Binary Pd-polyoxometalates and isolation of a ternary Pd-V-polyoxomolybdate active species for selective aerobic oxidation of alcohols

Article abstract of DOI:10.1002/chem.201303714

Binary Pd-polyoxometalates [Pd(dpa)₂]₃[PW ₁₂O₄₀]₂×12 DMSO (2), [Pd(dpa) ₂]₃[PMo₁₂O₄₀]₂×12 DMSO×2 H₂O (3), and [Pd(dpa)(DMSO)₂] ₂[HPMo₁₀V₂O₄₀]×4 DMSO (4) were synthesized by reaction of [Pd(dpa)(OAc)₂]×2 H₂O (1; dpa=2,2′-dipyridylamine) with three Keggin-type polyoxometalates and fully characterized by single-crystal and powder XRD analyses, IR spectroscopy, and elemental analyses. The synthesis is facile and straightforward, and the complicated ligand-modification procedure often used in the traditional charge-transfer method can be omitted. In 2-4, Pd complexes and polyoxometalate anions are coupled through electrostatic interaction. Compound 4 is more active than the other three compounds in the selective aerobic oxidation of alcohols at ambient pressure. Interestingly, during catalytic recycling of compound 4, unprecedented ternary Pd-V-polyoxometalate [Pd(dpa)₂{VO(DMSO) ₅}₂][PMo₁₂O₄₀]₂×4 DMSO (5), which was captured and characterized by single-crystal XRD, proved to be the true active species and showed high catalytic activity for the selective aerobic oxidation of aromatic alcohols (98.1-99.8 % conversion, 91.5-99.1 % selectivity). Moreover, on the basis of control experiments and EPR and UV/Vis spectra, a plausible reaction mechanism for the oxidation of alcohols catalyzed by 5 was proposed. Binary noble metal-POM compounds: The unprecedented ternary Pd-V-POM compound (POM=polyoxometalate) [Pd(dpa)₂{VO(DMSO) ₅}₂][PMo₁₂O₄₀]₂×4 DMSO, which was isolated during the catalytic recycling of [Pd(dpa)(DMSO) ₂]₂[HPMo₁₀V₂O₄₀]×4 DMSO (dpa=2,2′-dipyridylamine) and characterized by single-crystal X-ray diffraction, proved to be the true active species and showed high catalytic activity for the aerobic oxidation of aromatic alcohols (see figure). Copyright

Full text of DOI:10.1002/chem.201303714

DOI: 10.1002/chem.201303714
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&
Polyoxometalates
Binary Pd–Polyoxometalates and Isolation of a Ternary
Pd–V–Polyoxomolybdate Active Species for Selective Aerobic
Oxidation of Alcohols
Xianqiang Huang,[a, b] Xiaomei Zhang,[a] Dan Zhang,[a] Song Yang,[a] Xiao Feng,[a] Jikun Li,[a]
Zhengguo Lin,[a] Jie Cao,[a] Ran Pan,[a] Yingnan Chi,*[a] Bo Wang,*[a] and Changwen Hu*[a]
Abstract:
Binary
Pd–polyoxometalates
[Pd(dpa)2]3-
the other three compounds in the selective aerobic oxida-
tion of alcohols at ambient pressure. Interestingly, dur-
ing catalytic recycling of compound 4, unprecedented
[PW12O40]2·12DMSO (2), [Pd(dpa)2]3[PMo12O40]2·12DMSO·2H2O
(3), and [Pd(dpa)(DMSO)2]2[HPMo10V2O40]·4DMSO (4) were
synthesized by reaction of [Pd(dpa)(OAc)2]·2H2O (1; dpa=
2,2’-dipyridylamine) with three Keggin-type polyoxometa-
lates and fully characterized by single-crystal and powder
XRD analyses, IR spectroscopy, and elemental analyses. The
synthesis is facile and straightforward, and the complicated
ligand-modification procedure often used in the traditional
charge-transfer method can be omitted. In 2–4, Pd com-
plexes and polyoxometalate anions are coupled through
electrostatic interaction. Compound 4 is more active than
ternary
Pd–V–polyoxometalate
[Pd(dpa)2{VO(DMSO)5}2]-
[PMo12O40]2·4DMSO (5), which was captured and character-
ized by single-crystal XRD, proved to be the true active spe-
cies and showed high catalytic activity for the selective aero-
bic oxidation of aromatic alcohols (98.1–99.8% conversion,
91.5–99.1% selectivity). Moreover, on the basis of control ex-
periments and EPR and UV/Vis spectra, a plausible reaction
mechanism for the oxidation of alcohols catalyzed by 5 was
proposed.
Introduction
that [p-C5H5NC16H33]3{PO4[W(O)2(O2)]4} instead of the added
[p-C5H5NC16H33]3{PO4(WO3)4} was the real catalyst in the
epoxidation of olefins.[4] Mizuno et al. reported that
[g-H2SiW10O36Pd2(OAc)2]4ꢀ formed in situ, rather than the start-
ing materials Pd(OAc)2 and (TBA)4[g-SiW10O34(H2O)2], catalyzed
the hydration of nitriles.[5] It is crucial to identify these active
species, or better yet to isolate and fully characterize them.
This will ultimately help to understand the reaction mechanism
and guide the design of new catalysts.[6] Since the components
in catalytic systems are often complicated, direct observation,
isolation, and characterization of the catalytically active species
are very difficult and remain challenging. In this work, three
new binary Pd–POMs were designed, synthesized, and then
were used in the aerobic oxidation of alcohols at ambient
pressure. Interestingly, a novel ternary Pd–V–POM was cap-
tured and proved to be the catalytically active species in the
selective oxidation of alcohols.
Over the last decade, many chemists from various disciplines
have redirected their efforts to the field of catalytic reactions
due to their vital role in our modern society.[1] Polyoxometa-
lates (POMs) as a kind of nanosized metal oxide clusters have
exhibited excellent performance in both acidic and oxidation
catalysis, and they can be further fine-tuned in a controlled
way by choosing the constituent anions and countercations in
a myriad of distinct structures.[2] Excellent results have been
achieved by using POMs as catalysts.[3]
However, in many cases, the added POM catalysts were
not the true active species. For instance, Xi et al. found
[a] X. Huang, X. Zhang, D. Zhang, S. Yang, X. Feng, J. Li, Z. Lin, J. Cao, R. Pan,
Dr. Y. Chi, Prof. B. Wang, Prof. C. Hu
Key Laboratory of Cluster Science, Ministry of Education
School of Chemistry, Beijing Institute of Technology
Beijing 100081 (P. R. China)
Compared with traditional POMs, POMs coupled with noble-
metal complexes of organic ligands can bring together the
merits of both moieties, and the combination may lead to un-
expected yet desirable catalytic activity. In the past decade,
several binary noble metal–organic–POM systems have been
synthesized by Neumann et al. and showed great advantages
in several catalytic reactions.[7] For instance, PdII(15-crown-5-
phen)Cl2–H5PV2Mo10O40 (phen=1,10-phenanthroline) showed
high catalytic activity in the Wacker oxidation,[7a] and PtIILCl2–
H5PV2Mo10O40 (L=N-(2,6-diisopropylphenyl)pyrazin-2-ylmethan-
imine) could catalyze tandem pinacol coupling/rearrange-
Fax: (+86)10-68912631
E-mail: chiyingnan7887@bit.edu.cn
bowang@bit.edu.cn
cwhu@bit.edu.cn
[b] X. Huang
Shandong Provincial Key Laboratory of Chemical Energy Storage
and Novel Cell Technology, School of Chemistry and Chemical Engineering
Liaocheng University
Liaocheng, 252059 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303714.
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ment.[7e] In these cases, the noble metal–organic complexes
and POMs were coupled through electrostatic interaction, and
their syntheses were achieved by charge transfer from POMs
to the ligands of noble metal–organic complexes. Such a syn-
thetic strategy requires that the ligands can not only coordi-
nate to metal ions but also accept protons from POMs, and
these requirements make their preparation complicated and
time-consuming.[7a,b,e] Hence, we sought a new synthetic ap-
proach by which the rational design and controllable and
simple synthesis of noble metal–organic–POM catalysts can be
realized.
The synthetic pathway to binary Pd–POMs is elaborated in
Scheme 1: treatment of 2,2’-dipyridylamine (dpa) with
Pd(OAc)2 to give [Pd(dpa)(OAc)2]·2H2O (1), substituting the
acetate ligands in 1 with dpa or DMSO, and ion-exchange reac-
tions between 1 and three Keggin-type POMs in DMSO to
Figure 1. Crystal structures of 1–4 (W in 2; Mo in 3). Color codes: Pd lime,
Mo/V sea green, P pink, S yellow, C gray, O red, N blue. All H atoms and lat-
tice solvent molecules are omitted for clarity.
Scheme 1. Synthetic pathway to 1–4.
the classical saturated a isomer consisting of a central PO4 tet-
rahedron and four trimetallic Mo3O13 subunits. The notable fea-
ture of 3 is the cationic bis-chelated Pd complex, which was
generated by a coordinative substitution reaction of two ace-
tate anionic ligands by a neutral dpa ligand and has infre-
quently been reported.[9] The three crystallographically inde-
pendent Pd centers exhibit nearly square-planar geometry and
the PdꢀN distances are comparable with those found in 1.
In the family of Keggin-type POMs, H5PMo10V2O40 is famous
for its excellent oxidizing properties.[10] Therefore,
H5PMo10V2O40 was chosen as functional counteranion to couple
with 1. Single-crystal XRD showed that 4 contains two [Pd-
(dpa)(DMSO)2]2+ cations and one [HPMo10V2O40]4ꢀ polyanion
(Figure 1c). The cationic part of 4 was formed by replacing
acetate ligands in 1 by neutral DMSO. The four-coordinate
Pd2+ center is ligated by one dpa and two DMSO ligands and
adopts a nearly square-planar geometry. Although 4 was pre-
pared similarly to 2 and 3, its structure clearly differs from
those of 2 and 3 (Figure 1b and c). The structural difference re-
veals that the polyanion plays a crucial role in the assembly of
binary Pd–POMs.
yield
[Pd(dpa)2]3[PW12O40]2·12DMSO
(3),
(2),
and
[Pd(dpa)2]3-
[Pd(dpa)-
[PMo12O40]2·12DMSO·2H2O
(DMSO)2]2[HPMo10V2O40]·4DMSO (4). Compounds 1–4 were
fully characterized by single-crystal and powder XRD, FTIR
spectroscopy, and elemental analyses (see the Supporting In-
formation). The above approach is considerably simpler and
more straightforward than the traditional charge-transfer
method, since the complicated ligand-modification procedure
of the latter[7a,e,8] can be omitted. Moreover, the precipitation
rate of Pd–POMs can be controlled, and this is beneficial to ob-
taining single crystals, which can give accurate structural infor-
mation and help us to further understand the underlying cata-
lytic cycle.
Results and Discussion
Crystal structure analysis revealed that 1 consists of a palladium
cation, one dpa ligand, two acetate ligands, and two lattice
water molecules (Figure 1a). The Pd center is coordinated by
two chelating N atoms from one dpa ligand and two O atoms
from two acetate ligands in a distorted square-planar geome-
try.
Selective aerobic oxidation of alcohols is of interest for the
production of fine chemicals, because using O2 as an oxidant
offers significant environmental and economic benefits over
traditional stoichiometric oxidants.[11] Neumann et al. reported
Mixing DMSO solutions of 1 and H3PW12O40 and H3PMo12O40
followed by slow evaporation of the solvent led to the forma-
tion of crystals of 2 and 3, respectively. As 2 and 3 have similar
structure motifs, only the structure of 3 is described in detail
a
(PdCl2)0.5Q12{[WZn3(H2O)2][(ZnW9O34)2]} (Q=C16H33(CH3)3N+)
catalyst which gives satisfactory conversion and selectivity in
the aerobic oxidation of alcohols, but no crystal structure of
the catalyst was obtained and the reactions were carried out
under 2 atm pressure.[12] The lack of structural information will
here. Single-crystal XRD showed that
3 contains three
[Pd(dpa)2]2+ cations and two a-Keggin [PMo12O40]3ꢀ polyanions
(Figure 1b). The structure of the [PMo12O40]3ꢀ polyanion in 3 is
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drastically limit the understanding of the catalytic mechanism
and constrain the potential to further design and improve the
catalyst. We previously reported three imidazole-functionalized
hexanuclear oxidovanadium catalysts with well-defined crystal
structures that exhibited excellent selectivity in the aerobic oxi-
dation of alcohols under 2 atm pressure, but the conversion is
yet to be improved.[13]
To further improve the conversion of alcohols under milder
and greener conditions, the binary Pd–POMs with well-defined
crystal structures were explored under 1 atm pressure. The re-
actions were performed in a 10 mL flask equipped with a bal-
loon filled with O2 (1 atm). The effects of the catalysts on the
oxidation of benzyl alcohol to benzaldehyde are illustrated in
Figure 2a. The conversions of benzyl alcohol catalyzed by
Scheme 2. Transformation of DMSO in the aerobic oxidation of benzyl alco-
hol.
amount of dimethyl sulfide (Scheme 2a).[14] However, in the
catalytic reaction involving 4, dimethyl sulfone, not dimethyl
sulfide, was detected by gas chromatography after 1 h of reac-
tion (Scheme 2b), and the amount of dimethyl sulfone obvi-
ously increased over time (Figure 3). This suggested that
DMSO serves as a solvent rather than oxidant in our case. The
above results indicate that intramolecular cooperation be-
tween the Pd complex and POMs plays a crucial role in selec-
tive aerobic oxidation of benzyl alcohol.
Figure 3. Increasing amount of dimethyl sulfone over time in the oxidation
of benzyl alcohol (four runs). Reaction conditions: benzyl alcohol (1 mmol),
catalyst 4 (6.25 mmol), DMSO solvent (2 mL), 1308C, 1 atm O2.
Figure 2. a) Conversion of benzyl alcohol to benzaldehyde with different cat-
alysts. Reaction conditions: benzyl alcohol (1 mmol), catalyst (6.25 mmol),
DMSO (2 mL), 1308C, 1 atm O2, 8 h. b) Recycle tests for aerobic oxidation of
benzyl alcohol with 4 as catalyst. Reaction conditions: benzyl alcohol
(1 mmol), 4 (6.25 mmol), DMSO (2 mL), 1308C, 1 atm O2.
Next, we studied the long-term stability of 4 in a homoge-
neous system by carrying out the aerobic oxidation of benzyl
alcohol for four runs (Figure 2b). In the first run, the reaction
was completed after 8 h (99.8% conversion of benzyl alcohol
as determined by GC); then, 1 mmol benzyl alcohol was added
to the reaction solution, and the reaction was carried out
again under the same conditions. Surprisingly, the reaction
was completed after only 6 h. The third and the fourth runs
were basically as same as the second run.
1 and 2 were 23.3 and 27.6%, respectively. By introducing
[PMo12O40]3ꢀ and [HPMo10V2O40]4ꢀ polyanions with strong oxi-
dizing properties in 3 and 4, the conversion of benzyl alcohol
was dramatically increased. Specifically, using 4 without any
further optimization gave 99.8% conversion of benzyl alcohol.
In a previous report, [PMo12O40]3ꢀ was less effective in oxida-
tion of benzyl alcohol,[14] whereas 3, which combines a Pd
complex with [PMo12O40]3ꢀ, exhibited high catalytic activity. Al-
though H5PMo10V2O40 can efficiently catalyze the oxidation of
alcohol in DMSO under O2 atmosphere, the experiments of
Neumann et al. revealed that DMSO acted as oxidant in the re-
action, which was proved by the observation of a large
Thus, the question arises why the reaction time was short-
ened and whether a new active species was formed during the
course of reaction. Indeed, a new compound, namely, [Pd-
(dpa)2{VO(DMSO)5}2][PMo12O40]2·4DMSO (5), was isolated by re-
crystallization from CH3CN/DMSO. Single-crystal XRD analysis
revealed that
2559
5 , two [VO-
consists of one [Pd(dpa)2]2+
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without any further optimization, 5 exhibits remarkable catalyt-
ic activity for the aerobic oxidation of various aromatic alcohols
to the corresponding aldehydes or ketones with high conver-
sions of 98.1–99.8% and selectivities of 91.5–99.1% (Table 1,
entries 1–10 and Figure 5). However, aliphatic and cyclic alco-
hols were oxidized with low conversion (Table 1, entries 12 and
13). These preliminary results suggested that ternary 5 can fa-
cilitate the aerobic oxidation of aromatic and allylic alcohols.
Having more active sites than binary 4, ternary 5 exhibits
higher catalytic efficiency. In addition, recycle experiments
showed that 5 could be reused for three runs without obvious
loss of activity (Figure 6), and the IR spectrum of recovered 5
was identical to that of freshly prepared 5 (Figure S8 in the
Supporting Information).
Figure 4. Molecular structure of 5. Color codes: Pd lime, Mo sea green, V
light orange, P pink, S yellow, C gray, O red, N blue. All H atoms and lattice
solvent molecules are omitted for clarity.
(DMSO)5]2+, and two [PMo12O40]3ꢀ clusters as well as four free
DMSO molecules (Figure 4). The structure of [Pd(dpa)2]2+ in 5 is
similar to those in 2 and 3. In [VO(DMSO)5]2+, the V
atom exhibits an octahedral geometry and is sur-
rounded by six O atoms. The +4 oxidation state of
Table 1. Selective oxidation of alcohols catalyzed by
5
with O2 as oxidant.[a]
the V atom was confirmed by bond valence sum
analysis (Table S2 in the Supporting Information).[15]
As single-crystal XRD analysis cannot distinguish
Keggin-type [PMo12O40]3ꢀ from [HPMo10V2O40]4ꢀ, elec-
trospray ionization mass spectrometry (ESI-MS) was
used to determine the structural details (Figure S7 in
the Supporting Information), the strong peak at
m/z=607.54 can be assigned to [PMo12O40]3ꢀ, and
the peak at m/z=922.81 to [NaPMo12O40]2ꢀ. The ESI-
MS results show that the anion in 5 is [PMo12O40]3ꢀ
instead of [HPMo10V2O40]4ꢀ in precursor 4. The above
structural analysis shows that during the oxidation of
alcohols, VO2+ was removed from the parent
[HPMo10V2O40]4ꢀ and cationic species [VO(DMSO)5]2+
was formed, and at the same time the remaining
Entry
Alcohol
Product
Conversion
[%]
Selectivity
[%][b]
t
[h]
1
2
99.8
99.8
98.2
94.4
6
6
3
4
5
99.6
99.7
99.6
96.0
98.8
95.0
6
6
7
fragment reassembled into [PMo12O40]3ꢀ
. Similar
mononuclear vanadium species were observed by
Streb et al.[16] and Neumann et al.,[17] and they were
produced by the removal of VO2+ from V-containing
POMs under acidic conditions. A study on the trans-
formation mechanism from 4 to 5 is underway. Com-
pared with the two reported species, the most no-
ticeable feature of 5 is that three active centers, that
is, the Pd complex, the mononuclear vanadium clus-
ter, and the Keggin-type POM, which all have poten-
tial catalytic activity are combined together through
electrostatic interaction and, to the best of our
knowledge, such a ternary compound has not been
reported before. The isolation of 5 not only enriches
the structural data base of POMs, but also helps to
gain insight into catalytic process of POMs.
6
7
99.7
99.6
92.6
91.5
10
12
8
9
99.7
99.8
97.9
99.1
10
6
10
11
98.1
99.6
94.9
90.8
12
6
Isolated 5 was used as catalyst in the oxidation of
benzyl alcohol at ambient pressure, and the conver-
sion of benzyl alcohol reached 99.8% within 6 h. To
evaluate the scope and limitations of the current pro-
cedure, oxidation reactions of various aromatic, allyl-
ic, cycloaliphatic, and aliphatic alcohols were per-
formed with 5 (Table 1). Interestingly, the reaction of
the allylic alcohol (Table 1, entry 11) gave a similar
maximum conversion of 99.6%. It is noteworthy that,
12
13
18.4
16.5
92.5
65.3
12
12
[a] Reaction conditions: alcohol (1 mmol), 5 (6.25 mmol), DMSO (2 mL), O2 (1 atm),
1308C. [b] Selectivity to aldehydes or ketones. Trace amount of byproducts were ob-
served in the form of acids or ethers of corresponding alcohols.
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tion was finished in 10 h (99.2% conversion, 97.6% selectivity;
Figure S10 in the Supporting Information), which is clearly
slower than with 5 (99.8% conversion, 98.2% selectivity in 6 h).
Because the main difference between 3 and 5 is the [VO-
(DMSO)5]2+ centers, the difference in catalytic performance in-
dicates that vanadium species in the catalyst can enhance the
conversion rate and improve the performance. The above re-
sults mean that each active center in 5 plays an important role
in the oxidation reaction, but their combination in 5 can signif-
icantly enhance the catalytic activity compared each single
component. A study on the reaction kinetics with benzyl alco-
hol as model substrate revealed first-order dependence of the
reaction rate on the concentration of alcohol and catalyst (Fig-
ures S11 and S12 in the Supporting Information).
On the basis of the experimental results, the color change
during the reaction, the EPR and UV/Vis spectra, and compara-
ble studies reported previously,[18–20] a plausible reaction mech-
anism for the oxidation of alcohols catalyzed by 5 was pro-
posed (Figure 7). First, the VIVO(DMSO)5 unit of 5 (i.e., species
A) is oxidized to VVO(DMSO)4 by molecular oxygen, and one
DMSO ligand and one dpa ligand dissociate from A to gener-
ate species B.[21] Then, both PdII(dpa) and VVO(DMSO)4 of B oxi-
dize the alcohol to the corresponding aldehyde, and species B
is reduced to [Pd0(dpa){VIVO(DMSO)4}2]2[PMo12O40]2[ox] (species
C).[22] Owing to the presence of the chelating ligand, no palla-
dium black was observed during the reaction. Next, in species
C, reoxidation of Pd0(dpa) by [PMo12O40][ox] through intramolec-
ular electron transfer affords [PdII(dpa)(VIVO(DMSO)4)2]2-
[PMo12O40]2[red] (species D). Our designed experiments show
that the polyanion has the ability to oxidize the Pd0 species to
PdII species (Figure S13 in the Supporting Information). The ex-
istence of [PMo12O40][red] in D is supported by the observation
that the original yellow reaction solution turned blue, and the
blue color indicated the formation of a heteropoly blue. In ad-
dition, the EPR spectrum of the blue reaction solution showed
the presence of molybdenum(V) and vanadium(IV) species in D
Figure 5. Aerobic oxidation of substituted alcohols under the preliminary
optimization conditions.
Figure 6. Recycle experiments on 5 in the oxidation of benzyl alcohol. Reac-
tion conditions: benzyl alcohol (1 mmol), catalyst: blank or 5 (6.25 mmol),
DMSO solvent (2 mL), 1308C, 1 atmO2, 6 h.
Compound
active centers:
[PMo12O40]3ꢀ, and [VO(DMSO)5]2+
5
has three
[Pd(dpa)2]2+
,
.
To evaluate the influence of
each active center in the selec-
tive oxidation of benzyl alcohol,
the following control experi-
ments were designed and car-
ried out. When a mixture of [Pd-
(dpa)(OAc)2]·2H2O (1) and dpa in
a molar ratio of 1:1 was used
as catalyst, the conversion of
benzyl alcohol was 18.3%. The
conversion of benzyl alcohol
catalyzed by (TBA)3PMo12O40
reached 36.9% in 6 h (Figure S9
in the Supporting Information).
Figure 7. Plausible reaction pathway for oxidation of alcohol and the color changes of the reaction solution with
In the presence of 3, the reac- time.
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Conclusion
Crystalline binary Pd–POMs 2–4 were synthesized and used as
catalysts in the aerobic oxidation of alcohols. Notably, during
the catalytic reaction with 4, unprecedented ternary Pd–V–
POM 5 was isolated and characterized by single-crystal XRD. It
proved to be the true active species and showed high catalytic
activity for the aerobic oxidation of alcohols (98.1–99.8% con-
version, 91.5–99.1% selectivity). In the Pd–V–POM, three active
centers are combined by electrostatic interaction. Crystalliza-
tion and characterization of the active species sheds light on
the catalytic mechanism and may help to design new POM-
based catalysts. Studies on the use of Pd–V–POM in oxidative
desulfurization of petroleum and activation of CꢀH bonds are
under way.
Figure 8. EPR spectrum recorded at 123 K for [PdII(dpa)(VIVO(DMSO)4)2]2-
[PMo12O40]2[red] (species D; 0.3125 mm in DMSO).
Experimental Section
(Figure 8) with g=2.01, g =1.93, A =19.84 mT and g =2.00,
k
k
?
General methods and materials
A
=7.91 mT, respectively (Figure S14 in the Supporting Infor-
?
mation).[23] The formation of a heteropoly blue was further
proved by the UV/Vis spectrum with two peaks at lmax =713
and 980 nm (Figure 9), which are attributed to the typical in-
All reagents and solvents for synthesis were purchased from com-
mercial sources and used without further purification. The metal
content of compounds 1–5 was measured by inductively coupled
plasma optical emission spectroscopy (ICP-OES) on a JY-ULTIMA2
analyzer. The FTIR spectra were recorded from KBr pellets in the
range 4000–400 cmꢀ1 on Nicolet 170 SXFT/IR spectrometer. The XP
spectrum was recorded on an ESAY ESCA spectrometer with an
MgKa achromatic X-ray source. The EPR spectrum was recorded on
a Bruker ECS 300 spectrometer. The GC analyses were performed
on Shimadzu GC-2014C with a FID detector equipped with an Rtx-
1701 Sil capillary column. The GC mass spectra were recorded on
Agilent 7890A-5975C at an ionization voltage of 1200 V. C, H, and
N elemental analyses were conducted on PerkinElmer 240C ele-
mental analyzer. UV/Vis spectra were taken on a LabTech UV-Power
diode-array spectrometer.
Synthesis of 1
2, 2’-Dipyridylamine (51 mg, 0.3 mmol) and Pd(OAc)2 (67 mg,
0.3 mmol) were successively added to 5 mL of 0.5 molLꢀ1 HOAc/
NaOAc (pH 4.40) buffer solution. The resulting solution was stirred
at 508C for 30 min and then cooled to room temperature. The pre-
cipitate was collected by filtration, and the filtrate evaporated
slowly at room temperature. Yellow crystals were obtained after
about four days. Yield: 78%. Elemental analysis (%) calcd (found)
for C14H19N3O6Pd: C 38.95 (38.65), H 4.44 (4.59), N 9.73 (9.35), Pd
Figure 9. UV/Vis spectra of 5 (0.3125 mm) in the oxidation of benzyl alcohol
in DMSO. I) UV/Vis spectrum of the reaction solution before the reaction.
II) UV/Vis spectrum after 1 h of reaction. III) UV/Vis spectrum after 5 h of re-
action. IV) UV/Vis spectrum when reaction was completed.
~
24.65 (24.52); IR: n=3079 (Ar-H, m), 2971 (Aliph-H, m), 1649 (C=O,
m), 1587 (C=N, s), 1474 (C=C, s), 1334 (CꢀO, m), 1157 (s), 1024 (m),
441 cmꢀ1 (PdꢀN, m).
tervalence charge-transfer transition (d–d transition) of heter-
opoly blues.[24] The obvious redshift may be induced by the in-
teractions between organic molecules and the reduced poly-
Synthesis of 2
oxometalates.[17,25] Finally, [PMo12O40][red] of D is reoxidized by
H3PW12O40 (288.2 mg, 0.1 mmol) and 1 (43.2 mg, 0.1 mmol) were
successively added to DMSO (2 mL). The mixture was stirred at
room temperature for 1 h. The insoluble matter was removed
by filtration. The filtrate was evaporated slowly at room tempera-
ture, and orange crystals were obtained after about five days.
Yield: 82%. Elemental analysis (%) calcd (found) for
C84H126N18O92P2Pd3S12W24: C 12.55 (12.37), H 1.58 (1.69), N 3.14
[26]
O2 to yield [PdII(dpa)(VIVO(DMSO)4)2]2[PMo12O40]2[ox]
,
which
subsequently combines with dpa and DMSO to form A. Alter-
natively, the species D is possibly oxidized by O2 and produces
B to complete the catalytic cycle.
~
(3.05), Pd 3.97 (3.75), W 54.89 (54.62); IR: n=3072 (Ar-H, m), 1643
(s), 1585 (C=N, s), 1468 (C=C, s), 1080 (PꢀOa, s), 978 (WꢀOd, s), 895
(W-Ob-W, s), 815 (W-Oc-W, s), 462 cmꢀ1 (Pd-N, m).
Chem. Eur. J. 2014, 20, 2557 – 2564
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Table 2. Crystallographic data for 1–5.
1
2
3
4
5
Formula
Mr
crystal system
space group
T [K]
C14H19N3O6Pd
431.72
monoclinic
C2/c
C84H126N18O92P2Pd3S12W24
8038.29
triclinic
ꢀ
P1
296(2)
C84H130Mo24N18O94P2Pd3S12
5964.48
triclinic
P1
296(2)
C36H67Mo10N6O48PPd2S8V2
2913.48
triclinic
ꢀ
P1
296(2)
C48H102Mo24N6O96P2PdS14V2
5321.12
monoclinic
P21/c
296(2)
296(2)
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
19.217(14)
15.098(12)
15.046(19)
90
124.924(10)
90
3579(6)
8
1.599
12.0822(11)
14.4232(13)
23.824(2)
89.335(2)
86.5470(10)
80.1630(10)
8038.29
1
12.1077(10)
14.2545(12)
24.0988(19)
87.7510(10)
87.3760(10)
79.3720(10)
4081.5(6)
1
12.9923(4)
13.0966(4)
13.6296(4)
87.8250(10)
89.6330(10)
61.2080(10)
2030.79(11)
1
13.2041(6)
11.9098(6)
43.603(2)
90.00
94.5980(10)
90.00
6834.9(6)
2
2.629
b [8]
g [8]
V [ꢁ3]
Z
1calcd [gcmꢀ3
F(000)
]
3.269
3628
2.427
2880
2.372
1397
1744
5094
R1 [I>2s(I)]
0.0572
0.1533
0.0820
0.1826
1.116
0.0566
0.1779
0.0839
0.2741
0.0599
0.1913
0.0844
0.2690
0.0778
0.1506
0.0963
0.1567
0.0419
0.1388
0.0580
0.1648
wR2 [I>2s(I)]
R1 (all data)
wR2 (all data)
GOF
1.143
1.168
1.107
1.060
The reaction was carried out under 1 atm O2 with magnetic stirring
for the appropriate time at 1308C. After the reaction was complet-
ed, the resulting mixture was analyzed by GC-MS and GC.
Synthesis of 3
The synthetic procedure was the same as for 2, except that
H3PMo12O40 was used instead of H3PW12O40 Yield: 86%. Elemental
analysis (%) calcd (found) for C84H130Mo24N18O94P2Pd3S12: C 16.91
(16.78), H 2.20 (2.31), N 4.23 (4.32), Pd 5.35(5.24), Mo 38.61 (38.43);
Recycle experiment
~
IR: n=3072 (Ar-H, m), 1629 (s), 1587 (C=N, s), 1472 (C=C, s), 1062
Recycling of 5 was carried out for the aerobic oxidation of benzyl
alcohol. The reaction was carried out under the conditions in
Table 2. After the reaction was completed, 1 mmol of benzyl alco-
hol was added to the reaction solution, and the reaction was car-
ried out again under the same conditions. The catalytic reaction of
the third run was basically as same as that of the second run. The
recycle experiments showed that 5 could be reused in the aerobic
oxidation of benzyl alcohol without an appreciable loss of its high
catalytic performance [conversion of benzyl alcohol: 99.8 (first run),
99.5 (second run), 99.0% (third run)].
(PꢀOa, s), 957 (MoꢀOd, s), 878 (Mo-Ob-Mo, s), 802 (Mo-Oc-Mo, s),
462 cmꢀ1 (PdꢀN, m).
Synthesis of 4
H5PV2Mo10O40·34H2O was prepared by a literature method.[27] The
synthetic procedure of 4 was the same as for 2, except that
H5PV2Mo10O40was used instead of H3PW12O40. Yield: 85%. Elemental
analysis (%) calcd (found) for C36H67Mo10N6O48PPd2S8V2: C 14.84
(15.03), H 2.32 (2.09), N 2.88 (2.76), Pd 7.30 (7.16), Mo, 32.93
~
(32.90), V 3.50 (3.37); IR: n=3074 (Ar-H, m), 1631 (s), 1587 (C=N, s),
X-ray crystallography
1474 (C=C, s), 1075(s), 1057 (PꢀOa, s), 948 (MoꢀOd, s), 867 (Mo-Ob-
Mo, s), 797 (Mo-Oc-Mo, s), 462 cmꢀ1 (PdꢀN, m).
Single-crystal X-ray diffraction data for compounds 1–5 were col-
lected on a Bruker-AXS CCD diffractometer equipped with a graph-
ite-monochromated MoKa radiation (l=0.71073 ꢁ) at 296 K. All ab-
sorption corrections were applied by using the multiscan tech-
nique. The structures were solved by the direct method and re-
fined through full-matrix least-squares techniques on F2 by using
the SHELXTL 97 crystallographic software package.[28,29] The hydro-
gen atoms of the organic ligands were refined as rigid groups.
Crystallographic data for 1–5 are summarized in Table 2.
CCDC 947562 (1), 947563 (2), 947564 (3), 947565 (4) and 947566
(5) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
Synthesis of 5
Compound 4 (15 mg, 0.005 mmol) and benzyl alcohol (54 mg,
5 mmol) were successively added to 4 mL of DMSO. The solution
was stirred at 1308C for 2 h and then cooled to room temperature,
after which CH3CN (6 mL) was added. The solution was evaporated
at room temperature, and brown-red block crystals of 5 were ob-
tained. Yield: 28%. Elemental analysis (%) calcd (found) for
C48H102Mo24N6O96P2PdS14V2: C 10.83 (10.92), H 1.93 (1.76), N 1.58
~
(1.63), Pd 2.00 (2.09), Mo, 43.28 (43.11), V 1.91 (2.04); IR: n=3736
(m), 1586 (C=N, m), 1469 (C=C, s), 1063 (PꢀOa, s), 1025(V=O, m),
958 (MoꢀOd, s), 881 (Mo-Ob-Mo, s), 801(Mo-Oc-Mo, s), 614 (m), 496
(m), 460 cmꢀ1 (PdꢀN, m).
Acknowledgements
General procedure for 5-catalyzed oxidation of alcohols in
DMSO
This work was financially supported by the National Natural
Science Foundation of China (21231002, 21276026, 21173021,
21371024, 21201018), the 111 Project (B07012) and the Insti-
Compound 5 (33.2 mg, 6.25 mmol), DMSO (2 mL), and alcohol
(1 mmol) were added to a 10 mL flask with a magnetic stirring bar.
Chem. Eur. J. 2014, 20, 2557 – 2564
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Full Paper
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Received: September 23, 2013
Revised: November 16, 2013
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Chem. Eur. J. 2014, 20, 2557 – 2564
www.chemeurj.org
2564
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim