Cause for Unusually Large Thermal Hysteresis of Spin Crossover in [Fe(2-pic)3]Cl2·H2O

Thermodynamic properties of an iron(II) spin crossover complex [Fe(2-pic)3]Cl2·H2O (2-pic: 2-picolylamine or 2-aminomethylpyridine), for which an unusually large thermal hysteresis with a width of [Tc(↑) − Tc(↓) = 91 K] in its spin-state transition has been observed by 57Fe Mössbauer spectroscopy, were investigated by adiabatic heat capacity calorimetry. Contrary to the previous observation, the preliminary DTA result showed much smaller thermal hysteresis around 200 K, with a width of [Tc(↑) − Tc(↓) = 12 K]. The present heat capacity measurement by adiabatic calorimetry revealed that there exists a metastable low-spin low-temperature phase, besides the stable low-spin low-temperature phase and that this complex undergoes stabilization from the metastable low-spin phase to the stable low-spin phase accompanied by a large heat evolution when the sample is annealed around 200 K. The stable low-spin phase exhibited a phase transition to the high-spin high-temperature phase around 280 K upon heating. The high-spin state was found to be always undercooled down to ∼200 K. The large thermal hysteresis earlier reported for the title complex turns out to be apparent and is caused by the existence of the metastable low-spin phase. Moreover, another intermediate metastable (low-spin?) phase was found when the sample in the metastable low-spin phase was heated from 150 K at a heating rate of ∼5 K h−1. This metastable phase exhibited a phase transition to the undercooled high-spin phase at ∼265 K.