Three- and Four-Coordinate Homoleptic Iron(I)–NHC Complexes: Synthesis and Characterization

The use of the monodentate N-heterocyclic carbene (NHC) 1,3,4,5-tetramethylimidazol-2-ylidene (IMe₂Me₂) and its N,N-diethyl and N,N-diisopropyl analogues IEt₂Me₂ and IPr₂Me₂ enabled the preparation of the four-coordinate homoleptic iron­(I)–NHC complexes [(NHC)₄Fe]­[BPh₄] (NHC = IMe₂Me₂, 1; IEt₂Me₂, 2; IPr₂Me₂, 3) and the first three-coordinate homoleptic iron­(I)–NHC complex, [(IPr₂Me₂)₃Fe]­[BPh₄] (5). X-ray crystallographic studies revealed the tetrahedral coordination geometries of the cations in 1 and 2, the square-planar geometry in 3, and the trigonal-planar geometry in 5. ⁵⁷Fe Mössbauer spectroscopy and magnetic susceptibility measurements suggested that 1, 2, and 5 are high-spin (S = ³/₂) iron­(I) species and that 3 is a low-spin (S = ¹/₂) complex. ¹H NMR and UV–vis–NIR spectroscopies revealed the existence of tetrahedral–square-planar isomerization (for 2 and 3) and NHC dissociation (for 3) in solutions of 2 and 3.