Synthesis and properties of stable 2-metallanaphthalenes of heavier group 14 elements

Article abstract of DOI:10.1039/b718193k

The first stable neutral stannaaromatic compound, 2-stannanaphthalene 1a, was synthesized by taking advantage of an extremely bulky and efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of 1a were discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy, cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene 2a, which has a framework similar to that of 1a, was synthesized for comparison, and systematic elucidation was made for the properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).

Full text of DOI:10.1039/b718193k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and properties of stable 2-metallanaphthalenes of heavier
group 14 elements†‡§
Yoshiyuki Mizuhata,a Takahiro Sasamori,a Noriyoshi Nagahora,a,b Yasuaki Watanabe,c Yukio Furukawac and
Norihiro Tokitoh*a
Received 26th November 2007, Accepted 25th January 2008
First published as an Advance Article on the web 2nd July 2008
DOI: 10.1039/b718193k
The first stable neutral stannaaromatic compound, 2-stannanaphthalene 1a, was synthesized
by taking advantage of an extremely bulky and efficient steric protection group,
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of 1a were
discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy,
cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene 2a, which has a framework
similar to that of 1a, was synthesized for comparison, and systematic elucidation was made for the
properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).
lengths, and those for the two endocyclic E–C bonds in the sila- and
germa-benzenes were found to be essentially equal to each other,
Introduction
Aromatic compounds, i.e., [4n + 2]p electron ring systems, are
indicating their delocalized p-electrons. Recent progress in the
widely studied from the viewpoints of not only fundamental
chemistry of sila- and germa-aromatic compounds prompted us
chemistry but also application towards organic synthesis and
to synthesize a stable stannaaromatic compound, which might be
material science. In the past few decades, much attention has
interesting from the standpoints of not only systematic elucidation
been focused on the chemistry of heavier congeners of aromatic
of the properties of metallaaromatic systems but also verification
compounds, i.e., “metallaaromatic compounds”, and many results
about their generation and character have been reported. However,
of the concept of “aromaticity”. Although some stannole anions
and dianions, i.e., ionic stannaaromatic compounds, have been
the examples for the isolation of stable systems are limited to those
synthesized as stable compounds and fully characterized by Saito
containing Si or Ge atom.1–3 Recently, we have succeeded in the
synthesis and isolation of the first stable examples for neutral
sila- and germaaromatic compounds [sila-4 and germa-benzenes,5
1-silanaphthalene,6 2-sila-7 and 2-germa-naphthalenes8 (3b and
2b, respectively), 9-sila-9 and 9-germa-anthracenes10 and 9-sila-11
and 9-germa-phenanthrenes10] by taking advantage of kinetic
stabilization afforded by an efficient steric protection group, 2,4,6-
tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and evaluated their aro-
maticity based on their chemical and physical properties and
theoretical calculations.2 Their molecular structures were finally
determined by X-ray crystallographic analyses, which revealed
the planar geometries of the rings containing a Si or Ge atom.
The lengths of the endocyclic E–C bonds (E = Si, Ge) were inter-
mediate between the corresponding double-bond and single-bond
et al.,12,13 neutral stannaaromatic compounds are still elusive and
their properties have not been disclosed so far. The main reason
for the lack of neutral stannaaromatic compounds is probably due
to the difficulty in their synthesis and isolation responsible for the
extremely high reactivity of Sn–C double bonds.14–16 In fact, 9-Tbt-
9-stannaphenanthrene17 was found to undergo ready dimerization
at ambient temperature, most likely due to the insufficient
steric protection as contrasted with 9-Tbt-9-sila-11 or 9-germa-
phenanthrene,12 which are known to be stable compounds even at
100 ◦C. Here, we report the synthesis of the first stable neutral stan-
naaromatic compound, 2-stannanaphthalene 1a, which is kineti-
cally stabilized by the combination of a Tbt group on the tin atom
and a tert-Bu group on the neighboring carbon atom.18 Further-
more, 2-germanaphthalene 2a having a framework similar to that
of 1a was synthesized for comparison. Successful isolation of 1a
and 2a in addition to the previously reported 2b and 3b enabled us
to elucidate the properties of 2-metallanaphthalene systems con-
taining a heavier group 14 element (Si, Ge, or Sn) systematically.
aInstitute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto,
611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp; Fax: +81-774-
38-3209; Tel: +81-774-38-3200
bInstitute of Sustainability Science, Kyoto University, Gokasho, Uji, Kyoto,
611-0011, Japan, E-mail: nagahora@iss.iae.kyoto-u.ac.jp; Fax: +81-774-
38-3209; Tel: +81-774-38-3218
cDepartment of Chemistry, School of Science and Engineering, Waseda
University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan, E-mail:
furukawa@waseda.jp; Fax: +81-3-3208-7022; Tel: +81-3-5286-3244
Results and discussions
† Based on the presentation given at Dalton Discussion No. 11, 23–25 June
2008, University of California, Berkeley, USA.
Synthesis of the precursors
‡ Dedicated to Professor Renji Okazaki on the occasion of his 70th
birthday.
§ CCDC reference numbers for 1a and 2a 298027 and 669066. Electronic
supplementary information (ESI) available: Details of the optimized
structures for compound 1c–f, 2c-f, and 2-tert-Bu-naphthalene. See DOI:
10.1039/b718193k
Dehydrohalogenation reactions were efficient to synthesize the
metallaaromatic compounds reported previously.2,4–11 Therefore,
2-halo-2,2-dihydro-2-metallanaphthalenes 6a,b and 7a,b were pre-
pared as suitable precursors of 1a and 2a according to Schemes 1
and 2. Bromometallanes 6b and 7b were synthesized by the
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4409–4418 | 4409
©

Although we anticipated that this new method is applicable to
the preparation of the precursor of 2-silanaphthalene, the silicon
analogue 8b or 8c could not be synthesized by a similar method
using Tbt-substituted trihydrosilane or tribromosilane, probably
due to the steric hindrance. In the following section, therefore, the
comparison of the characters of 2-metallanaphthalenes was per-
formed using 2-silanaphthalene 3b7 with 1a and 2a as substrates.
Synthesis of 2-metallanaphthalenes 1a and 2a
2-Stannanaphthalene 1a and 2-germanaphthalene 2a were synthe-
sized by the dehydrobromination of the corresponding bromomet-
allanes 6b and 7b with LDA in hexane at −40 ◦C, respectively
(Scheme 3). Although the mixture of halogermanes 7a/7b was
available for the synthesis of 2a, the reaction of the mixture
of halostannanes 6a/6b with LDA afforded the mixture of the
substitution product, aminostannane 9, and 1a judging from
the 1H NMR due to the large space around the tin atom of
6a. Actually, when LTMP (lithium 2,2,6,6-tetramethylpiperidide),
more sterically hindered base than LDA, was used as a base, the
mixture of 6a/6b gave 1a efficiently. Compounds 1a and 2a were
obtained as yellow and colorless crystals, respectively. Both of
them were found to be thermally stable under an inert atmosphere
either in the solid state (1a: decomposed at 144 ◦C, 2a: mp. 236–
239 ◦C) or in solution (C6D6, at 80 ◦C for 1 h in a sealed tube).
Scheme 1 Generation of dilithio compound 5.
Scheme 2 Synthesis of the precursors. (a) TbtEX3 (E = Sn, Ge) were
prepared as mixtures of halides (X = Cl and Br). (b) LiAlH4 (excess),
THF, 0 ◦C. (c) NBS (1.1 equiv.), benzene, rt. (d) NBS (1.2 equiv.), benzene,
reflux. (e) Yields from 4.
Scheme 3 Synthesis of 2-metallanaphthalenes 1a and 2a. a - hexane,
−40 ◦C then rt.
reaction of Tbt-substituted trihalostannanes or trihalogermanes
with (E)-o-(2ꢀ-lithiovinyl)benzyllithium 5, which can be readily
generated from isotellurochromene 4 according to the method
reported by Sashida (Scheme 1).19 Since TbtEX3 (E = Sn, Ge)
were prepared as mixtures of halides (X = Cl and Br),¶ it is
necessary to reduce the initially obtained cyclic halostannanes
or halogermanes 6a,b and 7a,b with lithium aluminium hydride.
Careful bromination of the resulting hydrostannane 6c and
hydrogermane 7c with NBS afforded pure bromostannane 6b and
bromogermane 7b in good yields, respectively. This new synthetic
method for cyclic halometallanes 6b and 7b using 5 is easier and
milder than those reported for the synthesis of the precursors of
2b and 3c via 1,2,3,4-tetrahydro-2-metallanaphthalene skeletons.7,8
Crystal structures
The molecular structure of 1a was determined by X-ray crystal-
lographic analysis18 (Fig. 1), which revealed the planarity of the
2-stannanaphthalene moiety and the completely trigonal planar
geometry around the tin atom (the sum of the angles around the
tin atom is 360.0◦). In addition, the benzene ring of the Tbt group
was found to be almost perpendicular to the 2-stannanaphthalene
plane (89.9◦), and hence it is considered that there is very little
conjugative interaction between the two aromatic units.
The bond lengths of the 2-stannanaphthalene skeleton were
summarized in Table 1 together with the calculated values by using
the model compounds 1c–e. The lengths of the two endocyclic
¶ TbtEX3 (E = Sn or Ge, X = Cl and Br) were prepared by the reaction
of TbtLi (TbtBr + tert-BuLi) with ECl4. Bromo-substituted compounds
were generated by the halogen-exchange reaction between TbtECl3 and
LiBr in the reaction mixture.
˚
Sn–C bonds [2.029(6) and 2.081(6) A] are shorter than those
20
˚
of typical single bonds (av. 2.14 A). In particular, the former
4410 | Dalton Trans., 2008, 4409–4418
This journal is
The Royal Society of Chemistry 2008
©

˚
Table 2 Observed and calculated bond lengths (A) for 2-
germanaphthalenesa
bond
2b (obsd)b 2a (obsd) 2c (calcd) 2d (calcd) 2e (calcd)
C1-C8
C1-Ge
Ge–C2
C2-C3
C3-C9
C4-C9
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C2-C10
1.450(8)
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)
1.430(7)
—
1.427(12) 1.420
1.420
1.804
1.874
1.376
1.437
1.421
1.378
1.412
1.376
1.426
1.441
1.534
1.421
1.806
1.866
1.375
1.438
1.421
1.378
1.412
1.376
1.427
1.442
1.532
1.807(9)
1.876(8)
1.802
1.877
1.380(11) 1.376
1.415(12) 1.437
1.420(11) 1.421
1.353(13) 1.378
1.405(15) 1.413
1.360(14) 1.376
1.430(12) 1.426
1.460(12) 1.440
1.550(12) 1.534
Fig. 1 X-Ray structure of 1a (ORTEP drawing with 50% probability
level). Hydrogen atoms were omitted for clarity. (a) Top view. (b) Side
view.
˚
Table 1 Observed and calculated bond lengths (A) for 2-
stannanaphthalenes
a Calculated at the B3LYP/6–31G(d) level. b Ref. 8.
Bond
1a (obsd)
1c (calcd)a
1d (calcd)b
1e (calcd)b
in the 2-germanaphthalene ring has not been achieved so far, but
the observed bond lengths (including large errors) in 2a are almost
similar to those estimated by the theoretical calculations using the
model compounds 2c-e (Table 2), suggesting that this preliminary
structure of 2a reflects its real structure considerably. The sum
of the angles around the germanium atom was 359.6◦, indicating
the planarity of the 2-germanaphthalnene ring of 2a. In addition,
the angle between the benzene ring of the Tbt group and the 2-
germanaphthalene plane was 76.7◦, which value was smaller than
those of 1a (89.9◦) and 2b (88.8◦)8 due to steric repulsion between
Tbt and tert-Bu groups derived from the difference of the central
atom sizes. The bond lengths of 2a were in the same range of those
observed in 2b, suggesting little substituent effect of the tert-Bu
group on the 2-germanaphthalene ring.
C1-C8
C1-Sn
Sn–C2
C2-C3
C3-C9
C4-C9
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C2-C10
1.394(8)
2.029(6)
2.081(6)
1.372(9)
1.443(9)
1.417(9)
1.356(9)
1.415(10)
1.361(9)
1.419(9)
1.436(9)
1.522(9)
1.422
1.993
2.066
1.380
1.440
1.425
1.377
1.413
1.375
1.432
1.449
1.541
1.424
1.988
2.073
1.374
1.445
1.423
1.378
1.411
1.375
1.431
1.447
1.535
1.423
1.985
2.067
1.374
1.445
1.423
1.378
1.411
1.375
1.431
1.447
1.535
a Calculated at the B3LYP/6–31G(d) [TZ(2d) on Sn] level. b Calculated at
the B3LYP/6–31G(d) (LANL2DZ on Sn) level.
Sn–C bond length is close to the Sn–C double bond length of
=
NMR Spectra
the stable stannene Tbt(Mes)Sn CR2 [CR2 = fluorenylidene;
16
˚
2.016(5) A], which is stabilized by the conjugation of the Sn–
The 1H and 13C (and 119Sn) chemical shift values of the 2-
stanna- and 2-germa-naphthalenes, 1a and 2a, were summarized
in Tables 3 and 4, respectively, along with the calculated values by
C double bond with the fluorenylidene moiety. The C–C bond
lengths of the 2-stannanaphthalene ring of 1a [1.356(9)–1.443(9)
A] are also roughly intermediate between those of C–C double
˚
and single bonds, showing the bond-alternation nature similar
to that of parent naphthalene. These results suggest that the p-
electrons are delocalized in the 2-stannanaphthalene skeleton of
1a. Furthermore, theoretically optimized molecular geometry is
in good agreement with the experimental results, indicating the
effective p-conjugation in the Sn-containing aromatic ring systems.
Although the planar structure of 2-germanaphthalene 2a was
also suggested by the X-ray crystallographic analysis (Fig. 2), we
could not obtain sufficient data due to the twin character of their
crystals despite a number of trials using different single crystals.
Accurate experimental determination of bond lengths and angles
Table 3 Observed and calculated 119Sn, 1H, and 13C NMR chemical shifts
(ppm) for 2-stannanaphthalenes
Atom
1a (obsd)a
1c (calcd)b
1d (calcd)c
1e (calcd)c
Sn
264
9.28
8.75
7.65
7.05
7.21
7.68
147.4
174.0
142.2
135.4
120.0
125.3
128.0
147.3
125.9
39.7
150
8.42
8.91
7.80
7.17
7.35
7.66
136.5
169.9
149.6
136.6
119.4
125.4
128.8
151.0
126.9
41.8
273
8.58
8.57
7.71
7.01
7.25
7.59
139.4
173.8
143.2
136.9
120.4
126.0
128.7
148.8
127.1
41.2
123
9.25
8.79
7.88
7.38
7.31
7.64
147.9
175.4
143.4
137.0
121.9
125.3
129.1
149.9
129.2
42.5
H1
H3
H4
H5
H6
H7
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
a Measured in benzene-d6. b Calculated at the GIAO-B3LYP/6–
311G+(2d,p) (TZV on Sn)//B3LYP/6–31G(d) [TZ(2d) on Sn]
level. c Calculated at the GIAO-B3LYP/6–311G+(2d,p) (TZV on
Sn)//B3LYP/6–31G(d) (LANL2DZ on Sn) level.
Fig. 2 X-Ray structure of 2a (ORTEP drawing with 30% probability
level). Hydrogen atoms were omitted for clarity. (a) Top view (b) Side view.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4409–4418 | 4411
©

Table 4 Observed and calculated 1H and 13C NMR chemical shifts (ppm)
for 2-germanaphthalenes
Table 6 NICS(0) and (1) (ppm) values for 2-metallanaphthalenes 1f-3f
and naphthalenea
Atom
2a (obsd)a
2c (calcd)b
2d (calcd)b
2e (calcd)b
Sn (1f)
Ge (2f)
Si (3f)
Naphthalene
H1
H3
H4
H5
H6
H7
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
8.34
8.56
7.65
7.08
7.24
7.98
8.49
7.79
7.17
7.37
8.03
8.38
7.74
7.08
7.36
8.42
8.52
7.83
7.29
7.47
NICS(0)
NICS(1)
EC5-ring
C6-ring
EC5-ring
C6-ring
−6.7
−7.4
−8.4
−9.7
−7.2
−7.7
−8.9
−9.9
−7.3
−7.7
−8.9
−9.9
−8.0
−8.0
−10.5
−10.5
a Calculated at the GIAO-B3LYP/6–311+G(2d,p) (TZV on
7.70
7.61
7.62
7.66
Sn)//B3LYP/6–31G(d) (LANL2DZ on Sn) level.
135.8
160.1
140.7
133.6
120.8
125.8
127.1
143.6
126.6
39.7
129.5
159.9
139.7
135.0
120.4
126.6
127.7
146.8
127.1
40.0
130.5
161.2
139.7
134.9
121.0
126.4
127.6
145.9
128.3
40.6
136.6
165.0
140.4
134.9
121.9
126.2
128.0
146.5
129.3
40.7
compounds, as shown in the following section. On the other hand,
there is rather little variation of the chemical shift values of C3, C8,
and C9 carbon atoms. However, the deshielding of the a-carbon
atoms (C1 and C2) is progressive, taking the substitution of tert-Bu
into consideration. This reflects a similar trend in the H1 protons.
a Measured in benzene-d6. b Calculated at the GIAO-B3LYP/6–
311G+(2d,p)//B3LYP/6–31G(d) level.
Nucleus-independent chemical shifts (NICS)
An efficient computational probe for diatropic and paratropic
ring currents, associated with aromaticity and antiaromaticity,
respectively, may be the calculations for nucleus-independent
chemical shifts (NICS) values,22 which are computed as the
absolute magnetic shielding at ring centers. Generally, the NICS(0)
using the model compounds 1c-e and 2c-e. 2-Stannanaphthalene
1a showed the 119Sn NMR signal of the central Sn atom around
264 ppm in C6D6, which is characteristic of the low-coordinated
˚
1
and NICS(1) values are computed at the ring center and 1 A
tin atom. All of the H NMR signals of the protons of the 2-
above the ring centers, respectively, reflecting the ring current
effect of the p-electrons. The NICS(0) and NICS(1) values for 2-
metallanaphthalenes 1f-3f and naphthalene are shown in Table 6,
and the trends of NICS(0) and NICS(1) values are similar to
each other. Although the aromaticity of the SiC5 and GeC5 rings
is estimated to be 85–90% as compared with that of parent
naphthalene, the aromaticity of the SnC5 ring is slightly smaller
than those of Si and Ge analogues (about 80% to that of
naphthalene).
metallanaphthalene rings of 1a (7.05–9.28 ppm) and 2a (7.08–
8.56 ppm) were observed in the aromatic region, and the 13C
NMR signals of the metallanaphthalene ring carbons (1a: 120.0–
174.0 ppm; 2a: 120.8–160.1 ppm) appeared in the sp2 region. Thus,
these results clearly indicate the delocalized p-electronic systems
of 1a and 2a even in solution. The observed values are in good
agreement with the calculated ones.
1
The H and 13C chemical shift values of EC5 rings (E = Sn,
Ge, Si, and C) in the 2-metallanaphthalene skeletons are listed
in Table 5. The most striking feature is the large downfield shift
of the H1 protons, which increases as the central group 14 atom
goes from Si to Sn. The shifts of H3 also follow the same trend,
but the shift magnitudes are considerably smaller. As for H2,
we could not judge its trend accurately from only two values,
however it follows the trend in the H1. Similar trend has already
been observed in the cases of heavier congeners of pyridine.21 The
major reason of the downfield shift should be accounted by the
effect of the magnetic anisotropy of the increasingly large atom.
These upfield shifts are consistent with decreasing ring current
and hence decreasing aromaticity for the heavier metallaaromatic
Raman spectra
The Raman spectrum of 1a and 2a (Fig. 3a and 3c, respectively)
showed the most intense lines at 1332 and 1379 cm−1, respectively.
Simulated spectra obtained by the theoretical calculations using
model compounds (Fig. 3b and 3d) reproduced the observed
spectra almost exactly and enabled us to assign their vibration
modes. These most intense lines were assigned to the planar
skeletal-vibration modes, as shown in Fig. 4a for 1a, and their
calculated vibration modes considerably resemble those of 2-tert-
Bu-naphthalene (Fig. 4b), suggesting skeletal similarity among
2-metallanaphthalenes and naphthalene.
These vibration modes correspond to those for the most intense
lines of 2b,7 3b,8 and naphthalene, 1363, 1368, and 1382 cm−1,
respectively (Table 7), the values of which decreased with the
atomic number of the central atom. These results are considered
to be reasonable.
Table 5 1H and 13C chemical shifts (ppm) of EC5 rings of 2-
metallanaphthalenesa
Position
Sn (1a)
Ge (2a)
Ge (2b)b
Si (3b)c
Naphthalene
H1
H2
H3
C1
C2
C3
C8
C9
9.28
—
8.75
147.38
174.03
142.22
147.26
125.92
8.34
—
8.56
135.81
160.10
140.73
143.59
126.57
8.24
7.85
8.55
127.69
130.45
145.89
145.15
128.12
7.40
7.24
8.48
116.01
122.56
148.95
146.53
128.26
7.62
7.24
7.62
128.19
126.00
128.19
133.99
133.99
UV/vis Spectra
In Fig. 5 are shown the UV/vis spectrum of 2-metallanaphthalenes
1a, 2a, and 3b7 measured in hexane at room temperature, and their
absorption maxima containing those of parent naphthalene were
summarized in Table 8. By comparison of the absorption maxima
between 2a and 2b,8 the substitution effect of tert-Bu group is
a Measured in C6D6 at room temperature. b Ref. 8. c Ref. 7.
4412 | Dalton Trans., 2008, 4409–4418
This journal is
The Royal Society of Chemistry 2008
©

Table 7 The most intense lines (cm−1, obsd) in the Raman spectra of
2-metallanaphthalenes and naphthalene
Sn
Ge
Si
C
1a
1332
2a
1379
—
—
—
2b
3b
Naphthalene
1382
1363a
1368b
a Ref. 8. b Ref. 7.
Fig. 5 UV/vis spectra of 2-metallanaphthalenes (in hexane, rt).
(C << Si < Ge << Sn) reflects the energy gap estimated by the
theoretical calculations using non-substituted models 1f, 2f, and
3f (Fig. 6).
Fig. 3 Raman spectra of 2-stannanaphthalenes. (a) FT-Raman spectrum
of 1a measured with the excitation by He–Ne laser (532 nm). (b) Spectrum
of 1e simulated by the theoretical calculation at the B3LYP/6–31G(d)
(LANL2DZ on Sn) level. (c) FT-Raman spectrum of 2a measured with
the excitation by He–Ne laser (1064 nm). (d) Spectrum of 2c simulated by
the theoretical calculation at the B3LYP/6–31G(d) level.
Fig. 6 HOMO and LUMO energy levels of 2-metallanaphthalenes 1f, 2f,
and 3f. Calculated at the HF/6–31G(d) (LANL2DZ on Sn atom) level.
Cyclic voltammetry
Fig. 4 Calculated vibration modes of (a) 1a (1378 cm−1) and (b)
2-tert-butylnaphthalene (1424 cm−1).
A parent naphthalene is known to react with alkali metals
(Li, Na, K. . .) to give the corresponding radical anions (metal
naphthalenides).23,24 As described above, 2-metallanaphthalenes
were found to have sufficient aromaticity comparable to that of
parent naphthalene, judging from their structures and spectro-
scopic data. One can hit upon a natural question whether 2-
metallanaphthalenes can be reduced as well as parent naphthalene
or not.
considered to be negligible towards the absorption maxima. In the
Si and Ge cases, the spectra have almost similar shapes including
the characteristically strong absorptions at 326 (3b) and 340 (2a)
nm together with other small absorptions. The difference of the
absorbance between 2a and 3b might be derived form the electron-
donating character of the tert-Bu group. On the other hand, in the
Sn case, the spectra have a broadened and strong absorption at
393 nm. This value apparently shifts to a longer wavelength than
those for 2a, 2b, 3b, and naphthalene (Table 8). This trend of kmax
The electrochemical properties of 1a were furnished by cyclic
voltammetry, measurement of which was carried out in THF
with n-Bu4NBF4 as an electrolyte at room temperature. For
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4409–4418 | 4413
©

Table 8 Absorption maxima (nm) and absorbances (e) of 2-metallanaphthalenes and naphthalenea
1a
2a
2bb
3b
Naphthalene
295 (1.7 × 104)
393 (1.7 × 104)
—
275 (4.0 × 104)
340 (2.0 × 104)
384 (3.9 × 103)
269 (2 × 104)
335 (1 × 104)
386 (2 × 103)
267 (3.3 × 104)
326 (7.0 × 103)
369 (2.8 × 103)
387 (2.4 × 103)
221 (1.3 × 105)
286 (9.3 × 103)
312 (2.9 × 102)
a Measured in hexane at rt. b Ref. 8.
Table 9 Redox potentials in THF [V vs. Cp2Fe/Cp2Fe+]
Conclusions
Epa
Epc
E1/2
We have succeeded in the synthesis and isolation of the first
example of 2-stannanaphthalene 1a and 3-tert-Bu-substituted 2-
germanaphthalene 2a by taking advantage of an efficient steric
protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt).
The molecular structures of 2-metallanaphthalenes 1a and 2a were
discussed systematically with those of previously reported related
compounds on the basis of their X-ray crystallographic analysis
and NMR, UV/vis, and Raman spectra. X-Ray crystallographic
analyses demonstrated the planarity of the metallaaromatic rings
and the delocalization of p-electrons in the 2-metallanaphthalene
ring systems. These experimental results were in good agreement
with those obtained by theoretical calculations. The deshielding
1a
−2.19
−3.00
−2.77
−3.28
−2.03
−2.64
—
−2.94
−2.11
−2.82
—
−3.11
2a
3b
Naphthalene
comparison, the measurements of cyclic voltammetry for parent
naphthalene, 2-silanaphthalene 3b, and 2-germanaphthalene 2a
were performed under similar conditions. The voltammograms
were shown in Fig. 7 and summarized in Table 9.
1
effect observed in the H NMR spectra of 2-metallanaphthalene
systems strongly indicated the existence of the same ring current
effect as observed for parent naphthalene. The UV/vis and
Raman spectra of 2-metallanaphthalenes and parent naphthalene
showed patterns that closely resembled each other, indicating the
similarity in their molecular frameworks, though the intrinsic
bathochromic shifts (UV/vis) and low wavenumber shifts (Ra-
man) were observed for the heavy aromatics. The aromaticity
of 2-metallanaphthalenes was also evaluated by the calculations
of NICS values. Large negative NICS values strongly support
the ring current effect and aromatic stabilization in the 2-
metallanaphthalene systems. It should be noted that the concept
of aromaticity is applicable even in the case of Sn-containing
aromatic systems in addition to the Si and Ge systems.
On the other hand, these results clearly showed the similarities
between the Si- and Ge-containing systems and the uniqueness of
the Sn-containing system, e.g., the remarkable red-shift of 1a in
UV/vis spectra. These trends were considered to correlate with
the s and p orbital sizes of group 14 elements (C << Si < Ge
<< Sn). The efficient aromaticity and characteristic properties in
stannaaromatic systems are important aspects from the viewpoint
of “elements science” and may open the door to designing new
p-conjugated systems utilizing element characters.
Fig. 7 Cyclic voltammograms of 2-metallanaphthalenes and naphthalene
in THF.
As in the case of parent naphthalene, quasi-reversible one-
red
electron reduction waves were observed for 1a and 2a at E1/2
=
−2.11 and −2.82 V versus Cp2Fe/Cp2Fe+, respectively. The
reduction potentials of 1a and 2a were lower than that of
Experimental
red
naphthalene (E1/2 = −3.11). These well-defined reversible redox
waves indicate that the corresponding 2-stanna- and 2-germa-
naphthalene radical anions can be generated as stable chemical
species under such conditions.
General
All experiments were performed under an argon atmosphere unless
otherwise noted. Solvents used for the reactions were purified
by The Ultimate Solvent System (GlassContour Company).25
1H NMR (300 MHz), 13C NMR (76 MHz), and 119Sn NMR
(111 MHz) spectra were measured in CDCl3 or C6D6 with a JEOL
On the other hand,
a cyclic voltammogram for 2-
silanaphthalene 3b showed an irreversible reduction wave at
Epa = −2.77 V versus Cp2Fe/Cp2Fe+ (Fig. 7), indicating that
red
the anion radical species of 3b should be unstable under these
1
conditions.
JNM-AL300 spectrometer. In H NMR, signals due to CHCl3
4414 | Dalton Trans., 2008, 4409–4418
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The Royal Society of Chemistry 2008
©

(7.25 ppm) and C6D5H (7.15 ppm) were used as references, and
those due to CDCl3 (77 ppm) and C6D6 (128 ppm) were used
in 13C NMR. 119Sn NMR was measured with NNE technique
using SnMe4 as an external standard. Multiplicity of signals in
13C NMR spectra was determined by the DEPT technique. High-
resolution mass spectral data were obtained on a JEOL JMS-
SX102GC/MS spectrometer. WCC (wet column chromatogra-
phy) was performed on Wakogel C-200. PTLC (preparative thin-
layer chromatography) was performed with Merck Kieselgel 60
PF254 (Art. No. 7747). GPLC (gel permeation liquid chromatog-
raphy) was performed on an LC-908, LC-918, or LC-908-C60
(Japan Analytical Industry Co., Ltd.) equipped with JAIGEL
1H and 2H columns (eluent: chloroform or toluene). Electronic
spectra were recorded on a JASCO Ubest-50 UV/vis spectrometer.
Raman spectra were measured at room temperature on a Raman
spectrometer consisting of a Spex 1877 Triplemate and an EG
& G PARC 1421 intensified photodiode array detector. An NEC
GLG 108 He–Ne laser (532 and 1064 nm) was used for Raman
excitation. The electrochemical experiments were carried out with
an ALS 600A potentiostat/galvanostat using a glassy carbon disk
working electrode, a Pt wire counter electrode, and a Ag/0.01 M
AgNO3 reference electrode. The measurements were carried out
in THF solution containing 0.1 M n-Bu4NPF6 as a supporting
electrolyte with scan rates of 50–200 mVs−1 in a glovebox filled with
argon at room temperature. All melting points were determined
on a Yanaco micro melting point apparatus and were uncorrected.
Elemental analyses were carried out at the Microanalytical Lab-
oratory of the Institute for Chemical Research, Kyoto University.
All theoretical calculations were carried out using the Gaussian
9826 or 0327 programs. Isotellurochromene 4,28 TbtSiBr3,29 and
Preparation of 2-Tbt-2-bromo-3-tert-butyl-1,2-dihydro-2-
stannanaphthalene (6b) using TbtSnBr3
To a THF solution (20 mL) of 4 (601 mg, 2.01 mmol) was added n-
butyllithium (1.5 M in hexane, 2.7 mL, 4.1 mmol) at −78 ◦C. After
stirring at the same temperature for 10 min, THF solution (20 mL)
of TbtSnBr3 (1.83 g, 2.01 mmol) was added to the mixture. After
stirring for 3 h at −78 ◦C, the reaction mixture was warmed
to room temperature and stirred for 12 h. After removal of the
solvent, hexane was added to the residue. The resulting suspension
R
was filtered through Celiteꢀ, and the solvent was removed. The
residue was separated by GPLC (CHCl3) to afford 6b◦(380 mg,
0.411 mmol, 21%). 6b: colorless crystals; mp 162–165 C (dec.);
1H NMR (300 MHz, C6D6, rt): d −0.13 (s, 9H), −0.03 (s, 9H),
−0.01 (s, 9H), 0.04 (s, 18H), 0.11 (s, 9H), 1.25 (s, 1H), 1.39 (s,
9H), 1.59 (br s, 1H), 2.42 (s, 1H), 3.02 (d, J = 15.0 Hz, 1H), 3.24
(d, J = 15.0 Hz, 1H), 6.29 (br s, 1H), 6.44 (br s, 1H), 7.01–7.11
(m, 5H); 13C NMR (75 MHz, C6D6, rt): d 0.96 (q), 0.98 (q), 1.21
(q), 30.44 (t), 30.83 (d), 31.24 (d), 31.75 (d), 33.09 (q), 38.51 (s),
123.46 (d), 126.41 (d), 127.48 (d), 128.00 (d), 132.55 (d), 133.65
(d), 134.02 (s), 135.81 (s), 136.10 (s), 142.01 (d), 145.42 (s), 151.13
(s), 152.35 (s), 160.65 (s); 119Sn NMR (111 MHz, C6D6, rt): d
−93.0; High resolution FAB-MS m/z calcd for C40H7579BrSi6120Sn
([M]+): 922.2690, found: 922.2695. Anal. Calcd for C40H75BrSi6Sn:
C, 52.04; H, 8.19. Found: C, 52.31; H, 8.44.
Preparation of 2-Tbt-3-tert-butyl-1,2-dihydro-2-
stannanaphthalene (6c) using TbtSnX3
To a THF solution (16 mL) of 4 (493 mg, 1.64 mmol) was added n-
butyllithium (1.5 M in hexane, 2.2 mL, 3.3 mmol) at −78 ◦C. After
stirring at the same temperature for 10 min, THF solution (33 mL)
of TbtSnX3 (X = Cl or Br, ca. 75% purity; 1.63 g, ca 1.6 mmol
as X = Cl) was added to the mixture. After stirring for 3 h at
30
TbtSiH3 were prepared according to the reported procedures.
TbtEX3 (E = Sn and Ge, X = Cl or Br) were prepared by following
the syntheses of TbtECl3 (E = Sn31 and Ge32) and used without
strict purification (bromo-substituted compounds were generated
by the halogen-exchange reaction between TbtECl3 and LiBr in
the reaction mixture).
◦
−78 C, the reaction mixture was warmed to room temperature.
After removal of the solvent, hexane was added to the residue. The
R
resulting suspension was filtered through Celiteꢀ, and the solvent
was removed. The residue was separated by GPLC (CHCl3) to
afford the mixture of 6a and 6b (567 mg, 6a/6b = 7/3). To a THF
solution (10 mL) of this mixture was added lithium aluminium
hydride (88.5 mg, 2.33 mmol) at 0 ◦C. After stirring for 1 h at the
same temperature, ethyl acetate was added to the reaction mixture
at 0 ◦C. After removal of the solvent, hexane was added to the
Preparation of Tbt-tribromostannane
To a solution of TbtBr (5.00 g, 7.91 mmol) in THF (80 mL) was
added tert-butyllithium (7.9 mL, 2.2 M in pentane, 17.4 mmol) at
−78 ◦C. After the solution was stirred at the same temperature
for 1 h, SnBr4, (3.95 g, 9.01 mmol) was added at −78 ◦C. The
solution was stirred for 14 h, during which time it was warmed
to room temperature. After removal of the solvent, hexane was
added to the residue to precipitate inorganic salts, and the filtrate,
after evaporation of hexane, was purified by recrystalization from
acetonitrile/chloroform to afford TbtSnBr3 (3.06 g, 3.36 mmol,
42%). colorless crystals; mp 231–233 C; H NMR (300 MHz,
rt, CDCl3): d 0.07 (s, 18H), 0.11 (s, 18H), 0.14 (s, 18H), 1.41
(s, 1H), 2.33 (s, 1H), 2.46 (s, 1H), 6.36 (s, 1H), 6.48 (s, 1H);
13C NMR (75 MHz, rt, CDCl3): d 0.56 (q), 0.70 (q), 0.92 (q),
31.28 (d), 32.06 (d), 32.76 (d), 123.03 (d), 127.85 (d), 134.36
(s), 149.09 (s), 150.96 (s), 151.05 (s); 119Sn NMR (111 MHz,
rt, CDCl3): d −311.1; high resolution FAB-MS m/z calcd for
C27H5979Br281BrSi6120Sn: 909.9776 ([M]+), found: 909.9780 ([M]+).
Anal. Calcd for C27H59Br3Si6Sn: C, 35.61; H, 6.53. Found: C, 35.71;
H, 6.65.
R
residue. The resulting suspension was filtered through Celiteꢀ,
and the solvent was removed. This crude product was separated
by WCC (hexane) to afford 6c (509◦mg, 0.603 mmol, 37%, from 4).
6c: colorless crystals; mp 179–181 C (dec.); 1H NMR (300 MHz,
C6D6, rt): d 0.05 (s, 9H), 0.10 (s, 9H), 0.137 (s, 9H), 0.144 (s, 9H),
0.16 (s, 9H), 0.21 (s, 9H), 1.39 (s, 9H), 1.44 (s, 1H), 1.65 (br s, 1H),
2.34 (br s, 1H), 2.58 (d, 2J = 15.0 Hz, 1H), 2.78 (dd, 2J = 15.0 Hz,
3J = 0.9 Hz, 1H), 5.95 (d, 3J = 0.9 Hz, 1H), 6.52 (br s, 1H), 6.67
(br s, 1H), 6.91–7.10 (m, 5H); 13C NMR (75 MHz, C6D6, rt): d
0.96 (q), 1.00 (q), 1.18 (q), 17.21 (t), 30.63 (d), 32.12 (d), 32.24
(q), 32.88 (d), 37.81 (s), 122.29 (d), 125.98 (d), 127.12 (d), 127.35
(d), 132.95 (d), 133.50 (d), 135.91 (s), 135.95 (s), 139.61 (d), 143.92
(s), 151.79 (s × 2), 158.66 (s); 119Sn NMR (111 MHz, C6D6, rt):
d −293.3; high resolution FAB-MS m/z calcd for C40H76Si6120Sn
([M]+): 844.3585, found: 844.3589. Anal. Calcd for C40H76Si6Sn:
C, 56.91; H, 9.07. Found: C, 56.93; H, 9.23.
◦
1
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 4409–4418 | 4415
©

Preparation of 6b by the bromination reaction of 6c
high resolution FAB-MS m/z calcd for C40H7579Br74GeSi6 ([M]+):
876.2880, found: 876.2874. Anal. Calcd for C40H75BrGeSi6: C,
54.78; H, 8.62. Found: C, 54.80; H, 8.75.
A benzene (50 mL) solution of 6c (509 mg, 0.603 mmol) and
N-bromosuccinimide (118 mg, 0.663 mmol) was stirred for 1 h
at room temperature. After removal of the solvent, hexane was
added to the residue. The resulting suspension was filtered through
Attempted synthesis of 2-Tbt-2-bromo-3-tert-butyl-1,2-dihydro-2-
silanaphthalene (8b)
R
Celiteꢀ, and the solvent was removed to afford 6b (509 mg,
0.552 mmol, 91%).
To a THF solution (20 mL) of 4 (151 mg, 0.502 mmol) was
added n-butyllithium (1.5 M in hexane, 0.670 mL, 1.00 mmol)
at −78 ◦C. After stirring at the same temperature for 10 min,
THF solution (10 mL) of TbtSiBr3 (411 mg, 0.501 mmol) was
added to the mixture. After stirring for 3 h at −78 ◦C, the reaction
mixture was warmed to room temperature and stirred for 12 h.
After removal of the solvent, hexane was added to the residue.
Preparation of 2-Tbt-3-tert-butyl-1,2-dihydro-2-germanaphthalene
(7c)
To a THF solution (10 mL) of 4 (320 mg, 1.07 mmol) was
added n-butyllithium (1.5 M in hexane, 1.43 mL, 2.15 mmol) at
−78 ◦C. After stirring at the same temperature for 10 min, THF
solution (10 mL) of TbtGeX3 (X = Cl or Br, 0.832 g, ca 1.1 mmol as
X = Cl) was added to the mixture. After stirring for 3 h at −78 ◦C,
the reaction mixture was warmed to room temperature and stirred
for 12 h. After removal of the solvent, hexane was added to the
R
The resulting suspension was filtered through Celiteꢀ, and the
solvent was removed to afford a complicated mixture.
Attempted synthesis of 2-Tbt-3-tert-butyl-1,2-dihydro-2-
silanaphthalene (8c)
R
residue. The resulting suspension was filtered through Celiteꢀ,
and the solvent was removed. The residue was separated by GPLC
(CHCl3) to afford the mixture of 7a and 7b (318 mg, 7a/7b = 8/2).
To a THF solution (30 mL) of this mixture was added lithium
aluminium hydride (250 mg, 6.59 mmol) at 0 ◦C. After stirring for
10 h under reflux condition, ethyl acetate was added to the reaction
mixture at 0 ◦C. After removal of the solvent, hexane was added to
To a THF solution (10 mL) of 4 (301 mg, 1.00 mmol) was
added n-butyllithium (1.5 M in hexane, 1.30 mL, 1.95 mmol) at
−78 ◦C. After stirring at the same temperature for 10 min, THF
solution (15 mL) of TbtSiH3 (585 mg, 1.00 mmol) was added
to the mixture. After stirring for 1 h at −78 ◦C, the reaction
mixture was warmed to room temperature and stirred for 12 h.
After removal of the solvent, hexane was added to the residue.
R
the residue. The resulting suspension was filtered through Celiteꢀ,
R
The resulting suspension was filtered through Celiteꢀ, and the
and the solvent was removed. This crude product was separated
by WCC (hexane) to afford 7c (171◦mg, 0.214 mmol, 20%, from 4).
7c: colorless crystals; mp 192–194 C (dec.); 1H NMR (300 MHz,
323 K, CDCl3): d −0.02 (s, 27H), 0.03 (s, 18H), 0.04 (s, 9H), 1.21
solvent was removed to afford a complicated mixture.
Synthesis of 2-Tbt-3-tert-butyl-2-stannanaphthalene (1a) from
bromostannane 6b
2
(s, 9H + 1H), 1.27 (s, 2H), 2.51 (d, J = 16.5 Hz, 1H), 2.75 (dd,
3
3
2J = 16.5 Hz, J = 5.2 Hz, 1H), 5.26 (d, J = 5.2 Hz, 1H), 6.25
(br s, 1H), 6.39 (br s, 1H), 7.02 (s, 1H), 7.08–7.10 (m, 4H); 13C
NMR (75 MHz, rt, CDCl3): d 0.84 (q), 1.38 (q), 18.36 (t), 27.23
(d), 29.12 (d), 30.17 (d), 31.45 (q), 37.20 (s), 122.87 (d), 125.85 (d),
127.12 (d), 127.40 (d), 131.29 (s), 132.24 (d), 132.58 (d), 134.09 (s),
135.79 (s), 137.40 (d), 143.04 (s), 150.10 (s), 150.42 (s), 151.95 (s);
high resolution FAB-MS m/z calcd for C40H7674GeSi6: 798.3774
([M]+), found: 798.3777 ([M]+). Anal. Calcd for C40H76GeSi6: C,
60.19; H, 9.60. Found: C, 59.93; H, 9.75.
In a glovebox filled with argon, 6b (46.2 mg, 0.0500 mmol) was dis-
solved in hexane (2 mL, dried over K mirror and distilled by a trap-
to-trap method), and LDA (2.0 M in heptane/THF/ethylbenzene,
0.0300 mL, 0.0600 mmol) was added to the solution at −40 ◦C. Af-
ter stirring for 1 h at room temperature, the solvents were removed
under reduced pressure and hexane was added to the residue. The
R
resulting suspension was filtered through Celiteꢀ, and the solvent
was removed. The residue was recrystallized from hexane to give
1a (26.7 mg, 0.0317 mmol, 63%). 1a: yellow crystals; mp 144–
147 ◦C (dec.); 1H NMR (300 MHz, C6D6, rt): d 0.18 (br s, 54H),
1.06 (s, 1H), 1.53 (s, 9H), 2.04 (br s, 1H), 2.08 (s, 1H), 6.68 (br s,
1H), 6.81 (br s, 1H), 7.05 (dd, 3J = 9 Hz, 3J = 7 Hz, 1H), 7.21 (dd,
3J = 9 Hz, 3J = 7 Hz, 1H), 7.65 (d, 3J = 9 Hz, 1H), 7.68 (d, 3J =
9 Hz, 1H), 8.75 (s, 1H), 9.28 (s, 1H); 13C NMR (75 MHz, C6D6,
rt): d 0.91 (q), 1.55 (q), 30.84 (d), 34.93 (q), 39.36 (d), 39.72 (s),
39.90 (q), 119.97 (d), 122.12 (d), 125.30 (d), 125.92 (s), 126.76 (d),
127.93 (d), 135.39 (d), 141.58 (s), 142.22 (d), 146.02 (s), 147.26 (s),
147.38 (d), 150.92 (s × 2), 174.03 (s); 119Sn NMR (111 MHz, C6D6,
rt): d 264; high resolution FAB-MS m/z calcd for C40H75Si6120Sn
([M + H]+): 843.3506, found: 843.3531.
Preparation of 2-Tbt-2-bromo-3-tert-butyl-1,2-dihydro-2-
germanaphthalene (7b)
A benzene (20 mL) solution of 7c (171 mg, 0.214 mmol) and
N-bromosuccinimide (47.1 mg, 0.265 mmol) was stirred for 1 h
at room temperature. After removal of the solvent, hexane was
added to the residue. The resulting suspension was filtered through
R
Celiteꢀ, and the solvent was removed to afford 7b (155 mg,
◦
0.177 mmol, 83%). 7b: colorless crystals; mp 220–222 C (dec.);
1H NMR (300 MHz, 323 K, CDCl3): d −0.23 (s, 9H), −0.10 (s,
9H), −0.09 (s, 9H), −0.06 (s, 18H), −0.02 (s, 9H), 1.18 (s, 1H),
1.32 (s, 9H), 1.94 (br s, 1H), 2.90 (br s, 1H), 3.17 (d, 2J = 17.3 Hz,
2
1H), 3.26 (d, J = 17.3 Hz, 1H), 6.15 (br s, 1H), 6.29 (br s, 1H),
Attempted synthesis of 1a from a mixture of 6a/6b with LDA
6.93 (s, 1H), 6.97–7.09 (m, 4H); 13C NMR (75 MHz, rt, CDCl3):
d 0.85 (q), 0.86 (q), 1.02 (q), 1.22 (q), 1.43 (q), 26.85 (d), 27.05
(d), 30.30 (d), 31.76 (t), 32.69 (q), 37.71 (s), 123.69 (d), 126.36 (d),
127.43 (d), 127.88 (s), 128.40 (d), 131.89 (d), 132.57 (d), 133.90 (s),
134.30 (s), 140.13 (d), 144.68 (s), 149.98 (s), 151.01 (s), 152.07 (s);
In a glovebox filled with argon, 6a/6b [7/3, 46.4 mg, ca.
0.050 mmol (total)] was dissolved in hexane (2 mL, dried over K
mirror and distilled by a trap-to-trap method), and LDA (2.0 M
in heptane/THF/ethylbenzene, 0.0260 mL, 0.0520 mmol) was
4416 | Dalton Trans., 2008, 4409–4418
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The Royal Society of Chemistry 2008
©

added to the solution at −40 ◦C. After stirring for 1 h at room
temperature, the solvents were removed under reduced pressure
and hexane was added to the residue. The resulting suspension
was filtered through Celiteꢀ, and the solvent was removed to afford
R
almost pure 2a (40.3 mg, 0.0506 mmol).
R
was filtered through Celiteꢀ, and the solvent was removed. The
X-Ray data collection of 2-germanaphthalene 2a
1H and 119Sn NMR of the crude mixture showed the signals of 1a
(∼10%) and compound X (dSn = −163.6). However, compound
X could not be isolated and fully characterized. The 1H NMR of
compound X showed complicated signals (because of the signal
overlaps) in the region characteristic of alkyl protons, therefore we
proposed aminostannane 9.
Colorless and prismatic single crystals of 2a were grown by the slow
evaporation of its hexane and benzene solution in a glovebox filled
with Ar. The intensity data were collected on a RIGAKU Saturn70
CCD system with VariMax Mo Optic Mo Ka radiation (k =
0.71069 A) to 2hmax = 50◦ at 103 K. The structure of 2a was solved
˚
by direct methods (SHELXS-9733,34) and refined by full-matrix
least-squares procedures on F2 for all reflections (SHELXL-9734).
Two of the trimethylsilyl groups were disordered. All hydrogen
atoms were placed using AFIX instructions. All the non-hydrogen
atoms were refined anisotropically.
Synthesis of 1a from the mixture of 6a/6b with LTMP
In a glovebox filled with argon, 6a/6b [7/3, 48.4 mg, ca.
0.052 mmol (total)] was dissolved in hexane (2 mL, dried over
K mirror and distilled by a trap-to-trap method), and THF
solution (1 mL) of LTMP [0.065 mmol, prepared from HTMP
(12.7 mg, 0.0899 mmol) and n-BuLi (1.5 M in hexane, 0.0430 mL,
0.065 mmol)] was added to the solution at −40 ◦C. After stirring
for 0.5 h at room temperature, the solvents were removed under
reduced pressure and hexane was added to the residue. The
Crystallographic data for 2a. C40H74GeSi6 MW = 796.12;
orthorhombic; space group Pbca (#61); a = 9.2068(3), b =
3
˚
˚
˚
22.3394(7) A, c = 45.9538(19) A; V = 9451.5(6) A ; Z = 8; Dcalcd
=
1.119 g cm−3; l = 0.825 mm−1; 96496 reflections measured, 8303
unique [R(int) = 0.0884], 506 parameters, R1[I > 2r(I)] = 0.1103;
wR2 (all data) = 0.2572; GOF = 1.303.
R
resulting suspension was filtered through Celiteꢀ, and the solvent
was removed. The residue was recrystallized from hexane to give
1a (21.5 mg, 0.0255 mmol, 49%).
Acknowledgements
This work was partially supported by Grants-in-Aid for Creative
Scientific Research (No. 17GS0207), Science Research on Priority
Areas (No. 19027024, “Synergy of Elements”), Young Scientist
(B) (Nos. 18750030), the 21st Century COE Program B14 (Kyoto
University Alliance for Chemistry), and the Global COE Program
B09 (International Center for Integrated Research and Advanced
Education in Materials Science) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Synthesis of 2-Tbt-3-tert-butyl-2-germanaphthalene (2a) from
bromogermane 7b
In a glovebox filled with argon, 7b (80.7 mg, 0.0920 mmol) was
dissolved in hexane (4 mL, dried over K mirror and distilled by
a trap-to-trap method), and LDA (1.0 M in THF, 0.1010 mL,
0.1010 mmol) was added to the solution at −40 ◦C. After stirring
for 1 h at room emperature, the solvents were removed under
reduced pressure and hexane was added to the residue. The
Notes and references
R
resulting suspension was filtered through Celiteꢀ, and the solvent
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was removed. The residue was recrystallized from hexane to give
2a (49.5 mg, 0.0622 mmol, 68%). 2a: colorless crystals; mp 236–
◦
1
239 C; H NMR (300 MHz, rt, C6D6): d 0.15 (s, 18H), 0.20 (s,
36H), 1.54 (s, 1H), 1.55 (s, 9H), 2.75 (d, 2J = 14.6 Hz, 1H), 2.28
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d 0.94 (q), 1.22 (q), 1.56 (q), 2.03 (q), 30.95 (d), 34.24 (q), 35.19
(d), 35.52 (d), 38.26 (s), 120.80 (d), 122.36 (d), 125.80 (d), 126.57
(s), 127.12 (d), 128.54 (d), 133.55 (d), 135.00 (s), 135.81 (d), 140.73
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©