UC Berkeley

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).