N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I)

N-heterocyclic carbene complexes
of three- and four-coordinate
Fe­(I), [Fe­(LR)4]­[PF6] (LR = 1,3-R2-4,5-dimethylimidazol-2-ylidene, R = Me (2), Et (3), iPr (4))
and [Fe­(LMes)2(THF)]­[PF6] (5) (LMes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene),
were synthesized from successive reactions of [Fe­(toluene)2]­[PF6]2 with 1 equiv of KC8 and
LR (4 equiv for R = Me, Et, iPr; 2 equiv for
R = Mes). The coordination geometry of the iron atom in these complexes
varies depending on the nature of the R group in LR: a
tetrahedral geometry was observed for 2, a square-planar
one for 3 and 4, and a three-coordinate
T-shaped one for 5. In solution, 4 releases
LiPr ligand(s), while the LR ligands of the
other Fe­(I) complexes remain firmly bound. Tetrahedral 2 and T-shaped 5 contain a high-spin (S = 3/2) Fe­(I) center, whereas square-planar 3 and 4 contain Fe­(I) in the low-spin state (S = 1/2).