Crystal structures and electrochemical properties of nickel(II) complexes with N,N′,N′,S-tetradentate Schiff base ligands

The thiolate nickel complexes {2-[({2-[(2-aminoethyl-kappa N)(methyl)amino-kappa N]ethyl}imino-kappa N)methyl]benzenethiolato-kappa S}nickel(II) chloride, [Ni(C12H18N3S)]Cl (1), and [2-({[2-(piperazin-1-yl-kappa N-2(1),N-4)ethyl]imino-kappa N}methyl)benzenethiolato-kappa S]nickel(II) hexafluorophosphate dichloromethane monosolvate, [Ni(C13H18N3S)]PF6 center dot CH2Cl2 (2), were synthesized by the reactions of 2-(tert-butylthio)benzaldehyde, triamines, and nickel(II) salts. Both complexes have a nickel ion surrounded by an N,N',N '',S-tetradentate ligand, forming a square-planar geometry. The terminal N,N-chelating moiety is N,Ndialkylethane-1,2-diamine for 1 and 1-alkylpiperazine for 2. The N-Ni-N bite angle in the terminal N,N-chelate ring in 2 [76.05 (10)degrees] is much smaller than that in 1 [86.16 (6)degrees]. 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 tetradentate 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.