Scan Rate Dependent Spin Crossover Iron(II) Complex with Two Different Relaxations and Thermal Hysteresis fac-[Fe^II(HL^n-Pr)₃]Cl·PF₆ (HL^n-Pr = 2-Methylimidazol-4-yl-methylideneamino-n-propyl)

Solvent-free spin crossover Fe^II complex fac-[Fe^II(HL^n-Pr)₃]Cl·PF₆ was prepared, where HL^n-Pr denotes 2-methylimidazol-4-yl-methylideneamino-n-propyl. The magnetic susceptibility measurements at scan rate of 0.5 K min^-1 showed two successive spin transition processes consisting of the first spin transition T₁ centered at 122 K (T_1↑ = 127.1 K, T_1↓ = 115.8 K) and the second spin transition T₂ centered at ca. 105 K (T_2↑ = 115.8 K, T_2↓ = 97.2 K). The magnetic susceptibility measurements at the scan rate of 2.0, 1.0, 0.5, 0.25, and 0.1 K min^-1 showed two scan speed dependent spin transitions, while the Mössbauer spectra detected only the first spin transition T₁. The crystal structures were determined at 160, 143, 120, 110, 95 K in the cooling mode, and 110, 120, and 130 K in the warming mode so as to follow the spin transition process of high-spin HS → HS(T₁) → HS(T₂) → low-spin LS → LS(T₂) → LS(T₁) → HS. The crystal structures at all temperatures have a triclinic space group P1Ì... with Z = 2. The complex-cation has an octahedral N₆ coordination geometry with three bidentate ligands and assume a facial-isomer with Δ- and Λ-enantimorphs. Three imidazole groups of fac-[Fe^II(HL^n-Pr)₃]²⁺ are hydrogen-bonded to three Cl^- ions. The 3:3 NH(imidazole)···Cl^- hydrogen-bonds form a stepwise ladder assembly structure, which is maintained during the spin transition process. The spin transition process is related to the structural changes of the FeN₆ coordination environment, the order-disorder of PF₆^- anion, and the conformation change of n-propyl groups. The Fe-N bond distance in the HS state is longer by 0.2 Å than that in the LS state. Disorder of PF₆^- anion is not observed in the LS state but in the HS state. The conformational changes of n-propyl groups are found in the spin transition processes except for HS → HS(T₁) → HS(T₂).