Synthesis and Properties of a Thermochromic Spin Crossover FeII Complex: An Undergraduate Coordination Chemistry Laboratory Experiment

In this third-year undergraduate experiment, a coordination complex [Fe(NH2trz)3]Br2·H2O (NH2trz is 4-amino-1,2,4-triazole, C2H4N4) was synthesized and characterized by IR and UV–visible spectroscopies. This compound exhibits a spectacular reversible purple-white thermochromic transition upon heating–cooling with a large hysteresis centered at room temperature. The thermochromic transition is related to the spin crossover (SCO) from the low-spin state (LS, purple, S = 0, diamagnetic) to the high-spin state (HS, white, S = 2, paramagnetic). During this experiment, students discovered synthetic methods, such as anion metathesis and Ostwald ripening, and physical properties, such as magnetic properties, thermochromism, and bistability phenomenon, altogether leading to possible applications in optical-data storage and display.