Dicopper Alkyl Complexes: Synthesis, Structure, and Unexpected Persistence

Cationic μ-alkyl dicopper complexes [Cu2(μ-η1:η1-R)DPFN]NTf2 (R = CH3, CH2CH3, CH2C(CH3)3; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine NTf2– = N(SO2CF3)2–) were synthesized by treatment of the acetonitrile-bridged dicopper complex [Cu2(μ-η1:η1-NCCH3)DPFN](NTf2)2 with LiR or MgR2. Structural characterization by X-ray crystallography and NMR spectroscopy revealed that the alkyl ligands symmetrically bridge the two copper centers, and the complexes persist in room-temperature solution. Notably, the μ-methyl complex showed less than 20% decomposition after 34 days in room-temperature THF solution. Treatment of the μ-methyl complex with acids allows installation of a range of monoanionic bridging ligands. However, surprisingly insertion into the dicopper–carbon bond was not observed upon addition of a variety of reagents, suggesting that these complexes exhibit a fundamentally new reactivity profile for alkylcopper species. Electrochemical characterization revealed oxidation–reduction events that evidence putative mixed-valence dicopper alkyl complexes. Computational studies suggest that the dicopper–carbon bonds are highly covalent, possibly explaining their remarkable stability.