Cooperative Multifunctional Catalysts for Nitrone Synthesis: Platinum Nanoclusters in Amine-Functionalized Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201710164

Nitrones are key intermediates in organic synthesis and the pharmaceutical industry. The heterogeneous synthesis of nitrones with multifunctional catalysts is extremely attractive but rarely explored. Herein, we report ultrasmall platinum nanoclusters (PtNCs) encapsulated in amine-functionalized Zr metal–organic framework (MOF), UiO-66-NH₂ (Pt@UiO-66-NH₂) as a multifunctional catalyst in the one-pot tandem synthesis of nitrones. By virtue of the cooperative interplay among the selective hydrogenation activity provided by the ultrasmall PtNCs and Lewis acidity/basicity/nanoconfinement endowed by UiO-66-NH₂, Pt@UiO-66-NH₂ exhibits remarkable activity and selectivity, in comparison to Pt/carbon, Pt@UiO-66, and Pd@UiO-66-NH₂. Pt@UiO-66-NH₂ also outperforms Pt nanoparticles supported on the external surface of the same MOF (Pt/UiO-66-NH₂). To our knowledge, this work demonstrates the first examples of one-pot synthesis of nitrones using recyclable multifunctional heterogeneous catalysts.

Full text of DOI:10.1002/anie.201710164

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Angewandte
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Accepted Article
Title: Cooperative Multifunctional Catalysts for Nitrone Synthesis:
Platinum Nanoclusters in Amine-Functionalized Metal-Organic
Frameworks
Authors: Xinle Li, Biying Zhang, Linlin Tang, Tian Wei Goh, Shuyan
Qi, Alexander Volkov, Yuchen Pei, Zhiyuan Qi, Chia-Kuang
Tsung, Levi Stanley, and Wenyu Huang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710164
Angew. Chem. 10.1002/ange.201710164
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http://dx.doi.org/10.1002/ange.201710164

10.1002/anie.201710164
Angewandte Chemie International Edition
COMMUNICATION
Cooperative Multifunctional Catalysts for Nitrone Synthesis:
Platinum Nanoclusters in Amine-Functionalized Metal-Organic
Frameworks
Xinle Li,[ab] Biying Zhang,[a] Linlin Tang,[a] Tian Wei Goh, [ab] Shuyan Qi,[ac] Alexander Volkov,[a] Yuchen
Pei,[ab] Zhiyuan Qi,[ab] Chia-Kuang Tsung,[d] Levi Stanley,[a] and Wenyu Huang*[ab]
Abstract: Nitrones are key intermediates in organic synthesis and the
pharmaceutical industry. The heterogeneous synthesis of nitrones
with multifunctional catalysts is extremely attractive but rarely
explored. Herein, we report ultrasmall platinum nanoclusters (Pt NCs)
encapsulated in amine-functionalized UiO-66-NH2 (Pt@UiO-66-NH2)
as a multifunctional catalyst in the one-pot tandem synthesis of
nitrones. By virtue of the cooperative interplay among the selective
hydrogenation activity provided by the ultrasmall Pt NCs and Lewis
acidity/basicity/nanoconfinement endowed by UiO-66-NH2, Pt@UiO-
66-NH2 exhibits remarkable activity and selectivity, in comparison to
Pt/carbon, Pt@UiO-66, and Pd@UiO-66-NH2. Pt@UiO-66-NH2 also
outperforms Pt nanoparticles supported on the external surface of the
same MOF (Pt/UiO-66-NH2). To the best of our knowledge, this work
demonstrates the first examples of one-pot synthesis of nitrones using
recyclable multifunctional heterogeneous catalysts.
catalysts have been reported for the synthesis of nitrones,[8] and
these heterogeneous routes require the use of expensive
hydroxylamine precursors. Bearing these limitations in mind, we
envisioned that hydroxylamines, which are often unstable
intermediates,[9] could be generated in situ from the selective
hydrogenation of inexpensive and readily available nitroalkanes
in the presence of a heterogeneous catalyst. The subsequent
condensation of the resulting hydroxylamine with aromatic
aldehydes would produce nitrones. This rationally-designed
tandem reaction, combining two individual reactions
(hydrogenation and condensation) in a single pot, is highly
attractive as a synthetic route to nitrones, since it eliminates the
need to isolate and purify the unstable hydroxylamine
intermediates.[10] To accomplish the proposed tandem reaction,
we envisioned the development of
a
multifunctional
heterogeneous catalyst capable of promoting both reduction and
condensation process while minimizing undesired reaction
pathways.
Nitrones are versatile reagents central to a wide range of
cycloaddition reactions to form biologically active nitrogen
heterocycles.[1] Significant efforts have been devoted towards the
development of efficient syntheses of nitrones including the
condensation of N-methyl hydroxylamine hydrochloride with
aromatic aldehydes,[2] the oxidation of amines[3] or imines,[4] the
reduction of N-hydroxy amides,[5] the alkylation of oximes,[6] and
the Cope-type hydroamination of alkenes.[7] However, these
aforementioned methods are either non-catalytic or rely on
homogeneous transition metal catalysts that are difficult to reuse
and can contaminate the nitrone product. The development of
heterogeneous catalysts for nitrone synthesis is attractive due to
the relative ease with which heterogeneous catalysts can be
separated and recycled. Nevertheless, only a few heterogeneous
Metal-organic frameworks (MOFs) are a burgeoning class of
crystalline organic-inorganic porous materials that have attracted
increasing interest due to their large surface area, facile
tunability/tailorability, and their numerous applications in gas
storage and separation,[11] chemical sensors,[12] catalysis,[13] drug
delivery,[14] and proton conductivity.[15] Of particular interest to this
study, MOFs have been widely utilized as a host matrix for metal
nanoparticles (metal NPs@MOF), serving as advanced
heterogeneous catalysts with multiple functions.[16] Metal
NPs@MOF holds specific advantages as multifunctional catalysts
including (i) the uniform and small cavities of MOFs restrict the
overgrowth of metal NPs affording more surface metal sites; (ii)
multiple active sites (i.e., metal NPs and Lewis acid/base sites on
MOFs) enable the flexible design of multifunctional catalysts;[17]
(iii) the highly porous nature of MOFs facilitates rapid mass
transportation of substrates to access the encapsulated metal
NPs; and (iv) the nanoconfinement endowed by MOFs can lead
to enhanced catalytic performance. Despite intensive efforts
intended to elucidate the role of metal NPs@MOFs in
heterogeneous catalysis,[18] only a limited number of studies on
multifunctional NPs@MOF catalysts of tandem reactions have
been reported.[12,19] Therefore, the development of multifunctional
metal NPs@MOF catalysts for tandem catalysis is highly desired
but remains a significant challenge.
[a]
X.Li, B.Zhang, L.Tang, T. Goh, A.Volkov, Y.Pei, Z.Qi, Prof. L.
Stanley, Prof. W. Huang
Department of Chemistry, Iowa State University, Ames, IA 50011
(USA)
E-mail: whuang@iastate.edu
[b]
[c]
[d]
X.Li, T. Goh, Y.Pei, Z.Qi, Prof. W. Huang
Ames Laboratory, US Department of Energy
Ames, IA 50011 (USA)
S.Qi
Department of Chemistry, Beijing Normal University, Haidian,
Beijing 100875 (P.R China)
C.-K. Tsung
Herein, we report for the first time a facile, one-pot synthesis
of nitrones catalyzed by Pt NCs encapsulated inside a Zr-MOF,
UiO-66-NH2 (denoted as Pt@UiO-66-NH2). The multifunctional
Pt@UiO-66-NH2 catalyzes the tandem reaction of nitromethane
with aromatic aldehydes to form N-methyl-α-aryl nitrones with
Department of Chemistry, Boston College, Chestnut Hill, MA 02467
(USA)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201710164
Angewandte Chemie International Edition
COMMUNICATION
high activity and selectivity. This tandem reaction was achieved
by coupling the hydrogenation activity of Pt NCs, the Lewis
acidic/basic sites of UiO-66-NH2, and the nanoconfinement
endowed by the effective encapsulation of Pt NCs inside UiO-66-
NH2. By virtue of the cooperative interplay among ultrasmall Pt
NCs and UiO-66-NH2, Pt@UiO-66-NH2 exhibits superior catalytic
performance in the tandem nitrone synthesis in comparison to
Pt/carbon, Pt@UiO-66, Pt/UiO-66-NH2 (Pt NPs supported on the
external surface of UiO-66-NH2) and Pd@UiO-66-NH2. To the
best of our knowledge, this study demonstrates the first example
of one-pot tandem synthesis of nitrone using heterogeneous
multifunctional catalysts.
confirmed that most of the Pt NCs are indeed encapsulated inside
at different angles.[21]
Nitrogen sorption isotherms show that both UiO-66-NH2 and
2.0 wt% Pt@UiO-66-NH2 display a Type I curve (Figure S2),
suggesting the existence of micropores. Pt@UiO-66-NH2 has a
Brunauer–Emmett–Teller (BET) surface area of 869 m2 g−1 with a
micropore volume of 0.35 cm3 g−1. These values are smaller than
those of the parent UiO-66-NH2 (BET surface area of 931 m2 g−1
and micropore volume of 0.38 cm3 g−1). The decrease in the
surface area and pore volume of Pt@UiO-66-NH2 is attributed to
the partial occupation of the cavities in UiO-66-NH2 by the Pt NCs.
It is worth noting that the loaded metal occupies less than one
percent of the total internal volume of the MOF and the Pt@UiO-
66-NH2 remains highly porous for the diffusion of reactant and
product molecules.
OH
II.
Table 1. Identification of Reaction Conditions by Pt@UiO-66-NH2.[a]
H2
O
O
III.
I.
O
N
NO2
N
N
UiO-66-NH2
NO2
N
OH
O
NO2
H2
Pt NCs
UiO-66-NH2
H
O
N
Pt
NC2s
H
N
OH
O
entry
Pt
H2
solvent
conv.
(%)
99
99
93
98
90
98
95
94
31
/
nitrone sel.
Scheme1. A plausible mechanism of nitrone synthesis through hydrogenation
of nitromethane and condensation of benzaldehyde (solid rectangle) and
possible side reactions indicated by the dashed rectangles, including I) the
condensation reaction between benzaldehyde and nitromethane, II) the direct
hydrogenation of benzaldehyde, and III) further hydrogenation of the nitrone.
(psi)
200
200
200
200
200
200
100
500
200
200
(%)
97
98
94
80
60
91
82
94
86
/
1
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
0
Toluene
2[b]
3
Nitromethane
Trifluorotoluene
MeOH
1,4-dioxane
Cyclohexane
Toluene
Toluene
Toluene
Toluene
4[c]
5
6
7
8
a)
b)
9[d]
10
[a] Reaction conditions: benzaldehyde (0.1 mmol), nitromethane (1.5
mmol), toluene (1 mL), metal/substrate = 1 mol%, ambient temperature, 12
hours, stirring at 600 rpm. [b] 1 mL of nitromethane was used. [c]
Benzaldehyde dimethyl acetal was formed. [d] Nitromethane is 3 eq. of
benzaldehyde.
5 nm
10 nm
We next set out to evaluate Pt@UiO-66-NH2 in the tandem
reaction of benzaldehyde, nitromethane, and hydrogen at
ambient temperature. A plausible mechanism of this tandem
reaction involves two steps (Figure S3):[22] i) the selective
hydrogenation of nitromethane to yield N-methyl hydroxylamine
on the Pt NCs surface and ii) the subsequent condensation of
hydroxylamine and benzaldehyde to afford N-methyl-α-phenyl
nitrone by UiO-66-NH2 (solid rectangle in Scheme 1). Other
possible side reactions are indicated by the dashed rectangles in
Scheme 1, including I) the condensation of benzaldehyde and
nitromethane to form β-nitrostyrene, II) the direct hydrogenation
of benzaldehyde into benzyl alcohol, and III) the further
hydrogenation of N-methyl-α-phenyl nitrone to N-methyl-1-
phenylmethanimine. A variety of solvents, hydrogen pressures,
and feed ratios of nitromethane have been evaluated in an effort
to optimize reaction conditions. When toluene was used as
solvent with 200 psi H2 and 15 equivalents of nitromethane to
benzaldehyde, Pt@UiO-66-NH2 gave 99% conversion of
benzaldehyde and 97% selectivity for nitrone formation. Tandem
catalysis in nitromethane without toluene (entry 2) also led to high
conversion (99%) and selectivity (98%) to nitrone. Analogous
reactions in methanol, trifluorotoluene, cyclohexane, and 1,4-
dioxane occurred with decreased conversion and/or selectivity
Figure 1. TEM images of 2.0 wt% Pt@UiO-66-NH2 at different resolutions.
We used UiO-66-NH2 as the MOF host matrix, which
contains uniform and ultrasmall cavities (~1.2 nm). These cavities
could serve as rigid templates to encapsulate metal NCs with a
matching size.[13d] Following a reported protocol,[20] UiO-66-NH2
was prepared solvothermally from ZrCl4 and 2-aminoterephthalic
acid. The powder X-ray diffraction (PXRD) pattern of the as-
synthesized UiO-66-NH2 was identical to that of the simulated
UiO-66 (Figure S1). Using UiO-66-NH2 as the host, we
immobilized Pt NCs exclusively inside its cavities via a solution
impregnation approach developed by our group (Details in the
Supporting Information).[21] The PXRD analysis indicates that the
crystallinity of UiO-66-NH2 was well retained upon metal loading
and reduction (Figure S1). The actual Pt content in Pt@UiO-66-
NH2 was quantitatively determined to be 2.0 wt% by inductively
coupled plasma mass spectrometry (ICP-MS). Transmission
electron microscopy (TEM) images clearly show that the Pt NCs
are evenly dispersed over the UiO-66-NH2 crystals with a mean
diameter of ~1.1 nm and no observable aggregation on the
external surface of UiO-66-NH2 (Figure 1). Moreover, we have
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10.1002/anie.201710164
Angewandte Chemie International Edition
COMMUNICATION
(Table 1, entries 3-6). Decreasing the H2 pressure to 100 psi led
to a lower selectivity, while a higher H2 pressure of 500 psi did not
improve the selectivity (entries 7 and 8). It is notable that
decreasing the concentration of nitromethane from 1.5 M to 0.3 M
(15 to 3 equivalents) led to significantly decreased activity and a
lower selectivity at 86% (entry 9). UiO-66-NH2 alone resulted in
the negligible conversion of benzaldehyde, indicating that Pt NCs
are essential in the tandem catalysis (Table 1, entry 10). Time-
dependent studies of the tandem reaction under the optimum
conditions revealed that Pt@UiO-66-NH2 showed high selectivity
to nitrone initially. With the reaction proceeding, the activity
gradually increased without significant decrement of the
selectivity (Figure S4).
a comparison to investigate the effect of Pt NC location on
catalysis. Following the traditional impregnation method with
slight modification, we deliberately deposited Pt NPs on the
external surface of MOFs (designated as Pt/UiO-66-NH2) and the
TEM image of Pt/UiO-66-NH2 shows the average size of the Pt
NPs to be 3.8 ± 0.7 nm, which is larger than the largest cavity of
UiO-66-NH2 (Figure S6). Pt/UiO-66-NH2 showed an inferior
activity (69%) and selectivity (84%) in comparison to Pt@UiO-66-
NH2 (Figure 2). We also deposited PVP-capped Pt NPs (~2.7 nm)
onto the surface of MOF (Figure S7) using a polyol reduction
method (termed PVP-Pt/UiO-66-NH2). PVP-Pt/UiO-66-NH2
showed strikingly low activity (6%) and selectivity (60%, the major
byproduct is benzyl alcohol) in comparison to Pt@UiO-66-NH2
(Figure 2). The location of Pt in these two control samples and
Pt@UiO-66-NH2 was confirmed by the hydrogenation of two
Conv.
Sel. to nitrone
100
probe
molecules
of
different
sizes,
styrene
and
80
60
40
20
0
tetraphenylethylene (Figure S8). These results clearly highlight
the advantages of nanoconfinement effects in MOFs. Apart from
the nanoconfinement effect, we cannot completely exclude that
the smaller Pt particle size in the Pt@UiO-66-NH2 contributes to
the enhanced activity than those control catalysts.
The metal selection was also evaluated by replacing Pt with
Pd. Surprisingly, Pd@UiO-66-NH2, which possesses similar
ultrasmall NCs (Figure S9) did not lead to the formation of the
nitrone. Instead, high selectivity for benzyl alcohol (99%) was
observed under otherwise identical conditions (Figure 2). This
strikingly different catalytic performance clearly demonstrates the
unique properties of Pt NCs in this tandem reaction. We infer that
Pt and Pd NCs possess different adsorption strength concerning
nitromethane and benzaldehyde. For Pd NCs, benzaldehyde
molecules preferentially adsorb on the metal surface in
comparison to nitromethane, and thus, likely undergo the
hydrogenation to yield benzyl alcohol (reaction II in Scheme 1). In
contrast, absorption of nitromethane on the surface of Pt is
favored over benzaldehyde leading to the reduction of
nitromethane to form hydroxylamine and subsequent formation of
the nitrone upon condensation with benzaldehyde. These results
suggest that the competitive adsorption of reactants
(benzaldehyde and nitromethane) over different metal NPs (Pt
and Pd) could alter the product selectivity in this tandem reaction.
A similar effect has been experimentally and computationally
proven in selective phenol hydrogenation.[24] Kinetics studies of
tandem catalysis with varied benzaldehyde to nitromethane ratio
show that the reaction is first order to benzaldehyde and zero
order to nitromethane, indicating the nitromethane or its
hydrogenation products, hydroxylamine, is the dominant species
on the surface of the catalysts under investigated conditions
(Figure S10-11).[25] A more detailed study to reveal how catalytic
pathways vary in this tandem nitrone synthesis is ongoing.
Pt@UiO-66-NH2 is unique that can avoid the three
byproducts (I – III) showing in Scheme 1 and only lead to the
nitrone product. To test the scope of this multifunctional catalyst,
we conducted the tandem reaction of a variety of substituted
benzaldehydes with nitromethane over Pt@UiO-66-NH2 under
optimized conditions. As illustrated in Table 2, we found that
reactions of halogenated benzaldehydes (2-F, 4-Cl, and 4-Br) and
benzaldehydes containing electron-withdrawing groups (3-CN, 4-
CN) occurred with good-to-high conversions and selectivities.
Benzaldehydes containing electron-donating groups (4-Me, 4-
P
P
6
-N
P
d
@
t@
P
P
t/
t@
t
U
U
i
/
c
i
U
i
O
O
a
Ui
O
O
r
bon
-6
-6
6
-6
-
6
6
-
-N
6
H
H
NH
2
2
2
Figure 2. Tandem nitrone synthesis by various heterogeneous catalysts. PVP-
Pt/UiO-66-NH2, Pt/UiO-66-NH2, Pt@UiO-66 and Pd@UiO-66-NH2 gave benzyl
alcohol as the major product; Pt/Carbon gave β-nitrostyrene as the major
product.
To elucidate the origin of the excellent catalytic performance
of Pt@UiO-66-NH2 in this tandem reaction, we systematically
tested a series of control catalysts under identical conditions
(toluene, 200 psi H2, ambient temperature, 12 hours, Figure 2).
To probe the role of -NH2 substituents on the benzene
dicarboxylic acid (BDC) linker in this tandem reaction, we
synthesized UiO-66, an isoreticular analog of UiO-66-NH2, using
unsubstituted BDC as the linker. UiO-66 and UiO-66-NH2 have
the same structure but different chemical functionality. Pt@UiO-
66 prepared from UiO-66 gives similar Pt NCs size measured by
TEM (Figure S5). To our surprise, the nitrone selectivity of
Pt@UiO-66 is significantly lower (28%) than that of Pt@UiO-66-
NH2 (97%), and the major product formed is benzyl alcohol (53%).
This result highlights the critical role of –NH2 in the high selectivity
to nitrone in the tandem reaction, inhibiting the direct
hydrogenation of benzaldehyde. To probe the role of Lewis acidity
in the tandem catalysis, we added a base probe molecule
(triethylamine) when the reaction has proceeded for 2 hours. The
activity and selectivity decreased significantly compared to the
reaction without the base (Table S1), which demonstrates the
importance of Lewis acidity of MOFs on the catalytic performance.
Using Pt/carbon as the catalyst led to low selectivity for the nitrone
product (23%) and the formation of β-nitrostyrene (25%) as the
major product.
Nanoconfinement effects are often key factors in
heterogeneous catalysis to enhance selectivity.[23] To explore the
nanoconfinement effect rendered by the UiO-66-NH2, we
deposited Pt NPs on the external surface of UiO-66-NH2 via two
preparation protocols - traditional impregnation and loading
polyvinylpyrrolidone (PVP)-capped Pt NPs on MOFs, serving as
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10.1002/anie.201710164
Angewandte Chemie International Edition
COMMUNICATION
OMe) gave moderate conversions and selectivities, indicating the
generality of the multifunctional catalyst.
This work was partially supported by National Science Foundation
(NSF) grant CHE-1566445. We also thank the support from Iowa
State University. We thank Gordon J. Miller for the use of the XRD.
Table 2. Tandem Catalysis with Various Substituted Benzaldehyde and
Nitromethane over Pt@UiO-66-NH2.
O
Keywords: MOFs, tandem catalysis, heterogeneous catalysis,
cooperative catalysis, nanoparticles.
O
N
NO2
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2-F
4-Cl
4-Br
3-CN
4-CN
4-Me
4-OMe
Conv.(%)
Sel. to nitrone (%)
1
2
3
4
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99
87
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99
97
81
76
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58
47
88
82
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60
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noticeable decrease in activity and selectivity (Figure S14). A
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the conversion of benzaldehyde was observed (Figure S15).
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In conclusion, we have developed for the first time a one-pot
synthesis of nitrones with high activity and selectivity by virtue of
a
cooperative multifunctional heterogeneous catalyst. By
combining the in situ formation of N-methyl hydroxylamine and
subsequent condensation with aromatic aldehydes, the Pt@UiO-
66-NH2 catalyst shows excellent catalytic performance in the
tandem catalysis and significantly outperforms Pt/carbon,
Pt@UiO-66, Pt/UiO-66-NH2 and Pd@UiO-66-NH2, presumably
due to the synergetic cooperation among the ultrasmall Pt NCs,
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effects. Furthermore, the multifunctional Pt@UiO-66-NH2 catalyst
is recyclable. This facile and rational design will open new
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Acknowledgements
This article is protected by copyright. All rights reserved.

10.1002/anie.201710164
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Nitrones are key intermediates in
organic synthesis and the
Xinle Li, Biying Zhang, Linlin Tang, Tian
Wei Goh, Shuyan Qi, Alexander Volkov,
Yuchen Pei, Zhiyuan Qi, Chia-Kuang
Tsung, Levi Stanley, and Wenyu Huang*
pharmaceutical industry. We report
platinum nanoclusters (Pt NCs)
encapsulated in amine-functionalized
UiO-66-NH2 (Pt@UiO-66-NH2) as a
multifunctional catalyst in the one-pot
tandem synthesis of nitrones. By the
cooperative interplay among the
selective hydrogenation activity
provided by Pt NCs and Lewis
acidity/basicity/nanoconfinement
endowed by the MOF, Pt@UiO-66-
NH2 exhibits remarkable activity and
selectivity.
Page No. – Page No.
Cooperative Multifunctional Catalysts
for Nitrone Synthesis: Platinum
Nanoclusters in Amine-
Functionalized Metal–Organic
Frameworks
This article is protected by copyright. All rights reserved.