Electronic Modulation of Palladium in Metal Phosphide Nanoparticles for Chemoselective Reduction of Halogenated Nitrobenzenes

Article abstract of DOI:10.1002/asia.201801620

Tuning the electronic property of a transition metal plays an important role in the selective catalysis. Herein, the control synthesis of (Pd_xNi_y)-P nanoparticles is reported. The binding energy of Pd3d_5/2 as a function of

Full text of DOI:10.1002/asia.201801620

DOI: 10.1002/asia.201801620
Communication
Electronic Modulation of Palladium in Metal Phosphide
Nanoparticles for Chemoselective Reduction of Halogenated
Nitrobenzenes
[a]
[a]
[c]
[a]
[b]
[a]
Ming Zhao,* Baoming Feng, Xiaofei Qiao, Ning Zhong, Xuemei Ge,* and Yuan Ji
Reduction of nitrobenzenes to access to substituted anilines
is one of the critical issues in chemical transformations as the
products are key intermediates for the fine chemical, agro-
chemical, and pharmaceutical industries. The selective reduc-
tion of the nitro group over other reducible groups in the
same molecule is challenging. It is well known that Pd is a
highly active catalyst for the reduction of nitrobenzenes, and
many efforts have been made mainly based on the support or
ligand effects to control the selectivity of Pd-based catalysts.
Abstract: Tuning the electronic property of a transition
metal plays an important role in the selective catalysis.
Herein, the control synthesis of (Pd Ni )-P nanoparticles is
x
y
reported. The binding energy of Pd3d5/2 as a function of
x/y ratio is well tunable from 335.3 to 335.9 eV. The com-
position-induced electronic modulation was correlated
with the selective catalysis of (Pd Ni )-P in the reduction of
x
y
halogenated nitrobenzenes. The electro-deficiency of Pd
helped to improve the selectivity. The amorphous
However, they have still serious limitations in the reaction con-
(
Pd Ni )P /C performed an exceptional selectivity in
38 26 36
[17–24]
ditions or substrate generality.
For the production of halo-
comparison with other related (Pd-Ni)-P/C, Pd Ni /C, and
38
26
genated aniline from the corresponding halogenated nitroben-
Pd/C. Various halogenated nitrobenzenes (chlorides, bro-
mides, and iodide) were tolerant and the corresponding
halogenated anilines were obtained in high yields. This
work provides some clues for the rational design of bimet-
allic phosphides with covalent interactions to boost the
catalysis.
zene, the preservation of carbon-halide (CÀX) bond is necessa-
[25]
ry. Palladium and Ni-based materials are well known and ver-
satile catalysts for cross-coupling reactions, due to their high
[26–28]
ability to cleave C-X bonds.
These bonds become more in-
stable when coupled with nitro group. Therefore, it is still a
challenge using Pd- or Ni-based materials to catalyze the selec-
tive reduction of halogenated nitrobenzenes to the corre-
[29]
sponding halogenated anilines, compared with Pt catalysts.
It was reported that the electron-rich Pd could lower energy
Tuning the electronic property of transition metal plays an im-
portant role in the selective catalysis, and the electrons transfer
[3]
barrier for the cleavage of the CÀX bond. On the contrary,
electronic deficient Pd formed in M-P due to the Pd-P covalent
bonds. It inspired us that CÀX bonds would be stable in the
presence of M-P catalyst. For the fabrication of binary Pd-P cat-
alyst, excess amount of phosphine was used but metal rich
[1–7]
usually occurs by means of coordinated or covalent bonds.
Metal phosphides (M-P) including metal-P covalent bonds
[8–12]
have been extensively studied in heterogeneous catalysis.
They are synthesized by the co-reduction of metal precursor
[
12,13]
[12]
and phosphine in one pot.
The interaction between noble
compositions was obtained. It resulted in a deficient regula-
metal and metalloid element made the metal electronic defi-
cient and also formed a high energy barrier for subsurface
tion on the electronic modulation of Pd. In this work, we de-
signed the fabrication of ternary metal phosphide, that is,
(Pd Ni )-P, with a constant metal/P composition. The valent
[
14–16]
chemistry, segregation, and metal-hydride formation.
Thus,
x
y
M-P have been employed as a class of efficient hydrogenation
states of Pd were well tunable as a function of x/y. Based on
the adjustable electropositivity of Pd, the selective reduction
of halogenated nitrobenzenes with the series of (Pd Ni )-P cat-
[
8,10–12,16]
catalysts.
x
y
alysts was investigated.
The colloidal (Pd Ni )-P NPs were fabricated via an one-pot
[
a] Dr. M. Zhao, B. Feng, N. Zhong, Y. Ji
School of Chemical Engineering
China University of Mining and Technology
Xuzhou 221116 (P. R. China)
x
y
[13]
solution phase reduction method.
Specially, Pd(acac)2,
Ni(acac) , and PPh3 were co-reduced with a constant mole
2
E-mail: ming815zhao@163.com
ratio of M(acac) /PPh (M=Pd, Ni), 0.4 mmol/0.88 mmol. After
2
3
[
b] Dr. X. Ge
an incubation at 2708C, the coated NPs were deposited to
Vulcan XC-72 and annealed at 3008C for 2 h. From transmis-
sion electron microscope (TEM, Figures 1a–e), the robust syn-
thetic method allowed the formation of the NPs which were
well disperse on carbon. The particle size changed from 2.9 nm
to 7.5 nm in diameter as the content of Ni increased (Fig-
ure S1). Energy-dispersive X-ray (EDX) analysis determined the
compositions of the five (Pd Ni )-P NPs as (Pd Ni )P ,
Department of Food Science and Technology
College of Light Industry Science and Engineering
Nanjing Forestry University
Nanjing 210037 (P. R. China)
E-mail: gexuemei2008@126.com
[
c] X. Qiao
Wanhua Chemical Group Co., Ltd.
Yantai 264006 (P. R. China)
x
y
50 11 39
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
(
Pd Ni )P , (Pd Ni )P , (Pd Ni )P , and (Pd Ni )P , respec-
50 14 36 38 26 36 33 33 34 21 40 39
https://doi.org/10.1002/asia.201801620.
tively, which were in accord with the results from laser ablation
Chem. Asian J. 2019, 00, 0 – 0
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Communication
ciency and tunable d-band centres of Pd. Oppositely, the BEs
0
of P shift negatively (from 129.5 eV to 130 eV) with respect to
[31]
that of red phosphorus (130.4 eV). Moreover, the BEs of P2p
3À
in typical P O5 or PO4 in (Pd Ni )P , (Pd Ni )P , and
2
38 26 36
33 33 34
(
Pd Ni )P also shifted negatively by 0.4 eV in comparison
21 40 39
with those in (Pd Ni )P NPs and (Pd Ni )P NPs. In Fig-
50
11 39
50 14 36
ure S3, the peaks at 850–865 eV were assigned to Ni2p3/2, and
the two peaks located at 861.6 eV and 856.2 eV could be as-
signed to NiO and Ni(OH) , respectively. The BEs of Ni in
2
(
Pd Ni )P , (Pd Ni )P , and (Pd Ni )P NPs at 852.7 eV are
38 26 36 33 33 34 21 40 39
very close to the zero valance of metallic Ni (852.8 eV), indicat-
ing that the interactions between Ni and P are out of existence
[32,33]
or very weak.
These results demonstrate the main elec-
tronic transfer occurs between Pd and P. Palladium was posi-
tively charged while P was electron negative. Phosphorus with-
draws electrons from Pd atom via Pd-P covalent bonds, while
Ni atoms keep neutral and mainly exist as oxide states, that is,
NiO and Ni(OH) , at the particle surface. The electronic proper-
2
ties of Pd in (Pd Ni )-P is opposite to that of phosphine deco-
x
y
[1]
rated Pd NPs which has electron-rich character. As the con-
tent of P keeps constant as 36.5Æ2.5 at% in the five (Pd Ni )-P
x
y
materials, a lower Pd/P atomic ratio means the formation of
more Pd-P covalent bonds per Pd atom which results in more
electro-deficient Pd.
Figure 1. TEM images of (Pd50Ni11)P39/C (a), (Pd50Ni14)P36/C (b), (Pd38Ni26)P36/C
c), (Pd33Ni33)P34/C (d), and (Pd21Ni40)P39/C (e).
(
Inspired by the intrinsic electronic structures, it is expected
that the abilities for CÀX bond cleavage would be varied in
(
Pd Ni )-P/C. The selective reduction of p-chloro-nitrobenzene
x y
inductively coupled plasma mass spectrometry (LA-ICP-MS).
Due to the higher redox potential, Ni was incompletely re-
duced. Yet, EDX results indicated we had succeeded to synthe-
(p-CNB) to p-chloro-aniline (p-CAN) was elected as a model re-
action and introduced in an aqueous solvent. The NP sizes of
(Pd Ni )P /C and (Pd Ni )P /C are similar with that of Pd/C
50
11 39
50 14 36
size the M-P with
Pd Ni )
a
constant metal/P composition of
(Figure S4). They showed improved selectivity compared to
Pd/C (Figure 3a), due to the electronic deficiency of Pd. When
the BE of Pd3d was located at 335.86 eV, (Pd Ni )P /C con-
(
P
.
x
y 63.5Æ2.5 36.5Æ2.5
In order to investigate the electron interactions and possible
38
26 36
of metal-P covalent bonds, the chemical states of Pd, Ni, and P
in various (Pd Ni )-P/C were accessed with X-ray photoelectron
tributed to the best selectivity of 99% with an isolated yield
96%. A dived selectivity for p-CAN with (Pd Ni )P /C was ob-
x
y
33 33 34
spectroscopy (XPS). In Figure 2, the binding energies (BEs) of
Pd shift positively (from 335.3 eV to 335.94 eV with reference
to Pd3d5/2) with a decreased content of Pd, compared with
served probably ascribed to the cover of Pd atoms by abun-
[34]
dant Ni, as the particle size and electronic state were nearly
the same for (Pd Ni )P /C and (Pd Ni )P /C. It indicated
38
26 36
33 33 34
0
[30]
that of pure Pd (335.2 eV), which indicate the electron defi-
that a rational Pd/Ni ratio was crucial for the selective catalysis.
Another control experiment with Pd Ni /C (Figure S5) as the
38
26
catalyst, 80% of p-CAN was obtained (Figure 3b). It indicated
the necessity of P, or in another word, Pd-P covalent bonds.
The bimetallic catalyst did not lead to a higher selectivity com-
pared to Pd/C (Figures 3a and 3b). It also demonstrated Ni
was useless for the protection of C-Cl bond and the unique
catalysis of (Pd Ni )P /C arose from positively charged Pd.
38
26 36
When the reaction time was prolonged to 20 h, the isolated
yield of the p-CAN kept as high as 90% catalyzed by
(
Pd Ni )P /C (Figure 3c). It demonstrated the cleavage CÀCl
38
26 36
bond rarely occurred, indicating its robust catalysis in the se-
lective reduction of halogenated nitrobenzene. In contrast, Pd/
C easily cut CÀCl bond of aniline chloride to obtain aniline as
the main product after 20 h (Figure 3c). XPS analysis indicated
the electron deficiency of Pd in M-P, which could lead to a sup-
pression of Pd-hydride formation and an improved selectivity
[14]
for the reduction reaction. On the other hand, it was report-
Figure 2. XPS spectra of (Pd-Ni)-P/C in terms of Pd3d (left half part) and P2p
(right half part).
ed that CÀX bond was cut more easily with N-heterocyclic car-
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Communication
Table 1. The CÀX (X=Cl, Br, I) bonds were well tolerant with
(
Pd Ni )P /C (odd enties) compared with Pd/C (even entries).
38 26 36
With M-P catalyst, the products 3- and 2-chloro-aniline were
obtained in 94% and 86% yield, respectively (entry 3 and
Table 1. Selective reduction of various halogenated nitrobenzenes to the
corresponding halogenated anilines with (Pd38Ni26)P36/C and Pd/C as the
catalysts.
[
a]
Entry
Substrate
Catalyst
Pd [mol%]
Yield [%]
1
(Pd38Ni26)P36/C
0.2
96
2
Pd/C
0.2
84
3
4
(Pd38Ni26)P36/C
Pd/C
0.2
0.2
94
55
[
b]
5
6
(Pd38Ni26)P36/C
Pd/C
0.2
0.2
86
78
[
b]
7
8
(Pd38Ni26)P36/C
Pd/C
0.1(0.2)
0.2
95(86)
49
[
b]
[
b]
9
(Pd38Ni26)P36/C
Pd/C
0.1
0.2
75
20
[
b]
10
1
1
(Pd38Ni26)P36/C
Pd/C
0.1
0.2
88
43
[
b]
12
1
3
(Pd38Ni26)P36/C
Pd/C
0.1
0.2
50
Trace
[c]
14
Reaction conditions: p-CNB (2 mmol), NaBH
4
2
(4 mmol), EtOH/H O (2/
À1
0.5 mLmL ), room temperature, 10 h. [a] Isolated yield unless otherwise
Figure 3. a) Summary for the isolated yields of p-CAN and BEs of Pd3d with
different Pd-based catalysts (reaction time: 10 h for (Pd-Ni)-P/C and 15 h for
Pd/C). b) Comparison of the yields of p-CAN with Pd38Ni26/C, (Pd38Ni26)P36/C,
and (Pd38Ni26)P36/C-600 as the catalysts. c) Yield-time plots of the reduction
stated. [b] Proton NMR yields calculated with an internal standard of
CH Br . [c] Aniline was obtained as the main byproduct.
2
2
reaction with (Pd38Ni26)P36/C and Pd/C. Reaction conditions: p-CNB (2 mmol),
À1
catalyst (0.2 mol% Pd), NaBH
perature.
4
(4 mmol), EtOH/H
2
O (2/0.5 mLmL ), room tem-
entry 5). In contrast, lower yields were achieved with Pd/C
entry 2 and entry 4). In the presence of (Pd Ni )P /C, the se-
(
38
26 36
bene coordinated Pd/Al O due to the electro-negativity of
lective reductions of more challenging substrates 4- and 2-
bromo-nitrobenzene afforded bromo-anilines in 95% and 88%,
respectively (entries 7 and 11). However, less than 50% yields
of the two products were obtained with Pd/C. In entry 9, a
slightly decreased yield of 3-bromo-aniline was obtained with
(Pd Ni )P /C. But only 20% of this product was given with
2
3
[
3]
Pd. Thus, the high selectivity of (Pd Ni )P /C would be
38
26 36
mainly ascribed to the inefficient CÀCl bond cleavage ability
due to the electronic positivity of Pd. Another control experi-
ment was carried out with well crystallized (Pd Ni )P /C-600
38
26 36
as the catalyst, which was obtained by a simply thermal crys-
38
26 36
tallization of (Pd Ni )P /C at 6008C (see Section 6 in the Sup-
Pd/C (entry 10). In entry 13, the most instable C-I bond could
be compitable in the reduction reaction and 4-iodo-aniline was
isolated with a moderate yield of 50%. In contrast, trace
amount of the product was obtained with Pd/C (entry 14). The
catalytic performance between (Pd Ni )P /C and various re-
38
26 36
porting information for the detail). The (Pd Ni )P /C-600 ex-
38
26 36
hibited a similar selectivity (an isolated yield of 85% after 15 h)
with Pd/C probably due to the aggregations of Pd phase
during the high temperature annealing (Figure 3b). Compared
with (Pd Ni )P /C-600, the (Pd Ni )P /C catalyst show lower
38
26 36
ported Pd- or Ni-based catalysts in the reductions of halogen-
ated nitrobenzenes were compared and summarized in
Table S1. It also indicated that the (Pd Ni )P /C showed the
38
26 36
38 26 36
crystallization (an amorphous morphology, Figure S6a). The
entire coarse grained surface of the latter could decrease the
38
26 36
[
35]
hydriding behaviour of Pd, which might partially contributed
to the selectivity.
best selectivity.
In summary, five kinds of (Pd Ni )-P NPs were synthesized by
x
y
The reductions of various halogenated nitrobenzenes were
investigated with (Pd Ni )P /C and Pd/C and summarized in
feeding metal precursors and triphenylphosphine with a con-
stant mole ratio. It allows the investigation of the correlation
38
26 36
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Keywords: chemoselectivity · heterogeneous catalyst · metal
phosphide · palladium · para-chloronitrobenzene
[
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Manuscript received: November 3, 2018
6
Revised manuscript received: December 6, 2018
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Accepted manuscript online: December 11, 2018
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7410.
Version of record online: && &&, 0000
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Communication
COMMUNICATION
A series of (Pd Ni )-P nanoparticles
Ming Zhao,* Baoming Feng, Xiaofei Qiao,
Ning Zhong, Xuemei Ge,* Yuan Ji
x
y
were synthesized with tunable binding
energy of Pd3d. The composition in-
duced electronic modulation was corre-
lated with the selective catalysis in re-
duction of halogenated nitrobenzenes.
Thanks to the positively charged Pd,
&& – &&
Electronic Modulation of Palladium in
Metal Phosphide Nanoparticles for
Chemoselective Reduction of
(
Pd Ni )P /C performed an exceptional
Halogenated Nitrobenzenes
38
26 36
selectivity and the corresponding halo-
genated anilines were obtained in high
yields.
Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
5
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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