Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto=C2O2S2)

(n-C3H7)4N[FeIIFeIII(dto)3] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between FeII and FeIII occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P63, a=10.0618(5) Å, c=16.0424(7) Å, V=1406.54(12) Å3, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞, contracts by 0.1 Å at the charge-transfer transition temperature (TCT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at TCT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around TCT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C3H7)4N[FeIIFeIII(dto)3] is regarded as spin entropy driven phase transition.