Synthesis and characterization of several dicopper(I) complexes and a spin-delocalized dicopper(I,II) mixed-valence complex using a 1,8-naphthyridine-based dinucleating ligand

Article abstract of DOI:10.1021/ic0005339

The synthesis of dicopper(I) complexes [Cu₂(BBAN)(MeCN)₂](OTf)₂ (1), [Cu₂(BBAN)(py)₂](OTf)₂ (2), [Cu₂(BBAN)(1-Me-Bzlm)₂](OTf)₂ (3), [Cu₂(BBAN)(1-Me-Im)₂](OTf)₂ (4), and [Cu₂(BBAN)(μ-O₂CCPh₃)](OTf) (5), where BBAN = 2,7-bis((dibenzylamino)methyl)-1,8-naphthyridine, py = pyridine, 1-Me-Im = 1-methylimidazole, and 1-Me-BzIm = 1-methylbenzimidazole, are described. Short copper-copper distances ranging from 2.6151(6) to 2.7325(5) A were observed in the solid-state structures of these complexes depending on the terminal ligands used. The cyclic voltammogram of compound 5 dissolved in THF exhibited a reversible redox wave at E₁/₂ = -25 mV vs Cp₂Fe⁺/Cp₂Fe. When complex 5 was treated with 1 equiv of silver(I) triflate, a mixed-valence dicopper(I,II) complex [Cu₂(BBAN)(μ-O₂CCPh₃)(OTf)](OTf) (6) was prepared. A short copper-copper distance of 2.4493(14) A observed from the solid-state structure indicates the presence of a copper-copper interaction. Variable-temperature EPR studies showed that complex 6 has a fully delocalized electronic structure in frozen 2-methyltetrahydrofuran solution down to liquid helium temperature. The presence of anionic ligands seems to be an important factor to stabilize the mixed-valence dicopper(I,II) state. Compounds 1-4 with neutral nitrogen-donor terminal ligands cannot be oxidized to the mixed-valence analogues either chemically or electrochemically.