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

Abstract

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(SiMe 3 ) 2 } 2 ] 2 to yield the four-coordinate iron(II) complexes [LFeX 2 ] (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 1 H NMR and UV−vis spectroscopy, cyclic voltammetry, Mossbauer 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 [LFeX 2 ] are formed together with ferrous tetracarbene complexes [L 2 FeX 2 ] (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 [FeBr 2 (solv) x ], however, they transform into the ionic complexes [L 2 Fe(MeCN) 2 ][FeBr 4 ] (24−26), which have been characterized by single-crystal X-ray diffraction. They feature lowspin iron(II) (Mossbauer parameters δ ≈ 0.15 mm s −1 , ΔE Q ≈ 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 [LFeX 2 ] 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. ■ INTRODUCTIONN-heterocyclic carbenes (NHCs) have gained great popularity as ligands in organometallic chemistry and are finding widespread use in homogeneous catalysis. 1,2 They are usually viewed as strongly σ-donating ligands with little or negligible π back-bonding. Chemical and substitutional inertness is often mentioned among their particular advantages, and this is expected to be further amplified by incorporating the NHC moiety into a chelating scaffold. Mainly in the past decade a variety of iron complexes with monodentate NHC ligands 3−5 and several NHC ligated iron clusters have been published. 6 Some of the mononuclear iron NHC complexes 4,5 as well as species generated in situ from imidazolium salts and suitable iron precursors showed high activities in homogeneous catalysis. 7,8 However, only a f...

A Tetracarbene–Oxoiron(IV) Complex

“…19 While ESI‐MS shows [(L NHC )Fe(OTf) 2 ] + ( m / z =702.0, calcd m / z =702.0) and its fragments, X‐ray diffraction reveals that, instead of the anionic triflate counteranions, MeCN molecules are coordinated as axial ligands to give an octahedral environment for 1 in solid state (Figure 1, left). The well‐resolved NMR spectrum of 1 indicates low‐spin iron(II), and the Mössbauer isomer shift ( δ =0.23 mm s −1 ; Figure S1, see the Supporting Information) is in agreement with that of another octahedral low‐spin tetracarbene–iron(II) complex ( δ =0.15 mm s −1 ) 20. The large quadrupole splitting for 1 (Δ E Q =2.10 mm s −1 ) is unusual for the formally symmetric 3d 6 low‐spin configuration, but not unprecedented for complexes with strongly asymmetric covalent bonds 21–23.…”

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…13,14 We are interested in the properties and reactivity of low-valent late transition metal complexes, notably electron rich zero-valent palladium NHC complexes, as these often play a vital role in the catalytic cycles of many reactions. [15][16][17][18] Although a lot of research has been devoted to bis-carbene Pd II (halide) 2 complexes, usually obtained by treating a bisimidazolium salt with Pd(OAc) 2 at elevated temperatures, 7,[19][20][21][22][23] we are not aware of any previous reports on chelate [ …”

Iron complexes of a bidentate picolyl-NHC ligand: synthesis, structure and reactivity

“…2, the distorted tetrahedral geometry adopted by the Fe(II) centre is similar to those of the methylene-bridged biscarbene iron(II) dihalide complexes. [48][49][50] The bite angle of HL in 3 (89.1(1)°) is similar to that in 2. The Fe-C NHC (Fe1-C1 2.065(3) Å) and Fe-N pyridine (Fe1-N3 2.157(3) Å) bond lengths are shorter than those in 1 and more similar to those in 2.…”