Oxidation reactions catalyzed by polyoxomolybdate salts

Article abstract of DOI:10.5560/ZNB.2013-3033

Ionic compounds containing the polyoxomolybdate anion [Mo₆O ₁₉]^2- and [(n-C₄H₉) ₄P]⁺ (tetra-butylphosphonium), [(n-C₄H ₉)₃P(n-C₁₄H₂₉)]⁺ (tributyl (tetradecyl)phosphonium), [Bmim]⁺ (1-butyl-3- methylimidazolium) and [Dbmim]⁺ (1,2-dimethyl-3-butylimidazolium) cations were prepared and characterized, including the determination of three of the solid state structures by singlecrystal X-ray diffraction. These compounds were applied as catalysts for the epoxidation of olefins with urea hydrogen peroxide (UHP) as oxidant in the ionic liquid [Bmim]PF₆. Additionally, the oxidation of sulfides to sulfoxides with hydrogen peroxide (H₂O₂) in several solvents was investigated. The polyoxomolybdate catalysts showed a good performance for epoxidation of olefins as well as for oxidation of sulfides. Furthermore, the catalysts can be recycled several times in oxidation reactions. We present this methodology for the oxidation reaction in a simple, economically, technically, and environmentally benign manner.

Full text of DOI:10.5560/ZNB.2013-3033

Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
Bo Zhanga, Su Lia, Alexander Po¨thiga, Mirza Cokojaa, Shu-Liang Zangb,c, Wolfgang
A. Herrmanna, and Fritz E. Ku¨hna
a
Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische
Universita¨t Mu¨nchen, Ernst-Otto-Fischer-Straße 1, D-85747 Garching bei Mu¨nchen, Germany
School of Chemical and Materials Science, Liaoning Shihua University, Dandong Road, No.1,
b
113001 Fushun, P. R. China
Institute of Rare and Scattered Elements Chemistry, Liaoning University, Chongshan Middle
c
Road No. 66, 110036 Shenyang, P. R. China
Reprint requests to Prof. W. A. Herrmann and Prof. F. E. Ku¨hn. Tel: +49 89 289 13081. Fax: +49
89 289 13473. E-mail: wolfgangherrmann@ch.tum.de and fritz.kuehn@ch.tum.de
Z. Naturforsch. 2013, 68b, 587 – 597 / DOI: 10.5560/ZNB.2013-3033
Received January 30, 2013
Dedicated to Professor Heinrich No¨th on the occasion of his 85th birthday
2−
and [(n-C4H9)4P]+ (tetra-
Ionic compounds containing the polyoxomolybdate anion [Mo6O19
]
butylphosphonium), [(n-C4H9)3P(n-C14H29)]+ (tributyl (tetradecyl)phosphonium), [Bmim]+ (1-
butyl-3-methylimidazolium) and [Dbmim]+ (1,2-dimethyl-3-butylimidazolium) cations were pre-
pared and characterized, including the determination of three of the solid state structures by single-
crystal X-ray diffraction. These compounds were applied as catalysts for the epoxidation of olefins
with urea hydrogen peroxide (UHP) as oxidant in the ionic liquid [Bmim]PF6. Additionally, the oxi-
dation of sulfides to sulfoxides with hydrogen peroxide (H2O2) in several solvents was investigated.
The polyoxomolybdate catalysts showed a good performance for epoxidation of olefins as well as for
oxidation of sulfides. Furthermore, the catalysts can be recycled several times in oxidation reactions.
We present this methodology for the oxidation reaction in a simple, economically, technically, and
environmentally benign manner.
Key words: Catalysis, Ionic Liquids, Molybdenum, Oxidation, Polyoxomolybdate, Epoxides,
Sulfoxides
Introduction
imidazolium, and others), is thus regarded as a fea-
sible way to increase the reactivity of POMs in
Polyoxometalates (POMs) are an important and ionic liquids. So far, some examples of WCC-POM
structurally diverse class of inorganic metal oxide compounds (e. g. [Bmim]3[PW12O40] (Bmim = 1-
clusters [1], which are not only used as inorganic butyl-3-methylimidazolium),
[Bmim]4[SiM12O40]
components for novel materials, but also recog- and [Bmim]4[S2M18O62] (M = Mo, W), [(n-
nized as “green” industrial catalysts [2, 3]. Recently, C4H9)4N]4[Mo8O26] and [(n-C4H9)4N]2[W6O19])
ionic liquids (ILs) have received enormous atten- have been described and mainly used as electro-
tion in both academic and industrial research due chemicals [15 – 20]. However, only few POM salts
to their unique physicochemical properties and the have been investigated as catalysts. For example, the
resulting applicability in various fields [4 – 10]. Keggin-type POM anion [PW12O40]3− can be used
We and others have shown that certain anions as catalyst for esterification [21] and for epoxidation
exhibit an increased (catalytic) activity in ionic reactions in ionic liquids [22]. The compound [(n-
liquid media [11 – 14]. The concept of combining C4H9)4N]2[W6O19] was found to be a catalyst for
POM anions with “weakly coordinating cations” the synthesis of biscoumarins, which was investigated
(WCC), such as those typically used for ionic liquids by Davoodna [23]. From these results it appears that
(tetraalkylammonium and -phosphonium, pyridinium, tungsten-based WCC-POMs can exhibit excellent
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588
B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
catalytic performance because of controllable redox cellent yield using aqueous hydrogen peroxide as ox-
and acidic properties [24 – 26], which make them eco- idant and WCC-POMs as catalyst under mild reac-
nomical and environmentally acceptable. In contrast to tion conditions was also investigated. The WCC-POMs
tungsten-based WCC-POMs, which were widely used can be reused several times without significant loss of
as catalysts for oxidation reactions, molybdenum- activity.
based congeners, such as the Lindqvist-type POM
[Mo6O19]2−, are rather rare and have not been often Results and Discussion
used in oxidation catalysis so far. The findings of our
previous report on the catalytic oxidation of sulfides Synthesis of WCC-POMs
to sulfoxides using imidazolium tetrafluoroborate- and
perrhenate-based ionic liquids using H2O2 [11 – 14]
The WCC-[Mo6O19]2− salts with [(n-C4H9)4P] (1),
prompted us to investigate the catalytic properties of [(n-C4H9)3P(n-C14H29)] (2), [Bmim] (3) and [Bdmim]
molybdenum-based WCC-POMs for the epoxidation (4) cations were prepared by a modified literature pro-
of olefins and the selective oxidation of sulfides to cedure, which involves acid condensation in aque-
sulfoxides.
ous solution followed by addition of the precipitating
The catalytic olefin epoxidation is of high impor- cation (see Scheme 1) [46 – 48]. The success of the
tance in the chemical industry and also valuable for synthesis of WCC-POMs strongly depends on the pH
the synthesis of fine chemicals such as pharmaceuticals of the reaction solution, the solvent and the tempera-
and flavor & fragrance components [27 – 30]. A vast ture. The by-product (sodium halide) can easily be re-
number of coordination compounds have been applied moved by extraction with water. Recrystallization was
as catalysts for this type of reaction [31 – 33]. However, performed from acetonitrile. More detailed procedures
the typical molecular transition metal catalysts are too are given in the Experimental Section. All synthesized
expensive for a broad use and for upscaling, so that WCC-POMs are very stable and can be handled on air.
cheaper and recyclable catalysts are required. Organic They are highly soluble in CH3CN, but insoluble in
sulfoxides are also important synthetic intermediates water and methanol.
for the synthesis of various chemically and biologically
active molecules [34 – 36]. Numerous reports on the Characterization of the WCC-POM compounds 1–4
oxidation of sulfides to sulfoxides using homogeneous
transition metal catalysts in organic solvents have been
Thermogravimetric analysis (TGA), differential
published to date [37 – 45]. However, in most cases, scanning calorimetry (DSC) and IR spectroscopy data
the synthesis protocols involved formation of environ- of compounds 1–4 are given in Table 1. TGA indicates
mentally unfavorable by-products, and the catalysts are that all four compounds show negligible volatility and
rather difficult to recycle and to separate from the prod- high thermal stability with decomposition tempera-
ucts. We have now found that the above mentioned tures near 300 ◦C. [(n-C4H9)3P(n-C14H29)]2[Mo6O19]
drawbacks can be overcome with WCC-POMs, con-
firming that they qualify as good catalyst for oxidation
reactions.
In this work, we present the synthesis and charac-
terization of a series of WCC-POMs containing tetra-
butylphosphonium, tributyl(tetradecyl)phosphonium,
1-butyl-3-methylimidazolium, and 1-butyl-2,3-dime-
thylimidazolium cations along with the [Mo6O19]2−
dianion, including three X-ray single-crystal structure
determinations. The WCC-POMs show high stability
and selectivity for epoxidation of olefins with anhy-
drous urea hydrogen peroxide (UHP) as oxidant in the
ionic liquid [Bmim]PF6. Additionally, a highly effi-
cient method for selective oxidation of a series of sul-
fides to produce the corresponding sulfoxides in ex-
Scheme 1. Synthesis of the WCC-POMs 1–4.
Table 1. Melting points (Tm), decomposition temperatures
(Td) and characteristic IR data of compounds 1–4.
Compound
Tm (◦C) Td (◦C)
IR (cm−1
)
1
2
3
4
196
68
128
193
341
338
290
293
949
950
961
948
920
914
908
902
791
785
801
787
720
718
741
754
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
589
(2) is the only compound which has a melting point
below 100 ◦C (68 ◦C). The melting points of [(n-
C4H9)4P]2[Mo6O19] (1), [Bmim]2[Mo6O19] (3) and
[Bdmim]2[Mo6O19] (4) are 196, 128 and 193 ◦C, re-
spectively. Notably, the melting points of the phospho-
nium salts 1 and 2 decrease with increasing alkyl chain
length from butyl to tetradecyl. The lower degree of
crystal packing and long-range order, caused by the
long alkyl chain in 2, is most presumably the reason for
the significantly different melting points of compounds
1 and 2. The different melting points of compounds 3
and 4 are a consequence of the substitution of a proton
in the 2-position of the imidazolium ring by a methyl
group. A comparison of the structures of compounds 3
and 4 via Hirshfeld surface analysis [49, 50] revealed
that in compound 4 the [Mo6O19]2− anions form more
attractive contacts to the cations than in compound 3
(Supporting Information available online; see note at
the end of the paper for availability). Furthermore, the
degree of hydrogen-oxygen interactions in 4 is also
higher, leading to a higher melting point.
IR spectroscopy was used to identify the struc-
ture of the [Mo6O19]2− anion. In the region of
700 – 1100 cm−1, four characteristic bands at around
960 and 790 cm−1 are ascribed to v(Mo–Ot) and
v(Mo–Ob–Mo) modes of the [Mo6O19]2− anion (Ot
and Ob mark terminal and bridging oxo ligands, re-
spectively) [18, 51 – 53].
Crystal structures of WCC-POMs 1, 3 and 4
Crystals of the new compounds 1, 3 and 4 were
grown by slow evaporation of an acetonitrile solu-
tion at room temperature. Unfortunately, we were
so far not able to obtain crystals of compound 2
of a quality suitable for single-crystal X-ray diffrac-
tion. The structures of compounds 1, 3 and 4 reveal
that the [Mo6O19]2− anion consists of six distorted
MoO6 octahedra which are connected by edges and
a common vertex (Fig. 1) [54 – 56]. There are three
types of oxygen atoms in the anion (terminal oxy-
gen Ot, µ2-bridging oxygen Ob, and central oxygen
Oc). Thus, the Mo–O bond lengths can be grouped
into three sets (Table 2). Interestingly, the bond lengths
˚
Fig. 1 (color
online).
ORTEP
views
of
[(n-
Mo–Ob (1.8740(2) – 1.967(1) A) for 4 are longer than C4H9)4P]2[Mo6O19
]
(1) (above, symmetry code: −x,
y, 1/2 − z), [Bmim]2[Mo6O19] (3) (middle, symmetry
code: −x, y + 1/2, 1/2 − z) and [Bdmim]2[Mo6O19] (4)
(below, symmetry code: 1/2 − x, y + 1/2, 1/2 − z) showing
displacement ellipsoids at the 50% probability level. H
atoms are omitted for clarity.
˚
in the WCC-POMs 1 (1.868(1) – 2.002(1) A) and 3
˚
(1.862(2) – 2.015(2) A), but the bond lengths Mo–Ot
and Mo–Oc are similar. Further, a number of charge-
assisted hydrogen bonds CH···O exist between the
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
˚
Table 2. Selected bond lengths (A) of 1, 3
1
3
4
and 4.
Mo–Ot
Mo–Ob
Mo–Oc
1.672(2) – 1.688(1)
1.868(1) – 2.002(1)
2.313(2) – 2.319(1)
1.678(2) – 1.68(2)
1.862(2) – 2.015(2)
2.318(1) – 2.329(1)
1.6780(3) – 1.686(1)
1.8740(2) – 1.967(1)
2.3157(3) – 2.324(1)
Table 3. Epoxidation of cis-cyclooctene with different oxi-
dants and in different solvents at 60 ◦C using WCC-POM 1
as catalysta.
cation –CHn functions and the oxygen atoms of the an-
ion [Mo6O19]2− (Fig. 1).
Each polyoxoanion is surrounded by [(n-C4H9)4P],
[Bmim] or [Bdmim] cations, respectively, exhibiting
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
Solvent
Oxidant
H2O2 (35%)
TBHP
Conv. (%)b Yield (%)c
CH3CN
CH3CN
MeOH
[Bmim]BF4 H2O2 (35%)
[Bmim]BF4
[Bmim]BF4
29
30
70
17
9
29
30
70
17
9
˚
H···O distances in the range of 2.254 to 2.680 (A) for
˚
˚
1, 2.493 to 2.665 (A) for 3 and 2.358 to 2.680 (A) for
4, which indicate interactions between the polyoxoan-
ions and the cations via Coulomb forces and CH···O
hydrogen bonds. For compound 1, contacts between
the CH2 and CH3 groups of the butyl moieties and
the O(Ot, Ob) atoms of [Mo6O19]2− can be observed.
Compounds 3 and 4 also exhibit contacts between the
anion and the imidazolium ring hydrogen atoms and
the alkyl substituents of the cation.
H2O2 (35%)
TBHP
UHP
9
9
[Bmim]PF6 H2O2 (35%)
49
69
89
–
49
69
89
–
[Bmim]PF6
[Bmim]PF6
[Bmim]PF6
[Bmim]NTf2 H2O2 (35%)
[Bmim]NTf2
[Bmim]NTf2
TBHP
UHP
–
54
56
63
54
56
63
TBHP
UHP
a
Reaction conditions: cis-cyclooctene (2 mmol), catalyst 1 (1 mol-
%), oxidant (4 mmol), solvent (1 mL), 60 ◦C, 4 h; b the conversion to
cyclooctene oxide was determined by GC analysis; the yield was
Catalytic epoxidation of olefins
c
determined by GC analysis.
Compounds 1–4 were examined as catalysts of
the epoxidation of cis-cyclooctene. The organic and
ionic liquid solvents, as well as the oxidants were hibited both a high yield of 89% and a selectivity of
varied in order to determine the optimum reaction > 99% within 4 h (entry 9). Therefore, the usage of
conditions (Table 3). The yield of epoxide was in- water-free UHP is crucial for successful epoxidation.
deed found to be strongly dependent on these param- Furthermore, in the blank experiments, no reaction oc-
eter. Noteworthy, the (usually undesired) by-product curred in the absence of oxidant UHP, indicating that
1,2-cyclooctanediol was not found in the entire set the oxygen source of epoxides is not air (entry 10).
of experiments. The observed conversion was very Epoxidation of cis-cyclooctene at different tempera-
low (< 30%) with CH3CN and [Bmim]BF4 as sol- tures was investigated as well. The reaction conditions
vents (entries 1 – 2 and 4 – 6). On the other hand, were the same as those of entry 9 in Table 3.
when the reaction was carried out in methanol and
As indicated in Fig. 2, a lower temperature was dis-
in the ionic liquids [Bmim]PF6 and [Bmim]NTf2 advantageous for the oxidation reaction. The yield was
(NTf2 = bis(trifluoromethylsulfonyl)imide) as sol- only 69% within 4 h, and 72% after 24 h at 50 ◦C. Be-
vents, the yield of epoxide was significantly higher cause of the high yield (93% within 4 h, 97% after
(entries 3, 7 – 9 and 11 – 13). The nature of the ox- 24 h) at 70 ◦C, we chose 70 ◦C as the reaction temper-
idant also plays a crucial role in the reaction. Three ature for further experiments.
different oxidants [aqueous H2O2, tert-butyl hydroper-
Subsequently, in a comparative study the other syn-
oxide (TBHP) in n-decane and urea-hydrogen perox- thesized WCC-POMs 2–4 were also used as catalysts
ide (UHP)] were examined under comparable condi- in the epoxidation of cis-cyclooctene with UHP at
tions. From the results shown in Table 3, entries 7 – 13, 70 ◦C for 2 h. The results are shown in Table 4. Com-
it can be stated that a) aqueous hydrogen peroxide solu- pound 3 exhibits the highest reactivity and conversion
tions inhibit the catalytic reaction to some extent, and for the epoxidation of cis-cyclooctene (entry 3), most
b) UHP is superior to both TBHP and H2O2. Obvi- presumably due to the particularly high solubility in
ously, the olefin conversion significantly depends on [Bmim]PF6 at 70 ◦C. The other catalysts 1, 2 and 4
the content of water in the solution. The catalytic re- also exhibited good results, and the yields were 93, 90
action in the ionic liquid [Bmim]PF6 with UHP ex- and 94%, respectively (entries 1 – 2, 4).
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
591
Fig. 2 (color online). Effect of temperature and reaction time
for the catalytic epoxidation of cis-cyclooctene with com-
pound 1 as catalyst. Reaction conditions: cis-cyclooctene
Fig. 3 (color online). Recycling studies of the IL catalyst
mixture for the epoxidation of cis-cyclooctene.
(2 mmol), catalyst 1 (1 mol-%), UHP (4 mmol), [Bmim]PF6
(1 mL).
Table 4. Epoxidation of cis-cyclooctene with different cata-
lysts in [Bmim]PF6 at 70 ◦Ca.
The epoxidations of trans-β-methylstyrene (entry 4),
limonene (entry 5), cis-stilbene (entry 6) and (+)-
camphene (entry 7) were rather challenging due to
steric hindrance (26% after 3 h, 24% after 7 h, 25%
after 4 h, 9% after 8 h, respectively). The turnover fre-
quencies (TOFs) are in the range of 16 – 182 h−1. It is
noteworthy that no diol is detected in all investigated
reactions.
d
Entry
Catalyst
Conv. (%)b Yield (%)c
TOF (h−1
)
1
2
3
4
1
2
3
4
93
90
97
94
93
90
97
94
47
45
49
47
a Reaction conditions: cis-cyclooctene (2 mmol), catalyst (1 mol-%),
UHP (4 mmol), [Bmim]PF6 (1 mL), 70 ◦C, t = 2 h; b the conversion
was determined by GC analysis; c the yield was determined by GC
analysis; d determined after 2 h reaction time.
Selective catalytic oxidation of sulfides to sulfoxides
The catalyst recycling and reusability were studied
We investigated the oxidation reaction of thioanisole
as well. First, the product was extracted with n-hexane as a model substrate with different oxidants and in
and the ionic liquid phase containing the catalyst was various solvents. The results are presented in Table 6
washed with water to remove urea ([Bmim]PF6 and showing that the reaction was sensitive to the solvent.
WCC-POMs are insoluble in water). The IL was then In n-hexane (entry 1), CH2Cl2 (entry 2) and water
dried in high vacuum and used for the next catalytic (entry 4) rather low conversions and yields were ob-
run. The WCC-POMs remained active for at least three tained due to the poor solubility of the catalyst in these
catalytic runs. However, a slight decrease of conver- solvents. When the reaction was carried out in ace-
sion and yield was observed (Fig. 3). Most presumably, tonitrile (entry 3) high conversion (97%) and yield
this is resulting from the work-up procedure, since the (82%) were obtained within 40 min, but the selectiv-
IL catalyst solution is washed several times with n- ity of sulfoxide was relatively lower than when using
hexane and thereafter with water. Hence, it is reason- methanol, which was found to be a more efficient re-
able to assume that phase separation and subsequent action medium (95% yield, 97% conversion, entry 5).
decantation of the n-hexane and water phase might Note that methanol itself can act as catalyst of the ox-
have led to unintentional extraction of small amounts idation of thioanisol [57 – 59]. However, the reaction
of the IL.
time is usually significantly longer (18 h), whereas in
Catalyst 3 was applied for the epoxidation of var- our experiments using catalyst 3 in methanol, high con-
ious olefins with UHP as oxidant (Table 5). Cyclo- version (97%) was reached already after 40 min (entry
hexene was readily converted into the epoxide within 5). The conversion was only 41% without catalyst (en-
1.5 h with 88% yield (entry 2). Only moderate conver- try 9).
sion (46%) was observed for 1-octene (entry 3), which
Thus, it can safely be concluded that methanol is
is less prone to epoxidation than cis-cyclooctene. not catalyzing the oxidation in our case. The absence
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
Table 5. Epoxidation of olefins with UHP catalyzed by 3 at 70 ◦Ca.
d
Entry
1
Substrate
Product
Time (h)
2
Conv. (%)b Yield (%)c TOF (h−1
)
97
88
97
88
148
182
O
O
2
1.5
3
4
4
3
46
26
46
26
30
48
O
O
5
7
24
24
16
O
6
7
4
8
25
25
32
O
Ph
Ph
9
9
–
O
a
Reaction conditions: cis-cyclooctene (2 mmol), 3 (1 mol-%), UHP (4 mmol), [Bmim]PF6 (1 mL), 70 ◦C; b the conversion was determined
by GC analysis; c the yield was determined by GC analysis; d determined after 15 min reaction time.
of oxidant leads to a significant decrease of conversion
(28%, entry 7). An obvious decrease of oxidation ac-
tivity was observed by adding TBHP as oxidant (entry
6). The yield decreased to 46% after 4 h without any
solvent (entry 8).
In an effort to establish the scope of our protocol,
a series of sulfides with different substituents were
Scheme 2. Oxidation of sulfides to sulfoxides with aqueous
hydrogen peroxide in methanol using [Bmim]2[Mo6O19] (3)
as catalyst.
used (Scheme 2) in methanol. All oxidation reactions in very good selectivities (> 80%). TOFs were in the
were performed under the same conditions (Table 7) range of 165 – 380 h−1
and showed nearly quantitative conversions within Interestingly, sulfides with methyl groups (entry 1)
.
a very short time (< 3 h). The sulfoxides were obtained were found to be more easily oxidized within short
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
593
Table 6. Oxidation of thioanisole with different oxidants in
different solvents at 25 ◦Ca.
Entry
Solvent
Oxidant
Time
(h)
Conv.
(%)b
Yield
(%)c
1
2
3
4
5
6
7
8
9
n-hexane
CH2Cl2
CH3CN
H2O
MeOH
MeOH
MeOH
–
H2O2 (35%)
H2O2 (35%)
H2O2 (35%)
H2O2 (35%)
H2O2 (35 %)
TBHP
4
4
0.67
4
0.67
3
24
4
82
16
97
80
97
73
28
93
52d
57
14
82
67
95
71
27
46
41
–
H2O2 (35%)
H2O2 (35%)
MeOH
8
a
Fig. 4 (color online). Recycling of the catalyst 3 in the oxida-
tion of thioanisole with H2O2.
Reaction conditions: thioanisole (2 mmol), 3 (1 mol-%), H2O2
(35%, 2.1 mmol), solvent (1 mL) 25 ◦C; b determined by GC on the
crude reaction mixture; c isolated yield after column chromatogram.
with ethyl acetate, and dried in vacuum at room tem-
time (ca. 30 min) compared to other substrates with perature. Five catalytic runs were carried out and the
bulky substituents, indicating that the steric hindrance results are shown in Fig. 4. It has to be noted that no
is an important factor for the oxidation reaction. It was significant loss of conversion and yield was observed
a significant observation that functional groups such after five runs, indicating a rather steady reusability
as allyl (entry 6), hydroxo (entry 7) and ester moieties of the WCC-POM catalyst. In comparison to the re-
(entry 9) were not affected in this oxidation procedure. cycling of the IL catalyst mixture for the epoxida-
Noteworthy the WCC-POM catalyst 3 was very eas- tion of olefins, in this case the catalyst precipitation is
ily recovered by filtration when the reaction was com- a more convenient method to recover the catalyst with-
pleted. Therefore, the WCC-POMs-H2O2 system of- out a loss after each cycle.
fered a facile, rapid and highly selective method to ob-
tain sulfoxides.
Conclusion
The reusability of the catalyst WCC-POMs for ox-
idation of sulfides to sulfoxides was also studied. Af-
The epoxidation of olefins and the oxidation of
ter the oxidation of thioanisole using 3 as catalyst was sulfides to sulfoxides catalyzed by polyoxomolyb-
completed, ethyl acetate was added to the reaction mix- date salts containing weakly coordinating cations was
ture and the catalyst was precipitated, filtered, washed achieved under mild conditions. The catalysts are eas-
Table 7. Oxidation of sulfides to sulfoxides
with H2O2 as oxidant using 3 as catalyst in
MeOHa.
Entry
R1
R2
Time
(min)
Conv.
(%)b
Yield
(%)c
TOF
d
(h−1
)
1
2
3
4
5
6
7
8
Me
n-Bu
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
n-Bu
Me
30
40
40
40
40
40
45
60
70
150
70
60
96
96
97
93
95
95
90
88
87
93
90
89
95
94
95
92
94
93
87
87
86
83
85
86
190e
141
143
138
141
138
125
120
140
83
Et
CH(CH3)2
CH2–CH=CH2
CH2CH2OH
CH2OMe
CH2COOMe
Ph
9
10
11
12
Ph
Ph
Bz
Bzf
Bz
98
110
a
Reaction conditions: thioanisole (2 mmol), 3 (1 mol-%), H2O2 (35%, 2.1 mmol),
MeOH (1 mL) at 25 ◦C; b determined by GC or 1H NMR on the crude reaction mixture; c
isolated yield after column chromatography; d TOFs of the catalyst were calculated over
40 min; e TOFs of the catalyst were calculated over 30 min; f Bz = benzyl.
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
ily prepared, and can be recycled several times. The (m, 16H), 2.19 (m, 8H) 31P NMR (162 MHz, [D6]DMSO,
r. t., ppm): δ = 33.78. – Elemental analysis (%): calcd. C
WCC-POM catalysts show high stability and selectiv-
ity for the epoxidation of olefins in the ionic liquid
[Bmim]PF6 with anhydrous urea hydrogen peroxide
(UHP) as oxidant. For the selective oxidation of sul-
fides to the corresponding sulfoxides a method using
aqueous hydrogen peroxide as oxidant has also been
developed. Both catalytic reactions are cost-efficient.
27.48, H 5.19; found C 27.55, H 5.20.
Synthesis of [(n-C4H9)3P(n-C14H29)]2[Mo6O19] (2)
Na2MoO4·2H2O (4.8 g, 20 mmol) in H2O (20 mL) was
mixed with acetonitrile, and HCl (37%, 10 mL) was added.
The resulting mixture was refluxed for 1 h. After cool-
ing, the lower aqueous layer was discarded, and the upper
layer was treated with [(n-C4H9)3P(n-C14H29)]Cl (4.99 g,
10.4 mmol) in water (100 mL). The precipitate was filtered
and thoroughly washed successively three times with wa-
ter and ethanol. Recrystallization of the solid from ace-
tonitrile afforded pale-green crystals of [(n-C4H9)3P(n-
C14H29)]2Mo6O19. – IR (cm−1): ν = 436.3 (s), 591.4 (s),
718.9 (w), 785.5 (vs), 914.6 (w), 950.2 (vs), 1098.6 (w),
1459.6 (m), 1902.5 (w), 2850.2 (m), 2920.2 (m), 2956 (w). –
1H NMR (400 MHz, [D6]DMSO, r. t., ppm): δ = 0.89 (d,
3H), 1.00 (d, 9H), 1.29 (m, 18H), 1.39 (d, 2H), 1.56 (m,
8H), 1.70 (m, 8H), 2.40 (m, 8H). – 31P NMR (162 MHz,
[D6]DMSO, r. t., ppm): δ = 33.95. – Elemental analysis (%):
calcd. C 37.15, H 6.84; found C 36.67, H 6.43.
Experimental Section
General methods
All reactions were performed using standard Schlenk
techniques under an argon atmosphere. All solvents were
collected from purification systems and kept over molecu-
1
lar sieves. H NMR, 13C NMR and 31P NMR spectra were
recorded on a Bruker Avance DPX-400 spectrometer. IR
spectra were recorded on a Varian FTIR-670 spectrometer,
using a GladiATR accessory with a diamond ATR element.
Catalytic runs were monitored by GC methods on a Hewlett-
Packard instrument HP 5890 Series II equipped with a FID,
a Supelco column Alphadex 120 and a Hewlett-Packard in-
tegration unit HP 3396 Series II Elemental analyses were
performed with a Flash EA 1112 series elemental analyzer.
Thermogravimetric (TG) and differential scanning calori-
metric (DSC) analyses were conducted utilizing a Netzsch-
STA 409 PC system. Typically about 10 mg of each sam-
ple was heated from 25 to 1000 ◦C at 10 K min−1. Melting
points were determined by MPM-H2 melting point meters.
TLC was performed on silica gel 60F254 plates procured
form E. Merck. Silica gel (0.06 – 0.2 mm 60 A) was used
for column chromatography. All chemicals were purchased
from Acros and ABCR and used without further purification.
[Bmim]PF6, [Bmim]BF4 and [Bmim]NTf2 were synthesized
according to literature procedures [60, 61].
Synthesis of [Bmim]2[Mo6O19] (3)
Na2MoO4·2H2O (4.8 g, 20 mmol) was dissolved in
10 mL H2O, and an aqueous solution of HCl (37%, 5 mL)
was added. After stirring for 10 min, [Bmim]Br (1.6 g,
3.75 mmol) was added with vigorous stirring. The precipitate
was filtered and thoroughly washed successively three times
with water and ethanol. Recrystallization of the solid from
acetonitrile afforded yellow crystals of [Bmim]2Mo6O19. –
IR (cm−1): ν = 437.1 (m), 614.5 (m), 753.6 (s), 786.2 (vs),
878.6 (w), 911.8 (m), 952.3 (vs), 1164.3 (m), 1462.0 (w),
1568.3 (w), 1906.3 (w), 2870.2 (w), 2930.9 (w), 2959.3 (w),
3115.6 (w), 3144.8 (w). – 1H NMR (400 MHz, [D6]DMSO,
r. t., ppm): δ = 0.91 (d, 3H), 1.27 (m, 2H), 1.78 (m, 2H), 3.87
(s, 3H), 4.17 (t, 2H), 7.69 (s, 1H), 7.75 (s, 1H), 9.10 (s, 1H)
–
13C NMR (100 MHz, [D6]DMSO, r. t., ppm): δ = 13.76,
Synthesis of [(n-C4H9)4P]2[Mo6O19] (1)
19.28, 31.87, 36.23, 49.04, 122.70, 124.08, 136.95. – Ele-
mental analysis (%): calcd. C 16.57, H 2.78, N 4.83; found
C 16.69, H 2.61, N 4.64.
Na2MoO4 · 2H2O (4.8 g, 20 mmol) in H2O (20 mL) was
mixed with acetonitrile, and HCl (37%, 10 mL) was added.
The resulting mixture was refluxed for 1 h. After cooling,
the lower aqueous layer was discarded, and the upper layer
was treated with [(n-C4H9)4P]Br (2.7 g, 10.4 mmol) in wa-
Synthesis of [Bdmim]2[Mo6O19] (4)
Na2MoO4·2H2O (4.8 g, 20 mmol) was dissolved in
ter (100 mL). The precipitate was filtered and thoroughly 10 mL H2O, and an aqueous solution of HCl (37%, 5 mL)
washed successively three times with water and ethanol. Re- was added. After stirring for 10 min, [Bdmim]Br (1.75 g,
crystallization of the solid from acetonitrile afforded yellow 3.75 mmol) was added with vigorous stirring. The pre-
crystals of [(n-C4H9)4P]2Mo6O19. – IR (cm−1): ν = 434.9 cipitate was filtered and thoroughly washed successively
(s), 594.8 (m), 720.6 (m), 920.2 (w), 949.0 (vs), 1003.8(w), three times with water and ethanol. Recrystallization of
1100.9 (w), 1085.4 (w), 1378.8 (w), 1462.5 (w), 1919.5 (w), the solid from acetonitrile afforded pale-yellow crystals of
2157.9 (w), 2871.1 (w), 2929.5 (w), 2959.6 (w). – 1H NMR [Bdmim]2Mo6O19. – IR (cm−1): ν = 434.3 (s), 592.2 (s),
(400 MHz, [D6]DMSO, r. t., ppm): δ = 0.93 (d, 12H), 1.42 753.7 (s), 787.0 (vs), 948.6 (vs), 1093.2 (w), 1133.9 (w),
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B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
595
1247.0 (w), 1417.3 (w), 15334.6 (w), 1588.1 (w), 1901.0 (w), crystal was frozen under a stream of nitrogen. A matrix
2962.2 (w), 3140.6 (w). – 1H NMR (400 MHz, [D6]DMSO, scan using at least 20 centered reflections was used to deter-
r. t., ppm): δ = 0.92 (t, 3H), 1.29 (m, 2H), 1.70 (m, 2H), 2.59 mine the initial lattice parameters. Reflections were merged
(s, 3H), 3.76 (s, 3H), 4.11 (t, 2H), 7.61 (s, 1H), 7.64 (s, 1H) and corrected for Lorentz and polarization effects, scan
–
13C NMR (100 MHz, [D6]DMSO, r. t., ppm): δ = 9.60, speed, and background using SAINT [62]. Absorption cor-
13.87, 19.37, 31.64, 35.14, 47.79, 121.32, 122.78, 144.64. rections, including odd and even ordered spherical harmon-
– Elemental analysis (%): calcd. C 18.20, H 3.05, N 4.72; ics were performed using SADABS [63]. Space group as-
found C 18.14, H 2.94, N 4.65.
signments were based upon systematic absences, E statis-
tics, and successful refinement of the structures. Structures
were solved using Bruker APEX suite [64], and were re-
fined with all data using SHELXLE [65, 66]. Hydrogen atoms
were assigned idealized positions and refined using a riding
model with an isotropic displacement parameter 1.2 times
that of the attached carbon atom (1.5 times for methyl hy-
drogen atoms). If not mentioned otherwise, non-hydrogen
atoms were refined with anisotropic displacement parame-
ters. Full-matrix least-squares refinements were carried out
by minimizing Σw(Fo2 − Fc2)2 with the SHELXL-97 weight-
ing scheme [67], [68]]. Neutral atom scattering factors for
all atoms and anomalous dispersion corrections for the non-
hydrogen atoms were taken from International Tables for
Crystallography [69]. Images of the crystal structures were
generated by PLATON [70, 71] and Mercury [72, 73].
General procedure for the epoxidation of olefins
In a typical reaction, the catalyst (20 µmol) was dissolved
in solvent (1 mL). Substrate (2 mmol) was added, followed
by the addition of UHP (4 mmol, 0.3762 g). The reaction
mixture was extracted with n-hexane (5 × 1 mL) and then
monitored by quantitative GC analysis. Samples were taken
at regular time intervals. The resulting slurry was filtered and
the filtrate injected onto a GC column. The conversion of
olefins and the formation of epoxides were calculated from
calibration curves (r2 > 0.999) recorded prior to the reaction.
For the recycling experiment, 3 mL of water was added to the
mixtures after extracting the substrate and the product with
n-hexane. The upper phase was removed from the reaction by
means of cannulation. The IL phase was washed three times
with water and then dried in vacuum for 4 h. Fresh substrate
and UHP were then added for a new reaction cycle.
1: pale-yellow fragment, 2(C16H36P)·Mo6O19, Mr
=
1398.48, monoclinic, space group C2/c (no. 15), a =
˚
16.0547(3), b = 16.0680(3), c = 19.7281(4) A, β =
General procedure for the oxidation of sulfides
◦
3
˚
106.248(1) , V = 4885.94(16) A , Z = 4, λ(MoKα ) =
0.71073 A, µ = 1.6 mm−1, ρcalcd. = 1.90 g cm−3, T =
˚
Catalyst (20 µmol) and sulfide (2 mmol) were dissolved
in MeOH (1 mL), followed by dropwise addition of H2O2
(35%) (0.19 mL, 2.1 mmol) at room temperature. The
progress of the reaction was followed by TLC. After comple-
tion of the reaction, 3 mL of ethyl acetate was added to the
mixture to obtain the catalyst by filtration. The solvent was
removed under vacuum for 4 h and then the crude products
123(1)K, F(000) = 2792, θmax = 25.44◦, R1 = 0.0156
(4133 observed data), wR2 = 0.0370 (all 4475 data), GOF =
−3
˚
1.053, 414 parameters, ∆ρmax/min = 0.34 / −0.34 e A
.
3: light-yellow fragment, 2(C8H15N2)·Mo6O19, Mr =
1158.08, monoclinic, space group P21/c (no. 14), a =
◦
˚
8.546(2), b = 17.085(3), c = 11.075(2) A, β = 106.248(1) ,
1
3
˚
˚
were analyzed by GLC or H NMR using internal standard
V = 1529.5(5) A , Z = 2, λ(MoKα ) = 0.71073 A, µ =
2.5 mm−1, ρcalcd. = 2.52 g cm−3, T = 123(1) K, F(000) =
1116, θmax = 25.62◦, R1 = 0.0165 (2584 observed data),
wR2 = 0.0395 (all 2836 data), GOF = 1.064, 208 parame-
technology The sulfoxides were purified by column chro-
matography (silica gel using hexane-ethyl acetate 90 : 10
v/v). For the recycling experiment, ethyl acetate was added
to the reaction mixture after the reaction was completed and
the catalyst precipitated, filtered off, washed with ethyl ac-
etate, and dried in high vacuum at room temperature. All
−3
˚
ters, ∆ρmax/min = 0.34 / −0.27 e A
.
4: light-yellow fragment, 2(C9H17N2)·Mo6O19, Mr =
1186.13, monoclinic, space group P21/n (no. 14), a =
products were characterized by melting point, 1H NMR, 13
NMR and IR spectroscopy (see Supporting Information).
C
˚
11.0074(2◦), b = 10.7827(2), c = 13.5900(3) A, β =
3
˚
91.045(1) , V = 1612.72(5) A , Z = 2, λ(MoKα ) =
0.71073 A, µ = 2.3 mm−1, ρcalcd. = 2.44 g cm−3, T =
˚
Single-crystal X-ray structure determinations
123(1) K, F(000) = 1148, θmax = 25.47◦, R1 = 0.0256
(2902 observed data), wR2 = 0.0660 (all 2989 data), GOF =
The data were collected on an X-ray diffractometer
equipped with a CCD detector (APEX II, κ-CCD), a rotat-
ing anode (Bruker AXS, FR591) or a fine-focused sealed
−3
˚
1.161, 218 parameters, ∆ρmax/min = 1.36 / −0.58 e A
.
For more detailed information on all crystal structure de-
terminations, see the Supporting Information.
˚
tube with MoKα radiation (λ = 0.71073 A), and a graphite
monochromator by using the SMART software package [62].
The measurements were performed on single crystals coated
with Paratone oil and mounted on glass capillaries. Each
CCDC 892238 ([(n-C4H9)4P]2[Mo6O19]), CCDC
892239 ([Bmim]2[Mo6O19]), and CCDC 892240
([Bdmim]2[Mo6O19]) contain the supplementary crys-
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596
B. Zhang et al. · Oxidation Reactions Catalyzed by Polyoxomolybdate Salts
tallographic data for this paper. These data can be obtained are given as Supporting Information available online (DOI:
free of charge from The Cambridge Crystallographic Data 10.5560/ZNB.2013-3033).
Centre via www.ccdc.cam.ac.uk/data request/cif.
Acknowledgement
B. Z. thanks the TUM Graduate School for financial sup-
port. S.-L. Z. thanks the National Natural Science Foun-
Supporting information
Detailed information on all crystal structure determina- dation of China (21071073) and the Cooperation Project
tions and spectroscopic data characterizing the sulfoxides (21111130584) for the financial support.
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