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Dalton Transactions
DOI: 10.1039/C8DT00583D
Dalton Transactions
collected. An aliquot (5 ꢀL) was removed and promptly diluted in 1
mL acetonitrile, then sprayed using a home-built nano-ESI source.29
All ESI-MS experiments for 1b were performed in the negative ion
mode on a 4000 QTRAP triplequadrupole/linear ion trap mass
spectrometer (Sciex, Toronto, Canada). The characteristic
parameters of the MS during this study were set as follows: spray
voltage, 1500 V; curtain gas, 5 psi; declustering potential (DP), 20 V.
Mass analysis was achieved by using Q3 as a linear ion trap at a scan
rate of 1000 Da/s. Data shown here were typically averages of 50
scans. ESI-MS experimental conditions for 2b are provided in the
supplementary information. Data acquisition, processing, and
instrument control were performed using Analyst 1.6 software.
purchased from ACROS Organics. Hydrogen peroxide (30%) was
purchased from Macron Fine Chemicals and standardized via
iodometric titration. Oxygenation reaction samples were analyzed
using an Agilent 7890A GC system equipped with a flame ionization
detector. The separation of substrate and products was achieved
using an Agilent HP-5 column with dimensions of 30 m x 0.320 mm
with 25 micron film thickness. Reaction progress was monitored at
290 nm via UV-Vis spectroscopy on
Spectrophotometer.
a
JASCO V-670
Synthesis of (n-Bu4N)6[Mo7O24] (1b)
Ammonium heptamolybdate tetrahydrate (500 mg, 0.405 mmol)
was dissolved in water (4 mL). In a separate round bottom flask
tetrabutylammonium bromide (0.789 g, 2.43 mmol) was dissolved
in water (4 mL). The two solutions were mixed together and stirred
for 10 minutes, with a white precipitate forming immediately. The
precipitate was allowed to settle in the vial and collected by
filtration and dried. Yield: 81% based on Mo.
Conclusions
It has been shown in this contribution that commodity
chemical heptamolybdate is a highly efficient catalyst for
hydrogen peroxide oxygenation of organic sulfides in terms of
both the reaction rate and the hydrogen peroxide utility
(100%). Electrospray ionization mass spectrometry studies
revealed that the heptamolybdate ion undergoes significant
fragmentation under the catalytic reaction conditions. One of
the major ions detected in ESI-MS, [Mo2O11]2-, has been
identified as the main active species based on the
consideration of catalytic rates. Our success herein will
hopefully encourage further applications of ESI-MS and other
MS techniques35, 36 in polyoxometallate catalysis.
Synthesis of (n-Bu4N)6[Mo7O22(O2)2]·nH2O (2b)
Ammonium heptamolybdate tetrahydrate (2.472 g; 2.0 mmol) was
dissolved in 20 mL water, to which were slowly added with stirring
2.0
mL
30%
hydrogen
peroxide
(20
mmol)
and
tetrabutylammonium bromide (4.832 g, 15 mmol) in 10 mL water.
After 30 min, the yellow precipitate was collected by filtering
through a sintered glass filter, washed with 20 mL water, and air
dried to afford powder samples 3.98 g of 2b (yield 78% based on
Mo). Elemental analysis calcd (%) for C96H216N6Mo7O26: C, 45.4; H,
8.6; N, 3.3; Mo, 26.4. Found (%):C, 43.8; H, 9.0; N, 3.1; Mo, 25.6. IR
for 1 (KBr, cm-1): 3394 (s), 2964 (m), 2940 (m), 2874 (m), 1647 (m),
1483 (m), 1460 (m), 1381 (m), 1348 (w), 1281 (w), 1152 (w), 1158
(w), 1069 (w),1030 (w), 1004 (w), 949 (m), 922 (s), 903 (s), 852 (s),
797 (vs), 730 (s), 657 (vs), 583 (s), 556 (s).
Conflicts of interest
There are no conflicts to declare.
Catalytic conversion of sulfide
The catalyst (1b/2b) (0.005 mmol, 1 mol%) was dissolved in 5 mL of
acetonitrile. Substrate sulfide (0.5 mmol) and internal standard 1,2-
dichlorobenzene (0.4 mmol) were added to the reaction solution.
H2O2 (1 mmol) was added to the solution drop wise, and the
solution turned yellow. In the case of DBT additional equivalents of
H2O2 (2 mmol total) were added in order to achieve complete
conversion. Aliquots were taken at different time periods,
quenched using MnO2, and analyzed using GC.
Acknowledgements
We thank the financial support from Purdue University. Y. X. and H.
H. acknowledge the support from NSF CHE-1308114.
Notes and references
Initial rate kinetics of 1b and 2b
1
2
3
4
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Standard acetonitrile solutions utilized were prepared for 1b at
1.75 mM, 2b at 3.27 mM, MPS at 39 mM, and hydrogen
peroxide at 0.37 M. Solutions for kinetic studies were prepared
combining 200 μL of MPS and a specific volume of 1b in a
quartz cuvette, the volume of which was adjusted to 3.22 mL
with additional acetonitrile. The reaction was initiated by the
addition of 200 μL of the H2O2 stock solution. The absorbance
of the solution at 290 nm was measured every 20 seconds for
30 minutes. The initial in situ concentrations upon the addition
of H2O2 to the cuvette are 2.43 mM for MPS, 23 mM for H2O2,
with a range of concentrations for 1b (0.027-0.0326 mM) or 2b
(0.025-0.125 mM).
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Nano-electrospray ionization mass spectra of 1b and 2b
For a typical measurement, a 5 mL solution of 1b or 2b (0.005 mmol
in acetonitrile), 59 µL of MPS (0.5 mmol in acetonitrile) and 51 µL of
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2008, 283, 52-59.
H2O2 (0.5 mmol in acetonitrile) were combined.
These same
concentrations and volumes were used for the neat 1b/2b
(Experiment I) and 1b/2b plus H2O2 (Experiment II) spectra
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