Activating Azides and Alkynes for the Click Reaction with [Cu(aNHC)2I] or [Cu(aNHC)2]+ (aNHC = Triazole-Derived Abnormal Carbenes): Structural Characterization and Catalytic Properties

Neutral, iodido-containing copper(I) complexes [Cu(aNHC)(2)I] {aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 6) and 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 7)} and cationic, halide-free copper(I) complexes [Cu(aNHC)(2)](BF4) {aNHC = 1-benzyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 8), 3-methyl-1,4-diphenyl-1,2,3-triazol-5-ylidene (for 9), 3-methyl-1-[2-(methylthio)phenyl]-4-phenyl-1,2,3-triazol-5-ylidene (for 10), and 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (for 11)}, both containing two monodentate abnormal-carbene ligands (aNHC), were synthesized from [Cu(CH3CN)(4)](BF4) and the corresponding triazolium salts. It was possible to selectively synthesize both kinds of complexes by simply variing the counter-anion of the triazolium salts and keeping the metal precursor the same. All complexes were characterized by elemental analysis and spectroscopic methods. 6 and 11 were studied with single-crystal X-ray diffraction analyses. In 6, the copper(I) center is tricoordinated, and its geometry is in between trigonal planar and T-shaped. In halide-free 11, the copper(I) center is linearly coordinated by two abnormal-carbene ligands. All complexes were tested as catalysts in the Huisgen [3+2] cycloaddition reaction between azides and alkynes, and they showed excellent efficiencies under neat conditions. A comparison between the efficiencies of the halide-containing complexes 6 and 7 and the halide-free cases 8-11, shows that the halide-free Cu-aNHC complexes are significantly more efficient than their halide-containing counterparts. The best catalyst was used for a substrate screening by utilizing a variety of azides and a couple of alkynes. The efficiency of the catalyst was maintained with loadings as low as 0.005 mol-%. Mechanistic studies were carried out as well.