Structural bifurcation in the high→low‐spin and low→high‐spin phase transitions explains the asymmetric spin‐crossover in [FeL2][BF4]2 (L = 2,6‐Di{pyrazol‐1‐yl}isonicotinonitrile)

Polycrystalline [FeL2][BF4]2 (L = 2,6-di(pyrazol-1-yl)isonicotinonitrile) exhibits an abrupt hysteretic spin transition near 240 K, with a shoulder on the warming branch whose appearance depends on the sample history. The freshly isolated material is a ca 60:40 mixture of triclinic (HS1) and tetragonal (HS2) high-spin polymorphs, which are structurally closely related. Both HS1 and HS2 undergo a high→low-spin transition on cooling at 230±10 K. HS1 transforms to a triclinic low-spin phase with a doubled unit cell volume (LS3), while HS2 forms a monoclinic low-spin phase (LS4) with only small changes to its unit cell dimensions. Crystals of LS3 and LS4 both convert to HS1 on rewarming. The low→high-spin transition for LS4 occurs at ca 10 K higher temperature than for LS3, explaining the asymmetric thermal hysteresis. Powder diffraction, calorimetry and magnetic data show that multiple cycling about the spin-transition leads to slow enrichment of the HS1 and LS3 phases at the expense of HS2 and LS4. That is consistent with the HS2/LS4 fraction of the polycrystalline sample undergoing rare, bifurcated HS2→(LS3+LS4) and LS4→(HS1+HS2) phase transitions. The rate of enrichment of HS1/LS3 is sample- and measurement-dependent. Three other salts of this iron(II) complex and the coordination polymer [Ag(μ-L)]BF4 are also briefly described.