Insights Into the Exchange of N ‐Heterocyclic Carbene Ligands on Gold Nanoparticles: Necessity of Oxygen

Gold nanoparticles (AuNPs) stabilized by N ‐heterocyclic carbenes (NHCs) represent robust alternatives to thiol‐protected counterparts owing to the NHCs’ strong AuC bonds that provide enhanced stability. While NHC‐capped AuNPs with a single type of carbene have been reported many times, strategies to introduce multiple NHC ligands on the same nanoparticle remain unexplored. Here, we investigate NHC‐for‐NHC ligand exchange on AuNPs stabilized either by benzimidazol‐2‐ylidene or by mesoionic triazolylidene scaffolds, employing an in situ generated free carbene route. Prior to ligand exchange, the formation and stability of the studied NHCs from the corresponding azolium salts were examined and characterized by NMR spectroscopy. In situ NMR studies of the exchange reaction showed no evidence of ligand substitution under argon, the conventional atmosphere for free NHC intermediates. Although no exchange was detected by NMR, X‐ray photoelectron spectroscopy (XPS) analysis of the purified AuNPs revealed that ligand substitution had in fact occurred. Further investigation indicated that oxygen plays a key role in promoting the exchange, and that its involvement is also associated with oxidation of the gold core and the formation of NHC‐Au(I) complexes. Taken together, our results highlight the complexity of NHC‐for‐NHC exchange on AuNPs by the free carbene route and point to the need for alternative strategies to achieve controlled ligand substitution.