Synthesis, characterization, molecular modeling and antimicrobial activities of Copper(II) complex of 4-aminoantipyrine

A Copper(II) complex of 4-aminoantipyrine (4-AAP) was synthesized in ethanol and characterized using a combination of physicochemical and spectroscopic techniques, including solubility tests, melting point determination, Fourier Transform Infrared (FT-IR) spectroscopy, UV–Visible spectroscopy, and molecular modeling studies. The complex was soluble in ethanol, methanol, and DMSO, but insoluble in water. The melting points of the ligand (230 °C) and its complex (340 °C) indicate enhanced thermal stability upon coordination with Cu(II). FT-IR spectral data revealed shifts in the ν(C=O) and ν(N–H) bands, confirming that the ligand coordinates to the copper center through both the carbonyl oxygen and amine nitrogen atoms, acting as a bidentate donor. Additionally, water was found to participate in coordination, completing the metal’s coordination sphere. Density Functional Theory (DFT) calculations were performed at the B3LYP-D3/LACVP** level to optimize the geometry of the complex and evaluate its electronic properties. The theoretical data showed Cu–O and Cu–N bond lengths in the range of 1.84-2.26 Å and 1.89 Å, respectively, consistent with experimental observations. The HOMO–LUMO energy gap (2.78 eV) and global reactivity descriptors indicated good stability and moderate chemical reactivity of the complex, in agreement with literature reports on 4-AAP systems. The in vitro antimicrobial activity of the complex was tested against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa using the agar diffusion method. The complex exhibited markedly higher antibacterial activity than the free ligand, suggesting that coordination enhances the bioactivity of 4-AAP.