Structural Bifurcation in the High→Low-Spin and Low→High-Spin Phase Transitions Explains the Asymmetric Spin-Crossover in [FeL 2 ][BF 4 ] 2 (L=2,6-Di{pyrazol-1-yl}isonicotinonitrile)

• Published in: Angewandte Chemie International Edition 2024-12, p.e202416924
• Main Authors: Ahmed, Ahmed, Hall, Amy, Vasili, Hari Babu, Kulmaczewski, Rafal, Kulak, Alexander N, Cespedes, Oscar, Pask, Christopher M, Brammer, Lee, Roseveare, Thomas M, Halcrow, Malcolm A
• Format: Article
• Language: English
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202416924

Polycrystalline [FeL ][BF ] (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)]BF are also briefly described.