Mononuclear copper(II) complexes containing chelating ligand of 2-methyl-N-(pyridine-2-yl-methyl)propane-2-amine as chromotropic probes

Chromotropism is the change in color of a substance due to the physical and chemical properties of its ambient surrounding medium, such as temperature (thermochromism), solvent (solvatochromism), electrons (electrochromism), pressure (piezochromism), light (photochromism), pH (halochromism) and ion (ionochromism). Chromotropism properties of inorganic compounds have been studied during the past four decades. This phenomenon has many applications in thermo-sensitive, imaging, photo switching, sensor materials, pollutant sensor and as Lewis acid-base indicators [1]. Among the metal complexes whose color changes are due to the d–d transitions, the copper(II) ion can be an excellent candidate due to its diverse coordination numbers, various geometrical structures and the presence of a strong Jahn–Teller effect. A combination of the copper(II) ion with appropriate ligands has been reported to afford metal-ion responsive chromotropic molecules. Multidentate ligands stabilize the resultant complexes due to chelate effect. The metal binding is also more strengthened when a 5- or 6- membered chelate ring is formed [2,3]. Herein, the synthesis of two mononuclear copper(II) complexes of formula [L2CuX]X where L is a bidentate ligand of N-(pyridin-2-ylmethyl)propane-2-amine and X =Cl and Br (Fig. 1), is reported. The complexes are fully characterized by physic chemical techniques. The chromotropic behaviors of the complex, including thermochromism, solvatochromism, and halochromism, is investigated.