On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Heckenroth, Marion; Neels, Antonia; Garnier, M. G.; Aebi, Philipp; Ehlers, Andreas W.; Albrecht, Martin

doi:10.1002/chem.200900249

Cited by 105 publications

(75 citation statements)

References 119 publications

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“…38 Structural disparity between the two complexes arises predominantly because of the different Pd-Cl bond lengths, as a consequence of the stronger trans influence of the C4-bound carbene ligand. However, the http://doc.rero.ch 6 reactivity of the complexes varies considerably.…”

Section: C-h Bond Activation Of C2-substituted Imidazolium Saltsmentioning

confidence: 99%

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

et al. 2009

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theme are found via the replacement or displacement of one 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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Section: C-h Bond Activation Of C2-substituted Imidazolium Saltsmentioning

confidence: 99%

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

et al. 2009

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“…A reactivity sequence that may model this catalyst activation step was recently disclosed by reacting the palladium complexes with chlorine rather than with hydrogen. [14] Exposure of complexes similar to 4 to Cl 2 afforded instantaneously the doubly chlorinated imidazolium salt with [PdCl 4 ] 2À as counterion. Substituting chlorine in this process with hydrogen would provide the protonated imidazolium salt, as observed in substoichiometric catalytic runs, and [PdH 4 ] 2À as precursor for palladium nanoparticles.…”

Section: Mechanistic Investigationsmentioning

confidence: 99%

“…[10,13] This enhanced donor ability provides access to new reactivity pathways. Specifically, the high electron density imparted to the metal center is surmised to facilitate oxidative addition reactions, [14] which are a key step in the metal-mediated activation of H 2 .…”

Section: Introductionmentioning

confidence: 99%

Catalytic Hydrogenation Using Abnormal N‐Heterocyclic Carbene Palladium Complexes: Catalytic Scope and Mechanistic Insights

Heckenroth¹,

Khlebnikov²,

Neels³

et al. 2010

ChemCatChem

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Palladium complexes containing abnormally bound C4‐bound dicarbene ligands have been exploited for catalytic alkene hydrogenation. Comparison to normally C2‐bound homologues indicates that the carbene bonding mode critically influences the catalytic activity. Good catalytic performance in the hydrogenation of cis‐disubstituted olefins and non‐isomerizable terminal olefins under mild conditions (RT, 0.1 MPa H2) only occurs when the carbene is abnormally bound to the palladium center. Detailed mechanistic investigations using dynamic light scattering in connection with time‐dependent analysis of conversions, and also performance of substoichiometric catalytic experiments provide evidence that the catalysis is heterogeneous and that the abnormally bound carbene ligand has the role of an activator.

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“…[1][2][3][4][5] Only recently it has been shown that an abnormal binding mode via the C4 (or C5) position of imidazolylidenes yields extraordinary electron rich metal centers. [6][7][8][9][10][11][12][13][14][15][16][17] These discoveries led to a considerably increased interest in "non-classical" NHCs, i.e. carbenes that are not stabilized by two adjacent heteroatoms.…”

Section: Introductionmentioning

confidence: 99%

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

et al. 2010

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On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Cited by 105 publications

References 119 publications

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

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

Catalytic Hydrogenation Using Abnormal N‐Heterocyclic Carbene Palladium Complexes: Catalytic Scope and Mechanistic Insights

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

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