Spin transition and symmetry-breaking in new mononuclear FeII tren-complexes with up to 38 K hysteresis around room temperature

The structurally simple complex {FeII[tren(6F-py)3]}(BF4)2 [tren(6F-py)3 = tris(3-aza-4-(6-fluoro-2-pyridyl)-3-butenyl)amine] undergoes an abrupt spin transition (ST) with the critical temperature T1/2↓ = 243 K on cooling and T1/2↑ = 281 K on heating, with a 38 K wide hysteresis, while the ClO4− congener shows ST spanning the room temperature region at T1/2↓ = 267 K and T1/2↑ = 295 K with a 28 K wide hysteresis. Calorimetric data confirm the occurrence of a highly energetic ST process, while multi-temperature single crystal X-ray structural studies identify a concerted symmetry-breaking P21/n ↔ P21/c as the origin of the rare strong cooperativity and the large hysteresis loop. The analysis of the experimental data discloses two distinct interdependent events, namely, the combination of a gradual ST and an induced crystallographic phase transition, which endows the material with strong bistability and makes it an excellent platform for investigating new generations of ST based electronic and spintronic devices.