Carbon-sulfur bond cleavage and hydrodesulfurization of thiophenes by tungsten

Article abstract of DOI:10.1021/ja111034g

The reactions of W(PMe₃)₄ (η²-CH ₂PMe₂)H, W(PMe₃)₅H₂, W(PMe₃)₄H₄ and W(PMe₃) ₃H₆ towards thiophenes reveal that molecular tungsten compounds are capable of achieving a variety of transformations that are relevant to hydrodesulfurization. For example, sequential treatment of W(PMe₃)₄(η²-CH₂PMe ₂)H with thiophene and H₂ yields the butanethiolate complex, W(PMe₃)₄(SBuⁿ)H₃, which eliminates but-1-ene at 100 °C. Likewise, sequential treatment of W(PMe ₃)₄(η²-CH₂PMe₂)H with benzothiophene and H₂ yields W(PMe₃)₄(SC ₆H₄Et)H₃, which releases ethylbenzene at 100 °C. Moreover, W(PMe₃)₄(η²-CH ₂PMe₂)H desulfurizes dibenzothiophene to form a dibenzometallacyclopentadiene complex, [(κ²-C ₁₂H₈) W(PMe₃)](μ-S)(μ-CH ₂PMe₂)(μ-PMe₂)[W(PMe₃) ₃].

Full text of DOI:10.1021/ja111034g

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pubs.acs.org/JACS
Carbon-Sulfur Bond Cleavage and Hydrodesulfurization of
Thiophenes by Tungsten
Aaron Sattler and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S Supporting Information
b
Scheme 1
ABSTRACT: The reactionsofW(PMe3)4(η2-CH2PMe2)H,
W(PMe3)5H2, W(PMe3)4H4 and W(PMe3)3H6 towards
thiophenes reveal that molecular tungsten compounds are
capable of achieving a variety of transformations that are
relevant to hydrodesulfurization. For example, sequential
treatment of W(PMe3)4(η2-CH2PMe2)H with thiophene
and H2 yields the butanethiolate complex, W(PMe3)4-
(SBun)H3, which eliminates but-1-ene at 100 °C. Likewise,
sequential treatment of W(PMe3)4(η2-CH2PMe2)H with
benzothiophene and H2 yields W(PMe3)4(SC6H4Et)H3,
which releases ethylbenzene at 100 °C. Moreover, W(PMe3)4-
(η2-CH2PMe2)H desulfurizes dibenzothiophene to form a
dibenzometallacyclopentadiene complex, [(κ2-C12H8)W-
(PMe3)](μ-S)(μ-CH2PMe2)(μ-PMe2)[W(PMe3)3].
ydrodesulfurization (HDS) is the essential process for
removing sulfur-containing impurities from crude petro-
H
leum in order to obtain (i) cleaner fuels that minimize environ-
mental pollution and (ii) cleaner chemical feedstocks that are less
likely to poison the catalysts that are used for subsequent trans-
formations.1-3 The catalysts employed for hydrodesulfurization
are largely composed of molybdenum and tungsten sulfides
supported on alumina, and much effort has been directed
towards enhancing the efficiency of this process. It is difficult,
however, to achieve a molecular level understanding of the
catalytic cycle in view of the fact that the catalysts are
heterogeneous.4 For this reason, the reactivity of sulfur-contain-
ing compounds towards the metal centers of various molecular
systems has been the focus of much attention.5,6 However,
despite the fact that molybdenum and tungsten are the most
important components of HDS catalysts,1,2,3 the majority of these
studies has focused on precious metals such as rhodium, iridium,
palladium and platinum.5,6 Therefore, we describe here the first
report of desulfurization of thiophenes by molecular tungsten
compounds.
(PMe3)2(η2-CH2PMe2) (1), as illustrated in Scheme 1.9 The
formation of (η5-C4H5S)W(PMe3)2(η2-CH2PMe2) (1) repre-
sents the first example of thiophene C-S bond cleavage and hy-
drogen transfer by a molecular tungsten compound.10 Furthermore,
the butadiene-thiolate ligand of (η5-C4H5S)W(PMe3)2(η2-CH2-
PMe2) (1) may be hydrogenated at 60 °C to give the butanethiolate
complex, W(PMe3)4(SBun)H3 (2),11-13 which liberates but-1-ene
upon thermolysis at 100 °C (Scheme 1). As such, these observations
demonstrate that a molecular tungsten compound is capable of
achieving key steps in the hydrodesulfurization of thiophene.
It is pertinent to compare the above reactivity with that of the
corresponding molybdenum system14,15 because there are some
distinct differences. In particular, whereas only the butadiene-
thiolate complex (η5-C4H5S)W(PMe3)2(η2-CH2PMe2) (1) has
We have previously demonstrated that W(PMe3)4(η2-
CH2PMe2)H is a highly reactive molecule that is capable of
achieving a variety of bond activation reactions, as exemplified
by the cleavage of an aromatic C-C bond in quinoxaline.7 On this
basis, we have investigated the reactivity of W(PMe3)4(η2-
CH2PMe2)H towards thiophenes and have thereby discov-
ered that it is capable of cleaving the C-S bonds of thiophene, benzo-
thiophene and dibenzothiophene, as illustrated in Scheme 1.8 For
example, W(PMe3)4(η2-CH2PMe2)H reacts with thiophene at
60 °C to give the butadiene-thiolate complex (η5-C4H5S)W-
Received: December 8, 2010
Published: February 24, 2011
r
2011 American Chemical Society
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Scheme 2
Scheme 3
been isolated for the tungsten system, the reaction of Mo(PMe3)6
proceeds readily at room temperature to give the metallathia-
cycle, (κ2-C,S-C4H4S)Mo(PMe3)4, which rapidly converts to
the butadiene-thiolate complex (η5-C4H5S)Mo(PMe3)2(η2-
CH2PMe2) and the η5-thiophene derivative (η5-C4H4S)Mo-
(PMe3)3.14a,b Another interesting difference is that, whereas the
tungsten butadiene-thiolate complex (η5-C4H5S)W(PMe3)2-
(η2-CH2PMe2) (1) is readily hydrogenated to the butanethiolate
complex W(PMe3)4(SBun)H3 (2) at 60 °C, the molybdenum
counterpart (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2) does not re-
act with H2 under the same conditions. Thus, although the forma-
tion of the butadiene-thiolate complex from the reaction of thio-
phene with the molybdenum complex Mo(PMe3)6 is more facile
than that for the reaction of W(PMe3)4(η2-CH2PMe2)H, hydro-
genation of the butadiene-thiolate ligand occurs more readily
for the tungsten system.
In view of the fact that hydrogenation is an important compo-
nent of hydrodesulfurization, we have also investigated the reacti-
vity of the series of hydride complexes, W(PMe3)5H2, W(PMe3)4H4
and W(PMe3)3H6, towards thiophene (Schemes 2 and 3). Signifi-
cantly, these hydride complexes exhibit different reactivity from that
of W(PMe3)4(η2-CH2PMe2)H. For example, W(PMe3)5H2 reacts
with thiophene to give the κ1-thienyl complex, W(PMe3)4(κ1-CR-
C4H3S)H3 (3) (Scheme 2),16 a complex that is also generated by
the photochemical reaction of thiophene with W(PMe3)4H4
(Scheme 3).17,18 In contrast, thiophene reacts with W(PMe3)3H6
to give the butanethiolate complex W(PMe3)4(SBun)H3 (2), as
illustrated in Scheme 3.19
W(PMe3)4(κ1-CR-C4H3S)H3 (3) represents a kinetic product
that is also of sufficient thermodynamic stability to inhibit
access to an intermediate that is capable of achieving C-S
bond cleavage.
In support of this suggestion, treatment of Mo(PMe3)5H2
with d4-thiophene is accompanied by incorporation of hydrogen
into the R-position of free thiophene prior to formation of the
η5-thiophene and butadiene-thiolate complexes,14b whereas
no such exchange is observed for W(PMe3)5H2. Furthermore,
W(PMe3)4(κ1-CR-C4H3S)H3 (3) does not react with PMe3 to
regenerate W(PMe3)5H2. These observations, therefore, are
consistent with facile reversible oxidative addition of the C-H
bond of thiophene to molybdenum, but irreversible (on the time-
scale of the experiment) oxidative addition to tungsten.
Although the evidence presented above indicates that
W(PMe3)4(κ1-CR-C4H3S)H3 (3) does not undergo facile re-
ductive elimination of thiophene, elimination of thiophene may
be induced by treatment of W(PMe3)4(κ1-CR-C4H3S)H3 (3)
with H2, a reaction that is also accompanied by the formation
of W(PMe3)3H6 and the butanethiolate complex, W(PMe3)4-
(SBun)H3 (2), as illustrated in Scheme 2. Relative to W(PMe3)4-
(SBun)H3, the formation of W(PMe3)3H6 and thiophene is
inhibited by PMe3, thereby suggesting that the reductive elim-
ination of thiophene presumably occurs via a higher-valent
species such as W(PMe3)3(κ1-CR-C4H3S)H5, obtained by dis-
sociation of PMe3 and oxidative addition of H2. The formation of
the butanethiolate complex W(PMe3)4(SBun)H3 (2) in this
reaction is interesting because it demonstrates that although
W(PMe3)5H2 itself does not allow isolation of a product derived
from C-S bond cleavage, such a species may be obtained upon
subsequent treatment with H2 (Scheme 2).20
The fact that W(PMe3)4(η2-CH2PMe2)H and W(PMe3)5H2
behave differently towards thiophene suggests that the reactions do
not occur via a common [W(PMe3)5] intermediate. It is, therefore,
postulated that, whereas the reaction of W(PMe3)4(η2-CH2PMe2)H
proceeds via [W(PMe3)5], the reaction of W(PMe3)5H2 involves
initial dissociation of PMe3 to give 16-electron [W(PMe3)4H2]
followed by oxidative addition of a thiophene C-H bond. As such,
it is suggested that cleavage of the C-S bond, relative to a C-H
bond, is more favored for [W(PMe3)5] than [W(PMe3)4H2].
The formation of the κ1-thienyl complex W(PMe3)4(κ1-CR-
C4H3S)H3 (3) is also noteworthy because the corresponding
reaction of the molybdenum counterpart Mo(PMe3)5H2 with
thiophene does not yield Mo(PMe3)4(κ1-CR-C4H3S)H3, but rather
gives a mixture of the η5-thiophene and butadiene-thiolate
complexes, (η5-C4H4S)Mo(PMe3)3 and (η5-C4H5S)Mo(PMe3)2-
(η2-CH2PMe2).14b Thus, the molybdenum system exhibits a
greater propensity to cleave the C-S bond of thiophene
than does the tungsten system. We postulate that the origin
of this difference is that the tungsten κ1-thienyl complex
The reactivity of W(PMe3)4(η2-CH2PMe2)H, W(PMe3)5H2,
W(PMe3)4H4 and W(PMe3)3H6 towards other thiophenes has
also been investigated. For example, W(PMe3)4(η2-CH2PMe2)H
reacts with benzothiophene to give a mixture of products that
includes isomeric (κ1,η2-CH2CHC6H4S)W(PMe3)3(η2-CH2-
PMe2) (4) and (κ1,η2-CH2CC6H4S)W(PMe3)4 (5) (Scheme 1).
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Figure 1. Molecular structures of (κ1,η2-CH2CHC6H4S)W(PMe3)4H
(6) (left) and (κ1,η2-CH2CHC6H4S)W(PMe3)3(η2-CH2PMe2) (4)
(right).
Figure 3. Molecular structure of [(κ2-C12H8)W(PMe3)](μ-S)(μ-
CH2PMe2)(μ-PMe2)[W(PMe3)3] (9).
Interestingly, the corresponding reaction of W(PMe3)5H2 with
benzothiophene gives a different pair of isomers, namely (κ1,η2-
CH2CHC6H4S)W(PMe3)4H (6) and W(PMe3)4(κ1-CR-CCH-
SC6H4)H3 (7) (Scheme 2), that feature two more hydrogen atoms
than the products obtained from W(PMe3)4(η2-CH2PMe2)H.21
Thus, (κ1,η2-CH2CHC6H4S)W(PMe3)4H (6) is formally related
to (κ1,η2-CH2CHC6H4S)W(PMe3)3(η2-CH2PMe2) (4) by addi-
tion of H2 across the W-C bond, and is otherwise structurally very
similar (Figure 1).
ometallacyclopentadiene complex, [(κ2-C12H8)W(PMe3)](μ-S)-
(μ-CH2PMe2)(μ-PMe2)[W(PMe3)3] (9) with H2 at 60 °C
liberates biphenyl (Scheme 1).
Although we have not isolated any products from the reactions
of either W(PMe3)5H2, W(PMe3)4H4 or W(PMe3)3H6 with di-
benzothiophene, the latter two do, nevertheless, serve as effective
catalysts for the exchange of deuterium and hydrogen between di-
benzothiophene and C6D6.29 For example, W(PMe3)3H6 cata-
lyzes exchange into the R positions at 60 °C, the β and γ positions
at 80 °C, while the δ site remains undeuterated.30,31
(κ1,η2-CH2CHC6H4S)W(PMe3)3(η2-CH2PMe2) (4), (κ1,η2-
CH2CC6H4S)W(PMe3)4 (5) and (κ1,η2-CH2CHC6H4S)W-
(PMe3)4H (6) react with H2 at room temperature to give the
arylthiolate W(PMe3)4(SC6H4Et)H3 (8) (Scheme 1), which has
been structurally characterized by X-ray diffraction (Figure 2).
Furthermore, W(PMe3)4(SC6H4Et)H3 (8) is also obtained
upon treatment of benzothiophene with W(PMe3)4H4 and
W(PMe3)3H6, although the former requires photochemical acti-
vation (Scheme 3).22 Of most interest, however, upon heating,
W(PMe3)4(SC6H4Et)H3 (8) eliminates ethylbenzene (Scheme 1),
the principal product of hydrodesulfurization.23,24 As such, the
reactivity exhibited by this tungsten system provides a series of steps
by which benzothiophene may be hydrodesulfurized.13
In conclusion, the reactivity of W(PMe3)4(η2-CH2PMe2)H,
W(PMe3)5H2, W(PMe3)4H4 and W(PMe3)3H6, towards thio-
phenes has demonstrated that tungsten centers are capable of
achieving a variety of transformations relevant to hydrodesulfuri-
zation. For example, W(PMe3)4(η2-CH2PMe2)H cleaves the
C-S bond of thiophene to give the butadiene-thiolate complex,
(η5-C4H5S)W(PMe3)2(η2-CH2PMe2) (1), that may be subse-
quently hydrogenated to the butanethiolate complex, W(PMe3)4-
(SBun)H3 (2), which, upon heating, liberates but-1-ene. W(PMe3)4-
(η2-CH2PMe2)H also cleaves the C-S bond of benzothiophene to
give isomeric (κ1,η2-CH2CHC6H4S)W(PMe3)3(η2-CH2PMe2) (4)
and (κ1,η2-CH2CC6H4S)W(PMe3)4 (5). Both of these complexes
may be hydrogenated at room temperature to give the arylthio-
late W(PMe3)4(SC6H4Et)H3 (8) which, upon heating, liberates
the hydrodesulfurization product, ethylbenzene. Finally, W(PMe3)4-
(η2-CH2PMe2)H is also capable of desulfurizing dibenzothiophene
to form a dibenzometallacyclopentadiene complex, [(κ2-C12H8)W-
(PMe3)](μ-S)(μ-CH2PMe2)[W(PMe3)3] (9), which liberates
biphenyl upon heating in the presence of H2. The structural
characterization of these compounds, together with their inter-
conversions, provides a foundation for analyzing the reactions
that occur on tungsten-based HDS catalysts.
Significantly, W(PMe3)4(η2-CH2PMe2)H is also capable of
desulfurizing dibenzothiophene, a compound that is most resistant
to hydrodesulfurization.5 Specifically, W(PMe3)4(η2-CH2PMe2)H
reacts with dibenzothiophene to give the dinuclear dibenzome-
tallacyclopentadiene complex, [(κ2-C12H8)W(PMe3)](μ-S)
(μ-CH2PMe2)(μ-PMe2)[W(PMe3)3] (9) (Scheme 1),25 which
has been structurally characterized by X-ray diffraction (Figure 3).
While C-S bond cleavage of dibenzothiophene is precedented,
reactions that involve desulfurization are rare.5,26,27 Indeed, there
is only one other structurally characterized complex listed in the
Cambridge Structural Database that features a κ2-biphenyldiyl
ligandderived from dibenzothiophene, namely a nickel compound
synthesized by Jones.26,28 In addition, treatment of the dibenz-
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, computa-
b
tional data and crystallographic data in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
parkin@columbia.edu
’ ACKNOWLEDGMENT
We thank the U.S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-93ER14339) for support of this
Figure 2. Molecular structure of W(PMe3)4(SC6H4Et)H3 (8).
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research. The National Science Foundation (CHE-0840451) is
thanked for acquisition of an NMR spectrometer.
Inorg. Chim. Acta 2010, 10.1016/j.ica.2010.11.020. (c) Buccella, D.;
Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2008, 130, 16187–16189.
(15) For related studies, see: (a) Churchill, D. G.; Bridgewater,
B. M.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 178–179. (b) Churchill,
D. G.; Bridgewater, B. M.; Zhu, G.; Pang, K.; Parkin, G. Polyhedron 2006,
25, 499–512.
(16) For another example of selective C-H bond cleavage of thio-
phene by tungsten, see: Sakaba, H.; Yumoto, T.; Watanabe, S.; Kabuto, C.;
Kabuto, K. Chem. Lett. 2003, 32, 14–15.
(17) The reaction is also accompanied by the formation of
W(PMe3)3H6 due to reaction of W(PMe3)4H4 with the H2 that is liberated.
(18) W(PMe3)4H4 also serves as a catalyst for the photochemical
exchange of hydrogen and deuterium between both the R- and β-
positions of thiophene and C6D6. The isotopic exchange, however, is
inhibited by PMe3, which suggests that the reaction may occur via
reversible oxidative addition of benzene and thiophene C-H(D) bonds
to 16-electron [W(PMe3)3H4], rather than to [W(PMe3)4H2].
(19) This transformation is necessarily accompanied by ligand
redistribution. In an attempt to increase the yield of W(PMe3)4-
(SBun)H3, the reaction between W(PMe3)3H6 and thiophene was
performed in the presence of PMe3. However, under these conditions,
only W(PMe3)4H4 (which is thermally unreactive towards thiophene)
was obtained.
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(27) (a) Garcia, J. J.; Maitlis, P. M. J. Am. Chem. Soc. 1993,
115, 12200–12201. (b) Garcia, J. J.; Mann, B. E.; Adams, H.; Bailey,
N. A.; Maitlis, P. M. J. Am. Chem. Soc. 1995, 117, 2179–2186. (c) Matsubara,
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(28) For a ruthenium complex derived from dibenzothiophene in
which the biphenyldiyl fragment coordinates to two metal centers, see:
Chehata, A.; Oviedo, A.; Arꢀevalo, A.; Bernꢁes, S.; García, J. J. Organome-
tallics 2003, 22, 1585–1587.
(29) W(PMe3)4H4 requires photochemical activation to catalyze the
exchange.
(30) The R site is closest to the sulfur atom.
(11) Evidence for the presence of the butanethiolate ligand is provided
by the fact that treatment of W(PMe3)4(SBun)H3 with PhCO2H imme-
diately liberates BunSH.
(12) For other examples of hydrogenation of thiophene and benzothio-
phene to butanethiolate and ethylphenylthiolate ligands, respectively, see:
(a) Vicic, D. A.; Jones, W. D. Organometallics 1997, 16, 1912–1919. (b)
Rosini, G. P.; Jones, W. D. J. Am. Chem. Soc. 1992, 114, 10767–10775.
(13) For examples of desulfurization of thiophene and benzothio-
phene by other metals, see ref 14b and references therein.
(31) In support of these observations, DFT calculations on the
computationally simpler system in which the methyl groups of the PMe3
ligands are replaced by hydrogen atoms indicate that the δ isomer of
W(PH3)4(κ1-C-C6H3SC6H4)H3 is more than 10 kcal mol-1 less stable
than the R, β and γ isomers.
(14) (a) Janak, K. E.; Tanski, J. M.; Churchill, D. G.; Parkin, G. J. Am.
Chem. Soc. 2002, 124, 4182–4183. (b) Sattler, A.; Janak, K. E.; Parkin, G.
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