Diacenaphtho[1,2-b;1',2'-d]silole and-pyrrole

Article abstract of DOI:10.1246/cl.2011.1437

π-Extended silole and pyrrole analogs fused with two acenaphthylene units were synthesized from 2,2'-dibromo-1,1'-biacenaphthylene. The pyrrole was obtained as a stable compound, while the silole was thermally stable, but highly acid-sensitive purple crystals. The molecular structures reveal that both heteroles have a planar π-system and stack to form a head-to-tail dimer structure. Observed electronic and electrochemical properties obviously indicate low LUMO energy level of the silole and high HOMO level of the pyrrole as predicted by DFT calculations.

Full text of DOI:10.1246/cl.2011.1437

doi:10.1246/cl.2011.1437
Published on the web December 5, 2011
1437
Diacenaphtho[1,2-b;1¤,2¤-d]silole and -pyrrole
Yoshiharu Nagasaka,1 Chitoshi Kitamura,1 Hiroyuki Kurata,2 and Takeshi Kawase*1
1Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo,
2167 Shosha, Himeji, Hyogo 671-2280
2Department of Chemistry, Graduate School of Science, Osaka University,
1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043
(Received August 11, 2011; CL-110675; E-mail: kawase@eng.u-hyogo.ac.jp)
³-Extended silole and pyrrole analogs fused with two
acenaphthylene units were synthesized from 2,2¤-dibromo-1,1¤-
biacenaphthylene. The pyrrole was obtained as a stable
compound, while the silole was thermally stable, but highly
acid-sensitive purple crystals. The molecular structures reveal
that both heteroles have a planar ³-system and stack to form a
head-to-tail dimer structure. Observed electronic and electro-
chemical properties obviously indicate low LUMO energy level
of the silole and high HOMO level of the pyrrole as predicted by
DFT calculations.
The syntheses of the heteroles are outlined in Scheme 2.
Treatment of 1,2-dibromoacenaphthylene8 with 1 equiv of n-
BuLi in THF at ¹78 °C for 1.5 h and subsequent oxidation with
CuCl2 afforded 2,2¤-dibromo-1,1¤-biacenaphthylene (2) in 43%
yield.9 Treatment of 2 with 2.2 equiv of n-BuLi at ¹78 °C in
THF afforded 5 as a stable solid in 64% yield upon quenching
with chlorotrimethylsilane. The result proved the successful
generation of corresponding dilithium compound under the
reaction conditions. Quenching the dianion with dichlorodiphen-
ylsilane produced a purple solution upon usual aqueous work-up
and extraction with toluene. The extract was diluted with hexane
to precipitate diphenyl derivative 3a as purple crystals. It is a
fairly thermally stable compound, but chromatographic purifi-
cation using silica gel lead to decomposition to give 1,1¤-
biacenaphthylene (6)10 in low yields. Contrary to the purple
color of 3a, disilylated and unsubstituted 1,1¤-biacenaphthyl-
enes, 5 and 6, have pale yellow and yellow colors, respectively.
Based on recent intensive studies on the catalytic double N-
arylation of amines with organic halides,11,12 [Pd2(dba)3]/t-
Bu3P/NaOt-Bu was first employed for the synthesis of 4 from 2.
The pyrrole 4 was obtained as a sole isolated product in 26%
Recently, polycyclic conjugated systems involving heterole
unit(s) have been received much attention for application in
organic electronics such as organic field effect transistors
(OFET) and organic light-emitting diodes (OLEDs).1 Diace-
naphtheno[1,2-b;1¤,2¤-d]thiophene (1) is readily prepared by
heating acenaphthene with sulfur.2 Since the first publication in
1939, a variety of studies relating to derivatization,3 reactivities,4
and molecular structures5 have been performed to date. How-
ever, no other heterole analogs bearing a diacenaphtheno
skeleton have been synthesized, probably due to the lack of
appropriate synthetic method.6 According to recent development
of heterole synthesis,7 2,2¤-dibromo-1,1¤-biacenaphthylene (2)
could serve as a suitable precursor for the corresponding
heteroles. We herein discuss the successful synthesis of the ³-
extended silole 3a and pyrrole 4 from 2 (Scheme 1) and their
electronic and structural properties together with theoretical
studies.
TMS TMS
ii
5
Br
Br
Ph
Ph
iii
iv
Si
i
2
Br Br
S
3a
Ph
N
H H
2
1
Ph
N
R
R
Si
4
6
Scheme 2. Reaction conditions; i) 1 equiv n-BuLi, ¹78 °C,
1.5 h in THF, then, CuCl2 (43%); ii) 2.2 equiv n-BuLi, ¹78 °C,
30 min in THF, then, (CH3)3SiCl (64%); iii) 2.2 equiv n-BuLi,
¹78 °C, 30 min in THF, then, Ph2SiCl2 (50%); iv) C6H5NH2, 9
equiv t-BuONa, 0.5 equiv Cy3P, 8 mol % [Pd2(dba)3] in o-xylene
at 150 °C for 14 h (29%).
3a: R = Ph, 3b: R = H
4
Scheme 1. Diacenaphtheno[1,2-b;1¤,2¤-d]heteroles 1, 3, and 4
together with a precursor 2.
Chem. Lett. 2011, 40, 1437-1439
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1438
Figure 2. TD-DFT (RB3LYP/6-31G(d)) calculations of 4, 1,
and 3b.
Figure 1. UV spectra of 2, 3a, 4, 5, and 6 in CH2Cl2.
yield. When Cy3P was used as a ligand, the yield was improved
slightly (29%). The reaction at relatively low temperature
(toluene reflux) did not improve the yield. 4 was fairly stable,
1
dark reddish crystals. H and 13C NMR spectra of the heteroles
are consistent with the expected structures.
Figure 1 shows the absorption spectra of 3a and 4, and the
1,1¤-biacenaphthylenes 2, 5, and 6 in CH2Cl2. The spectra of all
compounds except 4 consist of roughly two absorption bands in
the measurable range. The longest absorption maximum of 3a
shows a large bathochromic shift (>100 nm) related to those of
5, 6, and the known thiophene 1 (457 nm).13 On the other hand,
the spectrum of 4 consists of three absorption bands. The longest
absorption band was observed at about 620 nm as a shoulder
peak, and the band almost disappeared in CH3CN (Figure S19).
The solvent effect would be ascribed to the charge-transfer
character of the transition.
The optimized structures of heteroles 3b, 4, and 1 together
with biacenaphthylenes 5 and 6 were calculated with the DFT
(RB3LYP/6-31G(d)) basis set and illustrated in Figures S2-S6.9
Parent silole 3b (R = H) was employed in place of 3a as a
model compound. The heteroles possess planar ³-skeletons,
while 5 and 6 have largely twisted structures around the 1,1¤-
pinch bonds (5 for 73° and 6 for 40°). The longest absorption of
6 was observed at 55 nm longer wavelength than that of 5. The
relatively large difference can mainly be account for by the
largely twisted conformation interrupting the conjugation. The
electronic properties of the heteroles were also calculated with
TD-DFT (RB3LYP/6-31G(d)).9,14 The energy diagrams are
shown in Figure 2, and Tables S1 and S2.9 The MO coefficient
structures of the HOMO and LUMO of 1 and 4 bearing unshared
electron pair(s) on the hetero atoms are quite similar to each
other. The HOMO-LUMO gap of 3b (2.40 eV) is considerably
smaller than those of 1 (3.09 eV) and 4 (3.01 eV). Particularly,
the LUMO energy level of 3b is considerably lower than those
of 1 and 4, whereas the differences of HOMO energies are
relatively small. The small band gap of 3b is probably due to the
participation of Si-C · bonds in the ³-conjugation of the silole
moiety.1a,1c This is in accord with the observed absorption
spectrum of 3a. On the other hand, the predicted second
absorption of 4 (487 nm, f = 0.004) mainly due to the HOMO-
LUMO+1 transition is close to the predicted first absorption
Figure 3. a) ORTEP drawing of
a dimer structure of
independent two molecules, and b) a molecular packing of 3a,
c) ORTEP drawing of a dimer structure of independent two
molecules, and d) a molecular packing of 4.
(498 nm, f = 0.12) mainly due to the HOMO-LUMO transition.
Because the large MO coefficient locates on the nitrogen atom
of 4 in the LUMO+1, the LUMO+1 energy level would be
susceptible to the polarity of solvents. Thus, the theoretically
assigned LUMO+1 would correspond to the actual LUMO in
CH2Cl2. In contrast to dibenzosilole6 and carbazole deriva-
tives,15 unfortunately, 3a and 4 show no fluorescence in both
solution and the solid state.
Good single crystals of 3a and 4 were collected by
recrystallization from hexane/CH2Cl2 solutions, respectively.
The crystal of 3a involves one hexane molecule in one unit cell.
The largely disordered hexane molecule increased the R1 value.
The ORTEP drawings reveal that they have a planar fused ring
system as expected (Figure 3).16 The selected averaged bond
lengths and angles of 3a and 4 together with 1 are listed in
Figures S7-S9.9 3a shows middle bond alternation in the silole
ring (1.386 and 1.474 ¡) in comparison to that of hexaphenyl-
silole (1.357 and 1.501 ¡)17 and the pyrrole ring of 4 (1.395 and
1.411 ¡). The bond lengths of 4 are almost comparable to the
Chem. Lett. 2011, 40, 1437-1439
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1439
thiophene ring of 1 (1.398 and 1.404 ¡).5 The two phenyl
substituents of 3a were tilted almost perpendicular to each other;
the protons of the phenyl groups direct toward the ³ plane of
the phenyl groups of neighbor molecules, which formed an
intermolecular CH-³ interaction network. Moreover, the silole
³ systems stack well in a head-to-tail manner; the intermolecular
distances are considerably short (3.35 and 3.41 ¡), which
indicated the strong ³-³ interaction. The molecules construct
a one-dimensional columnar structure in the crystal (Figure 3b).
On the other hand, the phenyl group of 4 is tilted 53.5° from the
pyrrole plane; the protons of phenyl group direct toward the
pyrrole ³ plane of a neighbor molecule. The resulting CH-³
interactions build a slipped parallel stacking in the crystal
(Figure 3d). The considerably short intermolecular distance
(3.39 ¡) should also indicate the operation of strong ³-³
interaction.
The redox properties of heteroles 1, 3a, 4, and the related
compounds were examined by cyclic voltammetry (CV) in DMF
and CH2Cl2 solutions (Table S69). The CV of 3a in DMF
exhibits two reversible reduction waves (¹1.60 and ¹2.10 V),
but no oxidation wave was observed in the measurable range
(>+1.2 V). On the other hand, the CV of 4 in CH2Cl2 exhibited
the first reversible and the second irreversible oxidation waves
(+0.35 and +1.06 V), and the reduction waves were not
observed. The electronic oxidation of 1 readily produced
polymeric materials as a film on the surface of the cathode
(Figure S129). The relatively high reversibility of oxidation
waves indicated that 4 formed stable cationic species.
D. M. Ho, R. A. Pascal, Jr., Tetrahedron 2004, 60, 2433.
J. A. Watson, Jr., R. A. Pascal, Jr., D. M. Ho, K. V. Kilway,
Tetrahedron Lett. 2000, 41, 5005.
O. C. Musgrave, J. M. S. Skakle, Acta Crystallogr., Sect. E:
Struct. Rep. Online 2004, 60, o359.
Very recently, Matano and Imahori’s group reported the synthesis
and electronic properties of diacenaphthophosphole derivatives
using a similar strategy. Y. Matano, A. Saito, T. Fukushima, Y.
Tokudome, F. Suzuki, D. Sakamaki, H. Kaji, A. Ito, K. Tanaka, H.
Imahori, Angew. Chem., Int. Ed. 2011, 50, 8016.
a) C. Xu, A. Wakamiya, S. Yamaguchi, J. Am. Chem. Soc. 2005,
127, 1638. b) L. Ilies, H. Tsuji, Y. Sato, E. Nakamura, J. Am.
Chem. Soc. 2008, 130, 4240. c) L. Huo, H.-Y. Chen, J. Hou, T. L.
Chen, Y. Yang, Chem. Commun. 2009, 5570. d) L. Ilies, H. Tsuji,
E. Nakamura, Org. Lett. 2009, 11, 3966. e) T. Agou, M. D.
Hossain, T. Kawashima, K. Kamada, K. Ohta, Chem. Commun.
2009, 6762. f) J.-H. Wan, W.-F. Fang, Z.-F. Li, X.-Q. Xiao, Z. Xu,
Y. Deng, L.-H. Zhang, J.-X. Jiang, H.-Y. Qiu, L.-B. Wu, G.-Q.
Lai, Chem.®Asian J. 2010, 5, 2290. g) L. Ilies, Y. Sato, C. Mitsui,
H. Tsuji, E. Nakamura, Chem.®Asian J. 2010, 5, 1376. h) T.
Agou, M. D. Hossain, T. Kawashima, Chem.®Eur. J. 2010, 16,
368. i) Y. Yabusaki, N. Ohshima, H. Kondo, T. Kusamoto, Y.
Yamanoi, H. Nishihara, Chem.®Eur. J. 2010, 16, 5581. j) T.
Ureshino, T. Yoshida, Y. Kuninobu, K. Takai, J. Am. Chem. Soc.
2010, 132, 14324. k) L. Li, C. Xu, S. Li, Tetrahedron Lett. 2010,
51, 622.
4
5
6
7
8
9
B. M. Trost, D. R. Brittelli, J. Org. Chem. 1967, 32, 2620.
See Supporting Information which is available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
10 Y. L. Chow, C. J. Colon, D. W. L. Chang, K. S. Pillay, R. L.
Lockhart, T. Tezuka, Acta Chem. Scand., Ser. B 1982, 36, 623.
11 a) Y. Dienes, M. Eggenstein, T. Kárpáti, T. C. Sutherland, L.
Nyulászi, T. Baumgartner, Chem.®Eur. J. 2008, 14, 9878. b) G.
Balaji, S. Valiyaveettil, Org. Lett. 2009, 11, 3358. c) G. Balaji,
D. I. Phua, W. L. Shim, S. Valiyaveettil, Org. Lett. 2010, 12, 232.
12 a) K. Nozaki, K. Takahashi, K. Nakano, T. Hiyama, H.-Z. Tang,
M. Fujiki, S. Yamaguchi, K. Tamao, Angew. Chem., Int. Ed. 2003,
42, 2051. b) K. Nakano, Y. Hidehira, K. Takahashi, T. Hiyama, K.
Nozaki, Angew. Chem., Int. Ed. 2005, 44, 7136.
In conclusion, heretofore unknown diacenaphtheno[1,2-
b;1¤,2¤-d]silole and -pyrrole derivatives were successfully pre-
pared from 2,2¤-dibromo-1,1¤-biacenaphthylene (2). The crystal-
lographic analyses revealed high planarity of the ³-systems, and
the electrochemical studies indicated a low energy LUMO level
of the silole 3a, and high energy HOMO level of 4 as predicted
by the DFT theoretical calculations. We are currently investigat-
ing the potential applications of these compounds for OFETs; the
results will be reported in due course.
13 R. D. Adams, B. Captain, J. L. Smith, Jr., J. Organomet. Chem.
2004, 689, 65.
14 The molecular geometries of the heteroles were fully optimized at
the RB3LYP/6-31G(d) level of theory embedded in the Gaussian
03 software package. The time-dependent calculations were
performed on the optimized geometries. Theoretical estimation of
S0 ¼ S1 absorption bands were performed with TD-RB3LP/
6-31G(d)// RB3LYP/6-31G(d) method.
We thank Prof. Takashi Kubo (Osaka University) for helpful
discussion. This work was supported by a Grant-in-Aid for
Scientific Research on Innovative Areas (No. 21108521 A01,
“pi-Space”) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
15 L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv.
Mater. 2011, 23, 926.
16 Selected data for 3a: Crystal system: Monoclinic, Space group:
P21/n, a = 22.8451(14) ¡, b = 8.2661(4) ¡, c = 29.5373(16)¡,
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
¹1,
¢ = 102.8594(16)°, V = 5437.9(5)¡3, Z = 4, ® = 0.11 mm
T = 200 K, R[F2 > 2·(F2)] = 0.096, wR(F2) = 0.270, S = 1.05,
Refl/Param. = 12394/721. And for 4: Crystal system: Triclinic,
ꢀ
References and Notes
Space group: P1, a = 8.0445(9) ¡, b = 11.184(1) ¡, c =
1
a) S. Yamaguchi, K. Tamao, Chem. Lett. 2005, 34, 2. b) A.
Fukazawa, S. Yamaguchi, Chem.®Asian J. 2009, 4, 1386. c) X.
Zhan, S. Barlow, S. R. Marder, Chem. Commun. 2009, 1948. d) Y.
Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009, 4332. e)
M. Wang, G. Zhang, D. Zhang, D. Zhu, B. Z. Tang, J. Mater.
Chem. 2010, 20, 1858.
11.525(1) ¡, ¡ = 83.288(3)°, ¢ = 74.390(3)°, £ = 72.725(3)°,
V = 952.84(16) ¡3, Z = 2, ® = 0.079 mm¹1, T = 223 K, R[F2 >
2·(F2)] = 0.0579, wR(F2) = 0.2043, S = 1.117, Refl/Param. =
12394/721. Crystallographic data reported in this manuscript
have been deposited with Cambridge Crystallographic Data
Centre as supplementary publication No. 842076 for 4 and
No. 842077 for 3a. Copies of the data can be obtained free of
charge on application to CCDC, 12, Union Road, Cambridge,
CB2 1EZ, U.K. (fax: +44 1223 336033); e-mail: deposit@ccdc.
cam.ac.uk.
2
3
D. B. Clapp, J. Am. Chem. Soc. 1939, 61, 2733.
a) K. Zimmermann, M. W. Haenel, Synlett 1997, 609. b) T. Kubo,
K. Yamamoto, K. Nakasuji, T. Takui, Tetrahedron Lett. 2001, 42,
7997. c) T. Kubo, M. Sakamoto, M. Akabane, Y. Fujiwara, K.
Yamamoto, M. Akita, K. Inoue, T. Takui, K. Nakasuji, Angew.
Chem., Int. Ed. 2004, 43, 6474. d) K. V. Kilway, K. A. Lindgren,
J. W. Vincent, J. A. Watson, Jr., R. G. Clevenger, R. D. Ingalls,
17 J. Chen, C. C. W. Law, J. W. Y. Lam, Y. Dong, S. M. F. Lo, I. D.
Williams, D. Zhu, B. Z. Tang, Chem. Mater. 2003, 15, 1535.
Chem. Lett. 2011, 40, 1437-1439
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/