Fig. 1. Stepwise reproduction of the CuAAC reaction; isolation of mono- and bis(copper) intermediates involved in both catalytic pathways.

(A) Stoichiometric reactions reproducing the different steps of the postulated CuAAC catalytic cycles, which allow for the isolation of a previously postulated π,σ-bis(copper) complex of type 1_Cu2 and of the never-mentioned bis(copper) triazole 2_Cu2. (B) Kinetic profiles of the stoichiometric reactions of 1_Cua and 1_Cu2a with benzyl azide, which reveal the critical rate acceleration when the bimetallic complex 1_Cu2a is used. (C) Molecular view of complexes 1_Cu2a (left, table S1), 2_Cu2a (center, table S2), and 1_Cu2b (right, table S3) in the solid state (for clarity, H atoms and anions are omitted).