C–C Coupling on Single-Atom-Based Heterogeneous Catalyst

Compared to homogeneous catalysis, heterogeneous catalysis allows for ready separation of products from the catalyst and thus reuse of the catalyst. C–C coupling is typically performed on a molecular catalyst which is mixed with reactants in liquid phase during catalysis. This homogeneous mixing at a molecular level in the same phase makes separation of the molecular catalyst extremely challenging and costly. Here we demonstrated that a TiO₂-based nanoparticle catalyst anchoring singly dispersed Pd atoms (Pd₁/TiO₂) is selective and highly active for more than 10 Sonogashira C–C coupling reactions (RCH + R′X → RR′; X = Br, I; R′ = aryl or vinyl). The coupling between iodobenzene and phenylacetylene on Pd₁/TiO₂ exhibits a turnover rate of 51.0 diphenylacetylene molecules per anchored Pd atom per minute at 60 °C, with a low apparent activation barrier of 28.9 kJ/mol and no cost of catalyst separation. DFT calculations suggest that the single Pd atom bonded to surface lattice oxygen atoms of TiO₂ acts as a site to dissociatively chemisorb iodobenzene to generate an intermediate phenyl, which then couples with phenylacetylenyl bound to a surface oxygen atom. This coupling of phenyl adsorbed on Pd₁ and phenylacetylenyl bound to O_ad of TiO₂ forms the product molecule, diphenylacetylene