Synthesis, structures, and magnetic properties of iron(II) complexes, [FeII(HLMe)2](ClO4)2 and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH (HLMe = 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine): Their structural distortion and spin states

Two new iron(II) complexes, [FeII(HLMe)2](ClO4)2 (1) and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH (2) were synthesized by selective crystallization in methanol and ethanol, respectively, where HLMe is a tridentate N3 ligand, 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine, and their structures and magnetic properties were investigated. Each compound contains the same octahedrally N6 coordinated complex-cation [FeII(HLMe)2]2+ and two ClO4− ions. The non-solvated compound 1 is the HS (S = 2) complex without spin transition over the 5–300 K temperature range, and has the homochiral 1D helical-chain structure constructed of the intermolecular NH···O− hydrogen bonds between ClO4− ions and neighboring imidazole NH groups of the [FeII(HLMe)2]2+ cations. The ethanol adduct 2 shows a gradual one-step spin transition between the HS (S = 2) and LS (S = 0) states, and has two crystallographically independent iron(II) sites with two NH(imidazole)···OH(EtOH)···O(ClO4−) hydrogen bonds of Fe(1) site and two NH(imidazole)···O(ClO4−) hydrogen bonds of Fe(2) site. There are no further intermolecular interactions to produce an extended structure in a crystal lattice of 2. The presence or absence of thermal spin transition is related to the structural distortion of each HS molecule of these perchlorate salts associated with their surroundings of intermolecular hydrogen bonds.

The 2:1 reaction of 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine (HLMe) and FeII(ClO4)2·6H2O gives non-solvated complex [FeII(HLMe)2](ClO4)2 and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH by selective crystallization in methanol and ethanol, respectively. The ethanol adduct shows a gradual one-step spin crossover, whereas the non-solvated complex is high-spin (HS) complex without spin transition. The single crystal X-ray studies reveal that the presence or absence of thermal spin transition is related to the structural distortion of each HS molecule of these perchlorate salts associated with their surroundings of intermolecular hydrogen bonds.Figure optionsDownload as PowerPoint slideHighlights
► Tridentate ligand, 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine is used.
► Two iron(II) complexes with two tridentate N3 ligands were prepared.
► Non-solvated and solvated (EtOH adduct) complexes are selectively crystallized.
► Non-solvated one is HS complex, while solvated one is SCO complex.
► Distortion of each HS molecule is related to the presence or absence of SCO.