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

Chiral nickel(II) and copper(II) Schiff base complexes with [N2O2] coordination derived from (1R,2R)- and (1S,2S)-1,2-diaminocyclohexane were prepared and characterized by H-1, C-13 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 Ig beta(2) values of a variety of known nickel(II) Schiff base complexes with different substituents R-1, R-2 and bridges X. No significant differences in Ig beta(2) with (R)-(+)-1-phenylethylamine were observed for RRNiIc and SSNiIc. Ligand SS2HIa (C20H30N2O6) crystallized in the orthorhombic system (P2(1)2(1)2(1)) with unit cell dimensions a = 11.035(2), b = 16.126(3), c = 24.676(5) Angstrom, and Z = 8; R = 0.045 and wR(2) = 0.115. The nickel complex RRNiIb (C18H14O2N2Ni . 0.5CH(2)Cl(2)) crystallized in the orthorhombic system (P2(1)2(1)2) with unit cell dimensions a = 18.404( 1), b = 18.657( 1), c = 5.621 (1) Angstrom, and Z = 4; R = 0.044 and wR(2) = 0.108. The structures were solved by direct methods and were refined by full-matrix least-squares procedures.