Influence of the halide and exposed facets on the visible-light photoactivity of bismuth oxyhalides for selective aerobic oxidation of primary amines

Bismuth oxyhalides BiOX (X = Cl, Br, I) are seldom applied as photocatalysts in organic synthesis. Herein, we investigated their potential for the aerobic oxidative coupling of benzylamine to N-benzylidenebenzylamine, as imines are important synthetic intermediates of pharmaceuticals and biologically active nitrogen-containing organic compounds. The influence of the halide and the exposed crystal facets on the photoactivity was investigated. BiOBr showed excellent photoactivity, surpassing BiOCl and BiOI, which have poor light absorption and oxidation ability, respectively. Three differently facetted BiOBr photocatalysts were synthesized. The {001}-facetted BiOBr exhibited higher intrinsic activity than the {010}- or {110}-samples due to efficient charge separation. However, surface area plays an important role as reactions usually occur at the catalyst surface. Because of their much larger surface area, the solvothermally-synthesized BiOBr microspheres with {110}-dominant exposed facet showed the highest photooxidative activity, with 100% conversion and 100% selectivity to N-benzylidenebenzylamine after 14 h visible light irradiation at room temperature using oxygen from atmospheric air. This work provides an economical, feasible, sustainable and green process for the synthesis of imines and illustrates the great potential of bismuth oxyhalides as photocatalysts for organic synthesis.