Solvent- and Additive-Free Dehydrogenation of N-Heterocycles with Oxygen Catalyzed by Polyoxovanadate-Based Metal–Organic Frameworks

N-heteroarenes are a family of organics with significant chemical and pharmaceutical applications. They are generally prepared by the catalytic oxidative dehydrogenation (ODH) of partially saturated N-heterocycles. In this work, we prepare and demonstrate the catalytic ODH applications of two polyoxovanadate-based metal–organic frameworks of the general formula {[MII(bibp)1.5][VV2O6]}·H2O (M = Ni 1, Co 2; bibp = 4,4′-bis(imidazol-1-ylmethyl)biphenyl). They are based on nonprecious metals, need no additives or organic solvents typically required for catalytic ODH, and utilize molecular O2 as the oxidant, thus possessing all the traits desirable for practical catalysis. Catalyst 1 shows tolerance for a range of substrates with different electronic and steric features, including 2,3-dihydro-1H-indole and tetrahydroquinolines substituted with various functional groups. Mechanistic studies supported primarily by evidence from electron paramagnetic resonance and X-ray photoelectron spectra suggest that the VV sites in 1 are catalytically responsible, first enabling the formation of the substrate-based radical species by a single electron transfer event while being converted into its mixed-valence form, followed by the production of the superoxide radical anion (O2•–) upon contact with O2. The reaction mixture containing O2•– and the initially formed substrate-based radical then undergoes a series of steps, including the hydrogen abstraction and formation of the hydroperoxyl radical, the production and tautomerization of the partially dehydrogenated intermediate, and finally a repeating cycle of the aforementioned steps, to achieve the high-yield conversion of substrates to the corresponding N-heteroarenes.