Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan.
Inorg Chem. 2010 Feb 15;49(4):1517-23. doi: 10.1021/ic9017344.
2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine (abbreviated as HL(Me)) is the 1:1 condensation product of 2-methyl-4-formylimidazole and 2-aminoethylpyridine and functions as a bidentate ligand to the iron(II) ion to produce the 3:1 complexes together with anions, [Fe(HL(Me))(3)]X(2) (X = PF(6) (1), ClO(4) (2), BF(4) (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and Mossbauer spectral measurements demonstrated that complexes 1, 2, and 3 showed a steep one-step spin crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states with small thermal hysteresis. Three complexes have an isomorphous structure and are crystallized in the same monoclinic space group, C2/c, both in the HS and LS states. The iron(II) ion has the octahedral coordination geometry of a facial isomer with N(6) donor atoms of three bidentate ligands, in which an imidazole and an imine nitrogen atom per ligand participate in the formation of the coordination bond, but the pyridine nitrogen is free from coordination. The complex cation fac-Fe(HL(Me))(3) is a chiral species with a Delta or Lambda isomer, and the adjacent Delta and Lambda isomers are linked alternately by an intermolecular imidazole-pyridine NH...N hydrogen bond to produce an achiral 1D chain. The two remaining imidazole moieties per complex are hydrogen-bonded to the anions that occupy the space among the chains. The SCO profile becomes steeper with the decrease of the anion size (73.0 A(3) for PF(6)(-), 54.4 A(3) for ClO(4)(-), and 53.4 A(3) for BF(4)(-)). The SCO transition temperature T(1/2) of the PF(6) (1), ClO(4) (2), and BF(4) (3) salts estimated from the magnetic susceptibility measurements are (T( downward arrow) = 151.8 K, T( upward arrow) = 155.3 K), (T( downward arrow) = 184.5 K, T( upward arrow) = 186.0 K), and (T( downward arrow) = 146.4 K, T( upward arrow) = 148.2 K), respectively, indicating that the T(1/2) value is not in accord with the anion size.
2-甲基咪唑-4-亚甲基氨基-2-乙基吡啶（缩写为 HL(Me)) 是 2-甲基-4-甲酰基咪唑和 2-氨基乙基吡啶 1:1 缩合产物，作为双齿配体与铁(II)离子配位，生成 3:1 的配合物与阴离子一起，[Fe(HL(Me))(3)]X(2)（X = PF(6) (1), ClO(4) (2), BF(4) (3)）。磁性研究、差示扫描量热法测量和穆斯堡尔光谱测量表明，配合物 1、2 和 3 表现出高自旋（HS，S = 2）和低自旋（LS，S = 0）状态之间的陡峭一步自旋交叉（SCO），具有小的热滞。三个配合物具有同构的结构，并且在同一单斜空间群 C2/c 中结晶，无论是在 HS 还是 LS 状态下。铁(II)离子具有面式异构体的八面体配位几何形状，三个双齿配体的 N(6)供体原子参与配位键的形成，其中每个配体的一个咪唑和一个亚胺氮原子参与配位键的形成，但吡啶氮原子不参与配位。配合物阳离子 fac-Fe(HL(Me))(3) 是一个手性物种，具有 Delta 或 Lambda 异构体，相邻的 Delta 和 Lambda 异构体通过分子间的咪唑-吡啶 NH...N 氢键交替连接，形成非手性的 1D 链。每个配合物的两个剩余的咪唑部分通过氢键与占据链间空间的阴离子结合。随着阴离子尺寸的减小（PF(6)(-) 为 73.0 A(3)，ClO(4)(-) 为 54.4 A(3)，BF(4)(-) 为 53.4 A(3)），SCO 谱变得更加陡峭。从磁性测量估计的 PF(6)（1）、ClO(4)（2）和 BF(4)（3）盐的 SCO 转变温度 T(1/2)分别为（T(downward arrow) = 151.8 K，T(upward arrow) = 155.3 K），（T(downward arrow) = 184.5 K，T(upward arrow) = 186.0 K）和（T(downward arrow) = 146.4 K，T(upward arrow) = 148.2 K），表明 T(1/2)值与阴离子大小不相符。
