What is the real catalyst in M/NHC systems? From molecular complexes to cocktail-type catalysis

Complexes of transition metals with N-heterocyclic carbene (NHC) ligands are often introduced and optimized as well-defined molecular precatalysts. However, the collected mechanistic evidence shows that M/NHC systems do not operate in a single universal manner. Depending on the metal, substrate, base, temperature, and ligand environment, the same precatalyst may function in a classical molecular mode with a preserved M – NHC bond, or it may evolve into an NHC-disconnected catalytic manifold composed of ionic metal species, clusters, and nanoparticles. The key mechanistic turning point is cleavage of the M – NHC framework, which can be initiated by NHC reductive elimination such as R–NHC, H–NHC, O–NHC, or N–NHC coupling, as well as by solvolysis or direct transformation. In this mini-review, the development of the mechanistic basis of M/NHC catalysis is outlined: from early evidence for facile lability of the M–NHC framework and the discovery of a new cocktail-type operating mode for Pd/NHC systems to the formulation of dual stabilization and fast/slow release scenarios. A complete classification of the operating states of M/NHC catalytic systems and a kinetic framework describing how these states are reached in practice are described. Taken together, these studies show that NHC ligands can stabilize both molecular active species and dynamic ensembles of metal particles of varying nuclearity, and that the balance between these possibilities is controlled predominantly by reaction conditions rather than by the static structural formula of the M/NHC precatalyst.