HIGH-PRESSURE SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDY OF 2 SPIN-CROSSOVER IRON(II) COMPLEXES - FE(PHEN)2(NCS)2 AND FE(BTZ)2(NCS)2

Crystal structures of Fe(Phen)2(NCS)2, form II (phen = 1,10-phenanthroline) and Fe(Btz)2(NCS)2 (Btz = 2,2'-bi-4,5-dihydrothiazine) were determined at room temperature by X-ray diffraction, under a pressure of P congruent-to 1.0 GPa. For bath compounds, the space group is orthorhombic, Pbcn (Z = 4). Lattice constants of Fe(Phen)2(NCS)2 are a = 12.656(3), b = 9.848(2), and c = 16.597(4) angstrom. Data were refined (159 parameters) to R(w) = 0.043 for 631 observed reflections (F(o) > 4sigma(F(o)))); for Fe(Btz)2(NCS)2, a = 12.839(4), b = 10.454(3), and c = 16.362(4) angstrom, with R(w) = 0.056 (154 parameters for 662 observed reflections). Both structures reveal that the iron(II) complexes are in the low-spin state. The molecular configurations are very similar to those obtained at ambient pressure and low temperature: similar shortenings of Fe-N(ligand) bond lengths as well as a more regular shape of the [Fe-N6] octahedra are observed, compared to those at ambient pressure and room temperature. Lattice parameters have been obtained over the pressure range [1000 HPa to 1.3 GPa]: a pronounced change of derivative in the curve a vs P relative to Fe(Phen)2(NCS)2 is clearly evidenced at P(c) congruent-to 0.6 GPa, the pressure at which the singlet-quintet spin transition is expected to occur, according to recent high-pressure magnetic susceptibility, infrared, and X-ray absorption studies. Fe(Btz)2(NCS)2 does not reveal such a lattice parameter anomaly. Only a slight change of slope in the curve representing the evolution of the unit cell volume as a function of pressure is observed at 0.4-0.5 GPa. The spin conversion is found to occur more gradually in the latter compound than in the former. Compressibility data are used to estimate the volume variation associated with the spin transition in the case of Fe(Phen)2(NCS)2: it is very close to the one obtained as a function of temperature. Bulk moduli are comparable for both complexes, but Fe(Phen)2(NCS)2 exhibits a strong lattice anisotropy compared to Fe(Btz)2(NCS)2. These parameters are discussed in relation with the cooperativity of the spin transition.