Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 138

Au 25 (SG) 18
NaBH 4

HO NO 2

HO NH 2

PNP

d n 9 r 4 n g | 3

Scheme 5.5 Hydrogenation reaction of 4-nitrophenol to 4-aminophenol catalyzed
by Au 25 (SR) 18 nanocluster catalysts.

5.5.3.2 Hydrogenation of Nitrophenol
Yamamoto et al. 41 investigated and compared the catalytic properties of
Au 25 (SG) 18 nanoclusters and dimethylformamide (DMF)-stabilized gold
nanoclusters (not of atomic precision) in the reduction reaction of 4-nitro-
phenol to 4-aminophenol by NaBH 4 in water at room temperature
(Scheme 5.5). The reduction process was detected by UV-visible spectra. The
Au 25 (SG) 18 nanoclusters exhibited higher catalytic activity than DMF-capped
clusters in the nitrophenol reduction (pseudo-first-order rate constant
8 10 3 s 1 for Au 25 (SG) 18 vs.3 10 3 s 1 for DMF-capped Au clusters). High
catalytic activity of Au 25 (SG) 18 catalyst can be achieved even at a very low
catalyst concentration (e.g., 1.0 mM). No induction time was observed in the
case of Au 25 (SG) 18 despite -SG being a strongly binding ligand due to less
steric hinderance, while the DMF-stabilized Au clusters exhibited an in-
duction time (

4000 s), which was attributed to the impedance by the
surface DMF layer on the clusters when reactants access the cluster sur-
face. 41 For Au 25 (SG) 18 nanoclusters, the unique core-shell structure seems to
pose less steric hinderance and thus renders the catalytic active sites more
accessible to reactants.
Recently, Shivhare et al. 42 also demonstrated intact Au 25 (SR) 18 (where
R ¼ C 6 H 13 ,C 8 H 17 ,andC 12 H 25 ) nanoclusters for the reduction of nitrophenol
by NaBH 4 in mixed THF/water and under N 2 atmosphere. The Au 25 (SR) 18
nanoclusters were more stable and retained their structural integrity towards
NaBH 4 treatment, while the larger Au B 180 (SC 6 H 13 ) B 100 nanoclusters were not.

B

.

5.5.4 Catalytic Carbon-Carbon Coupling Reaction
Gold nanoclusters also hold great promise for the carbon-carbon coupling
reactions, such as Ullmann-type homo-coupling and Sonogashira cross-
coupling reactions. The catalytic application in C-C coupling reaction was
due to the good capability of gold nanocluster catalysts in activating the C-I
bond of iodobenzene and the terminal C-H bond of phenylacetylene. Below
we illustrate the C-C reactions.

5.5.4.1 Ullmann-type Homo-coupling Reaction
Gold nanoparticles are good catalysts for activating the C-I bond of the
iodobenzene in the carbon-carbon homo-coupling process. Li et al. 43 in-
vestigated the catalytic activity of the oxide-supported Au 25 (SR) 18 catalysts for