Organotin-oxomolybdate coordination polymer as catalyst for synthesis of unsymmetrical organic carbonates

Article abstract of DOI:10.1039/c0gc00765j

Efficient and greener synthesis of unsymmetrical organic carbonates is of great importance. In this work, two organotin-oxometalates, Bu ₂SnMoO₄ and (Bu₃Sn)₂MoO₄, were prepared and their catalytic performance for the transesterification of diethyl carbonate (DEC) with alcohols to synthesize unsymmetrical organic carbonates was studied. It was found that (Bu₃Sn)₂MoO ₄ was very active and selective for the transesterification of DEC and various alcohols, including alkyl, cyclic, and aryl alcohols due to the synergetic effect between the groups of [MoO₄]^2- and [Bu₃Sn]⁺ in the catalyst. The yields of the corresponding carbonates could reach 98% at the suitable conditions. The catalyst was reused five times and the activity and selectivity were not changed. We believe that the highly efficient, versatile, greener, inexpensive, selective and stable catalyst has great potential applications in the synthesis of various unsymmetrical organic carbonates from DEC and alcohols. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00765j

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PAPER
Organotin-oxomolybdate coordination polymer as catalyst for synthesis of
unsymmetrical organic carbonates
Jinliang Song, Binbin Zhang, Tainbin Wu, Guanying Yang and Buxing Han*
Received 2nd November 2010, Accepted 6th January 2011
DOI: 10.1039/c0gc00765j
Efficient and greener synthesis of unsymmetrical organic carbonates is of great importance. In this
work, two organotin-oxometalates, Bu2SnMoO4 and (Bu3Sn)2MoO4, were prepared and their
catalytic performance for the transesterification of diethyl carbonate (DEC) with alcohols to
synthesize unsymmetrical organic carbonates was studied. It was found that (Bu3Sn)2MoO4 was
very active and selective for the transesterification of DEC and various alcohols, including alkyl,
cyclic, and aryl alcohols due to the synergetic effect between the groups of [MoO4]2- and [Bu3Sn]+
in the catalyst. The yields of the corresponding carbonates could reach 98% at the suitable
conditions. The catalyst was reused five times and the activity and selectivity were not changed.
We believe that the highly efficient, versatile, greener, inexpensive, selective and stable catalyst has
great potential applications in the synthesis of various unsymmetrical organic carbonates from
DEC and alcohols.
carbonates have been synthesized from primary or secondary
Introduction
alcohols and CO2 via unstable methanesulfonyl carbonates.11
Dialkyl organic carbonates are very attractive from a green
chemistry point of view.1 They have been used as intermediates
Recently, Chi and co-workers reported the synthesis of organic
carbonates via alkylation of metal carbonate with various alkyl
for the synthesis of pharmaceuticals and fine chemicals,2 as
halides and sulfonates in ionic liquids.12
excellent polar aprotic solvents,3 as monomers for organic
The unsymmetrical organic carbonates can also be prepared
glass, synthetic lubricants and plasticizers,4 as octane enhancers
by transesterification of DEC with various alcohols. Different
for gasoline,5 in agriculture to produce herbicides, acaricides,
heterogeneous catalysts, including MCM-41-TBD,13 Mg/La
fungicides and seed disinfectants.6
metal oxide,14 CsF/a-Al2O3,15 nano-crystalline MgO16 and
Conventional synthetic methods for organic carbonates use
metal–organic frameworks17 have been used for the reactions.
toxic and harmful chemicals such as phosgene, pyridine and
However, development of highly efficient synthetic routes for
carbon monoxide.7 To avoid the use of the toxic reagents, in re-
the production of unsymmetrical organic carbonates is an
cent years, symmetrical organic carbonates, especially dimethyl
interesting topic.
carbonate (DMC) and diethyl carbonate (DEC), have been
Recently, tri-n-butyltin derivatives have been reported by
synthesized by CO2, epoxides and alcohols,8a–8e or synthesized
Fischer and co-workers.18 These compounds, having the general
directly from CO2 and alcohols.8f
formula [(R3Sn)2MO4] [R = Me, Et, n-Pr, n-Bu and phenyl
Compared to symmetrical organic carbonates, unsymmet-
(Ph); M = Mo and W], are formed through the coordination
rical organic carbonates are more useful molecules than the
of the tetrahedral [MO4]2- subunits and [R3Sn]+ as the linking
symmetrical ones, but the synthetic routes are more complex.
spacers (Scheme 1). These compounds are interesting candidates
Several methods have been developed for the production of
as catalysts or catalyst precursors and have been used in
unsymmetrical organic carbonates. For example, electrochem-
some oxidation reactions, such as epoxidation of olefins,19
ical synthesis of organic carbonates from CO2 and alcohols has
sulfoxidation,20 N-oxidation of primary aromatic amines,21 etc.
been conducted in ionic liquids.9 Some unsymmetrical organic
It is interesting to extend the application of these compounds as
carbonates have been produced by the coupling of alcohols, CO2
catalysts for more chemical reactions.
and alkyl halides in the presence of Cs2CO3.10 Furthermore,
In this work, we prepared two organotin-oxometalates,
Bu2SnMoO4 and (Bu3Sn)2MoO4, and their catalytic perfor-
mance for the transesterification of DEC with various al-
cohols to synthesize unsymmetrical organic carbonates was
studied. It was found that the organotin-oxometalates, especially
(Bu3Sn)2MoO4, are very active, selective and stable for the
Beijing National Laboratory for Molecular Sciences (BNLMS), Centre
for Molecular Science, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, China. E-mail: hanbx@iccas.ac.cn;
Fax: +86-10-62562821
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Scheme 1 Representation of the proposed structure of (R3Sn)2MO4.
reactions. To the best of our knowledge, this is the first
application of organotin-oxometalates to catalyze the transes-
terification of alcohols with dialkyl organic carbonates.
Results and discussion
Characterization of the catalysts. The organotin-
oxometalates, Bu2SnMoO4 and (Bu3Sn)2MoO4, were
characterized by X-ray diffraction (XRD), Fourier transform
infrared (FT-IR), and thermogravimetric (TG) techniques.
The powder XRD patterns are presented in Fig. 1. The XRD
pattern of the (Bu3Sn)2MoO4 was in good agreement with that
reported by other authors21 The patterns of the Bu2SnMoO4
and (Bu3Sn)2MoO4 were quite different, suggesting that the
tin-bound group exerts a considerable structure directing effect.
Meanwhile, the powder XRD patterns of Bu2SnMoO4 and
(Bu3Sn)2MoO4 showed characteristic peaks of MoO4 in the
range of 2q (22), which are similar to the data reported in the
literature.21 FT-IR spectra of Bu2SnMoO4 and (Bu3Sn)2MoO4
showed all characteristic bands of organic and metal organic
functionalities present in the coordination polymers. The strong
band at 810 cm-1 in the FT-IR spectra of the two compounds
(Fig. 2) was assigned to the asymmetric stretch of the [MoO4]2-
oxoanion and this is in agreement with published result.20
Fig. 2 FT-IR spectra for (Bu3Sn)2MoO4 (a) and Bu2SnMoO4 (b).
The observed strong peaks at about 1460 cm-1 and 2900 cm-1
should be assigned to the stretch of n-butyl (n-Bu) The TG
thermograms of the two compounds (Fig. 3) indicated that they
had very good thermal stability. The weights of the Bu2SnMoO4
and (Bu3Sn)2MoO4 remained almost constant up to 240 ◦C and
250 ◦C, respectively. The decomposition of the coordination
polymers occurred at about 240 ◦C and 250 ◦C, respectively,
which is much higher than the temperature use◦d in the reaction.
The weight loss which occurred at about 650 C resulted from
the decomposition of oxides of Sn or Mo.
Catalytic performance of different catalysts. The activity and
selectivity of various catalysts were tested for the transesterifica-
tion of 1-octanol and DEC, and the results are given in Table 1. In
the absence of the catalyst, the desired unsymmetrical carbonate
was not formed (Table 1, entry 1). Similarly, the reaction
did not occur when only the precursors of the organotin-
oxometalates, such as Na2MoO4·2H2O, BuSnCl3, Bu2SnCl2,
Bu3SnCl, were used as the catalysts (Table 1, entries 2–5). The
(NH4)6Mo7O24·4H2O alone just gave a moderate yield (Table 1,
entry 6). To our delight, when Na2MoO4·2H2O and Bu3SnCl
were used at the same time, the reaction took place with about
30% yield of the desired product (Table 1, entry 7). The insoluble
nature of Na2MoO4·2H2O in the reaction system resulted in slow
formation rate of (Bu3Sn)2MoO4. Therefore, the yield of the de-
sired product catalyzed by physical mixture of Na2MoO4·2H2O
and Bu3SnCl was lower than that catalyzed by (Bu3Sn)2MoO4.
On the base of this finding, we synthesized two organotin-
oxometalates (Bu2SnMoO4 and (Bu3Sn)2MoO4) as the catalysts
for the transesterification reaction. Surprisingly, the reaction
proceeded with much higher yield (Table 1, entries 8 and 9) and
Fig. 1 Powder X-ray diffraction pattern for (Bu3Sn)2MoO4 (a) and
Bu2SnMoO4 (b).
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Table 1 Reaction of 1-octanol and DEC catalyzed by different
Table 2 Effect of reaction temperature on the reaction of 1-octanol
catalystsa
and DECa
Entry
T/◦C
Yield(%)b
Selectivity(%)c
1
2
3
4
100
110
120
130
0
—
99
99
99
Entry
Catalyst
Yield(%)b
Selectivity(%)c
17
51
98
1
2
3
4
5
6
7d
8
9
None
0
0
0
0
—
—
—
—
—
97
97
99
99
Na2MoO4·2H2O
BuSnCl3
a Typical reaction conditions: a flask of 10 ml, 33 mmol DEC, 2 mmol
1-octanol, 15 mg (Bu3Sn)2MoO4, reaction time 2 h. b The yields were
determined by GC. c The byproduct was mainly octanoyl ethyl ether,
corresponding di-ether and product of the alcohol oxidation.
Bu2SnCl2
Bu3SnCl
(NH4)6Mo7O24·4H2O
Na2MoO4·2H2O + Bu3SnCl
Bu2SnMoO4
0
47
30
80
98
(Bu3Sn)2MoO4
The effects of catalyst amount and temperature on the reaction.
The influence of the amount of (Bu3Sn)2MoO4 on the trans-
esterification of 1-octanol with DEC was investigated under
the identical reaction conditions (Fig. 4). The selectivity to the
corresponding unsymmetrical organic carbonate was about 99%
as the amount of the catalyst was in the range of 5 to 20 mg. The
byproducts were mainly octanoyl ethyl ether, corresponding di-
ether and product of the alcohol oxidation. The yield increased
with the amount of the catalyst and reached a 98% as the amount
of the catalyst was increased up to 15 mg. Therefore, 15 mg of
the catalyst would be an appropriate amount at the reaction
conditions.
a Typical reaction conditions: a flask of 10 ml, 33 mmol DEC, 2 mmol
1-octanol, 15 mg catalyst, reaction temperature 130 ◦C, reaction time 2
h. b The yields were determined by GC. c The by-product was mainly
octanoyl ethyl ether, corresponding di-ether and product of alcohol
oxidation. d 5 mg Na2MoO4·2H2O and 13 mg Bu3ClSn were used, which
could form 15 mg (Bu3Sn)2MoO4.
Fig. 4 Effect of catalyst amount on the transesterification of 1-octanol
with DEC. Reaction conditions: a flask of 10 ml, 33 mmol DEC, 2 mmol
1-octanol, reaction temperature 130 ◦C, reaction time 2 h.
Furthermore, we found that the yield of the corresponding
unsymmetrical organic carbonate was strongly affected by the
reaction temperature using (Bu3Sn)2MoO4 as the catalyst. As
shown in Table 2, the yield increased with temperature in the
range of 100 to 130 ◦C and a yield of 98% was achieved at
130 ◦C.
Reactions of DEC with different alcohols. The catalytic
performance of (Bu3Sn)2MoO4 for the reactions of DEC with
various kinds of alcohols including alkyl, cyclic, and aryl
alcohols was also examined. (Bu3Sn)2MoO4 showed satisfactory
activity and selectivity for the transesterification reactions, as can
be known from the data in Table 3. The activity of the primary
alkyl alcohols was very high and they were all transformed
into the corresponding unsymmetrical carbonates with good
selectivity (Table 3, entries 1–6). The activity order of alkyl
alcohols was primary alcohol > secondary alcohol > tertiary
Fig. 3 Thermogram of (Bu3Sn)2MoO4 (a) and Bu2SnMoO4 (b).
the selectivity to the corresponding unsymmetrical carbonate
was 99% in the presence of Bu2SnMoO4 or (Bu3Sn)2MoO4.
In addition, (Bu3Sn)2MoO4 had the best catalytic activity
(Table 1, entry 9). This increased activity is likely to be due
to the synergetic effect between the groups of [MoO4]2- and
[Bu3Sn]+, which existed in the organotin-oxometalate. Therefore,
(Bu3Sn)2MoO4 was used as the catalyst for further study.
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a
Table 3 Synthesis of unsymmetrical organic carbonates from DEC and various alcohols catalyzed by (Bu3Sn)2MoO4
Entry
1
Alcohol
Product
Time(h)
2
Yield(%)b
99
Selectivity(%)c
99
2
3
4
5
6
7
2
2
2
2
2
8
98
98
97
98
97
98
99
98
99
98
98
98
8
9
8
4
0
—
97
94
10
2
98
98
11
12
8
8
96
81
97
98
13
8
90
95
14
15
4
8
93
79
97
90
a Typical reaction conditions: a flask of 10 ml, 33 mmol DEC, 2 mmol alcohol, 15 mg (Bu3Sn)2MoO4, reaction temperature 130 ◦C. b The yields were
determined by GC. c The by-product was mainly corresponding ether, di-ether and product of the alcohol oxidation.
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alcohol (Table 3, entries 3, 7 and 8), which could result mainly
from steric hindrance of the three types of alcohols and tertiary
alcohols have the largest steric hindrance. The aryl alcohols
(Table 3, entries 13–15) had lower activity than the alkyl ones
because the p electron of the benzene ring could be combined
with the activity center of the catalyst, which reduced the
interaction chance of the reactants and the catalyst, and this
effect of p electron also existed in the alkyl alcohols (Table 3,
entries 9 and 10). The aryl alcohols also showed the same activity
order of primary alcohol > secondary alcohol (Table 3, entries
14 and 15). For the cyclic alcohols (Table 3, entries 11 and
12), the lower activity was due to the steric hindrance of the
ring.
Reusability of the catalyst. Experiments were also carried
out to examine the recyclability of the catalyst using 1-hexanol
as the substrate. In each cycle, (Bu3Sn)2MoO4 was recovered by
distillation at reduced pressure. Then, the catalyst was reused
directly for the next run. The yields of the corresponding
carbonate for the five repeated runs are shown in Fig. 5. There
was no considerable decrease in the yield and selectivity of the
corresponding carbonate after five cycles, indicating that the
catalyst was very stable.
Scheme 2 Plausible mechanism for synthesis of unsymmetrical organic
carbonates catalyzed by the (Bu3Sn)2MoO4.
Conclusions
(Bu3Sn)2MoO4 is an very efficient catalyst for the synthesis of
unsymmetrical organic carbonates through the transesterifica-
tion of DEC with various alcohols due to the synergetic effect
between the groups of [MoO4]2- and [Bu3Sn]+ in the catalyst. The
yields of the unsymmetrical organic carbonates can be as high
as 98%. The activity of alkyl alcohols follows the order primary
alcohol > secondary alcohol > tertiary alcohol, which result
mainly from the difference of steric hindrance of the alcohols.
The aryl alcohols also show the same activity order of primary
alcohol > secondary alcohol. The catalyst can be reused at least
five times without reducing the activity and selectivity. We believe
that the catalyst has great potential applications in the synthesis
of various unsymmetrical organic carbonates from DEC and
alcohols.
Fig.
5
Reuse of the catalyst. Reaction conditions: 15 mg
(Bu3Sn)2MoO4, 2 mmol 1-hexanol, 33 mmol DEC, reaction temperature
130 ◦C, reaction time 2 h.
Experimental
Mechanism. A plausible mechanism was proposed based on
a typical transesterification process (Scheme 2), where the more
nucleophilic reagent displaces the less nucleophilic one or the
less volatile compound displaces the more volatile one when
both the reagents have similar nucleophilicity. In the present
process, the nucleophilic displacement of the ethoxy group
by a second molecule of the alcoholic reagent probably leads
to the corresponding unsymmetrical carbonate. As shown in
Scheme 2, the catalytic cycle may be initiated by activation
of an alcohol through hydrogen bonding formed between
hydroxyl group and [MoO4]2- in the catalyst and meanwhile,
DEC was activated by the group [Bu3Sn]+ [(a) in Scheme 2].
Then, the activated alcohol attacked the activated carbonyl
group of DEC [(b) in Scheme 2]. Finally, the corresponding
unsymmetrical carbonates were formed through an intermediate
[(c) in Scheme 2]. From the discussion above, we think that the
synergistic effect of [MoO4]2- and [Bu3Sn]+ existing in the catalyst
may be the main reason for the high activity of the catalyst
system.
Materials
Diethyl carbonate, n-Bu3SnCl, n-Bu2SnCl2, n-BuSnCl3 were
purchased from Alfa Aesar. The alcohols, Na2MoO4·2H2O,
(NH4)6Mo7O24·4H2O, acetone were all analytical grade and
purchased from Beijing Chemical Reagents Company. All
chemicals were used as received.
Methods
FT-IR spectra were recorded on a spectrometer (Bruker Tensor
27, Germany) and the sample was prepared by the KBr pellet
method. XRD measurements were conducted on an X-ray
diffractometer (D/MAX-RC, Japan) operated at 40 kV and
200 mA with Cu-Ka (l = 0.154 nm) radiation. TG analysis
was performed on a thermogravimetric analysis system (Netzsch
STA 409 PC/PG, Germany) in N2 atmosphere at a heating rate
of 20 ◦C min-1.
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Catalyst synthesis
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Synthesis of [(n-Bu3Sn)2MoO4]. n-Bu3SnCl (10 mmol) was
dissolved in a solution of water (6 mL) and acetone (27 mL).
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Acknowledgements
The authors are grateful to National Natural Science Founda-
tion of China (21003133, 20973177), Ministry of Science and
Technology of China (2009CB930802), and Chinese Academy
of Sciences (KJCX2.YW.H16).
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