Dicopper(I) Complexes of a Binucleating, Dianionic, Naphthyridine Bis(amide) Ligand

The dinucleating ligand, 1,8-naphthyridine-2,7-bis(2,6-diisopropylphenyl)carboxamide (NBDA), was synthesized by palladium-catalyzed aminocarbonylation. This ligand was treated with two equivalents of mesitylcopper in the presence of [ⁿBu₄N]X (X = Cl, N₃) to give the anionic complexes [ⁿBu₄N][Cu₂(NBDA)(μ-Cl)] and [ⁿBu₄N][Cu₂(NBDA)(μ-N₃)]. Treatment of H₂NBDA with mesitylcopper and two equivalents of xylyl isocyanide led to the formation of a charge-neutral dicopper(I) complex, [Cu₂(NBDA)(CNXyl)₂], displaying two isocyanide ligands, each terminally bound to one of the copper atoms. The complexes were characterized by NMR and IR spectroscopy, as well as by single-crystal X-ray diffraction analysis. Electrochemical characterization of the complexes using cyclic voltammetry revealed a reversible ligand-based reduction between –1.65 and –2.0 V vs. Fc/Fc⁺. DFT calculations suggest a more ionic bonding character and weaker Cu–Cu interactions in the NBDA complexes compared to those with other 1,8-naphthyridine-based ligands. This is congruent with intermetallic separations of over 3 Å induced by relatively strong coordination of the copper atoms to the amide nitrogen donor atoms observed in the solid state molecular structures.