Trinuclear copper(I) pyrazolate [Cu(3,5-(CF3)(2)Pz)](3) was employed in a mechanistic study of azide-alkyne cycloaddition (CuAAC). It was shown that the copper complex operates as a bifunctional catalytic system (copper source and Bronsted base) under mild conditions (RT and air atmosphere) at low catalyst loading (1 mol%). The rate-determining step of the reaction is the first C-N bond formation between azide and acetylene (azide migratory insertion) but not the copper(I) acetylide formation as commonly assumed. It was supported by the observed kinetic isotope effect equaling 1.1 and by DFT calculations. The reaction has a second-order dependence on the catalyst concentration implicating the two copper(I) centers participate in the metallacycle formation step from "bis-butterfly" tetranuclear complex Cu4L4(RC CH)(2) in the rate-determining step. DFT calculations proved that the pyrazolate ligand acts as a Bronsted base and deprotonates the acetylene molecule providing successful catalysis. A plausible catalytic cycle of the reaction has constructed based on DFT calculations. (C) 2020 Elsevier Inc. All rights reserved.