Synthesis and structure of a kinetically stabilized 2-germanaphthalene: The first stable neutral germaaromatic compound

Article abstract of DOI:10.1021/om010881y

The first stable neutral germaaromatic compound, i.e., the 2-germanaphthalene 1a bearing an efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), was successfully synthesized by the reaction of the corresponding bromogerman

Full text of DOI:10.1021/om010881y

Organometallics 2001, 20, 5507-5509
5507
Syn th esis a n d Str u ctu r e of a Kin etica lly Sta bilized
2-Ger m a n a p h th a len e: Th e F ir st Sta ble Neu tr a l
Ger m a a r om a tic Com p ou n d
Norio Nakata, Nobuhiro Takeda, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, J apan
Received October 9, 2001
Ch a r t 1
Summary: The first stable neutral germaaromatic com-
pound, i.e., the 2-germanaphthalene 1a bearing an
efficient steric protection group, 2,4,6-tris[bis(trimethyl-
silyl)methyl]phenyl (Tbt), was successfully synthesized
by the reaction of the corresponding bromogermane 4
with lithium diisopropylamide. The molecular structure
and aromaticity of 1a were discussed on the basis of its
NMR, UV-vis, and Raman spectra, X-ray crystal-
lographic analysis, and theoretical calculations.
In recent years much attention has been focused on
germaaromatic compounds,1 namely, Ge-containing [4n
+ 2]-π-electron ring systems, since they are among the
heavier congeners of aromatic hydrocarbons,2 which
play very important roles in organic chemistry. As for
ionic germaaromatic compounds, cyclotrigermenium
cations3 and germole dianions4 have been successfully
synthesized as stable compounds and fully character-
ized, while the synthesis of neutral germaaromatic
compounds is little known. Ma¨rkl et al. have already
reported the synthesis of 1,4-di-tert-butylgermaben-
zene,5 but the generation was confirmed only by the
formation of its [2 + 2] dimer and the trapping reaction
with diene. Furthermore, they described the spectro-
scopic detection of 1,4-dialkylgermabenzenes in the gas
phase by VTPES (variable-temperature photoelectron
spectroscopy) experiments.6 However, no isolation of a
neutral germaaromatic compound has been reported,
probably due to its high reactivity. On the other hand,
we have recently succeeded in the synthesis and isola-
tion of kinetically stabilized silabenzene7 and 2-si-
lanaphthalene,8 the first examples of stable neutral
silaaromatic compounds, by taking advantage of an
efficient steric protection group, 2,4,6-tris[bis(trimeth-
ylsilyl)methyl]phenyl (Tbt). With these stable systems
in hand, we have revealed their molecular structures
and reactivities and have discussed the aromaticity of
silaaromatic compounds. The successful results in the
silaaromatic systems naturally prompted us to extend
this chemistry to the heavier metallaaromatic systems
of group 14 elements. Here, we report the synthesis and
structure of the 2-germanaphthalene 1a kinetically
stabilized by the Tbt group, the first stable neutral
germaaromatic compound (Chart 1).
1,2,3,4-Tetrahydro-2-germanaphthalene 2 was syn-
thesized from homophthalic anhydride in four steps
(Scheme 1).9 Bromination of 2 with excess NBS followed
by reduction with LiAlH4 gave an inseparable mixture
of the expected vinylgermane 3a and overbrominated
vinylgermane 3b. Treatment of the mixture of vinylger-
manes 3a and 3b with t-BuLi and successive addition
of saturated aqueous NH4Cl afforded the vinylgermane
3a as a pure compound. Finally, careful bromination of
3a with NBS (1 equiv) resulted in the quantitative
formation of the corresponding bromogermane 4, which
is a suitable precursor of 2-germanaphthalene 1a
(Scheme 1).
(1) For reviews of germaaromatic compounds, see: (a) Barrau, J .;
Escudie´, J .; Satge´, J . Chem. Rev. 1990, 90, 283. (b) Lee, V. Y.;
Sekiguchi, A.; Ichinohe, M.; Fukaya, N. J . Organomet. Chem. 2000,
611, 228.
(2) Minkin, V. J .; Glukhovtsev, M. N.; Simkin, Y. B. Aromaticity
and Antiaromaticity; Electronic and Structural Aspects; Wiley: New
York, 1994.
(3) (a) Sekiguchi, A.; Tsukamoto, M.; Ichinohe, M. Science 1997, 275,
60. (b) Ichinohe, M.; Fukaya, N.; Sekiguchi, A. Chem. Lett. 1998, 1045.
(c) Sekiguchi, A.; Fukaya, N.; Ichinohe, M. Phosphorus, Sulfur, Silicon
Relat. Elem. 1999, 59, 150. (d) Sekiguchi, A.; Fukaya, N.; Ichinohe,
M.; Ishida, Y. Eur. J . Inorg. Chem. 2000, 1155.
(4) (a) West, R.; Sohn, H.; Powell, D. R.; Mu¨ller, T.; Apeloig, Y.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1002. (b) Choi, S.-B.; Boudjouk,
P.; Qin, K. Organometallics 2000, 19, 1806.
(5) Ma¨rkl, G.; Rudnick, D. Tetrahedron Lett. 1980, 21, 1405.
(6) Ma¨rkl, G.; Rudnick, D.; Schulz, R.; Schweig, A. Angew. Chem.,
Int. Ed. Engl. 1982, 21, 221.
(7) (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.
(8) (a) Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; Schleyer,
P. v. R.; J iao, H. J . Am. Chem. Soc. 1997, 119, 6951. (b) Wakita, K.;
Tokitoh, N.; Okazaki, R.; Nagase, S.; Schleyer, P. v. R.; J iao, H. J .
Am. Chem. Soc. 1999, 121, 11336.
2-Germanaphthalene 1a was synthesized by the de-
hydrobromination of bromogermane 4 with LDA in THF
at room temperature. 2-Germanaphthalene 1a is ther-
mally stable under an inert atmosphere either in the
solid state (mp 143-145 °C) or in solution (benzene, at
100 °C for 1 h in a sealed tube), and no dimerization or
decomposition was observed. However, 1a is highly
moisture sensitive and undergoes addition of water to
(9) Fang, X.; Larson, D. L.; Portoghese, P. S. J . Med. Chem. 1997,
40, 3064.
10.1021/om010881y CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/22/2001

5508 Organometallics, Vol. 20, No. 26, 2001
Communications
Sch em e 1a
a
Conditions: (a) LiAlH4, THF, 0 °C, 96%; (b) Ph3P, CBr4,
Et2O, room temperature, 68%; (c) TbtGeCl3, Mg, THF, room
temperature; (d) LiAlH4, THF, 0 °C, 33% for two steps; (e) NBS
(7.1 equiv), BPO (cat.), benzene, reflux; (f) LiAlH4, THF, 0 °C;
(g) t-BuLi, THF, -78 °C; (h) saturated aqueous NH4Cl, room
temperature, 62% for four steps; (i) NBS, (1.0 equiv), BPO
(cat.), benzene, reflux, 99%; (j) LDA (1.3 equiv), THF, room
temperature, ca. 98%.
F igu r e 1. ORTEP drawing of 2-germanaphthalene 1a
(major fragment, 30% probability).
signals of the germanaphthalene ring carbons (120.75-
145.89 ppm) were located in the sp2 region.
Sch em e 2
The molecular geometry of 1a was finally determined
by X-ray crystallographic analysis (Figure 1),11 in which
the structure was successfully refined more adequately
by the separation of the overlapped disorder of the
2-germanaphthalene moieties (2:1 ratio).12 The struc-
tural analysis revealed the planarity of the 2-ger-
manaphthalene ring and the completely trigonal planar
geometry around the germanium atom. In addition, the
benzene ring of the Tbt group is almost perpendicular
to the 2-germanaphthalene ring, and hence it is con-
sidered that there is very little conjugative interaction
between the two aromatic rings. The bond lengths of
the two Ge-C bonds in the 2-germanaphthalene ring
(1.803(5) and 1.859(5) Å) are between those of calculated
Ge-C double (1.767 Å) and single bonds (1.908-2.001
Å).13 In particular, the former Ge-C bond length is close
to the Ge-C double bond length of the previously
reported germene Mes2GedCR2 (CR2 ) fluorenylidene;
1.803(4) Å),14 which is stabilized by the conjugation of
Ta ble 1. Obser ved a n d Ca lcu la ted 1H a n d 13C NMR
Ch em ica l Sh ifts (p p m ) for 2-Ger m a n a p h th a len esa
1a (R ) Tbt, obsda) 1b (R ) H, calcdb) 1c (R ) Ph, calcdb,c
)
H1
H2
H3
H4
H5
H6
H7
C1
C2
C3
C4
C5
C6
C7
C8
C9
8.24
7.85
8.55
7.60
7.03
8.13
7.48
8.50
7.49
7.02
7.62
7.18
8.40
7.44
6.92
7.20
7.65
7.22
7.45
7.10
7.36
127.69
130.45
145.89
133.71
120.75
126.26
128.40
145.15
128.12
135.55
128.34
145.51
133.17
120.94
125.53
127.15
145.61
127.69
127.46
125.74
145.09
132.48
119.43
124.80
127.08
145.20
126.43
(10) Calculations were carried out using the Gaussian 98 program:
Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A., J r.;
Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen, W.; Wong, M.
W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.; Pople, J . A.
Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(11) Crystal data of 1a : Rigaku/MSC Mercury CCD, Mo KR radia-
tion, graphite monochromator, reflections recorded at 93 K from a
colorless prismatic crystal with dimensions 0.40 × 0.40 × 0.35 mm3 to
2θmax ) 50°, structure solved by direct methods (SIR97)17 and refined
by full-matrix least-squares procedures on F2 (SHELX-97),18 formula
C36H66GeSi6, mol wt 740.02, triclinic, space group P1h, Z ) 2, a )
11.033(4) Å, b ) 12.112 (5) Å, c ) 17.851(6) Å, R ) 72.423(15)°, â )
86.91(2)°, γ ) 75.064(9)°, V ) 2196.6(14) Å3, Dcalcd ) 1.119 g cm-3, µ )
0.883 mm-1; R1(I > 2σ(I)) ) 0.053, wR2(all data) ) 0.138 for 7709
reflections, 479 parameters, and 50 restraints.
a
b
See Chart 1. Measured in benzene-d6. c Calculated at the
GIAO-B3LYP/6-311G(d)(6-311G(3d) on Ge)//B3LYP/6-31G(d) level.
d
The phenyl group is fixed in such a way that it is perpendicular
to the 2-germanaphthalene.
its GedC moiety, giving the corresponding hydroxyger-
mane 5 in a quantitative yield (Scheme 2).
1
The molecular structure of 1a was confirmed by H
and 13C NMR spectroscopy using 2D NMR techniques
together with NOE and decoupling experiments. The
1
assignments of H and 13C NMR signals are listed in
Table 1 along with the calculated values for some model
compounds, i.e., 2-germanaphthalene 1b and 2-phenyl-
2-germanaphthalene 1c, where the observed values are
in good agreement with the calculated ones.10 Thus,
these results clearly indicate the delocalized π-electronic
system of 1a . All the 1H NMR signals of the 2-ger-
manaphthalene ring protons (7.03-8.55 ppm) of 1a
were observed in the aromatic region, and the 13C NMR
(12) Recently, we reported the analogous separation by overlapped
disorder of a 2-silanaphthalene bearing a Tbt group: Wakita, K.;
Tokitoh, N.; Okazaki, R. Bull. Chem. Soc. J pn. 2000, 73, 2157.
(13) Grev, R. S.; Schaefer, H. F., III. Organometallics 1992, 11, 3489.
(14) Lazraq, M.; Escudie´, J .; Couret, C.; Satge´, J .; Dammel, R. A.
Angew. Chem., Int. Ed. Engl. 1988, 27, 828.

Communications
Organometallics, Vol. 20, No. 26, 2001 5509
and 386 nm (ꢀ 2 × 103)) most likely assignable to the
E1, E2, and B bands, respectively. These are red-shifted
compared to those for naphthalene (221 (ꢀ 1.33 × 105),
286 (ꢀ 9.3 × 103), and 312 nm (ꢀ 2.9 × 102))16 and
2-silanaphthalene (267 (ꢀ 2 × 104), 327 (ꢀ 7 × 103), and
387 nm (ꢀ 2 × 103)),8 respectively, suggesting the
aromatic character of this conjugated ring system
containing the germanium atom similar to that of
naphthalene and 2-silanaphthalene.
In summary, we have succeeded in the synthesis and
characterization of 2-germanaphthalene 1a , the first
example of a stable neutral aromatic compound contain-
ing a germanium atom. Further investigations on the
physical and chemical properties of 1a are currently in
progress.
Ta ble 2. Obser ved a n d Ca lcu la ted Bon d Len gth s
(Å) of 2-Ger m a n a p h th a len es
1a (R ) Tbt,
1b (R ) H,
1c (R ) Ph,
obsda)
calcdb)
calcdb)
C1-Ge1
Ge1-C2
C2-C3
C3-C9
C9-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-C1
C8-C9
1.803(5)
1.859(5)
1.368(7)
1.429(8)
1.416(8)
1.337(12)
1.391(15)
1.368(9)
1.413(8)
1.450(8)
1.430(7)
1.810
1.851
1.374
1.433
1.423
1.376
1.414
1.374
1.429
1.421
1.446
1.812
1.853
1.374
1.433
1.422
1.376
1.414
1.375
1.429
1.421
1.446
a
The major fragment; two overlapped fragments were re-
b
strained to have the same structure. Calculated at the B3LYP/
6-31G(d) level.
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for COE Reseach on Elements
Science (No. 12CE2005) and Grants-in-Aid for Scientific
Research (Nos. 11304045 and 11166250) from the
Ministry of Education, Culture, Sports, Science and
Technology of J apan. We are grateful to Prof. Shigeru
Nagase, Institute for Molecular Science, and Prof. Yukio
Furukawa, Waseda University, for the theoretical cal-
culations and the measurement of FT-Raman spectra,
respectively. Mr. M. Shimada, Kyushu University,
should also be acknowledged for his pioneering work on
the synthesis of the precursor 2.
the Ge-C double bond with the fluorenylidene moiety.
The C-C bond lengths (1.337(12)-1.450(8) Å) of the
2-germanaphthalene ring are also roughly intermediate
between those of C-C double and single bonds. These
results suggest that the π-electrons in the 2-germanaph-
thalene ring system of 1a are delocalized. Furthermore,
theoretical calculations for the model compounds 1b and
1c were also performed for comparison (Table 2).10 The
experimentally obtained bond lengths are in fairly good
agreement with those theoretically calculated.
The Raman spectrum of 1a showed the most intense
Raman signal at 1363 cm-1, which corresponds to the
most intense signals of 1382 and 1368 cm-1 for naph-
thalene and 2-silanaphthalene,8 respectively. This stron-
gest vibrational frequency observed for 1a is in good
agreement with the theoretically calculated one (1356
cm-1 for 1c, computed at the B3LYP/6-31G(d) level and
scaled by 0.96).15 Furthermore, the calculated vibration
modes of 1c considerably resemble those of naphthalene,
suggesting the structural similarity between 2-ger-
manaphthalene 1a and naphthalene.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving physical properties of compounds 1a and 2-5, crystal-
lographic data with all bond lengths, bond angles, and thermal
and positional parameters for 1a , and the observed Raman
shifts for 1a (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
OM010881Y
(16) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. In Spectrometric
Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991.
(17) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
SIR97: a New Tool for Crystal Structure Determination and Refine-
ment. Submitted for publication.
(18) Sheldrick, G. M. SHELX-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,
1997.
The UV-vis spectrum of 1a in hexane showed three
absorption maxima (269 (ꢀ 2 × 104), 335 (ꢀ 1 × 104),
(15) For the scaling factor of 0.96, see: Bauschlicher, C. W., J r.;
Partridge, H. J . Chem. Phys. 1995, 103, 1788.