Activation of the carbon-sulfur bonds in benzothiophenes by precoordination of transition metals to the carbocyclic ring

Article abstract of DOI:10.1021/om9802865

Coordination of an electrophilic transition-metal fragment, ML_n, to the carbocyclic ring of benzothiophene (BT) to form (η⁶-BT)ML_n^m+ activates a C-S bond to cleavage by the weak nucleophile Pt(PPh₃)₃, with concomitant insertion of Pt(PPh₃)₂. The rate of formation of the resulting metallathiacyclic insertion products, {(η⁶-BT·Pt(PPh₃)₂}ML _n^m+, depends on the metal fragment in the order ML_n = Ru(C₆Me₆)²⁺, Mn(CO)₃⁺ > FeCp⁺, RuCp⁺ ? Cr(CO)₃, with no reaction occurring in the absence of a ML_n activating group. All of the unsubstituted benzothiophene complexes undergo regiospecific cleavage of the olefinic C-S bond rather than the aryl C-S bond, which is likely a consequence of steric congestion that would exist if insertion had occurred at the latter site. The X-ray structures of the metallathiacycles {(η⁶-BT·Pt(PPh₃)₂}Mn(CO) ³⁺ and {(η⁶-BT·Pt(PPh₃)₂}FeCp⁺ are reported. The complexes (η⁵-2,5-dimethylthiophene)Mn(CO)₃⁺ and (η⁶-dibenzothiophene)Mn(CO)3+ also undergo rapid C-S bond cleavage with metal insertion in the presence of Pt(PPh₃)₃. The results suggest that π-adsorption of a thiophenic substrate on a catalyst surface in hydrodesulfurization reactions is a viable way to facilitate C-S bond cleavage, as well as subsequent desulfurization and hydrogenolysis.

Full text of DOI:10.1021/om9802865

3316
Organometallics 1998, 17, 3316-3322
Activa tion of th e Ca r bon -Su lfu r Bon d s in
Ben zoth iop h en es by P r ecoor d in a tion of Tr a n sition
Meta ls to th e Ca r bocyclic Rin g
Conor A. Dullaghan, Xiao Zhang, David L. Greene, Gene B. Carpenter, and
Dwight A. Sweigart*
Department of Chemistry, Brown University, Providence, Rhode Island 02912
Chiara Camiletti
Dipartmento di Chimica Fisica e Inorganica, Viale Risorgimento 4, Bologna 40126, Italy
Edward Rajaseelan
Department of Chemistry, Millersville University, Millersville, Pennsylvania 17551
Received April 14, 1998
Coordination of an electrophilic transition-metal fragment, MLn, to the carbocyclic ring of
m+
benzothiophene (BT) to form (η6-BT)MLn activates a C-S bond to cleavage by the weak
nucleophile Pt(PPh3)3, with concomitant insertion of Pt(PPh3)2. The rate of formation of
the resulting metallathiacyclic insertion products, {(η6-BT‚Pt(PPh3)2}MLnm+, depends on the
+
metal fragment in the order MLn ) Ru(C6Me6)2+, Mn(CO)3 > FeCp+, RuCp+ . Cr(CO)3,
with no reaction occurring in the absence of a MLn activating group. All of the unsubstituted
benzothiophene complexes undergo regiospecific cleavage of the olefinic C-S bond rather
than the aryl C-S bond, which is likely a consequence of steric congestion that would exist
if insertion had occurred at the latter site. The X-ray structures of the metallathiacycles
{(η6-BT‚Pt(PPh3)2}Mn(CO)3+ and {(η6-BT‚Pt(PPh3)2}FeCp+ are reported. The complexes (η5-
+
+
2,5-dimethylthiophene)Mn(CO)3 and (η6-dibenzothiophene)Mn(CO)3 also undergo rapid
C-S bond cleavage with metal insertion in the presence of Pt(PPh3)3. The results suggest
that π-adsorption of a thiophenic substrate on a catalyst surface in hydrodesulfurization
reactions is a viable way to facilitate C-S bond cleavage, as well as subsequent desulfur-
ization and hydrogenolysis.
In tr od u ction
C-S bonds are two of the key steps in HDS. A number
of nucleophilic 16-electron organometallic fragments are
known to insert into thiophenic carbon-sulfur bonds,
possibly after initial coordination to the sulfur.3 In the
case of benzothiophene (BT, 1) regiospecific metal
insertion into the olefinic carbon-sulfur bond, C(2)-S,
to give metallathiacycle 2 is known2-4 for Fe-, Ru-, Rh-,
Ir-, and Pt-based fragments. The metal has never been
observed to insert into the aryl carbon-sulfur bond,
C(8)-S, in free unsubstituted BT. Of particular rel-
evance to the present study is the chemistry reported
Hydrodesulfurization (HDS) is an important indus-
trial process whereby sulfur is removed from crude
petroleum feedstocks and converted to H2S.1 This
treatment is necessary to prevent catalyst poisoning in
subsequent refinement steps and to reduce sufur oxide
emissions from the combustion of fossil fuels. Current
HDS technology, which utilizes alumina-supported metal
sulfides (e.g., Mo/Co) as catalysts, is adequate for the
treatment of most sulfur-containing compounds. How-
ever, unsaturated heterocycles of the thiophenic type,
especially substituted benzothiophenes (BTs) and di-
benzothiophenes (DBTs), are resistant to desulfurization
and, as a consequence, constitute much of the residual
sulfur contamination in fossil fuels. For this reason
there is substantial interest in the development of
homogeneous models for thiophene desulfurization.2
Attachment of the thiophenic substrate to the metal
catalyst or activator and subsequent cleavage of the
(2) (a) Angelici, R. J . Polyhedron 1997, 16, 3073. (b) Bianchini, C.;
Meli, A. J . Chem. Soc., Dalton Trans. 1996, 801. (c) Bianchini, C.; Meli,
A. Acc. Chem. Res. 1998, 31, 109. (d) J ones, W. D.; Vicic, D. A.; Chin,
R. M.; Roache, J . H.; Myers, A. W. Polyhedron 1997, 16, 3115. (e)
Stafford, P. R.; Rauchfuss, T. B.; Verma, A. K.; Wilson, S. R. J .
Organomet. Chem. 1996, 526, 203.
(3) (a) Dong, L.; Duckett, S. B.; Ohman, K. F.; J ones, W. D. J . Am.
Chem. Soc. 1992, 114, 3328. (b) Bianchini, C.; Herrera, V.; J imenez,
M. V.; Meli, A.; Sanchez-Delgado, R.; Vizza, F. J . Am. Chem. Soc. 1995,
117, 8567.
(4) (a) Garcia, J . J .; Mann, B. E.; Adams, H.; Bailey, N. A.; Maitlis,
P. M. J . Am. Chem. Soc. 1995, 117, 2179. (b) Garcia, J . J .; Arevalo, A.;
Montiel, V.; Del Rio, F.; Quiroz, B.; Adams, H.; Maitlis, P. M.
Organometallics 1997, 16, 3216. (c) Garcia, J . J .; Arevalo, A.; Capella,
S.; Chehata, A.; Hernandez, M.; Montiel, V.; Picazo, G.; Del Rio, F.;
Toscano, R. A.; Adams, H.; Maitlis, P. M. Polyhedron 1997, 16, 3185.
(d) Iretskii, A.; Adams, H.; Garcia, J . J .; Picazo, G.; Maitlis, P. M. J .
Chem. Soc., Chem. Commun. 1998, 61.
(1) (a) Sanchez-Delgado, R. A. J . Mol. Catal. 1994, 86, 287. (b)
Startsev, A. N. Catal. Rev.sSci. Eng. 1995, 37, 353. (c) Vasudevan, P.
T.; Fierro, J . L. G. Catal. Rev.sSci. Eng. 1996, 38, 161. (d) Petroleum
Chemistry and Refining; Speight, J . G., Ed.; Taylor & Francis, Inc.:
Washington, DC 1998. (e) Hydrotreating Technology for Pollution
Control; Occelli, M. L.; Chianelli, R., Eds.; Marcel Dekker: New York,
1996.
S0276-7333(98)00286-6 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 07/03/1998

Activation of C-S Bonds in Benzothiophenes
Organometallics, Vol. 17, No. 15, 1998 3317
system to reductive insertion of Mn(CO)4 into the
C(aryl)-S bond according to eq 2. (An analogous reac-
tion occurs with DBT.9) We reasoned that precoordi-
nation to the carbocyclic ring in 8 differentially weakens
the C(aryl)-S bond over the C(2)-S bond, thus account-
ing for the reversal in regioselectivity compared to
insertions with free BTs (which give 2).
One may anticipate that the availability of multiple
metal binding sites in a catalyst could provide for
cooperativity in the key steps in HDS: C-S bond
cleavage with metal insertion, hydrogenolysis of σ
bonds, and desulfurization. Indeed, in homogeneous
model studies, it is known2c,10 that multimetallic sys-
tems are superior to monometallic ones for the stoichio-
metric hydrodesulfurization of thiophenic molecules.
The chemistry summarized by eq 2 established that the
reactivity of the heterocyclic ring in BT can be signifi-
by Maitlis and co-workers,4 who found that Pt(PEt3)3
undergoes oxidative insertion into a carbon-sulfur bond
in thiophene (T), BT, and DBT to afford platinum
metallathiacycles 3-5. Steric effects play a significant
role in these reactions, as evidenced by the fact that 2,5-
dimethylthiophene does not undergo the insertion reac-
tion.
Coordination of a thiophenic molecule to a metal on
an HDS catalyst surface can occur via the sulfur atom
(η1-S bonding) or via the π-system.2a,5 It is easy to see
that coordination to the sulfur could facilitate C-S bond
scission. The effect of complexation to the π-system on
the energetics of C-S cleavage is perhaps less obvious.
Since HDS catalyst surfaces are multimetallic, it follows
that one metal could activate the thiophenic substrate
by π-coordination, making it easier for a second metal
to insert into a C-S bond. Thus, Rauchfuss et al.
reported6 that η5-coordination of Ru(C6Me6)2+ to thio-
phene activates the system to C-S cleavage upon
reaction with a reducing agent and a second (electro-
philic) metal. In a process that may be related mecha-
nistically, we showed7 that η5-coordination of the
Mn(CO)3+ moiety to thiophene (6) affords the bimetallic
7 when reduced under CO (eq 1). With benzothiophene,
+
cantly influenced by coordination of Mn(CO)3 to the
carbocyclic ring. Ordinarily, complexation of the type
shown in 8 electrophilically activates the arene ring to
nucleophilic addition reactions, and indeed, common
organic nucleophiles react with 8 in this manner to
afford the expected cyclohexadienyl complexes.11 Equa-
tion 2 differs from a nucleophilic addition reaction in
that electrons, rather than a nucleophile, are being
added, with the result that a metal fragment is trans-
ferred from one molecule into a C-S bond of another.
This result prompted us to examine the direct reaction
of 8 with a metal nucleophile. The nucleophile chosen,
Pt(PPh3)3, does not react with free BT. It was found,
however, that Pt(PPh3)3 reacts rapidly with 8 to give
C-S insertion according to eq 3.
+
we demonstrated7a,8 that precoordination of Mn(CO)3
to the carbocyclic ring regiospecifically activates the
(5) (a) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259. (b)
Huckett, S. C.; Angelici, R. J . Organometallics 1988, 7, 1491. (c) Choi,
M.-G.; Angelici, R. J . Organometallics 1992, 11, 3328. (d) Benson, J .
W.; Angelici, R. J . Organometallics 1993, 12, 680. (e) Angelici, R. J .
Bull. Soc. Chim. Belg. 1995, 104, 265. (f) Rudd, J . A.; Angelici, R. J .
Inorg. Chim. Acta 1995, 240, 393.
(6) Dailey, K. M. K.; Rauchfuss, T. B.; Rheingold, A. L.; Yap, G. P.
A. J . Am. Chem. Soc. 1995, 117, 6396.
(7) (a) Dullaghan, C. A.; Sun, S.; Carpenter, G. B.; Weldon, B.;
Sweigart, D. A. Angew. Chem. 1996, 108, 234; Angew. Chem., Int. Ed.
Engl. 1996, 35, 212. (b) Dullaghan, C. A.; Carpenter, G. B.; Sweigart,
D. A.; Choi, D. S.; Lee, S. S.; Chung, Y. K. Organometallics 1997, 16,
5688.
In this paper, we demonstrate that this activating
effect is a general one. Thus, precoordination of any of
the electrophilic metal fragments Ru(C6Me6)2+
,
Mn(CO)3+, FeCp+, RuCp+, or Cr(CO)3 to the π-system
of T, BT, and DBT results in activation of a C-S bond
(9) Zhang, X.; Dullaghan, C. A.; Carpenter, G. B.; Sweigart, D. A.;
Meng, Q. J . Chem. Soc., Chem. Commun. 1998, 93.
(10) (a) Bianchini, C.; Meli, A. J . Chem. Soc., Dalton Trans. 1996,
801. (b) Bianchini, C.; J imenez, M. V.; Meli, A.; Moneti, S.; Patinec,
V.; Vizza, F. Organometallics 1997, 16, 5696.
(11) Lee, S. S.; Chung, Y. K.; Lee, S. W. Inorg. Chim. Acta 1996,
253, 39.
(8) (a) Zhang, X.; Dullaghan, C. A.; Watson, E. J .; Carpenter, G. B.;
Sweigart, D. A. Organometallics 1998, 17, 2067. (b) Sun, S.; Dullaghan,
C. A.; Sweigart, D. A. J . Chem. Soc., Dalton Trans. 1996, 4493. (c)
Dullaghan, C. A.; Zhang, X.; Walther, D.; Carpenter, G. B.; Sweigart,
D. A.; Meng, Q. Organometallics 1997, 16, 5604.

3318 Organometallics, Vol. 17, No. 15, 1998
Dullaghan et al.
Ch a r t 1
generate the simple metallathiacycle 19 from the reac-
tion of free BT and Pt(PPh3)3 is due to a high kinetic
barrier or due to thermodynamic instability.
The reaction of Pt(PPh3)3 with BT precoordinated to
the fragments FeCp+, RuCp+, and Ru(C6Me6)2+ led
cleanly to insertion products 15-17 (Chart 1; eq 5) in
isolated yields of 76%, 80%, and 80%, respectively. With
to facile oxidative insertion by Pt(PPh3)3. It is suggested
that this effect may be relevant to the chemistry
occurring in heterogeneous HDS reactions.
Resu lts a n d Discu ssion
Ben zoth iop h en e Com p lexes. The known ben-
zothiophene complexes depicted in Chart 1 were syn-
thesized in order to study the effect of π-coordination
on the ease of C-S bond cleavage and metal insertion
upon addition of the nucleophilic reagent Pt(PPh3)3. As
stated above, it is known4 that Pt(PEt3)3 inserts revers-
ibly into the C(2)-S bond of free BT to give 4. By way
of contrast, we found (as shown by 31P NMR) that
Pt(PPh3)3 does not react with BT, even after refluxing
for 12 h in CH2Cl2. Maitlis4a noted previously that
Pt(PPh3)3 does not react with free thiophenes under
conditions that lead to insertion with Pt(PEt3)3. That
Pt(PPh3)3 is less nucleophilic than Pt(PEt3)3 is, of course,
not surprising. It has been shown, for example, that
Pt(PEt3)3 is ca. 1000 times more reactive than Pt(PPh3)3
toward the electrophile MeI.16
(η6-BT)FeCp+ (11), an approximate half-life of 1 h at
room temperature was estimated from 1H NMR data
in CD2Cl2. On the basis of the synthetic time scales and
spectroscopic observations, the following qualitative
order was found for the reactivity of the cationic BT
+
complexes toward Pt(PPh3)3: Ru(C6Me6)2+, Mn(CO)3
> FeCp+, RuCp+. The mechanistic implications of this
order are discussed below.
Verification of the proposed structures of the products
of eq 5 was obtained from single-crystal X-ray diffraction
studies of [10]BF4 and [15]PF6, Table 1. The cationic
portions of these molecules are illustrated in Figures 1
and 2. The most significant feature of these structures
is that the Pt(PPh3)2 moiety is inserted into the C(2)-S
bond. Previously, we showed that Mn(CO)4 inserts
regiospecifically into the C(aryl)-S bond in (η6-BT)Mn-
(CO)3+, as in eq 2. A cursory examination of Figures 1
and 2 suggests that steric congestion would have been
substantial had insertion into the C(aryl)-S bond
occurred, although such a species cannot be ruled out
as a reaction intermediate (vide infra). The metal-
lathiacyclic ring in 10 is quite planar (mean deviation
) 0.041 Å) and is nearly coplanar with the carbocyclic
ring; the dihedral angle between these two planes is
only 3.9°. The platinum atom in 10 adopts the usual
square-planar geometry, although the P(1)-Pt(1)-P(2)
angle is larger (96.5°) and the C(2)-Pt(1)-P(1) angle
is smaller (85.2°) than the ideal angle of 90° due,
presumably, to steric constraints imposed by the large
phosphine ligands. Overall, the benzometallathiacycle
portion of 10 is similar to that reported4a by Maitlis for
the Pt(PEt3)2 insertion product of free BT (4).
Although Pt(PPh3)3 is inert toward free BT, it was
+
found to react rapidly with (η6-BT)Mn(CO)3 (8) at
room-temperature according to eq 3 to afford the inser-
tion product 10 in an isolated yield of 82%. IR spectra
showed that under typical conditions the reaction has
a half-life of less than 1 min. Derivatives of 10 bearing
a 7-Et or a 2-Me substituent were synthesized by the
same procedure. The platinum metallathiacycle 10, as
well as the analogues shown in Chart 1 (vide infra), are
air stable but moderately light sensitive. Of the various
metallathiacycles prepared, the derivative of 10 with a
2-Me substitutent was found to be the least stables
probably due to unfavorable steric interactions (vide
+
infra). The Mn(CO)3 moiety could be removed from
10 by refluxing for 1 h in MeCN, as shown in eq 4. This
does not prove, however, whether the inability to
(12) Sun, S.; Yeung, L. K.; Sweigart, D. A.; Lee, T.-Y.; Lee, S. S.;
Chung, Y. K.; Switzer, S. R.; Pike, R. D. Organometallics 1995, 14,
2613.
(13) Lee, C. C.; Igbal, M.; Gill, U. S.; Sutherland, R. G. J . Organomet.
Chem. 1985, 288, 89.
(14) Nolan, S. P.; Martin, K. L.; Stevens, E. D.; Fagan, P. J .
Organometallics 1992, 11, 3947.
(15) Fischer, E. O.; Goodwin, H. A.; Kreiter, C. G.; Simmons, H. D.;
Sonogashira, K.; Wild, S. B. J . Organomet. Chem. 1968, 14, 359.
(16) Pearson, R. G.; Figdore, P. E. J . Am. Chem. Soc. 1980, 102,
1541.
Complex 15 (Figure 2) was found to crystallize with
one molecule of well-ordered solvent (Et2O). The plati-
num center in 15 is square planar, with much of the
same angles as in 10. There are, however, significant
structural differences between these two complexes.
Thus, the metallathiacyclic ring in 15 adopts a decidedly
nonplanar conformation (mean deviation ) 0.16 Å). The

Activation of C-S Bonds in Benzothiophenes
Organometallics, Vol. 17, No. 15, 1998 3319
Ta ble 1. Cr ysta llogr a p h ic Da ta for Com p lexes [10]BF 4 a n d [15]P F 6
[10]BF4
[15]PF6‚Et2O
formula
fw
C
47H36BF4MnO3P2PtS
C53H51F6FeOP3PtS
1193.85
298
0.710 73
triclinic
P1h
10.8906(2)
15.2881(2)
15.7410(2)
75.2590(10)
78.3060(10)
87.83
2481.69(6)
2
1.598
3.307
1192
1079.60
298
0.710 73
monoclinic
P21/n
10.6371(3)
30.6432(6)
13.4551(3)
temp, K
wavelength, Å
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
93.8890(10)
4375.7(2)
4
1.639
3.661
Z
d
calcd, g cm-3
µ, mm-1
F(000)
2128
cryst dimens, mm
θ range, deg
no. of reflns collected
no. of indep reflns
data/restraints/params
GOF on F2
0.28 × 0.16 × 0.13
0.22 × 0.20 × 0.05
1.68-26.53
26 298
10091 (Rint ) 0.0403)
10091/27/595
1.042
1.33-23.25
19 757
6256 (Rint ) 0.0946)
6253/21/541
1.049
final R indices [I > 2σI]
R indices (all data)
R1 ) 0.0649, wR2 ) 0.1365
R1 ) 0.1222, wR2 ) 0.1648
R1 ) 0.0410, wR2 ) 0.1019
R1 ) 0.0529, wR2 ) 0.1084
F igu r e 2. Crystal structure of the cation in [15]PF6‚Et2O.
Selected bond distances (Å) and angles (deg) are Pt(1)-
C(2) 2.056(5), C(2)-C(3) 1.287(8), C(3)-C(9) 1.456(9), C(9)-
C(8) 1.423(9), C(8)-S(1) 1.775(6), Pt(1)-S(1) 2.2888(13),
Pt(1)-P(1) 2.2952(12), Pt(1)-P(2) 2.3474(13), C(2)-Pt(1)-
S(1) 89.65(16), C(8)-S(1)-Pt(1) 111.1(2), C(3)-C(2)-
Pt(1) 133.9(5), C(2)-Pt(1)-P(1) 86.63(16), P(1)-Pt(1)-P(2)
96.69(4), S(1)-Pt(1)-P(2) 87.10(5).
F igu r e 1. Crystal structure of the cation in [10]BF4.
Selected bond distances (Å) and angles (deg) are Pt(1)-
C(2) 2.052(14), C(2)-C(3) 1.26(2), C(3)-C(9) 1.47(2), C(9)-
C(8) 1.37(2), C(8)-S(1) 1.75(2), Pt(1)-S(1) 2.279(4), Pt(1)-
P(1) 2.297(3), Pt(1)-P(2) 2.346(3), C(2)-Pt(1)-S(1) 90.0(4),
C(8)-S(1)-Pt(1) 113.4(5), C(3)-C(2)-Pt(1) 136.2(13), C(2)-
Pt(1)-P(1) 85.2(4), P(1)-Pt(1)-P(2) 96.53(12), S(1)-Pt(1)-
P(2) 88.81(13).
situation is illustrated in Figure 3, which provides edge-
on views of 10 and 15. Experimental and theoretical17
studies suggest that the energy barrier(s) linking planar
and various accessible nonplanar conformations in
thiophenic metallathiacycles can be low and that steric
factors play a dominant role in this regard. An illustra-
tion of this point is provided by complexes 9, 20, and
21. The first two of these complexes have highly
nonplanar metallathiacyclic rings, while the third adopts
a planar conformation.8c It is most probable that steric
factors are important in 9 and 20 but not in 21. The
influence of steric effects is further indicated by the fact
that 20 has a very different nonplanar conformation
from that in 9 as a result of the methyl substituent. In
the case of 15, the observed nonplanarity could result
from modest steric interaction between a PPh3 ligand
(17) Blonski, C.; Myers, A. W.; Palmer, M.; Harris, S.; J ones, W. D.
Organometallics 1997, 16, 3819.

3320 Organometallics, Vol. 17, No. 15, 1998
Dullaghan et al.
by Pt(PPh3)3. A plausible mechanism involves initial
η1 coordination of Pt(PPh3)3 to the sulfur atom, followed
by insertion of the platinum into a C-S bond (which
can also be viewed as oxidative addition of a C-S group
to Pt). At some point along the reaction pathway, a
PPh3 ligand is lost, most likely prior to or in concert
with the actual insertion. In accordance with this, the
reaction of Pt(PPh3)3 with 14 was found to be signifi-
cantly inhibited by the presence of excess PPh3 (vide
supra).
Although the details of the mechanism of eq 5 are
unknown, it is apparent that initial formation of an η1-S
intermediate cannot be the rate-determining step. This
follows from the observation that the most electron-
withdrawing MLn fragments are the most activating,
which would not be expected if the rate-determining
step were coordination of the sulfur atom to Pt(PPh3)n.
It was noted above that insertion of Mn(CO)4 into 8
occurs regiospecifically at the C(aryl)-S bond (eq 2). In
contrast, Pt(PPh3)3 inserts regiospecifically into the
C(2)-S bond in 8 and 11-14 (eq 5), most likely because
of steric effects. We are of the opinion that precoordi-
nation of MLn to BT selectively weakens that C(aryl)-S
bond, and it is possible that C(aryl)-S cleavage is
favored kinetically in eq 5 to afford an intermediate that
isomerizes to the thermodynamically preferred C(2)-
Pt-S product. Such an isomerization with a Mn(CO)4
fragment has been shown8c to be facile at room temper-
ature in the conversion of 20 to 21. In addition, it is
shown below that Pt(PPh3)3 inserts into (η6-DBT)Mn-
(CO)3+, which necessarily involves cleavage of a C(ar-
yl)-S bond (albeit at the one farther away from the
Mn(CO)3 group). Possibly related to this is the observa-
tion that (η6-2-MeBT)Mn(CO)3+ reacted with Pt(PPh3)3
to give a product that could be isolated but was
thermally unstable in comparison to the insertion
F igu r e 3. Edge-on views of insertion products 10 and 15.
For clarity, only the phenyl ring carbon atoms directly
bonded to the phosphorus atoms are shown.
and the Cp ring or could be the result of solid-state
packing.
In contrast to the insertion of Pt(PPh3)3 into BT
complexes containing cationic metal fragments (8, 11-
13), the reaction with (η6-BT)Cr(CO)3 (14) took place
much more slowly and required heating. The reaction
in refluxing CH2Cl2 with 1.6 equiv of Pt(PPh3)3 to give
the insertion product 18 occurred with a half-life of ca.
3 h and was terminated after approximately 75%
completion due to the onset of decomposition. In the
presence of 3 equiv of PPh3, the rate was greatly
retarded, with the half-life increasing to ca. 30 h (after
which the reaction was discontinued). Evidence was
obtained that the insertion of Pt(PPh3)3 into 14 is a
reversible process, in analogy to that reported4a for 4.
Thus, refluxing 18 in CH2Cl2 for 20 h with 7.6 equiv of
PPh3 resulted in ca. 20% deinsertion to produce 14; it
was not determined if the reaction had reached equi-
librium.
product (10) with (η6-BT)Mn(CO)3 (8) while reaction
+
of (η6-2,7-Me2BT)Mn(CO)3+ with Pt(PPh3)3 gave a prod-
uct that could not be isolated. NMR, IR, MS, and
elemental analysis data clearly indicate that (η6-2-
+
MeBT)Mn(CO)3 reacts with Pt(PPh3)3 to insert Pt-
(PPh3)2 into a C-S bond, but we cannot prove which
C-S bond is cleaved. However, consideration of 31P
NMR data (see Experimental Section) show that com-
plexes 10 and 15-18 all have very similar δ and J Pt-P
values. The 31P NMR parameters of the insertion
Mech a n ism of In ser tion in to Ben zoth iop h en e
Com p lexes. The experimental data show that the
relative rate of Pt(PPh3)3 insertion into BT complexes
according to eq 5 depends on the MLn fragment as
+
product with (η6-2-MeBT)Mn(CO)3 are noticeably dif-
ferent and, in fact, are close to those found for the
+
insertion product obtained from (η6-DBT)Mn(CO)3
,
follows: Ru(C6Me6)2+, Mn(CO)3+ > FeCp+, RuCp+
.
which must contain a C(aryl)-Pt-S link. Thus, we
Cr(CO)3. On the basis of extensive studies,18 it is
expected that coordination of the MLn fragment to BT
should electrophilically activate the carbocyclic ring to
attack by nucleophiles in the order indicated above. For
believe that the 2-methyl group in (η6-2-MeBT)-
+
Mn(CO)3 forces insertion to occur at the C(aryl)-S
bond. Significantly, this suggestion accounts for the
inability to isolate an insertion product with (η6-2,7-Me2-
example, Mn(CO)3 is predicted to be 104 times more
+
+
BT)Mn(CO)3
.
activating than FeCp+. However, the reaction in ques-
tion, eq 5, does not involve nucleophilic attack on the
carbocyclic ring but rather insertion into the olefinic
C(2)-S bond of the heterocyclic ring. Nevertheless, the
data indicate that the same qualitative reactivity order
prevails for eq 5. The MLn moieties in 8 and 11-14
function to activate a C-S bond to nucleophilic cleavage
Of relevance to the present paper is a recent study
by Angelici and co-workers5f on the ability of the CpRe-
(CO)2 fragment to coordinate to the sulfur and to the
C(2)dC(3) double bond in benzothiophenes. In com-
parison to results with free BT, it was found that η6-
coordination of the Cr(CO)3 group promotes CpRe(CO)2
binding to the double bond. This led to the suggestion
that η6 adsorption of BT on a metal site in a HDS
catalyst should promote coordination of the double bond
(18) (a) Kane-Maguire, L. A. P.; Honig, E. D.; Sweigart, D. A. Chem.
Rev. 1984, 84, 525. (b) Pike, R. D.; Sweigart, D. A. Synlett 1990, 565.

Activation of C-S Bonds in Benzothiophenes
Organometallics, Vol. 17, No. 15, 1998 3321
to another metal site. In this manner, hydrogenation
of the C(2)dC(3) bond would be facilitated, as proposed
to occur in an initial step of a major HDS pathway for
BT. In view of our results with complexes 8 and 11-
14, it is suggested that η6 adsorption of BT also
promotes C-S bond cleavage, which is a prelude to
desulfurization (and hydrogenolysis) in HDS chemistry.
Th iop h en e a n d Diben zoth iop h en e Com p lexes.
Insertion of a variety of metal fragments into a C-S
bond in free thiophene (T) has been reported.2-4 Pt-
(PEt3)3 was shown4 to insert into T, 2-MeT, and 3-MeT
to afford derivatives of 3, but no reaction was observed
with the sterically encumbered thiophene 2,5-Me2T (22).
bond to rapid insertion by the weak nucleophile Pt-
(PPh3)3. Cleavage of a C-S bond via metal insertion is
a key step in HDS chemistry, and the results presented
herein suggest that π-adsorption of the thiophenic
substrate on the catalyst surface may facilitate this
process, as well as subsequent desulfurization and
hydrogenolysis.
Exp er im en ta l Section
Ma ter ia ls. Standard reagents were purchased from com-
merical sources and used without further purification. The
following compounds were synthesized by the indicated lit-
erature method: [(η6-R-BT)Mn(CO)3]BF4 (R ) H, 2-Me,
7-Et),12 [(η5-2,5-Me2T)Mn(CO)3]BF4,19 (η6-BT)Cr(CO)3,15 [(η6-
BT)FeCp]PF6,13 [(η6-BT)RuCp]PF6,14 [(η6-BT)Ru(C6Me6)][BF4]2,8a
[(η6-DBT)Mn(CO)3]BF4,12 and Pt(PPh3)3.20 Activated neutral
alumina (Brockmann I, 150 mesh) was deactivated by adding
water to form a 10% w/w mixture. 31P NMR chemical shifts
are relative to a 85% phosphoric acid external reference.
Cr ysta l Str u ctu r e Deter m in a tion s. The crystal struc-
tures of [10]BF4 and [15]PF6‚Et2O were determined with a
Siemens P4 diffractometer equipped with a CCD area detector
and controlled by SMART version 4 software. Data reduction
was carried out by SAINT version 4 and included profile
analysis; this was followed by absorption correction using the
program SADABS. Data were collected at 25 °C with Mo KR
radiation. The structures were determined by direct methods
and refined on F2 using the SHELXTL version 5 package.
Hydrogen atoms were introduced in ideal positions, riding on
the carbon atom to which they are bonded; each was refined
with isotropic temperature factors ranging from 20% to 50%
greater than that of the ridden atom. All other atoms were
refined with anisotropic thermal parameters.
We found that Pt(PPh3)3 is unreactive toward free T but
readily inserts when the π-system is precoordinated to
+
Mn(CO)3+. The activating effect of Mn(CO)3 is dra-
matically illustrated with 2,5-Me2T, which reacts rap-
idly at room temperature according to eq 6.
Pt(PPh3)3 also inserts rapidly into (η6-DBT)Mn(CO)3
+
In ser tion of P t(P P h 3)3 in to (η6-BT)Mn (CO)3 (8). Pt-
+
to afford 23. The alternative structure 24 was judged
(PPh3)3 (0.300 g, 0.305 mmol) was added to a suspension of
[(η6-BT)Mn(CO)3]BF4 (0.120 g, 0.30 mmol) in CH2Cl2 (8 mL)
under N2 at room temperature. An immediate color change
from bright yellow to orange occurred. After 15 min, the
volume was reduced to 3 mL and the product precipitated by
addition of Et2O. Reprecipitation from acetone with Et2O
afforded pure [10]BF4 as a yellow powder. [10]BF4: yield 82%
(0.279 g). IR (acetone): νCO 2058 (s), 2000 (s, br) cm-1
.
1H
NMR (CD3C(O)CD3): δ 7.52-7.05 (m, 32H, Ph, H2,3), 6.94 (br,
1H), 6.71 (br, 1H), 6.60 (br, 1H), 5.29 (br, 1H). 31P NMR
(CD3C(O)CD3): δ 26.49 (dd, J P-P ) 22 Hz, J P-Pt ) 3414 Hz),
22.15 (dd, J P-P ) 22 Hz, J P-Pt ) 1900 Hz). 31P NMR (CD2-
Cl2): δ 26.04 (dd, J P-P ) 23 Hz, J P-Pt ) 3428 Hz), 21.75 (dd,
J P-P ) 23 Hz, J P-Pt ) 1901 Hz). MS FAB: 992 (M+ - BF4-),
730 (M+ - PPh3). Anal. Calcd for C47H36O3MnP2PtSBF4: C,
52.29; H, 3.36. Found: C, 52.03; H, 3.44. A crystal of [10]-
BF4 suitable for X-ray diffraction was grown by vapor diffusion
of Et2O into an acetone solution of the complex at -15 °C over
a period of several days.
unlikely based on steric considerations and on a com-
parison of 31P NMR data of the product with that of the
cyclohexadienyl complex obtained after hydride addition
to the coordinated arene ring (25, 26). The 31P NMR
chemical shifts on average changed by only 0.74 ppm,
which is more consistent with the transformation 23 f
25 than with 24 f 26. In a control experiment, hydride
addition to the arene ring in the BT complex 10 was
found to change the 31P NMR chemical shifts by an
average of 1.7 ppm.
+
The Mn(CO)3 was removed from 10 according to eq 4 by
dissolving [10]BF4 (0.065 g, 0.060 mmol) in CH3CN (10 mL)
and heating to reflux under N2 for 1 h. After this time, an IR
spectrum showed complete conversion of the manganese to
(MeCN)3Mn(CO)3+. The solvent was removed in vacuo, and
the residue was redissolved in CH2Cl2, loaded unto a dry
activated alumina column, and eluted with acetone. The
solvent was removed, and the oily residue of 19 was washed
with hexanes. For 19: yield 60% (0.030 g). 1H NMR (CD2-
(19) (a) Lesch, D. A.; Richardson, J . W.; J acobson, R. A.; Angelici,
R. J . J . Am. Chem. Soc. 1984, 106, 2901. (b) Chen, J .; Young, V. G.;
Angelici, R. J . Organometallics 1996, 15, 325. (c) Lee, S. S.; Lee, T. Y.;
Choi, D. S.; Lee, J . S.; Chung, Y. K.; Lee, S. W.; Lah, M. S.
Organometallics 1997, 16, 1749.
(20) Yoshida, T.; Matsuda, T.; Otsuka, S. Inorg. Synth. 1990, 28,
123.
Con clu sion s
Coordination of any of a variety of transition-metal
fragments to the π-system of thiophene, benzothiophene,
and dibenzothiophene results in activation of a C-S

3322 Organometallics, Vol. 17, No. 15, 1998
Dullaghan et al.
Cl2): δ 7.86 (m, 1H), 7.71-6.80 (m, 35H). 31P NMR (CD2Cl2):
δ 28.07 (dd, J P-P ) 21.5 Hz, J P-Pt ) 3246 Hz), 26.18 (dd, J P-P
) 21.5 Hz, J P-Pt ) 1852 Hz).
(dd, J P-P ) 24 Hz, J P-Pt ) 1921 Hz). Anal. Calcd for C56H54
RuP2PtSB2F8: C, 52.11; H, 4.22. Found: C, 51.83; H, 4.29.
In ser tion of P t(P P h 3)3 in to (η6-BT)Cr (CO)3 (14).
-
A
Insertion of Pt(PPh3)3 into (η6-R-BT)Mn(CO)3 (R ) 2-Me,
+
solution of (η6-BT)Cr(CO)3 (0.022 g, 0.081 mmol) in CH2Cl2 (20
mL) under N2 was warmed to reflux, and Pt(PPh3)3 (0.130 g,
0.132 mmol) was then added. After the mixture was refluxed
for 10 h, the solution was concentrated to 3 mL and loaded
unto a deactivated alumina column. Hexanes was used to
elute impurities and then Et2O/hexanes (1:2) to elute (BT)Cr-
(CO)3 (0.005 g). The product (18) was finally eluted with Et2O/
hexanes (3:1) and obtained as a yellow solid after drying. 18:
yield 84% (0.052 g) based on the amount of 14 consumed. IR
(CH2Cl2): νCO 1946 (s), 1868 (s, br) cm-1. IR (hexanes): νCO
7-Et) was performed similarly to the method described above
for the synthesis of [10]BF4. [{η6-2-MeBT‚Pt(PPh3)2}Mn(CO)3]-
BF4: yield 89% (0.160 g). IR (CH2Cl2): νCO 2056 (s), 2002 (s),
1992 (s) cm-1 1H NMR (CD3C(O)CD3): δ 7.80-7.05 (m, 30H,
.
Ph), 6.57 (m, 1H), 6.43 (m, 1H), 6.24 (m, 1H), 6.15 (m, 1H),
5.37 (m, 1H), 2.21 (s, Me). 31P NMR (CD3C(O)CD3): δ 25.81
(dd, J P-P ) 21 Hz, J P-Pt ) 3156 Hz), 16.03 (dd, J P-P ) 21 Hz,
J P-Pt ) 2074 Hz). HR MS: M+ (m/z) calcd 1006.1045, obsd
1006.1023. Anal. Calcd for C48H38O3MnP2PtSBF4: C, 52.72;
H, 3.50. Found: C, 52.71; H, 3.74. For [{η6-7-EtBT‚Pt-
(PPh3)2}Mn(CO)3]BF4: yield 90% (0.090 g). IR (CH2Cl2): νCO
1952 (s), 1881 (s, br) cm-1
.
1H NMR (CD2Cl2): δ 7.80-7.14
(m, 30H, Ph), 7.06 (m, H2), 6.50 (m, H3), 5.79 (d, J ) 6.7, H7),
5.65 (dd, J ) 1.2, 6.6 Hz, H4), 5.38 (dt, J ) 1.3, 6.4 Hz, H6),
5.05 (dt, J ) 1.2, 6.3 Hz, H5). 31P NMR (CD3C(O)CD3): δ 26.95
(dd, J P-P ) 21 Hz, J P-Pt ) 3348 Hz), 23.33 (dd, J P-P ) 21 Hz,
J P-Pt ) 1874 Hz). Anal. Calcd for C47H36O3CrP2PtS: C, 57.03;
H, 3.67. Found: C, 57.38; H, 4.06.
2056 (s), 1996 (s, br) cm-1 1H NMR (CD3C(O)CD3): δ 7.84-
.
7.10 (m, 32H, Ph, H2,3), 6.82 (m, H4,7), 6.56 (m, H6), 6.41 (m,
H5), 2.62 (m, CH2), 1.09 (t, J ) 7.1 Hz, Me). 31P NMR (CD3C-
(O)CD3): δ 26.55 (dd, J P-P ) 21 Hz, J P-Pt ) 3418 Hz), 24.11
(dd, J P-P ) 21 Hz, J P-Pt ) 1916 Hz). Anal. Calcd for C49H40O3-
MnP2PtSBF4: C, 53.13; H, 3.64. Found: C, 53.03; H, 3.47.
In ser tion of P t(P P h 3)3 in to (η6-BT)F eCp + (11). Pt-
(PPh3)3 (0.050 g, 0.055 mmol) was added to a solution of [(η6-
BT)FeCp]PF6 (0.020 g, 0.050 mmol) in CH2Cl2 (5 mL) under
N2 at room temperature. The color changed from bright yellow
to orange-red and ultimately to orange. After 2 h, the solvent
was removed in vacuo and the orange residue of [15]PF6 was
recrystallized from CH2Cl2/Et2O. [15]PF6: yield 76% (0.043
g). 1H NMR (CD2Cl2): δ 7.57-7.30 (m, 18H, Ph), 7.30-7.10
(m, 13H, Ph, H2), 6.68 (br, H3), 6.34 (br, H5 or H6), 6.16 (br,
H5 or H6), 5.81 (m, 2H, H4,7), 4.50 (s, 5H, Cp). 31P NMR (CD2-
Cl2): δ 25.48 (dd, J P-P ) 22 Hz, J P-Pt ) 3350 Hz), 21.91 (dd,
J P-P ) 21.5 Hz, J P-Pt ) 1880 Hz), -143.97 (sept, J P-F ) 709
Hz). Anal. Calcd for C49H41FeP3PtSF6: C, 52.56; H, 3.69.
Found: C, 52.45; H, 3.84. A crystal of [15]PF6‚Et2O suitable
for X-ray diffraction was grown by vapor diffusion of Et2O into
an acetone solution of the complex at -15 °C over a period of
several days.
In ser tion of P t(P P h 3)3 in to (η5-2,5-Me2T)Mn (CO)3+ a n d
(η6-DBT)Mn (CO)3+. The procedure followed for these reac-
tions is essentially the same as that described above for 11,
with the reaction time being 15 min in both cases. [{η5-2,5-
Me2T‚Pt(PPh3)2}Mn(CO)3]BF4 (see eq 6): yield 80% (0.112 g).
IR (CH2Cl2): νCO 2039 (s), 1977 (s), 1966 (s) cm-1
.
1H NMR
(CD3C(O)CD3): δ 7.60-7.15 (m, 30H, Ph), 6.90 (m, 1H), 4.68
(m, 1H), 2.41 (s, Me), 1.57 (m, Me). 31P NMR (CD3C(O)CD3):
δ 19.57 (dd, J P-P ) 20.5 Hz, J P-Pt ) 1984 Hz), 16.42 (dd, J P-P
) 20.5 Hz, J P-Pt ) 3638 Hz). HR MS: M+ (m/z) calcd 970.1046,
obsd 970.1019. Anal. Calcd for C45H38O3MnP2PtSBF4: C,
51.08; H, 3.62. Found: C, 51.32; H, 3.52. [{η6-DBT‚Pt-
(PPh3)2}Mn(CO)3]BF4 ([23]BF4): yield 88% (0.180 g). IR (CH2-
Cl2): νCO 2058 (s), 2006 (s), 1996 (s) cm-1 1H NMR (CD2Cl2):
.
δ 7.58 (d, J ) 7.2 Hz, 1H), 7.50-7.20 (m, 33H, Ph), 6.32 (1H),
6.18 (1H), 5.92 (1H), 5.30 (1H). 31P NMR (CD2Cl2): δ 24.64
(dd, J P-P ) 20 Hz, J P-Pt ) 3116 Hz), 15.75 (dd, J P-P ) 19.5
Hz, J P-Pt ) 2110 Hz). MS FAB: 1042 (M+). Anal. Calcd for
In ser tion of P t(P P h 3)3 in to (η6-BT)Ru Cp + (12) a n d (η6-
BT)Ru (C6Me6)2+ (13). The procedure followed for these
reactions is essentially the same as that described above for
11, with reaction times being 2 h for 12 and 20 min for 13.
[16]PF6: yield 80% (0.114 g). 1H NMR (CD2Cl2): δ 7.43-7.30
(m, 18H, Ph), 7.30-7.15 (m, 13H, Ph, H2), 6.50 (m, H3), 6.41
(m, H5 or H6), 6.02 (m, H5 or H6), 5.76 (m, 2H, H4,7), 4.92 (s,
5H, Cp). 31P NMR (CD2Cl2): δ 26.46 (dd, J P-P ) 22 Hz, J P-Pt
) 3350 Hz), 22.47 (dd, J P-P ) 21.5 Hz, J P-Pt ) 1880 Hz). HR
MS: M+ (m/z) calcd 1020.1095, obsd 1020.1081. Anal. Calcd
for C49H41RuP3PtSF6: C, 50.47; H, 3.55. Found: C, 50.52; H,
3.55. For [17][BF4]2: yield 80% (0.135 g). 1H NMR (CD2Cl2):
δ 8.11 (m, H2), 7.49-7.21 (m, 30H, Ph), 6.43 (t, J ) 5.7 Hz,
H6), 6.36 (t, J ) 5.7 Hz, H5), 6.27 (d, J ) 6.0 Hz, H7), 6.22
(m, H3), 6.07 (d, J ) 5.8 Hz, H4), 2.14 (s, C6Me6). 31P NMR
(CD2Cl2): δ 25.29 (dd, J P-P ) 24 Hz, J P-Pt ) 3446 Hz), 21.26
C
51H38O3MnP2PtSBF4: C, 54.22; H, 3.39. Found: C, 53.59;
H, 3.62.
Ack n ow led gm en t. This work was supported by
grants from the National Science Foundation (Grant No.
CHE-9705121) and the Petroleum Research Fund,
administered by the American Chemical Society.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond lengths and angles, anisotropic displacement
parameters, and hydrogen coordinates for [10]BF4 and [15]-
PF6‚Et2O (49 pages). Ordering information is given on any
current masthead page.
OM9802865