Color-change behavior of a bis(benzimidazole)-coordinated nickel-dichlorido complex induced by the adsorption of pyridine or ammonia vapor

Pyridine (py) and ammonia (NH₃) have been widely used as raw materials in manufacturing processes; however, both are volatile, and their vapor is detrimental to human health. To limit the exposure of those who work with py and NH₃ vapor, the development of effective techniques to sense atmospheric levels of py and NH₃ in order to decrease their concentration when required is important. In the present study, we found that crystals of bis(benzimidazole)NiCl₂ (1, bis(benzimidazole) = phenylbis(benzimidazol-2-yl)methane)) adsorb py and NH₃ vapor with a concomitant color change from purple to green (py) or light purple (NH₃). Powder X-ray diffraction, UV–Vis diffuse reflectance, and IR spectroscopic studies revealed that these color changes are induced by the formation of trans-[NiCl₂(py)₄] (2) or [Ni(NH₃)₆]Cl₂ (3). A time-dependent analysis of the py-vapor adsorption indicated that the formation of 2 from 1 proceeds non-uniformly in the solid. Crystals of 1 were furthermore found to adsorb py or NH₃ even at low concentrations (py: ~ 6 ppm; NH₃: ~ 33 ppm), albeit that a color change was not observed in these cases.