Reduction of 1-chloro-1,2,3,4,5-pentaphenylsilole: Formation of silole monoanion and dianion

Article abstract of DOI:10.1016/j.jorganchem.2003.09.040

The reduction of 1-chloro-1,2,3,4,5-pentaphenylsilole, (C₄Ph₄SiPhCl, 1) with 2 equiv lithium gave the pentaphenylsilole anion [C₄Ph₄SiPh]^- (2), silole dianion [C₄Ph₄Si]^2-

Full text of DOI:10.1016/j.jorganchem.2003.09.040

Journal of Organometallic Chemistry 689 (2004) 134–138
www.elsevier.com/locate/jorganchem
Reduction of 1-chloro-1,2,3,4,5-pentaphenylsilole: formation
of silole monoanion and dianion
Honglae Sohn *
Department of Chemistry, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501759, Republic of Korea
Received 28 May 2003; accepted 29 September 2003
Abstract
The reduction of 1-chloro-1,2,3,4,5-pentaphenylsilole, (C4Ph4SiPhCl, 1) with 2 equiv lithium gave the pentaphenylsilole anion
ꢀ 2ꢀ
[
C4Ph4SiPh] (2), silole dianion [C4Ph4Si] (3), and hexaphenylsilole C4Ph4SiPh2 (4). 2, 3, and 4 from the reaction mixture were
characterized by 29Si NMR spectroscopy. The 29Si chemical shift of 3.7 ppm for 2 is shifted upfield as compared to that of previously
reported t -butyltetraphenylsilole anion Li[C4Ph4SitBu], but shifted downfield compared to that of the other silole monoanion such
as Li[C4Me4SiSiMe3], indicating the delocalization of silole anion through the 5-membered ring. Derivatization of the reaction
mixture with iodomathane gave C4Ph4SiPh2 (4), C4Ph4SiMePh (5), and C4Ph4SiMe2 (6), which were characterized by H, 13C, and
29Si NMR spectroscopy. The silole dianion 3 could be either from the continuous reduction of 1 with lithium or from the dis-
1
proportionation of 2. The reduction of 1 with excess lithium in THF gave the silole dianion [C4Ph4Si]2 in about 70% yield.
ꢀ
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Reduction; Delocalization; Silole monoanion; Silole dianion
1
. Introduction
porting materials in devices [19] such as light-emitting
diodes (LEDs) [20–22] or in chemical sensors [23,24].
þ
2ꢀ
In recent times, a considerable amount of interest has
X-ray crystal structures of [Li (THF) ][C Ph Si
5
]
4
4
2
þ
2ꢀ
developed with regard to anions of siloles and their
structure, bonding, reaction, and possible aromaticity
leading to novel optical properties for a variety of ap-
plications [1–14]. Analogous of cyclopentadiene, siloles
are p-electron systems containing a single silicon atom
as part of a cyclic 5-membered ring. Characteristic fea-
tures of siloles include a low reduction potential and a
low-lying LUMO due to r –p conjugation arising from
the interaction between the r orbital of silicon, and the
p orbital of the butadiene moiety of the five membered
ring [15,16]. Since silole and germole dianions (RC)4Si
and (RC)4Ge2 , R ¼ Ph and Me, were initially studied
by X-ray crystallography [3–6], siloles and germoles
have received growing attention, both because of their
unusual electronic and optical properties [17,18] and
because of their possible application as electron trans-
[3] and [K(18-crown-6) ] 2[C4Me4Si ] [5] indicate that
the silole dianions are highly delocalized and aromatic.
In contrast, several crystal structures of silole monoan-
ion have been published and showed the localization of
5-membered ring. For example, [K(18-crown-
6)][C Me SiSiMe ] [6] has a localized structure. Only
4
4
3
one silole monoanion Li[C Ph SitBu] has been pub-
4
4
ꢁ
ꢁ
ꢁ
lished as a delocalized structure by the interpretation of
13C and 29Si NMR data [7a], however its crystal struc-
ture has not been reported to date.
ꢁ
2
ꢀ
ꢀ
Ph
Ph
Si
Recently, it has been reported that the methylsilole
ꢀ
anion [C Ph SiMe] dimerized to the tricyclic diallylic
4 4
*
Tel.: +82-622307372; fax: +82-622344326.
E-mail address: hsohn@chosun.ac.kr (H. Sohn).
2
ꢀ
dianion [C4Ph4SiMe]
by head-to-tail, 2 + 2 cycload-
2
0
022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.09.040

H. Sohn / Journal of Organometallic Chemistry 689 (2004) 134–138
135
dition [25]. The possible mechanism for the tricyclic
diallylic dianion could take place in the mesomeric forms
having Si@C double bond character of methylsilole
monoanion. In addition, the reduction of methylsilole
products 4, 5, and 6 were separated by preparative scale
1
size exclusion chromatography and characterized by H,
13C, and 29Si NMR spectroscopy.
The isolated yield of the products 4, 5, and 6 is 25%,
13%, and 25%, respectively. The ratio of isolated prod-
uct distribution of 4:5:6 is 2:1:2, showing that the for-
mation quantity of silole dianion 3 and hexaphenylsilole
4 is equal. The silole dianion 3 could be either from the
continuous reduction of 1 with lithium or from the
disproportionation of 2. Fig. 1 shows two possible
mechanisms, over-reduction and disproportionation.
Disproportionation exhibits the equal formation of sil-
ole 3 and 4. However, the formation of the diphenyl-
silole 4 may result from the reaction of 1 with
phenyllithium, which is produced during the over-re-
duction of 2 to the silole dianion 3. This mechanism
would also produce 3 and 4 in equal amounts. We are
not able to observe the phenyl derivative. Therefore, any
mechanisms are not ruled out at this stage.
[C4Ph4SiMeH] would involve the presence of the equilib-
rium between carboanion and silyl anion [26]. Therefore, it
seems of interest to investigate the pentaphenylsilole
monoanion.
Ph
Ph
Ph
Ph
Me Ph
Si
2
Ph
Ph
Si
Si
Me
Ph
Ph
Me
2
. Results and discussion
Herein the reduction of 1-chloro-1,2,3,4,5-pentaphe-
nylsilole, (C4Ph4SiPhCl, 1) to the pentaphenylsilole an-
While silole 5 forms after quenching, indicating the
presence of the pentaphenylsilole anion 2, the silole 6 is
2
ꢀ
expected from the silole dianion [C Ph Si] (3). Using a
4 4
ꢀ
ion [C4Ph4SiPh] (2), and the derivatization products;
10 mm NMR tube containing 1 (0.248 g, 0.50 mmol)
2
9
C4Ph4SiPh2 (4), C4Ph4SiMePh (5), and C4Ph4SiMe2 (6)
is reported. The reduction of 1 with 2 equiv of lithium in
THF at )78 °C gave a dark red solution. The addition
of excess iodomethane to the solution produced mixture
products of 4, 5, and 6. (Scheme 1) The derivatization
and lithium (0.007 mg, 1.0 mmol) in THF-d , Si NMR
8
study was carried out for the dark red solution. 29Si
NMR spectrum of dark red solution showed three 29Si
resonance signals at 67.0, 3.7, and –6.0 ppm which
shows the presence of silole dianion 3, silole anion 2, and
a
Ph
Ph
Ph
Ph
+
Ph
Ph + Ph
Ph
Si
Ph Cl
1
Si
Si
Si
+
Ph
Li
+
Ph
Ph
2Li
2
3
4
b
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph + Ph
Ph
Ph
Ph
+
Si
Si
Si
Ph
Me
Me
Me
Ph
Ph
Scheme 1. Reduction of C4ph4SiPhCl, 1 , with 2 equiv lithium. Reagents and conditions: (a) 2Li/THF/)78 °C – RT/2 h; (b) excess MeI/THF/)78 °C –
RT/3 h.
i
Ph
Ph + Ph
Ph
Ph
Si
Si
Ph
Ph
Ph
Ph
Ph
+
2Li
3
4
Si
+
Ph
Li
Ph
Ph
Ph
Ph
Ph
Ph
Ph + Ph
Ph
Ph
Ph
2
ii
1
+
PhLi
Si
Si
Si
+
Ph
Ph
2Li
+
2
Li
Fig. 1. Two possible mechanisms for the equal formation of 3 and 4: (i) disproportionation; (ii) over-reduction.

136
H. Sohn / Journal of Organometallic Chemistry 689 (2004) 134–138
hexaphenylsilole C4Ph4SiPh2 (4), respectively. Silole di-
anion 3 and hexaphenylsilole 4 were independently
synthesized and characterized by 29Si NMR spectros-
copy, and gave the two 29Si chemical shifts of 67.0 and –
represents a new synthetic route to the silole dianion via
slightly more air stable monohalogenated silole starting
materials.
6
.0 ppm for silole dianion 3 and hexaphenylsilole 4,
respectively. The 29Si chemical shift of 3.7 ppm for 2 is
shifted upfield as compared to that of previously re-
ported t-butyltetraphenylsilole anion Li[C Ph SitBu]
3. Experimental
3.1. General
4
4
(
25.1 ppm) [7], but shifted downfield compared to that
of the other silole monoanion such as Li[C4Me4SiSiMe3]
)45.38 ppm) [6]. It is believed that this is caused by the
All synthetic manipulations were carried out under an
atmosphere of dry argon gas using standard vacuum-
line Schlenk techniques. All solvents were degassed and
purified before use according to standard literature
methods: diethyl ether, hexanes, tetrahydrofuran, and
toluene were purchased from Aldrich Chemical Co. Inc.
and distilled from sodium/benzophenone. All other re-
agents (Aldrich, Gelest) were used as received or dis-
tilled before use. NMR data were collected with Bruker
(
delocalization of silole anion through the 5-membered
ring. It should be noted that one of the products ob-
served after quenching is the silole dianion 3 that is
formed as a result of cleavage of Si–C bond of the
phenyl-silicon moiety.
AC-300, 400, or 500 MHz spectrometers (300.1 MHz for
1
Ph
Ph
Ph
Ph
H NMR, 75.5 MHz for 13C NMR and 99.2 MHz for
Ph
Ph
Si
Si
Si
29Si NMR). The NMR solvent THF-d8 was purified by
vacuum distillation from Na/K alloy. Chloroform-d was
stirred over CaH2 for 1 day, transferred by vacuum
distillation onto P O , stirred for 2 h, and then vacuum-
ꢀ
Methylsilole anion [C Ph SiMe] (8) dimerized to the
4
4
2
5
2
2
ꢀ
tricyclic diallylic dianion [C Ph SiMe] (9) by head-to-
4
distilled for purification. Chemical shifts are reported in
parts per million (d ppm); downfield shifts are reported
as positive values from tetramethylsilane (TMS) stan-
dard at 0.00 ppm. The H and 13C chemical shifts were
3
4
tail, 2 + 2 cycloaddition [25], which means that the anion
of 8 is delocalized over the silicon-containing cyclic 5-
membered ring. However, the dimerization of penta-
phenylsilole anion, [C4Ph4SiPh] (2), did not take place.
Possible reasons for this are that 2 is less nucleophilic
1
ꢀ
referenced relative to CHCl (d ¼ 77:0 ppm) as an in-
2
9
ternal standard, and the Si chemical shifts were refer-
13
ꢀ
than [C4Ph4SiMe] and that the steric hindrance of
enced to an external TMS standard. C NMR were
recorded as proton decoupled spectra, and 29Si NMR
spectra were acquired using an inverse gate pulse se-
quence with a relaxation delay of 30 s. High-resolution
electron-impact ionization mass spectrometry was per-
formed on an MS80 Kratos spectrometer.
phenyl group at silicon in 2 is too large to allow di-
merization. Unfortunately, attempts to obtain crystals
of 2 for the X-ray analysis were not successful.
The reduction of 1 with excess lithium in THF also
gave dark red solution. (Scheme 2) However, after re-
moval of unreacted lithium, the derivatization of reac-
tion mixture with iodomethane gave 6 in about 70%
yield, respectively. The 29Si NMR spectrum of latter
solution gave only single resonance at 67 ppm that is the
silole dianion 3. The pentaphenylsilole anion 2 and
hexaphenylsilole 4 can be further reduced to give silole
dianion 3 in latter reaction, due to the good leaving
group of phenyl moiety in pentaphenylsilole anion 2.
Although the standard synthetic route for the for-
mation of silole dianion is via the dehalogenation of
dihalosiloles, the observation that monohalogenated
silole also results in the formation of a silole dianion
3.2. Preparation of 1-chloro-1,2,3,4,5-pentaphenylsilole
1-Chloro-1,2,3,4,5-pentaphenylsilole (1) and 1,1,2,3,
4,5-hexaphenylsilole (4), were prepared as described in
the literature [12], by adding the corresponding phenyl-
trichlorosilane and diphenyldichlorosilane, respectively.
1,1-dilithio-2,3,4,5-tetraphenylsilole (3) was prepared as
described in the literature [3]. The NMR spectra of 6
agreed with those reported earlier [27].
1
Selected data for 1. H NMR (300.133 MHz, CDCl ):
3
d ¼ 6:80–7:10 (m, 20H, Ph), 7.35–7.50 and 7.75–7.85
Ph
Ph
Ph
Ph
Ph
Ph
Ph
excess Li
MeI
Ph
Ph
Ph
Si
Si
Si
Ph
Cl
+
Me
Me
2Li
Scheme 2. Reduction of 1 with excess Li.

H. Sohn / Journal of Organometallic Chemistry 689 (2004) 134–138
137
(
7
(
1
m, 5H, Ph); 13C{H} NMR (75.403 MHz, CDCl3 ðd ¼
to remove the residual lithium metal. The solution was
kept at )78 °C. Excess iodomethane was added by a
syringe in one portion at )78 °C. The mixture was kept
at )78 °C for 5 min before the cooling bath was re-
moved, then the solution was allowed to warm up slowly
to room temperature and stirred for 3 h to give a yellow
solution. Then the volatiles were removed under reduced
pressure. The residue was extracted with hexane and
filtered. The solution was concentrated and cooled to
)20 °C for the crystallization. The product 6 was iso-
lated in about 70 % yield.
7:00Þ): d ¼ 126:36 (C), 126.91 (C), 127.67 (C), 127.95
C), 128.48 (C), 129.19 (C), 129.68 (C), 130.22 (C),
31.15 (C), 134.37 (C), 135.64 (C), 137.15 (C), 137.74
(
C), 156.26 (C); 29Si NMR (INEPT, 99.363 MHz,
þ
CDCl ): d ¼ 5:00: MS(EI): m/z (%): 496 (24) [M ],
3
High-resolution MS: calcd. for C H SiCl 496.1414
25
34
found 496.1432.
3.3. Reduction of 1-chloro-1,2,3,4,5-pentaphenylsilole
with 2 equiv Li
1
(2.48 g, 5.0 mmol) and lithium (69 mg, 10 mmol)
3.5. NMR study of 2
was stirred in 50 ml of THF at )78 °C, then the solution
was allowed to warm up slowly to room temperature
and stirred for an additional 2 h to give a dark red so-
lution. The solution was kept at )78 °C. Excess
iodomethane (ca. 20 mmol) was added by a syringe in
one portion at )78 °C. The mixture was kept at )78 °C
for 5 min before the cooling bath was removed, then the
solution was allowed to warm up slowly to room tem-
perature and stirred for 3 h to give a yellow solution.
Then the volatiles were removed under reduced pres-
sure. The residue was extracted with small portions of
toluene (a total of 50 ml) and filtered. The products 4, 5,
and 6 were separated by preparative size exclusion
chromatography. Each solution was concentrated and
cooled to )20 °C for the crystallization.
To a 10 mm NMR tube containing 1 (0.248 g, 0.50
mmol) in THF-d8 was added lithium (0.007 mg, 1.0
mmol), then the NMR tube was sealed under vacuum.
The solution was kept at )78 °C, then the solution was
allowed to warm up to room temperature and stirred for
an additional 2 h to give a dark red solution. 13C NMR
spectrum was measured, however it was not clear to
analyze the products, respectively. 29Si NMR (inversed
gated decoupling, 99.36 MHz, THF-d8/reference; ex-
ternal TMS): d ¼ ꢀ6:0, 3.7, 67.0.
Acknowledgements
1
,1,2,3,4,5-hexaphenylsilole (4). Greenish-yellow
The author greatly acknowledges the advice and
useful discussions provided by Professor Robert West
University of Wisconsin-Madison).
crystals (isolated yield ¼ 25%). Selected data; M.p. 190–
1
1
with those reported earlier [9]; C{H} NMR (75.403
91 °C (lit. 186 –187) [9]. H NMR spectra of 4 agreed
(
1
3
MHz, CDCl3 ðd ¼ 77:00ÞÞ: d ¼ 125:61 (C), 126.35 (C),
1
27.42 (C), 127.74 (C), 128.23 (C), 128.32 (C), 129.19
C), 129.96 (C), 130.11 (C), 131.59 (C), 136.08 (C),
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