Three new vic-dioxime ligands: Synthesis, characterization, spectroscopy, and redox properties of their mononuc ear nickel(II) complexes

Nickel(II) complexes with three new vic-dioxime reagents, N-(ethyl-4-amino-1-piperidine carboxylate)phenylglyoxime (L₁H ₂), N-(ethyl-4-amino-1-piperidirie carboxylate)glyoxime (L ₂H₂) and N,N′-bis(ethyl-4-amino-1-piperidine carboxylate)glyoxime (L₃H₂), have been prepared. Mononuclear nickel(II) complexes with a metal/ligand ratio of 1:2 were prepared using Ni(II) salt. All these nickel(II) complexes are nonelectrolytes as shown by their molar conductivities (Λ_M) in DMF solution at 10 ^-3 M concentration. The ligands are soluble in common solvents such as DMSO, DMF, CHCl₃, and C₂H₅OH. The ligands and their Ni(II) complexes were characterized by elemental analyses, FT-IR, UV-visible, ¹H NMR, ¹³C NMR, magnetic susceptibility measurements, cyclic voltammetry, and molar conductivities (Λ _M). The cyclic voltammetric measurements show that [Ni(L ₁H)₂] and [Ni(L₂H)₂] complexes exhibit almost similar electrochemical behavior, with two reduction and two oxidation processes based on either metals or oxime moities, while [Ni(L ₃H)₂·2H₂O] complex displays irreversible, with one reduction and one oxidation processes based on oxime moity. This main difference could be attributed to the highly polarized [Ni(L₃H)₂·2H₂O] complex that has four carboxylate groups attached to piperidine on the oxime moieties.