Selective Hydrogenation of Alkyne by Atomically Precise Pd6 Nanocluster Catalysts: Accurate Construction of the Coplanar and Specific Active Sites

Atomically precise nanoclusters are promising model catalysts to understand the relationship between structure and catalytic activity. However, designing efficient active sites remains challenging because the highly covered ligands obscure the metal sites. Herein, we reported a Pd6(S-Adm)6(PPh3)(PPh) nanocluster, which shows high selectivity in the semihydrogenation of aromatic alkyne. The Pd6 nanocluster has a unique chair structure, where the pan is composed of the coplanar Pd atoms, the back is composed of two S-Adm ligands, and the legs are composed of four S-Adm, one PPh3, and one PPh ligands. Experiments reveal that the high selectivity of intact Pd6 nanocluster is attributed to the synergistic effect of thiols and phosphines, modulating the electron properties and benefiting the proper hydrogen dissociation ability and desorption of product. Interestingly, it was found that the exposed coplanar Pd atoms could provide specific active sites for the adsorption of C≡C and benzene ring. DFT calculations show that phenylacetylene and styrene adsorb on the coplanar Pd6 much weaker than on the Pd(111) surface, allowing the styrene to be desorbed before further hydrogenation. The phenyl adsorption constrains that the hydrogenation can occur only on the coplanar Pd6, which is more facile for phenylacetylene than for styrene, and results in semihydrogenation.