Reactivity of Mo(PMe3)6 towards benzothiophene and selenophenes: New pathways relevant to hydrodesulfurization

Article abstract of DOI:10.1021/ja8070545

Mo(PMe₃)₆ cleaves a C-S bond of benzothiophene to give (κ²-CHCHC₆H₄S)Mo(PMe₃)₄, which rapidly isomerizes to the olefin-thiophenolate and 1-metallacyclopropene-thiophenolate complexes, (κ¹,η²-CH₂CHC₆H₄S)Mo(PMe₃)₃(η²-CH₂PMe₂) and (κ¹,η²-CH₂CC₆H₄S)Mo(PMe₃)₄. The latter two molecules result from a series of hydrogen transfers and are differentiated according to whether the termini of the organic fragments coordinate as olefin or η²-vinyl ligands, respectively. The reactions between Mo(PMe₃)₆ and selenophenes proceed differently from those of the corresponding thiophenes. For example, whereas Mo(PMe₃)₆ reacts with thiophene to give η⁵-thiophene and butadiene-thiolate complexes, (η⁵-C₄H₄S)Mo(PMe₃)₃ and (η⁵-C₄H₅S)Mo(PMe₃)₂(η₂-CH₂PMe₂), selenophene affords the metallacyclopentadiene complex [(κ²-C₄H₄)Mo(PMe₃)₃(Se)]₂[Mo(PMe₃)₄] in which the selenium has been completely abstracted from the selenophene moiety. Likewise, in addition to (κ¹,η²-CH₂CC₆H₄Se)Mo(PMe₃)₄ and (κ¹,η²-CH₂CHC₆H₄Se)Mo(PMe₃)₃(η²-CH₂PMe₂), which are counterparts of the species observed in the benzothiophene reaction, the reaction of Mo(PMe₃)₆ with benzoselenophene yields products resulting from C-C coupling, namely [κ²,η⁴-Se(C₆H₄)(CH)₄(C₆H₄)Se]Mo(PMe₃)₂ and [μ-Se(C₆H₄)(CH)C(CH)₂(C₆H₄)](μ-Se)[Mo(PMe₃)₂][Mo(PMe₃)₂H]. Copyright

Full text of DOI:10.1021/ja8070545

Published on Web 11/08/2008
Reactivity of Mo(PMe3)6 towards Benzothiophene and Selenophenes: New
Pathways Relevant to Hydrodesulfurization
Daniela Buccella, Kevin E. Janak, and Gerard Parkin*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received September 5, 2008; E-mail: parkin@columbia.edu
Scheme 1
Hydrodesulfurization (HDS) is the critical industrial process by
which sulfur is removed from compounds present in crude petroleum
feedstocks prior to their use as fuel.1,2 Of these compounds, thiophenes
belong to the class of molecules that are most resistant towards HDS,
such that they feature prominently in model studies to provide
mechanistic insight into the industrial process.2,3 Since molybdenum
is an essential component of an HDS catalyst, it is of particular
relevance to define its coordination chemistry with respect to thiophenes.
In this regard, we have demonstrated that [Me2Si(C5Me4)2]MoH2 and
Mo(PMe3)6 exhibit interesting reactivity towards thiophene, including
examples of κ1-coordination, η5-coordination, C-S bond cleavage,
and formation of a butadiene-thiolate derivative.4,5 Here we describe
the reactivity of Mo(PMe3)6 towards benzothiophene and selenophenes
that reveals new pathways which are relevant to mechanisms of
hydrodesulfurization.
We previously demonstrated that Mo(PMe3)6 reacts with thiophene
to give the isomeric η5-thiophene and butadiene-thiolate com-
plexes, (η5-C4H4S)Mo(PMe3)3 and (η5-C4H5S)Mo(PMe3)2(η2-
CH2PMe2), respectively. We now report that the corresponding
reaction of Mo(PMe3)6 with benzothiophene follows a different
course, thereby increasing our knowledge of the array of organic
species that may exist and interconvert on the surface of an HDS
catalyst. Specifically, Mo(PMe3)6 cleaves the C-S bond of ben-
zothiophene to give paramagnetic (κ2-CHCHC6H4S)Mo(PMe3)4 (1),
which rapidly isomerizes to the olefin-thiophenolate and 1-metallacy-
clopropene-thiophenolate complexes, (κ1,η2-CH2CHC6H4S)Mo-
(PMe3)3(η2-CH2PMe2) (2) and (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3),
as illustrated in Scheme 1. While (κ2-CHCHC6H4S)Mo(PMe3)4 (1)
is simply the product of C-S bond cleavage,6 (κ1,η2-CH2CHC6H4S)-
Mo(PMe3)3(η2-CH2PMe2) (2) and (κ1,η2-CH2CC6H4S)Mo(PMe3)4
(3) are the results of more complex sequences involving hydrogen
transfers, such that the termini of the thiolate fragments coordinate
Via η2-olefin7 and η2-vinyl8,9 modes, respectively. Of particular
note, the latter coordination mode is without precedent for
complexes derived from benzothiophene.10
lacyclopropene (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) is favored at high
temperatures (80 °C). This situation reflects a kinetic preference
because the isolated complexes do not interconvert thermally on a
comparable time scale.12 Furthermore, the relative amounts of the two
isomers are influenced by the presence of PMe3, such that the formation
of the metallacyclopropene (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) is
inhibited by increasing the concentration of PMe3. These observations
indicate that an important difference in the mechanisms for formation
of (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) and (κ1,η2-CH2CHC6H4S)Mo-
(PMe3)3(η2-CH2PMe2) (2) is that isomerization to the metallacyclo-
propene (3) requires a sequence involving reversible dissociation of a
PMe3 ligand prior to the rate-determining step.
Deuterium labeling studies employing d1-2-D-benzothiophene
and d1-3-D-benzothiophene demonstrate that a methylene hydrogen
atom of both (κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2)
and (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) is derived from a PMe3
ligand. Furthermore, the reaction of Mo(PMe3)6 with d1-2-D-
benzothiophene at room temperature yields the isotopomer of (κ1,η2-
CHDCHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2) in which the deute-
rium is selectively located cis to the hydrogen on the adjacent carbon
The composition of the thiolate ligand of (κ1,η2-CH2CHC6H4S)-
Mo(PMe3)3(η2-CH2PMe2) (2) bears a close analogy to that of (η5-
C4H5S)Mo(PMe3)2(η2-CH2PMe2), in the sense that both are a result
of C-S cleavage and hydrogen transfer. An important difference,
however, resides with their coordination modes. Thus, whereas the
ligand derived from thiophene coordinates in a flat η5-manner (i.e., as
a butadiene-thiolate ligand), that derived from benzothiophene
coordinates in a puckered κ1,η2-manner (i.e., as an olefin-thiolate
ligand).11 As a corollary of the different coordination modes, (κ1,η2-
CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2) possesses an additional
PMe3 ligand to maintain an 18-electron configuration.
The relative amounts of (κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-
CH2PMe2) (2) and (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) in the product
mixture depend critically on the reaction conditions. For example,
(κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2) is favored if the
reaction is performed at low temperatures (10 °C), while the metal-
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 16187–16189 16187

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
atom.13,14 This labeling pattern is consistent with a mechanism that
involves oxidative addition of C-H bond of a PMe3 ligand,
followed by C-H bond reductive elimination of
a
{(κ2-
CHCHC6H4S)MoH} intermediate.15,16
Both (κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2) and
(κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3) react with H2 at room temper-
ature to yield Mo(PMe3)4(SC6H4Et)H (4),17,18 thereby achieving
partial hydrogenation of benzothiophene on a molybdenum center.
The relative reactivity of the two isomers, however, differ consider-
ably. Thus, whereas hydrogenation of the former occurs within
minutes, hydrogenation of the latter requires a period of days to
proceed to completion.
The reactivity of Mo(PMe3)6 towards selenophene derivatives
is pertinent towards understanding the mechanisms of HDS because
the products obtained could correspond to different stages of the
reaction coordinate for the thiophene system. As such, an analysis
of the reactivity of the selenophene systems furnishes insight into
the types of transformations that may be achieved by molybdenum
as a component of an HDS catalyst surface. It is, therefore,
interesting that Mo(PMe3)6 reacts with selenophene to give the
metallacyclopentadiene complex [(κ2-C4H4)Mo(PMe3)3(Se)]2-
[Mo(PMe3)4] (5) in which the selenium has been completely
abstracted from the selenophene moiety (Scheme 2).19,20
X-ray diffraction demonstrates that [(κ2-C4H4)Mo(PMe3)3-
(Se)]2[Mo(PMe3)4] (5) possesses an almost linear chain of
Mo-Se-Mo-Se-Mo atoms (Figure 1), in which the Mo-Se bond
lengths for the central molybdenum atom [2.4617(4) and 2.4712(4)
Å] are longer than those for the outer molybdenum atoms [2.3472(4)
and 2.3512(4) Å]. These Mo-Se bond lengths are comparable to
the terminal ModSe bonds in trans-Mo(PMe3)4(Se)2 [2.381(1) Å
and 2.385(1) Å]21 but considerably shorter than a Mo-Se single
bond;22 as such, it is evident that there is a degree of multiple
bonding in the Mo-Se-Mo-Se-Mo chain. An analysis of the
π-molecular orbitals indicates that there is a more significant
π-bonding component for the outer Mo-Se bonds because the in-
phase combinations of selenium 4pz and 4py orbitals of the two
selenium atoms are of appropriate symmetry to interact with 4d
orbitals of the outer molybdenum atoms but are of incorrect
symmetry for the 4dxz and 4dyz orbitals of the central molybde-
num.23,24
The ability of molybdenum to extract selenium from selenophene
and generate the κ2-butadienediyl C4H4 ligand suggests that a similar
process could occur for thiophene on an HDS catalyst surface. The
synthesis of [(κ2-C4H4)Mo(PMe3)3(Se)]2[Mo(PMe3)4] (5), therefore,
indicates that consideration should also be given to HDS mechanisms
that feature cleavage of both C-S bonds prior to hydrogenation. In
this regard, Jones has reported an interesting nickel system where
dibenzothiophene (but not benzothiophene or thiophene) undergoes
desulfurization without any form of hydrogenation.25,26
The reaction of Mo(PMe3)6 with benzoselenophene is
considerably more complex than the reaction with benzothio-
phene, with four products having been isolated. Thus, in addi-
tion to (κ1,η2-CH2CHC6H4Se)Mo(PMe3)3(η2-CH2PMe2) (6) and
(κ1,η2-CH2CC6H4Se)Mo(PMe3)4 (7), which correspond to
two of the species observed in the benzothiophene reaction,
products resulting from C-C coupling, namely [κ2,η4-Se(C6H4)(CH)4-
(C6H4)Se]Mo(PMe3)2 (8) and [µ-Se(C6H4)(CH)C(CH)2(C6H4)]-
(µ-Se)[Mo(PMe3)2][Mo(PMe3)2H] (9), are also formed (Scheme 3).
X-ray diffraction studies demonstrate that [κ2,η4-Se(C6H4)-
(CH)4(C6H4)Se]Mo(PMe3)2 (8) possesses a novel [Se(C6H4)(CH)4-
(C6H4)Se] ligand that may be conceptually viewed as being derived
from C-C coupling of two ring-opened benzoselenophene mol-
ecules. While a similar ligand derived from thiophene has been
observed to link two rhodium centers,27 an interesting feature of
[κ2,η4-Se(C6H4)(CH)4(C6H4)Se]Mo(PMe3)2 (8) is that the
[Se(C6H4)(CH)4(C6H4)Se] ligand coordinates to a single molybde-
num center and effectively occupies four coordination sites of an
octahedral geometry. Specifically, the two selenolate donors occupy
trans sites while the central butadiene component occupies two cis
sites.
The asymmetric dinuclear complex [µ-Se(C6H4)(CH)C(CH)2-
(C6H4)](µ-Se)[Mo(PMe3)2][Mo(PMe3)2H] (9), which also features
a C-C coupled ligand derived from benzoselenophene, can be
Figure 1. Molecular structure of [(κ2-C4H4)Mo-(PMe3)3(Se)]2[Mo(PMe3)4]
(5).
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C O M M U N I C A T I O N S
(9) Compounds with η2-C(R)dCR2 ligands are also referred to as 1-metalla-
cyclopropene complexes, a description that better portrays the bonding
situation (See: Casey, C. P.; Brady, J. T.; Boller, T. M.; Weinhold, F.;
Hayashi, R. K. J. Am. Chem. Soc. 1998, 120, 12500–12511. ). In accord
with this description, the Mo-C bond lengths of (κ1,η2-
viewed as originating from [Se(C6H4)(CH)4(C6H4)Se] Via a se-
quence that involves both C-Se and C-H bond cleavage reactions.
The bonding in this complex is highly delocalized, but a salient
feature is that the “quaternary” carbon of the [Se(C6H4)-
(CH)C(CH)2(C6H4)] ligand interacts to a similar degree with both
molybdenum atoms, with bond lengths [2.110(2) Å and 2.160(2)
Å] that are comparable to the Mo-aryl interaction [2.107(2) Å].
Furthermore, the Mo-Mo separation [2.7394(3) Å] is in the range
for a single bond, although bond distance alone is not a sufficient
criterion for establishing bond orders in such systems.28
In summary, the reactions of Mo(PMe3)6 with benzothiophene,
selenophene, and benzoselenophene reveal reaction pathways that
are pertinent to the mechanisms of hydrodesulfurization. Of
particular note, cleavage of the C-S bond of benzothiophene results
in the formation of three isomers, namely (κ2-CHCHC6H4S)-
Mo(PMe3)4 (1), (κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (2),
and (κ1,η2-CH2CC6H4S)Mo(PMe3)4 (3), thereby providing evidence
for the types of interconversions that may occur on the surface of
an HDS catalyst. Furthermore, Mo(PMe3)6 undergoes a novel
reaction with selenophene to give the metallacyclopentadiene
complex [(κ2-C4H4)Mo(PMe3)3(Se)]2[Mo(PMe3)4] (κ1,η2-CH2-
CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) (5), thereby demonstrating that
the molybdenum is capable of completely abstracting selenium from
the heterocycle.
CH2CC6H4S)Mo(PMe3) are 1.953(1) and 2.294(1) Å, corresponding to
4
single and double bonds, respectively.
(10) A related ligand derived from thiophene, namely κ1,κ1-CH2CCHCHS, has
been reported, but the vinyl group only coordinates in a κ1-manner with a
five-membered thiametallacyclopentene motif. See ref 5a.
(11) Mononuclear compounds that feature the [(κ1,η2-CH2CHC6H4S)M] motif
have been described for metals other than molybdenum. For example, see:
(a) Bianchini, C.; Frediani, P.; Herrera, V.; Jime´nez, M. V.; Meli, A.;
Rinco´n, L.; Sa´nchez-Delgado, R.; Vizza, F. J. Am. Chem. Soc. 1995, 117,
4333–4346. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Moneti,
S.; Herrera, V.; Sa´nchez-Delgado, R. A. J. Am. Chem. Soc. 1994, 116,
4370–4381.
(12) Although the relative stabilities of (κ1,η2-CH2CHC6H4S)Mo(PMe3)3-(η2-
CH2PMe2) and (κ1,η2-CH2CC6H4S)Mo(PMe3)4 are not known experimen-
tally, density functional theory calculations (B3LYP and cc-pVTZ(-f)/
LACV3P basis sets) predict that the latter is more stable by 7.2 kcal
mol-1
.
(13) While the deuterium labeling of (κ1,η2-CHDCHC6H4S)Mo(PMe3)3(η2-
CH2PMe2) is stereoselective at room temperature, upon heating at 60 °C
the deuterium scrambles statistically into both sites of the methylene group.
Significantly, this exchange occurs without accessing the η2-vinyl complex
(κ1,η2-CH2CC6H4S)Mo(PMe3)4.
(14) It is pertinent to note that this labeling pattern is not observed upon formation
of the butadiene-thiolate complex (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2)
Via reaction of Mo(PMe3)6 with d4-thiophene (see ref 4c), thereby indicating
that the mechanisms for formation of (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2)
and (κ1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) are not analogous.
(15) For an example of isomerization of {(κ2-CHCHC6H4S)MH} to {(κ1,η2-
CH2CHC6H4S)M}, see ref 11.
(16) In this regard, it is pertinent to note that the formation of a butadiene-
thiolate complex Via addition of D+ or D- to η5-thiophene complexes occurs
with the deuterium being respectively incorporated into cis and trans sites
relative to the adjacent CH group. These labeling patterns are consistent
with C-S bond cleavage within a η4-C4H4DS species for which the
deuterium is located respectively in the endo (for D+) and exo (for D-)
positions. See: (a) Luo, S.; Rauchfuss, T. B.; Gan, Z. J. Am. Chem. Soc.
1993, 115, 4943–4944. (b) Luo, S.; Rauchfuss, T. B.; Wilson, S. R. J. Am.
Chem. Soc. 1992, 114, 8515–8520. (c) Hachgenei, J. W.; Angelici, R. J. J.
Organomet. Chem. 1988, 355, 359–378.
Acknowledgment. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-93ER14339) for support
of this research.
Supporting Information Available: Experimental details, compu-
tational data and crystallographic data in CIF format. This material is
available free of charge Via the Internet at http://pubs.acs.org.
(17) For another example involving hydrogenation of a κ1,η2-CH2CHC6H4S
ligand, see: Bianchini, C.; Herrera, V.; Jimenez, M. V.; Meli, A.; Sa´nchez-
Delgado, R.; Vizza, F. J. Am. Chem. Soc. 1995, 117, 8567–8575.
(18) For a related tungsten alkoxide compound, W(PMe3)4(OC6H4Et)H3, see:
Rabinovich, D.; Zelman, R.; Parkin, G. J. Am. Chem. Soc. 1992, 114, 4611–
4621.
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(19) [(κ2-C4H4)Mo(PMe3)3(Se)]2[Mo(PMe3)4] may be conceptually viewed to
be a result of Mo(PMe3)6 trapping two [(κ2-C4H4)Mo(PMe3)3(Se)] moieties.
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(21) Murphy, V. J.; Parkin, G. J. Am. Chem. Soc. 1995, 117, 3522–3528.
(22) For example, the Mo-Se bond length in (κ1,η2-CH2CC6H4Se)Mo(PMe3)4,
which is predicted to be a single bond on the basis of the 18-electron
configuration, is 2.7101(4) Å (see Supporting Information).
(23) z corresponds to the 2-fold axis and x corresponds to the Se-Mo-Se axis
for the central molybdenum.
(24) Note that the in-phase combinations of 4pz and 4py orbitals of the two
selenium atoms are of appropriate symmetry to interact with 4d orbitals of
both the inner and outer molybdenum atoms.
(5) For reviews of compounds with butadiene-thiolate (thiapentadienyl)
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(26) Jones has also reported the extraction of sulfur from thiophene by the iridium
hydride complex [Cp*IrH3]2, in the presence of ButC2H3, although in this
case the transformation is also accompanied by hydrogenation to give the
butadiene complex [Cp*Ir]2(µ-S)(µ-C4H6). See: Jones, W. D.; Chin, R. M.
J. Am. Chem. Soc. 1994, 116, 198–203.
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of (η1,η2-CH2CHC6H4S)Mo(PMe3)3(η2-CH2PMe2) indicate that there is a
significant metallacyclopropane component to the bonding description, with
d(Mo-C) ) 2.268(2) and 2.304(1) Å and d(C-C) ) 1.420(2) Å.
(8) Frohnapfel, D. S.; Templeton, J. L. Coord. Chem. ReV. 2000, 206-207,
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