Dicopper Cu(I)Cu(I) and Cu(I)Cu(II) Complexes in Copper-Catalyzed Azide–Alkyne Cycloaddition

A discrete, dicopper μ-alkynyl complex, [Cu 2 (μ-η 1:η 1 - C≡C(C 6 H 4 )CH 3 )DPFN]NTf 2 (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; NTf 2 – = N(SO 2 CF 3 ) 2 – ), reacts with p -tolylazide to yield a dicopper complex with a symmetrically bridging 1,2,3-triazolide, [Cu 2 (μ-η 1:η 1 -(1,4-bis(4-tolyl)-1,2,3-triazolide))DPFN]NTf 2 . This transformation exhibits bimolecular reaction kinetics and represents a key step in a proposed, bimetallic mechanism for copper-catalyzed azide–alkyne cycloaddition (CuAAC). The μ-alkynyl and μ-triazolide complexes undergo reversible redox events (by cyclic voltammetry), suggesting that a cycloaddition pathway involving mixed-valence dicopper species might also be possible. Synthesis and characterization of the mixed-valence μ-alkynyl dicopper complex, [Cu 2 (μ-η 1:η 1 - C≡C(C 6 H 4 )CH 3 )DPFN](NTf 2 ) 2, revealed an electronic structure with an unexpected partially delocalized spin, as evidenced by electron paramagnetic resonance spectroscopy. Studies of the mixed-valence μ-alkynyl complex’s reactivity suggest that a mixed-valence pathway is less likely than one involving intermediates with only copper(I).