Ionic Liquid-Regulated Titanium Oxo Clusters Catalyze Selective Oxidation of Anilines to Azoxybenzenes

Selective oxidation of aniline is a highly effective way to produce azoxybenzene. Herewith, we constructed a series of ionic liquid-regulated titanium oxo clusters via a solvothermal method. The as-synthesized Ti oxo clusters were investigated by FT-IR, elemental analysis, ICP-AES, TGA, UV–vis, and high-resolution transmission electron microscopy (HRTEM). The ionic liquid ([C4imC2COOH][H2PO4], C4imC2COOH = 1-(2-carboxyethyl)-3-butyl imidazolium) played a crucial role not only in stabilizing titanium oxo clusters but also in enhancing the catalytic activity. Notably, the full conversion of aniline and excellent selectivity to azoxybenzene (more than 95%) were achieved with Ti oxo-[C4imC2COOH][H2PO4] as the titanium oxo-cluster catalyst. Moreover, the reaction was carried out smoothly at room temperature without any other additives. Mechanistic studies proved that the reaction was proceeded through a catalytic mechanism rather than a radical route. Besides, the further characterization provided clear evidence that the Ti-OOH species is an active intermediate involved in aniline's selective oxidation. The hydrogen bond interaction between the −NH2 group and C2-proton of the imidazolium cation played an important role in producing N-phenylhydroxylamine, a key intermediate, which was then oxidized into nitrosobenzene. The condensation between N-phenylhydroxylamine and nitrosobenzene afforded azoxybenzene quantitatively. The superior catalytic performance as well as the green and mild reaction conditions make this catalytic system highly promising for the oxidative coupling of aromatic amines in further applications.