Three new copper(II) isothiosemicarbazone complexes with the Schiff base compound 2-hydroxyacetophenone S- benzyl-isothiosemicarbazone hydrochloride (H2L⋅HCl) are synthesized in good yields. The complexes are fully characterized by FT-IR, UV–Vis, 1H NMR and 13C NMR spectroscopies. In the FT-IR spectra of the complexes, disappearance of the peaks characteristic of the OH and NH bond stretching vibration in the ligand confirms the formation of the isothiosemicarbazone complexes. Solid-state structures of the complexes are also determined by single crystal X-ray diffraction which showed the copper atom adopts a distorted square-planar geometry in all compounds. In complexes 1 and 2, the tetradentate N2O2 ligand is bonded to the central metals but in 3 a tri- dentate H2L ligand and a chloride ligand occupy the coordination sites. Interaction between human serum al- bumin (HSA) and the three copper complexes is examined experimentally and theoretically. All three complexes show a tendency to bind to HSA protein and the complexes quench the fluorescence emission of HSA signifi- cantly. The results of docking calculations indicate that the interactions between the complexes and the protein are hydrophobic. In 1 the cleft between the three domains of HSA is the most suitable site and in 2 and 3 the upper part of the interdomain region of HSA is the most probable site. Density functional theory (DFT) studies are successfully used to investigate the ground state, frequency assignment and natural bond order.

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