Synthesis, characterization, hirshfeld surface analysis, and chromotropic behavior of a novel copper(II) complex with amide-pyridyl ligand: solvato-, halo-, and thermochromism and azide ion sensing

This work describes the synthesis and characterization of a new copper(II) complex, [CuL(NO₃)]NO₃, obtained from the reaction of 3,3′-((pyridin-2-ylmethyl)azanediyl)dipropanamide (L) with Cu(NO₃)₂•3H₂O in methanol. The ligand (L) acts as an N₂O₂ donor set. Various techniques, including FT-IR, UV–Vis spectroscopy, conductivity measurements, and elemental analysis, confirmed the formation of the complex. The geometrical property of this compound was determined using X-ray crystallography, which revealed a mononuclear, six-coordinate Cu(II) center with a bidentate nitrate ligand occupying the remaining coordination sites. The intermolecular interactions were studied through Hirshfeld surface analysis. The complex exhibits good solubility in water and polar organic solvents. Interestingly, it displays solvatochromism (color change based on solvent) and thermochromism (color change based on temperature) in DMSO, attributed to structural changes and reversible interactions with solvent molecules. The complex demonstrates pH-dependent color changes in an aqueous solution, acting as a potential off–on-off absorption switch. This behavior probably arises from the deprotonation of the hemilabile amide groups and switching between Cu–O(amide) and Cu-NH(amide) coordination modes. Furthermore, the complex exhibits exceptional selectivity towards the N₃⁻ anion, showing a distinct color change in its presence. This makes it a promising “naked-eye” indicator for N₃⁻ detection.