Synthesis and Characterization of Di- and Tetracarbene Iron(II) Complexes with Chelating N-Heterocyclic Carbene Ligands and Their Application in Aryl Grignard–Alkyl Halide Cross-Coupling

A series of new and known bis­(imidazolium) chloride and
bromide
salts bridged by either a methylene group (1–8, 10a,b) or an ethylene group (9a,b) and bearing different N substituents (Me,
Et, Bn, tBu, Mes) have been reacted with [Fe­{N­(SiMe3)2}2]2 to yield the four-coordinate
iron­(II) complexes [LFeX2] (11–20; X = Cl, Br; L = chelating bis­(imidazolylidene) ligand).
Molecular structures of six of these complexes have been characterized
by X-ray crystallography, and selected examples have been characterized
by 1H NMR and UV–vis spectroscopy, cyclic voltammetry,
Mössbauer spectroscopy, and SQUID magnetometry. In all cases
the iron­(II) is found in a distorted-tetrahedral environment; it is
in the high-spin state and shows large quadrupole splittings in the
range 3.67–4.03 mm·s–1 (δ = 0.73–0.81
mm·s–1). Subtleties of the metric parameters depend
on the bridging unit between the two imidazolylidene groups, the peripheral
N substituents, and the coligand (Cl or Br). In case of rather small
(Me, Et) or flexible (Bn) N substituents the dicarbene species [LFeX2] are formed together with ferrous tetracarbene complexes
[L2FeX2] (21–23), which are difficult to separate and could not be isolated in pure
form. When the latter are dissolved in MeCN in the presence of residual
[FeBr2(solv)x], however, they transform into the ionic complexes [L2Fe­(MeCN)2]­[FeBr4] (24–26), which have been characterized by single-crystal X-ray diffraction.
They feature low-spin iron­(II) (Mössbauer parameters δ
≈ 0.15 mm s–1, ΔEQ ≈ 1.36 mm s–1) and distorted-octahedral
structures with the two MeCN ligands in a cis configuration. Selected
examples of the new dicarbene complexes [LFeX2] have been
tested as catalysts for the standard cross-coupling reaction between p-tolylmagnesium bromide and bromo- or chlorocyclohexane.
They show moderate activity that appears to be generally lower than
for related complexes with two monodentate NHC ligands, but the activities
clearly depend on the peripheral N substituents and the linker between
the two imidazolylidene groups; the best results are obtained for
complex 19, which features a long ethylene bridge and
bulky Mes substituents, and hence the most shielded metal center.