Synthesis, structure, and reactivity of a thermally stable dialkylgermylene

Article abstract of DOI:10.1246/BCSJ.20190338

The dialkylgermylene Trp*₂Ge: (Trp*: peripherally extended sterically demanding alkyl groups based on the triptycyl (Trp) framework), was obtained from the reaction of Trp*Li with GeCl₂￠ dioxane. The structure of Trp*₂Ge: was unequivocally determined by NMR and UV-vis spectroscopy, mass spectrometry, and a single-crystal X-ray diffraction analysis. Moreover, the reactivity of Trp*₂Ge: was examined.

Full text of DOI:10.1246/BCSJ.20190338

Advance Publication Cover Page
Synthesis, Structure, and Reactivity of a Thermally Stable Dialkylgermylene
Fumiaki Suzuki, Ryohei Nishino, Mariko Yukimoto, Koh Sugamata, and Mao Minoura*
Advance Publication on the web December 5, 2019
doi:10.1246/bcsj.20190338
©
2019 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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Publication manuscripts.

When one Dis group in Dis Ge: is exchanged with Tsi,
2
3
a
DisTsiGe: exists as a monomer in the solid state (Chart 2).
Moreover, Kira et al. have reported a stable dialkylgermylene
Synthesis, Structure, and Reactivity of
3
b
#
that bears helmet ligands on the germanium atom. But despite
a Thermally Stable Dialkylgermylene
4
numerous studies conducted on aryl-substituted germylenes,
3
only two isolated monomeric dialkylgermylenes are listed in
the Cambridge Structural Database, and synthetic examples of
6
Fumiaki Suzuki, Ryohei Nishino,
germylenes with aliphatic carbon-based ligands on the
§
Mariko Yukimoto , Koh Sugamata,
2c,e,h-o,3
germanium atom remain scarce.
and Mao Minoura*
Department of Chemistry, College of Science,
Rikkyo University, 3-34-1 Nishi-Ikebukuro,
Toshima-ku, Tokyo 171-8501
M. Minoura
E-mail: minoura@rikkyo.ac.jp
Received: November X, 2019; Accepted:
Web Released:
Chart 2. Hitherto reported dialkylgermylenes.
Recently, our group has designed and synthesized Trp*, i.e.,
a novel triptycene (Trp)-based aliphatic bulky group that bears
bulky fused ring-type substituents at the periphery of the Trp
Abstract
7
framework. Using Trp*, we have demonstrated that it is
The dialkylgermylene Trp* Ge: (Trp*: peripherally
2
possible to synthesize and isolate highly reactive species, such
extended sterically demanding alkyl groups based on the
triptycene (Trp) framework), was obtained from the reaction of
Trp*Li with GeCl ·dioxane. The structure of Trp* Ge: was
7a
as an alkylsulfenic acid (Trp*SOH) and a tetraalkyldisilenes
7b
(
[Trp*CH (Trp)Si] ) . In the present study, we took advantage
2 2
2
2
of the steric protection of the Trp* group to synthesize the
unequivocally determined by NMR and UV-vis spectroscopy,
mass spectrometry, and a single-crystal X-ray diffraction
thermally stable germylene Trp* Ge: (1), in which the divalent
2
germanium atom is bound to aliphatic carbon ligands.
analysis. Moreover, the reactivity of Trp* Ge: was examined.
2
Keywords: Germylene, Steric Protection, X-ray Analysis
1
. Introduction
Since the first stable germylene has been reported by
1
Lappert et al., the chemistry of divalent organogermanium
2
compounds and related congeners has been developed steadily.
Due to the high intrinsic reactivity and instability of
2
k
germylenes, most of the hitherto reported isolated germylenes
are stabilized kinetically by bulky substituents on the
Chart 3. A stable alkylsulfenic acid (Trp*SOH) and a stable
tetraalkyldisilene ([Trp*CH (Trp)Si] ) with Trp* substituent(s).
3
,4
2
2
germanium atom,
stabilized germylenes have also been reported (Chart 1). The
albeit that some thermodynamically
5
2
. Results and Discussion
first stable germylene, Dis Ge:, is monomeric in solution, but
2
The synthesis of germylene 1 was achieved by the reaction
1
b
dimerizes in the solid state (Chart 1a).
of GeCl ·dioxane with 2 equiv. of Trp*Li, which was obtained
2
from Trp*Br and tert-BuLi at –78 °C. After removal of the
volatiles in vacuo, the residue was dissolved in benzene and
filtered to eventually afford 1 as an orange solid in 55% yield
(
Scheme 1). Although isolated 1 is moisture-sensitive, it is
1
thermally extremely stable (mp > 300 °C), and the H NMR
spectrum of 1 in toluene-d remained unchanged after one week
8
8
at 110 °C.
Scheme 1. Synthesis of dialkylgermylene 1.
Single crystals of 1 were obtained from recrystallization
from benzene, and the solid-state structure of 1 was determined
by a single-crystal X-ray diffraction analysis. The molecular
structure showed a two-coordinate Ge atom and intermolecular
contacts around the Ge atom were not observed (shortest
intermolecular Ge···Ge distance: 14.3899(6) Å), which indicates
that the germylene is monomeric in the solid state. The bond
Chart 1. a) Lappert’s germylene Dis Ge:. b) Selected
2
kinetically
stabilized
germylenes.
c)
Selected
thermodynamically stabilized germylenes.

lengths [Ge-C1: 2.055(2); Ge-C2: 2.048(3) Å] and the bond
angles [C1-Ge-C2: 110.4(1) °] around the Ge atom are
comparable to those of previously reported germylenes. The
three-membered germacyclopropene (germirene) 7 via a [1 +
1
3
2]-cycloaddition. Isolated 7 is air-stable, and single crystals
were obtained from recrystallization from benzene, which
allowed determining the molecular structure of 7 (Figure 3).
During the solid-state thermal reaction of 7 at 200 °C in vacuo,
detectable quantities of 1 were regenerated, although 6 is
thermally very stable.
3
front view of the space-filling model of 1 (Figure 1) seems to
exhibit sufficient space for reactions around the Ge atom. The
side and bottom views show an inter-meshing of the Trp*
groups that should provide effective steric protection. The
UV-vis spectrum of a THF solution of 1 showed an absorption
maximum at 458 nm (ε = 500), which was assigned to the
n(Ge)-4pπ(Ge) transition as it is close to that of a previously
3
b
reported diakygermylene. Again, this result indicates that 1 is
9
monomeric in solution.
Scheme 2. Reactivity of 1 with small molecules.
Figure 3. ORTEP drawing of 6 (left) and 7 (right) with
thermal ellipsoids at 50% probability.
3
. Conclusions
Figure 1. a) ORTEP drawing of 1 with thermal ellipsoids at
Dialkylgermylene Trp* Ge: (1) was synthesized and isolated
2
5
0% probability. b) Space-filling models of 1.
thanks to the kinetical stabilization of the Trp* group. The
structure of 1 was fully characterized. Isolated 1 exhibits
1
13
In the H and C NMR spectra of a benzene-d solution of 1,
remarkable thermal stability and reacts with MeOH, H O, EtBr,
6
2
only one set of Trp* signals was observed and the six aromatic
rings of the two Trp* groups are magnetically equivalent at
room temperature. Line-broadening was not observed, not even
CS , and acetylene. Further studies into the reactivity of 1 and
the synthesis of other reactive main-group-element compounds
with Trp* groups are currently in progress in our laboratory.
2
at –60 °C in toluene-d . These phenomena are probably due to
8
the correlative rotation around the two C-Ge bonds given the
meshing-gear conformation of the Trp* groups.
4. Experimental
Synthesis of Trp* Ge: (1). A THF solution (20 mL) of 2
2
(
(
492 mg, 0.743 mmol) at –78 °C was treated with tert-BuLi
1.00 mL, 1.56 mmol, 1.56 M in pentane) and stirred for 5 min.
Then the mixture was stirred for 2.5 min at –50 °C. This THF
solution of Trp*Li was cooled to –78 °C and treated with a THF
solution (3 mL) of GeCl ·dioxane (122 mg, 0.520 mmol). The
2
reaction mixture was allowed to warm to room temperature,
where it was stirred for 1 h. Then, the solvent was removed
under reduced pressure, and the residue was recrystallized from
benzene to give 1 (253 mg, 0.204 mmol, 55%) as orange
1
crystals, which were isolated by filtration: mp > 300 °C; H
NMR (400 MHz, C D ): δ 0.85 (s, 36H), 1.17 (s, 36H),
6
6
1
6
.40-1.42 (m, 12H), 1.45-1.48 (m, 12H), 5.54 (s, 2H), 7.59 (s,
1
3
H), 7.75 (s, 6H); C NMR (100 MHz, C D ): δ 32.0, 32.1, 34.3,
6
6
1
Figure 2. H NMR spectrum of 1. All aromatic protons are
34.4, 35.5, 35.6, 55.4, 64.9, 122.1, 122.4, 140.9, 141.2, 146.3,
146.7; UV-vis (THF) λmax = 458 nm (ε = 500 cm M );
HRMS-FAB (m/z): [M] calcd. for C H Ge, 1240.7819;
110
-
1
-1
labeled with the corresponding colored circles and squares.
7
4
8
8
Isolated 1 exhibited the characteristic reactivity of a divalent
group-14-element species (Scheme 2). Reactions of 1 with an
found, 1240.7774.
excess of MeOH or H O afforded the corresponding adducts 3
ꢀ X-ray Crystal Structure Analysis. The single-crystal X-ray
diffraction analyses of 1, 6, and 7 were carried out at the
BL40XU and BL02B1 beam lines of the SPring-8 synchrotron.
All frame images (Dectris) were converted to the SFRM format
2
and 4, whereby the formation of Trp* GeH2 was not
2
1
0,11
observed.
In the reaction of 1 with EtBr, the C-Br bond-
insertion product 5 was obtained in good yield. The reaction of 1
1
4
with CS furnished three-membered thioxothiagermirane 6 (74%
using Henkankun-R. Data reduction was performed using
2
yield), whose thioxothiagermirane structure was experimentally
confirmed for the first time (Figure 3). Such three-membered
ring systems have been postulated as reactive intermediates in
Bruker SAINT. Structures were solved by direct methods
(SHELXT) and refined against F by weighted full-matrix
least-squares (SHELXL). Crystallographic data of 1, 6, and 7
were deposited at the CCDC under reference numbers
1966229-1966231.
1
2
2
the reaction between the in-situ-generated germylene and CS ,
2
1
2b,c
by Okazaki et al.
Treatment of 1 with acetylene afforded the

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General experimental procedures and materials, as well as
X-ray crystallographic and spectroscopic data are available in
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Acknowledgements
This work was supported by the Japan Society for the
Promotion of Science (JSPS) via KAKENHI grant 19K05425,
and by the Collaborative Research Program of the Institute for
Chemical Research, Kyoto University (grants 2019-115 and
2
018-106). We would also like to express our gratitude to the
SPring-8 synchrotron facility, where synchrotron radiation
experiments were performed at the BL40XU and BL02B1
beamlines with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) under proposal numbers 2017A1647,
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