Light-activated Ullmann homocoupling of aryl halides catalyzed using gold nanoparticle-functionalized potassium niobium oxides

Lamellar, or layered, potassium niobium oxide perovskites are a class of underdeveloped semiconductors in organic photocatalysis that offer the inherent advantages of larger particle size and ease of recoverability as compared to traditional semiconductor materials. Using photochemical methodologies, gold nanoparticle-functionalized potassium niobium oxides are synthesized. Nanocomposite characterization using UV-visible spectroscopy, X-ray diffraction and TEM confirms nanoparticle deposition on the perovskite surface, with an average nanoparticle diameter of 17 nm. High resolution imaging and X-ray diffraction also confirmed the crystallinity of the niobium oxide support with an estimated interlayer spacing of 10 angstrom. Given the importance of carbon-carbon bond formation in organic synthesis, the Ullmann homocoupling of aryl halides is examined as a probe reaction for the application of this new class of nanocomposites. The influence of nanoparticle dopant, semiconductor support, reaction solvent, reaction time and aryl substitution are examined and shows that UVA-activation of gold nanoparticle/potassium niobium oxides promotes carbon-carbon bond formation in as little as 1 hour with yields as high as 98%, with high recyclability of the catalyst. The experimental methodology shows good versatility for a series of substituted iodobenzenes. The suggested mechanism involves a single electron transfer from the nanoparticle to facilitate aryl-halide bond activation to drive adsorption of the aryl halide starting material onto the gold nanoparticle surface. The positive results obtained with this lamellar nanocomposite may prove useful as a more cost-effective alternative in future carbon-carbon couplings.