Linear and T‑Shaped Iron(I) Complexes Supported by N‑Heterocyclic Carbene Ligands: Synthesis and Structure Characterization

The use of the N-heterocyclic carbene (NHC) ligands 1,3-bis­(2′,6′-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene (cyIDep), 1,3-bis­(2′,6′-diethylphenyl)-imidazolin-2-ylidene (sIDep), and its N-mesityl analogue sIMes enables the preparation of the two-coordinate homoleptic iron­(I)-NHC complexes [(cyIDep)2Fe]­[BArF4] (3, ArF denoted for 3,5-di­(trifluoromethyl)­phenyl) and [(sIDep)2Fe]­[BArF4] (4) and the T-shaped iron­(I)-NHC complex [(sIMes)2Fe­(THF)]­[BPh4] (5, THF = tetrahydrofuran). Complexes 3–5 were prepared via the sequential protocol of control reduction of iron­(II) dihalides by KC8 in the presence of the corresponding NHC ligands followed by halide-abstraction with NaBAr4. Spectroscopic characterization, including single-crystal X-ray diffraction studies and 57Fe Mössbauer spectroscopy, in combination with density functional theory calculations, suggest their high-spin nature. Solution property study (absorption spectroscopy and cyclic voltammetry) indicates that 3 and 5 keep their corresponding two- and three-coordinate nature in THF solution, and 4 might reversibly coordinate a THF molecule to form, presumably, the T-shaped species [(sIDep)2Fe­(THF)]­[BArF4]. The isolation of 3 and 4 demonstrates the accessibility of homoleptic two-coordinate iron­(I)-NHC complexes.