Pressure Effect and Crystal Structure Reinvestigations on the Spin Crossover System:  [Fe(bt)<sub>2</sub>(NCS)<sub>2</sub>] (bt = 2,2‘-Bithiazoline) Polymorphs <b>A</b> and <b>B</b>

The crystal structure of [Fe(bt)<sub>2</sub>(NCS)<sub>2</sub>] (<b>A</b>) was determined by X-ray diffraction at 293 and at 150 K in order to analyze the structural changes associated with the spin transition. The space group is <i>P</i>1̄ with <i>Z</i> = 2 at both temperatures. Lattice constants are as follows:  <i>a</i> = 8.5240(4), <i>b</i> = 11.0730(6), <i>c</i> = 12.5300(8) at 293 K and <i>a</i> = 8.1490(4), <i>b</i> = 11.4390(5), <i>c</i> = 12.1270(6) at 150 K. The iron(II) atom lies at the center of a distorted octahedron [FeN<sub>6</sub>] defined by two bt ligands arranged in a cis conformation. The two remaining coordination positions are occupied 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 <b>A</b>, Δ<i>V</i> = Δ<i>V</i><sub>SCO</sub> = 28 Å<sup>3</sup> per formula unit and is accompanied by a hysteresis of 10 K. χ<sub>M</sub><i>T</i> vs <i>T</i> curves at atmospheric pressure for <b>A</b> show an abrupt spin transition with <i>T</i><sub>c</sub><sup>↓</sup> = 176 K and <i>T</i><sub>c</sub><sup>↑</sup> = 187 K. The thermodynamic parameters associated with the spin transition are Δ<i>H</i> = 8.4 ± 0.4 kJ mol<sup>-1</sup> and Δ<i>S</i> = 46.5 ± 3 J K mol<sup>-1</sup>. 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 phase transition or a change in the bulk modulus of the complex. Polymorph <b>B</b> crystallizes in the <i>C</i>2/<i>c</i> space group with an average Fe−N bond length of 2.168(2) Å and φ = 14.7° at 293 K. <b>B</b> remains in the HS configuration even at pressures of 1.06 GPa.