One-Pot Synthesis of Au11(PPh2Py)7Br3 for the Highly Chemoselective Hydrogenation of Nitrobenzaldehyde

Article abstract of DOI:10.1021/acscatal.5b02116

In this study, the gold clusters Au₁₁(PPh₃)₇Cl₃ and Au₁₁(PPh₂Py)₇Br₃ (PPh₂Py = diphenyl-2-pyridylphosphine) are synthesized via a one-pot procedure based on the wet chemical reduction method. The Au₁₁(PPh₃)₇Cl₃ cluster is found to be active in the chemoselective hydrogenation of 4-nitrobenzaldehyde in the presence of hydrogen (H₂) and a base (e.g., pyridine). Interestingly, the cluster with the functional ligand PPh₂Py shows similar activity without losing catalytic efficiency in the absence of the base. The structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via UV-vis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found that one ligand (PPh₃ or PPh₂Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site for the catalytic reaction. Then, the H-H bond of the H₂ molecule becomes activated with the aid of either free amine (base) or ligand PPh₂Py which is attached to the gold clusters. This work demonstrates the promise of the functional ligand PPh₂Py in the catalytic hydrogenation to reduce the amount of materials (free base: e.g., pyridine) that ultimately enter the waste stream, thereby providing a more environmentally friendly reaction medium.

Full text of DOI:10.1021/acscatal.5b02116

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Article
One-pot Synthesis of Au11(PPh2Py)7Br3 for the Highly
Chemoselective Hydrogenation of Nitrobenzaldehyde
Chao Liu, Hadi Abroshan, Chunyang Yan, Gao Li, and Prof. Masatake Haruta
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02116 • Publication Date (Web): 18 Nov 2015
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OneꢀPot Synthesis of Au11(PPh2Py)7Br3 for the Highly
Chemoselective Hydrogenation of Nitrobenzaldehyde
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Chao Liu†, Hadi Abroshan‡, Chunyang Yan†, Gao Li†,*, and Masatake Haruta†
†Gold Catalysis Research Centre, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, Dalian 116023, China.
‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.
ABSTRACT: In this study, gold clusters Au11(PPh3)7Cl3 and Au11(PPh2Py)7Br3 (PPh2Py = diphenylꢀ2ꢀpyridylphosphine) are synꢀ
thesized via oneꢀpot procedure based on the wet chemical reduction method. The Au11(PPh3)7Cl3 cluster is found to be active in
chemoselective hydrogenation of nitrobenzaldehydes in the presence of hydrogen (H2) and a base (e.g., pyridine). Interestingly, the
cluster with functional ligand PPh2Py shows similar activity without losing the catalytic efficiency in the absence of the base. The
structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via
UVꢀvis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found
that one ligand (PPh3 or PPh2Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site
for the catalytic reaction. Then, the H–H bond of H2 molecule becomes activated with the aid of either free amine (base) or ligand
PPh2Py which is attached to the gold clusters. This work demonstrates promise of functional ligand PPh2Py in the catalytic hydroꢀ
genation to reduce the amount of materials (free base e.g., pyridine) that ultimately enter the waste stream, thereby providing a
more environmentally friendly reaction media.
KEYWORDS: Au nanocluster, hydrogenation, chemoselective, DFT, amine
the hydrogenation reaction. In our very recent work, we preꢀ
sented that thiolateꢀprotected Aun(SR)m clusters (–SR repreꢀ
sents thiolate ligand, n and m varied from 15 to 99 and from
14 to 42, respectively) exhibit excellent catalytic activity in the
chemoselective hydrogenation of aldehyde (e.g., nitrobenzalꢀ
dehyde) to alcohols in the presence of pyridine.20 Of note,
catalytic active sites are located on the gold clusters and pyriꢀ
dine was found to act as the effective additive (promoter).
■ INTRODUCTION
Gold clusters have become one of the most promising materiꢀ
als for applications in a wide range of nanoscience and nanoꢀ
technology owing to their quantumꢀsize effects.1ꢀ4 The gold
clusters are stabilized by a wide range of organic ligands such
as thiolate,5,6 polymers,7 carbenes,8 amines,9 alkynes,10,11 and
phosphines.12ꢀ13 Atomically precise gold clusters (i.e., molecuꢀ
lar purity, formulated as AunLm, where L stands for ligand)
have attracted considerable attention due to sizeꢀdependent
properties that differ substantially from corresponding bulk
materials.6 The gold clusters have shown great potential in a
wide range of applications such as optoelectronic nanodevices,
biosensors, nanoelectronics, and novel catalysts.14ꢀ19
It is widely acknowledged that there should be significant
changes in the operation of the chemical industry to reduce
their negative impact on the environment. This tends towards
what is commonly known as ‘Green Chemistry’ with princiꢀ
ples formulated by the U.S. Environmental Protection Agency
(US EPA) to design chemical products and processes that reꢀ
duce or eliminate the generation of hazardous substances.
Though pyridine is used as a solvent or additive in numerous
chemical industries, it is toxic and not environmentallyꢀ
friendly even at low concentrations. Thus, it would be exꢀ
tremely worthwhile and desirable to synthesize new gold clusꢀ
ters with high catalytic performance in the absence of any
amine additive (e.g., pyridine). In this study, we report on
catalytic activity of gold clusters capped by functional ligands
which contain pyridine group (e.g., PPh2Py). A key novel asꢀ
pect of our present work is to bring pyridine groups close to
surface gold to assist reactants’ activation instead of using free
base in reaction media. Such a novelty not only eliminates use
of free base (e.g., pyridine) for the reactions but also markedly
improves the probability of reactants’ collision on the surface
of catalysts, thereby increasing the reaction rate.
The gold clusters have been demonstrated to exhibit excelꢀ
lent catalytic performance in selective oxidation, hydrogenaꢀ
tion, and carbonꢀcarbon coupling reactions.17ꢀ21 The stabilizing
organic ligands play important roles in catalytic activity of the
clusters.22ꢀ24 For example, Leeuwen and coworkers reported on
the application of the secondary phosphine oxide (SPO,
(Naph)POH(tꢀBu), Naph = naphthyl)ꢀprotected gold cluster
(core size: ca. 1.24 nm) in the chemoselective hydrogenation
of α, βꢀunsaturated aldehyde to α, βꢀunsaturated alcohol using
o
H2 gas (40 bar) at 60 C.24 It is speculated that the SPO ligand
plays a crucial role in the hydrogenation mechanism via the
ligand–metal cooperative effects.
Pyridine was found to be an effective functional group for
activation of reactants in the nanogoldꢀcatalyzed hydrogenaꢀ
tion reactions.20,25 According to a work by Yan et al., gold
nanoparticles catalyze the selective semihydrogenation of alꢀ
kynes to alkenes using organosilanes with water as the hydroꢀ
gen source.25 They showed that an amine additive (e.g., pyriꢀ
dine) is required to suppress association of hydrogen atoms in
Herein, we present oneꢀpot synthesis of an atomically preꢀ
cise gold cluster Au11(PPh2Py)7Br3 and investigate its catalytic
performance in the chemoselective hydrogenation of 4ꢀ
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spectrometer.
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Transꢀ2ꢀ[3ꢀ(4ꢀtertꢀbutylphenyl)ꢀ2ꢀmethylꢀ2ꢀ
nitrobenzaldehyde to 4ꢀnitrobenyl alcohol with 100% selectivꢀ
o
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ity in the presence of H2 (20 bar) at 80 C in water without
propenyldidene]malononitrile was used as the matrix in
MALDIꢀMS. Typically, 1 mg matrix and 0.1 mL analyte stock
solution were mixed in 100 ꢁL CH2Cl2. 10 ꢁL of the solution
was applied to the steel plate, and then airꢀdried prior to
MALDIꢀMS experiment.
using any additives. Further, we demonstrate the significant
role of PPh2Py ligand in the catalytic hydrogenation through a
combined approach of experiment and theory.
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Typical procedure for chemselective hydrogenation reꢀ
action. In a typical selective hydrogenation reaction, 4ꢀ
nitrobenzaldehyde (0.05 mmol), pyridine (0.1 mmol),
Aun(SPh)m/CeO2 catalyst (100 mg, 1 wt% cluster loading)
were added to an reactor (Parr instrument company, 22 mL
capacity, series 4700) under 20 bar H2. Of note, in the case of
the Au11(PPh2Py)7Br3/CeO2 catalyst, no pyridine was present.
■ EXPERIMENTAL METHODS
Synthesis of the Au clusters. In the case of
Au11(PPh2Py)7Br3, a freshly prepared solution of NaBH4 (15.2
mg, 0.4 mmol) in ethanol (1 mL) was added dropwise to a
stirred solution of [Au(PPh2Py)Cl] (40 mg, 0.13 mmol) and
TOABr (85 mg, 0.15 mmol) in ethanol (10 mL). The solution
color gradually turned to dark brown. After 12 h of stirring (at
600 rpm), the mixture was poured into water (10 mL). The
dark solid was collected by centrifugation at 4,000 rpm and
washed several times with hexane:CH2Cl2 (v/v = 5:1) to reꢀ
move excess PPh2Py ligand and TOABr. The precipitated
product was extracted by methanol (2 mL), followed by cenꢀ
trifugation again (at 10,000 rpm, 5 min) to remove insoluble
components. The final products were obtained after drying the
solution by rotary evaporation and stored in refrigerator. The
production yield of Au11(PPh2Py)7Br3 clusters based on conꢀ
sumption of Au(PPh2Py)Cl was found to be ca. 25%. It is
worth mentioning that the yield of the same clusters in the
absence of TOABr was ca. 15%. These results show that the
use of a phase transfer catalyst like TOABr, which facilitates
the migration of reactants between solution phases, leads to a
higher production rate of Au11(PPh2Py)7Br3 clusters.
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The reaction was carried out at 80 C for 10 h as indicated in
Table 1. After the catalytic reaction, the mixture was extracted
by ethyl acetate. The crude product was obtained after the
removal of the solvent. The conversion of 4ꢀnitrobenzaldehyde
and the selectivity for the 4ꢀnitrobenzyl alcohol product were
determined by 1H NMR (300 MHz) spectroscopic analysis. In
the recycling tests, the Au11(PPh2Py)7Br3/CeO2 catalyst was
collected by centrifugation (at 1,000 rpm) for 5 min after the
reaction and reꢀused in a fresh reaction.
Computational details. All calculations were performed
using the Gaussian09 package with B3PW91 functional and
LANL2DZ basis set (for Au) which have previously been
shown to perform well to predict the structure of gold clusꢀ
ters.26ꢀ34 For other elements the corresponding valence basis
sets 6ꢀ31G** together with the appropriate StuttgartꢀDresden
effective core potentials (ECPs) were employed.35ꢀ38 The enerꢀ
gy change (∆E) through a given reaction is determined by ꢂE
= Σ Epro ꢀ Σ Erea, where Σ Epro and Σ Erea are total energy of
isolated products and reactants in the reaction, respectively.
The integral equation formalism polarizable continuum model
(IEFPCM) was applied to study solvation effect on the stabiliꢀ
zation of the clusters and complexes in water.39
The thiolateꢀprotected Aun(SPh)m clusters are prepared by a
reported literature method.20 As shown in Figure S1 in the
Supporting Information, the Au25(SPh)18, Au36(SPh)24, and
Au99(SPh)42 clusters are wellꢀcharacterized by UVꢀvis specꢀ
troscopy and matrixꢀassisted laser desorption ionization mass
spectrometry (MALDIꢀMS).
Preparation of CeO2ꢀsupported gold cluster catalysts.
Typically,
2
mg
Aun(SPh)m,
Au11(PPh3)7Cl3,
or
■ RESULTS AND DISCUSSION
Au11(PPh2Py)7Br3 clusters were dissolved in 10 mL dicholoꢀ
methane, and then 200 mg CeO2 powder was added. After
stirring for 12 h at room temperature, the Aun(SPh)m/CeO2,
Au11(PPh2Py)7Br3/CeO2, and Au11(PPh3)7Cl3/CeO2 catalysts
were collected by centrifugation (at 1,000 rpm) and dried in
vacuum. The catalysts were then annealed at 100 oC for ~ 1 h
in a vacuum oven. Fourierꢀtransform infrared (FTꢀIR) analysis
was performed for the Au11(PPh2Py)7Br3/CeO2 catalyst before
Synthesis and characterization of Au11(PPh2Py)7Br3 clusꢀ
ter. The gold clusters were obtained by a facile oneꢀphase
process. In
a typical synthesis, Au(I)(PPh2Py)Cl and
tetraoctylammonium bromide (TOABr) were dissolved in
ethanol. The solution was stirred at room temperature. Next,
sodium borohydride (NaBH4) dissolved in ethanol was added
successively into the reaction mixture. The final products were
obtained simply by centrifugation and purification.
o
and after the 100 C treatment in vacuum. The FTꢀIR spectra
of the catalyst before and after the thermal treatment are superꢀ
imposable (Figure S2 in the Supporting Information), strongly
indicating that the Au11(PPh2Py)7Br3 clusters on the surface of
CeO2 indeed remain intact. This is line with previous studies
that the CeO2ꢀsupported Aun(SPh)m clusters remain intact after
the thermal treatment.11,20,21
Transmission electron microscopy (TEM) imaging of the
asꢀobtained Au11(PPh2Py)7Br3 cluster shows that the cluster’s
size is ca. 0.7–0.8 nm (Figure 1). Further, the gold cluster was
analyzed by optical spectroscopy and electrospray ionization
mass spectrometry (ESIꢀMS). The UVꢀvis spectrum of the
Au11(PPh2Py)7Br3 cluster shows a singleꢀband at 406 nm and a
shoulder between 450 and 550 nm (Figure 2A, red profile)
similar to Au11(PPh3)7Cl3 cluster (Figure 2A, black proꢀ
file).40,41 Of note, peak at 406 nm for Au11(PPh2Py)7Br3 is
overall blueꢀshifted compared to that of Au11(PPh3)7Cl3 (412
nm). This is mainly due to electronic effect of the protecting
organic ligand as it has also been observed in the case of
Au25(SR)18 (ꢀSR = thiolate ligand).42
Characterization of gold nanoclusters. The UVꢀVis specꢀ
tra of the clusters (dissolved in methanol) were acquired on a
HewlettꢀPackard (HP) Agilent 8453 diode array spectrophoꢀ
tometer at room temperature. Electrospray ionization (ESI)
mass spectra were obtained using a Waters QꢀTOF mass specꢀ
trometer equipped with a Zꢀspray source. The sample was
dissolved in toluene (1 mg/ml) and then mixed with a dry
methanol solution of CsOAc (50 mM) by a 1:1 vol ratio. FTꢀ
IR measurements were recorded on a Brukers/Tensor 27 inꢀ
strument (resolution, 1 cm−1; scans, 8; range, 1000−4000
cm−1). MALDIꢀMS analysis was performed with a PerSeptive
Biosystems Voyager DE superꢀSTR timeꢀofꢀflight (TOF) mass
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that the structure of Au11(PPh2Py)7Br3 cluster should be basiꢀ
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cally similar to that of Au11(PPh3)7Cl3. The skeleton of
Au11clusters often has an approximate C3v symmetry (incomꢀ
plete centered icosahedron) that all the surface gold atoms are
bound to either phosphine ligands or halogen atoms (Figure S5
in the Supporting Information).41 In the case of Au11(PPh3)7Cl3
cluster, average atomic distance between surface Au atoms and
C atom at the 2ꢀposition of the phenyl ring is ~3.62 Å (Figure
S5 in the Supporting Information). Therefore, it is worthwhile
to replace such C atom by N atom and examine if phosphine
ligands with a functional group like 2ꢀpyridine may promote
the catalytic hydrogenation reaction without the aid of free
base.
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Figure 1. TEM image of asꢀobtained Au11(PPh2Py)7Br3 cluster
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(core diameter ≈ 0.7ꢀ0.8 nm). Scale bar: 10 nm.
Catalytic performance of the Au11(PPh2Py)7Br3 cluster in
the chemoselective hydrogenation. The catalytic hydrogenaꢀ
tion is carried out under the following conditions: 0.1 mmol 4ꢀ
nitrobenzaldehyde, 100 mg CeO2ꢀsupported gold cluster cataꢀ
lysts (1 wt% cluster loading), 1 mL water, 20 bar H2 for 10 h
(see details in the Experimental section). We chose CeO2 oxꢀ
ide as support for the gold clusters. According to our previous
work, CeO2 leads to a better stability and higher catalytic effiꢀ
ciency of the supported clusters than other oxideꢀsupports
(e.g., TiO2, SiO2) in the catalytic hydrogenation reaction.20
To verify molecular formula of the asꢀobtained clusters, the
monodisperse productgold cluster is further characterized
by ESIꢀMS and formulated indeed as Au11(PPh2Py)7Br3 (Figꢀ
ure 2B). ESIꢀMS is considered as a "soft" ionization method
and usually does not induce fragmentation of cluster during
the analysis.43 It is worth pointing out that no peaks are obꢀ
served when no caesium acetate is present (Figure S3 in the
Supporting Information). Therefore, cesium acetate (CsOAc,
dissolved in methanol) was added to the solution to impart
charges to the cluster. During the ESIꢀMS process, the Cs+
ions attach to the cluster to form positively charged [clusꢀ
ter+Csx]x+ adducts. As shown in Figure 2B, there are three
prominent peaks in range of 1500ꢀ6000 m/z (no peaks are
found for m/z > 6,000); the observed peaks are centered at m/z
= 4,381.87 (peak I), 2,257.43 (peak II), and 2,125.84 Da (peak
III). In the zoomꢀin spectrum, the spacing of the experimental
isotopic patterns of peak I is one, indicating that the ion carries
+1 charge (Figure S4A in the Supporting Information). The
spacing of the experimental isotopic patterns of peak II and III
are 0.5, implying these ions are +2 charged (Figure S4B and
S4C in the Supporting Information). The ESIꢀMS peaks are
assigned as follows: peak I at m/z = 4381.87 Da to
[Au11(PPh2Py)7Br3Cs1]+ (theoretical m/z: 4382.13 Da, deviaꢀ
Table 1. Comparison of the catalytic performance of the CeO2ꢀ
supported gold clusters and complexes in the selective hydroꢀ
genation of 4ꢀnitrobenzaldehyde to 4ꢀnitrobenzyl alcohol.a
Au NCs
OHC
NO2
HOH2C
NO2
H2
T
Conv. Select.
Au NCs
Base
(oC)
80
80
80
80
80
80
80
80
80
80
60
60
60
60
60
(%)b
n.r.
n.r.
n.r.
n.r.
90
(%)b
ꢀ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Au11(PPh3)7Cl3
Au25(SPh)18
Au36(SPh)24
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Au99(SPh)42
Au11(PPh2Py)7Br3
Au11(PPh2Py)7Br3
Au11(PPh3)7Cl3
Au25(SPh)18
Au36(SPh)24
Au99(SPh)42
Au11(PPh2Py)7Br3
Au11(PPh3)7Cl3
Au25(SPh)18
ꢀ
100
100
100
100
100
100
100
100
100
100
100
20
Py
Py
Py
Py
Py
ꢀ
Py
Py
Py
Py
93
91
97
95
92
24
16
15
tion: ꢀ0.26 Da), peak II at m/z
= 2257.43 Da to
[Au11(PPh2Py)7Br3Cs2]2+ (theoretical m/z: 2257.55 Da, deviaꢀ
tion: ꢀ0.12 Da), and peak III at m/z 2125.84 Da to
[Au11(PPh2Py)6Br3Cs2]2+ (theoretical m/z: 2125.88 Da, deviaꢀ
tion: ꢀ0.04 Da). Of note, the experimental isotopic patterns of
the peaks match quite well with simulated ones (Figure S4 in
the Supporting Information).
Au36(SPh)24
Au99(SPh)42
AuI(PPh2Py)Cl
14
12
The similarities in both the molecular formula (Au11L7X3, L
= PPh3 or PPh2Py, X = Cl or Br) and UVꢀvis spectra indicate
16
80
ꢀ
81
(80)c
24
17
AuI(PPh3)Cl
18d Au11(PPh2Py)7Br3
80
80
Py
ꢀ
84
87
(76)c
100
aReaction conditions: 100 mg CeO2ꢀsupported AunLm clusꢀ
ter catalyst (ca. 1 wt% cluster loading), 1 mL water, 0.05
mmol 4ꢀnitrobenzaldehyde, 0.1 mmol pyridine (Py), 20 bar
b
H2, 10 h; The conversion (Conv.) of 4ꢀnitrobenzaldehyde
and selectivity (Select.) for 4ꢀnitrobenzyl alcohol were deꢀ
c
termined by NMR analysis. The data given in the parenꢀ
theses is selectivity for 4ꢀaminobenzaldehyde. dReaction
conditions: 3 g Au11(PPh2Py)7Br3/CeO2 catalyst (~30 mg
Au11(PPh2Py)7Br3 loading) in 30 mL water, 5 mmol 4ꢀ
o
nitrobenzaldehyde, 20 bar H2 at 80 C, 20 h. n.r. = no reacꢀ
Figure 2. (A) UVꢀvis spectra of the Au11(PPh2Py)7Br3 and
Au11(PPh3)7Cl3 clusters. (B) Positive mode ESI mass spectrum of
the Au11(PPh2Py)7Br3 clusters in the presence of CsOAc.
tion.
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Scheme 1. (A) Structure of Au11(PPh3)7Cl3 cluster.41 Detachment of a halide anion and a phosphine ligand results in (B)
[Au11(PPh3)7Cl2]+ and (C) Au11(PPh3)6Cl3 with an open metal site on the cluster. Color codes: Au, green; P, purple; Cl, cyan. All the
carbon and hydrogen atoms are omitted for clarity.
First, we find that no conversion (nitrobenzaldehyde ꢀ niꢀ
trobenzyl alcohol) is achieved by using Au11(PPh3)7Cl3/CeO2
catalyst in the absence of the base (Table 1, entry 1). Catalytic
activity of thiolateꢀprotected gold clusters (e.g., Au25(SPh)18,
Au36(SPh)24, and Au99(SPh)42) were also examined under the
same reaction conditions. As shown in Table 1 (entries 2ꢀ4),
no product formation was observed in these cases, as well.
However, such catalysts yield a high conversion in the presꢀ
ence of two molar equivalent of pyridine at 80 oC (Table 1,
entries 7ꢀ10), in consistent with our previous studies.21 The
most salient feature of Table 1 is the gold clusters with funcꢀ
tional ligand PPh2Py exert a major influence on the conversion
of nitrobenzaldehyde. When Au11(PPh2Py)7Br3 is used, the
conversion reaches to 90% even in the absence of free base
like pyridine (Table 1, entry 5). The turnover frequency (TOF
= [reacted mol of nitrobenzaldehyde]/[(mol of Au cluster) ×
(reaction time)]) of the reaction is found to be ca. 20 hꢀ1. Sevꢀ
eral reasons can lead to a low TOF including; (1) slow diffuꢀ
sion/adsorption of the reactants towards the catalyst, and (2)
low number of active site per catalyst. In our case, we think
the later reason is important to consider as there is one active
site per catalyst (see the proposed mechanism below). Of note,
the catalytic activity of the Au11(PPh2Py)7Br3 does not change
when free pyridine was added into the reaction media (Table
1, entry 6). These results strongly indicate that the PPh2Py
ligands are indeed promoter for the catalytic reaction using
Au11(PPh2Py)7Br3. The lower temperatures reduce the reaction
conversion (Table 1, 60 oC vs. 80 oC). However,
Au11(PPh2Py)7Br3 shows a better catalytic activity among all
the gold clusters considered in this study at the lower temperaꢀ
noparticles, which confirm the heterogeneous nature of the
catalysis by the gold clusters.
To test the performance of the catalyst in large scale syntheꢀ
sis, we examined the activity of the Au11(PPh2Py)7Br3/CeO2 in
a scaled up catalytic system under reaction conditions as indiꢀ
cated in Table 1, entry 18. The reaction conversion and selecꢀ
tivity for 4ꢀnitrobenzyl alcohol product were found to be 87%
and 100%, respectively (Table 1, entry 18). These results are
close to those reported as entry 5 of Table 1 for the smaller
scale (90% and 100%, respectively). Therefore, the
Au11(PPh2Py)7Br3/CeO2 is characterized as a good catalyst for
large scale catalysis.
Next,
we
investigated
the
recyclability
of
Au11(PPh2Py)7Br3/CeO2 catalyst. The Au11(PPh2Py)7Br3/CeO2
catalyst was collected by centrifugation (at 1,000 rpm for 5
min) after the reaction, washed with water and ethyl acetate,
and dried in an oven. The catalyst thus prepared was then reꢀ
used in a fresh reaction medium under the identical reaction
o
conditions at 80 C. The recycled catalyst showed the same
activity and selectivity as the fresh catalyst (Figure S6). No
appreciable loss of catalytic activity and selectivity was obꢀ
served after 4 cycles (higher cycles were not tested). Thus, the
Au11(PPh2Py)7Br3/CeO2 catalyst appears to be relatively robust
and holds promise as a practically useful catalyst in the
chemoselective hydrogenation process.
Structure and characterization of Au11(PPh3)6Cl3 species.
Extensive studies on bare gold clusters without ligands atꢀ
tached have revealed that lowꢀcoordinated naked gold atoms
on the surface are the main active sites for catalytic reacꢀ
tions.19,44ꢀ46 All the surface gold atoms of Au11(PPh2Py)7Br3
and Au11(PPh3)7Cl3 clusters are bound to phosphine ligands
(PPh3 and PPh2Py) and halogen atoms (Cl or Br). This raises
an important question as to the nature of the active sites for
hydrogenation reactions catalyzed by the gold clusters. Thereꢀ
fore, we first speculated that the gold clusters may lose either a
phosphine ligand or a halogen atom under reaction conditions
to provide an open metal site for the catalytic reaction
(Scheme 1). DFT calculations on framework of
Au11(PPh3)7Cl3,41 revealed that the open metal site produced
via removal of a halide anion (noted as Au1, Scheme 1B) is
inert toward H2 adsorption in the presence or absence of a base
(e.g., NH3). However, removal of a phosphine ligand provides
an open metal site (noted as Au2, Scheme 1C) which was
found to adsorb H2 in the presence of the base. Of note, there
is no strong interaction between H2 and Au2 in the absence of
ammonia, in agreement with our experimental results (Table 1,
entry 2). As a result, our DFT calculations propose that hydroꢀ
genation of benzaldehyde is mainly catalyzed by
o
ture (60 C). Notably, almost 100% chemoselectivity for the
alcohol product was observed in all the catalytic reactions
(Table 1). These experimental results indicate that the PPh2Py
ligands on the gold clusters play a crucial role to promote the
catalytic hydrogenation.
Further, the conventional gold nanoparticles (bared), supꢀ
ported on CeO2 were tested in the hydrogenation of nitrobenꢀ
zaldehydes under the identical reaction conditions (in the presꢀ
ence of pyridine and H2). A 100% selectivity towards nitroꢀ
benzyl alcohol product was obtained, which is similar to those
achieved by Au11 clusters. To make sure that the observed
activity of Au11 clusters is due to a heterogeneous catalysis and
not a homogeneous one, we examined catalytic activity of two
gold complexes: AuI(PPh2Py)Cl and AuI(PPh3)Cl. Results
show that the homogeneous catalysts lead to hydrogenation of
nitro group over aldehyde (Table 1, entries 16 and 17). These
results are totally different compared to those obtained using
Au11 clusters (Table 1, entries 5ꢀ15) and ligand free gold naꢀ
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Au11(PPh3)6Cl3 species which should be generated from its indicate the Au11(PPh2Py)6Br3 species may also exist in the
1
2
3
4
parent cluster Au11(PPh3)7Cl3.
reaction media while Au11(PPh2Py)7Br2 is not observed. These
results strongly indicate that one ligand (PPh3 or PPh2Py) reꢀ
moval is the first step to expose the core of the gold clusters to
reactants, thereby providing an active site for the catalytic
reaction.
5
6
7
8
9
Hydrogenation mechanism catalyzed by Au11 cluster. We
next propose a mechanism for hydrogenation of benzaldehyde
catalyzed by Au11(PPh3)6Cl3 and Au11(PPh2Py)6Br3 species
which are generated from their parent clusters Au11(PPh3)7Cl3
and Au11(PPh2Py)7Br3. DFT calculations are performed to
rationalize the mechanism of hydrogenation reaction catalyzed
by the Au11 clusters promoted by amines (e.g., pyridine). The
framework of the Au11(PPh3)7Cl3 cluster is adopted from its
crystal structure for the calculations.41 Our ESIꢀMS and UVꢀ
vis results strongly suggest that Au11(PPh3)7Cl3 and
Au11(PPh2Py)7Br3 clusters must basically have the same moꢀ
lecular structure. Therefore, mechanistic insights offered by
DFT using Au11(PPh3)7Cl3 framework can be informative for
the case of Au11(PPh2Py)7Br3 cluster, as well. We later comꢀ
ment on the differences between Au11(PPh3)7Cl3 and
Au11(PPh2Py)7Br3 clusters. Specifically, the roles played by
the functional ligand PPh2Py are discussed in details.
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Upon removal of a phosphine ligand from Au11(PPh3)7Cl3
to form Au11(PPh3)6Cl3 (Re, Figure 4), HꢀH bond of a H2 molꢀ
ecule is activated on site Au2 (Scheme 1C) of the cluster with
the help of a base like ammonia (Re ꢀ Im1, Figure 4). Note,
pyridine group is replaced by ammonia to reduce the computaꢀ
tional demand. According to DFT calculations, the adsorption
energy (ꢂE) of H2—NH3 is ꢀ8.0 kcal/mol in gas phase (Table
2). While the gasꢀphase results offer useful insight, it is
worthwhile to account solvation effects for both qualitative
and quantitative understanding of reactions pathways in soluꢀ
tion. Specifically, in a polar environment like H2O, charged
species become stabilized through electrostatic interactions
with the solvent, compared to the neutral ones. The polarizable
continuum model (PCM) calculations indicate that ꢂE of Re
ꢀ Im1 in water is ꢀ22.0 kcal/mol which shows the product
state (Im1) becomes considerably stabilized compared to the
Figure 3. (A) Positive mode ESI mass spectra of Au11(PPh3)7Cl3
cluster in the presence of ammonia (condition: 80 C, 4h). (B)
Thermal stability of Au11(PPh3)7Cl3 cluster in a closed cuvette
(dissolved in ethanol at 80 °C for 12h).
o
To test the notion driven from DFT calculations, we preꢀ
pared an ethanol solution of the Au11(PPh3)7Cl3 cluster in the
o
presence of ammonia string at 80 C. The UVꢀvis spectrum of
the Au cluster was monitored for 12 h (Figure 3B). We obꢀ
served that the distinct peak at 412 nm gradually redꢀshifted
by 9 nm, which indicates a slight structural change in the gold
cluster. The ESIꢀMS results of the cluster are shown in Figure
3A. Only one peak at m/z = 1989.66 Da is observed in the
range of 1500 to 4000 of ESIꢀMS profile (in positive mode).
The spacing of the experimental isotopic pattern of the peak is
0.5 (Figure S7 in the Supporting Information) which indicates
an ion with a net charge of +2. The molecular ion is assigned
to [Au11(PPh3)6Cl3Cs]2+ (theoretical m/z: 1989.50 Da, deviaꢀ
tion: +0.16 Da) with the experimental isotopic patterns matchꢀ
ing well with simulated ones (Figure S7 in the Supporting
Information). These results strongly imply that one phosphine
ligand is removed from the gold cluster in the presence of a
reactants because of the highly chargeꢀseparated nature of the
+
NH4 …H
̅
moiety. We note that upon adsorption of H2—NH3,
the H–H atomic distance is elongated to 0.82 Å, much longer
than that for an isolated H2—NH3 in the gas phase (0.75 Å).
This bond length is even extend to 1.83 Å in PCM calculations
+
indicating that H2 bond is completely broken to form NH4
with H– on site Au2. The atomic distances are summarized in
Figure 4.
According to the experimental results, we assume pyridine
group of the phosphine ligands of Au11(PPh2Py)6Br3 acts as a
base like ammonia to activate the H2 bond via its lone pair of
electrons. The ammonium ion formed in previous step may
become solvated in the reaction media and act as a donor for
protonation of aldehyde (Im1 ꢀ Im2, Figure 4). This step was
found to be endothermic by +10.7 kcal/mol in water, while it
o
base (e.g., ammonia) at 80 C, in good agreement with our
DFT results. The ESIꢀMS results (Figure 2B, peak III) also
Table 2. DFT results for energies (kcal/mol) for elementary steps of proposed mechanism in Figure 4.
a
b
a
Au11(PPh3)6Cl3
Au11(PPh3)6Cl3
[Au11(PPh3)6Cl3]+
ꢂ
E
(
(
Re
Im1
Im2
Σ ꢂEc
ꢀ
ꢀ
ꢀ
Im1
Im2
Re
)
)
)
ꢀ8.0
+107.9
ꢀ95.6
+ 4.3
ꢀ22.0
+10.7
ꢀ7.6
ꢀ22.8
+72.8
ꢀ45.7
+ 4.3
ꢂE
ꢂE
(
ꢀ18.9
aResults obtained in gas phase calculations. bResults obtained using PCM calculations.
cΣ ꢂE =
ꢂE
(
Re
ꢀ
Im1) + ꢂ
E (Im1 ꢀ Im2) + ꢂE (Im2 ꢀ Re).
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Page 6 of 8
Re). This must be more significant if solvation effect is taken
into account. Since charged reactant (PhCHOꢀNH4 ) is conꢀ
+
1
siderably smaller than charged product ([Au11(PPh3)6Cl3]+),
we expect the reactant becomes more stabilized than the prodꢀ
ucts in water. Note that Born solvation free energy,
½(1−1/ε)e2/a, is inversely proportional to the ion diameter 2a
(ε and e are solvent dielectric constant and elementary charge,
respectively). As a result, the Im2 ꢀ Re step in a polar solꢀ
vent like H2O becomes less exothermic compared to the gas
phase.
2
3
4
5
6
7
8
9
On the basis of the experimental and DFT results on natural
and charged Au11(PPh3)6Cl3, we further propose a mechanism
for the catalytic hydrogenation reaction catalyzed by
Au11(PPh2Py)7Br3 (Scheme 2). At the first step, one PPh2Py
ligand is removed from Au11(PPh2Py)7Br3 to yield
Au11(PPh2Py)6Br3 species. This step exposes a gold atom to
the reactants (e.g., aldehyde and H2). Then, H2 adsorbs onto
the open metal site with subsequent activation of the HꢀH
bond with the help of pyridine group of phosphine ligands on
Au clusters. The pyridine group abstracts a proton from H2 to
form pyridinium (PyH+), leaving H– on the core (Au2 site) for
the ensuing hydrogenation (Scheme 2, Step I). Similar proton
transfer was indeed found in the case of gold nanoparticle
protected by functional ligand P(Naph)(tꢀBu)OH where the H2
was activated on gold with the help of –OH group of the
phosphine ligand.26 Next, nitrobenzaldehyde gets physically
absorbed on the surface of the cluster via interactions of ꢀ
CHO groups with PyH+ and H– (Scheme 2, Step II). Finally,
H+ (PyH+) and H– will be transferred to NO2PhCHO to proꢀ
duce NO2PhCH2OH product (Scheme 2, Step III).
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60
Figure 4. The atomic distances in the case of neutral
Au11(PPh3)6Cl3 are as follows, a = 0.75 and 0.75, b = 2.57 and
2.62, c = 0.82 and 1.83, and d = 2.01 and 1.04 Å in gas phase and
PCM calculations, respectively. The atomic distances obtained in
the case of positively charged cluster [Au11(PPh3)6Cl3]+ in gas
phase calculations are a = 0.75, b = 2.57, c = 1.76, and d = 1.05
Å. The Au11(PPh3)6Cl3 species was drawn in the spaceꢀfilling
mode according to the crystal structure of the Au11(PPh3)7Cl3
cluster.27 Color code: Au, green; P, purple; Cl, cyan. All the carꢀ
bon and hydrogen atoms are omitted for clarity.
is +107.9 kcal/mol in the gas phase. This means that solvation
can facilitate the elongation of the H—H bond (c in Figure 4)
+
to produce charged species (NH4 and H
̅
). This is followed
by reduction of protonated benzaldehyde to benzyl alcohol via
an exothermic step by 95.6 and 7.6 kcal/mol in the gas phase
and PMC calculations, respectively (Im2 ꢀ Re). A big differꢀ
ence between the PMC and the gas phase results is expected
+
since reactants (Au11(PPh3)6Cl3—H
̅
and PhCHO—NH4 ) are
charged, hence become more stabilized in water compared to
neutral products (Au11(PPh3)6Cl3 and PhCH2OH—NH3).
We briefly pause here for perspective. The ESIꢀMS results
indicate that removal of a phosphine ligand from clusters
Au11(PPh3)7Cl3 and Au11(PPh2Py)7Br3 result in the formation
of [Au11(PPh3)6Cl3]+ and Au11(PPh2Py)6Br3, respectively. Note
that the former one carries charge of +1, while the latter one is
neutral. We think this is mainly due to the protection of the
open metal site (Au2, Scheme 1C) by the loneꢀpair on nitroꢀ
gen (Py) of the nearby ligands PPh2Py (Scheme 2, Re2). Coꢀ
ordination of Au2 site with the nitrogen leads to stabilization
of molecular orbitals of the Au11(PPh2Py)6Br3 cluster which in
turn prevents oxidation of the cluster. In the case of
Au11(PPh3)6Cl3, the open metal site (Au2) is easily accessible
by solvents which may result in the oxidation of the cluster to
form [Au11(PPh3)6Cl3]+. In view of the protic nature of the
solvents studied in the present work, gold oxidation by H+ is
prudent to consider. Although the redox potential (E0) for
such reaction is not high enough for spontaneous oxidation of
gold, E0 decreases significantly for gold clusters compared to
bulk Au. This is mainly due to nonlinear relationship between
cluster size and E0.48,49
Scheme 2. The Au11(PPh2Py)7Br3 and Au11(PPh2Py)6Br3 species
was drawn in the spaceꢀfilling mode according to the crystal
structure of the Au11(PPh3)7Br3 cluster.41 Color code: Au, green;
P, purple; Br, cyan. Ph2Py of other phosphine ligands are omitted
for clarity.
In order to account the oxidation state of the cluster in the
hydrogenation reaction, we performed DFT calculations on
[Au11(PPh3)6Cl3]+ cluster in gas phase. The calculated change
in energy (ꢂE) for different steps of the proposed mechanism
in Figure 4 are given in Table 2. Comparison of such energetꢀ
ics and interꢀatomic distances (Figure 4) for Au11(PPh3)6Cl3
and [Au11(PPh3)6Cl3]+ reveals that oxidation of the cluster can
facilitate the activation of the H–H bond (c in Figure4) to proꢀ
■
CONCLUSIONS
We report a facile oneꢀpot synthesis of the functionalized ligꢀ
andꢀprotected gold cluster formulated as Au11(PPh2Py)7Br3.
The asꢀprepared gold cluster is wellꢀdetermined by UVꢀvis
spectroscopy and electrospray ionization mass spectrometry.
The CeO2ꢀsupported cluster catalyst shows good catalytic perꢀ
formance, including activity and selectivity in the chemoselecꢀ
tive hydrogenation of nitrobenzaldehyde in the absence of
amine base. A perfect 100% selectivity for the nitrobenzyl
+
duce charged species (NH4 and Hꢀ) in gas phase (Re ꢀ Im1
ꢀ Im2). However, reduction of the aldehyde to alcohol seems
to be less favorable when [Au11(PPh3)6Cl3]+ is used (Im2 ꢀ
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■ ASSOCIATED CONTENT
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Supporting Information. The experimental and simulated isoꢀ
topic distribution patterns of species [Au11(PPh2Py)7Br3Cs1]+,
[Au11(PPh2Py)7Br3Cs2]2+, and [Au11(PPh3)6Cl3Cs]2+, recovery and
reuse of the Au11(PPh2Py)7Br3/CeO2 catalyst, and FTꢀIR spectra
of Au11(PPh2Py)7Br3/CeO2 catalyst before and after the thermal
treatment. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Email: gaoli@dicp.ac.cn
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENT
G.L. acknowledges financial support by the starting funds of the
Dalian Institute of Chemical Physics and the “Thousand Youth
Talents Plan”.
■
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