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 CC 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.