Abrupt Spin Transition with Thermal Hysteresis of Iron(III) Complex [Fe^III(Him)₂(hapen)]AsF₆ (Him = Imidazole, H₂hapen = <i>N</i>,<i>N</i>′‑Bis(2-hydroxyacetophenylidene)ethylenediamine)

The solvent-free spin crossover iron­(III) complex [Fe^III(Him)₂­(hapen)]­AsF₆ (Him = imidazole, H₂hapen = <i>N</i>,<i>N</i>′-bis­(2-hydroxyacetophenylidene)­ethylenediamine), exhibiting thermal hysteresis, was synthesized and characterized. The Fe^III ion has an octahedral coordination geometry, with N₂O₂ donor atoms of the planar tetradentate ligand (hapen) and two nitrogen atoms of two imidazoles at the axial positions. One of two imidazoles is hydrogen-bonded to the phenoxo oxygen atom of hapen of the adjacent unit to give a hydrogen-bonded one-dimensional chain, while the other imidazole group is free from hydrogen bonding. The temperature dependencies of the magnetic susceptibilities and Mössbauer spectra revealed an abrupt spin transition between the high-spin (<i>S</i> = 5/2) and low-spin (<i>S</i> = 1/2) states, with thermal hysteresis