A polyoxometalate-based PdII-coordinated ionic solid catalyst for heterogeneous aerobic oxidation of benzene to biphenyl

Article abstract of DOI:10.1039/c2cc31919e

An ionic solid catalyst by pairing Keggin polyoxometalate-anions with Pd^II-coordinated nitrile-tethered ionic liquid cations was synthesized, characterized, and tested for aerobic oxidation of benzene to biphenyl. A unique heterogeneous intramolecular electron transfer mechanism is proposed to understand its high activity.

Full text of DOI:10.1039/c2cc31919e

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Showcasing research from Professor Jun Wang’s
laboratory, Nanjing University of Technology, China.
As featured in:
II
A polyoxometalate-based Pd -coordinated ionic solid catalyst for
heterogeneous aerobic oxidation of benzene to biphenyl
Low-temperature heterogeneous aerobic oxidation of benzene
to biphenyl is achieved over an ionic solid catalyst via a unique
intramolecular electron transfer mechanism. The strategy for
synthesizing the ionic catalyst is pairing Keggin Polyoxometalate-
II
anions with nitrile-tethered Pd -coordinated ionic liquid cations.
See Jun Wang et al.,
Chem. Commun., 2012, 48, 5721.
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COMMUNICATION
II
A polyoxometalate-based Pd -coordinated ionic solid catalyst for
heterogeneous aerobic oxidation of benzene to biphenylw
a
b
a
a
a
a
Pingping Zhao, Yan Leng, Mingjue Zhang, Jun Wang,* Yajing Wu and Jun Huang
Received 16th March 2012, Accepted 12th April 2012
DOI: 10.1039/c2cc31919e
An ionic solid catalyst by pairing Keggin polyoxometalate-anions
II
with Pd -coordinated nitrile-tethered ionic liquid cations was synthe-
We recently prepared a series of solid-state POM-based IL catalysts
15
for heterogeneous organic transformations. Accordingly, we
sized, characterized, and tested for aerobic oxidation of benzene to
biphenyl. A unique heterogeneous intramolecular electron transfer
mechanism is proposed to understand its high activity.
think it rational to prepare a composite catalyst by pairing
II
POM-anions with Pd -coordinated IL-cations such that the
II
catalyst contains Pd and POM sites in an IL-like structure. The
resultant ionic solid may be a novel heterogeneous catalyst for
aerobic oxidation of benzene to biphenyl.
As an important intermediate for synthesizing polymers and
liquid crystals, biphenyl is industrially produced by vapour phase
II
This communication reports a POM-based Pd -coordinated
1,2
3
4
ionic catalyst [(C CNpy) Pd(OAc) ] HPMoV (Scheme 1) prepared
3
dimerization of benzene. Ullmann and Suzuki reactions also
produce biphenyl, but lead to by-products like copper iodide or
boric salts. Therefore, a greener low-temperature liquid-phase
coupling reaction of benzene affording biphenyl becomes an
attractive topic. Actually, Pd-catalyzed liquid-phase aerobic
coupling of benzene has been studied extensively; however, the
2
2 2
2
1
by pairing the nitrile-tethered pyridinium IL-cation with the
3a
4À
Keggin POM-anion HPMo V O
10
(HPMoV ), followed by
2
2
40
coordinating palladium acetate Pd(OAc)2 with the nitriles.
Characterization results of CHN elemental analysis, TG,
1 13
FT-IR, UV-vis, ESR, SEM, XRD, MS, H and C NMR
5
(ESIw), and the catalytic tests demonstrate that the catalyst leads
system is a high pressure homogeneous reaction with low yields.
to a highly efficient heterogeneous system for aerobic oxidation
of benzene to biphenyl. An intramolecular electron transfer
mechanism is proposed to understand its much higher activity
The high valence metal ions in polyoxometalates (POMs)
are known to be catalytically active sites for various oxidation
6
reactions. In Pd-catalyzed acetoxylation, acetalization and
7
, POM was used as a cocatalyst to
over the earlier-reported homogeneous Pd–POM–O system.
2
Wacker-type oxidation with O
2
0
reoxidize Pd to Pd . Neumann and Ettedgui coordinated
II 8
9
The elemental analysis (found: C 19.02 wt%, N 3.96 wt%,
II
H 2.00 wt%; calculated: C 19.07 wt%, N 4.04 wt%,
H 2.06 wt%) and the TG result (Fig. S1, ESIw) (weight loss found:
28.2 wt%; calculated: 30.0 wt%) confirm the chemical composition
of [(C CNpy) Pd(OAc) ] HPMoV . The SEM image (Fig. S2,
5 2 40
phenanthroline-crown ether with both Pd and H PMo10V O
0
for accelerating electron transfer from Pd to Pd in Wacker
II
1
0
oxidation of alkenes. Scheuermann et al. combined palladium
with a non-POM cocatalyst, which afforded the novel hybrid
II
Pd -complex of 1,4-quinone-substituted bis-(pyrazol-1-yl)methane.
3
2
2 2
2
ESIw) illustrated micron-sized particles, as a solid up to 180 1C.
The XRD pattern (Fig. S3, ESIw) showed an amorphous phase for
this solid containing large-volume cations and anions, different
Thus far only the homogeneous Pd–POM–O
2
system has been
11
reported to afford biphenyl from oxidative coupling of benzene,
1
6
II
5 2 40
from the crystal structure of its parent H PMoV O . Moreover,
wherein the POM and Pd individually acted as the cocatalyst and
major catalyst. To the best of our knowledge, no low-temperature
heterogeneous catalysis system has appeared as yet.
the solid is insoluble in commonly used solvents such as water,
acetic acid, DMSO, and even in the oxidative media of molecular
oxygen or hydrogen peroxide. These features allow trying it as a
heterogeneous catalyst for aerobic oxidation of benzene.
On the other hand, functionalized ionic liquids (ILs) have
12
attracted growing interest, and the coordinating group-tethered
1
3
ILs may immobilize metal catalysts. Further, POM-anions
14
have been used as counteranions for task-specific IL catalysts.
a
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemistry and Chemical Engineering, Nanjing University
of Technology, Nanjing 210009, China.
E-mail: junwang@njut.edu.cn; Fax: +86-25-83172261;
Tel: +86-25-83172264
School of Chemical and Material Engineering, Jiangnan University,
Wuxi 214122, China
b
w Electronic supplementary information (ESI) available: Experimental
details, optimization of reaction conditions and catalyst characteriza-
tions. See DOI: 10.1039/c2cc31919e
II
Scheme 1 Synthesis of the POM-based Pd -coordinated ionic
3 2 2 2 2
catalyst [(C CNpy) Pd(OAc) ] HPMoV .
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5721–5723 5721

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Table 1 Catalytic performance of various catalysts for the oxidative
coupling reaction of benzene with O
a
2
b
c
Solubility in
reaction
Y Sel
(%) (%)
Entry Catalyst
1
2
3
4
5
6
7
8
[(C
Pd
3
CNpy) Pd(OAc)
2
2
]
2
HPMoV
2
Insoluble
Insoluble
Soluble
18.3 88.0
8.2 93.9
10.4 83.5
7.1 88.8
16.1 91.0
18.4 81.8
2.2 93.4
2.8 89.8
2
HPMoV
2
H
H
5
PMoV
PMoV
2
O
O
40-Pd(OAc)
40-Pd(OAc)
2
11b
Fig. 1 (A): FT-IR spectra of (a) H
(c) [(C CNpy) Pd(OAc) HPMoV
(b) [C CNpy] HPMoV and (c)
5
PMo10
V
2
O
40, (b) [C
3 4 2
CNpy] HPMoV ,
5
2
2
Soluble
[(C
[(C
[(C
[(C
3
3
3
3
CNpy)
CNTMA)
CNmim) Pd(OAc)
CNpy) PdCl HPMoV
2
2
Pd(OAc)
2
]
2
PMoV
HPMoV
HPMoV
1
Insoluble
Insoluble
Insoluble
Insoluble
3
2
2
]
2
2
and (d) [C
3
CNpy]Cl. (B): UV-vis
2
Pd(OAc)
2
]
2
2
spectra of (a) H PMo V O
5
10
2
40
3
4
2
2
2
]
2
2
[(C
3 2 2 2 2
CNpy) Pd(OAc) ] HPMoV .
2
2
]
2
a
Reaction conditions: catalyst (0.043 mmol), benzene (30 mmol),
O (3.4 ml), O (3 atm), 100 1C, 7 h; for entry 3:
2
40 (0.043 mmol), Pd(OAc) (0.086 mmol). Yields based
À1
into two bands at 1055 and 1076 cm , implying the distortion of
POM’s Keggin framework due to the extension of the conjugated
HAc (6.6 ml), H
PMo10
2
2
b
H
5
V
2
O
c
18
p electrons from cations. For [(C CNpy) Pd(OAc) ] HPMoV ,
3 2 2 2 2
the 1055 cm band further split into 1048 and 1062 cm by the
II
involvement of the larger Pd -complex cation. In addition, the
on benzene. Selectivity to biphenyl; byproducts: o-, m-, p-terphenyls,
and phenol.
À1
À1
Pd-free sample [C
3 4 2
CNpy] HPMoV presented a similar nitrile
À1
Catalytic performance of [(C
oxidative coupling of benzene with O
observed liquid–solid biphasic catalysis system exhibited a yield of
8.3% for biphenyl with a selectivity of 88.0% (entry 1; see Fig. S4,
3
CNpy)
2 2 2 2
Pd(OAc) ] HPMoV for
stretching vibration at 2248 cm to that of the IL precursor
II
CNpy]Cl (curve d), but for the Pd -complex sample, the band
2
is shown in Table 1. The
[C
3
À1
shifted to 2325 cm . This is a strong indication for the
II
19
formation of Pd -coordinated nitrile structure. These IR
results demonstrate well the electron extension from the
1
ESIw for influence of reaction conditions). In contrast, the inorganic
salt Pd HPMoV , a heterogeneous catalyst prepared by replacing
II
pyridinium-based Pd –nitrile complexed organic cations to
2
2
II
the four protons of H PMoV O with two Pd ions, gave a
the inorganic POM-anions. Also, the strong and broad IR
À1
5
2 40
remarkably lower yield of 8.2% (entry 2). The conventional
homogeneous Pd–POM–O system tested under similar conditions
entry 3) showed a yield of 10.4%. The previous data, 7.1% yield
obtained at 90 1C, 1 atm O , 15 h in a homogeneous Pd–POM–O
band at 3452 cm (Fig. S5, ESIw) for H–O vibrations implies
the intermolecular hydrogen bonding networks, which may
2
(
account for the solid nature of the ionic compound
2
0
2
2
[(C
In the UV-vis spectra shown in Fig. 1(B), the broad
absorption band at 432 nm for H PMo10 40 is assigned
3 2 2 2 2
CNpy) Pd(OAc) ] HPMoV .
1
1b
system, are also included in Table 1 (entry 4). Very clearly,
the present heterogeneous system is much more active than the
5
5+
2
V O
2À
21
homogeneous Pd–POM–O
As the single V-containing POM-anion PMo11
PMoV ) was used instead of the double V-containing PMoV
2
analogues.
to the charge transfer O - V
contrast, for the intermediate [C CNpy]
shifted to 388 nm due to a stronger charge transfer aided by
in Keggin POM. In
4
À
V
1
O
40
3
4
HPMoV , the band
2
(
1
2
2
0
(entry 5), or pyridine was substituted with trimethylamine (TMA)
the neighbouring IL-cations. Moreover, for the Pd-bearing
[(C CNpy) Pd(OAc) ] HPMoV , the band was narrower and
in the IL-cation (entry 6), the control samples [(C CNpy) -
2
3
3
2
2 2
2
Pd(OAc) ] PMoV and [(C CNTMA) Pd(OAc) ] HPMoV were
2
further shifted to 379 nm, accompanying a large broad one at
V
2
2
1
3
2
2 2
IV
IV
heterogeneous catalysts and exhibited good yields and selectivities.
This observation suggests quite a flexibility for the chemical
composition of this kind of ionic solid catalysts. However, when
pyridine was replaced with imidazole (mim) in the IL-cation, the
500–800 nm for V - V . The existence of the reduced V species
is due to the extremely strong electron transfer to POM-anions
II
22
from the Pd -complex organic cations. Furthermore, the
much sharper and stronger ESR signals for [(C CNpy)
Pd(OAc) HPMoV (Fig. S6, ESIw) confirm the coexistence
3
2
-
resultant [(C
of 2.2% (entry 7). Lastly, the control sample [(C
HPMoV obtained by replacing the palladium salt Pd(OAc)
3
CNmim)
2
Pd(OAc)
2
]
2
HPMoV
2
gave a very low yield
CNpy) PdCl
with
2
IV
]
2
2
23
V
3
2
2
]
2
-
of V /V species.
2
2
From the above-mentioned spectral results, one can conclude
that the redox capability of POM-anions in [(C CNpy) -
PdCl also presented a much lower yield of 2.8% (entry 8). This
2
3
2
result is in agreement with the lower activity of PdCl2 over
Pd(OAc) ] HPMoV should have been significantly tuned by
2 2 2
Pd(OAc) in the homogeneous oxidative coupling of benzene with
2
the intramolecular charge transfer from the ionically linked
II
Pd -coordinated organic cations, which is responsible for its
11b
H PMo V O as the cocatalyst.
4
10 1 40
2
4
Spectral characterization was conducted to obtain insight
into the structural features and intermolecular electronic
high activity for aerobic oxidation of benzene.
A reaction mechanism over the [(C CNpy) Pd(OAc)
catalyst is proposed in Scheme 2, wherein the coexistence of V /V
species arising from the intramolecular electron transfer from the
3
2
2 2 2
] HPMoV
V
IV
3 2 2 2 2
interactions of [(C CNpy) Pd(OAc) ] HPMoV . As shown in
Fig. 1(A), the IR bands for the Keggin structure of neat
À1
II
II
H
5
PMo10
V
2
O
40 occurred at 1060, 961, 864 and 788 cm
reduced state of Pd -complex cation (Pd -complex[red]) to the
oxidized state of POM-anion (V-POM[ox]) has been evidenced
earlier. It is because of the intramolecular electron transfer that
the transitional active sites of cations and anions are created: the
1
curve a). The four bands were detected for the two POM–IL
7
(
hybrids (curves b and c), indicating that the Keggin structure
of the POM moiety was well preserved. However, for
À1
II
II
[
C CNpy] HPMoV , the P–O vibration at 1060 cm split
3
oxidized state of the Pd -complex cation (Pd -complex[ox]) with the
4
2
5
722 Chem. Commun., 2012, 48, 5721–5723
This journal is c The Royal Society of Chemistry 2012

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The authors thank greatly the Key Programme of National
Natural Science Foundation of China (No. 21136005).
Notes and references
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Scheme 2 Proposed intramolecular electron transfer process for
(C CNpy) Pd(OAc) HPMoV -catalyzed heterogeneous aerobic oxi-
dation of benzene to biphenyl.
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reduced state of the POM-anion (V-POM[red]). It is suggested that
II
phenyl cations are produced by the attack of the Pd -complex
6
7
[ox]
25
À
on the benzene rings, while the active oxygen species O2 are
formed from the activation of molecular oxygen by V-POM[red].
II
Simultaneously, Pd -complex[ox] with V-POM[red] returns to the
II
original state of the Pd -complex[red] with POM[ox]. In this case,
À
9
1
the two reactive intermediates, phenyl cation and O
2
, would
readily react, giving biphenyl. This heterogeneous intramolecular
electron transfer route reflects the rational design of the catalyst
II
involving two active centers Pd and V-POM within an ionic solid
1
structure. This new route is essentially different from the previous
homogeneous intermolecular electron transfer process, catalyzed
1
1
2
6
individually by Pd(OAc) and V-POM (Scheme S1, ESIw). In
2
addition, the much lower activity of the imidazolium-based
catalyst [(C CNmim) Pd(OAc) ] HPMoV than the pyridinium
2 2
3
2
2
one is mostly because of the hindering of the intramolecular
electron transfer by the strong interaction between the highly
2
7
1
active C-2 protons of imidazole rings and nitriles.
2
After reaction, [(C CNpy) Pd(OAc) HPMoV was easily
3
2
2
]
2
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of 96.8%. The reused catalyst showed a similar selectivity
(86.4%) to the fresh one, but the yield dropped to 6.8%.
The durable structure of the recovered catalyst suggested by the
unchanged IR curve (Fig. S7, ESIw) seems responsible for the
stable selectivity. The very slight leached amount of Pd (1.8 wt%)
revealed by the ICP-AES analysis for recovered catalyst cannot
account for the largely decreased yield. Moreover, the elemental
analysis for the recycled catalyst found C 23.67 wt%, N 3.16 wt%,
and H 2.82 wt%. Compared to the fresh catalyst, the recovered
one showed a slight increase in H content, but a significant
increase in C content, which indicates that the large decrease in
yield over the recycled catalyst is possibly due to the coking during
the heterogeneous reaction. To increase the catalyst’s reusability,
further research is still required.
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1
2
In this study, we design, synthesize, characterize, and catalyti-
II
cally evaluate a new Pd -coordinated POM–IL hybrid catalyst.
2
The catalyst is synthesized by pairing Keggin POM-anions with
II
Pd -coordinated nitrile-tethered IL-cations. The novelty of this
II
strategy is the involvement of Pd and V-POM active sites in an
2
2
2
ionic solid, leading to a heterogeneous aerobic coupling of benzene.
The catalyst shows much higher activity and easier recoverability
than the previous homogeneous Pd–POM–O
2
system. A unique
26 J. E. Lyons, in: Oxygen Complexes and Oxygen Activation by
Transition Metal Complexes, ed. A. E. Martell and D. T. Sawyer,
Plenum Press, New York, 1988, pp. 233–251 and references therein.
intramolecular electron transfer mechanism is proposed to under-
stand the reaction results. Despite the decrease in yield over the
recovered catalyst, we think this result takes a step toward the
heterogeneous aerobic oxidative coupling of benzene to biphenyl.
2
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5721–5723 5723