Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N‑Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution

The salt elimination reactions of (IPr₂Me₂)₂FeCl₂ (IPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr₂Me₂)₂FeAr₂] (Ar = Ph, 3; C₆H₄-p-Me, 4; C₆H₄-p-^tBu, 5; C₆H₃-3,5-(CF₃)₂, 6) in good yields. X-ray crystallographic studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and ⁵⁷Fe Mössbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, respectively. Solution property studies, including solution magnetic susceptibility measurement, variable-temperature ¹H and ¹⁹F NMR, and absorption spectroscopy, on 3–6, as well as an ⁵⁷Fe Mössbauer spectrum study on a frozen tetrahydrofuran solution of tetrahedral [(IPr₂Me₂)₂⁵⁷FePh₂] suggest the coexistence of tetrahedral and trans square planar structures in solution phase. Density functional theory calculations on (IPr₂Me₂)₂FePh₂ disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron­(II) center.