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)

Abstract

AbstractPolycrystalline [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 new triclinic low‐spin phase with a doubled unit cell volume (LS3), while HS2 forms a monoclinic low‐spin phase (LS4) with similar unit cell dimensions to HS2. Single crystals of LS3 and LS4 both convert to HS1 on rewarming. The low→high‐spin transition for LS4 is ca 10 K higher in 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 differed between these experiments, implying it is sample and/or measurement‐dependent. Three other salts of this iron(II) complex and the coordination polymer [Ag(μ‐L)]BF4 are also briefly described.