Scan Rate Dependent Spin Crossover Iron(II) Complex with Two Different
Relaxations and Thermal Hysteresis fac-[FeII(HLn‑Pr)3]Cl·PF6 (HLn‑Pr = 2‑Methylimidazol-4-yl-methylideneamino‑n‑propyl)
Solvent-free spin crossover FeII complex fac-[FeII(HLn‑Pr)3]Cl·PF6 was
prepared, where HLn‑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 (T1↑ = 127.1 K, T1↓ = 115.8 K) and the second spin transition T2 centered at ca. 105 K (T2↑ = 115.8 K, T2↓ = 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 P1̅ 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-[FeII(HLn‑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).