Synthesis of Mono- and Binuclear Cu(II) Complexes Bearing Unsymmetrical Bipyridine–Pyrazole–Amine Ligand and Their Applications in Azide–Alkyne Cycloaddition

Mono- and binuclear Cu­(II) complexes bearing an unsymmetrical bipyridine–pyrazole–amine ligand were synthesized and characterized using X-ray diffraction. The mononuclear complex could be converted to the corresponding binuclear complexes under basic conditions due to the lability of the pyrazolyl N–H. Both complexes proved to be effective catalysts for azide–alkyne cycloaddition to form triazoles, with the binuclear complex exhibiting higher catalytic activity than the corresponding mononuclear one. The binuclear complex was effective at catalyst loadings as low as 0.0125 mol %, making it one of the most active catalysts for this reaction to date. Therefore, this catalyst was applied in the synthesis of potentially biologically active molecules. At 0.1–0.3 mol % catalyst loading, three precursors of Sorafenib analogs were synthesized in excellent yields. A one-pot variant of the reaction, generating the azide in situ, could also be performed using the binuclear complex as the catalyst. The transition metal complex bearing an unsymmetrical ligand may exhibit excellent catalytic activity, which represents a direction for developing new highly active catalysts.