An iron(II) complex shows bistable spin-state switching characteristic with T1/2 centred at room temperature

An iron(II) complex (1·CH3CN) composed of a tridentate all nitrogen coordinating ligand—ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt)—shows bistable spin-state switching characteristic with thermal hysteresis width (ΔT1/2) = 44 K and switching temperature (T1/2) = 298 K in the first cycle. Crystal structures of the LS and HS forms of the complex reveal that spin-state switching induces a pronounced angular distortion, creating an energy barrier separating the LS and HS states. Traversing the barrier requires substantial molecular rearrangement in the presence of constraints imposed by the crystal lattice, rendering the spin-state switching of 1·CH3CN hysteretic in the solid-state. The rare observation of bistable SCO with T1/2 centred at room temperature (RT) renders the complex an ideal model system to study reversible spin-state tuning under ambient conditions.