Syntheses, molecular structure, and electrochemical investigations of cobalt(II), copper(II), palladium(II), and zinc(II) complexes with 3-methylpyrazole

Complexes of M(II) with 3-methylpyrazole as ligand were prepared at ambient temperature using MCl2 metal salts (M = Cu, Co, Pd, and Zn). Reactions led to the formation of octahedral (Cu, Co), square-planar (Pd), and tetrahedral (Zn) mononuclear complexes. All reactions were straightforward and no appreciable amount of side products was detected. X-ray structures were determined for all the complexes. The cyclic voltammetric investigations indicate the involvement of proton during electron transfer reactions of these complexes. Mononuclear complexes of 3-methylpyrazole with general formulas (3-Mepz)(4)CuCl2 (1), (3-Mepz)(4)-CoCl2 (2), (3-Mepz)(2)PdCl2 (3), and (3-Mepz)(2)ZnCl2 (4) were prepared by reaction of the corresponding MCl2 salt (M = Cu, Co, Pd, and Zn) with 3-methylpyrazole in appropriate amounts using acetonitrile as solvent at ambient temperature. The X-ray crystal structure determination reveals that 1 and 2 possess octahedral geometry, while 3 and 4 are square planar and tetrahedral, respectively. All the synthesized compounds have the MCl2 fragment, thus making the synthesized compounds attractive synthons for further transformation. The cyclic voltammograms of the synthesized complexes were obtained and the voltammetric signatures of 1, 2, and 4 showed a single irreversible pH-dependent cathodic peak, while 3 has two reversible cathodic peaks. Involvement of protons accompanying the electron transfer processes was ascertained from differential pulse voltammetric results, indicating peak potential shift as a function of pH.