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

Four bis[2‐{pyrazol‐1‐yl}‐6‐{pyrazol‐3‐yl}pyridine] ligands have been synthesized, with butane‐1,4‐diyl (L (1)), pyrid‐2,6‐diyl (L (2)), benzene‐1,2‐dimethylenyl (L (3)) and propane‐1,3‐diyl (L (4)) linkers between the tridentate metal‐binding domains. L (1) and L (2) form [Fe(2)(μ−L)(2)]X(4) (X(−)=BF(4) (−) or ClO(4) (−)) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe(2)(μ−L (1))(2)][BF(4)](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 [Fe(2)(μ−L (2))(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 L (1)−L (3) vary with the ligand linker group, by mass spectrometry and (1)H NMR spectroscopy. Gas‐phase DFT calculations imply the butanediyl linker conformation in [Fe(2)(μ−L (1))(2)](4+) influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.