Synthesis and characterization of two isomers of 14π-electron germaaromatics: Kinetically stabilized 9-germaanthracene and 9-germaphenanthrene

Article abstract of DOI:10.1021/om060371+

The first stable 9-germaanthracene and 9-germaphenanthrene were successfully synthesized by taking advantage of kinetic stabilization utilizing effective steric protection groups, Tbt and Bbt groups (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Bbt = 2,6-bis[bis(trimethylsilyl)methylJ4-[tris(trimethylsilyl)methyl]phenyl). The similarities and differences of the structures and properties between the two isomers of germaaromatics are discussed.

Full text of DOI:10.1021/om060371+

Organometallics 2006, 25, 3533-3536
3533
Communications
Synthesis and Characterization of Two Isomers of 14π-Electron
Germaaromatics: Kinetically Stabilized 9-Germaanthracene and
9-Germaphenanthrene
Takahiro Sasamori,† Koji Inamura,† Wataru Hoshino,† Norio Nakata,† Yoshiyuki Mizuhata,†
Yasuaki Watanabe,‡ Yukio Furukawa,‡ and Norihiro Tokitoh*,†
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan, and
Department of Chemistry, School of Science and Engineering, Waseda UniVersity, 3-4-1 Okubo,
Shinjuku-ku, Tokyo 169-8555, Japan
ReceiVed May 1, 2006
Scheme 1
Summary: The first stable 9-germaanthracene and 9-ger-
maphenanthrene were successfully synthesized by taking ad-
Vantage of kinetic stabilization utilizing effectiVe steric protec-
tion groups, Tbt and Bbt groups (Tbt ) 2,4,6-tris[bis(trimeth-
ylsilyl)methyl]phenyl, Bbt ) 2,6-bis[bis(trimethylsilyl)methyl]-
4-[tris(trimethylsilyl)methyl]phenyl). The similarities and dif-
ferences of the structures and properties between the two
isomers of germaaromatics are discussed.
Bbt. The experimental and theoretical studies on the structures,
spectroscopic properties, and reactivities of these kinetically
stabilized sila- and germaaromatics evidenced the high aroma-
ticity of the monosila- and monogermaaromatic systems.1g Very
recently, the stable stannaaromatics,8 2-stannanaphthalene, was
also isolated and fully characterized, suggesting the considerable
aromaticity of a 10π-electron ring system containing a much
heavier group 14 atom, an Sn atom.9 Thus, the concept of kinetic
stabilization is evidenced to be of great use for the construction
of stable heaVy aromatics. On the other hand, anthracene and
phenanthrene are attractive 14π-electron aromatic systems due
to their unique electrochemical and photochemical properties
from the viewpoint of material science.10 Hence, it should be
of great importance to elucidate the similarities and differences
between two types of extended π-electron aromatic systems
containing a heavier group 14 atom, i.e., the linear type (acene
system) and the zigzag type (phen system) such as anthracene
and phenanthrene (Scheme 1). We report here the synthesis of
the two structural isomers of 14π-electron germaaromatic
systems, 9-germaanthracene 1 and 9-germaphenanthrene 2,
utilizing a Tbt or Bbt group as a steric protection group.
There has been much interest in the chemistry of “heaVy
aromatics”, [4n+2] π-electron ring systems containing a heavier
group 14 element (Si, Ge, Sn, Pb),1 from the standpoint of
comparison with the parent aromatic hydrocarbon compounds,
which play very important roles in organic chemistry. Although
heaVy aromatics have been known to be highly reactive and
undergo ready dimerization or oligomerization under ambient
conditions so far, we have succeeded in the synthesis of the
first stable neutral sila- and germaaromatics,1g i.e., silabenzene,2
1- and 2-silanaphthalenes,3,4 9-silaanthracene,5 germabenzene,6
and 2-germanaphthalene,7 by taking advantage of kinetic
stabilization using efficient steric protection groups, Tbt and
* To whom correspondence should be addressed. Phone: +81-774-38-
3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp.
† Kyoto University.
‡ Waseda University.
(1) (a) Choi, S. B.; Boudjouk, P.; Wei, P. R. J. Am. Chem. Soc. 1998,
120, 5814. (b) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000,
33, 704. (c) Lee, V. Y.; Sekiguchi, A.; Ichinohe, M.; Fukaya, N. J.
Organomet. Chem. 2000, 611, 228. (d) Nishinaga, T.; Izukawa, Y.; Komatsu,
K. J. Am. Chem. Soc. 2000, 122, 9312. (e) Saito, M.; Haga, R.; Yoshioka,
M. Chem. Commun. 2002, 1002. (f) Saito, M.; Haga, R.; Yoshioka, M.
Chem. Lett. 2003, 32, 912. (g) Tokitoh, N. Acc. Chem. Res. 2004, 37, 86.
(h) Ichinohe, M.; Igarashi, M.; Sanuki, K.; Sekiguchi, A. J. Am. Chem.
Soc. 2005, 127, 9978. (i) Saito, M.; Haga, R.; Yoshioka, M.; Ishimura, K.;
Nagase, S. Angew. Chem., Int. Ed. 2005, 44, 6553.
(2) (a) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Angew. Chem.,
Int. Ed. 2000, 39, 634. (b) Wakita, K.; Tokitoh, N.; Okazaki, R.; Takagi,
N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 5648. (c) Shinohara, A.;
Takeda, N.; Sasamori, T.; Matsumoto, T.; Tokitoh, N. Organometallics
2005, 24, 6141.
In consideration of previous reports,1g the reactions of the
corresponding precursors bearing a leaving group (halogeno or
(6) (a) Nakata, N.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2002,
124, 6914. (b) Nakata, N.; Takeda, N.; Tokitoh, N. J. Organomet. Chem.
2003, 672, 66. (c) Nakata, N.; Takeda, N.; Tokitoh, N. Angew. Chem. Int.
Ed. 2003, 42, 115.
(7) (a) Nakata, N.; Takeda, N.; Tokitoh, N. Organometallics 2001, 20,
5507. (b) Nakata, N.; Takeda, N.; Tokitoh, N. Chem. Lett. 2002, 818. (c)
Nakata, N.; Takeda, N.; Tokitoh, N. Organometallics 2003, 22, 481.
(8) This is the first example of neutral stannaaromatics, although ionic
stannaaromatic compounds such as stannole dianion derivatives have been
already synthesized and characterized; see ref 1i and: Todorov, I.; Sevov,
S. C. Inorg. Chem. 2005, 44, 5361.
(3) (a) Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; Schleyer, P. v.
R.; Jiao, H. J. Am. Chem. Soc. 1997, 119, 6951. (b) Wakita, K.; Tokitoh,
N.; Okazaki, R.; Nagase, S.; Schleyer, P. v. R.; Jiao, H. J. Am. Chem. Soc.
1999, 121, 11336. (c) Wakita, K.; Tokitoh, N.; Okazaki, R. Bull. Chem.
Soc. Jpn. 2000, 73, 2157.
(4) (a) Takeda, N.; Shinohara, A.; Tokitoh, N. Organometallics 2002,
21, 4024. (b) Shinohara, A.; Takeda, N.; Sasamori, T.; Tokitoh, N. Bull.
Chem. Soc. Jpn. 2005, 78, 977.
(5) (a) Takeda, N.; Shinohara, A.; Tokitoh, N. Organometallics 2002,
21, 256. (b) Shinohara, A.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc.
2003, 125, 10804.
(9) Mizuhata, Y.; Sasamori, T.; Takeda, N.; Tokitoh, N. J. Am. Chem.
Soc. 2006, 128, 1050.
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H.; Liao, C. H.; Tsai, C. H.; Chen, C. H. Appl. Phys. Lett. 2004, 85, 3301.
10.1021/om060371+ CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/13/2006

3534 Organometallics, Vol. 25, No. 15, 2006
Communications
Scheme 2
Scheme 3
and Raman spectra) and finally established by X-ray crystal-
lographic analysis.13
The X-ray crystallographic analyses (Figure 1) of 1b and 2
show a completely planar geometry for their central germaaro-
matic moieties, which are almost perpendicular to the aromatic
rings of the Bbt (86° for 1b) and Tbt (87° and 86° for 2) groups,
indicating that there is almost no conjugative stabilization effect
between the substituents and the central germaaromatic rings.
It should be noted that 9-germaphenanthrene 2 has two
independent molecules in the unit cell. Two types of intermo-
lecular C-H π-interactions are confirmed between the two
molecules, forming a unique tetrameric packing structure, as
shown in Figure 2; one is between the two 9-germaphenanthrene
rings (the distance between the C atom and the aromatic ring is
ca. 3.54 Å), and the other is between the methyl group and the
9-germaphenanthrene ring (the distance between the C atom
and the aromatic ring is ca. 3.41 Å).14 In contrast, 9-germaan-
thracene 1b shows no apparent intermolecular interactions in
its packing structure. In Figure 3 are shown selected structural
parameters of 1b and 2 together with calculated ones for the
model compounds bearing a Dmp (2,6-dimethylphenyl) group.13
In the case of both 1b and 2, the C-C and Ge-C bond lengths
are the middle values between the corresponding single- and
double-bond lengths,15 indicating the existence of π-electron
delocalization on their germaaromatic rings, as in the case of
1-Tbt-germabenzene6 and 2-Tbt-2-germanaphthalene.7 The ob-
served structural parameters of 1b and 2 are in good agreement
with those calculated for their model compounds and show bond
alternations in the central germaaromatic ring reflecting the
corresponding canonical structures16 as in the case of anthracene
and phenanthrene, respectively. While the two Ge-C bond
lengths [1.855(2) and 1.862(2) Å] of the 9-germaanthracene ring
of 1b are close to each other due to its bilateral symmetry, those
of the 9-germaphenanthrene skeleton of 2 are different from
each other [1.797(3) and 1.883(3) Å for molecule A; 1.792(3)
and 1.886(3) Å for molecule B], reflecting the most contributory
trifluoromethanesulfonyl group) with an appropriate base should
be effective for the construction of heaVy aromatics. 9-Trifluo-
romethanesulfonyl-9-Tbt-9,10-dihydro-9-germaanthracene (6a)
was prepared as a precursor for 9-germaanthracene 1a according
to Scheme 2. Treatment of 6a with 1.1 equiv of lithium
diisopropyl amide (LDA) in benzene at rt afforded germaan-
thracene 1a as red crystals in 27% isolated yield. The similarity
of the spectroscopic data (1H NMR and UV/vis) to those of
9-Tbt-9-silaanthracene5 supported the structure of 1a. However,
9-germaanthracene 1a undergoes gradual decomposition at
ambient temperature to afford an insoluble colorless solid, in
contrast to 9-Tbt-9-silaanthracene,5 which is stable up to 100
°C in C6D6. Therefore, further investigation on the properties
of 1a could not be performed. Next, we introduced a Bbt group
on the central germanium atom instead of a Tbt group.11 Thus,
triflate 6b, prepared by the same method as that for 6a, was
treated with 1.1 equiv of LDA to gave 9-Bbt-9-germaanthracene
(1b) as red crystals in 84% yield. 9-Germaanthracene 1b
possesses sufficient stability at ambient temperature to be
handled under an inert atmosphere, although it is slightly light-
sensitive. Heating of the C6D6 solution of 1b at 70 °C for 70 h
gave [4+4] head-to-tail dimer 7 in 83% yield. Hence, 9-ger-
maanthracene 1b is less stable than 9-Tbt-9-silaanthracene,
which undergoes dimerization in C6D6 at 110 °C for 15 days
to afford the corresponding [4+4] dimer.5 On the other hand,
the dehydrobromination reaction of 9-bromo-9-Tbt-9,10-dihy-
dro-9-germaphenanthrene (10), which was prepared according
to Scheme 3, using LDA in THF afforded 9-Tbt-9-ger-
maphenanthrene (2) as pale yellow crystals in 89% yield. It
should be noted that 2 is stable up to 100 °C in C6D6 in contrast
to 9-Tbt-9-stannaphenanthrene, which undergoes ready [2+2]
dimerization at room temperature.12 The structures of 1b and 2
were identified by spectroscopic analyses (NMR, UV/vis, mass,
(13) The experimental procedures and spectral data for 1b and 2 and
theoretical calculations for their model compounds are shown in the
Supporting Information.
(14) Although the degree of T-shaped CH-π interaction should be
discussed on the basis of the distance between the center of the aromatic
ring and the aromatic plane, it is somewhat difficult to define the center of
the 9-germaphenanthrene ring due to the slight deformation of the skeleton.
It was reported that the T-shaped CH-π interaction in a benzene dimer
should be highly effective when the distance between the center of the
aromatic ring and the aromatic plane is ca. 5.0 Å; that is, that between the
C atom at the edge and the aromatic plane is ca. 3.6 Å, see: (a) Pawliszyn,
J.; Szczesniak, M. M.; Scheiner, S. J. Phys. Chem. 1984, 88, 1726. (b)
Felker, P. M.; Maxton, P. M.; Schaeffer, M. W. Chem. ReV. 1994, 94, 1787.
(c) Sinnokrot, M. O.; Sherrill, C. D. J. Am. Chem. Soc. 2004, 126, 7690.
(d) Sinnokrot, M. O.; Sherrill, C. D. J. Phys. Chem. A 2004, 108, 10200.
(e) Sato, T.; Tsuneda, T.; Hirao, K. J. Chem. Phys. 2005, 123, 104307.
(15) The calculated Ge-C single and double bond lengths are ca. 1.91-
2.00 and ca. 1.77 Å, respectively. See: Grev, R. S.; Schaefer, H. F.
Organometallics 1992, 11, 3489.
(11) It has been reported that 1-Bbt-1-silanaphthalene is more stable than
1-Tbt-1-silanaphthalene toward [4+2] dimerization; see ref 4b. In the course
of our studies on kinetic stabilization of the double-bond compounds
between heavier group 15 elements, it was found that the central moieties
of BbtEdEBbt (E ) P, Sb, Bi) are slightly more congested than those of
TbtEdETbt; see: (a) Sasamori, T.; Arai, Y.; Takeda, N.; Okazaki, R.;
Furukawa, Y.; Kimura, M.; Nagase, S.; Tokitoh, N. Bull. Chem. Soc. Jpn.
2002, 75, 661. (b) Sasamori, T.; Takeda, N.; Tokitoh, N. J. Phys. Org.
Chem. 2003, 16, 450.
(12) Mizuhata, Y.; Takeda, N.; Sasamori, T.; Tokitoh, N. Chem. Lett.
2005, 34, 1088.

Communications
Organometallics, Vol. 25, No. 15, 2006 3535
Figure 1. ORTEP drawing of 1b (a) and 2 (b) with thermal
ellipsoid plots (50% probability). Hydrogen atoms are omitted for
clarity.
Figure 3. Observed and calculated (italic) bond lengths (Å) for
(a) 9-germaanthracenes and (b) 9-germaphenanthrenes. The calcu-
lated values are obtained for the Dmp (2,6-dimethylphenyl)-
substituted model compounds at the B3LYP/6-31G(d) level.
than those of olefins (ca. 4.5-7.0 ppm).17 In addition, the
calculated chemical shifts for the model compounds bearing less
hindered substituents on the Ge atoms are in good agreement
with the observed values, indicating that the steric effect of Tbt
and Bbt on the central germaaromatic rings should be negli-
gible.13 Thus, the NMR spectral properties of 1b and 2 are
similar to each other. Electronic spectra should be of great use
in the evaluation of the extension of π-electrons on the aromatic
rings. In the UV/vis spectra in hexane, the longest absorption
maxima (λmax) corresponding to the HOMO-LUMO electron
transitions of 1b (520 nm) and 2 (409 nm) are red-shifted as
compared with those of 1-Tbt-germabenzene6 (326 nm) and
2-Tbt-2-germanaphthalene7 (386 nm), respectively. That is, the
π-electron conjugation can be extended as the n value of [4n+2]
π-electron aromatic systems increases.18 Furthermore, the long-
est λmax of 9-germaanthracene 1b is apparently longer than that
of 2, reflecting their colors in solution (red for 1b and pale
yellow for 2; see Figure S6 in the Supporting Information).
Hence, it is evidenced that the 14π-electron systems of
germaaromatics can be extensively conjugated in the linear type
but not so effectively in the zigzag type.
Figure 2. Tetrameric packing structure of 9-germaphenan-
threne 2.
canonical structure of a 9-germaphenanthrene skeleton.16 Thus,
the Ge-C bond of the 9,10-position of 9-germaphenanthrene 2
features higher double-bond character than the other Ge-C bond
of 2.
In the 1H and 13C NMR spectra of 1b and 2, all of the signals
corresponding to the central 9-germaanthracene and 9-ger-
maphenanthrene skeletons were observed within the aromatic
In conclusion, two isomers of 14π-electron germaaromatic
systems, 9-germaanthracene 1b and 9-germaphenanthrene 2,
were synthesized as stable crystalline compounds. Their simi-
larities and differences are revealed on the basis of the structural
and spectroscopic analyses. Further investigations on their
photochemical and electrochemical properties, reactivities, and
applications as arene ligands to transition metals are currently
in progress.
1
region, 7-8 ppm for the H NMR spectra and 110-150 ppm
for the 13C NMR spectra. In particular, the protons of the 10-
positions of 1b and 2 appeared as characteristic singlet signals
at 7.73 and 8.13 ppm, respectively, which are apparently lower
(16) In the case of anthracene and phenanthrene, the slight bond alter-
nations can be seen, reflecting their most dominant canonical structures as
shown below. Similarly, one can see their slight bond alternations in the
germaaromatic rings of 1b and 2, which should be reasonably explained in
terms of their dominant canonical structures as well as their all-carbon
analogues.
(17) For example, the olefinic proton of triphenylethylene is observed
at 6.96 ppm [SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/(National
Institute of Advanced Industrial Science and Technology, 2006)].
(18) The longest absorption maxima of silaaromatic systems are appar-
ently red-shifted as compared with those of the corresponding all-carbon
systems. However, those of germaaromatic systems are observed in the
region similar to that for the corresponding silaaromatic systems. The
systematic comparisons of the longest absorption maxima between the
kinetically stabilized sila- and germaaromatic systems with all-carbon
systems are shown in the Supporting Information.

3536 Organometallics, Vol. 25, No. 15, 2006
Communications
Supporting Information Available: X-ray crystallographic data
of 1 and 15 in CIF format, experimental procedures, and spectral
data. These materials are available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This work was partially supported by
Grants-in-Aid (Nos. 17GS0207, 12CE2005, 14078213, and
16750033) and the 21st Century COE on Kyoto University
Alliance for Chemistry from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. T.S. thanks the Kinki
Invention Center for the research grant.
OM060371+