Chiral undecagold clusters: synthesis, characterization and investigation in catalysis

Enantiopure undecagold clusters protected by chiral atropisomeric diphosphine ligands (P boolean AND P) have been synthesized by the stoichiometric reduction of the corresponding (P boolean AND P)(AuCl)(2) complexes with NaBH4. The molecular mono-disperse [Au-11(P boolean AND P)(4)Cl-2] Cl species have been thoroughly characterized using an array of analytical techniques. P-31 NMR experiments suggested the presence of a slow intramolecular ligand exchange process. Circular dichroism measurements showed that enantiomeric clusters display mirror-image chiroptical activity. Such undecagold clusters containing two chloride ligands bound to the peripheral Au(I) atoms were expected to display a carbophilic Lewis acidity similar to the well-documented molecular Au(I) complex catalysts. Chloride abstraction, performed to generate active Au+ sites, induced the Au-11 cluster evolution to larger gold clusters and nanoparticles, together with Au(I) complexes, which, in fact, perform the catalysis. This result was corroborated by running an asymmetric tandem hydroarylation-carbocyclization reaction, for which the enantiomeric excesses obtained with Au-11 clusters are similar to those reported using Au(I) complexes.