Carbon-sulfur bond cleavage of benzothiophene by Cp?Co(C2H4)2

Article abstract of DOI:10.1016/j.ica.2015.07.040

Cp^?Co(C₂H₄)₂ was reacted with benzothiophene and C-S activation of the vinyl-sulfur bond was observed. The resulting product, [Cp^?Co]₂(μ-κ²-C,S-C₈H₆S) was characterized by ¹H NMR, ¹³C{¹H} NMR, ¹³C DEPT-135 NMR, and ¹H-¹³C 2D HSQC spectroscopies. The broad Cp^? methyl peaks in these spectra suggested a fluxional structure.

Full text of DOI:10.1016/j.ica.2015.07.040

Accepted Manuscript
Carbon-Sulfur Bond Cleavage of Benzothiophene by Cp∗Co(C2H4)2
Natalie H. Chan, James H. Roache, William D. Jones
PII:
S0020-1693(15)00390-4
http://dx.doi.org/10.1016/j.ica.2015.07.040
ICA 16638
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Reference:
Inorganica Chimica Acta
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29 June 2015
29 July 2015
Please cite this article as: N.H. Chan, J.H. Roache, W.D. Jones, Carbon-Sulfur Bond Cleavage of Benzothiophene
by Cp∗Co(C2H4)2, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.07.040
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Graphical Abstract
Carbon-Sulfur Bond Cleavage of Benzothiophene by Cp*Co(C2H4)2
Natalie H. Chan, James H. Roache, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, NY 14627
Highlights:
Carbon-sulfur bond cleavage is achieved in benzothiophene
Assignment of NMR resonances of κ2-benzothiophene
X-ray structure of a dinuclear C-S activation product
1

Carbon-Sulfur Bond Cleavage of Benzothiophene by Cp*Co(C2H4)2
Natalie H. Chan, James H. Roache, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, NY 14627
*email: jones@chem.rochester.edu
ABSTRACT:
Cp*Co(C2H4)2 was reacted with benzothiophene and C–S activation of the vinyl-sulfur bond
was observed. The resulting product, [Cp*Co]2(µ-κ2-C,S-C8H6S) was characterized by 1H NMR,
13C{1H} NMR, 13C DEPT-135 NMR, and 1H-13C 2D HSQC spectroscopies. The broad Cp*
methyl peaks in these spectra suggested a fluxional structure.
Keywords: desulfurization; cobalt; C–S cleavage
Introduction
The removal of sulfur from natural gas and refined petroleum products is commonly referred
to as hydrodesulfurization (HDS). This process prevents sulfur from poisoning catalysts in
automobiles and reduces the emission of SO2 in combusting fossil fuels. Recently, the U.S.
Environmental Protection Agency finalized their regulations in Tier 3 Vehicle Emissions and Fuel
Standards Program that federal gasoline will be required to meet an annual average standard of 10
ppm of sulfur by 2017 [1]. Therefore, there is a pressing need to develop catalysts containing
inexpensive and abundant metal centers for C−S bond activation in organosulfur compounds,
which is the elementary step in HDS. First row transition metals are thus attractive choices for
serving as models for C−S bond activation from this perspective.
Most of the research in this area focuses on homogeneous modeling of the HDS process using
various metal catalysts. The sulfur compounds of interest are aromatic heterocycles such as
thiophene, benzothiophene, and dibenzothiophene due to their lower reactivity as compared to
thiols, sulfides, and disulfides. The end result in most of these reactions is metal insertion into the
carbon-sulfur bond either through oxidative addition or migration of the α-carbon forming a
metallothiobenzene ring with the transition metal. Bianchini, et al., discovered an unprecedented
coordination pathway for benzothiophene leading to the metal inserted metallocycle product using
[(triphos)Ir(C6H6)]BPh4 as a precursor [2]. Upon reaction of this complex with an excess of
2

benzothiophene at 25 °C, they observed the formation of an η3 bound benzothiophene complex
with the evolution of benzene (Scheme 1). Benzothiophene was coordinated to the iridium
complex through the sulfur atom and the unsubstituted carbon-carbon double bond. Heating this
complex in THF above 40 °C led to iridium insertion into the carbon-sulfur bond. Based on 31P
and 1H NMR spectroscopy, no intermediates were observed in either the coordination of the
benzothiophene or the metal insertion step. X-ray diffraction analysis of the iridium inserted
product showed a planar metallocycle similar to metallocycles observed with other metal
complexes [3], and the bond lengths within the metallocycle are indicative of a delocalized π-
system. The relatively low temperature at which the metal insertion occurs can be accounted for
by considering the η3 bound benzothiophene complex to be on the pathway to C–S bond cleavage.
This example illustrates that the coordination pathway is dependent on the transition-metal
complex used, but, the end result remains the same, metal insertion into the carbon-sulfur bond.
Scheme 1:
Earlier studies in our group demonstrate that reactions of (C5Me5)Rh(PMe3)(Ph)H with
thiophene, benzothiophene, and dibenzothiophene at 65 °C lead to products arising from
oxidative addition at the carbon-sulfur bond (Scheme 2) [4]. Mechanistic studies with deuterated
thiophenes have shown that the reaction proceeds through η1-S bound and η2-C,C bound
intermediates [5]. Further experiments provided insight into the steric and electronic effects that
control insertion of rhodium into one C–S bond vs. another [6-8] DFT studies of the selectivity
seen in these insertions provided evidence for an η2-C,S species prior to C–S cleavage [9,10]. An
3

analogous dimethylthiophene insertion product with iridium was reported by Angelici, Cp*Ir(κ2-
C,S-C4H2Me2)(PMe3), which forms by reaction of Cp*Ir(η4-thiophene) with PMe3 [11].
Scheme 2:
Our group has also investigated the activation of thiophene using precursors for the reactive
[M(dippe)] fragment where M = Ni [12,13], Pd [14,15], and Pt [14]. The [Pt(dippe)] fragment
showed unique reactivity in that it was observed to insert into the hindered C–S bond of 4,6-
dimethyldibenzothiophene (eq 1) [16]. DFT studies of substituted thiophenes were also carried
out, showing good agreement with experimental regioselectivities [17-19]. Reactions of the nickel
and palladium fragments with benzothiophene and dibenzothiophene have also been reported [12-
14].
(1)
Our group also reported carbon–sulfur cleavage by a cobalt complex. Reaction of
(C5Me5)Co(C2H4)2 with thiophene at 70 °C results in the formation of a dinuclear cobalt species
1
in which the carbon-sulfur bond of thiophene was cleaved (eq 2) [20]. H NMR studies of this
dinuclear molecule revealed a fluxional structure with a broad C5Me5 resonance which separated
into two distinct peaks upon cooling to -50°C. This same cobalt complex was also reacted with
4

dibenzothiophene yielding a similar dinuclear complex (eq 3) [21]. Unfortunately, upon cooling to
–80 °C, the broad C5Me5 resonance could not be resolved. It has been postulated that these
reactions occur through the loss of ethylene followed by insertion into the carbon–sulfur bond.
The carbon–sulfur inserted product then reacts with another cobalt fragment forming the
dinuclear adduct.
(2)
(3)
In this report, the C−S bond cleavage in benzothiophene, a common constituent of petroleum-
related deposits, by Cp*Co(C2H4)2 is investigated and the activation product is characterized by
1H NMR, 13C{1H} NMR, 13C DEPT-135 NMR, and 1H-13C 2D HSQC spectroscopies, and by
single crystal X-ray analysis.
Benzothiophene C–S Bond Activation
Three equivalents of benzothiophene was reacted with Cp*Co(C2H4)2 for 95 h at 100 °C in
benzene. The reaction yielded a black crude product (Cp*Co)2(µ-κ2-C,S-C8H6S), 1, in 80 % yield.
Based on the NMR spectra recorded and previous work on similar organosulfur compounds, we
propose an analogous structure to that seen with thiophene and dibenzothiophene (eq 4).
(4)
A single crystal X-ray structure of 1 was determined. The molecule crystallizes in space group
P2/n with Z = 2, indicating that the molecule lies on a special position. Solution of the structure
revealed that 1 lies on a 2-fold axis, with the κ2-benzothiophene being disordered over two
orientations. The 2-fold lies perpendicular to the Co–Co axis. Figure 1 shows the resulting
5

structure. Even with the disorder present, one can see differences in the binding of the two cobalt
atoms to sulfur. The Co1-S1 bond is substantially shorter (2.073 Å) than the Co1A-S1 bond
(2.209 Å). Similarly, the Co1-C18 bond is shorter (1.91 Å) than the Co1A-C18 bond (2.05 Å).
The Co1A-C17 bond is also long (2.19 Å), consistent with the formulation of the complex as Co1
being in the +3 oxidation state, with Co1A in the +1 oxidation state. This same type of disorder
was observed in the thiophene and dibenzothiophene X-ray structures [20,21].
Fig. 1. Structure of (Cp*Co)2(κ2-S,C-C8H6S), 1. Ellipsoids are shown at the 50% level.
The 1H NMR spectrum of the product was recorded (Figure 2). The Cp* resonance integrates
as 30 protons to the 6 hydrogens of the benzothiophene ligand, in agreement with the dinuclear
structure. The Cp* peak was unusually broad, suggesting a dynamic exchange which gives an
apparent symmetry in the averaged NMR spectrum. This process was assigned as a fluxional
process in which the η2-coordinated double bond has interchanged cobalt atoms (eq 5). In the 1H
NMR spectrum, the resonances at δ 4.10 and 9.27 are assigned to the vinylic protons of the ring-
opened benzothiophene. The resonance at δ 4.10 was believed to be Ha, as it is shielded by its
proximity to two cobalt centers, but this assignment is tentative (vide infra). The 13C{1H} NMR
spectrum of 1 was also examined, and showed a broad resonance at δ 88.90 that was presumed to
be the central Cp* carbons and two singlets at δ 178.1 and 86.66 for the Cα/β or Cβ/α carbons of
the κ2-benzothiophene (Figure 3).
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(5)
Fig. 2. 1H NMR spectrum of 1 in benzene-d6.
To determine the vinylic carbon assignments, 1H-13C HSQC and 13C{1H} DEPT spectra were
recorded (Figures 4 and 5). Using the data from the 2D HSQC spectrum, the aromatic peaks and
the broad Cp* methyl peak in the 13C NMR spectrum could be assigned. The pairs of
upfield/downfield signals for the vinylic hydrogens in the 1H and 13C spectra could be correlated,
but we were once again unsure of assigning the peaks at δ [9.27, 178.14] and [4.10, 86.72] to
Ha/Cα and Hb/Cβ or vice versa. Since Cα and the Cp* ring carbons are both attached to cobalt
centers, both of them have the potential to give a broad peak in the 13C NMR spectrum due to
quadrupolar relaxation from the cobalt. A 13C DEPT-135 spectrum differentiated between these
peaks (Figure 4). The absence of the peak at δ 89.90 in the 13C DEPT-135 spectrum confirms that
this peak is due to the quaternary Cp* ring carbons. The two resonances at δ 178 and 86 are
present in both 13C NMR and the DEPT-135 spectra, implying that these are the vinylic peaks. In
both 13C and DEPT-135 spectra, the resonance at δ 86.7 has a significantly higher intensity than
7

the peak at δ 178.1. The resonance at δ178 is almost 3 times as broad as the resonance at δ 86
(ν = 340 vs. 110 Hz).
½
13C{1H} (C6D6)
Cp*
Cβ
Cp*
Grease
Starting
material
Cα
Fig. 3. 13C{1H} NMR spectrum of 1 in benzene-d6.
Fig. 4. 1H-13C HSQC spectrum of 1 in benzene-d6.
8

Fig. 5. 13C{1H} DEPT spectrum of 1 in benzene-d6.
Since Cα is bonded to two quadrupolar cobalt atoms, it would be expected to have a much shorter
relaxation time and be broadened more than Cβ. Therefore, the 1H/13C resonances at δ
9.27/178.14 are assigned to Ha/Cα and those at δ 4.10/86.72 are assigned to Hb/Cβ.
Conclusions
Cp*Co(C2H4)2 was synthesized from Co2(CO)8 and reacted with benzothiophene. The
resulting product, [Cp*Co]2(µ-κ2-C,S-C8H6S) was characterized by 1H NMR, 13C{1H} NMR, 13C
DEPT-135 NMR and 1H-13C 2D HSQC. The broad Cp* methyl peaks in these spectra are
consistent with a fluxional structure, as seen with the DBT analog [21].
Experimental
General Considerations. Dry and deoxygenated dichloromethane and hexanes were
collected from an Innovative Technology PS-MD-6 solvent purification system. Dry and oxygen-
free benzene-d6, THF and ethyl ether were vacuum distilled from purple solutions of Na and
benzophenone under a nitrogen atmosphere. CDCl3 and xylenes was used without further
purification. Cp*Co(C2H4)2 was synthesized from Co2(CO)8 as reported in the literature [22].
NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer. Chemical shift values in
9

NMR spectra were referenced internally to the residual solvent resonances. Elemental analysis
was obtained from the CENTC Elemental Analysis Facility at the University of Rochester.
Reaction of (C5Me5)Co(C2H4)2 with Benzothiophene. (C5Me5)Co(C2H4)2 (0.8 mmol, 200
mg) and three equivalents of benzothiophene (2.4 mmol, 324 mg) were dissolved in 50 mL of
benzene. The orange-brown solution was placed in a 100 °C oil bath for 4 d. A 1H NMR
spectrum of the crude reaction mixture revealed that the cobalt starting material had been
consumed. The benzene was removed under vacuum, and the crude product was transferred to a
sublimation vessel. The crude product was sublimed at 60 °C for 3.5 h to remove any excess
benzothiophene starting material. The sample was recrystallized from CH2Cl2/toluene/hexane to
yield 170 mg product (80%). 1H NMR (500 MHz, benzene-d6, 25 °C) of crude [Cp*Co]2(µ-η2,η3-
C8H6S): δ 9.27 (d, J = 8.0 Hz, 1H), 7.06 (d, J = 7.4 Hz, 1H), 6.87 (d, J = 7.4 Hz, 1H), 6.70 (t, J
= 7.4 Hz, 1H), 6.55 (t, J = 7.4 Hz, 1H), 4.10 (d, J = 8.0 Hz, 1H), 1.68 (s, 30H). 13C{1H} NMR
(125 MHz, benzene-d6, 25 °C): δ 178.14 (s, CHα), 149.99 (s, C), 138.45 (s, C), 126.85 (s, CH),
125.62 (s, CH), 124.54 (s, CH), 124.07 (s, CH), 88.90 (br s, C5Me5), 86.66 (s, CHβ), 10.59 (br s,
C5Me5). 13C DEPT-135 NMR (125 MHz, benzene-d6, 25 °C): δ 178.19 (s, CHα), 126.87 (s, CH),
125.64 (s, CH), 124.56 (s, CH), 124.09 (s, CH), 86.68 (s, CHβ), 10.62 (br s, C5Me5). A 1H−13C
2D HSQC NMR spectrum (500 MHz, benzene-d6, 25 °C) of crude product was recorded. Anal.
.
Calcd. (found) for Co2C28H36S 1/4CH2Cl2: % C, 62.40 (62.73); % H, 6.77 (7.10); % N, 0.00
(0.044).
Acknowledgement
This work was funded by National Science Foundation, grant CHE-1360985. The author
wishes to thank Prof. William D. Jones, Dr. Sumit Chakraborty and Dr. Sandip Sur for helpful
discussions.
Supporting Information Available.
Supporting information give preparative details for Cp*Co(C2H4)2 and X-ray structural data
for 1. A listing of crystallographic information including data collection parameters, bond lengths,
fractional atomic coordinates, and anisotropic thermal parameters (PDF) for complex 1 (CCDC
deposition #1408675).
10

References
[1] U.S. Environmental Protection Agency Fact Sheet.
http://www.epa.gov/otaq/documents/tier3/420f14007.pdf (accessed May 10, 2015) .
[2] C. Bianchini, A. Meli, M. Peruzzini, F. Vizza, S. Moneti, C. Herrera, R.A. Sanchez-Delgado,
J. Am. Chem. Soc. 116 (1994) 4370-4381.
[3] C. Blonski, A.W. Myers, M. Palmer, S. Harris, W.D. Jones, Organometallics 16 (1997) 3819-
3827.
[4] W.D. Jones, L. Dong, J. Am. Chem. Soc. 113 (1991) 559-564.
[5] L. Dong, S.B. Duckett, K.F. Ohman, W.D. Jones, J. Am. Chem. Soc. 114 (1992) 151-160.
[6] A.W. Myers, W.D. Jones, S.M. McClements, J. Am. Chem. Soc. 117 (1995) 11704-11709.
[7] A.W. Myers, W.D. Jones, Organometallics 15 (1996) 2905-2917.
[8] A.W. Myers, L. Dong, T.A. Ateşin, R. Skugrud, C. Flaschenriem, W.D. Jones, Inorg. Chim.
Acta, 361 (2008) 3263-3270.
[9] T.A. Ateşin, W.D. Jones, Organometallics 27 (2008) 3666-3670.
[10] T.A. Ateşin, W.D. Jones, Inorg. Chem. 47 (2008) 10889-10894.
[11] J. Chen, L.M. Daniels, R.J. Angelici, Polyhedron 9 (1990) 1883-1891.
[12] D.A. Vicic, W.D. Jones, J. Am. Chem. Soc. 119 (1997) 10855-10856.
[13] D.A. Vicic, W.D. Jones, J. Am. Chem. Soc. 121 (1999) 121, 7606-7617.
[14] T.A. Ateşin, S.S. Oster, K. Skugrud, W.D. Jones, Inorg. Chim. Acta 359 (2006) 2798-2805.
[15] L. Munjanja, W.W. Brennessel, W.D. Jones, Organometallics 34 (2015), 1716-1724.
[16] D.A. Vicic, W.D. Jones, Organometallics 17 (1998) 3411-3413.
[17] T.A. Ateşin, W.D. Jones, Organometallics 27 (2008) 53-60.
[18] T.A. Ateşin, A.C. Ateşin, K. Skugrud, W.D. Jones, Inorg. Chem. 47 (2008) 4596-4604.
[19] T.A. Ateşin, S. Kundu, K. Skugrud, K. Lai, B.D. Swartz, W.D. Jones, Organometallics 30
(2011) 4578-4588.
[20] W.D. Jones, R.M. Chin, Organometallics 11 (1992) 2698-2700.
[21] W.D. Jones, R.M. Chin, J. Organomet. Chem. 472 (1994) 311-316.
[22] S.A. Frith, J.L. Spencer, W.E. Geiger, J. Edwin, (η5- Pentamethylcyclopentadienyl)-Cobalt
complexes, in: Inorganic Syntheses, John Wiley & Sons, Inc., NY, 2007; pp. 273-280.
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