A tetra-dentate ligand L [3,3'-((pyridin-2-ylmethyl)azanediyl)dipropanamide] was synthesized and characterized by spectroscopic and structural methods. Reaction of L with two different copper(II) halides [CuX2; X = Br, Cl] in a similar condition yields two different compounds of [LCuCl]Cl, 1 and [CuLBr]2[CuBr4]•CH3OH•H2O, 2, both being characterized by several physicochemical techniques. Single crystal X-ray studies reveal that the Cu(II) centers in the cationic complexes are in a square pyramidal N2O2X (X=Cl and Br) environment. Compound 1 is chromotropic and its solvato-, halo- and ionochromism properties were investigated. It was found that the observed positive solvatochromism is due to structural change followed by solvation of the vacant sites of the complex. The complex demonstrated distinct reversible spectral change over the pH range 1.3–12.1 and can act as pH-induced off–on–off absorption switch through deprotonation and the Cu―O to Cu―N bond rearrangement of the coordinated amide groups in aqueous solution. The complexes showed noticeable ionochromism and are sensitive and selective towards cyanide and azide anions in the presence of other pseudo-halide anions.
