Sonogashira Coupling Catalyzed by Gold Nanoparticles: Does Homogeneous or Heterogeneous Catalysis Dominate?

Abstract

A variety of measurements indicate that Au-nanoparticle catalyzed Sonogashira coupling of iodobenzene and phenylacetylene is predominantly a heterogeneous process. Large gold particles are much more selective than small ones, consistent with this view. Substantial leaching of Au into the solution phase occurred during reaction, but the resulting supernatant liquid exhibited immeasurably low catalytic activity: TONs for the nanoparticles were orders of magnitude higher than those for the leached Au, once more pointing to the primacy of heterogeneous chemistry. These properties were independent of the support material, implying that they are intrinsic to metallic Au nanoparticles. Reaction data and quantitative analysis of the solid and solution phases by XPS and ICP-MS respectively showed that catalytic activity ceased when all the metallic Au had dissolved. Conversely, when starting with a soluble Au complex, a long inactive induction phase was followed by sharp onset of reaction and steadily increasing catalytic activity, consistent with the eventual nucleation and growth of gold particles. Again, the implication is that, for the nanoparticle catalyzed reaction, heterogeneous catalysis is by far the most important process.