N/N Bridge Type and Substituent Effects on Chemical and Crystallographic Properties of Schiff-Base (Salen/Salphen) Niii Complexes

Abstract

In total, 13 ligands R-salen (N,N’-bis(5-R-salicylidene)ethylenediamine (where R = MeO, Me, OH, H, Cl, Br, NO2) and R-salphen (N,N’-bis(5-R-salicylidene)-1,2-phenylenediamine (where R = MeO, Me, OH, H, Cl, Br) and their 13 nickel complexes NiRsalen and NiRsalphen were synthesized and characterized using IR (infrared) spectroscopy, mass spectrometry, elemental analysis, magnetic susceptibility, NMR (nuclear magnetic resonance), UV-vis (ultraviolet-visible) spectroscopy, cyclic voltammetry, and X-ray crystal diffraction. Previous studies have shown that all complexes have presented a square planar geometry in a solid state and as a solution (DMSO). In electrochemical studies, it was observed that in N/N aliphatic bridge complexes, the NiII underwent two redox reactions, which were quasi-reversible process, and the half-wave potential followed a trend depending on the ligand substituent in the 5,5’-R position. The electron-donor substituent—as -OH, and -CH3 decreased the E1/2 potential—favored the reductor ability of nickel. The crystals of the complexes NiMesalen, NiMeOsalen, NiMeOsalphen, and Nisalphen were obtained. It was shown that the crystal packaging corresponded to monoclinic systems in the first three cases, as well as the triclinic for Nisalphen. The Hirshfeld surface analysis showed that the packaging was favored by H∙∙∙H and C∙∙∙H/H∙∙∙C interactions, and C-H∙∙∙O hydrogen bridges when the substituent was -MeO and π-stacking was added to an aromatic bridge. Replacing the N/N bridge with an aromatic ring decreased distortion in square-planar geometry where the angles O-Ni-N formed a perfect square-planar.