The Coordination chemistry and cyclic voltammetry exploration of Cu(II), Co(II), Ni(II) and Zn(II) complexes of novel (E)-3,4-dichloro-N-(2-(1-(pyridin-2-yl)ethylidene)hydrazine-1-carbonothioyl) benzamide ligand

Abstract
A novel thiosemicarbazone derivative (DEHC-H), (E)-3,4-dichloro-N-(2-(1-(pyridin-2-yl)ethylidene) hydrazine-1-carbonothioyl) benzamide was synthesized after three steps. Firstly, the reaction between acetyl pyridine with hydrazine hydrate at room temperature obtaining (E)-2-(1-hydrazonoethyl)pyridine. Secondly, 3,4-dichlorobenzoyl chloride was refluxed with potassium thiocyanate in acetonitrile solvent forming 3,4-dichlorobenzoyl isothiocyanate. The last step of the reaction is summarized by the refluxing the mixture of (E)-2-(1-hydrazonoethyl) pyridine and 3,4-dichlorobenzoyl isothiocyanate in acetonitrile solvent. A series of eight new metal complexes, Copper(II), Cobalt(II), Nickel(II) and Zinc(II) chloride and acetate complexes of the thiosemicarbazone ligand (DEHC-H) were accomplished in DMF/H2O or CHCl3/CH3OH mixture solvents. The {[(DEHC)CuCl], [(DEHC)CoCl], [(DEHC)NiCl], [(DEHC-H)ZnCl2], [(DEHC-H)2Cu](CH3COO)2, [(DEHC-H)2Co](CH3COO)2, [(DEHC-H)2Ni](CH3COO)2 and [(DEHC-H)2Zn](CH3COO)2} were prepared by the reaction of the free ligand with chloride and acetate metal salts, CuCl2.2H2O, CoCl2.6H2O, NiCl2.6H2O, ZnCl2, Cu(CH3COO)2.H2O, Co(CH3COO)2.4H2O, Ni(CH3COO)2.4H2O and Zn(CH3COO)2.2H2O. The preparation of the metal complexes was achieved in 1:1 and 1:2 mole ratios of (Metal salt : free Ligand). The identities of the (DEHC-H) ligand and its metal chloride and acetate complexes were characterized and confirmed by 1HNMR, 13CNMR, UV-visible, Infrared, mass spectroscopy, elemental analysis, solubility test, magnetic moments and conductivity measurements. The spectroscopic techniques and measurements indicate that metal chloride complexes were formed with the neutral structural formula [(DEHC)MCl], where M= Copper(II), Cobalt(II) and Nickel(II) and with the neutral structural formula [(DEHC-H)ZnCl2] for Zinc(II) complexes in spite of two mole ratios were examined. Moreover, the acetate complexes were formed as anionic formula [(DEHC-H)2M](CH3COO)2. The employed techniques indicate that thiosemicarbazone chloride complexes with formula [(DEHC)MCl], one tridentate [(DEHC) ligand coordinating in the thiole form with the metal ion and the fourth site was the chloride anion to form square planar geometry. While Zinc(II) complex [(DEHC-H)ZnCl2] show coordinating of the one tridentate [(DEHC-H) ligand as thione where the fourth and the fifth sites coordinating with the Zinc(II) ion to adopt a square pyramidal geometry. The characterization of the all acetate complexes [(DEHC-H)2M](CH3COO)2 pointed to two moles of tridentate [(DEHC-H) ligands in thione form coordinating with the metal ions to fill six sites forming octahedral geometry as well as to two moles of acetate ions as counter ions. It is worth noting that tridentate thiosemicarbazone [(DEHC-H) ligand coordinates through the nitrogen pyridyl, thionic sulfur and the azomethine nitrogen atoms. The observed magnetic moments data exhibit diamagnetic properties of Zinc(II) chloride and acetate complexes and Nickel(II) square planar chloride complexes. While Cu(II) and Co(II) square planar chloride compounds and Copper(II), Cobalt(II) and Nickel(II) octahedral acetate complexes appeared as paramagnetic species. The electrochemical behavior of a set Copper(II), Cobalt(II) and Nickel(II) chloride and acetate complexes of free ligand were recorded using cyclic voltammetry technique. The voltammetric analysis of Copper(II) chloride and acetate complexes displays a quasi-reversible one electron reduction in the range –1.0728 to –1.1439 volt versus Fc/Fc+ ascribable to the Copper(II)/Copper(I) redox couple. The Co(II) and Ni(II) chloride and acetate compounds display one and two irreversible oxidation processes respectively in the range +0.419 to +0.479 in Co(II) compounds and +0.449 to +0.491 as well as to +0.656 to +0.721 volt versus Fc/Fc+ in Ni(II) compounds.