Temperature- or Pressure-Induced Structure Changes of a Spin Crossover Fe(II) Complex; [Fe(bpy)2(NCS)2]

The crystal structures of polymorph II of diisothiocyanatobis(2,2′-bipyridine)iron(II), [Fe(bpy)2(NCS)2], in the high- (298 K) and low-spin states (175, 110 K) were determined by an X-ray diffraction method. The crystal is orthorhombic, space group Pcnb and Z=4 over the entire temperature range. At 298 K, a=13.2044 (6), b=16.4820 (7), c=10.1086 (7) Å, U=2200.0 (2) Å3, Dc=1.462 g cm−3 and at 110 K, a=12.9842 (9), b=16.1067 (12), c=9.8938 (8) Å, U=2069.1 (3) Å3, Dc=1.555 g cm−3. The structures were refined to R=0.047, 0.044, and 0.042 at 298, 175, and 110 K, respectively. The difference in the average Fe–N bond lengths between high- and low-spin states is 0.17 Å. The pressure dependence of the absorption spectra was also measured up to 8.5 GPa using a diamond anvil cell. These results indicate that with increasing pressure the high-spin state at 1 atm starts to change to the low-spin state at 0.3 GPa and that its low-spin state reverts again to the high-spin state at around 3.3 GPa.