Intramolecular Si-Si bond activation by the iron carbonyl unit in an (η4-Silole)tricarbonyliron complex

Article abstract of DOI:10.1021/om701095j

Intramolecular activation of the Si-Si bond linked to an η⁴-silole ligand by a metal center was observed when 1,1-bis(trimethylsilyl)tetraphenylsilole was heated with Fe(CO)₅ in refluxing p-xylene, to produce η⁴-(1-trimethylsilyl-1- methyltetraphenylsilole)Fe(CO)₃ as the final product.

Full text of DOI:10.1021/om701095j

Copyright 2008
Volume 27, Number 10, May 26, 2008
American Chemical Society
Communications
Intramolecular Si-Si Bond Activation by the Iron Carbonyl Unit in
an (η4-Silole)tricarbonyliron Complex
Zhensheng Zhang, Jianli Gu, Chengxun Zhang, and Huailin Sun*
Department of Chemistry and the State Key Laboratory of Elemento-Organic Chemistry, Nankai
UniVersity, Tianjin 300071, People’s Republic of China
ReceiVed October 30, 2007
intriguing reactions in recent years.3–9 To date, many studies
of such reactions have focused on systems containing Si-Si
Summary: Intramolecular actiVation of the Si-Si bond linked
to an η4-silole ligand by a metal center was obserVed when
1,1-bis(trimethylsilyl)tetraphenylsilole was heated with Fe(CO)5
in refluxing p-xylene, to produce η4-(1-trimethylsilyl-1-meth-
yltetraphenylsilole)Fe(CO)3 as the final product.
bonds directly linked to transition metals, initially observed
independently by Pannell’s and Ogino’s groups in 1986.4,5 Also
studied have been systems in which the Si-Si bonds are linked
indirectly to a metal atom through an η1-methylene ligand, a
Si-Si bonds are very thermally stable, sometimes even
comparable to a C-C bond, but can easily be activated by
transition metals.1,2 This property has most frequently been
observed in intramolecular processes, which has led to many
(4) (a) Pannell, K. H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti,
S. Organometallics 1986, 5, 1056–1057. (b) Pannell, K. H.; Rozell, J. M.;
Hernandez, C. J. Am. Chem. Soc. 1989, 111, 4482–4485. (c) Pannell, K. H.;
Sharma, H. K.; Kapoor, R. N.; Cervantes-Lee, F. J. Am. Chem. Soc. 1997,
119, 9315–9316.
* To whom correspondence should be addressed. E-mail: sunhl@
nankai.edu.cn.
(5) (a) Tobita, H.; Ueno, K.; Ogino, H. Chem. Lett. 1986, 1777–1780.
(b) Ueno, K.; Tobita, H.; Shimoi, M.; Ogino, H. J. Am. Chem. Soc. 1988,
110, 4092–4093. (c) Tobita, H.; Ueno, K.; Shimoi, M.; Ogino, H. J. Am.
Chem. Soc. 1990, 112, 3415–3420.
(1) West, R. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1983;
Vol. 9, pp 365-397.
(6) (a) Pannell, K. H.; Rice, J. R. J. Organomet. Chem. 1974, 78, C35-
C39. (b) Sharma, S.; Kapoor, R. N.; Cervantes-Lee, F.; Pannell, K. H.
Polyhedron 1991, 10, 1177–1187. (c) Pannell, K. H.; Kobayashi, T.; Kapoor,
R. N. Organometallics 1992, 11, 2229–2235. (d) Zhang, Y.; Cervantes-
Lee, F.; Pannell, K. H. J. Am. Chem. Soc. 2000, 11, 2229–2235.
(7) Nakadaira, Y.; Kobayashi, T.; Sakurai, H. J. Organomet. Chem. 1979,
165, 399–405.
(2) For reviews on Si-Si bond activation by transition metals, see: (a)
Schubert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 419–421. (b) Sharma,
H. K.; Pannell, K. H. Chem. ReV. 1995, 95, 1351–1374. (c) Suginome, M.;
Ito, Y. J. Chem. Soc., Dalton Trans. 1998, 1925–1934.
(3) Some recent papers for intramolecular activation of Si-Si bonds: (a)
Zirngast, M.; Marschner, C.; Baumgartner, J. Organometallics 2006, 25,
4897–4908. (b) Zhang, Y.; Cervantes-Lee, F.; Pannell, K. H. Organome-
tallics 2003, 22, 2517–2524. (c) Ueno, K.; Asami, S.; Watanabe, N.; Ogino,
H. Organometallics 2002, 21, 1326–1328. (d) Naka, A.; Ishikawa, M.; Cha,
S. H.; Lee, K. K.; Kwak, Y. W. J. Organomet. Chem. 2002, 645, 47–53.
(e) Tobita, H.; Sato, T.; Okazaki, M.; Ogino, H. J. Organomet. Chem. 2000,
611, 314–322. (f) Suginome, M.; Kato, Y.; Takeda, N.; Oike, H.; Ito, Y.
Organometallics 1998, 17, 495–497. (g) Schubert, U.; Pfeiffer, J.; Sto¨hr,
F.; Sturmayr, D.; Thompson, S. J. Organomet. Chem. 1997, 548, 57–63.
(h) Nlate, S.; Herdtweck, E.; Fischer, R. A. Angew. Chem., Int. Ed. Engl.
1996, 35, 1861–1863. (i) Mitchell, G. P.; Tilley, T. D. Organometallics
1996, 15, 3477–3479. (j) Suginome, M.; Oike, H.; Ito, Y. J. Am. Chem.
Soc. 1995, 117, 1665–1666.
(8) (a) Sun, H.; Xu, S.; Zhou, X.; Wang, H.; Yao, X. J. Organomet.
Chem. 1993, 444, C41-C44. (b) Sun, H.; Zhang, Z.; Pan, Y.; Yang, J.;
Zhou, X. Inorg. Chem. 2003, 42, 4076–4081. (c) Sun, H.; Pan, Y.; Huang,
X.; Guo, Z.; Zhang, Z.; Zhang, H.; Li, J.; Wang, F. Organometallics 2006,
25, 133–139. (d) Sun, H.; Gu, J.; Zhang, Z.; Lin, H.; Ding, F.; Wang, Q.
Angew. Chem., Int. Ed. 2007, 46, 7498–7500.
(9) The reaction also takes place for complexes with a Ge-Ge bond
and an Ru-Ru bond; see: (a) Zhou, X.; Xie, W.; Xu, S. Chin. Chem. Lett.
1996, 7, 385–386. (b) Xie, W.; Wang, B.; Dai, X.; Xu, S.; Zhou, X. J. Chem.
Soc., Dalton Trans. 1999, 1141–1146. (c) Zhang, Y.; Xu, S.; Zhou, X.
Organometallics 1997, 16, 6017–6020. (d) Zhang, Y.; Wang, B.; Xu, S.;
Zhou, X.; Sun, J. J. Organomet. Chem. 1999, 584, 356–360.
10.1021/om701095j CCC: $40.75
2008 American Chemical Society
Publication on Web 04/24/2008

2150 Organometallics, Vol. 27, No. 10, 2008
Communications
Scheme 1. Synthesis of the Complex
Chart 1
complex 5 but the new complex 6, in which one of the Si-Si
bonds has been lost.14b
The 1H NMR spectrum showed the presence of two types of
SiMe groups in a 1:3 ratio, in accord with the structure of 6.
The IR spectrum exhibited strong absorption of the carbonyl
ligands. The molecular structure of 6, determined by X-ray
crystal diffraction (Figure 1), clearly demonstrated the presence
of the methyl group at the endo position.15 The silole ligand
coordinates to the iron atom in an η4 fashion. The silicon atom
of the silole ring is bent up from the plane of the diene unit
with a dihedral angle of 34.5°, as has usually been observed in
such complexes (within the range of 8.9-44.5°).10
reaction first reported by Pannell and co-workers in 1974.6
Sakurai and co-workers in 1979 reported that the Si-Si bonds
in η4-1,2-disilacyclohexadiene complexes could also be acti-
vated.7 However, study of the η4 systems to date has been
restricted to this single example. In 1993, we reported that the
Si-Si bond in Si-Si-bridged bis(cyclopentadienyl)diiron com-
plexes could be activated.8,9 During the mechanistic study of
this reaction, it was recognized that oxidative addition to a
coordinatively unsaturated iron atom was responsible for the
easy activation of the Si-Si bond in this and in the systems
noted above.8c Thus, it is very likely that such oxidative addition
might constitute a general mode of Si-Si bond activation in
iron complexes.
The observed result showed that the endo Si-Si bond has
been activated and that the trimethylsilyl group on the silole
ligand has been replaced by a methyl group. It is noteworthy
that the reaction occurred only at the endo Si-Si bond but not
at the exo Si-Si bond. This indicates that the Si-Si bond
activation has taken place intramolecularly after the iron
carbonyl group coordinated to the silole ligand, as indicated in
Scheme 1.
Silacyclopentadienes (or siloles) are known η4 ligands in
transition-metal complexes, which have been studied extensively
over the past few years.10 Complexes containing Si-Si bonds
linked to silole ligands, e.g., 1-3 (Chart 1), were reported by
Corriu and co-workers,11 but for none of them was Si-Si bond
cleavage by a metal species reported. On the basis of our
knowledge of the Si-Si bond reactivity, we thought that this
may be due to the fact that the approach of the Si-Si bond by
the metal species was hindered in such compounds,12 since the
metal always was coordinated to the silole ligand from the side
away from the Si-Si bond. However, if two silyl substituents
were attached to the silicon atom of the silole ring, interaction
of one of these Si-Si bonds with the metal species would be
able to occur on coordination of the metal from either sides of
the silole ring. We report here that this idea has merit and has
led to the first observation of such Si-Si bond activation in an
η4-silole complex.
By analogy to the reaction of η4-1,2-disilahexadiene systems,7
the mechanism of the present transformation can be rationalized
as shown in Scheme 2. First, the η4-η2 slippage of the silole
(14) (a) Synthesis of 4 by method B: to a suspension of 1,4-
dilithiotetraphenylbuta-1,3-diene in 100 mL of diethyl ether at-78 °C,
obtained from 5.50 g (31 mmol) of diphenylacetylene and 0.216 g (31 mmol)
of lithium according to the literature procedure,13 was added 3.80 g (15.5
mmol) of 2,2-dichlorohexamethyltrisilane. A 100 mL portion of THF was
added, and the resulting mixture was heated to reflux for 2 h. It was cooled
to room temperature and hydrolyzed with water. The organic layer was
separated, washed with water, and dried over anhydrous sodium sulfate.
Removal of the solvent, followed by separation through a column (silica,
petroleum ether), gave 864 mg of 4 (5.7% yield) contaminated by a
byproduct characterized as 1,4-bis(trimethylsilyl)tetraphenyldutadiene. The
origin of the byproduct has not yet been determined. Pure 4 was obtained
by fractional crystallization of the product in pentane at-20 °C. (b) Synthesis
of 6: 200 mg (0.38 mmol) of 4 and l mL (ca. 7 mmol) of Fe(CO)5 were
added to a flask containing 20 mL of p-xylene. The mixture was heated to
reflux with stirring. The reaction required 100 h, as determined by 1H NMR
monitoring of its progress by periodically withdrawing samples from the
reaction flask. After the mixture was cooled to room temperature, the solvent
was removed under reduced pressure (unreacted Fe(CO)5 should be collected
in a liquid nitrogen trap!) to give a solid residue. Separation of this product
mixture by column chromatography (Al2O3, 4/1 petroleum ether/dichlo-
romethane) gave 60 mg (25% yield) of red crystals of 6, mp 190-192 °C.
Anal. Calcd for C35H32O3FeSi2: C, 68.62; H, 5.26. Found: C, 68.76; H,
5.34. 1H NMR (CDCl3): δ 0.36 (s, 9H, SiMe3), 0.92 (s, 3H, Me), 6.77-
7.24 (m, 20H, Ph). 13C NMR (CDCl3): δ-1.2 (Me), 0.5 (SiMe3), 63.6
(diene), 111.0 (diene), 125.5 (Ph), 127.5 (Ph), 1127.6 (Ph), 127.9 (Ph), 130.5
(Ph), 133.0 (Ph), 134.7 (Ph), 140.2 (Ph), 211.4 (CO). 29Si NMR (CDCl3):
δ-20.1 (SiMe), 18.4 (SiMe3). IR (KBr): νco 2033 (s), 1968 (s), 1933 (s)
Synthesis of the silole compound 4 was accomplished by
starting from 1,4-dilithiotetraphenylbuta-1,3-diene, either by the
three-step literature procedures (Scheme 1; path A)13 or by
simple treatment with 2,2-dichlorohexamethyltrisilane (B).14a
Reaction of 4 with pentacarbonyliron was performed in refluxing
p-xylene. The product isolated, however, was not the expected
(10) Colomer, E.; Corriu, R. J. P.; Lheureux, M. Chem. ReV. 1990, 90,
265–282.
(11) Carre, F.; Colomer, E.; Corey, J. Y.; Corriu, R. J.; Guerin, C.;
Henner, B. J. L.; Kolani, B.; Man, W. W. C. Organometallics 1986, 5,
910–917.
(12) A similar case has been reported in the case of 1,1,1,3,3,3-
hexamethyltrisilylene-bridged bis(cyclopentadienyl) complexes; see: Sun,
H.; Zhang, Z. Polyhedron 2007, 26, 1211–1216.
cm-1. IR (CS2): νco 2035 (s), 1973 (vs) cm-1
.
(13) (a) Joo, W.-C.; Hong, J.-H.; Choi, S.-B.; Son, H.-E.; Kim, C. H. J.
Organomet. Chem. 1990, 391, 27–36. (b) Hong, J.-H.; Boudjouk, P.;
Castellino, S. Organometallics 1994, 13, 3387–3389. (c) West, R.; Sohn,
H.; Powell, D. R.; Mu¨ller, T.; Apeloig, Y. Angew. Chem., Int. Ed. 1996,
35, 1002–1004.
(15) Crystallographic data for 6: Mo KR (λ ) 0.710 73 Å) radiation,
monoclinic, space group P21/n, a ) 8.965(3) Å, b ) 19.433(5) Å, c )
18.239(4) Å, R ) 90°, ꢀ ) 95.184(6)°, γ ) 90°, Z ) 4, full-matrix least
squares on F2, GOF 0.974, R1 ) 0.0669/wR2 ) 0.1217 for 5571 reflections
(I > 2σ(I)), R1 ) 0.1700/wR2 ) 0.1658 for all data.

Communications
Organometallics, Vol. 27, No. 10, 2008 2151
the η3-silapropenyl complex 7 as the key intermediate. Migration
of the methyl group between two silicon centers of 7 produces
the dimethylsilylene complex, which would easily lose the
silylene, accompanied by recombination of the C-C double
bond of the silole ligand.
The formation of an η3-silapropenyl intermediate is the critical
step for Si-Si bond activation. Although η1-silylene, η2-silene,
and η4-silatrimethylenemethane intermediates have been well
documented, the formation of an η3-silapropenyl intermediate
via Si-Si bond activation has not been easy to verify.16,17 Even
though such intermediates have been suggested in reactions of
η4-1,2-disilacyclohexadiene and other systems,7,18 the isolation
of stable η3-silapropenyl complexes had not been successful
until 2003, when Sakaba and co-workers isolated the first stable
complex of this type,19 which ultimately provided a solid
foundation for both the previous and present explanation of the
Si-Si bond activation reactions.
The migration of the methyl group between two silicon atoms
is another important process. Similar reactions recently have
been of interest.20 Previous attention has mainly been concen-
trated on alkyl or aryl migration in silyl(silylene) complexes.
The only example of a silyl(η3-silapropenyl) complex was
mentioned by Sakurai et al. in the η4-1,2-disilacyclohexadiene
systems, which involved only the migration of the ring skeleton
group and resulted in ring contraction. In the present case, the
migration of a substituent on the tertiary silyl group might
provide an opportunity for more convenient study of such
processes.
Figure 1. Molecular structure of 6 with thermal ellipsoids at the
30% probability level. Selected bond lengths (Å) and angles (deg):
Si(1)-Si(2) ) 2.385(2), Si(1)-C(32) ) 1.870(6), Si(1)-C(4) )
1.860(5), Si(1)-C(7) ) 1.873(6), C(4)-C(5) ) 1.427(7), C(5)-C(6)
) 1.447(7), C(6)-C(7) ) 1.431(7), Fe(1)-C(1) ) 1.772(7),
Fe(1)-C(2) ) 1.790(7), Fe(1)-C(3) ) 1.795(7), Fe(1)-C(4) )
2.128(5), Fe(1)-C(5) ) 2.060(5), Fe(1)-C(6) ) 2.085(5),
Fe(1)-C(7))2.189(5);C(1)-Fe(1)-C(2))96.4(3),C(1)-Fe(1)-C(3)
) 88.8(3), C(2)-Fe(1)-C(3) ) 95.5(3), C(4)-Si(1)-C(7) )
85.4(2), C(32)-Si(1)-Si(2) ) 107.7(2).
Acknowledgment. This study was supported by the NSFC
(Nos. 20372036 and 20421202), the Natural Science Foun-
dation of Tianjin (No. 05YFJMJC06800), and the Ministry
of Education of China (No. 03406).
Scheme 2. Proposed Mechanism
Supporting Information Available: CIF file giving X-ray
diffraction data for 6. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM701095J
(16) (a) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Am. Chem. Soc.
1976, 98, 7453–7454. (b) Sakurai, H.; Kamiyama, Y.; Mikoda, A.;
Kobayashi, T.; Sasaki, K.; Nakadaira, Y. J. Organomet. Chem. 1980, 201,
C14-C18. (c) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Organomet.
Chem. 1980, 184, 13–30.
(17) Radnia, P.; Mckennis, J. S. J. Am. Chem. Soc. 1980, 102, 6349–
6351.
(18) Dai, X.; Kano, N.; Kako, M.; Nakadaira, Y. Chem. Lett. 1999, 717–
718.
(19) Sakaba, H.; Watanabe, S.; Kabuto, C.; Kabuto, K. J. Am. Chem.
Soc. 2003, 125, 2842–2843.
ligand creates a vacant coordination site at the iron atom.
Oxidative addition of the Si-Si bond then takes place, giving
(20) Tobita, H.; Matsuda, A.; Hashimoto, H.; Ueno, K.; Ogino, H.
Angew. Chem., Int. Ed. 2004, 43, 221–224, and references cited therein.