Search

B Understanding dynamic interplay b between the molecular and ionic forms of metal/N-heterocyclic carbene (M/NHC) complexes is the key to develop efficient catalysts with N-heterocyclic carbene ligands. B Understanding dynamic interplay b between the molecular and ionic forms of metal/N-heterocyclic carbene (M/NHC) complexes is the key to develop efficient catalysts with N-heterocyclic carbene ligands. Carbene ligands, dynamic catalysis, electrospray ionization, mass spectrometry, Mizoroki-Heck reaction. [Extracted from the article]

N‐Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M–NHC binding in this metal–ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M−NHC bond (rather than its stabilization) could be more important for high‐performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal–NHC binding and [PdX3]−[NHC‐R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC‐R]+ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well‐defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC‐free anionic Pd in polar media through either Ph–NHC coupling or reversible H–NHC coupling. The present study formulates novel ideas for M/NHC catalyst design. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue are Valentine P. Ananikov and co‐workers. The image depicts the dynamic behaviour of a Pd/NHC catalytic system with easy transition from molecular to ionic complex. Read the full text of the article at 10.1002/chem.201903221. [ABSTRACT FROM AUTHOR]