02 February 2011
Spanish scientists have proved that gold alone can catalyse cross-coupling reactions following a claim made last year that palladium impurities in the gold are essential for the catalysis.
Avelino Corma from the Polytechnic University of Valencia and his colleagues used kinetic and theoretical studies to prove their findings, published in 2007, that gold, coupled with cerium oxide, catalyses the reaction alone.1 'Gold nanoparticles are active enough to promote the reaction, regardless of the presence or absence of palladium,' says Corma.
The paper opposing their claim, by Antonio Echavarren from Spain's Institute of Chemical Research of Catalonia and colleagues, says that palladium impurities are crucial for the process.2 In their report, Echavarren's team revisited the reaction carried out by Corma, coupling phenylacetylene with iodobenzene in a palladium-free setting with the gold-cerium oxide catalyst. They found that gold alone failed to catalyse the reaction that time.
But Corma tested the reaction again with the same catalyst and found that it did proceed. As he explained in the paper, the gold complex decomposed in solution to form gold nanoparticles, after which the Sonogashira coupling occurred.
Rupture of the C-I bond in iodobenzene on a gold nanoparticle. Au atoms are yellow, C atoms orange, H atoms white, I atoms red
'It's the gold nanoparticles that are crucial and this was missed in the Echavarren paper,' says Graham Hutchings, an expert in heterogeneous catalysis and surface science from Cardiff University, UK. 'Corma's paper sets the record straight for this interesting reaction. Until the 1980s, everyone would agree that as gold was the most noble of metals, it could not be a catalyst but now we know different.'
'Sonogashira coupling is important because it's used to prepare compounds for use from electronics to medicine,' says Corma. But he adds that the next challenge is to get a better understanding of the reaction mechanism to define the differences between the reaction pathways for gold and palladium. 'It's necessary to revisit other reactions catalysed by gold to assess the role of nanoparticles and palladium impurities on the overall catalytic activity.'