28 August 2019

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis

N-Heterocyclic carbene ligands (NHC) 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 [PdX 3]–[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 cation 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 via either Ph-NHC coupling or reversible H-NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

Reference: Chem. Eur. J., 2019, ASAP

DOI: 10.1002/chem.201903221

On-line version: https://onlinelibrary.wiley.com/doi/abs/10.1002/ch...