Experimental and DFT study of the bis-Schiff base N,N′-pyridyl-imine ligand as a probe for Hg2+ in DMSO solvent

Mercury contamination in the natural environment and from industrial products and their by-products poses well-documented public health concerns due to its toxicity, persistence, and bioaccumulation. The present study reports on the synthesis and properties of the symmetrical bis-Schiff base ligand, ( 1E,1E′ )- N,N′ -(1,4-phenylene)bis(1-(pyridin-2-yl)methanimine) (“ NPIL” ), which exhibits a rapid and distinct colorless to dark yellow colorimetric change upon complexation with Hg 2+ ions in DMSO media, enabling naked-eye detection. UV–Vis titration in DMSO revealed a strong association constant ( K a = 4.8 × 10 3 M −1 ) and a low detection limit (LOD) of 0.027 μM and a quantification limit (LOQ) of 23 μM. The LOD is substantially below Health Canada's recommended value of 2.48 μM. The coordination of Hg 2+ via the ligand's imine and pyridine nitrogen atoms, as ascertained from 1 H NMR titration experiments in DMSO‑ d 6 suggested 2:1, 2:2, and 4:4 metal-to-ligand stoichiometries. Density functional theory (DFT) calculations were performed for several conformations of the host ligand NPIL and its complexes with HgCl₂ in DMSO. The binding interaction energies showed that of four possible “ A - D " conformations of NPIL examined, a 2:1 HgCl 2 complex with the All-cis D conformation (−113.30 kJ·mol −1 ) is more stable than the 2:1 HgCl 2 complex with the Cis-trans-cis C conformation (−111.50 kJ·mol −1 ) due to strong ligand-to-metal charge transfer and stabilizing noncovalent interactions. The combination of high sensitivity, selective optical response, and its simple synthesis makes NPIL a suitable candidate as a cost-effective UV–vis probe for mercury ion detection.