STRUCTURAL-CHANGES ASSOCIATED WITH THE SPIN TRANSITION IN FE(PHEN)2(NCS)2 - A SINGLE-CRYSTAL X-RAY-INVESTIGATION

The crystal structure of Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline) was determined by X-ray diffraction at ≈293 and ≈130 K, in order to detect the structural changes associated with the singlet ↔ quintet spin transition. The space group is Pbcn with Z = 4 at both temperatures. Lattice constants are as follows: a = 13.1612 (18), b = 10.1633 (11), and c = 17.4806 (19) Å at ≈293 K and a = 12.7699 (21), b = 10.0904 (25), and c = 17.2218 (30) Å at ≈130 K. The data were refined (167 parameters) to R = 3.4% (4.1%) at ≈293 K (≈130 K) for 1050 (1115) observed independent reflections (Fo2 > 2σ(Fo)2). In disagreement with most of the predictions, the transition is found to be accompanied neither by a change in the crystal symmetry nor by an order-disorder transition involving (NCS)− groups. Only a large reorganization of the iron(II) environment is detected. The main structural modifications, when passing from the high- to the low-spin form, consist of an important shortening of the Fe-N(phen) and Fe-N(CS) distances (by 0.20 (mean value) and 0.10 Å, respectively) and a noticeable variation of the N-Fe-N angles, leading to a more regular shape of the [Fe–N6] octahedron. The temperature dependence of χmT (χm = molar magnetic susceptibility), determined on a polycrystalline sample of Fe(phen)2(NCS)2, shows the existence of a sharp transition centered at Tc ≈ 176.5 K. The higher and lower limits of χmT (3.41 and 0.58 cm3•mol−1•K) indicate that the compound is in the high-spin state at room temperature, whereas ≈17% of the high-spin isomer is retained in the low-spin form at low temperature. On account of this magnetic behavior and of the crystallographic data, the compound is assumed to be in the crystalline form “II”, by analogy with Fe(bpy)2(NCS)2 (bpy = 2,2′-bipyridine). © 1990, American Chemical Society. All rights reserved.