N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

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Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

ion of the chloride ligands, compound 2 was further reacted with MeMgCl, yielding the bimetallic compound 3 in which the biscarbene ligand is bridging two Cr(II) atoms, also bridged by two methyl ligands. Recent studies by Arnold and co-workers showed that the incorporation of an anionic functional group (alkoxide, amido, or amino) in an NHC unit allows the preparation of tridentate ligands capable of coordinating to d0 early transition metals such as yttrium, titanium, and zirconium.87 The lithium aminodicarbene chloride complex 5 reacts with Y[N(SiMe3)2]3 affording the yttrium(III) complex 6 (Scheme 10). Hexacoordinated titanium (7) and zirconium (8) complexes were synthesized by reaction of 4 with M(NEt2)4 (M ) Ti, Zr) (Scheme 10). Complexes 7 and 8 constituted the first examples of group 3 and 4 metal complexes with monoanionic biscarbene ligands. Another interesting example of early transition metal NHC-based complexes was described by Smith and coworkers.88 By the reaction shown in Scheme 11, the tris(NHC)borate 989,90 afforded a tricarbonyl complex of Mn(I) with a tripodal tris-NHC ligand, 10. The analysis of the ν(CO) bands on the IR spectrum of this complex showed that 9 is the most electron-donating tripodal ligand compared with all others that have been bound to the same Mntricarbonyl fragment. Compound 10 is air-sensitive and is easily oxidized to a homoleptic Mn(IV) complex (11, Scheme 11), which is the first example of a Mn(IV)-NHC complex reported to-date. 4.2. Catalytic Applications Although many catalytic applications of late transition metal complexes bearing poly-NHC ligands have been described, those of early transition metals are restricted to a few examples. In 2003, Gibson and co-workers reported Cr(III)-based ethylene oligomerization precatalysts that incorporate a CNC-pincer carbene ligand.91,92 More recently, McGuinness shed some light on the mechanism of the reaction with the mentioned pincer complexes in combination with MAO.93 The Cr(II) and Cr(III) complexes described by Theopold were tested for polymerization of ethylene with a MAO cocatalyst, showing low activities and a broad weight distribution of the polymers.85 In particular, Cr(II) compounds such as 2 (Scheme 9) were unreactive to ethylene when exposed to MAO, showing that the more Lewis acidic Cr(III) performs better in this reaction. The authors pointed out the ease with which the Cr(III) complexes are reduced to Cr(II), arguing that very strong σ-donating but soft NHC ligands may have a stronger affinity for the softer Cr(II) rather than the harder Cr(III). Thus, they concluded that this ligand system may be better suited for lower oxidation state chromium chemistry and that, due to the inactivity of the Cr(II) compounds tested, it would not lead to successful results in ethylene polymerization. 5. Poly-NHC Ligands in Complexes of Group 8 Metals

Complexes with poly(N-heterocyclic carbene) ligands: structural features and catalytic applications.

With four site: Catalytic olefin hydrogenation is observed with palladium complexes when the N-heterocyclic carbene ligand is changed from the normal C2- to the unusual C4-bonding mode (see scheme). In a cis coordinating framework, the electron density at the metal center is substantially increased as a result of the exceptionally strong donor ability of C4-bound carbenes. (Chemical Equation Presented) © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

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Neutral Ligands with Exceptional Donor Ability for Palladium‐Catalyzed Alkene Hydrogenation

https://doc.rero.ch/record/13137/files/albrecht_cnr.pdf

Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H 2 activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

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On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes.

Copper(I) complexes comprising different N-heterocyclic carbene ligands were prepared via in situ deprotonation and metallation. Depending on the wingtip groups on the carbene ligand (alkyl vs. aryl, chelating or monodentate), a variety of different structural motifs were identified, such as a trigonal planar geometry (alkyl wingtips) and an unprecedented see-saw-type structure (pyridinyl wingtip groups). While aryl wingtip groups increase the stability of the complexes, alkyl substituents induce rapid demetallation in the presence of moisture. The reactivity of these complexes was used to establish a carbene-transfer protocol, which is illustrated by the formation of new cyclic thiourea compounds (transfer to sulfur) and new (carbene)ruthenium(II) complexes (transfer to ruthenium). This suggests that (carbene)copper(I) complexes could become valuable alternatives to (carbene)silver(I) complexes for synthesizing (carbene)metal systems via transmetallation.

Synthesis, Structural Diversity, and Ligand‐Transfer Potential of (Carbene)copper(I) Complexes

http://doc.rero.ch/record/11877/files/Heckenroth_Marion_-_Synthesis_and_structural_analysis_of_palladium_20090508.pdf

Marion Heckenroth

Papers

Neutral Ligands with Exceptional Donor Ability for Palladium‐Catalyzed Alkene Hydrogenation

Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes.

Synthesis, Structural Diversity, and Ligand‐Transfer Potential of (Carbene)copper(I) Complexes

Synthesis and structural analysis of palladium biscarbene complexes derived from bisimidazolium ligand precursors