Electron transfer-oxygen transfer oxygenation of sulfides catalyzed by the H5PV2Mo10O40 polyoxometalate

Article abstract of DOI:10.1021/ja105183w

The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H₅PV₂Mo₁₀O₄₀ and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H ₅PV₂Mo₁₀O₄₀ to yield a cation radical-reduced polyoxometalate ion pair, R₂^+·, H₅PV^IVV^VMo₁₀O₄₀, that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe ^+· and H₅PV^IVV^VMo ₁₀O₄₀) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for V^IV). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO²⁺ moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET-OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt₂)_x(solv)_5-x]²⁺ cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90%) but also some disulfide (10%) via carbon-sulfide bond cleavage.

Full text of DOI:10.1021/ja105183w

Published on Web 07/29/2010
Electron Transfer-Oxygen Transfer Oxygenation of Sulfides Catalyzed by the
H5PV2Mo10O40 Polyoxometalate
Alexander M. Khenkin,† Gregory Leitus,‡ and Ronny Neumann*,†
Department of Organic Chemistry and Chemical Reseach Support Unit, Weizmann Institute of Science,
RehoVot, Israel 76100
Received June 14, 2010; E-mail: Ronny.Neumann@weizmann.ac.il
Scheme 1. Oxygenation of ArSMe to ArS(O)Me with
Abstract: The oxygenation of sulfides to the corresponding
sulfoxides catalyzed by H5PV2Mo10O40 and other acidic vanado-
molybdates has been shown to proceed by a low-temperature
electron transfer-oxygen transfer (ET-OT) mechanism. First, a
sulfide reacts with H5PV2Mo10O40 to yield a cation radical-reduced
polyoxometalate ion pair, R2+•,H5PVIVVVMo10O40, that was identi-
fied by UV-vis spectroscopy (absorptions at 650 and 887 nm
for PhSMe+• and H5PVIVVVMo10O40) and EPR spectroscopy
(quintet at g ) 2.0079, A ) 1.34 G for the thianthrene cation
radical and the typical eight-line spectrum for VIV). Next, a
precipitate is formed that shows by IR the incipient formation of
the sulfoxide and by EPR a VO2+ moiety supported on the
polyoxometalate. Dissolution of this precipitate releases the
H5PV2Mo10O40 under Anaerobic Conditions
18
Reaction of PhSMe (85 mM) and H5PV2Mo10
O
(30 mM,
40
∼50% enrichment),1 yielded PhS(18O)Me that was 41% 18O-labeled.
Oxidation of PhSMe (85 mM) in the presence of H3PMo12O40,
H3PW12O40, H5PV2W10O40, or (n-BuN)5PV2Mo10O40 (30 mM)
showed no formation of PhS(O)Me, although with H3PMo12O40 the
polyoxometalate was reduced, as evidenced by the formation of a
green reduced species. With other acidic vanadomolybdates, such
as H5SiVMo11O40, H4PVMo11O40, and H5PV2Mo10O40, PhS(O)Me
was also formed [Table S1 in the Supporting Information (SI)]. In
aerobic reactions, PhSMe (850 mM) was reacted in the presence
of H5PV2Mo10O40 (10 mM) at 70 °C under 1 bar O2 for 15 h in
CH3NO2. A conversion of 57% with 48 turnovers was observed,
with both PhS(O)Me (90%) and PhSSPh (10%) obtained as
products. The formation of the latter is probably from the cation
radical intermediate.9
sulfoxide product. ET-OT oxidation of diethylsulfide yielded
2+
crystals containing [V(O)(OSEt2)x(solv)5-x
]
cations and poly-
oxometalate anions. Under aerobic conditions, catalytic cycles
can be realized with formation of mostly sulfoxide (90%) but also
some disulfide (10%) via carbon-sulfide bond cleavage.
During the past decade, we have shown that H5PV2Mo10O40 can
catalyze oxygenation reactions of arenes and alkyl arenes,1 primary
alcohols and vicinal diols,2 and CO3 by an outer-sphere electron
transfer-oxygen transfer (ET-OT) mechanism,4 which is a
homogeneous low-temperature analogue of the heterogeneous, high-
temperature Mars-van Krevelen oxygenation.5 The salient property
of these reactions is that contrary to the general paradigm in organic
and bioorganic chemistry that higher-valent oxo species are more
reactive than lower-valent ones, in the ET-OT reactions the species
containing lower-valent VIV-O are reactive while the VV-O species
are not; reduction of the polyoxometalate precedes the oxygen
transfer. Although the oxygenation of sulfides to sulfoxides with
H2O2 is a relatively facile reaction, similar reactions with O2 using
metal-based catalysts are not so.6 With polyoxometalates, mostly
synthetic aspects were stressed in the oxidation of sulfides to
sulfoxides with TBHP7 and with O2 using both iron- and vanadium-
containing poloxometalates.8 In this paper, we describe our research
on the ET-OT oxidation of sulfides with H5PV2Mo10O40, including
the identification of the electron-transfer step and the “suicidal”
formation of sulfoxides that under anaerobic conditions yields
The reaction of PhSMe (42 mM) with H5PV2Mo10O40 (1.5 mM)
at room temperature (RT) instead of 70 °C under Ar led to a green
solution exhibiting a visible spectrum with λmax ) 650 and 887
nm, as shown in Figure 1. This spectrum is hypothesized to arise
from a strongly red-shifted ion pair complex between PhSMe+• and
the reduced H5PVVVIVMo10O40. Previously, PhSMe+• was shown
to absorb at λmax ) 530 nm,9 but similar UV-vis spectra of sulfide-
based cation radicals were reported when measured under acidic
conditions;10 the documented maximum for H5PVVVIVMo10O40 is
at ∼700 nm.11
[V(O)(OSR2)x(solv)5-x]
2+. In the presence of O2 and a suitable
solvent, mainly sulfoxides but also disulfides are formed.
Reactions of ArSMe (85 mM) and H5PV2Mo10O40 (30 mM) in
acetic acid at 70 °C for 1 h under Ar gave a green precipitate
(Scheme 1). Isolation of the solids and their dissolution in DMSO
showed the 100% selective formation of ArS(O)Me in 140-200%
yields based on H5PV2Mo10O40.
Figure 1. Visible spectrum of H5PVVVIVMo10O40-PhSMe+•. A strong UV
peak at ∼300 is not shown.
The kinetic behavior of H5PVVVIVMo10O40-ArSMe+• formation
with various substrates showed a good Hammett correlation with
† Department of Organic Chemistry.
‡ Chemical Reseach Support Unit.
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C O M M U N I C A T I O N S
Figure 4. (left) EPR spectrum of the precipitate from the reaction of PhSMe
and H5PV2Mo10O40. (right) IR spectra of (bottom) H5SiVMo11O40 and (top)
the precipitate from PhSMe and H5SiVMo11O40.
Figure 2. Hammett plot for the electron-transfer reaction between ArSMe
and H5PV2Mo10O40. Reaction conditions: 42 mM ArSMe, 1.5 mM
H5PV2Mo10O40, 2 mL of AcOH, RT, Ar. The value of r2 was 0.984. Inset:
time profile for the reaction of PhSMe with H5PV2Mo10O40.
Figure 5. Ball-and-stick structure of [V(O)(OSEt2)4(MeCN)]2+
-
2
[V(O)(OSEt2)2(OH2)3]2+[PMo12O40]3-2 ·2H2O·MeCN. Only one cation and
anion are shown. P, green; Mo, brown; O, red; V, purple; N, blue; C, black;
S, yellow.
nitromethane, these compounds do not precipitate and there is
turnover in the presence of O2 (see above) with re-formation of
H5PV2Mo10O40 (31P NMR). The removal of VO2+ from the vicinal
isomers of H5PV2Mo10O40 upon reduction was observed by EPR
spectroscopy and has been discussed elsewhere,17 as was the likely
identity of oxygen atom involved in the OT step.3
Figure 3. EPR spectrum resulting from the reaction of thianthrene and
H5PV2Mo10O40.
F ) -3.6 (Figure 2), as expected for the formation of a one-
electron-oxidized species in this step.12
On the basis of the evidence presented, a sequence of reactions
that are involved in the ET-OT oxidation of sulfides can be
suggested (Scheme 2). An outer-sphere ET reaction between R2S
and H5PV2Mo10O40 leads to an ion pair (step a). This is followed
by an OT reaction to yield R2SO with removal of VO2+ from the
Keggin structure (step b), which is postulated to be the green
precipitate. In the presence of a suitable solvent (e.g., CH3NO2),
the sulfoxide is liberated with the likely formation of the reduced
H7PVIV2Mo10O40 (step c).
To solidify the hypothesis of the formation of H5PVVVIV-
Mo10O40-PhSMe+•, we sought to observe such a species in solution
using thianthrene, since it is known to form a stable radical cation
with a known EPR spectrum.12 In Figure 3 one is able to observe
both a rather weak eight-line signal attributable to VIV in
H5PVVVIVMo10O40 and a signal at g ) 2.0079, A ) 1.34 G due to
the thianthrene cation radical. This spectrum consists of five lines
in an integral ratio of 1:4:6:4:1.13 A similar EPR spectrum was
observed with diphenyl sulfide (see the SI).
From Scheme 1, a question arises regarding the identification of
the green precipitate. The EPR spectrum (Figure 4 left) is very
similar to the published spectrum of a VO2+ species supported on
a polyoxometalate with an axially symmetric g tensor of V(IV)
with hyperfine splitting due to interaction of an unpaired electron
with the nuclear spin of 51V (I ) 7/2).14 Though the peak intensity
was strong, there was no signal attributable to PhSMe+•, suggesting
that PhS(O)Me was present in the precipitate. The S-O vibrations
of sulfoxides are typically at 1050-1100 cm-1, but the P-O bond
of H5PV2Mo10O40 absorbs at these wavenumbers.15 As stated above,
H5SiVMo11O40 also reacted anaerobically with PhSMe to form
PhS(O)Me via a green precipitate. Indeed, in that case the S-O
vibration was observed at 1087 cm-1 (Figure 4 right).
Scheme 2. Proposed Pathway for ET-OT Oxidation of Sulfides
Acknowledgment. This research was supported by the Israel
Science Foundation and the Kimmel Center for Molecular Design.
Lev Weiner is thanked for the EPR measurements. R.N. is the
Rebecca and Israel Sieff Professor of Organic Chemistry.
Supporting Information Available: Complete experimental section,
additional spectra and explanations, and a CIF file. This material is
available free of charge via the Internet at http://pubs.acs.org.
Attempts to grow crystals from the reactions with PhSMe failed,
but the reaction of EtSEt with H5PV2Mo10O40 at RT did yield single
crystals after 60 days in MeCN.16 The X-ray structure (Figure 5)
shows that VO2+ was removed from H5PV2Mo10O40 and that two
cationic species, [V(O)(OSEt2)4(MeCN)]2+ and [V(O)(OSEt2)2-
(OH2)3]2+, were formed. The oxygenation reaction is apparently
“suicidal” in acetic acid, although in other solvents, such as
References
(1) (a) Khenkin, A. M.; Neumann, R. Angew. Chem., Int. Ed. 2000, 39, 4088.
(b) Khenkin, A. M.; Weiner, L.; Wang, Y.; Neumann, R. J. Am. Chem.
Soc. 2001, 123, 8531.
(2) Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2008, 130, 14474.
(3) Goldberg, H.; Kaminker, I.; Goldfarb, D.; Neumann, R. Inorg. Chem. 2009,
48, 7947.
9
J. AM. CHEM. SOC. VOL. 132, NO. 33, 2010 11447

C O M M U N I C A T I O N S
(4) Neumann, R. Inorg. Chem. 2010, 49, 3594.
(12) Shaik, S.; Wang, Y.; Chen, H.; Song, J.; Meir, R. Faraday Discuss. 2010,
145, 1.
(5) (a) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41. (b)
Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New
York, 1980.
(6) (a) Riley, D. P.; Shumate, R. E. J. Am. Chem. Soc. 1984, 106, 3179. (b)
Huynh, M. H. V.; Witham, L. M.; Lasker, J. M.; Wetzler, M.; Mort, B.;
Jameson, D. L.; White, P. S.; Takeuchi, K. J. J. Am. Chem. Soc. 2003,
125, 3089.
(7) Gall, R. D.; Faraj, M.; Hill, C. L. Inorg. Chem. 1994, 33, 5015.
(8) (a) Okun, N. A.; Anderson, T. M.; Hill, C. L. J. Am. Chem. Soc. 2003,
125, 3194. (b) Kozhevnikov, I. V.; Simagina, V. I.; Varnakova, G. V.;
Matveev, K. I. Kinet. Catal. 1979, 20, 506.
(13) Giordan, J.; Bock, H. Chem. Ber. 1982, 115, 2547.
(14) (a) Bayer, R.; Marchal, C.; Liu, E. X.; Teze, A.; Herve, G. J. Mol. Catal.
A: Chem. 1996, 110, 65. (b) Poppl, A.; Manikandan, P.; Kohler, K.; Maas,
P.; Strauch, P.; Bottcher, R.; Goldfarb, D. J. Am. Chem. Soc. 2001, 123,
4577.
(15) Rocchiccioli-Deltcheff, C.; Fournier, M.; Frank, R.; Trouvenot, R. Inorg.
Chem. 1983, 22, 207.
(16) The “green precipitates” formed in the oxidation of sulfides probably do
not have the same structure as these crystals, since complete Keggin species
are likely to be re-formed only by a series of reactions in solution. The
EPR spectra of the green precipitate and crystals are similar.
(17) Kaminker, I.; Goldberg, H.; Neumann, R.; Goldfarb, D. Chem.sEur. J.
[Online early access]. DOI: 10.1002/chem.201000944. Published Online:
July 19, 2010.
(9) Ioele, M.; Steenken, S.; Bachiocchi, E. J. Phys. Chem. A 1997, 101,
2979.
(10) Shine, H. J.; Piette, L. J. Am. Chem. Soc. 1962, 84, 4798.
(11) Altenau, J. J.; Pope, M. T.; Prados, R. A.; So, H. Inorg. Chem. 1975, 14,
417.
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