Mixed ligand copper(II) chelates derived from an O, N, S- donor tridentate thiosemicarbazone: Synthesis, spectral aspects, FMO, and NBO analysis

Five new copper(II) chelates [(Cu(bmct))(2)] (1), [Cu(bmct)(phen)] (2), [Cu(bmct)(bipy)] (3), [Cu(bmct)(4,4'-dmbipy)] (4) and [Cu(bmct)(5,5'-dmbipy)] (5) with 5-bromo-3-methoxysalicylaldehyde-N(4)-cyclohexylthiosemicarbazone (H(2)bmct) as the chelating ligand and 1,10-phenanthroline, 2,2'-bipyridine, 4,4'-dimethylbipyridine, 5,5'-dimethylbipyridine as coligands have been synthesized and characterized by different physicochemical techniques like CHNS analysis, molar conductivity and magnetic studies, IR, UV/Vis and EPR spectral studies. In all the complexes, the thiosemicarbazone exists in thioiminolate form and coordinates to the metal through azomethine nitrogen, thioiminolate sulfur, and phenolate oxygen. EPR spectra in polycrystalline state at 298 K showed that compounds 1, 4, and 5 are isotropic, 2 is axial and 3 is rhombic in nature. In DMF at 77 K, compound 1 showed hyperfine lines in the parallel and perpendicular regions as well as superhyperfine lines due to the interaction of copper center with azomethine nitrogen of the ligand. Complex 2, in which g(parallel to)>g(perpendicular to)> 2.0023 suggests a distorted square pyramidal structure. To analyze the stability of the complexes, quantum chemical parameters like hardness, softness, polarizability, electrophilicity, electronegativity, and dipole moment were calculated and discussed within the framework of electronic structure principles known as Maximum Hardness, Minimum Polarizability and Minimum Electrophilicity Principles. Besides, the intramolecular donor-acceptor interactions for all complexes were evaluated by using NBO analysis. All calculations proved that Compound 3 is the most stable chelate among them. (C) 2020 Elsevier Ltd. All rights reserved.