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

The synthesis of dicopper(I) complexes [Cu2(BBAN)(MeCN)2](OTf)2 (1), [Cu2(BBAN)(py)2](OTf)2 (2),
[Cu2(BBAN)(1-Me-BzIm)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) Å 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) Å 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.