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-κN)(meth-yl)amino-κN]eth-yl}imino-κN)meth-yl]benzene-thiol-ato-κS}nickel(II) chloride, [Ni(C12H18N3S)]Cl (1), and [2-({[2-(piperazin-1-yl-κ2 N 1,N 4)eth-yl]imino-κN}meth-yl)benzene-thiol-ato-κS]nickel(II) hexa-fluoro-phosphate di-chloro-methane monosolvate, [Ni(C13H18N3S)]PF6·CH2Cl2 (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.