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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)

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journal contribution
posted on 03.08.2015, 00:00 by Takeshi Fujinami, Koshiro Nishi, Daisuke Hamada, Keishiro Murakami, Naohide Matsumoto, Seiichiro Iijima, Masaaki Kojima, Yukinari Sunatsuki
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).

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