Now showing 1 - 6 of 6
  • Publication
    Non-classical N-Heterocyclic carbene complexes
    (RSC Publishing, 2010) ;
    The expansion of the concept of N-heterocyclic carbenes as ligands for transition metals to mesoionic ligand systems has led to the discovery of a wide range of non-classical carbene-type ligands. These non-classical carbene-type ligands are characterised by a significantly lower heteroatom stabilisation of the (putative) free carbene, a situation that also affects the ligand donor properties and hence the reactivity of the coordinated metal centre. Based on the unique impact of non-classical carbene-type ligands, a number of attractive transition metal-catalysed processes have been disclosed in recent years, predominantly in the area of cross-coupling reactions, hydrogenations, and olefin metathesis
      623
  • Publication
    Chelating C4-bound imidazolylidene complexes via oxidative addition of imidazolium salts to palladium(0)
    Oxidative addition of donor-functionalised 4-iodoimidazolium salts to palladium(0) provides a selective route for the preparation of chelating abnormal N-heterocylic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate the imidazolium C2 position to be protected, leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N-heterocyclic carbene ligand was unsuccessful when palladium was bound to the C4 carbon, sequential metallation of first the C2 position via transmetallation followed by C4–I oxidative addition afforded a dimetallic complex comprising two palladium centres bridged by a single NHC ligand.
    Scopus© Citations 37  555
  • Publication
    Rhodium-mediated activation of an alkane-type C–H bond
    (RSC Publishing, 2010-01-14) ; ;
    Abnormal C4-bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp3)–H bond, whereas no such intramolecular C–H bond breaking is observed when the carbene binds normally through the C2 carbon.
    Scopus© Citations 33  444
  • Publication
    Rhodium carbene complexes as versatile catalyst precursors for Si–H bond activation
    (Wiley-VCH, 2012-01-09) ;
    Rhodium(III) complexes comprising monoanionic C,C,C-tridentate dicarbene ligands activate Si–H bonds and catalyse the hydrolysis of hydrosilanes to form silanols and siloxanes with concomitant release of H2. In dry MeNO2, selective formation of siloxanes takes place, while changing conditions to wet THF produces silanols exclusively. Silylethers are formed when ROH is used as substrate, thus providing a mild route towards the protection of alcohols with H2 as the only by-product. With alkynes, comparably fast hydrosilylation takes place, while carbonyl groups are unaffected. Further expansion of the Si–H bond activation to dihydrosilanes afforded silicones and polysilylethers. Mechanistic investigations using deuterated silane revealed deuterium incorporation into the abnormal carbene ligand and hence suggests a ligand-assisted mechanism involving heterolytic Si–H bond cleavage.
    Scopus© Citations 49  643
  • Publication
    Smooth C(alkyl)-H Bond Activation in Rhodium Complexes Comprising Abnormal Carbene Ligands
    Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C–H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the Calkyl–H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C–H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C–H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N–C–N amidinium unit and a metalla-allyl type M–C–C fragment.
    Scopus© Citations 38  424
  • Publication
    Abnormal N-heterocyclic Carbenes: More than just Exceptionally Strong Donor Ligands
    (CSIRO Publishing, 2011-08-19) ;
    Complexes comprising a so-called abnormal carbene ligand, which displays pronounced mesoionic character, have recently been shown to be competent catalyst precursors for bond activation processes and oxidative transformations, including base-free alcohol oxidation and water oxidation. In this highlight we propose that these abnormal carbene ligands are not just useful spectator ligands but also actively participate in the bond activation step. This mode of action is partially based on the exceptionally strong donor properties of the ligand and, specifically, on the mesoionic character of these abnormal carbenes. The mesoionic properties provide a reservoir for charges and holes and thus induce efficient ligand-metal cooperativity, which is beneficial in particular for oxidation catalysis that involves concerted proton and electron transfer processes.
    Scopus© Citations 97  532