Syntheses, characterization, redox behavior and Lewis acidity of chiral nickel(II) and copper(II) Schiff base complexes

Abstract

Chiral nickel(II) and copper(II) Schiff base complexes with [N 2O 2] coordination derived from (1 R.2 R)- and (1 S.2 S)-1,2-diamino- cyclohexane were prepared and characterized by 1H, 13C NMR, IR and mass spectroscopy and polarimetric measurement in the case of the ligands. Cyclovoltammetric and spectrophotometric measurements revealed that the redox behavior and the Lewis acidity are influenced strongly by the substituents of the ligand. Substituents with high electron-withdrawing effect stabilize the lower oxidation state and promote the addition of axial ligands. The new nickel complex NiIc fits in a diagram which presents the correlation between the reduction potentials and lg β 2 values of a variety of known nickel(II) Schiff base complexes with different substituents R t, R 2 and bridges X. No significant differences 1g β 2 with ( R)-(+)-1-phenylethylamine were observed for RRNiIc and SSNiIc. Ligand SS2HIa (C 20H 30N 2O 6) crystallized in the orthorhombic system ( P2 12 12 1) with unit cell dimensions a = 11.035(2), b = 16.126(3), c = 24.676(5) A ̊ , and Z = 8; R = 0.045 and wR 2 = 0.115. The nickel complex RRNiIb (C 18H 14O 2N 2Ni · 0.5CH 2Cl 2) cyrstallized in the orthorhombic system ( P2 12 12) with unit cell dimensions a = 18.404(1), b = 18.657(1), c = 5.621(1) A ̊ , and Z = 4; r = 0.044 and wR 2 = 0.0108 . The structures were solved by direct methods and were refined by full-matrix least-squares procedures.