Facile syntheses of monodisperse ultrasmall Au clusters

During our effort to synthesize the tetrahedral Au-20 cluster, we found a facile synthetic route to prepare monodisperse suspensions of ultrasmall Au clusters Au-N (N < 12) using diphosphine ligands. In our monophasic and single-pot synthesis, a Au precursor ClAu(I)PPh3 (Ph = phenyl) and a bidentate phosphine ligand P(Ph)(2)(CH2)(M)P(Ph)(2) are dissolved in an organic solvent. Au(I) is reduced slowly by a borane-tert-butylamine complex to form Au clusters coordinated by the diphosphine ligand. The Au clusters are characterized by both high-resolution mass spectrometry and UV-vis absorption spectroscopy. We found that the mean cluster size obtained depends on the chain length M of the ligand. In particular, a single monodispersed Au-11 cluster is obtained with the P(Ph)(2)(CH2)(3)P(Ph)(2) ligand, whereas P(Ph)(2)(CH2)(M)P(Ph)(2) ligands with M = 5 and 6 yield Au-10 and Au-8 clusters. The simplicity of our synthetic method makes it suitable for large-scale production of nearly monodisperse ultrasmall Au clusters. It is suggested that diphosphines provide a set of flexible ligands to allow size-controlled synthesis of Au nanoparticles.