Tetranuclear and trinuclear copper(I) pyrazolates as catalysts in copper mediated azide-alkyne cycloadditions (CuAAC).

Homoleptic, tetranuclear copper(I) pyrazolates {[3,5-(-Bu)Pz]Cu}, {[3-(CF)-5-(-Bu)Pz]Cu}, and {[4-Br-3,5-(-Pr)Pz]Cu} are excellent stand-alone catalysts for azide-alkyne cycloaddition reactions (CuAAC). This work demonstrates that a range of pyrazolates, including those with electron donating and electron-withdrawing groups to sterically demanding substituents on the pyrazolyl backbones, can serve as effective ligand supports on tetranuclear copper catalysts. However, in contrast to the tetramers and also highly fluorinated {[3,5-(CF)Pz]Cu}, trinuclear copper(I) complexes such as {[3,5-(-Pr)Pz]Cu} and {[3-(CF)-5-(CH)Pz]Cu} supported by relatively electron rich pyrazolates display poor catalytic activity in CuAAC. The behavior and degree of aggregation of several of these copper(I) pyrazolates in solution were examined using vapor pressure osmometry. Copper(I) complexes such as {[3,5-(CF)Pz]Cu} and {[3-(CF)-5-(-Bu)Pz]Cu} with electron withdrawing pyrazolates were found to break up in solution to different degrees producing smaller aggregates while those such as {[3,5-(-Pr)Pz]Cu} and {[3,5-(-Bu)Pz]Cu} with electron rich pyrazolates remain intact. In addition, kinetic experiments were performed to understand the unusual activity of tetranuclear copper(I) pyrazolate systems.