Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-Pyrazol-1-yl)Pyridine) Complexes.

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes - [Fe(bpp-COOEt)2 ](X)2 ⋅CH3 NO2 (X=ClO4 , 1; X=BF4 , 2). Stable spin-state switching - T1/2 =288 K; ΔT1/2 =62 K - is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO - T1/2 =331 K; ΔT1/2 =43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations - from the Jahn-Teller-distorted HS state to the less distorted LS state - and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.