Visible Light Gold Nanocluster Photocatalyst: Selective Aerobic Oxidation of Amines to Imines

Article abstract of DOI:10.1021/acscatal.6b03509

This work demonstrates the synthesis of an efficient photocatalyst, Au₂₅(PPh₃)₁₀Cl₂(SC₃H₆SiO₃)₅/TiO₂, for selective oxidation of amines to imines. The photo

Full text of DOI:10.1021/acscatal.6b03509

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Article
Visible Light Gold Nanocluster Photocatalyst:
Selective Aerobic Oxidation of Amines to Imines
Haijun Chen, Chao Liu, Min Wang, Chaofeng Zhang, Nengchao
Luo, Yehong Wang, Hadi Abroshan, Gao Li, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03509 • Publication Date (Web): 10 Apr 2017
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Visible
Light
Gold
Nanocluster
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Photocatalyst: Selective Aerobic Oxidation
of Amines to Imines
a, c
b
a
a
a
Haijun Chen, Chao Liu, Min Wang , Chaofeng Zhang , Nengchao Luo ,
a
d
b,
a,
Yehong Wang , Hadi Abroshan , Gao Li *, Feng Wang *
a
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
People’s Republic of China
b
Gold Catalysis Research Centre, State Key Laboratory of Catalysis, Dalian Institute
of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic
of China
c
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China
d
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
ABSTRACT: This work demonstrates the synthesis of an efficient photocatalyst,
Au25(PPh
3
)
10Cl
2
(SC
3
H
6
SiO
3
)
5
/TiO
2
, for selective oxidation of amines to imines. The
[(SC Si(OC
photocatalyst is prepared via hydrolysis of Au25(PPh
3
)10Cl
2
3
H
6
2
H
5
) ]
3 5
nanoclusters in the presence of P25 support. The gold nanoclusters exhibit good
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photocatalytic activity using visible light and under mild thermal conditions for the
selective oxidation with molecular oxygen (O ). The turnover frequency (TOF) of 4-
2

1
methylbenzylamine oxidation is found to be 1522 h , which is considerably higher
than that conventional gold catalysts. The gold nanoclusters present good recyclability
and stability for the oxidation of a wide range of amines. The superior activity of the
photocatalyst is associated with its unique electronic structure and framework. The
catalytically active sites are deemed to be the exposed gold atoms upon detaching
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protecting ligands, i.e., PPh . The Hammett parameter suggests that the photocatalytic
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process involves the formation of carbocation intermediate species. Further, Au-H
species were confirmed by TEMPO (2,2,6,6-tetramethylpiperidinyloxy) as a trapping
agent.
KEYWORDS: Gold nanoclusters, Au25, photocatalysis, selective organic
transformation, benzylamine oxidation
1
. INTRODUCTION
Atomically precise gold nanoclusters have recently emerged as novel promising
1
catalysts owing to their unique molecular and electronic configurations. The
nanoclusters display intrinsically different catalytic properties in comparison with the
2
conventional (bare) gold nanoparticles. The chemical nature of the nanoclusters often
result in superior or new properties for catalytic reactions, including oxidation,
3
hydrogenation and carbon-carbon bond formation reactions. Because of their
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nanometric size, the gold nanoclusters usually exhibit a non-metallic behavior with
energy quantization manifested in their HOMO–LUMO gap, opposite to localized
2
b
surface plasmon resonance (LSPR) of the conventional nanoparticles. The electrons
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of the gold nanoclusters can be excited from HOMO to LUMO upon illumination,
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resulting in the formation of electron-hole pairs just as semiconductors do. The
generation and subsequent recombination of the electron–hole pairs using solar energy
are of great interest for photocatalytic and photovoltaics applications. For example, the
catalytic activity of gold nanoclusters have been demonstrated in solar energy
harvesting for power generation, pollutant degradation, and hydrogen production.4b, 5
Among gold nanoclusters, Au25(PPh
3
5
)10(SR) Cl
2
nanoclusters, here “-SR”
representing thiolate ligand, show a distinct absorption spectrum in the visible region.6
This nanocluster with a HOMO-LUMO gap of 1.9 eV and a unique framework helping
phosphine ligand dynamically opens up an exciting possibility for practical applications
7
in solar energy harvesting. Light-induced electrons with large reduction potentials can
easily activate O
2
to mediate energy, thus facilitating oxidation reactions. Using
emission spectra and photocurrent experiments, Kamat et al. showed that the electrons
of gold clusters excited by visible light can be injected into the conduction band of TiO
2
8
(
cluster’s support). The Au25(PPh
3
)
10(SR)
5
Cl nanocluster can absorb light, leading to
2
the separation of electrons and holes. The excited electrons would later be injected into
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the supports (e.g., P25) because the LUMO energy of Au25 cluster (-0.63 V) is higher
than the conduction edge (-0.09 V) of P25.10
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Selective oxidation of amines to imines is of significant importance due to the
versatile applications of the product (imines) in fine chemicals and pharmaceuticals.11
Tada and coworkers developed a plasmonic gold photocatalyst which gave 4.5%
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conversion of benzylamine to imine in the absence of solvent. Herein, we investigate
the photocatalytic performance of the P25-supported Au25(PPh SiO Cl
nanoclusters in the selective oxidation of amines to imines with O under mild
3
)10(SC H
3 6
3
)
5
2
2
o
-1
conditions (ca. 30 C). A TOF of 1522 h obtained on this gold nanocluster
photocatalyst is much higher than the conventional gold nanoparticles. To our
knowledge, there have been few reports so far regarding the photocatalytically selective
oxidation of organic compounds by well-defined Au clusters except in the oxidation of
sulfides.13
2. EXPERIMENTAL SECTION
Synthesis of Au25Cl
Seventy six mg of Au(PPh
5 mL of CH Cl and 5 mL of ethanol) under ultrasonic condition, followed by stirring
for 20 min. Then, 6 mg of NaBH dispersed in 2 mL of ethanol was added dropwise.
2
(PPh
3
)
10(SC
3
H
6
SiO
3
5
) /P25 Photocatalyst
3
)Cl and 76 µL of MPTES was dissolved in a mixed solution
(
2
2
4
After 30 min of reaction, 38 µL of MPTES was injected into the system. The reaction
was terminated when the absorption features of Au25 clusters appeared in the UV-vis
spectrum of the solution. The solvent was evaporated by rotary evaporation. The
Au25(PPh
3
)10[SC
3
H
6
Si(OC
2
H
5
)
3
]
5
Cl nanoclusters were obtained by extraction with
2
acetonitrile. And then redissolved in 9 mL of chloromethane. About 6 mL of the cluster
solution was diluted with 24 mL of dichloromethane, and then 1 g of P25 was added.
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After stirring for 12 h, the obtained solids were collected by centrifugation and washed
by dichloromethane three times. The Au25(PPh
dried in vacuum at 60 °C, and denoted hereafter as [Au25]/P25. The synthesis of anatase
and rutile supported Au25(PPh (SR) (R= SC SiO ) nanoclusters followed
almost the same procedure, except P25 was replaced by anatase and rutile (denoted here
after as [Au25]/A-TiO and [Au25]/R-TiO ).
3
)
10Cl
2
[SC
3
H
6
3
SiO ]
5
/P25 solids were
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3
)
10Cl
2
5
3
H
6
3
2
2
Synthesis of AuNP/P25
0.2 g of P25 impregnated with CH
3
OH was dispersed in 50 mL of H O, 0.2 mL of
2
HAuCl (24.3 mM) was added with stirring for 10 min. Then the suspension was
4
irradiated under high-voltage mercury lamp for 30 min. The obtained solid was washed
with water and dried in vacuum at 60 °C for 10 h. Then, AuNP/P25 was obtained.
Photocatalytic Oxidation of Amines
In a typical oxidation reaction, 2 mg of photocatalyst was placed in a 5 mL quartz tube
with a magnetic stirrer. The system was first furnished with O
2
. This followed by
CN. The tube
addition 0.2 mmol of benzylamine, 0.1 mmol of p-xylene, 1 mL of CH
3
was sealed and irradiated with a 9.6 W LED light (455 nm). After the reaction, the
catalysts were separated by a membrane filter. The remaining liquid was examined by
GC and GC-MS. The conversion and selectivity were determined using internal
standard method.
Characterization
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The UV-vis spectra were acquired on a Hewlett-Packard Agilent 8453 diode array
spectrophotometer at room temperature. Diffuse reflectance UV-vis spectra were
recorded by PerkinElmer Lamada 750 UV/VIS/NIR Spectrometer. FTIR spectra were
collected on a Bruker Tensor 27 instrument. The transmission electron microscopy
(TEM) was performed on a JEM-2100 microscope at an accelerating voltage of 200 kV.
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Solid state P magic-angle-spinning (MAS) NMR experiment was conducted in a
Bruker Avance III 500 MHz NMR spectrometer equipped with a 4 mm standard bore
probe head, using a magnetic field of 11.7 T at 300 K. The sample was packed in the
ZrO
2
rotor closed with Kel-F cap, which was then spun at 8 kHz rate. A total of 5000
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scans were recorded with 5 s recycle delay for each sample. P MAS chemical shift is
referenced to the resonances of ADP standard (δ=0.81 ppm). Ultrahigh resolution mass
spectrum was acquired using a Bruker SolariX FT-ICR-MS equipped with a 15.0 T
superconducting magnet and an ESI ion source. Sample for ESI FT-ICR-MS analysis
was continuously infused into the ESI unit by syringe infusion at a flow rate of 120 μL
−
1
h . The ESI needle voltage was set to 4.5 kV. The samples was analyzed in positive
ionization mode with broadband detection. Ions accumulated in a hexapol ion trap for
0.2 s before being introduced into the ICR cell. 4 M words of data were recorded per
broadband mass scan. The lower mass limit was set to m/z = 152.45 Da and the upper
mass limit to m/z = 10000 Da. The concentration of gold was determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES; ICPS-8100, Shimadzu). The
2
1
specific surface area of P25 (72.9 m ·g ) was measured by Micromeritics ASAP 2020
surface area analyzer (Quantachrome). The apparent quantum efficiency (AQE)
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measurement was carried out using a Pyrex top-irradiation-type reaction vessel and a
300 W xenon lamp fitted with a filter. The number of photons reaching the reaction
solution was measured using a calibrated Si photodiode (LS-100, EKO Instruments Co.,
LTD.), and the AQE was calculated according to the following equation:
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amounts of products formed
AQE =
amounts of photons irradiated
We consider one absorbed photons will produce one product. All the AQE
measurements were based on a product yield of less than 20%.
Computational details.
Geometry optimization of Au25(PH
3
)
10(SH) as a model of the nanocluster was
5
performed using the Gaussian09 package with the B3PW91 hybrid density functional.
The 6-31G** basis set was employed for H, P, S and Cl.[14] The LANL2DZ basis set
was used for gold atoms.[15] The energy required for removal of a ligand from the
nanocluster is estimated as the energy difference ΔE = E(NC-L) + E(L) – E(NC), where
E(NC), E(NC-L), and E(L) are total energies of the isolated nanocluster, the
nanocluster with one ligand removed, and the ligand, respectively. The integral
equation formalism polarizable continuum model (IEFPCM) was applied to account for
the solvation effect.[16]
3
. RESULTS AND DISCUSSION
.1 Synthesis and Characterization of [Au25]/P25 Catalysts.
The Au25(PPh Si(OC Cl nanocluster was synthesized via a one-
3
3
)10[SC
3
H
6
2
H
5
3
) ]
5
2
phase size-focusing method. In a typical procedure, the gold nanoclusters were prepared
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through a simple reduction of Au(PPh )Cl with sodium borohydride in the presence of
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excess MPTES (3-mercaptopropyltriethoxysilane). The final products were obtained
by centrifugation and purification. The yield of the obtained Au25 nanocluster was ~ 52%
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based on the Au(PPh )Cl consumption.
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The as-syhtnesized gold nanoclusters were firstly characterized by ultraviolet–
visible spectroscopy (UV-vis). As shown in Figure 1a, red line, the distinct absorption
peaks of the free Au25(PPh
ethanol) appeared at 415, 445 and 675 nm, consistent with reportedly neat
Au25(PPh Cl
nanoclusters in literatures (Figure S1).6, 17 This is different from
3
)10(SC
3
H
6
Si(OC
2
H )
5 3
)
5
Cl
2
nanoclusters (dispersed in
3
)
10(SR)
5
2
the local surface plasmon resonance (LSPR) effect of Au nanoparticle (AuNP)
supported P25 (Figure 1a, green line). For addtional insights, we performed TD-DFT
2+
calculations of [Au25(PH
Results indicated that the first peak at 675 nm (1.84 eV) is attributed to HOMO–LUMO
excitation of the Au25(PPh Cl nanoclusters. The other absorption peaks and
shoulder are found to correspond to the mixtures of electronic transition of HOMO-m
LUMO+n, which m and n are positive integer. To determine the exact composition
of the gold nanoclusters, Fourier transform-ion cyclotron resonance-mass spectrometry
3
)
10Cl
2
5
(SH) ] , as a model of the nanocluster (Figure S2).
3
)
10(SR)
5
2

(FT-ICR-MS) with the ESI soft ionization technique was employed, as it usually does
18
not lead to cluster fragmentation. The mass signal was centered at m/z = 4402.2
(where
z
=
2, Figure S3), corresponding to the doubly charged
2+
[
Au25(PPh
3
)
10(SC
3
6
H Si(OC
2
H
5
)
3
)
5
Cl ] (theoretical value m/z = 4402.5, deviation: ±
2
0
.3). The TEM image showed that the size of the metal core of the free standing gold
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nanoclusters is ~ 1.2 nm (Figure 1b), matching well with the framework of
Au25(PPh
Next, the Au25(PPh
hereafter) was
Au25(PPh (SC
3
)
10(SR)
5
Cl
.3e, 3j
2
3
)
10Cl
2
(SC
obtained
Si(OC
3
H
6
SiO
via
nanoclusters with hydroxyl group of P25. Of note,
3
)
5
/P25 photocatalyst (denoted as [Au25]/P25,
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a
hydrolysis process of
3
)10Cl
2
3
H
6
2
H
5
)
)
3 5
4
b
the P25 nanocrystals comprise two phases, anatase and rutile. The diffuse reflectance
UV-vis spectrum of [Au25]/P25 showed similar optical peaks (especially the
characteristic peak at 675 nm) with that of the unsupported nanoclusters (Figure 1a blue
line vs. red line), implying that the [Au25] nanoclusters keep intact on the P25 surface.
FT-IR analysis indicated that both the phosphine and thiolate ligands ligate on the
nanoclusters (Figure S4). Further, the [Au25]/P25 catalysts were analyzed by solid state
31P magic angle spinning (MAS) NMR spectroscopy (Figure 1c). Only one single peak
at δ = 33.2 ppm was observed in the NMR analysis, indicating that the PPh ligands are
3
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bonded with the gold atoms. Furthermore, TEM analysis showed that the [Au25] is
immobilized on the P25 surface and the diameter of cluster is still ~1.2 nm (Figure 1d
and S5), which is same as that of the unsupported clusters, also suggesting that the
[Au25] nanocluster remains intact on the P25 surface. Of note, the ICP analysis showed
that the gold content of the [Au25]/P25 was ~ 1.04 wt%. Next, the as-prepared
[
Au25]/P25 was investigated in the aerobic photocatalytic oxidation of amines to imines
without any other pretreatments.
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Figure 1. a) UV-vis of Au25(PPh
ethanol), and diffuse reflectance UV-vis spectra of the [Au25]/P25 photocatalyst,
AuNP/P25 and P25 support. b) TEM image of Au25(PPh Si(OC Cl . c)
Solid state P MAS NMR spectrum of the [Au25]/P25. d) TEM image of the [Au25]/P25.
3
)
10(SC
3
H
6
Si(OC
2
H
5
)
3
)
5
Cl
2
nanoclusters (dispersed in
3
)10(SC
3
H
6
H
2 5
)
3
)
5
2
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3
.2 Photocatalytic Properties of [Au25]/P25
In the previous literatures, the TiO
photocatalytic oxidation of benzylamine via a surface complex mechanism. Therefore,
we firstly investigated and compared the catalytic performance of three different TiO
semiconductors (P25, A-TiO , and R-TiO ; A represents anatase and R rutile) and the
corresponding TiO -supported gold nanocluster catalysts ([Au25]/P25, [Au25]/A-TiO
and [Au25]/R-TiO ) in the photocatalytic oxidation of benzylamine. The photocatalysis
was carried out under the conditions as indicated in Table 1. About 12-16%
2
oxides exhibited some activity in the aerobic
2
0
2
2
2
2
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,
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benzylamine conversion with ≥ 99% selectivity for imine was achieved when using P25,
A-TiO
supported on the TiO
conditions. The conversion was improved to 45% for [Au25]/R-TiO
TiO , and 80% for [Au25]/P25. In all cases, the selectivity was above 99% (Table 1,
entries 1-3). In a control experiment without TiO and the gold cluster, no activity was
2
and R-TiO
2
as the catalysts (Table 1, entries 4-6), while the [Au25] catalysts,
showed much better catalytic activity under the identical reaction
, 67% for [Au25]/A-
2
10
11
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2
2
observed at all (Table 1, entry 7). The [Au25]/P25 catalyst showed no activity in the
absence of light irradiation and only 3% conversion under Ar atmosphere (Table 1,
entries 8 and 9). Thus, it indicates that the photocatalysis is indeed promoted by oxygen
and is associated with the nanoclusters.
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1
1
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1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1. Catalytic performance of various catalysts in the photo-oxidation of
benzylamine.a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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entry
1
catalyst
conv. (%)b
sel. (%)b
99
[Au25]/A-TiO
2
67
45
80
14
16
12
0
2
[Au25]/R-TiO
[Au25]/P25
P25
2
99
3
99
4
99
5
A-TiO
2
99
6
R-TiO
2
99
7
−
−
8c, e
9d, e
10
[Au25]/P25
[Au25]/P25
AuNP/P25
0
−
3
99
14
14
99
11
[Au25]/P25-300f
99
aReaction conditions: 2 mg of catalyst, 0.2 mmol of benzylamine, 1 mL of
CH CN, 0.1 mmol of p-xylene as internal standard, O 1 atm, 455 nm light, 2 h.
bThe conversion (conv.) and selectivity (sel.) for imines are determined by GC.
3
2
c
d
e
f
In absence of light, 40 °C . Protected by Ar. 10 mg catalyst, 1.5 h. Annealed at
3
00 °C for 4 h in a H atmosphere.
2
Next, for the purpose of control, we prepared a P25-supported gold nanoparticle
(
AuNP/P25) by photoreduction method. The average size of gold NP was ~ 2.0 nm
Figure S6). Surprisingly, the AuNP/P25 catalyst only gave 14% conversion (entry
(
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0), which was close to the pure P25 catalyst under the same reaction conditions. The
[Au25]/P25 catalyst, annealed at 300 °C for 4 h and noted as [Au25]/P25-300, showed
low catalytic activity (14% conversion) similar to that of the AuNP/P25 catalyst
10
11
12
13
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(
Table 1, entry 11). Of note, after the heat treatment, the structure of [Au25]
nanoclusters was destroyed and only LSPR effect was showed in Figure S7. Further,
we measured the action spectra of [Au25]/P25, which show that the apparent quantum
efficiency at different irradiation wavelengths were in agreement with the absorption
of [Au25]/P25 (Figure S8). These results may indicate the reaction was
photocatalyzed by the excitation of gold cluster. We also measured the average AQE
using the wavelength range from 500 to 800 nm and a result of 4.7% (5.2% at 460
nm) was obtained. These results indicate the photo-excited electrons of gold cluster
were facilitated to the oxidation because the complex between surface hydroxyl of
20
TiO
2
and benzylamine scarcely absorb light longer than 500 nm. Consequently, the
distinct LSPR effect of AuNP (Figure 1, green line) and surface complex mechanism
cannot attribute to the photocatalytic oxidation reaction, indicating that the
photocatalyzed oxidation is mainly induced by [Au25] cluster with suitable electronic
property and structure.
Further, the reaction kinetics of [Au25]/P25 and P25 catalysts were studied by
monitoring the conversion of benzylamine at different time interval. It was found
that > 50% benzylamine was converted within 90 min when the reaction was
catalyzed by [Au25]/P25 catalyst, while plain P25 merely gave a 7% conversion, as
depicted in Figure 2. After an irradiation time for 2 h, the benzylamine conversion
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reached up to 82% for [Au25]/P25 and 14% for P25. The superior catalytic activity of
[Au25]/P25 than pure P25 indicates a different reaction pathway, discussed vide infra.
The TOF of the [Au25]/P25 photocatalyst (TOF = (reacted mol of amine)/[(mol of Au
0
1
2
3
4
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9
0
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9
0
1
atoms) × reaction time in hour)]) was calculated to be 878 h based on a benzylamine
conversion of 14%, which is much higher than those catalyzed by AuNP and other
metals (Table S1).
1
00
80
100
80
60
40
20
0
Au25(PPh
3 5 2
)10(SR) Cl /P25
P25
6
4
2
0
0
0
0
0
20
40
60
80 100 120 140
Time (min)
Figure 2. Time-dependent catalytic performances of [Au25]/P25 and P25 in the
photocatalyzed oxidation of benzylamine. (Reaction conditions: 2 mg of catalyst, 0.2
mmol of benzylamine, 1 mL of CH
1 atm, ~30 °C, 455 nm LED).
3
CN, 0.1 mmol of p-xylene as internal standard,
O
2
The catalytic activity of [Au25]/P25 was further examined with various amines
derivatives. As shown in Table 2, both the benzylamines with electron-withdrawing
and electron-donating groups gave good conversions (73–99%), excellent selectivity to
corresponding imines (Table 2, entries 2-6). The 2-bromobenzylamine was relatively
inert to be converted (73% conversion, Table 2, entry 4), which reveals the presence of
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steric effect. Heterocyclic amine containing of sulfur could also be oxidized to the
corresponding imine in excellent yield (Table 2, entry 7), albeit the sulfur species
usually poison metal catalysts. Over-oxidation occurred which resulted in a methylene
decreased in the final product for phenylethylamine oxidation; a conversion of 69%
with a selectivity of 87% to 1-phenyl-N-(2-phenylethylidene)methanamine was
10
11
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2
1
obtained (Table 2, entry 8), consistent with the previous results. It is worthy to note
that the TOF was calculated to be 1522 h1 based on the 4-methylbenzlamine
conversion of 26%. Furthermore, we conducted the catalyst recyclability for the
benzylamine photocatalytic oxidation. Only <10% decreasing activity was observed
after three cycles all with the selectivity more than 99% (Figure 3).
Table 2. Photocatalytic oxidation of a range of amine substrates over [Au25]/P25
catalyst.a
1
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aReaction conditions: 10 mg of catalyst, 0.2 mmol of benzylamine, 1 mL of CH
CN,
3
b
c
0
.1 mmol of p-xylene as internal standard, O
2
1 atm, 455 nm LED, 1.5 h. 2 h, 3 h.
dTOF was calculated based on reaction for 20 min using 2 mg catalyst, conversion
e
between 13-26%. 30min.
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40
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2
0
0
fresh
1
2
3
Cycle times
Figure 3. Cycling tests of benzylamine oxidation catalyzed by [Au25]/P25.
3.3 Photocatalytic Mechanism Investigation
To get more insight into the role of photo-generated radical species in these
photocatalytic redox reactions, the controlled experiments were conducted. Figure 4
showed the effect of different radical scavengers over photocatalytic oxidation of
benzylamine under visible light. Significant decreasing activities for the photocatalysis
2
2
were observed when K
2
S
2
O (scavenger for electrons ) and ammonium oxalate (AO,
8
2
3
scavenger for holes ) were added into the photocatalysis system. These results clearly
demonstrate that both photo-generated holes and electrons are initial drivers for the
photocatalytic reactions. The [Au25] nanoclusters supported on P25 absorbing the light
can act as a narrow band gap semiconductor leading to separation of electrons and
4b
holes. The electrons can be injected into the conduction band of P25, thus enhancing
the separation efficiency of electrons and holes.4c, 24 The more efficient separation of
carriers can facilitate the oxidation reaction. A reasonably near linearity relationship
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between log(k
X
/k
H
) and the Brown-Okamoto constant  was established for the
oxidation of para-substituted benzylamine (F, Cl, H, CH
3
, OCH ) as shown in Figure
3
S10. The slop of the plot ( value) is negative, indicating that the reaction is probably
0
1
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0
via carbocation intermediate species.
60
40
20
0
K S O
8
AO
None
2
2
Figure 4. Controlled experiments using different radical scavengers for the
photocatalytic selective oxidation of benzylamine over [Au25]/P25. AO= ammonium
oxalate.
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Table 3. Photocatalytic amine oxidation over various catalysts.a
entry
catalyst
additive
conv. (%) sel. (%)
1
2
3
4
5
6
7
[Au25]/P25
–
98
33
53
85
44
20
2
99
99
95
99
99
99
99
Au25(SC
2
H
4
Ph)18/P25 –
PPh b
AuNP/P25
3
Au11(PPh
3
)
7
Cl
3
/P25
–
Au101(PPh
3
)
21Cl
5
/P25 –
TEMPOc
TEMPOc
[Au25]Cl /P25
2
–
aReaction conditions: 10 mg of catalyst, 0.2 mmol of benzylamine, 1 mL
of CH
3
CN, 0.1 mmol of p-xylene as internal standard, O 1 atm, 455 nm
2
LED, 1.5 h.bnPPh3:nAu=27.6, 0.4 mmol of TEMPO ,under Ar.
c
Ph)18/P25 catalyst (Figure S11).3j,
H
Next, for comparison, we prepared a Au25(SC
Intriguingly, the Au25(SC Ph)18/P25 showed a very low catalytic activity (Table 3,
entry 2). On the other hand, the AuNP/P25 catalyst exhibited no increase in catalytic
activity in the presence of excess PPh (Table 3, entry 3), suggesting that the PPh
cannot influence the catalytic activity. Therefore, the electronic property and
2
4
2
5
2
H
4
3
3
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framework of [Au25] nanocluster should account for the high catalytic activity. Further,
the Au11(PPh
presented in previous studies (Figure S12).26 The Au11(PPh
/P25 catalysts gave conversion of 85% and 44%, respectively (Table
3
)
7
Cl
3
and Au101(PPh
3
)21Cl
5
nanoclusters were synthesized using protocols
3
)
7
Cl /P25 and
3
0
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0
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4
5
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7
8
9
0
Au101(PPh
3
)
21Cl
5
3, entries 4 and 5) in the benzylamine photocatalytic oxidation, which are considerably
lower than that for P25-supported [Au25] (98%). It is also worth noting that no direct
size dependence of the cluster catalyst was observed. Further, we conducted the
photocatalysis of benzylamine over [Au25]/P25 using TEMPO to abstract hydrogen
under Ar atmosphere. It is pointed out that TEMPO can abstract hydrogen from the
27
metal surface instead of from the substrate, which plays a similar role as oxygen. A
20% conversion was obtained when TEMPO was present without oxygen (Table 3,
entry 6). Of note, TEMPO itself (without cluster catalyst) cannot oxidize benzylamine,
demonstrating that the Au-H species were indeed generated during the photocatalytic
reaction.
Table 4. DFT results on the energy (eV) required for the removal a ligand from
2+
[
Au25(PH
3
)
10(SH)
5
Cl
2
]
cluster as a model of the nanocluster in gas and solution
(acetonitrile) phases.
PH
1.2
1.3
3
-SH
10.1
2.7
-Cl
8.2
0.9
Gas phase
Solution phase
Then, we briefly discuss the possible mechanism for the photocatalytic reaction
over the gold nanocluster. The [Au25] clusters are comprised of two biicosahedral Au13
units which are surrounded by PPh and MPTES ligands (Figure S13). The DFT
3
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simulations suggest that the benzylamine reactant cannot be adsorbed on the surface of
gold clusters. Therefore, we initially speculated that there should be partial ligand
removal under the reaction conditions, providing active sites for the oxidation. Our
previous studies have found that the phosphine ligands on the gold nanoclusters (e.g.,
10
11
12
13
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[Au11(PPh
3
)
7
Cl
3
]+ and [Au25(C≡CPh)
5
(PPh
), strongly evidenced by the experiments and
, MPTES, and Cl) are ligated
3
)
10Cl
2
]2+ clusters) can be partially detached
in the presence of amine (e.g., NH
3
28
theoretical calculations. Three organic ligands (i.e., PPh
3
on the gold nanoclusters and can be lost during the catalytic process. We here choose
2
+
[
Au25(PH
3
)
10(SH)
5
Cl
2
] as a model for the simulations to rationalize the possible ligand
removal process in the presence of amine. Table 4 showed the energy (eV) required for
2
+
removing a ligand of different types from [Au25(PH
3
)
10(SH)
5
Cl ] nanocluster in gas
2
and solution (acetonitrile, using polarizable continuum model) phases. Results
-
indicated the detachment of a Cl or PH
3
ligands required 0.9 and 1.3 eV/mol,
respectively, which are considerably lower than that for removing a thiolate ligand (2.7
-
eV/mol). These results indicate Cl and phosphine ligands have higher potentials to
leave the nanoclusters, thereby providing gold open metal sites. We think that the
-
removal of a phosphine ligand is more favorable than Cl in solution phase, especially
3
+
-
due to the high charge density of [Au25] formed upon removal of a Cl . This may not
be captured in DFT calculations since we replaced PPh with PH in our model as well
3
3
as overestimation of the solvation effect by continuum approach.
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Ⅰ
Ⅱ
Ⅲ
Scheme 1. Proposed mechanism for photo-oxidation of benzylamine catalyzed by
TiO -suppported [Au25] clusters. Color codes: Au, green; exposed Au, yellow; P, purple;
2
S, magenta; Cl, light green; Ti, gray; O, pink. Partial carbon and hydrogen atoms are
omitted for clarity.
Finally, the reaction mechanism can be rationalized as follows (Scheme 1). [Au25
]
cluster absorbs light which leads to separation of electrons and holes. The excited
•

electron is injected into the P25 support, and O
2
is reduced to O
2
species (Scheme S1).
29
This has been already confirmed by electron spin resonance in a preceding work. The
holes can abstract an electron from the benzylamine to yield a benzylamine radical
3
0
cation, which was also reported previously. and then the -H of benzylamine radical
cation can be abstracted by the exposed gold atom on the nanocluster surface, and then
an Au-H intermediate is formed accompanied by producing a carbocation intermediate.
•

Next, the O
2
species would abstract H from the Au-H and the amine to produce Ph-
(Scheme S1). H was detected by the KMnO as shown in Figure
CH=NH and H
2
O
2
2
O
2
4
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S14. Further, if H
using O
intermediate. The H
2
O
2
was used as oxidant, the reaction proceeded more quickly than
2
as oxidant (Figure S15), also indicating peroxide species may be the active
would be soon decomposed by oxiding benzyalamine that give
2
O
2
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water and benzaldimine.30b notably, the Ph-CH=NH intermediates are very active in the
photocatalysis reaction of benzylamine.12, 30a. The benzaldimine is subjected to
nucleophilic attack by benzylamine to form aminal.30b Finally, the aminal group can
eliminate the ammonia to afford the imine product.
4
. CONCLUSION
In
summary,
we
Si(OC
have
Cl
synthesized
atomically
precise
Au25(PPh
3
)10[SC
3
H
6
2
H
5
)
3
]
5
2
nanoclusters supported on P25 exhibiting
superior selective visible light photocatalytic oxidation of amines to imines with
molecular oxygen. The high catalytic activity arises from the photo-excited separation
of electrons and holes of the Au25(PPh
3
)
10(SR)
5
Cl nanoclusters instead of the LSPR
2
effect. The turnover frequency reaches up to 1522 h1 in the 4-methylbenzylamine
oxidation. The catalytic active sites are the exposed gold atoms due to the partial
removal of PPh ligand. The photocatalysis proceeds a carbocation intermediate and
3
Au-H intermediate species. This work may lay a firm foundation of visible light
photocatalytic selective organic transformation over gold nanoclusters.
ASSOCIATED CONTENT
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Supporting Information.
The Supporting information is free of charge on the ACS Publications website at
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Additional characterization data, including Uv-vis, FT-ICR-MS and IR spectra,
TEM image, Diffuse reflectance UV-vis spectroscopy, details regarding data
processing, kinetics data, Au25 schematic drawing(PDF).
AUTHOR INFORMATION
Corresponding Authors
*
Tel. / Fax: (+86) 0411-84379762. E-mail: wangfeng@dicp.ac.cn (F. Wang)
*Tel / Fax: (+86) 0411-82463017. E-mail: gaoli@dicp.ac.cn (G. Li)
Author Contributions
The manuscript was written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
ORCID
Feng Wang: 0000-0002-9167-8743
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (21422308,
2
1690080 and 21601178), the Strategic Priority Research Program of CAS
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(
XDB17020300) and the starting funds of “Thousand Youth Talents Plan” (GL) for
financial support.
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Table of contents
TOF reach up to 1522 h-1
4
55 nm
e
h+
O2•
O2•

O2
e
2
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