The Commensurate Modulated Structure of The Thermal Quenching Meta-stable State In Spin Crossover Complex [Fe(abpt)2(NCS)2]

Department of Chemistry, National Taiwan University
â—‹Chou-Fu Sheu Szu-Miao Chen Shih-Chi Wang Gene-Hsiang Lee Liu Yi-Hung Yu Wang

One polymorph (C) of a spin crossover complex [Fe(abpt)2(NCS)2] (TC = 86 K) is structurally characterized, the c axis is tripled to form a commensurate modulated structure in the thermal quenching meta-stable high spin state at 25 K, which is obviously different from that of the thermodynamically stable state at 300 K. The asymmetric unit contains two crystallographic independent iron sites (Fe1 and Fe2), both located at inversion centers. Each iron atom is octahedrally coordinated by six nitrogen atoms: four from two abpt ligands and two from NCS- ligands in a trans conformation. The molecular packing around each Fe site is very different. The packing around Fe 1 is similar to that of polymorph A, where neighboring abpt ligands form quasi 1-D chain through π-π interaction. On the contrary, the packing around Fe 2, the neighboring abpt are nearly perpendicular to each other, no π-π interaction is possible. The reason for phase transition is most likely due to the different anisotropic thermal contraction in the different packing layer. The diffraction evidence of the commensurate structure is shown below, the three dimensional structures will be presented.