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 (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.