Ligand-protected Au4Ru2and Au5Ru2nanoclusters: Distinct structures and implications for site-cooperation catalysis

Article abstract of DOI:10.1039/d0cc04692b

We report two ligand-protected Au4Ru2 and Au5Ru2 nanoclusters with distinct atomic-packing modes and electronic structures, both of which act as ideal model catalysts for identifying the catalytically active sites of catalysts on the nanoclusters. Au5Ru2 exhibits superior catalytic performances to Au4Ru2 for N-methylation of N-methylaniline to N-methylformanili, which is likely due to the site-cooperation catalysis of Au5Ru2. This journal is

Full text of DOI:10.1039/d0cc04692b

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Ligand-protected Au Ru and Au Ru
4
2
5
2
nanoclusters: distinct structures and
Cite this:DOI: 10.1039/d0cc04692b
implications for site-cooperation catalysis†
Received 7th July 2020,
Accepted 13th September 2020
a
a
a
a
a
Yongnan Sun, Dan Yang, Yuying Zhang, Weigang Hu, Xinglian Cheng,
a
b
a
Xu Liu,
Mingyang Chen
and Yan Zhu
*
DOI: 10.1039/d0cc04692b
rsc.li/chemcomm
We report two ligand-protected Au
with distinct atomic-packing modes and electronic structures, both explored in catalysis science.
of which act as ideal model catalysts for identifying the catalytically nanoclusters, bimetal nanoclusters further expand the studied
active sites of catalysts on the nanoclusters. Au Ru exhibits system in catalysis due to the fact that the surface composition
superior catalytic performances to Au Ru for N-methylation of and the structure of the nanoclusters can be tailored by the
N-methylaniline to N-methylformanili, which is likely due to the introduction of foreign metal atoms, which thus can tune
4 2 5 2
Ru and Au Ru nanoclusters gold atoms and m = the number of ligands) have been widely
1
2,13
Compared with homo-metal
5
2
4
2
1
4
site-cooperation catalysis of Au
5 2
Ru .
the electronic and catalytic properties of the nanoclusters.
The so-called Au-doped bimetallic nanoclusters can be facilely
Single-atom catalysts possess extraordinary atom utilization obtained by substituting one or more gold atoms with noble
ability and significant research efforts have been devoted to metal elements (e.g., Ag, Pt, and Pd) or non-precious metal
1
–3
15
single-atom catalysis.
Nevertheless, the chemical reactions elements (such as Cu, Cd, and Hg). However, ruthenium, one
occurring on multiple-active-sites cannot be currently determined of the most active catalysts in heterogeneous and homogeneous
by using single-atom catalysts or single-site metal catalysts. The reactions, has been rarely reported as a dopant that can be
1
6
emergence of atomically precise metal nanoclusters provides an doped into gold nanoclusters.
exciting opportunity to investigate and compare single-site and Herein, we synthesized two gold–ruthenium bimetallic
site-cooperation catalysis, since their atomic-level structures have nanoclusters, namely Au Ru (PPh ) (SC H Ph) and Au Ru (PPh ) -
4
2
3 2
2
4
8
5
2
3 2
4,5
been solved by single-crystal X-ray analysis.
These metal (PMeC ) Cl . (PMeC = 1,2,3,4,5-pentamethylcyclopentadienyl),
5 2 2 5
nanoclusters are ideally composed of an exact number of metal whose total structures have been solved. The two Au Ru2
x
atoms, which is typically different from that in single-atom (x = 4 and 5) nanoclusters are exploited as model catalysts for
6
catalysts. Metal nanoclusters as well-defined catalysts are promis- deciphering single-site and dual-site catalysis.
ing to establish a precise relationship between the catalytic
properties and the atomic-packing structures, which is a central nanoclusters is shown in Fig. 1a and b. Au
The structural comparison of the two Au
x
Ru
Ru
2
(x = 4 and 5)
4
2
presents a
issue that remains unaddressed in conventionally heterogeneous crown-like structure, in which eight S atoms bridge four Au
7
catalysts. In particular, only a minor change in the composition atoms and two Ru atoms to form a distorted octahedral
or the structure of these metal nanoclusters can lead to drastic structure (Fig. 1a). The apex Ru atoms are protected by two
8,9
changes in their physicochemical properties.
PPh ligands with a Ru–P bond average length of 2.204 Å. Each
3
Significant advances have been made in the precise syn- Ru atom is surrounded by four S atoms and the Ru–S bond
thesis of metal nanoclusters and the identification of their total lengths are in the range of 2.368–2.390 Å. A gold atom binds
1
0,11
structures.
Within this field, homo-metal nanoclusters to two S atoms, where the Au–S bond distance is 2.310 and
1
6
such as Au
n
(SR)
m
(where SR is the ligand; n = the number of 2.318 Å, respectively. Notably, Ru atoms are not bound with
Au atoms (that is, no Au–Ru bond) and the two Ru atoms are
a
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
10093, China. E-mail: zhuyan@nju.edu.cn
not coordinated with each other (no Ru–Ru bond), and thus
each Ru atom may be regarded as a single-Ru-site. Interestingly,
Au Ru shows a distinct atomic-packing pattern. As shown
2
b
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, China
5
2
5 2
in Fig. 1b, the Au Ru cluster is a regular decahedron with five
†
Electronic supplementary information (ESI) available. CCDC 2014573. For ESI
Au atoms forming the plane of a standard five-membered ring
and two Ru atoms at the apex of the roof ridge. Two Ru atoms
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0cc04692b
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Fig. 2 (a) Normalized Ru K-edge XANES profiles of Au
4
Ru
and Au
2
and Au
5 2
Ru .
(
b) Normalized Au L3-edge XANES profiles of Au
4
Ru
.
2
5
2
Ru . (c) Ru 3p
4 2 3 2 2 4 8 5 2
Fig. 1 Crystal structures of (a) Au Ru (PPh ) (SC H Ph) and (b) Au Ru -
and (d) Au 4f XPS profiles of Au Ru and Au Ru
4
2
5
2
(
PPh
3 3 5 2 2
) (PMeC ) Cl . Color labels: magenta = Au; green = Ru; yellow = S;
grey = P; blue = Cl. The H atoms are omitted for clarity. (c) ESI-MS and
(
(
d) UV-vis spectra of Au
PMeC Cl . The inset shows the photograph of Au
Cl
4
Ru
2
(PPh
3
)
2
(SC
2
H
4
Ph)
8
and Au
5
Ru
2 3 3
(PPh ) -
and Au species of the two clusters. As shown in Fig. 2a, both
Au Ru and Au Ru show a higher absorption energy and a
stronger white line intensity than Ru foil, indicating that the
valence states of Ru in the Au Ru and Au Ru clusters are more
, and five Au atoms are capped by positive than those of Ru . Compared with Au Ru , the adsorp-
and two chlorine atoms, respectively. The average tion threshold of Au Ru is closer to that of Ru(IV), meaning
Au–Ru–Au angle is 62.41, where the Au–Ru bond average that the two Ru in the Au Ru cluster are more positively
5
)
2
2
Ru
4 2
and Au Ru
5 2
dissolved in CH
2
2
.
4
2
5
2
4
2
5
2
0
are coordinated with PMeC
three PPh
5
4
2
3
5
2
5
2
distance is 2.780 Å. The two Ru atoms form the Ru–Ru bond charged. This is likely caused by transferring more electrons
with a bond length of 2.559 Å. Therefore, unlike the Au Ru cluster, from Ru to PMeC than from Ru to PPh . The Au L -edge
constitute a dual-site Ru–Ru. profiles in the Au Ru and Au Ru clusters seem to be similar
Ru and Au Ru clusters are further (Fig. 2b), but the white line intensities of Au in Au Ru and
Ru are higher than those of the Au foil but lower than those
analysis. A prominent peak is observed at m/z 2745 corresponding of the previously reported Au
4
2
5
3
3
the two Ru atoms in Au
5
Ru
2
4
2
5
2
The formulas of the Au
4
2
5
2
4
2
confirmed by electrospray ionization mass spectrometry (ESI-MS) Au
5
2
+
1.62 17
,
implying that the gold
are slightly positive.
To further determine the surface composition and metal
UV-vis absorption spectrum of Au Ru shows a major absorption valence states of the two Au Ru and Au Ru clusters, X-ray
+
to the [Au
4 2 3 2 2 4 8 4 2 5 2
Ru (PPh ) (SC H Ph) + Cs] adduct, and the m/z 2316 charge states in Au Ru and Au Ru
+
peak is assigned to [Au Ru (PPh ) (PMeC ) Cl ꢀ e] (Fig. 1c). The
5
2
3 3
5 2
2
4
2
4
2
5
2
band at 640 nm and two humps at 349 and 453 nm (Fig. 1d), photoelectron spectroscopy (XPS) was carried out. Ru 3p spectra
respectively, which are basically consistent with the computed instead of Ru 3d were detected to avoid the interference from
gaps between the highest occupied molecular orbitals and the the C 1s signal. From the Ru 3p XPS profiles of Au
4
Ru
2
and
16
0
lowest unoccupied molecular orbitals. The optical spectrum of Au Ru (Fig. 2c), we can differentiate the Ru 3p peaks into Ru ,
5 2
d+
4+
the Au
5
Ru
2
cluster presents a prominent peak at 575 nm and a Ru (0 o d o 4) and Ru . The peaks at binding energies (BEs)
0
broad band at 375 nm (Fig. 1d). As shown in Fig. S1 (ESI†), the of 461.2 and 483.2 eV are assigned to Ru , those at BE = 462.4
d+
575 nm band is attributed to the excitation from the high-lying and 484.4 eV are attributed to Ru , and those at BE = 464.5 and
4
+ 18,19
occupied orbitals to the low-lying unoccupied orbitals. The high- 486.5 eV are attributed to Ru ,
suggesting that the average
lying occupied orbitals near the SOMO (singly occupied molecular chemical valences of Ru in the two samples are positive. On the
orbital) are contributed by the delocalized orbital density from Au basis of the relative amounts of the Ru species calculated from
0
and Ru, and also by Cl and PPh
are dominated by C Me and PPh
tion involves the excitation from the metals (Au and Ru) to C
The 372 nm band can be a mix of various types of excitations. The Au
3
. The low-lying unoccupied states the XPS data (Table S1, ESI†), Au
5
Ru
. Therefore, the 575 nm excita- and higher content of Ru compared with Au
Me. that the valence state of Ru in Au Ru is higher than that in
Ru , which matches well with the XANES results. Besides
2
has lower content of Ru
4
+
5
3
4
Ru
2
, indicating
5
5
2
4
2
excitation from the occupied metal orbital to the unoccupied this, no obvious difference of the Au 4f spectra is observed in the
metal orbital appears to contribute significantly to the 372 nm two samples with a small amount of positive charge, which is
band. The optical characteristics of the two clusters suggest the consistent with the Au L
electronic structure differences between the two clusters. We recognized that the two Au
3
-edge analysis.
2
Ru (x = 4 and 5) clusters
x
X-ray absorption near edge structure (XANES) measure- showed structural and electronic differences, especially in the
ments were performed to measure the charge states of the Ru Ru atoms. Thus, it should be possible to utilize the two AuRu
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Fig. 3 Catalytic performances of the Au
N-methylation of substituted aromatic amines with CO
4
Ru
2
and Au
5
Ru
and phenylsilane:
a) conversion of the substrates and (b) the selectivity of the products.
Reaction conditions: 0.5 mmol substrate, 3 mg catalyst, 0.03 mmol tpy,
PhSiH (5 equiv.), toluene (5 mL), CO (15 bar), 50 1C, 10 h. The products
2
catalysts for
2
(
2
were determined by GC using M-xylene as an internal standard.
Fig. 4 Time-resolved in situ FT-IR spectra for the N-methylation reaction
5 2
of N-methylaniline on the Au Ru catalyst.
clusters to explore the differences between the single-site
activity and the dual-site activity for a suitable model reaction.
Here the N-methylation reaction of N-methylaniline with CO
was used as a probe reaction. As shown in Fig. 3a, the clusters with time extension under the reaction conditions.
2
used to detect the possible changes of the ligands in the two
2
0
N-methylaniline (MA) conversion on the Au Ru catalyst can Clearly, the IR band intensity of the ligands did not change with
5
2
be up to 98% with 91% selectivity for N-methylformanilide the extension of the treatment time, which indicated that the
MFA) as the major product and a small amount of N,N- ligands of the clusters were hardly shed under the reactions
dimethylaniline (DMA) as a by-product. By contrast, the MA (Fig. S4, ESI†). Therefore, this further indicated that the Au Ru
conversion on the Au Ru cluster catalyst was very low (38%) and Au Ru clusters were intact throughout the reactions. It is
with MFA as the major product. Notably, the optimal reaction worth pointing out that the pure ligands had no activity for the
(
5
2
4
2
4
2
0
0
0
00
conditions were obtained using 2,2 :6 ,2 :2 -tert-pyridine (tpy) N-methylation reactions.
as the reaction promoter (Fig. S2, ESI†).
Time-resolved in situ FT-IR spectroscopy was utilized to
Furthermore, as various substituted secondary anilines were monitor the evolution of the reaction intermediates. As pre-
ꢀ1
used as reactants, a similar activity trend was observed, that sented in Fig. 4, the band at 1390 cm , assigned to the d(N–H)
is, Au Ru always exhibited a higher conversion of MA deformation vibration on the secondary amine, gradually
than Au Ru . When the secondary anilines containing appeared fragile with the reaction time, implying that MA was
electron-donating 4-methoxy- or 4-methyl- groups were used as constantly consumed. Moreover, there was a new IR band
5
2
4
2
2
1
ꢀ1
reactants, the conversions slightly improved (498.5%). Due appearing at 1258 cm , assigned to the n(C–O) stretching
ꢀ
to the larger steric hindrance effect of ortho-substitution vibration of the HCOO species, indicating the formation of
2
2,23
compared to para-substituted secondary anilines, the conver- formate species over the Au Ru catalyst.
Additionally, new
5
ꢀ
2
1
sion of electron-donating 2-methyl-N-methylaniline to the bands at 1678 and 1320 cm assigned to n(CQO) and n(C–N)
2
4,25
corresponding products required a longer reaction time. When were also observed, respectively.
electron-withdrawing 4-chloro-substituted n-methylaniline was Au Ru catalyst can indeed convert MA into MFA under mild
used as a reactant, the product selectivity showed little change, conditions. Based on the in situ FT-IR results, a plausible
but the conversion decreased obviously. reaction pathway was proposed. On the Au Ru catalyst, the
As a comparison, single-site Au or Ru catalysts such as N-methylation of MA with CO and PhSiH may be carried out
AuPPh Cl, Ru(PPh Cl and Ru(PMeC Cl had little catalytic through two steps. First, CO was activated by Au Ru to react
The result verified that the
5
2
5
2
2
3
3
)
3
2
5
)
2
2
2
5
2
activity under identical reaction conditions (Table S2, ESI†). with H on PhSiH to form formate species on the catalyst
This indicates that the N-methylation reaction might not be surface. Second, MFA was formed by the reaction of formate
initiated by using these single-site metal catalysts. Meanwhile, intermediates with MA. Due to the insufficient hydrogenation
3 5 2 2
with AuPPh Cl and Ru(PMeC ) Cl as a mixed catalyst, abilities of the two cluster catalysts and the insufficient redu-
N-methylaniline was hardly converted. The result suggested cibility of PhSiH, it is difficult for MFA to be further hydro-
that the synergistic effect between Au and Ru or Ru and Ru of genated to DMA on the cluster catalysts, thus leading to high
the catalysts may boost the N-methylation reaction. Further, selectivity for MFA.
owing to the similar electronic properties of gold atoms on the
4 2
In summary, AuRu bimetallic nanoclusters, that is, Au Ru
two clusters, it is reasonable to believe that the superior and Au Ru , provide us an ideal platform to assess the nature of
5
2
catalytic activity of Au Ru is mainly determined by the dual- single-site and dual-site catalysts. The Au Ru cluster with an
5
2
5
2
site Ru–Ru or Au–Ru, rather than the Au–Au sites.
4 2
Ru–Ru or Au–Ru dual-site outperformed the Au Ru cluster
The two clusters were robust during the reactions, which was with a single-Ru site for the N-methylation reactions. We
supported by the UV-vis spectra of the spent clusters (Fig. S3, anticipate that this study will benefit the rational design and
ESI†). In situ infrared Fourier transform (FT-IR) spectroscopy was synthesis of bimetallic nanoclusters, as well as represent an
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important step for further exploring heterogeneous catalysis at 11 J. Yan, B. K. Teo and N. Zheng, Acc. Chem. Res., 2018, 51, 3084–3093.
1
1
1
1
1
1
2 T. Higaki, Y. Li, S. Zhao, Q. Li, S. Li, X. Du, S. Yang, J. Chai and
R. Jin, Angew. Chem., Int. Ed., 2019, 58, 8291–8302.
3 X. Yuan, X. Dou, K. Zheng and J. Xie, Part. Part. Syst. Charact., 2015,
32, 613–629.
4 S. Yang, J. Chai, Y. Song, J. Fan, T. Chen, S. Wang, H. Yu, X. Li and
M. Zhu, J. Am. Chem. Soc., 2017, 139, 5668–5671.
an atomic precision.
We acknowledge financial support from the National Natural
Science Foundation of China (21773109, 91845104, and
U1930402).
5 S. Wang, Q. Li, X. Kang and M. Zhu, Acc. Chem. Res., 2018, 51,
2784–2792.
6 Y. Sun, W. Pei, M. Xie, S. Xu, S. Zhou, J. Zhao, K. Xiao and Y. Zhu,
Chem. Sci., 2020, 11, 2440–2447.
7 Y. Zhang, J. Zhang, B. Zhang, R. Si, B. Han, F. Hong, Y. Niu, L. Sun,
L. Li, B. Qiao, K. Sun, J. Huang and M. Haruta, Nat. Commun., 2020,
Conflicts of interest
There are no conflicts to declare.
11, 558.
Notes and references
18 P. Zhou, X. Hou, Y. Chao, W. Yang, W. Zhang, Z. Mu, J. Lai, F. Lv,
K. Yang, Y. Liu, J. Li, J. Ma, J. Luo and S. Guo, Chem. Sci., 2019, 10,
5898–5905.
19 C. Zhang, J. Sha, H. Fei, M. Liu, S. Yazdi, J. Zhang, Q. Zhong, X. Zou,
N. Zhao, H. Yu, Z. Jiang, E. Ringe, B. I. Yakobson, J. Dong, D. Chen
and J. M. Tour, ACS Nano, 2017, 11, 6930–6941.
20 Y. Li, X. Fang, K. Junge and M. Beller, Angew. Chem., Int. Ed., 2013,
52, 9568–9571.
1
L. Zhang, M. Zhou, A. Wang and T. Zhang, Chem. Rev., 2020, 120,
83–733.
6
2
3
C. Ye, N. Zhang, D. Wang and Y. Li, Chem. Commun., 2020, 56, 7687–7697.
X. Yang, A. Wang, B. Qiao, J. Li, J. Liu and T. Zhang, Acc. Chem. Res.,
2
013, 46, 1740–1748.
4
5
6
I. Chakraborty and T. Pradeep, Chem. Rev., 2017, 117, 8208–8271.
Y. Du, H. Sheng, D. Astruc and M. Zhu, Chem. Rev., 2020, 120, 526–622. 21 M. Rozenberg, G. Shoham, I. Reva and R. Fausto, Phys. Chem. Chem.
R. Jin, C. Zeng, M. Zhou and Y. Chen, Chem. Rev., 2016, 116,
0346–10413.
H. Qian, M. Zhu, Z. Wu and R. Jin, Acc. Chem. Res., 2012, 45,
470–1479.
Y. Negishi, K. Nobusada and T. Tsukuda, J. Am. Chem. Soc., 2005,
27, 5261–5270.
Phys., 2005, 7, 2376–2383.
22 D. Yang, W. Pei, S. Zhou, J. Zhao, W. Ding and Y. Zhu, Angew. Chem.,
Int. Ed., 2020, 59, 1919–1924.
23 X. Su, W. Lin, H. Cheng, C. Zhang, Y. Li, T. Liu, B. Zhang, Q. Wu,
X. Yu and F. Zhao, RSC Adv., 2016, 6, 103650–103656.
24 W. Lin, H. Cheng, Q. Wu, C. Zhang, M. Arai and F. Zhao, ACS Catal.,
2020, 10, 3285–3296.
1
7
8
9
1
1
X. Cai, W. Hu, S. Xu, D. Yang, M. Chen, M. Shu, R. Si, W. Ding and
Y. Zhu, J. Am. Chem. Soc., 2020, 142, 4141–4153.
0 Z. Wu, Angew. Chem., Int. Ed., 2012, 51, 2934–2938.
25 S. E. Collins, D. L. Chiavassa, A. L. Bonivardi and M. A. Baltanas,
Catal. Lett., 2005, 103, 83–88.
1
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