Synthesis, activation, and decomposition of N-heterocyclic carbene-containing complexes

NHC–transition metal complexes are described in terms of their synthesis, stability and utility as catalytic precursors. After a review of the more common methods of synthesis, the known decomposition reactions are described. These include displacement of the NHC from the metal coordination sphere, reductive elimination of the NHC and adjacent ligands, and C–H/C–C or C–N activation of the NHC ligand, in addition to other pathways such as migratory insertion reactions. With these decomposition routes in mind, the approaches required to prepare NHC–metal complexes that are resistant to degradation are depicted. Finally, useful methods for generating active catalysts from NHC–metal complexes are described, including, in some cases, results gleaned during studies of decomposition pathways. Taken together, these sections will provide the reader with a detailed knowledge of the expected stability of typical NHC complexes, in addition to methods of generating active catalysts from them