N-Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis§

Authors

  • Prof. Dr. Wolfgang A. Herrmann,

    Corresponding author
    1. Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
    • Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
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  • Martina Elison,

    1. Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
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  • Jakob Fischer,

    1. Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
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  • Christian Köcher,

    1. Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
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  • Georg R. J. Artus

    1. Anorganisch-chemisches Institut der Technischen Universität München Lichtenbergstraße 4, D-85747 Garching (Germany) Fax: Int. code +(89)2891-3473
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  • Carbenes may be defined as “divalent” carbon compounds. More precisely, the carbene carbon is linked to two adjacent groups by covalent bonds and possesses two nonbonding electrons, which may have antiparallel spins (singlet state) or parallel spins (triplet state). In other compounds with formally “divalent” carbons (e.g., carbon monoxide, isocyanides), resonance stabilization of the “divalent” carbon by neighbouring groups is extremely strong. The term carbene, conceived by Doering, Winstein and Woodward, seems well suited to describe the class of compounds under discussion, but the carbene nomenclature conflicts with established rules of the International Union of Pure and Applied Chemistry (IUPAC). The nomenclature for metal-carbene complexes was introduced by E. O. Fischer [2a–d] and became common in the organometallic literature.

  • Heterocyclic Carbenes, Part 5. Part 4: see ref. [20].

  • §

    Dedicated to Professor Max Herberhold on the occasion of his 60th birthday

Abstract

Stable mono- and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloro- and acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene). A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia–the liquid ammonia route–is described. This method yields not only the known free monimeric 1,3-dimethylimidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethyl-imidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhI PdII, OsII and IrI are presented. Reaction of the potentially chelating ligand 1,1′-(1,2-ethylene)-3,3′-dimethyldiimidazoline-2,2′-diylidene with [(COD)-RhCl]2 yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, Os) are reported. N-Heterocyclic carbene complexes of Group 8–10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal–carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis-basic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.

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