Thiosemicarbazone Structures Including Nickelophilic Interaction as Well as Both Hydrogen Bonding and π–π Stacking Interactions: NLO, Electrochemical, Chromism, and Spectroelectrochemical Properties

Nickelophilic complexes were formed from salicylaldehyde thiosemicarbazone and nitro-substituted aniline through second-sphere coordination interactions. The structures of the complexes were characterized by elemental analysis, IR, ¹H NMR, UV-vis spectroscopy, and single-crystal X-ray diffraction. Additionally, the structural, spectroscopic, electronic, and solvatochromic properties were investigated, and the nonlinear optic properties were studied using density functional theory (DFT) calculations and diffuse reflectance spectroscopy. The energy gap values suggested that the complexes exhibited semiconductor-like behavior. X-ray crystallographic studies revealed that the two phenolato oxygen atoms and two azomethine nitrogen atoms of the doubly deprotonated ONNO tetradentate Schiff base occupy the corners of a square-planar geometry around the metal atom. The structures were found to be stabilized by hydrogen bonding and π-π stacking, in addition to Ni···Ni interactions. Electrochemical characterizations of the starting material and complexes were carried out to evaluate the influence of the 2-nitroaniline and 2,4-dinitroaniline coligands as well as the coordination of the starting material with the Ni-(II) cation, which exhibited very complex reduction processes with reduction responses significantly influenced by the applied waveforms of the voltammetric excitations. Additionally, complexation also influenced the in situ spectroelectrochemical responses of the starting material.