Moderate oxidation levels of Ru nanoparticles enhance molecular oxygen activation for cross-dehydrogenative-coupling reactions: Via single electron transfer

Article abstract of DOI:10.1039/c7ra05726a

For converting a homogeneous synthetic chemistry reaction to a heterogeneous one, using nanocatalysts, it is essential to investigate the underlying principles and the associated dominant factors of methodologies between inorganic nanoparticles and organi

Full text of DOI:10.1039/c7ra05726a

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Moderate oxidation levels of Ru nanoparticles
enhance molecular oxygen activation for cross-
dehydrogenative-coupling reactions via single
electron transfer†
Cite this: RSC Adv., 2017, 7, 33078
b
Mu Lin,‡a Lin-Xiu Dai,‡a Jun Gu,‡a Li-Qun Kang,a Yu-Hao Wang,a Rui Si,
a
a
a
a
a
*
*
Ze-Qiong Zhao, Wen-Chi Liu, Xuefeng Fu, Ling-Dong Sun, Ya-Wen Zhang
a
*
and Chun-Hua Yan
For converting a homogeneous synthetic chemistry reaction to a heterogeneous one, using nanocatalysts,
it is essential to investigate the underlying principles and the associated dominant factors of methodologies
between inorganic nanoparticles and organic substrates. Here it is reported that novel, hydrothermally
synthesized ruthenium (Ru) nanoparticles performed differently in molecular oxygen activation via single
electron transfer for cross-dehydrogenative-coupling reactions, following a volcano shaped relationship
between their oxidation levels and catalytic activity. Characterizations and a systematic mechanism study
indicated that Ru3+ sites contributed to more superoxo- or peroxo-like species being formed from
molecular oxygen on Ru nanoparticles with a moderate oxidation level, and the Ru3+ were the highly
active species which accelerate this cross-dehydrogenative-coupling reaction via single electron transfer.
Received 21st May 2017
Accepted 13th June 2017
DOI: 10.1039/c7ra05726a
rsc.li/rsc-advances
signicance to develop highly efficient nanocatalysts for
important organic reactions.
Introduction
Carbon–carbon bond formation is the essential link in all
organic molecules, and is a central part of many chemical
syntheses, such as in natural products, medicines, and organic
materials.15–20 Building a C–C linkage directly via the cleavage of
dual C–H bonds using molecular oxygen as the terminal oxidant,
called aerobic cross-dehydrogenative coupling (CDC), has been
successfully developed into the most atom-economic, clean, and
efficient strategy in synthetic organic chemistry.15,21,22 Homoge-
neous transition metal catalyzed activation of C–H bonds for C–C
bond formation, has proved to be versatile for the CDC reaction,
using metals such as cobalt (Co), copper (Cu), iron (Fe), nickel (Ni),
palladium (Pd), rhodium (Rh), and ruthenium (Ru).23–25 However,
for widely investigated Pd or Cu catalyzed CDC reactions, most of
the reports only provided possible reaction mechanisms without
enough experimental evidence. In past decades, the development
of heterocatalysis means that nanoparticles (NPs) play more and
more important roles in organic syntheses, and many NPs (e.g.,
Fe3O4, Cu6Se4.5, CuFe2O4, ruthenium(IV) oxide (RuO2), RuO2/gra-
phene, and so) exhibit higher catalytic performance than their salts
or complexes in CDC reactions.26–29 However, the complicated
structure of the active sites makes the elucidation of the reaction
mechanism in a heterogeneous system challenging.30–32 Thus,
understanding the relationship between the structure effects
(electronic and geometric) of NPs and the catalytic performance
towards important organic reactions is necessary to illustrate the
With the development of controlled synthesis of nanocatalysts
with different sizes, shapes, composition, structure, oxidation
state and so on, remarkable progress on the investigation of the
relationship between the structure of catalytic active sites and
the catalytic performance has been made, which provides more
opportunities to design and synthesize nanocatalysts with high
activity, selectivity and stability.1–8 In recent years, nanocatalysts
in organic chemistry have attracted much attention.9–14 Homo-
geneous catalysts possess higher catalytic activity than that of
heterogeneous catalysts because of the easy accessibility of
reactants to active sites and higher selectivity by tuning the
steric and electronic properties of ligands. There is no doubt
that because of the separation of the catalysts from the reaction
solutions it is too difficult to reuse these catalysts, resulting in
the contamination of the products and the waste of noble metal
catalysts. For green and sustainable chemistry, it is of great
aBeijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare
Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare
Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China. E-mail: yan@pku.edu.cn;
ywzhang@pku.edu.cn; sun@pku.edu.cn
bShanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, Shanghai 201204, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra05726a
‡ These authors contributed to this work equally.
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reaction mechanism and to enable the screening of highly efficient
heterocatalysts.
In this paper it is reported that novel Ru NPs performed
differently in triggering CDC reactions. The catalytic mecha-
nism was studied and it was found that the content of Ru3+ and
its oxidation levels were crucial for the catalytic activity, and
a volcano shaped relationship was established. On the basis of
systematic characterizations, adsorbed superoxo- or peroxo-like
species depending on the content of Ru3+ drove the CDC reac-
tions, which was the essence of the as-observed structure–
activity relationship.
Fig. 1 Ru-nanocatalyzed CDC reactions. Reaction conditions: 8 mol%
of Ru NPs, 1 equiv. of 1, 4 equiv. of 2, 48 mL acetic acid (AcOH), 0.025 M
in concentration. Isolated yield of 3a after column chromatography
and the yield of product 4a was determined using 1H-NMR with 4-
chloroacetophenone as an internal standard.
Results and discussion
Ru NPs used in this work were prepared using a hydrothermal
strategy, in which ruthenium chloride (RuCl3$xH2O), formalde-
hyde and poly(vinylpyrrolidone) (PVP) were used as precursor,
reducing agent and particle stabilizer, respectively. Sodium oxalate
(Na2C2O4), which can form stable complexes with Ru2+ and Ru3+
ions,33 was also introduced to avoid the rapid and complete
reduction of the Ru precursor. Therefore, it was expected that Ru
NPs with a considerable amount of Ru atoms with a positive
valency would be obtained. The Ru NPs obtained aer 8 h (denoted
as Ru-NP-1) mainly contains 70 wt% of hexagonal-closest-packed
(hcp) ultra-thin (thickness of 1.4 nm) plates with two {0001} fac-
ets as a basal surface, and the other 30 wt% of the NPs was
comprised of poorly crystallized nanospheres. Transmission elec-
tron spectroscopy (TEM) images, and X-ray diffraction (XRD)
patterns were presented in the ESI; Fig. S1 and S2,† and the size
distributions of all the samples are listed in Table S1.†
The Ru NPs were introduced into the CDC reaction of tetrahy-
droisoquinolines via oxidative C(sp3)–H functionalization.34–36
With the optimization of reaction conditions (Table S5; ESI†), the
Ru-nanocatalyzed CDC reaction of tetrahydroisoquinolines and
nucleophiles such as indole and nitromethane performed with
excellent selectivity and reactivity (Fig. 1). These Ru NPs converted
the traditionally homogeneous CDC reaction with tough oxidants,
energy supplement or/and high temperature into a heterogeneous
process with a simple procedure, namely, just stirring under
ambient conditions (room temperature, cheap solvents methanol
(CH3OH)/water (H2O), open ask, and free of light or thermal
energy supplement). Turnover frequency (TOF) was as high as 33.6
sꢀ1 under 60 ꢁC. The recycling capability of Ru NPs was also
examined, and the yields of recovered Ru NPs changed slightly.
The listed yields of product 4a were 97%, 93%, 87%, 96%, 86%,
77% aer six catalytic cycles.
(ICP-AES) in the reaction solvent mixture of CH3OH–H2O aer
removal of Ru NPs. Thirdly, no signicant changes in the
morphology and oxidation level were observed, as revealed by
the TEM image and X-ray photoelectron spectroscopy (XPS)
spectra shown in Fig. S3 and S4 (ESI†), indicating that Ru NPs
were stable during this reaction. Finally, the recovered solution
was also used aer removing Ru NPs to catalyse the model
substrates and no reaction occurred. Thus, it was veried that
the Ru NPs did not leach to form a homogeneously active
catalyst.
A study of the mechanism of the heterogeneous CDC reac-
tion helped to understand the catalytic behaviour and function
of NPs towards organic substrates. The results of control
experiments used to test the importance of oxygen are pre-
sented in Table S7.† The results conrmed that oxygen is
necessary in this reaction and more oxygen yields a faster
conversion rate of substrates. Furthermore, electron para-
magnetic resonance (EPR) spectroscopy showed the existence of
superoxide radicals (O2cꢀ) that were generated when oxygen
molecules captured one electron in this reactive system, but no
It is difficult to distinguish homogeneous and heteroge-
neous catalysis, because of the possibility that metal ions may
leach from a heterogeneous catalyst into solution and act as the
homogeneous active species. Therefore, it was necessary to test
the Ru ions leached from NPs in the reaction system. Firstly, as
revealed by using in situ X-ray adsorption spectroscopy (see
Fig. 2), the spectra of the sample at different reaction times did
not show obvious differences, indicating that the oxidation level
of the Ru element in the NPs remained unchanged during the
catalytic reaction. Secondly, no Ru element was detected using
Fig. 2 Ru K-edge in situ X-ray adsorption spectroscopy monitoring of
Ru-NP-1 during the catalytic reaction. The spectra were taken before
the reaction and during the reaction at of 1.5 h, 2.5 h, 3.5 h, 4.5 h and
9 h. The spectra of the Ru foil and RuO2 powders were used as
inductively coupled plasma – atomic emission spectroscopy references.
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structure and the activity of the Ru NPs in the solution, EPR was
used to detect radicals on the surface of Ru NPs which possibly
mediated the SET process (Fig. 4). First, very broad and inten-
sive signals with g tensor components g1 ¼ 2.19, g2 ¼ 2.04 and g3
¼ 1.88 ascribed to the Ru3+ species were observed on Ru-NP-1. If
the as-synthesized Ru-NP-1 was post-reduced using hydrogen
(H2) under hydrothermal conditions (denoted as Ru-NP-2), the
signal of Ru3+ became weaker than that of Ru-NP-1 with the
same amount of Ru element. The RuO2 powders considered as
the over-oxidized sample showed little sign of Ru3+ species.
Meanwhile, the SET reactions were used to test the catalytic
activity of the three Ru catalysts. Among the Ru nanocatalysts,
Ru-NP-1 showed the highest TOF value (up to 33.6 sꢀ1), and the
reaction achieved the nal product with full conversion and an
excellent yield as high as 94% (Table 1). Thus, it was supposed
that the content of Ru3+ in the Ru catalysts was related to the
catalytic activity.
Ru NPs with different Ru3+ contents were synthesized by just
prolonging the time of synthesis of Ru-NP-1 to 16 h and 24 h,
denoted as Ru-NP-3 and Ru-NP-4, respectively. As shown in
Fig. S1 (ESI) and Table S1 (ESI†), the thickness of the plate-
shaped NPs in these two samples remained the same as that
of Ru-NP-1 (1.4 nm), and the morphology selectivity of the plates
were also similar (77 wt% for Ru-NP-3 and 78 wt% for Ru-NP-4).
The EPR results showed that the Ru3+ content of Ru catalysts
with the same mass was Ru-NP-1 > Ru-NP-3 > Ru-NP-2 > Ru-NP-4
> RuO2 (see the magnied detail in Fig. 4). Table 1 compares the
apparent TOF of different catalysts, which is dened as the
average number of reactant molecules consumed on one
surface Ru site per second (see ESI† for the calculation process).
Different Ru nanocatalysts exhibited quite different catalytic
activities under the same ambient conditions. The higher the
content of Ru3+ in the nanocatalysts is, the higher the catalytic
activity toward CDC reactions is.
Fig. 3 EPR spectrum of [DMPO–OOH] formed in the standard reac-
tive system of Ru-nanocatalyzed CDC reactions (microwave
frequency: 9056.103 MHz, microwave power: 4.0 mW). There are
classical six peaks of the signals corresponding to [DMPO–OOH]. The
calculated hyperfine splittings are g0 (2.002), aH (12.5 G), aN (14.0 G).
radicals were detected whenꢀthe substrates or the catalyst was
absent (Fig. 3).37 Thus, O2 + e ¼ O2cꢀ, and this process not only
suggests that this CDC reaction proceeds as a single-electron-
transfer (SET) catalysis, but also conrmed that oxygen was
the electron acceptor.
Some previous work has shown that some adsorbed radicals,
especially reactive oxygen species, can form on Ru-based hetero-
or homogeneous catalysts and are active sites for the catalytic
reaction.38,39 In order to study the relationship between the
More importantly, an obvious sigꢀnal ascribed to adsorbed
superoxo- or peroxo-like species (*O2c )40–42 was observed in the
EPR spectrum of Ru-NP-1, the sample with highest content of
Ru3+ species. The *O2cꢀ species might form because of the
electron transfer from the Ru sites on the surface of Ru NPs to
oxygen molecules, as shown by:
(Run+
)
+ O2 / (Ru(n+1)+
)
surf
ꢀ *O2cꢀ
surf
This process is more likely to occur on Ru3+ sites on the
surface of Ru NPs. The signals of the *O2cꢀ species were not
detected for Ru-NP-2 and RuO2, which had a lower Ru3+ content.
If benzoquinone was added to the reaction as a radical
quencher of *O2cꢀ species, no reactions occurred on Ru-NP-1
(entry 4 in Table S7, ESI†), indicating that the *O2cꢀ species
mediated the SET process. Based on the previous characteriza-
tions, it is proposed that the oxidation level of Ru nanocatalysts
determines the content of Ru3+ sites on which *O2cꢀ species can
Fig. 4 EPR spectra of Ru-NP-1 (red curve), Ru-NP-2 (orange curve),
Ru-NP-3 (green curve) and Ru-NP-4 (blue curve) all as solid powders
and RuO2 powders (black curve) of the same mass collected at 123 K. form and mediate the SET process. Therefore, Ru nanocatalysts
For Ru-NP-1, a signal was superimposed on the signal of Ru3+ species
with a moderate oxidation level may trigger the CDC reactions
of tetrahydroisoquinolines and achieve excellent catalytic
performances.
(g1 ¼ 2.23, g2 ¼ 2.04, g3 ¼ 1.86), which was ascribed to the superoxo-
or peroxo-like species (*O2cꢀ) adsorbed on the Ru catalyst, as shown
in the inset.
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Table 1 Catalytic properties of the cross-dehydrogenative-coupling reactions with different Ru catalystsa
Entry
Catalyst
Equivalentb [mol%]
Time [h]
Conversion [%]
Yieldc [%]
TOFd [sꢀ1
]
1
2
3
4
5
6
Ru-NP-1
Ru-NP-1
Ru-NP-2
Ru-NP-3
Ru-NP-4
RuO2
8
8
8
8
8
80
42
2
72
72
72
45
100
100
41
73
74
94
93
34
57
45
18
1.6
33.6e
0.28
0.45
0.10
0.091
30
a
Reaction conditions: 0.10 mmol of 1a, 4 equiv. of 2a, 48 mL of 0.025 M AcOH, solvent: H2O/CH3OH (1 : 1), room temperature. b Molar ratio of Ru
c
d
element to reagent 1a. ICP-AES was used for the quantication of the Ru element. Isolated yield aer column chromatography. The TOF was
obtained with the conversion of substrate 1a being lower than 15%. The methods to calculate the number of catalytic sites are shown in the
ESI. e Reaction temperature was 60 ꢁC.
The content of Ru3+ depends on the oxidation level of Ru scattering reected the sequence of oxidation levels, which
NPs. To investigate the relationship between the catalytic showed the same trend as XANES. Combined with the activity
activity and oxidation levels of Ru NPs, an X-ray adsorption ne data of the SET reactions, Ru-NP-1 with a moderate oxidation
structure (XAFS) analysis of different Ru nanocatalysts was level showed the highest catalytic activity.
performed. Fig. 5a shows the X-ray absorption near edge
structure (XANES) spectra of Ru-NP-1, Ru-NP-2, Ru-NP-3, Ru-NP-
4, Ru foil and RuO2. The higher energy of the Ru K-edge reects
its higher oxidation level. Thus, the oxidation levels of different
Ru NPs were in the sequence of Ru-NP-3 > Ru-NP-1 > Ru-NP-2 >
Ru-NP-4. This result was consistent with the R-space extended
X-ray absorption ne structure (EXAFS) data shown in Fig. 5b.
Fourier transformation (F.T.) of the k2-weighted extended
˚
˚
EXAFS data showed two main peaks around 2.0 A and 2.7 A,
attributed to Ru–O and Ru–Ru scattering, respectively.43 The
ratio of the peak intensity of Ru–O scattering to Ru–Ru
Fig. 6 XPS spectra of Ru-NP-1, Ru-NP-2, Ru-NP-3, Ru-NP-4 and RuO2
powders. (a) The spectra in the C-1s and Ru-3d regions. Each spectrum
was deconvoluted into C-1s peaks at 284.8 eV, 285.8 eV and 287.6 eV
(violet curves), which were assigned to C atoms in the C–H, C–N
and C]O forms, respectively, and Ru-3d doublets (3d5/2 and 3d3/2
,
D ¼ 4.17 eV) with the binding energy of Ru 3d5/2 peak at 279.5–279.9 eV,
281.2–281.4 eV and 282.6–282.8 eV which were assigned to Ru(0) (red
curves), Ru(IV) (green curves) and Ru(VI) (blue curves) states, respec-
Fig. 5 (a) XANES spectra and (b) R-space EXAFS spectra and the curve tively.45–47 The fitting results of the Ru-3d spectra are shown in Table 2.
fitting for Ru K-edge of Ru foil (red curve), Ru-NP-1 (blue curve), Ru- (b) The spectra in the Ru-3p3/2 region. The red, green and blue vertical
NP-2 (green curve), Ru-NP-3 (pink curve), Ru-NP-4 (orange curve) lines indicate the binding energy of Ru(0), Ru(IV) and Ru(VI) states,
and RuO2 (black curve).
respectively.48,49
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Table 2 XPS data fitting results of all the catalysts in Ru-3d region
Ru(0)
Ru(IV)
Ru(VI)
Sample
BE (eV) a
Ratio (%)
BE (eV)
Ratio (%)
BE (eV)
Ratio (%)
Ru-NP-1
Ru-NP-2
Ru-NP-3
Ru-NP-4
RuO2 powders
Ru-NP-1 aer catalysis
279.8
279.7
279.7
279.5
—
30
50
18
69
0
281.2
281.1
281.2
281.1
281.3
281.4
50
33
63
31
64
50
282.7
282.7
282.7
—
282.8
282.9
20
17
18
0
36
20
279.9
30
a
BE: binding energy of Ru 3d5/2 electrons.
Next, XPS was used to investigate the surface oxidation level process. The volcano shaped relationship between the catalytic
of the Ru NPs. Considering the probing depth of XPS and the activity and the oxidation level conrmed the as-proposed
ultra-thin nature of the Ru NPs, most Ru atoms in the sample process on how Ru NPs catalyze the CDC reactions: the *O2cꢀ
could be detected in the analysis. As shown in Fig. 6a, the C-1s species formed on Ru3+ sites on the surface of Ru NPs were the
and Ru-3d regions of the samples were deconvoluted into three active sites to mediate the SET process and connected the
C-1s peaks at 284.80 eV (C–H), 285.80 eV (C–N) and 287.63 eV inorganic nanocatalysts and organic substrates. For the
(C]O), and three Ru-3d doublets (3d5/2 and 3d3/2, D ¼ 4.17 eV) completely reduced Ru NPs, a few Ru3+ sites existed on the
with the binding energy (BE) of Ru 3d5/2 peak at 279.5–279.9 eV,
281.2–281.4 eV and 282.6–282.8 eV, assigned to the Ru(0), Ru(IV)
and Ru(VI) states, respectively. The tting results of the XPS
spectra of Ru-3d electrons are listed in Table 2. Compared with
Ru-NP-1, the post-reduced sample (Ru-NP-2) showed a signi-
cantly lower oxidation level. The oxidation level of Ru-NP-3 was
higher than that of Ru-NP-1, probably because of the oxidizing
effect of the oxygen sealed in the reactor, on the Ru NPs from 8 h
to 16 h. As the reaction time was extended to 24 h, the Ru-NP-4
obtained showed greatly enhanced crystallinity (Fig. S2, ESI†)
and became inert and were unable to be oxidized.44 Thus, Ru-
NP-4 represented the lowest oxidation level. The XPS spectra
in the Ru-3p regions showed the same trend of the oxidation
levels of different Ru catalysts, as shown in Fig. 6b.
When the TOF values of all the Ru catalysts were plotted
against their oxidation level (quantied as the percentage of
surface Ru atoms with positive valences among all the surface
Ru atoms deduced from the Ru-3d XPS spectra), a volcano
shaped relationship was found, as shown in Fig. 7. The highest
Fig. 7 Plot of the TOF values of all the catalysts against their oxidation
level quantified as the percentage of surface Ru atoms with positive
catalytic activity was achieved at a moderate oxidation level. The valencies among all the surface Ru atoms deduced from the Ru-3d
XPS spectra. A volcano shaped relationship between the TOF values
and the oxidation level is indicated by the red line.
catalytic performances and the oxidation level of Ru NPs with
other morphologies synthesized by hydrothermal methods were
also investigated, and the results obtained agreed well with the
volcano shaped relationship, as discussed in Fig. S5 and S6 (ESI)
and Tables S2 and S6 (ESI†).
In this work, Ru nanocatalysts were synthesized using
a hydrothermal method. With the understanding of the growth
mechanism of the Ru NPs33 (Fig. S11–S13, ESI†), aer the
nucleation and growth steps, Ru NPs were oxidized mainly
using subcritical water in hydrothermal synthesis,50–53 together
with the generation of active *O2cꢀ species at the surfaces of the
Ru NPs as revealed by the EPR measurements. Ru(0) was still
the body constituent to support the structure and morphology
of these Ru nanocatalysts with different oxidation levels. In the
interface, the oxidized Ru nanocatalysts triggered the CDC
reaction of tetrahydroisoquinoline derivatives via the SET
Scheme 1 Mechanism of SET reactions catalyzed by Ru NPs.
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surface, whereas the Ru3+ sites rst increased with increasing cycle and enhance the activities of Ru NPs for the CDC
degree of oxidation of Ru NPs, and then decreased as the Ru4+ reactions. The detailed explanations of the organic mecha-
sites became dominant on the surface. The content of Ru3+ sites nism including control experiments and proposed pathways
was highest on Ru NPs with a moderate oxidation level, which are shown in Table S7 (ESI) and Fig. S14 (ESI†).
consequently generated more *O2cꢀ species by the electron
The general mechanism of the Ru-nanocatalyzed CDC reac-
transfer from oxygen molecules to the Ru3+ sites and the highest tions was further explored using the standard conditions [8 mol%
catalytic activity for the CDC reaction was exhibited with the Ru-NP-1, 0.10 mmol of N-phenyl tetrahydroisoquinoline deriva-
sample of Ru-NP-1.
tives, 4 equiv. of indoles, 10–48 mL of AcOH, H2O/CH3OH (1 : 1) as
Based on the previous discussion, it is now possible to solvent, room temperature, concentration of 0.025 M (ESI†)] in the
systematically clarify the mechanism as shown in Scheme 1. optimization studies. Signicantly, all the positions of the
In the SET process, Ru NPs with superoxo- or peroxo-like substituted nucleophilic indoles were reactive and the yields were
species captured one single electron from tetrahy- generally high (Fig. 8 3b–3j). The product (3d) that substitutes the
droisoquinoline derivative 1a with lone pair electrons to indole at the 4-position has not been discovered before. In addi-
generate aminium radical cation A and transferred it to tion, the substrates substituted N-phenyl tetrahydroisoquinolines
oxygen from the air to form the superoxide anion radical at all positions of N-aryl group perform high reactivity and the
(O2cꢀ), which is the impetus of this reaction. Then radical obtained products (3k–3o) are also rst discovered. The reaction
intermediate B could be achieved by several conversions proceeds satisfactorily resulting in the formation of the crossing-
following the formation of aminium radical ion A and attack coupling products with excellent yields (4a and 5). All the experi-
by nucleophiles yielding the nal complex molecules. In this mental results mentioned previously, shows that Ru-NP-1 with
heterogeneous catalysis, it is supposed that a large amount of a moderate oxidation level presented excellent catalytic perfor-
reactive oxygen radicals on the surface of Ru NPs, that are mance and a broad applicability, by virtue of the powerful single
associated with their oxidation level, could drive the catalytic electron grabbing capacity under ambient conditions.
Conclusions
A novel Ru nanocatalyst with a different oxidation level was
synthesized using a hydrothermal strategy. The Ru NPs with
a moderate oxidation level exhibited excellent reactivity and
selectivity for the CDC reaction of tetrahydroisoquinolines
under ambient conditions. This heterogeneous catalytic
process was free of energy supplement, sacricial reductants
or sacricial oxidants compared to homogeneous catalysis,
and can be conducted at the gram scale. It was proposed and
conrmed that there was a heterogeneous catalytic mecha-
nism of Ru-nanocatalyzed CDC reactions. Ru NPs rstly
capture one electron from amine substrates to generate an
aminium radical cation and subsequently transfer the elec-
tron to the oxygen. Then, the active aminium radical cation
could react with nucleophiles to form complex structures.
The active aminium radical cation can be obtained for
a series of available substrates. The catalytic behaviors
present a volcano shaped correlation to the oxidation level of
the NPs. The Ru3+ content was the highest for Ru NPs with
a moderate oxidation level, which consequently generated
more *O2cꢀ species by electron transfer from Ru3+ sites to
oxygen molecules and exhibited a higher catalytic activity to
trigger this SET process.
By introducing catalytically active NPs into the SET
process of CDC reactions, not only were the underlying
principles and the associated dominant factors of nano-
Fig. 8 Ru NP catalyzed CDC reactions. Reaction conditions: 8 mol% catalysis for the development of efficient and versatile
of Ru nanocatalyst, 1 equiv. of 1, 4 equiv. of 2, 10–48 mL of AcOH,
0.025 M in concentration. Isolated yields were obtained after column
approaches to heterogeneous catalysis under ambient
conditions studied, but the results also provided a thorough
insight into the relationship between the inorganic nano-
catalysts and organic methodologies. Extended research with
other NPs is necessary to facilitate the development of
chromatography, except that the yields of products 4a and 5 were
determined using 1H-NMR using 4-chloroacetophenone as an internal
standard. a Recovered 10% of 1d, 88% of yield based on the recovered
starting material (brsm). b Solvent of 3h, 4a and 5 is CH3OH.
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heterogeneous catalysts with high reactivity and selectivity in
an even larger array of chemical reactions.
Notes and references
1 G. A. Somorjai, Chem. Rev., 1996, 96, 1223.
2 T. S. Ahmadi, Z. Wang, T. C. Green, A. Henglein and M. A. El-
Sayed, Science, 1996, 272, 1924.
3 Y. N. Xia, Y. J. Xiong, B. Lim and S. E. Skrabalak, Angew.
Chem., Int. Ed., 2009, 48, 60.
Experiments
Hydrothermal synthesis of Ru nanocatalysts
In a typical synthesis, 100 mg of PVP and 80 mg of Na2C2O4 were
rst dissolved in 10 mL of water, followed by addition
0.24 mmol RuCl3$xH2O, 0.062 mL of hydrochloric acid (1 M)
and 0.1 mL of formaldehyde solution (40 wt%). The solution
was then diluted to 15 mL, loaded in to a 25 mL Teon-lined
container and sealed in a matched steel autoclave. The auto-
clave was then heated in an oven kept at 160 ꢁC for 8 h, 16 h or
24 h for the synthesis of Ru-NP-1, Ru-NP-3 and Ru-NP-4,
respectively. Aer the reaction, the autoclave was cooled natu-
rally to 25 ꢁC and 45 mL of acetone was added and the product
was then collected by centrifugation at 7800 rpm for 10 min.
The yield of Ru NPs using the hydrothermal method was
measured using ICP-AES and found to be about 85%.
A post-reduction process generating Ru-NP-2 was also used to
lower the oxidation level of Ru-NP-1. The as-obtained Ru-NP-1 was
dispersed in 15 mL of water containing 100 mg of PVP. The
dispersion was loaded into a 50 mL Teon-lined reactor which was
then lled with 2.0 MPa of H2. The reduction process was kept at
150 ꢁC for 4 h. Aer the reaction, 45 mL acetone was added and the
NPs were collected by centrifugation at 7800 rpm for 10 min.
4 C. Wang, H. Daimon, T. Onodera, T. Koda and S. H. Sun,
Angew. Chem., Int. Ed., 2008, 47, 3588.
5 X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Nature, 2005, 437,
121.
6 Y. Wu, D. S. Wang and Y. D. Li, Chem. Soc. Rev., 2014, 43,
2112.
7 X. Q. Huang, Z. P. Zhao, J. M. Fan, Y. M. Tan and N. F. Zheng,
J. Am. Chem. Soc., 2011, 133, 4718.
8 A. R. Tao, S. Habas and P. D. Yang, Small, 2008, 4, 310.
9 K. M. Bratlie, H. Lee, K. Komvopoulos, P. D. Yang and
G. A. Somorjai, Nano Lett., 2007, 7, 3097.
10 Y. Wu, S. F. Cai, D. S. Wang, W. He and Y. D. Li, J. Am. Chem.
Soc., 2012, 134, 8975.
11 S. B. Wang, W. Zhu, J. Ke, M. Lin and Y. W. Zhang, ACS
Catal., 2014, 4, 2298.
12 L. X. Dai, W. Zhu, M. Lin, Z. P. Zhang, J. Gu, Y. H. Wang and
Y. W. Zhang, Inorg. Chem. Front., 2015, 2, 949.
13 M. Lin, L. Q. Kang, J. Gu, L. X. Dai, S. B. Tang, T. Zhang,
Y. H. Wang, L. D. Li, X. Y. Zheng, W. Zhu, R. Si, X. F. Fu,
L. D. Sun, Y. W. Zhang and C. H. Yan, Nano Res., 2017, 10,
922.
14 L. J. Durndell, C. M. A. Parlett, N. S. Hondow, M. A. Isaacs,
K. Wilson and A. F. Lee, Sci. Rep., 2015, 5, 9425.
15 C. S. Yeung and V. M. Dong, Chem. Rev., 2011, 111, 1215.
16 J. M. Brown and N. A. Cooley, Chem. Rev., 1988, 88, 1031.
17 Y. N. Wu, J. Wang, F. Mao and F. Y. Kwong, Chem.–Asian J.,
2014, 9, 26.
18 C. C. C. Johansson Seechurn, M. O. Kitching, T. J. Colacot
and V. Snieckus, Angew. Chem., Int. Ed., 2012, 51, 5062.
19 S. A. Girard, T. Knauber and C. J. Li, From C–H to C–C Bonds:
Cross-Dehydrogenative-Coupling, 2014.
20 X. Lu, B. Xiao, Z. Q. Zhang, T. J. Gong, W. Su, J. Yi, Y. Fu and
L. Liu, Nat. Commun., 2016, 7, 11129.
21 C. Liu, H. Zhang, W. Shi and A. Lei, Chem. Rev., 2011, 111,
1780.
22 W. Shi, C. Liu and A. Lei, Chem. Soc. Rev., 2011, 40, 2761.
23 A. Meijere and F. Diederich, Metal-Catalyzed Cross-Coupling
Reactions, Wiley-VCH, Weinheim, 2004.
General procedure of Ru-nanocatalyzed CDC reactions
The concentration of a Ru NP colloidal CH3OH solution was
determined using ICP-AES. Ru nanocatalysts with the same Ru
content (8 mol% of the substrate content) were added into the
solvents [H2O/CH3OH (1 : 1)] with 0.10 mmol of tetrahy-
droisoquinoline derivatives, 4 equiv. of indoles and 10–48 mL of
AcOH. The mixture (concentration: 0.025 M) was stirred under
room temperature. The reaction tube was immersed in water
bath under room temperature. When the thin layer chroma-
tography (TLC) indicated the disappearance of the starting
material, the reaction mixture was ltered through a thin pad of
silica gel. The lter cake was washed with petroleum ether/ethyl
acetate, and the combined ltrate was concentrated. The crude
product was puried using ash column chromatography on
silica gel to give the nal product.
24 C. A. Parrodi and P. J. Walsh, Angew. Chem., Int. Ed., 2009, 48,
467925.
Acknowledgements
The Authors wish to thank the National Natural Science Foun-
dation of China (No. 21461162001, 21371011, 21621061,
21229101, 21573005 and 21331001), the National Basic
Research Program of China (No. 2012CBA01204), the National
Key Research and Development Program of China (no.
2016YFB0701100), the Beijing Natural Science Foundation (No.
2162019), and the 7th China Postdoctoral Science Foundation
Funded Project (No. 2014T70009) for nancial support. ML was
also partly supported by a Postdoctoral Fellowship of Peking-
Tsinghua Center for Life Sciences.
25 Z. Shao and F. Peng, Angew. Chem., Int. Ed., 2010, 49, 9566.
26 S. Gupta, H. Joshi, N. Jain and A. K. Singh, J. Mol. Catal. A:
Chem., 2016, 423, 135.
27 R. Hudson, S. lshikawa, C. J. Li and A. Moores, Synlett, 2013,
24, 1637.
28 T. Zeng, G. Song, A. Moores and C. J. Li, Synlett, 2010, 13,
2002.
29 Q. Y. Meng, Q. Liu, J. J. Zhong, H. H. Zhang, Z. J. Li, B. Chen,
C. H. Tung and L. Z. Wu, Org. Lett., 2012, 14, 5993.
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Paper
RSC Advances
30 G. Ertl, Angew. Chem., Int. Ed. Engl., 1990, 29, 1219.
43 T. Mitsudome, Y. Takahashi, T. Mizugaki, K. Jitsukawa and
K. Kaneda, Angew. Chem., Int. Ed., 2014, 53, 8348.
44 J. Ohyama, T. Sato, Y. Yamamoto, S. Arai and A. Satsuma, J.
Am. Chem. Soc., 2013, 135, 8016.
¨
31 H.-J. Freund, G. Meijer, M. Scheffler, R. Schlogl and M. Wolf,
Angew. Chem., Int. Ed., 2011, 50, 10064.
32 G. Ertl, Angew. Chem., Int. Ed., 2008, 47, 3524.
33 A. X. Yin, W. C. Liu, J. Ke, W. Zhu, J. Gu, Y.-W. Zhang and 45 A. Lewera, W. P. Zhou, C. Vericat, J. H. Chung, R. Haasch,
C.-H. Yan, J. Am. Chem. Soc., 2012, 134, 20479.
34 C.-J. Li, Acc. Chem. Res., 2009, 42, 335.
A. Wieckowski and P. S. Bagus, Electrochim. Acta, 2006, 51,
3950.
`
35 Q. Y. Meng, J.-J. Zhong, Q. Liu, X.-W. Gao, H.-H. Zhang, 46 V. Mazzieri, F. Coloma-Pascual, A. Arcoya, P. C. L'Argentiere
T. Lei, Z.-J. Li, K. Feng, B. Chen, C.-H. Tung and L.-Z. Wu,
J. Am. Chem. Soc., 2013, 135, 19052.
36 K. Godula and D. Sames, Science, 2006, 312, 67.
and N. S. Figoli, Appl. Surf. Sci., 2003, 210, 222.
47 R. Bavand, A. Yelon and E. Sacher, Appl. Surf. Sci., 2015, 355,
279.
´
´
37 J.-L. Clement, N. Ferre, D. Siri, H. Karoui, A. Rockenbauer 48 R. Kotz, H. J. Lewerenz and S. Stucki, J. Electrochem. Soc.,
and P. Tordo, J. Org. Chem., 2005, 70, 1198. 1983, 130, 825.
38 M. Matsushita, K. Kamata, K. Yamaguchi and N. Mizuno, J. 49 Z. L. Liu, J. Y. Lee, M. Han, W. X. Chen and L. M. Gan, J.
Am. Chem. Soc., 2005, 127, 6632. Mater. Chem., 2002, 12, 2453.
39 D. Moonshiram, J. W. Jurss, J. J. Concepcion, T. Zakharova, 50 H. Hayashi, A. Ueda, A. Suino, K. Hiro and Y. Hakuta, J. Solid
I. Alperovich, T. J. Meyer and Y. Pushkar, J. Am. Chem. Soc.,
2012, 134, 4625.
40 G.-D. Lei and L. Kevan, J. Phys. Chem., 1991, 95, 4506.
41 H. Tsunoyamo, N. Ichikuni, H. Sakurai and T. Tsukuda, J.
Am. Soc. Chem., 2009, 131, 7086.
42 M. D'Arienzo, J. Carbajo, A. Bahamonde, M. Crippa,
S. Polizzi, R. Scotti, L. Wahba and F. Morazzoni, J. Am. Soc.
Chem., 2011, 133, 17652.
State Chem., 2009, 182, 2985.
51 S. A. Simakov and Y. Tsur, J. Nanopart. Res., 2007, 9, 403.
52 S. Yu, S. Li, X. Ge, M. Niu, H. Zhang, C. Xu and W. Li, Ind.
Eng. Chem. Res., 2014, 53, 2238.
53 N. Lock, M. Bremholm, M. Christensen, J. Almer, Y.-S. Chen
and B. B. Iversen, Chem.–Eur. J., 2009, 15, 13381.
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