Easy Access to Metal-N-Heterocyclic Carbene Catalysts
Metal-N-heterocyclic carbene (NHC) complexes have been widely studied in the last decade and have been ubiquitously employed in homogeneous catalysis, especially in cross-coupling chemistry.1 Out of the numerous findings emerging from these studies are the significant advantages associated with metal-ligand stoichiometry control.
Palladium(II) complexes of the type [Pd(NHC)(ƞ3-R-allyl)Cl] (R-allyl = allyl, metallyl, cinnamyl, indenyl) have shown high catalytic activity in many important C-C and C-heteroatom bond formations.2 The original synthetic approach made use of a strong base (e.g. KOtBu) to generate the free carbene followed by addition of a suitable palladium precursor such as [Pd(ƞ3-allyl)(μ-Cl)]2 or [Pd(ƞ3-cinnamyl)(μ-Cl)]2.3 The method yielded complexes that are, much to our surprise, air- and moisture-stable, as phosphine congeners had been reported as sensitive. In order to render the synthesis of these pre-catalysts even more facile, cost-effective, sustainable and accessible, we have probed the effect of the nature of the base on numerous metal-centered systems bearing NHC ligands.4
Recently, we and others have reported on the use of an external weak base as an effective method to generate Au- and Cu-NHC complexes.5-6 In the course of our studies, an unusual intermediate was isolated, an “ate” complex, that could be easily observed within minutes of mixing the metal precursor, copper or gold and now palladium, and the imidazolium salt in air. Action of a weak base upon this intermediate led to the well-defined M-NHC complex (M= Au and Cu) (Figure 1). We suspected that such “ate” complexes might be more prevalent than imagined in the context of the organometallic chemistry of M-NHC complexes and might even offer a uniquely simple approach to catalysis.