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 [Cu-2(BBAN)(MeCN)(2)](OTf)(2) (1), [Cu-2(BBAN)(py)(2)] (OTf)(2) (2), [Cu-2(BBAN)(1-Me-BzIm)(2)](OTf)(2) (3), [Cu-2(BBAN)(1-Me-Im)(2)](OTf)(2) (4), and [Cu-2(BBAN)(mu -O2CCPh3)](OTf) (5), where BBAN = 2,7-bis((dibenzylamino)methyl)-1,8-naphthyridine, py = pyridine, 1-Me-Im = 1-methylimidazole, and 1-Me-BzIm = l-methylbenzimidazole, are described. Short copper-copper distances ranging from 2.6151(6) to 2.7325(5) Angstrom 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 E-1/2 = -25 mV vs Cp2Fe+/Cp2Fe. When complex 5 was treated with 1 equiv of silver(I) triflate, a mixed-valence dicopper(I,II) complex [Cu-2(BBAN)(mu -O2CCPh3)(OTf)](OTf) (6) was prepared. A short copper-copper distance of 2.4493(14) Angstrom 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,IJ) state. Compounds 1-4 with neutral nitrogen-donor terminal ligands cannot be oxidized to the mixed-valence analogues either chemically or electrochemically.