Ahmed, A., Hall, A., Vasili, H.B. et al. (7 more authors) (2025) 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). Angewandte Chemie, 137 (5). e202416924. ISSN 0044-8249
Abstract
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 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.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Keywords: | iron; spin-crossover; crystal engineering; phase transitions; in situ diffraction |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Engineering & Physical Sciences (Leeds) > School of Chemistry (Leeds) > Inorganic Chemistry (Leeds) The University of Leeds > Faculty of Engineering & Physical Sciences (Leeds) > School of Physics and Astronomy (Leeds) > Condensed Matter (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 10 Feb 2025 17:22 |
Last Modified: | 10 Feb 2025 17:22 |
Status: | Published |
Publisher: | Wiley |
Identification Number: | 10.1002/ange.202416924 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:223038 |