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(mu-L)(2)]X-4 (X-=BF4- or ClO4-) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe-2(mu-L-1)(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 [Fe-2(mu-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 H-1 NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe-2(mu-L-1)(2)](4+) influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

Automated summary

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands were synthesized and their complexes with iron (II) were studied. The results show that the torsion of the linker group can influence the conformation and spin state of the complexes.