Heteroleptic Bis(N-heterocyclic carbene)Copper(I) Complexes: Highly Efficient Systems for the [3+2] Cycloaddition of Azides and Alkynes

The first examples of heteroleptic bis-N-heterocyclic carbene (NHC) copper(I) complexes and a mixed NHC–phosphine Cu complex are reported. These complexes are easily synthesized from the reaction of [Cu(OH)(NHC)] with various imidazol(idin)ium or phosphonium tetrafluoroborate salts. These cationic heteroleptic bis-NHC Cu complexes are highly active systems for the azide–alkyne cycloaddition leading to the formation of 1,2,3-triazoles. The mechanism of this transformation was investigated, and information gathered suggests that only one NHC remains coordinated to the metal center during catalysis.