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

The crystal structure of [Fe(bt)₂(NCS)₂] (A) 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 P1̄ with Z = 2 at both temperatures. 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 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 A, ΔV = ΔV_SCO = 28 ų per formula unit and is accompanied by a hysteresis of 10 K. χ_MT vs T curves at atmospheric pressure for A show an abrupt spin transition with T_c^↓ = 176 K and T_c^↑ = 187 K. The thermodynamic parameters associated with the spin transition are ΔH = 8.4 ± 0.4 kJ mol^-1 and ΔS = 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 phase transition or a change in the bulk modulus of the complex. Polymorph B crystallizes in the C2/c space group with an average Fe−N bond length of 2.168(2) Å and φ = 14.7° at 293 K. B remains in the HS configuration even at pressures of 1.06 GPa.