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

Abstract

Solvent-free spin crossover Fe^II complex <i>fac</i>-[Fe^II(HL^{<i>n</i>‑Pr})₃]­Cl·PF₆ was prepared, where HL^{<i>n</i>‑Pr} denotes 2-methylimidazol-4-yl-methylideneamino-<i>n</i>-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 <i>T</i>₁ centered at 122 K (<i>T</i>_1↑ = 127.1 K, <i>T</i>_1↓ = 115.8 K) and the second spin transition <i>T</i>₂ centered at ca. 105 K (<i>T</i>_2↑ = 115.8 K, <i>T</i>_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 <i>T</i>₁. 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(<i>T</i>₁) → HS­(<i>T</i>₂) → low-spin LS → LS­(<i>T</i>₂) → LS­(<i>T</i>₁) → HS. The crystal structures at all temperatures have a triclinic space group <i>P</i>1̅ with <i>Z</i> = 2. The complex-cation has an octahedral N₆ coordination geometry with three bidentate ligands and assume a <i>facial</i>-isomer with Δ- and Λ-enantimorphs. Three imidazole groups of <i>fac</i>-[Fe^II(HL^{<i>n</i>‑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 <i>n</i>-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 <i>n</i>-propyl groups are found in the spin transition processes except for HS → HS­(<i>T</i>₁) → HS­(<i>T</i>₂)