The
two-coordinate ring-expanded N-heterocyclic carbene copper(I)
complexes [Cu(RE-NHC)<sub>2</sub>]<sup>+</sup> (RE-NHC = 6-Mes, 7-<i>o</i>-Tol, 7-Mes) have been prepared and shown to be effective
catalysts under neat conditions for the 1,3-dipolar cycloaddition
of alkynes and azides. In contrast, the cationic diamidocarbene analogue
[Cu(6-MesDAC)<sub>2</sub>]<sup>+</sup> and the neutral species [(6-MesDAC)CuCl]<sub>2</sub> and [(6-MesDAC)<sub>2</sub>(CuCl)<sub>3</sub>] show good
activity when the catalysis is performed on water