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. 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.