Synthesis, spectroscopic, electrochemical characterization, density functional theory (DFT), time dependent density functional theory (TD-DFT), and antibacterial studies of some Co(II), Ni(II), and Cu(II) chelates of (E)-4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-1-(3-hydroxynaphthalen-2-yl)methylene) thiosemicarbazide Schiff base ligand

In this study, mononuclear Co(II), Ni(II), and Cu(II) chelates of the potentially tridentate O, N, S donor ligand (E)-4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-1-(3-hydroxynaphthalen-2-yl)methylene) thiosemicarbazide (H₂L) were synthesized and characterized by elemental analyses, conductivity measurements, Fourier transform infrared spectroscopy, UV–visible spectroscopy, magnetic susceptibility, electron spin resonance (ESR) spectral analysis, thermal thermogravimetric analyses, cyclic voltammetry, and theoretical density functional theory (DFT) and time dependent density functional theory (TD-DFT) studies. The potential donors O (phenolic) and N (azomethine) are found in a syn configuration due to a strong intramolecular hydrogen bond [O–H …. N(1)] as shown in the ¹H NMR spectrum of the free ligand. In addition, analytical results revealed the formation of the metal chelates in a ligand-to-metal molar ratio of 1:1. The organic ligand (H₂L) coordinated to the metal center through azomethine nitrogen, phenolate oxygen, and thione/thiolate sulfur groups with the formation of a five- and six-membered rings. The magnetic and spectral results supported the formation of tetrahedral geometry around the Co(II) and Ni(II) centers, while Cu(II) centers were assumed to have a square planar geometry. The time dependent density functional theory (TD-DFT) calculation outputs were used to expect and explain the experimental bathochromic and hypsochromic shifts resulting from the ligand chelation. The antibacterial activity of the compounds under study was evaluated in vitro against Staphylococcus aureus and Escherichia coli demonstrating Gram-positive and Gram-negative bacteria, respectively. It was found that among synthesized compounds, Cu(II) chelate 5 showed the most inhibitory activity against both Gram-positive and Gram-negative bacteria when compared to the organic ligand as well as to the other prepared chelates.