Stepwise Spin Transition and Hysteresis of a Tetrameric Iron(II) Complex, fac ‐[Tris(2‐methylimidazol‐4‐ylmethylidene‐ n ‐hexylamine)]iron(II) Chloride Hexafluorophosphate, Assembled by Imidazole···Chloride Hydrogen Bonds

A spin‐crossover iron(II) complex, fac ‐[Fe II (HL n ‐Hx ) 3 ]Cl · PF 6 ( 1 ), that exhibits stepwise spin transition and thermal hysteresis was synthesized by using the ligand 2‐methylimidazol‐4‐ylmethylidene‐ n ‐hexylamine (HL n ‐Hx ). Four complex cations ( fac ‐[Fe II (HL n ‐Hx ) 3 ] 2+ ) and four Cl – anions associate through twelve imidazole ··· Cl – H bonds to give a tetrameric cubane‐like assembly, and one PF 6 – anion is encapsulated within the cavity of the cubane. Magnetic susceptibility measurements showed a three‐step spin transition, and the spin‐crossover profile became less pronounced as the sweep rate increased. At a sweep rate of 0.5 K min –1 , the midpoint temperatures of the three steps are T c (↑) = 131.2, 144.2, and 154.2 K in the heating mode and T c (↓) = 116.8, 137.8, and 151.8 K in the cooling mode. The respective thermal hystereses were evaluated to be 14.4, 6.3, and 2.4 K wide.