New copper(i) complexes of bulky 5-substituted-2-iminopyrrolyl ligands as catalysts for azide–alkyne cycloaddition

Abstract

Five dinuclear copper(I) complexes of the type [Cu{κN,κN′-5-R-NC₄H₂-2-C(H)N(2,6-iPr₂C₆H₃)}]₂ (1a–e; R = 2,4,6-iPr₃C₆H₂ (a), R = 2,6-Me₂C₆H₃ (b), R = 3,5-(CF₃)₂C₆H₃ (c), R = 2,6-(OMe)₂C₆H₂ (d), R = CPh₃ (e)) were prepared by the reaction of the respective 5-R-2-iminopyrrolyl potassium salts KLa–e and [Cu(NCMe)₄]BF₄ in moderate yields. These new copper(I) complexes were characterized by NMR spectroscopy, elemental analysis and, in selected cases, by single crystal X-ray diffraction and their structural and electronic features further analyzed by DFT calculations and cyclic voltammetry, respectively. X-ray diffraction studies reveal dimeric Cu structures assembled by 2-iminopyrrolyl bridging ligands adopting a transoid conformation (complexes 1a and 1d), while complexes 1c and 1e displayed a cisoid conformation of those moieties, with respect to the Cu(I) centers. Additionally, VT-¹H NMR and ¹H–¹H NOESY NMR experiments on complexes 1a–e exhibited complex fluxional processes in solution, assigned to a conformational inversion of the respective Cu₂N₄C₄ metallacycles in all complexes but 1c, accompanied by a cisoid–transoid isomerization in the cases of complexes 1d,e. The Cu(I) complexes were also analyzed by cyclic voltammetry, where all complexes exhibit two oxidation processes, where the first oxidation is reversible, with the exception of 1b and 1c, which also show the highest oxidation potentials. The oxidation potentials follow clear trends related to the structural parameters of the complexes, in particular the Cu⋯Cu distance and the Cu₂N₄C₄ macrocycles torsion angles. All new 5-substituted-2-iminopyrrolyl Cu(I) complexes 1a–e served as catalysts for azide–alkyne cycloaddition (CuAAC) reactions, being able to generate the respective 1,2,3-triazole products in yields as high as 82% and turnover frequencies (TOFs) as high as 859 h⁻¹, after optimizing the conditions. The activity, as measured by the TOF, is in accordance with the oxidation potential of the corresponding complexes, the easier the oxidation, the higher the TOF value. Complex 1-H, where R = H, proved to be a poor catalyst for the same reactions, indicating that the 5-substitution in the ligand framework is crucial in stabilizing any potential catalytic species.