One-dimensional Spin-crossover iron(II) complexes bridged by intermolecular imidazole-pyridine NH· · · N hydrogen bonds, [Fe(HLMe)3]X2 (HLMe = (2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine; X = PF6, ClO4, BF4)

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]X₂ (X = PF₆ (1), ClO₄ (2), BF ₄ (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and M̈ossbauer 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 N₆ 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]²⁺ is a chiral species with a δ or δ isomer, and the adjacent δ and δ 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 SCOprofile becomes steeper with the decrease of the anion size (73.0 Å ³ for PF₆-, 54.4 Å ³ for ClO₄-, and 53.4 Å ³ for BF₄-). The SCO transition temperature T_1/2 of the PF ₆ (1), ClO₄ (2), and BF₄ (3) salts estimated from the magnetic susceptibility measurements are (T↓ = 151.8 K, T↑ = 155.3 K), (T =184.5K, T = 186.0 K), and (T = 146.4 K, T =148.2K), respectively, indicating that the T_1/2 value is not in accord with the anion size.