Modeling aspects of hydrodesulfurization by molybdenum hydride compounds: Desulfurization of thiophene and benzothiophene and C-S bond cleavage of dibenzothiophene

Article abstract of DOI:10.1016/j.ica.2010.11.020

The molybdenum hydride complexes Mo(PMe₃)₅H ₂ and Mo(PMe₃)₄H₄ are capable of cleaving the C-S bonds of thiophene, benzothiophene and dibenzothiophene. For example, Mo(PMe₃)₅H₂ reacts with thiophene to give the η⁵-thiophene and butadiene-thiolate complexes, (η⁵-C₄H₄S)Mo(PMe₃)₃ and (η⁵-C₄H₅S)Mo(PMe₃) ₂(η²-CH₂PMe₂). These complexes are also obtained from the reaction between Mo(PMe₃) ₄H₄ and thiophene under photochemical conditions, whereas at elevated temperatures thiophene is desulfurized to liberate but-1-ene. Similarly, Mo(PMe₃)₄H₄ desulfurizes benzothiophene at elevated temperatures to liberate ethylbenzene, while the arylthiolate complex Mo(PMe₃)₄(SC₆H ₄Et)H₃ is obtained photochemically. Furthermore, Mo(PMe₃)₄H₄ cleaves the C-S bond of dibenzothiophene to give [η⁶,κ¹-C ₆H₅C₆H₄S]Mo(PMe₃) ₂H.

Full text of DOI:10.1016/j.ica.2010.11.020

Inorganica Chimica Acta 369 (2011) 197–202
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Modeling aspects of hydrodesulfurization by molybdenum hydride compounds:
Desulfurization of thiophene and benzothiophene and C–S bond cleavage of
dibenzothiophene
⇑
Aaron Sattler, Kevin E. Janak, Gerard Parkin
Department of Chemistry, Columbia University, New York, NY 10027, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 23 November 2010
The molybdenum hydride complexes Mo(PMe ) H and Mo(PMe ) H4 are capable of cleaving the C–S
bonds of thiophene, benzothiophene and dibenzothiophene. For example, Mo(PMe ) H reacts with thi-
3 5 2
3
5
2
3 4
5
5
5
ophene to give the
g
-thiophene and butadiene–thiolate complexes, (
-CH PMe ). These complexes are also obtained from the reaction between Mo
and thiophene under photochemical conditions, whereas at elevated temperatures thiophene
is desulfurized to liberate but-1-ene. Similarly, Mo(PMe desulfurizes benzothiophene at elevated
temperatures to liberate ethylbenzene, while the arylthiolate complex Mo(PMe (SC Et)H is
4 4 3 3
g -C H S)Mo(PMe ) and (g -
Dedicated with respect to Professor Robert
G. Bergman in recognition of his numerous
pivotal contributions to organometallic
chemistry.
2
4
C H
5
S)Mo(PMe
3
2
) (
g
2
2
3 4 4
(PMe ) H
3 4 4
) H
)
3 4
H
6 4
3
Keywords:
Hydrodesulfurization
Molybdenum
obtained photochemically. Furthermore, Mo(PMe
[g ,j -C H C H S]Mo(PMe ) H.
6 5 6 4 3 2
3
)
H
4 4
cleaves the C–S bond of dibenzothiophene to give
6
1
Ó 2010 Elsevier B.V. All rights reserved.
Thiophene
Benzothiophene
Dibenzothiophene
Hydrodesulfurization (HDS) is the important industrial process
by which sulfur-containing impurities are removed from crude
petroleum feedstocks prior to their use as fuels [1–8].1 As such, con-
siderable effort hasbeen directed towardsobtaining an understanding
of this complex process at a mechanistic level by investigating the
reactivity of various sulfur-containing compounds, and thiophenes
observed that (
g
5-C
) can also be obtained via the reaction of Mo
with thiophene (Scheme 1). Furthermore, Mo(PMe
reacts with benzothiophene at 60 °C to give, inter alia, the 1-metal-
4 4 3 3 4 5 3 2
H S)Mo(PMe ) and (g H S)Mo(PMe )
5-C
2
(g
2 2
-CH PMe
,2
3
(PMe )
H
5 2
3 5 2
) H
1
2
lacyclopropene-thiophenolate complex
(PMe , as illustrated in Scheme 1, a complex that is also obtained
from the reaction of Mo(PMe with benzothiophene [13].
Monitoring of the reaction between Mo(PMe and thio-
phene by H NMR spectroscopy provides evidence for a paramag-
2 6 4
(j ,g -CH CC H S)Mo
3 4
)
2
in particular, towards various metal centers [9]. In this regard, we
3 6
)
have focused our attention on the reactivity of thiophenes towards
molybdenum compounds because molybdenum is an important com-
ponent of HDS catalysts.1 For example, we have demonstrated that
3 5 2
) H
1
,2
5
netic intermediate prior to formation of (
g
-C
). The same intermediate
can also be observed in the reaction between Mo(PMe and thio-
phene (Scheme 2), and since the reaction with Mo(PMe is much
faster than the corresponding reaction with Mo(PMe , the
4 4 3 3
H S)Mo(PMe )
5
2
[
Me Si(C Me
2 5 4
) ]MoH
2
2
[10,11] and Mo(PMe
3
)
6
[12,13] are capable of
4 5 3 2 2 2
and (g -C H S)Mo(PMe ) (g -CH PMe
reacting with thiophenes to give derivatives that feature a variety of
coordination modes, as illustrated in Fig. 1. Since an essential
component of HDS involves hydrogenation, we sought to extend our
3 6
)
)
3 6
) H
3 5 2
investigation of the trimethylphosphine compound Mo(PMe
examining the reactivity of thiophenes towards related hydride com-
plexes, namely Mo(PMe and Mo(PMe
We have previously reported that Mo(PMe
3
)
6
by
intermediate can be isolated if the reaction mixture is immediately
cooled to À15 °C before it has the opportunity to convert to
5
5
2
)
3 5
H
2
3
4
) H
4
.
4 4 3 3 4 5 3 2 2 2
(g -C H S)Mo(PMe ) and (g -C H S)Mo(PMe ) (g -CH PMe ).
3
)
6
reacts with thio-
Significantly, the paramagnetic complex has been identified as
5
2
phene to give the
g
-thiophene and butadiene–thiolate complexes,
octahedral (
accord with (
j
-C
-C
4
H
4
S)Mo(PMe
S)Mo(PMe
S)Mo(PMe
3
)
4
by X-ray diffraction (Fig. 2). In
5
5
2
2
(g
-C
4
4
H S)Mo(PMe
3
)
3
and
(g
-C
4 5
H
S)Mo(PMe
3
)
2
(g
-CH
2
PMe
2
),
j
4
H
-C
4
3
)
4
being an intermediate, it rapidly
5
5
respectively [12]. As an extension of this study, we have now
converts to
(g
4
H
4
)
3 3
and
(g
-C
4
H
5
S)Mo(PMe
3
)
2
2
5
(g
-CH
2
PMe
g
2
) at room temperature. Interestingly, the
g
-thiophene
5
complex (
the absence of PMe
the favored product in the presence of PMe
-C
4
H
4
S)Mo(PMe
3 3
) is formed very selectively (>95%) in
⇑
Corresponding author.
E-mail address: parkin@columbia.edu (Gerard Parkin).
For recent reviews, see Refs. [1–4].
5
3
, whereas the
g
-butadiene–thiolate complex is
. It is, therefore, suggested
1
2
3
2
5
For review articles concerned with modeling HDS, see Refs. [5–8].
4 4 3 4 4 4 3 3
that conversion of (j -C H S)Mo(PMe ) to (g -C H S)Mo(PMe )
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.020

1
98
A. Sattler et al. / Inorganica Chimica Acta 369 (2011) 197–202
3 6 2 5 4 2 2
Fig. 1. Compounds derived from the reactions of Mo(PMe ) and [Me Si(C Me ) ]MoH with thiophenes.
occurs via initial reversible dissociation of PMe
PMe occurs after (orpossiblyduring) therate determining step inthe
transformation that gives the
mechanistic scenario is consistent with the substantial temperature
dependence of the product distribution [12], in which the relative
3
, while dissociation of
g
5-thiopheneligandof (
tion of Mo(PMe with d
incorporation for the corresponding reaction of Mo(PMe
feature is that the reaction between Mo(PMe and d
is accompanied by incorporation of hydrogen into the
g
5-C
4
H
4
S)Mo(PMe
-thiophene, whereas there is negligible
. Another
-thiophene
-position of
free thiophene, thereby suggesting the existence of an additional
pathway that involves reversible -C–H bond cleavage prior to
3 3
) obtainedfrom the reac-
3
3
5
) H
2
4
5
g
-butadiene–thiolate complex. This
3 6
)
)
3 5
H
2
4
a
5
5
amount of (
g
-C H S)Mo(PMe )
4 4 3 3
increases with temperature due to
prior
a favorable entropy term resulting from dissociation of PMe
to the rate determining step.
Another interesting feature of this system is that the formation
-thiophene complex by treatment of Mo(PMe
ophene does not simply proceed by displacement of PMe
3
a
5
5
formation of either (
4 4 3 3 4 5 3 2
g -C H S)Mo(PMe ) or (g -C H S)Mo(PMe )
2
(g
-CH
The above observations suggest that there are mechanistic dif-
ferences pertaining to the reactions of thiophene with Mo(PMe
and Mo(PMe . In this regard, while Mo(PMe can generate
[Mo(PMe ] as a viable 16-electron intermediate, Mo(PMe
can generate both [Mo(PMe ] and the dihydride [Mo(PMe
via reductive elimination of H and dissociation of PMe , respec-
tively. For example, the conversion of Mo(PMe to Mo(PMe
upon treatment with PMe is an example of reductive elimination
of H [15], while the conversion of Mo(PMe to Mo(PMe
upon treatment with H is an example of a reaction that occurs
via dissociation of PMe [16]. The subtle differences in the reactiv-
ity of Mo(PMe and Mo(PMe towards thiophene are, there-
fore, suggested to be a consequence of the ability of the latter to
access [Mo(PMe ]. For example, [Mo(PMe ] provides a
simple means by which facile H incorporation into the -position
2 2
PMe ).
5
of the
g
3
)
6
with thi-
ligands
3
3 6
)
by thiophene, but rather occurs via a more complex mechanism
3
5
) H
2
3 6
)
that involves reversible C–S bond cleavage. Thus, C–S bond cleav-
3
)
5
3 5 2
) H
) H ]
3 4 2
5
age of thiophene facilitates
g
-coordination, a counter-intuitive
3
)
5
3
result that may be of relevance to other systems [14]. Also of note,
in the presence of protiothiophene (C S), the d -isotopologue
S)Mo(PMe
2
3
4
H
4
4
3
)
H
5 2
3 6
)
2
5
(j
-C
4
D
4
S)Mo(PMe
3
)
4
converts selectively to (
g
-C
4
D
4
3
)
3
,
3
5
thereby demonstrating that the transformation to the
g
-thiophene
2
3
)
5
H
2
3 4 4
) H
complex does not involve complete reductive elimination of the C–S
bond and release of thiophene. Thus, it is postulated that reductive
2
3
x
coupling of the C–S bond leads directly to an
g
-thiophene adduct
3
)
6
3 5 2
) H
5
that subsequently transforms to the
WhilethereactivityofMo(PMe
ophene are similar, there are some interestingdifferences. For exam-
ple, the reaction of Mo(PMe with d -thiophene is accompanied by
the formation of an isotopomer of (
CH PMe ) in whichthe hydrogen from the PMe
selectively to the carbon adjacent to the sulfur of the
thiolate ligand, whereas the corresponding reaction of Mo(PMe
g
-thiophene derivative.
3
)
6
andMo(PMe towardsthi-
3
)
5
H
2
3
)
H
4 2
3 4 2
) H
1
a
3
)
6
4
of deuterothiophene can be achieved, and thereby result in differ-
ent selectivities for the H incorporation into the
g
5
2
1
5
g
-C
4
D
4
HS)Mo(PMe
3
)
2
(g
-
g
-thiophene and
5
2
2
3
ligand is transferred
-butadiene–thiolate ligands of the products derived from the
5
g
-butadiene–
reactions with d
4
-thiophene, as compared to that for Mo(PMe
and thio-
does not react with thiophene
under mild conditions. However, treatment of Mo(PMe with
3 6
) .
)
3 5
H
2
In contrast to the facile reaction between Mo(PMe
3 5 2
) H
1
is accompanied by hydrogen ( H) incorporation into other sites
phene, the tetrahydride Mo(PMe
3 4 4
) H
5
1
of the
g
-butadiene–thiolate ligand (Scheme 3). Furthermore,
H
3 4 4
) H
incorporation (mainly into the
a
-site)4 is also observed for the
d
4
-thiophene does result in incorporation of hydrogen into the
6
a-position of free thiophene at 100 °C, analogous to the reaction
3
C–S cleavage of 2-cyanothiophene has been shown to occur prior to isomerization
a thienyl–cyanide derivative, thereby providing another example of facile
to
5
2H NMR spectroscopy demonstrates that deuterium is incorporated into the PMe
3
reversibility of the C–S cleavage of thiophenes. See Ref. [14].
4
1
The identification of the H NMR spectroscopic signals of the
a
- and b-sites of the
ligands, in addition to the hydride site of Mo(PMe
3
)
4
4
H , which is also formed during
5
5
g
-thiophene ligand of
(
g
-C
H
4 4
S)Mo(PMe
3
)
3
were identified by using 2-D-d
1
-
the reaction.
6
thiophene.
The deuterium is incorporated into both hydride and PMe sites.
3

A. Sattler et al. / Inorganica Chimica Acta 369 (2011) 197–202
199
Scheme 2.
Scheme 1.
with Mo(PMe
Mo(PMe converts thiophene to, inter alia, but-1-ene (Scheme 4),
thereby demonstrating that the system is capable of C–S bond cleav-
age of thiophene, in addition to hydrogen transfer. C hydrocarbons
3 5 2
) H . Furthermore, at higher temperatures (120 °C),
7
3 4 4
) H
4
are the principal products of thiophene hydrodesulfurization and
the formation of butenes has also been observed upon reaction of thi-
ophene with a molecular molybdenum–cobalt–sulfide cluster in the
presence of H
pounds formed from the thermal reaction between Mo(PMe
and thiophene have not been identified, other than the formation of
2
[17,18]. Although the molybdenum containing com-
3 4 4
) H
5
small quantities of the butadiene–thiolate complex, (
g
-C
4 5
H S)Mo
2
(
PMe
3 2 2 2 3 4 4
) (g -CH PMe ), Mo(PMe ) H reacts with thiophene under
5
photochemical activation to generate (
(
g
-C
4 4 3 3
H S)Mo(PMe ) and
5
2
g
-C
mixture of products (Scheme 5).
Similar to the reaction of thiophene, Mo(PMe
C–S bonds of benzothiophene at 120 °C to liberate ethylbenzene
4
H
5
S)Mo(PMe
3
2
) (
g
-CH
2
PMe
2
)
as two components of a
3
)
H
4 4
cleaves both
2-C
Fig. 2. Molecular structure of (
j
4 4 3 4
H S)Mo(PMe ) .
8
(
Scheme 4) which is a principal product of hydrodesulfurization
[
19]. The desulfurization of thiophene and benzothiophene de-
scribed here is noteworthy because such transformations are not
unknown for mononuclear molybdenum compounds. While the
molybdenum containing compounds from the thermal reaction be-
well precedented in homogeneous systems [18,20–26],9 and are
,10
tween Mo(PMe
the photochemical reaction generates, inter alia, the arylthiolate
complex, Mo(PMe (SC Et)H [13], the formation of which is im-
proved in the presence of H (Scheme 5).
Mo(PMe also reacts with dibenzothiophene to give [
S]Mo(PMe H which is a result of C–S bond cleavage and
3 4 4
) H and benzothiophene have not been identified,
7
The formation of but-1-ene is not prevented if the reaction is carried out in the
)
3 4
H
6 4
3
presence of mercury.
8
2
The formation of ethylbenzene is not prevented in the presence of mercury.
For desulfurization of thiophene, see Refs. [20–22].
For desulfurization of benzothiophene, see Refs. [23–26].
g j
6, 1-
9
3
)
H
4
4
10
C H
6 5
C H
6 4
3 2
)

2
00
A. Sattler et al. / Inorganica Chimica Acta 369 (2011) 197–202
Scheme 3.
Scheme 4.
hydrogen transfer (Scheme 5). The molecular structure of [
S]Mo(PMe H has been determined by X-ray diffraction,
as illustrated in Fig. 3.
The formation of [
because it represents the first example of cleavage of the C–S bond
of dibenzothiophene by a molybdenum compound. In this regard,
g
6,
j
1-
complex [
(PMe with 2,6-Ph
the structures of [
(Ph)O]Mo(PMe
Mo–O and C–O bonds allow the arylthiolate group to lie closer to
g
6,
j
1-C
6
H
5
C
6
H
3
(Ph)O]Mo(PMe
3 2
) H via reaction of Mo
C H
6 5
C H
6 4
3
)
2
3
)
6
2 6 3
C H OH [39]. The most notable difference in
6
1
6
1
g
,
j
-C
6 5 6 4 3 2 6
H C H S]Mo(PMe ) H and [g ,j -C
g
6,
j
1-C
6
H
5
C H
6
4
S]Mo(PMe
3
)
2
H is noteworthy
H C
5
6
H
3
3 2
) H is that the longer Mo–S and C–S versus
6
the plane of the
ipso carbon of the
(PMe H is closer to planarity (
two crystallographically independent molecules) than is that in
g -arene moiety, as illustrated in Fig. 4. Thus, the
6
6
1
we previously reported that although [Me
capable of cleaving the C–S bonds of thiophene and benzothio-
phene, [Me Si(C Me ]MoH reacts with dibenzothiophene to give
-adduct, [Me Si(C
2
Si(C
5
4
Me )
2
]MoH
2
is
g
-arene moiety of [
6 5 6 4
g ,j -C H C H S]Mo
3
)
2
R
C–C–C = 359.8° and 360.0° for
2
5
4
)
2
2
1
1
6
1
only the
j
2
5
4
Me )
2
]Mo(j
-DBT) [10]. C–S bond
[
g
,
j
-C
ingly, the
(PMe H (171.4° and 173.2° for two crystallographically indepen-
dent molecules) is closer to 180° than is the value for [
(Ph)O]Mo(PMe H (164.3°).
In summary, the molybdenum hydride complexes Mo
(PMe and Mo(PMe are capable of cleaving the C–S bonds
of thiophene, benzothiophene and dibenzothiophene under vary-
ing conditions. Of particular note, Mo(PMe is capable of
6 5 6 3 3 2
H C H (Ph)O]Mo(PMe ) H (RC–C–C = 357.4°). Correspond-
g -C6(centroid)–Cipso–Caryl angle for [g ,j -C H C H S]Mo
6 5 6 4
6
6
1
cleavage of dibenzothiophene has, nevertheless, been observed
for other metals but is not typically accompanied by subsequent
hydrogen transfer [27–34].
3 2
)
6
1
g ,j -
For comparison purposes, there are no other examples of struc-
turally characterized complexes listed in the Cambridge Structural
C
6
H
5
C
6
H
3
3 2
)
6
1
database that feature [
g
,j
-C
6
H C
5 6
4
H S] ligands, although there
3
)
5
H
2
3 4 4
) H
are several examples of molybdenum compounds that possess re-
6
1
lated [g
,j
-C
H
6 5
ArS] ligands. These compounds, however, are not
3 4 4
) H
obtained via C–S bond cleavage of dibenzothiophene derivatives,
but are rather obtained via reactions with the arylthiolate ligand
desulfurizing thiophene and benzothiophene to liberate but-1-
ene and ethylbenzene, respectively, while it cleaves the C–S bond
of dibenzothiophene to give [g ,j -C H C H S]Mo(PMe ) H.
6 5 6 4 3 2
6
1
[
35–38]. We have also synthesised the closely related aryloxide

A. Sattler et al. / Inorganica Chimica Acta 369 (2011) 197–202
201
Scheme 5.
6 5 6 4 3 2
g j H C H S]Mo(PMe ) H
1-C
Fig. 4. Comparison of the molecular structures of [
6,
Fig. 3. Molecular structure of [
g
6,
j
1-C
H
6 5
C
6
4
H S]Mo(PMe
3
2
) H.
6
1
(
blue) and [g ,j -C H C H (Ph)O]Mo(PMe ) H (red). (For a color version of this
6 5 6 3 3 2
figure, the reader is referred to the web version of this article.)
Acknowledgement
Appendix A. Supplementary material
We thank the US Department of Energy, Office of Basic Energy
Sciences (DE-FG02-93ER14339) for support of this research.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.ica.2010.11.020.

2
02
A. Sattler et al. / Inorganica Chimica Acta 369 (2011) 197–202
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