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

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Abstract

A series of new and known bis(imidazolium)
\nchloride and bromide salts bridged by either a methylene
\ngroup (1−8, 10a,b) or an ethylene group (9a,b) and bearing
\ndifferent N substituents (Me, Et, Bn, tBu, Mes) have been
\nreacted with [Fe{N(SiMe3)2}2]2 to yield the four-coordinate
\niron(II) complexes [LFeX2] (11−20; X = Cl, Br; L = chelating
\nbis(imidazolylidene) ligand). Molecular structures of six of
\nthese complexes have been characterized by X-ray crystallography,
\nand selected examples have been characterized by 1H
\nNMR and UV−vis spectroscopy, cyclic voltammetry, Mössbauer
\nspectroscopy, and SQUID magnetometry. In all cases the iron(II) is found in a distorted-tetrahedral environment; it is in the
\nhigh-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
\nmetric parameters depend on the bridging unit between the two imidazolylidene groups, the peripheral N substituents, and the
\ncoligand (Cl or Br). In case of rather small (Me, Et) or flexible (Bn) N substituents the dicarbene species [LFeX2] are formed
\ntogether with ferrous tetracarbene complexes [L2FeX2] (21−23), which are difficult to separate and could not be isolated in pure
\nform. When the latter are dissolved in MeCN in the presence of residual [FeBr2(solv)x], however, they transform into the ionic
\ncomplexes [L2Fe(MeCN)2][FeBr4] (24−26), which have been characterized by single-crystal X-ray diffraction. They feature lowspin
\niron(II) (Mössbauer parameters δ ≈ 0.15 mm s−1, ΔEQ ≈ 1.36 mm s−1) and distorted-octahedral structures with the two
\nMeCN ligands in a cis configuration. Selected examples of the new dicarbene complexes [LFeX2] have been tested as catalysts for
\nthe standard cross-coupling reaction between p-tolylmagnesium bromide and bromo- or chlorocyclohexane. They show moderate
\nactivity that appears to be generally lower than for related complexes with two monodentate NHC ligands, but the activities clearly
\ndepend on the peripheral N substituents and the linker between the two imidazolylidene groups; the best results are obtained for
\ncomplex 19, which features a long ethylene bridge and bulky Mes substituents, and hence the most shielded metal center.

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