Spin Transition Regime in New One-Dimensional Polymeric Iron(II) Compounds. Importance of the Water Content for the Thermal and Optical Properties

A new series of polymeric chainlike materials exhibiting the spin crossover phenomenon has been synthesized in aqueous conditions using different iron(II) salts and 4-(formylamino)-1,2,4-triazole (fatrz). The magnetic and thermogravimetric behaviors of the compounds of general formula [Fe(fatrz)3](A)2·xH2O are presented for 1 (A = trifluoromethanesulfonate and x = 3), 2 (A = tetrafluoroborate and x = 2), 3 (A = p-tolylsulfonate and x = 3), and 4 (A = nitrate and x = 1). In 1, the high (S = 2) ↔ low (S = 0) spin transition of the iron(II) ions occurs sharply in the close vicinity of 0 °C. Reflectivity measurements show that this transition can be cycled and used as a temperature sensor. However, 1 loses water when warmed above 320 K, whereby its transitions become more gradual and shift to lower temperatures. Calorimetric parameters for the spin transition of 1 have been obtained and indicate a strongly cooperative phenomenon, with ΔH = 17.19 kJ mol-1 and ΔS = 64.26 J K-1 mol-1 in the warming mode. 2 has a more gradual transition in the range 265−285 K. 3 and 4 are low spin at room temperature and possess large apparent hysteresis due to a loss of water upon the first L → H spin change. The reabsorption of water in 3 makes it a possible reusable temperature threshold sensor. The prominent role of water molecules in the whole family of chainlike spin crossover materials is discussed.