Gold nanocluster-catalyzed semihydrogenation: a unique activation pathway for terminal alkynes.

We report high catalytic activity of ultrasmall spherical Au25(SC2H4Ph)18 and rod-shaped Au25(PPh3)10(C≡CPh)5X2 (X = Br, Cl) nanoclusters supported on oxides for the semihydrogenation of terminal alkynes into alkenes with >99% conversion of alkynes and ∼100% selectivity for alkenes. In contrast, internal alkynes cannot be catalyzed by such "ligand-on" Au25 catalysts; however, with "ligand-off" Au25 catalysts the internal alkynes can undergo semihydrogenation to yield Z-alkenes, similar to conventional gold nanoparticle catalysts. On the basis of the results, a unique activation pathway of terminal alkynes by "ligand-on" gold nanoclusters is identified, which should follow a deprotonation activation pathway via a R'-C≡C-[AunLm] (where L represents the protecting ligands on the cluster), in contrast with the activation mechanism on conventional gold nanocatalysts. This new activation mode is supported by observing the incorporation of deprotonated -C≡CPh as ligands on rod-shaped Au25(PPh3)10(C≡CPh)5X2 nanoclusters under conditions similar to the catalytic reaction and by detecting the R'-C≡C-[Aun(SC2H4Ph)m] via FT-IR spectroscopy.