Iron(III) spin-crossover compounds with a wide apparent thermal hysteresisaround room temperature

The magnetic properties of the spin-crossover compounds, [Fe(qsal)(2)]NCSe . MeOH (1) and [Fe(qsal)(2)]NCSe . CH2Cl2 (2), have been measured. We have discovered that both compounds 1 and 2 exhibit a wide thermal hysteresis lo op of 140 K (T(1/2)up arrow = 352 K and T(1/2)down arrow = 212 K) and 180 K (T(1/2)up arrow = 392 K and T(1/2)down arrow = 212 K), respectively, in th e first cycle. Thermogravimetric analysis shows that solvent molecules esca pe from compounds 1 and 2 around 340 and 395 K, respectively. This means th at the hysteresis loops observed for the first cycle are only apparent ones . Following the first loop, they show a two-step spin-crossover in warming mode. The so-called "step 1" and "step 2" are centered around T(1/2(S1))up arrow = 215 K and T(1/2(S2))up arrow = 282 K, respectively. On the other ha nd, a one-step spin-crossover occurs at T(1/2)down arrow = 212 K in cooling mode. The hysteresis widths can be estimated to be 3 K (step 1) and 70 K ( step 2), assuming that the widths in steps 1 and 2 are defined as the diffe rences between T(1/2(S1))up arrow and T(1/2)down arrow, and T(1/2(S2))up ar row and T(1/2)down arrow, respectively. The hysteresis width of 70 K in ste p 2 is one of the widest values reported so far for spin-crossover complexe s. It is thought that the cooperativity operating in the complexes arises m ainly from the intermolecular T interactions between quinoline and phenyl r ings. Using a previously repored model, we are able to simulate the hystere sis loop with a two-step spin-crossover in warming mode and a one-step tran sition in cooling mode.