Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 140

After optimization of the conditions, the reaction was carried out under N 2
atmosphere at 160 1C using DMF as solvent and K 2 CO 3 as base. A high
conversion of p-iodoanisole (up to 96.1%) with excellent selectivity for cross-
coupling product (up to 88.1%) was obtained when it was catalyzed by
Au 25 (SR) 18 /CeO 2 catalyst (Table 5.2, entry 1). The Au 25 (SR) 18 nanocluster
catalyst performed much better than the larger sized 2-3 nm Au nanoclus-
ters (Table 5.2, entry 1 vs. 5) and CeO 2 -supported Au nanoparticles (bare,
B

d n 9 r 4 n g | 3

20 nm). Support effects were also investigated, and no distinct effect of the
oxide supports was observed (i.e., CeO 2 , SiO 2 , TiO 2 , and MgO). In the
recyclability test, the conversion of p-iodoanisole exhibited no significant
loss, while the selectivity was decreased from 88.1% to 64.5% after five
cycles. The drop in selectivity should be caused the gradual degradation of
nanoclusters in the multiple recycling tests, as larger Au clusters give a much
lower selectivity. Moreover, density functional theory (DFT) calculation of the
reactant adsorption shows that both reactants (i.e., iodobenzene and phe-
nylacetylene) prefer to adsorb on the open facet with the phenyl ring facing a
surface gold atom. A total adsorption energy reaches 0.90 eV when the two
reactants co-adsorb on the Au 25 (SR) 18 catalyst. The DFT results suggested
that the catalytic active site is associated with the Au 25 (SR) 18 , which is con-
sistent with the experimental results.

Table 5.2 The catalytic performance of Au 25 (SR) 18 (R ¼ CH 2 CH 2 Ph)
nanoclusters supported on various oxides as catalysts for
Sonogashira cross-coupling reaction of p-iodoanisole and
phenylacetylene. (a) Top view of one of the two open facets of
Au 25 (SR) 18 nanoclusters where three external gold atoms are
exposed; the other facet is on the back side. (b) Top view of the
co-adsorption of phenylacetylene and iodobenzene on the surface
of the Au 25 (SR) 18 nanoclusters. R 0 ¼ OCH 3 . Au, yellow; S, blue; C,
gray; H, white; I, green.

.

Adapted with permission from ref. 26.