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·PF₆ (1), that exhibits stepwise spin transition and thermal hysteresis was synthesized by using the ligand 2-methylimidazol- 4-ylmethylidene-n-hexylamine (HLn-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 PF₆ - anion is encapsulated within the cavity of the cubane. Magnetic susceptibility mea- surements showed a three-step spin transition, and the spincrossover profile became less pronounced as the sweep rate increased. At a sweep rate of 0.5 Kmin^-1, the midpoint temperatures of the three steps are Tc(↓) = 131.2, 144.2, and 154.2 K in the heating mode and Tc(↑) = 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.