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In addition, we have screened different H2O2/substrate Acknowledgements
(O/S) molar ratios for the oxidation of DBT. When the O/S
We are grateful to the financial support from the National
Science Foundation of China (20771034 and 21401042) and
the National Key R&D Program of China (2018YFB0605802).
molar ratio was varied from 1 : 1 to 2 : 1 to 3 : 1, the degree
of selectivity varied from 69% to 86% to 99%, respectively
(entries 2, 8 and 9). Based on these results, the O/S molar
ratio of 3 : 1 was used for the following studies. As
previously reported, the solvent plays an important role in
determining the catalytic activity and selectivity in many
catalytic oxidation reactions with H2O2.19 Then, we
investigated the oxidation of DBT in different solvents, and
the results showed that the reaction is sensitive to the
solvent. For example, using H2O, CH3CH2OH, and CCl4 as
solvents, the conversions are 8.1%, 16% and 20%,
respectively, and the selectivities toward sulfones are all less
than 80%, which may be due to the low solubility of the
catalyst in these solvents (entries 10–12). CH3CN turned out
to be the best solvent providing the highest conversion
(99%) and the best selectivity (99%).
Notes and references
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Sulfoxidation with different substrates. Having established
the optimized reaction conditions for selective oxidation of the
model substrate, we extended the study to the other thiophenic
compounds
(3-methylbenzothiophene),
BT
(benzothiophene),
and
3-MBT
4-MDBT
(4-methyldibenzothiophene), the results of which are
summarized in Table 3. The reaction shows satisfactory general
applicability for the production of sulfones. Generally, the
oxidation reactivity decreased in the order DBT > BT, which is
attributed to the significantly lower electron density on the
sulfur atom of BT (the electron densities on the sulphur atoms
are reported to be 5.739 and 5.758 for BT and DBT,
respectively).20,21 It is possible to conclude that good to
excellent conversions and selectivities, ranging from 90% to
100%, were obtained with the four substrates for all the S/C
molar ratios tested (200 and 1000). In this article, by
comparing between the ODS performance of different catalysts,
the catalytic efficiency of complex 1 is amediocre (Table S5†).
Conclusions
In summary, a new phosphotungstate-supported rhenium
carbonyl derivative, [N(CH3)4]6H4ijP2W17O62{ReIJCO)3}2]·25H2O,
was obtained by using a conventional mixed-solvent solution
method. Solution behavior examined via ESI-MS showed that
complex 1 was stable in CH3CN medium. In addition, the
homogeneous catalytic performance of 1 in the selective
oxidation of thiophenes using H2O2 as an oxidant in CH3CN
was also demonstrated. The successful isolation of
compound 1 provided new scope for obtaining novel POM-
based organometallic compounds with intriguing properties.
In the following work, we will concentrate on exploring the
synthesis of novel structural analogues.
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Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2019
CrystEngComm, 2019, 21, 7322–7328 | 7327