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Di‐Iron(II) [2+2] Helicates of Bis‐(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

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  • Rafal Kulmaczewski
  • Isaac T. Armstrong
  • Pip Catchpole
  • Emily S. J. Ratcliffe
  • Hari Babu Vasili
  • Stuart L. Warriner
  • Oscar Cespedes
  • Malcolm A. Halcrow
Article numbere202202578
<mark>Journal publication date</mark>10/02/2023
<mark>Journal</mark>Chemistry - A European Journal
Issue number9
Number of pages11
Publication StatusPublished
Early online date27/12/22
<mark>Original language</mark>English


Four bis[2‐{pyrazol‐1‐yl}‐6‐{pyrazol‐3‐yl}pyridine] ligands have been synthesized, with butane‐1,4‐diyl (L1), pyrid‐2,6‐diyl (L2), benzene‐1,2‐dimethylenyl (L3) and propane‐1,3‐diyl (L4) linkers between the tridentate metal‐binding domains. L1 and L2 form [Fe2(μ−L)2]X4 (X−=BF4− or ClO4−) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2(μ−L1)2][BF4]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin‐crossover, with examples of stepwise switching and partial spin‐crossover to a low‐temperature mixed‐spin form. Salts of [Fe2(μ−L2)2]4+ are high‐spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1−L3 vary with the ligand linker group, by mass spectrometry and 1H NMR spectroscopy. Gas‐phase DFT calculations imply the butanediyl linker conformation in [Fe2(μ−L1)2]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.