Crystal structures and electrochemical properties of nickel(II) complexes with ,','',-tetra-dentate Schiff base ligands.

The thiol-ate nickel complexes {2-[({2-[(2-amino-ethyl-κ)(meth-yl)amino-κ]eth-yl}imino-κ)meth-yl]benzene-thiol-ato-κ}nickel(II) chloride, [Ni(CHNS)]Cl (), and [2-({[2-(piperazin-1-yl-κ , )eth-yl]imino-κ}meth-yl)benzene-thiol-ato-κ]nickel(II) hexa-fluoro-phosphate di-chloro-methane monosolvate, [Ni(CHNS)]PF·CHCl (), were synthesized by the reactions of 2-(-butyl-thio)-benzaldehyde, tri-amines, and nickel(II) salts. Both complexes have a nickel ion surrounded by an ,','',-tetra-dentate ligand, forming a square-planar geometry. The terminal ,-chelating moiety is ,-di-alkyl-ethane-1,2-di-amine for and 1-alkyl-piperazine for . The N-Ni-N bite angle in the terminal ,-chelate ring in [76.05 (10)°] is much smaller than that in [86.16 (6)°]. Cyclic voltammograms of and in aqueous media indicated that the reduction and oxidation potentials of are more positive than those of . The smaller bite angle of the terminal piperazine chelate in reduces the electron-donating ability of the tetra-dentate ligand, resulting in a positive shift of the redox potentials. Both complexes exhibit catalytic activity for proton reduction, and the piperazine moiety in is effective in reducing the overpotential.