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

A discrete, dicopper μ-alkynyl complex, [Cu2(μ-η1:η1-CC­(C6H4)­CH3)­DPFN]­NTf2 (DPFN = 2,7-bis­(fluoro-di­(2-pyridyl)­methyl)-1,8-naphthyridine; NTf2– = N­(SO2CF3)2–), reacts with p-tolylazide to yield a dicopper complex with a symmetrically bridging 1,2,3-triazolide, [Cu2(μ-η1:η1-(1,4-bis­(4-tolyl)-1,2,3-triazolide))­DPFN]­NTf2. 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, [Cu2(μ-η1:η1-CC­(C6H4)­CH3)­DPFN]­(NTf2)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).