| Abstract
| - The crystal structure of [Fe(bt)2(NCS)2] (A) was determined by X-ray diffraction at 293 and at 150 K in order toanalyze the structural changes associated with the spin transition. The space group is P1̄ with Z = 2 at bothtemperatures. Lattice constants are as follows: a = 8.5240(4), b = 11.0730(6), c = 12.5300(8) at 293 K and a= 8.1490(4), b = 11.4390(5), c = 12.1270(6) at 150 K. The iron(II) atom lies at the center of a distorted octahedron[FeN6] defined by two bt ligands arranged in a cis conformation. The two remaining coordination positions areoccupied by two isothiocyanate anions. The average bond lengths of 2.159(4) Å (293 K) and 1.951(2) Å (150 K)clearly indicate the change in spin configuration. The trigonal distortion parameter φ has a value of 9.6° and 5.5°at 293 and 150 K, respectively. For A, ΔV = ΔVSCO = 28 Å3 per formula unit and is accompanied by a hysteresisof 10 K. χMT vs T curves at atmospheric pressure for A show an abrupt spin transition with Tc↓ = 176 K and Tc↑= 187 K. The thermodynamic parameters associated with the spin transition are ΔH = 8.4 ± 0.4 kJ mol-1 andΔS = 46.5 ± 3 J K mol-1. The thermal dependence of the magnetic susceptibility at different pressures,0.1−0.91 GPa, points out an unusual behavior, which can only be understood in terms of a crystallographic phasetransition or a change in the bulk modulus of the complex. Polymorph B crystallizes in the C2/c space group withan average Fe−N bond length of 2.168(2) Å and φ = 14.7° at 293 K. B remains in the HS configuration even atpressures of 1.06 GPa.
- The crystal structure determination of [Fe(bt)2(NCS)2] polymorph A at 150 K and a detailed analysis of the unit cell volume in the spin transition region are reported. Structural reinvestigations of both polymorphs, A and B, at 300 K and the influence of hydrostatic pressures up to 1 GPa on the spin crossover properties are also discussed.
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