Synthesis, crystal structure and sulphide ion sensing study of a Cu(II) complex of aroyl hydrazone

Herein, we report a Cu(II) complex {[Cu^II(HL)(H₂O)₂]Cl} of a benzoylhydrazone Schiff base ligand (4-((2-benzoylhydrazineylidene)methyl)-3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-1-ium chloride) (H₂L). The molar conductance in solution, magnetic susceptibility, ESI-MS and EPR spectroscopy have been performed to justify the structure of the complex. The solid-state structure of the complex has also been solved by single crystal X-ray diffractometry. Cu(II) centre is penta-coordinated with imine-N, phenoxide-O and deprotonated amide-O donor of the ligand and two H₂O molecules occupying the coordination sphere forming a distorted square pyramidal geometry (τ = 0.32). We have studied the anion sensing property of the complex by monitoring the changes in the UV–Vis and fluorescence spectra of the complex in aqueous Tris-HCl buffer medium with incremental addition of various anions as tetrabutylammonium/sodium salts. The complex shows appreciable sensitivity toward sulphide ion, among various other anions, with association constant (K_b) for binding of the complex with S²⁻ and lowest detection limit (L.O.D) value of 6.87 × 10⁴ M⁻¹ and 6.9 × 10⁻⁷ M, respectively. Thus, the complex can be used as an efficient S²⁻ probe. The mechanism of sensing is found to be displacement of the fluorescent ligand from the Cu(II) complex by the sulphide ion.