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

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT${}_{1/2}$) 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)${}_2$⋅CH${}_3$NO${}_2$ (X=ClO${}_4$, 1; X=BF${}_4$, 2). Stable spin-state switching – T${}_{1/2}$=288 K; ΔT${}_{1/2}$=62 K – is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO – T${}_{1/2}$=331 K; ΔT${}_{1/2}$=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.