A Planar π‑Conjugated Naphthyridine-Based N‑Heterocyclic Carbene Ligand and Its Derived Transition-Metal Complexes

HL·(PF6) (L = 2,4-dimethyl-8-phenyl­[1,2,4]­triazolo­[4,3-a]­[1,8]­naphthyridin-9-ylidene) was synthesized from the
nucleophilic reaction of 7-chloro-2,4-dimethyl-1,8-naphthyridine with
phenylhydrazine and subsequent acidification, anion exchange, and
condensation with triethyl orthoformate. Its silver, copper, cobalt,
and nickel complexes [Ag2(L)2(CH3CN)2]­(PF6)2 (1), [CuL­(CH3CN)2]­(PF6) (2), [CuL­(phen)]­(PF6) (3), [CuL­(dppe)]­(PF6) (4), [Co­(L)2(CH3CN)2]­(PF6)2 (5), and [Ni­(L)3]­(PF6)2 (6) have been synthesized and fully characterized
by NMR, elemental analysis, and X-ray diffraction analysis. The copper
complex 2 exhibits excellent catalytic activity in the
Cu­(I)-catalyzed azide–alkyne cycloaddition reaction of 2,2,6,6-tetramethylpiperidinyl-1-oxy-tethered
alkynes in an air atmosphere at 50 °C.