2 and 1:1 and the ligand is in a deprotonated form. The redox behaviour was explored by cyclic voltammetry. L 1H 6 complexes show Cu(II)/Cu(I) couples and quasireversible waves associated with the Cu(III)/Cu(II) process. The reduction/oxidation potential depends on the structure and conformation of the central atom in the coordination compounds.