Exposing the Delocalized Cu?S π Bonds on the Au24Cu6(SPhtBu)22 Nanocluster and Its Application in Ring-Opening Reactions

Article abstract of DOI:10.1002/anie.201907609

Bimetallic nanomaterials are of major importance in catalysis. A Au-Cu bimetallic nanocluster was synthesized that is effective in catalyzing the epoxide ring-opening reaction. The catalyst was analyzed by SCXRD and ESI-MS and found to be Au₂₄Cu₆(SPhtBu)₂₂ (Au₂₄Cu₆ for short). Six copper atoms exclusively occupy the surface positions in two groups with three atoms for each, and each group was bonded with three thiolate ligands to give a planar motif reminiscent of a benzene ring. In the epoxide-ring opening reaction, Au₂₄Cu₆ exhibited superior catalytic activity compared to other homometallic and Au-Cu alloy NCs, such as Au₂₅ and Au_38?xCu_x. Control experiments and DFT calculations revealed that the π conjugation among the Cu?S bonds played a pivotal role. This study demonstrates a unique π conjugation established among the Cu?S bonds as a critical structural motif in the nanocluster, which facilitates the catalysis of a ring-opening reaction.

Full text of DOI:10.1002/anie.201907609

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Accepted Article
Title: Exposing the delocalized Cu-S π bonds on Au24Cu6(SPhtBu)22
nanocluster and its application in ring opening reaction
Authors: Manzhou Zhu, Jinsong Chai, Sha Yang, Ying Lv, Hanbao
Chong, and Haizhu Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907609
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10.1002/anie.201907609
Angewandte Chemie International Edition
COMMUNICATION
Exposing the delocalized Cu-S π bonds on Au24Cu6(SPhtBu)22
nanocluster and its application in ring opening reaction
Jinsong Chai,+ Sha Yang,+ Ying Lv, Hanbao Chong, Haizhu Yu,* and Manzhou Zhu*
Abstract: Bimetallic nanomaterials are of major importance in
catalysis. In this study, we synthesized a novel Au-Cu bimetallic
nanocluster and found it effective to catalyze the epoxide ring
opening reaction. The catalyst was analyzed by SCXRD and ESI-MS
to be Au24Cu6(SPhtBu)22 (Au24Cu6 for short). Six copper atoms were
found to exclusively occupy the surface positions in two groups with
three atoms for each, and each group was bonded with three thiolate
ligands to give a planar motif reminiscent of a benzene ring. In the
epoxide ring opening reaction, Au24Cu6 exhibited superior catalytic
activity compared to other homometallic and Au-Cu alloy NCs, such
as Au25 and Au38-xCux. Control experiments and density functional
theory (DFT) calculations revealed that the π conjugation among the
Cu-S bonds played a pivotal role. This study demonstrates a unique
π conjugation established among the Cu-S bonds as a critical
structural motif in the nanocluster, which facilitates the catalysis of a
novel type of ring opening reaction.
nanoclusters, the mechanisms for both reactions were
successfully elucidated.19,20 Tsukuda et al. improved the
performance of an Au25 catalyst for the aerobic oxidation of
benzyl alcohol by doping it with Pd, a change ascribed to
electron transfer from Pd to Au in the Pd1Au24 cluster.21 These
successes greatly intrigued us, and we wondered whether the
scope of nanocluster catalysis could be broadened further.
The introduction (doping) of a foreign metal (e.g. Ag, Cu, Pt,
Pd, etc.) atom(s) to gold nanoclusters has been shown to
perturb their electronic structure.22-27 Zheng et al. found that Cu
atoms present multiple coordination with ligands, and
preferentially locate on the nanocluster surface.28,29 Similar
observations were observed in
a series of Au-Cu alloy
nanoclusters, reported by our group.30-31 In a continuation of
these studies, we use the gold and copper salts to prepare new
Au-Cu alloy nanoclusters that bear catalytically active Cu atoms
on the surface. This could give rise to peripheral Cu-ligand
motifs, which are expected to facilitate novel catalytic reactions.
Herein, a new Au-Cu bimetallic NC, i.e. Au24Cu6(SPhtBu)22 is
synthesized, crystallized, and characterized by SCXRD. The
structure of Au24Cu6(SPhtBu)22 comprises a 30-atom metal core
(24 gold atoms and 6 copper atoms), which is protected by 22
thiolate ligands. The 6 copper atoms are doped onto the surface
of the alloy nanocluster and exist in two groups, and each group
is bonded to three thiolate ligands to form a planar, trimeric
Cu3(SR)3 structure that resembles a phenyl group. The exposure
of the planar Cu3(SR)3 motif is pivotal to the catalytic activity of
Au24Cu6 in the epoxide ring opening reaction. Theoretical
calculations have identified the active site to be the planar
Cu3(SR)3 and demonstrated that Cu(d)-S(p) π-bonding and the
conjugation between different π-moieties are mainly responsible
for the catalytic activity.
Owing to the high surface area and unique properties, metal
nanoparticles have emerged as an important category of
heterogeneous catalysts for
a
wide range of chemical
transformations.1,2 Recent studies have shown that catalytic
activity can be enhanced via tuning the composition, size, shape,
and surface structure of the nanoparticles.3-5 However,
a
detailed, mechanistic explanation of the catalytic performance of
nanoparticles is still lacking, in part due to the mysterious nature
of the surface layer of the catalyst. Ultrasmall metal nanoclusters
(size < 2 nm), which have excellent stability, molecular-like
spectroscopic properties, well-defined atomic packing modes,
and are amenable to characterization by single crystal X-ray
diffraction (SCXRD) techniques, have emerged as a means of
studying catalytic mechanisms and thereby improving catalytic
activity.6-11
The Au24Cu6(SPhtBu)22 alloy cluster was prepared by an in
situ two-phase ligand exchange method.30 CuCl2 and reduced
glutathione were dissolved in water. Then, an aqueous solution
of HAuCl4·4H2O was added and the mixture was stirred. After 20
mins, a solution of borane-tert-butylamine complex and 4-tert-
butylbenzenethiol in toluene was added quickly. After 4 hours,
the crude product (in toluene) was washed several times with
methanol and collected by centrifugation to give a pure product
(yield: 5% based on Au atom) (Figure S1a). The product shows
two main peaks at 380 and 550 nm in UV-Vis (Figure S2).
Crystals were obtained by layering methanol onto a CH2Cl2
solution of the cluster, with a yield of ~1% based on the initial Au
atom (Figure S1b). SCXRD measurements determined its
formula to be Au24Cu6(SPhtBu)22. The Au24Cu6 NC crystallizes
in a triclinic crystal system (space group P-1). The details of
crystal data and the refinement are provided in the Supporting
Information.
To date, a variety of metal nanoclusters have been found to
exhibit
excellent
catalytic
performance
in
oxidation,
hydrogenation, and carbon−carbon coupling reactions.12-15
Specifically, the electronic structure of nanoclusters, which is
amenable to modeling by theoretical calculations,16-18 greatly
influences their catalytic activities. In this context, both precise
elucidation of the catalytic mechanism and the rational
modifications to the electronic structure of the nanocluster
catalyst become possible. For example, Pt1Au24(SC6H13)18 and
Au11(PPh2Py)7Br3 nanoclusters show higher activity than the
traditional
catalysts
in
hydrogen
production
and
nitrobenzaldehyde hydrogenation. Based on the structures of
[*]
J. Chai,[+] S. Yang,[+] Y. Lv, Prof. H. Yu, Prof. M. Zhu
Department of Chemistry and Centre for Atomic Engineering of
Advanced Materials, Anhui Province Key Laboratory of Chemistry
for Inorganic/Organic Hybrid Functionalized Materials,
Anhui University, Hefei, Anhui, 230601, China
E-mail: yuhaizhu@ahu.edu.cn; zmz@ahu.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201907609
Angewandte Chemie International Edition
COMMUNICATION
within the planar Cu3(SR)3 structure can be seen clearly in
several highest occupied molecular orbitals such as HOMO-15,
HOMO-16, and HOMO-18. Figure 2 shows an illustrative figure
on the conjugation between different π-orbitals in one Cu3S3
structure, and the full details are given in Figure S5.
Figure 2. Schematic diagrams of the π-bonding of selected HOMOs of
Au24Cu6 nanocluster.
Electrospray ionization mass spectrometry (ESI-MS), thermal
gravimetric analysis (TGA), nuclear magnetic resonance (NMR)
and X-ray photoelectron spectroscopy (XPS) were used to
confirm the formula of the Au24Cu6 nanocluster, and the details
are given in Figures S6-S12.
Figure 1. The core (a), surface motif (b) and hexagons (c) of the Au24Cu6 alloy
nanocluster (color labels: magenta/light green = Au, orange = Cu, cyan = S; C
and H atoms are not shown).
We used the epoxide ring opening reaction as a model
reaction to investigate the catalytic activity of the Au24Cu6. After
the optimization of catalytic conditions (Figure S13-S14), the
catalytic activity of the Au24Cu6 was tested. For comparison, the
capacity of other nanoclusters (including the popular Au25,34 and
The structure of Au24Cu6(SPhtBu)22 (Figure S3a) is distinct
from the Au30(S-Adm)18 and Au30S(StBu)18 nanoclusters, albeit
all of them contain the same number of metal atoms.32,33
Au24Cu6 consists of a 30-atom metal core (24 gold and 6 copper
atoms), with 22 peripheral thiolate ligands. As shown in Figure
S3b, each unit cell of this NC contains a pair of enantiomers (for
clarity, only one of them is depicted in Figure S3b). Interestingly,
all six Cu atoms are doped onto the staple motifs, and therefore
the structure of the NC could be viewed as comprising an Au18
kernel flanked by a pair of Au3Cu3S9 staple motifs on each side
and four bridging thiolate ligands (Figure S3b). In detail, the Au18
kernel is composed of one Au13 unit (Figure 1a, top left) and one
Au11 unit (top right), that share six Au atoms (pale blue spheres).
In each Au3Cu3S9 motif (Figure 1b), the Cu atoms are all triply
coordinated to the S atoms. However, the three Cu atoms in
each motif are also cyclically bonded to three S ligands to form a
planar Cu3(SR)3 structure that closely resembles a phenyl group
(Figure 1c). The structure of the two Cu3(SR)3 motifs are almost
the same (Figure S4). For clarity, we take the left Cu3(SR)3 motif
as an example for a detailed analysis. The bond lengths of Cu-S
are in the range of 2.188-2.374 Å. The trimeric ring motif adopts
a planar conformation, and the S−Cu−S angles are 103.54°,
115.20° and 116.38°, respectively. The angle sum of the
hexagon ring is 716.55°.
31
35
the alloy Au38-xCux and Ag28Cu12 nanoclusters) were also
tested for the same reaction. NMR analysis was used to analyze
the product (Figure S15-S16). Detailed results are given in Table
1. In the absence of a catalyst (entry 1), a yield of 3% was
achieved after ~12 hours. The Au-SPhtBu complex (Figure S17)
exhibited a low catalytic activity (entry 2), and the yield is similar
to that of the blank reaction. To our delight, the Au24Cu6 NC
(single crystal) showed remarkable activity, yielding conversion
of 96% after 12 hours (Table 1, entry 4 and Figure S18), similar
to the SBA-15 (Table 1, entry 9). In contrast, Au25 showed quite
poor catalytic activity (4% conversion, entry 3). This result
indicates that Au atoms are unlikely to be the active site of
Au24Cu6. The conversion was also very low (3%) when another
Au-Cu alloy cluster, Au38-xCux, was tested under the same
conditions. This suggests that the specific structural motifs,
rather than the general electron transfer between the Cu and Au
atoms, account for the significantly higher catalytic activity of
Au24Cu6. Interestingly, another Ag-Cu alloy Ag28Cu12, which
bears similar Cu3S3 cyclic motifs to Au24Cu6(SPhtBu)22 but in a
chair-like configuration, showed poor catalytic activity (entry 6,
7% conversion). In addition, the Cu3(SR)3(iPr3P)3 complex36
(Cu3(SR)3(iPr3P)3 complex was characterized by ESI-MS and
TGA shown in Figure S19) also showed a lower catalytic activity
(Table 1, entry 10). Taking the results together, we conclude that
the planar nature of the Cu3S3 motifs is essential for effective
catalysis and the superior catalytic activity of Au24Cu6 was
suggested to originate from the synergy between gold and
copper atoms.
The planar Cu3(SR)3 structure as a component on the surface
of nanoclusters is unprecedented and intriguing. We used DFT
calculations to explore the electronic structure of Au24Cu6 with
such a unique type of surface motif. The optimized geometry of
Au24Cu6 showed the Cu3(SR)3 motif to be planar, which is
consistent with the SCXRD analysis. In addition, the Cu(d)-S(p)
π-bonding and anti-bonding interactions were found to
significantly contribute to the molecular orbitals. Particularly, the
π-bonding and the conjugation between different π-moieties
Recycling tests of the Au24Cu6 catalyst showed that the
catalytic activity maintained after three cycles (entries 7 and 8).
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10.1002/anie.201907609
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The UV-vis absorption and ESI-MS spectra of Au24Cu6 alloy
recorded before and after the catalysis proved it to be stable
under the reaction conditions (Figure S20).
Table 1. Epoxide ring opening reaction under different
catalysts.[a]
Tss-1. After that, the C-O bond gradually forms to generate the
transition state Tss-2, in which the C-O bond is shortened to
2.37 Å (from 3.43 Å in Tss-1). The further approach of the
MeOH results in an automatic H transfer from the hydroxyl group
of MeOH to the O atom of epoxide, and the energy decreases
continuously throughout this process. Finally, the release of the
product (Tss-3 → Prod) is thermodynamically favored by 1.2 eV,
with simultaneous regeneration of the catalyst Au24Cu6.
From Figure 3, the Au24Cu6 catalyzed epoxide ring opening
reaction occurs via the C-O bond cleavage of styrene oxide,
successive C(epoxide)-O(alcohol) bond formation and H-transfer,
and the final catalyst regeneration steps. The second step is the
rate-determining step, and the overall energy demand is 1.07 eV
T
Time
(hours)
Con.
(%)[b]
Sel.
(oC)
Entry Catalyst
(%)[b]
o
which is achievable under the experimental condition (60 C). In
1
2
None
60
60
12
12
addition, according to Mulliken charge analysis (Table S2), the
Cu3S3 motif actually acts as the electron donor, transferring an
electron to the epoxide substrate to activate its C-O bond.
Although other nanoclusters such as Ag28Cu12 also has Cu3S3
motif but in a chair-like configuration. The DFT result shows that
using the Ag28Cu12 as the catalyst, the relative energy is higher
than that of Au24Cu6 by 0.65 eV (Figure S21). Therefore,
Au24Cu6 NC with the planar nature of the Cu3S3 motifs exhibited
superior catalytic activity compared to other NCs.
<3
4
>98
>98
Au-SPhtBu
3
Au25
60
60
60
60
60
60
60
60
12
12
12
12
12
12
12
12
4
>96
3
>98
>98
>98
>98
>98
>98
>98
>98
4
Au24Cu6
Au38-xCux
Ag28Cu12
5
6
7
2
97[c]
98[d]
>98
32
7
Au24Cu6
3
8
Au24Cu6
9
SBA-15
Cu3
10
[a] Reaction conditions: epoxide (0.5 mmol), CH3OH (1.5
mL), catalyst (1 mg).
[b] Determined by GC analysis.
[c] Catalyst cycle second times.
[d] Catalyst cycle third times.
The Au24Cu6 was further examined for epoxide ring opening
reaction of various epoxide derivatives and alcohols (Table S1).
This nanocluster indeed shows high catalytic activity for the
epoxide ring opening.
To investigate the catalytic mechanism of Au24Cu6, we carried
out DFT calculations. It was found that the planar Cu3S3 motif is
the active site. The left and the right Cu3S3 motifs were found to
show comparable reactivity (Figure S21), and thus only the
energy profiles of the left Cu3S3 motif were given in Figure 3 for
clarity reasons. The DFT calculations indicate that the approach
of the substrate to the Au atom is significantly more energy-
demanding (Figure S22). Also, epoxide is preferentially activated
compared to MeOH (Figure S23). As shown in Figure 3, the
activation of the epoxide could occur via two modes (Prec1 and
Prec2), both of which result in 2C-O bond dissociation. In Prec1,
only one Cu atom participates in the 2C-O activation, and the
formed O-Cu bond slightly distorts the planarity of the Cu3S3
motif. In contrast, two Cu atoms participate in the activation
process in Prec2, and the formed O-Cu and 2C-Cu bonds
significantly distort the planar Cu3S3 structure into a chair-like
one. The relative energy of Prec2 is higher than that of Prec1 by
0.3 eV, even though the 2C atom is stabilized by coordination
with the Cu atom. This phenomenon is attributed to the
perturbation of the π-interactions in Prec2 and is disfavored.
From Prec1, the energy of the reaction system decreases with
the approaching of MeOH until the formation of the intermediate
Figure 3. Energy profile for Au24Cu6 catalyzed epoxide ring opening reaction.
In summary,
a
novel Au-Cu bimetallic nanocluster
Au24Cu6(SPhtBu)22 is obtained, and the cluster is found to
catalyze the epoxide ring opening reaction, which is the first
report of
a nanocluster catalyst for this reaction. The
composition and atomic structure of Au24Cu6(SPhtBu)22 were
verified by a variety of characterization techniques, including
SCXRD, ESI-MS, TGA, and XPS. All six copper atoms are
present on the surface of the alloy nanocluster as two phenyl-
like Cu3(SR)3 structures. DFT calculations demonstrated that the
Cu3(SR)3 structure, the Cu(d)-S(p) π-bonding, and the
conjugation between different π-moieties in Au24Cu6(SPhtBu)22
are all essential for catalysis. One Cu atom acts as the active
site to promote C(benzyl)-O bond dissociation. Subsequently,
the approach of MeOH results in the formation of a C(alcohol)-
O(epoxide) bond, associated with an automatic H transfer from
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Entry for the Table of Contents
COMMUNICATION
We synthesized Au24Cu6(SPhtBu)22
and found it effective to catalyze the
epoxide ring opening reaction.
Jinsong Chai,+ Sha Yang,+ Ying Lv,
Hanbao Chong, Haizhu Yu,* and
Manzhou Zhu*
Page No. – Page No.
Exposing the delocalized Cu-S π
bonds on Au24Cu6(SPhtBu)22
nanocluster and its application in ring
opening reaction
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