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

The thiol-ate nickel complexes {2-[({2-[(2-amino-ethyl-κ N )(meth-yl)amino-κ N ]eth-yl}imino-κ N )meth-yl]benzene-thiol-ato-κ S }nickel(II) chloride, [Ni(C 12 H 18 N 3 S)]Cl ( 1 ), and [2-({[2-(piperazin-1-yl-κ 2 N 1 , N 4 )eth-yl]imino-κ N }meth-yl)benzene-thiol-ato-κ S ]nickel(II) hexa-fluoro-phosphate di-chloro-methane monosolvate, [Ni(C 13 H 18 N 3 S)]PF 6 ·CH 2 Cl 2 ( 2 ), were synthesized by the reactions of 2-( tert -butyl-thio)-benzaldehyde, tri-amines, and nickel(II) salts. Both complexes have a nickel ion surrounded by an N , N ', N '', S -tetra-dentate ligand, forming a square-planar geometry. The terminal N , N -chelating moiety is N , N -di-alkyl-ethane-1,2-di-amine for 1 and 1-alkyl-piperazine for 2 . The N-Ni-N bite angle in the terminal N , N -chelate ring in 2 [76.05 (10)°] is much smaller than that in 1 [86.16 (6)°]. Cyclic voltammograms of 1 and 2 in aqueous media indicated that the reduction and oxidation potentials of 2 are more positive than those of 1 . The smaller bite angle of the terminal piperazine chelate in 2 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 2 is effective in reducing the overpotential.