Reaction of a stable digermyne with acetylenes: Synthesis of a 1,2-digermabenzene and a 1,4-digermabarrelene#

Article abstract of DOI:10.1246/bcsj.20160269

Reactions between acetylenes and a stable digermyne bearing 4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2 (Tbb) groups afforded the corresponding stable 1,2-digermabenzenes together with the respective 1,4-digermabarrelenes. The properties of the obtained products and the reaction mechanism are discussed on the basis of experimental and theoretical results. Especially, the aromaticity of the newly obtained 1,2-digermabenzene has been discussed on the detailed calculations, revealing its aromatic character to some degree.

Full text of DOI:10.1246/bcsj.20160269

Advance Publication Cover Page
Reaction of a Stable Digermyne with Acetylenes: Synthesis of a
1,2-Digermabenzene and a 1,4-Digermabarrelene
Tomohiro Sugahara, Jing-Dong Guo, Takahiro Sasamori,* Yusaku Karatsu, Yukio Furukawa,
Arturo Espinosa Ferao, Shigeru Nagase, and Norihiro Tokitoh
Advance Publication on the web September 2, 2016
doi:10.1246/bcsj.20160269
© 2016 The Chemical Society of Japan

Reaction of a Stable Digermyne with Acetylenes: Synthesis of a 1,2-Digermabenzene
#
and a 1,4-Digermabarrelene
Tomohiro Sugahara,1 Jing-Dong Guo,1, 2 Takahiro Sasamori,1* Yusaku Karatsu,3 Yukio Furukawa, 3 Arturo Espinosa
Ferao,4 Shigeru Nagase,2 and Norihiro Tokitoh1
1
Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan
2
Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto, Kyoto 606-8103, Japan
3
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo,
Shinjuku-ku, Tokyo 169-8555, Japan
4
Departmento de Química Orgánica, Universidad de Murcia, Campus de Espinardo, 30100 Murcia, Spain
E-mail: sasamori@boc.kuicr.kyoto-u.ac.jp
Takahiro Sasamori
Takahiro Sasamori received Ph.D. Degree from Kyushu University in 2002. He got a position as Assistant Professor in
2003 and as Associate Professor in 2009 at Institute for Chemical Research, Kyoto University. His main research fields
are organometallic and main group element chemistry.
Abstract
Me3Si
Me3Si
SiMe3
SiMe3
Ar
Reactions between acetylenes and a stable digermyne
bearing 4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2 (Tbb) groups afforded
the corresponding stable 1,2-digermabenzenes together with the
respective 1,4-digermabarrelenes.
obtained products and the reaction mechanism are discussed on
the basis of experimental and theoretical results.
E
E
Ar
R
1: E = Si, Ar = Bbt
2a: E = Ge, Ar = Bbt
2b: E = Ge, Ar = Tbb
The properties of the
Bbt: R = C(SiMe3)3
Tbb: R = t-Bu
Chart 1. Stable disilyne and digermynes
1. Introduction
Modifications of unsaturated hydrocarbons such as
alkenes and alkynes are of great importance in organic
synthesis, as most methods for the introduction of functional
groups start with modifications of a C–C multiple bond. Most
modifications of unsaturated hydrocarbons thereby require
transition metal catalysts to activate the C–C π-bonds.8
Moreover, low-coordinated species of heavier group 14
elements (divalent species or multiple-bonded compounds)
have recently attracted considerable attention as potential
transition-metal-free reaction initiators for unsaturated
hydrocarbons. Such low-coordinated species of heavier group
14 elements have already shown high electrophilic reactivity
toward small unsaturated organic molecules such as ketones,
alkenes, or alkynes.9 For example, Power et al. reported that
the stable digermyne 3 as well as the distannyne 4 smoothly
react in the absence of any transition metal catalyst with two
molecules of ethylene at room temperature to afford the
Multiple-bond compounds of heavier group 14 elements
represent the heavier homologues of unsaturated organic
compounds. For a long time, these compounds were considered
hardly isolable as stable, monomeric compounds under ambient
conditions, on account of their inherently high propensity
towards auto-oligomerization and their high reactivity towards
addition reactions with moisture and/or aerobic oxygen.1
However, this is only correct in the absence of appropriate
stabilization methods, as the introduction of bulky substituents
on the heavier group 14 elements offers kinetic protection, and
their presence renders such multiple-bond compounds isolable
and stable under ambient conditions. Especially homonuclear
triple-bond compounds of heavier group 14 elements, i.e., the
heavier analogues of acetylene, have attracted much attention
on account of their unique chemical and physical properties
that arise from their trans-bent structures, which stand in strong
contrast to the linear structures of carbon-based alkynes.1,2
Power and coworkers have achieved the synthesis and isolation
of stable digermynes (ArGe≡GeAr), distannynes (ArSn≡SnAr),
and diplumbynes (ArPb≡PbAr) using bulky m-terphenyl
ligands.2,3 In 2004, Sekiguchi and Wiberg independently
reported the synthesis of stable disilynes bearing bulky silyl
groups.4 Later on, we reported the synthesis of the stable
diaryldisilyne 15 and the diaryldigermynes 2,6 which bear the
corresponding 4-membered-ring cycloadducts
5
and 6,
respectively (Scheme 1).10 The cycloaddition of Sn-analogue 6
was found to be thermally reversible to release 4 together with
two molecules of ethylene. The stereoselective [2+2]
cycloaddition of the stable disilyne RSiSi≡SiRSi (7; RSi
=
Si[CH(SiMe3)2](i-Pr)) with cis- and trans-2-butens to generate
disilenes 8 was reported by Sekiguchi and coworkers.11
Recently, we reported the reaction of stable disilyne 1 and
digermyne 2a with ethylene to furnish cyclic products 9 and
10.12,13 Interestingly, 10 was converted into 11 at high ethylene
pressures, while 11 underwent a retro-cycloaddition to release
10 and ethylene under ambient conditions.13 Furthermore, upon
treatment with ethylene at low temperature in THF, 10
furnished four-membered ring species 12, which is an analogue
of 5. It can thus be concluded that 11 and 12 should be the
kinetic and thermodynamic products for the reaction between
bulky
aryl
substituents
Bbt
or
Tbb
Tbb
[Bbt
=
=
2,4-[CH(SiMe3)2]2-4-[C(SiMe3)3]-C6H2;
4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2] (Chart 1). So far, several
stable examples of compounds with triple bonds between
heavier group 14 elements have been synthesized and these
have allowed us study their intrinsic nature and properties.3-7
Accordingly, these species are no longer laboratory curiosities,
but key components for advanced synthetic projects.

2a and ethylene, respectively. In order to validate this
hypothesis, we conducted systematic investigations on the
reactivity of a disilyne and a digermyne with ethylene.
1,4-digermabarrelene 14, which could be isolated and fully
characterized. Herein, we report the reaction of the digermyne
TbbGe≡GeTbb (2b) with acetylene in detail and we discuss the
reaction mechanism on the basis of experimental and
theoretical results.
The reaction of disilynes (e.g. 1 and 7) and digermynes
(e.g. 2) with acetylene are of great interest, as have been
reported to afford the corresponding stable 1,2-disila- and
1,2-digermabenzenes.11,14,15 In the case of the carbon analogues,
the trimerization of acetylene to afford a six-membered cyclic π
-conjugated system (i.e. an aromatic ring system) usually
requires an appropriate transition metal catalyst and severe
conditions, as for example in the well-known Reppe reaction.
In contrast, a cyclic six-membered π-conjugated aromatic ring
system containing two heavier group 14 elements can be
generated under ambient conditions in the absence of any
transition metal catalysts from the reaction of two molecules of
acetylene and one heavier dimetallyne (vide supra).
R
R
Tbb
Tbb
Ge Ge
R
E
E
R'
E = Si
E
E
R'
E = Ge
R
R'
R' = H, Ph, SiMe3
1: E = Si, R = RSi
7: E = Si, R = Bbt
13
2b: E = Ge, R = Tbb
Scheme 2. 1,2-dimetallabenzenes from the reaction between
dimetallynes and alkynes
H2C CH2
ArDip
2. Results and Discussion
Ge Ge
ArDip
ArDip
Ge
Ge
ArDip
Reaction of digermyne 2b with acetylene.
5
3
The synthesis of TbbBr and its subsequent application to
the stabilization of other reactive main group element species is
reported elsewhere.16 Digermyne 2b was prepared by a
synthesis similar to that of BbtGe≡GeBbt.6 Dibromodigermene
Tbb(Br)Ge=Ge(Br)Tbb (15) was obtained from the reaction of
TbbLi with GeBr2·dioxane, followed by a subsequent reduction
with KC8 in benzene at room temperature to furnish digermyne
2b in 99% yield as stable red crystals (Scheme 3).
ArDip
H2C CH2
Sn
Sn
ArDip
Sn Sn
ArDip
ArDip
Δ or
vacuum
6
4
ArDip = 2,6-(2,6-diisopropylphenyl)2-C6H3
RSi
Tbb
Tbb
Br
MeHC CHMe
GeBr2·dioxane
KC8
Si
E
Si
Si
E
Si
Ge Ge
Ge
Ge
Tbb
[TbbLi]
RSi
RSi
THF
–78 ºC
benzene
r.t.
RSi
7
8
Tbb
Br
15
2b
RSi = Si(i-Pr)[CH(SiMe3)2]2
Tbb
Tbb
Tbb
Ge
Bbt
H2C CH2
Ge Ge
Bbt
E
E
Bbt
2b
+
Ge
Tbb
E = Si
hexane
r.t.
Bbt
1: E = Si
2a: E = Ge
9: E = Si
11: E = Ge
13
14
H2C CH2
Scheme 3. Synthesis of digermyne 2b and its subsequent
reaction with acetylene to afford 1,2-digermabenzene 13 and
1,4-digermabarrelene 14.
E = Ge
H2C CH2
H2C CH2
Ge Ge
Bbt
Bbt
Ge Ge
The structural parameters of digermene 15 and
digermyne 2b were unambiguously determined by
single-crystal X-ray diffraction analysis (Figure 1).17 In both
cases, two independent molecules were found in the unit cell,
and each independent molecule contained a crystallographic
center of symmetry. The Ge–Ge bonds in digermene 15
[2.4064(8) and 2.3969(8) Å] are slightly longer than those in
carbon-substituted digermenes, indicating a weakened Ge=Ge
Bbt
Bbt
10
12
Scheme 1. Reactions of dimetallynes with alkenes
Among the abundance of unsaturated organic compounds,
benzene occupies a truly outstanding position. The replacement
of carbon atoms in the benzene ring with heavier group 14
elements, generating so-called heavy aromatics,15 has generated
much interest, even though this class of compounds is known to
be highly susceptible towards auto-oligo- and polymerizations.
So far, treatment of disilynes or digermynes with acetylene
remains the only synthetic method to generate 1,2-disila- or
1,2-digermabenzenes.11,14,15 Such transformations are not only
of great interest due to their unique reactivity, but the resulting
heavy aromatic systems that include two heavy atoms are also
highly interesting with respect to structural aspects, such as
bond-alternation, planarity of the aromatic ring, and other
features that may be affected by the double bond character of
the E=E bonds.
bond.6 Conversely, digermyne 2b exhibits
a trans-bent
structure with Ge–Ge–C bond angles of ~130°, which is
comparable to previously reported stable digermynes.3a,6 The
observed Ge≡Ge bonds in 2b [2.2410(9) and 2.2221(9) Å] are
comparable to those of previously reported stable digermynes
[e.g., 2.2060(7) Å for BbtGe≡GeBbt (2a) or 2.2850(6) Å for
ArDipGe≡GeArDip (3)].3a,6 The Raman spectrum of 2b revealed
a strong Raman shift at 408 cm–1, which is comparable to that
of 2a (398 cm–1).6b At the B3PW91/6-311G(3d,p) level of
theory, a Ge–Ge vibrational frequency of 406 cm–1 was
calculated for 2b. The spectral and structural features observed
for 2b suggest a considerable triple-bond character for the
Ge≡Ge bond in 2b, similar to those in 2a and 3.18
In a preliminary report, we described that the reaction of
2b
with
acetylene
furnished
the
corresponding
1,2-digermabenzene (13) as the main product.15 However, this
reaction also generated another unprecedented product,

Subsequently, both the [2+1] cycloaddition affording
intermediate INT-1, and the isomerization of INT-1 to INT-2
by migration of a carbon atom would occur as low-barrier or
barrierless processes, respectively.
(a)
Br1
Ge1
Tbb
Tbb
Ge2*
Br2*
insertion
Ge1*
Br1*
Ge
Ge
Ge Ge
Ge Ge
(a)
TS0
(8.2)
Tbb
Tbb
Tbb
Tbb
2b
(0.0)
INT-1
(–14.2)
INT-2
(–27.3)
Ge2
Br2
Tbb
Ge
Ge
Tbb
reactant complex
molecule A
molecule B
(b)
small
barrier
rotation
Ge
Ge
Ge Ge
Ge Ge
Tbb
Tbb
Tbb
Tbb
Tbb
Tbb
Ge2*
Ge2
Ge1*
INT-4
(–44.2)
INT-2"
(–17.6)
INT-2'
(–21.7)
Ge1
small
barrier
TS1
(–17.9)
[2+2]
insertion
molecule A
molecule B
Tbb Ge
Ge Tbb
Ge Ge
Tbb
Tbb
Figure 1. Molecular structure of (a) dibromodigermene 15 and
(b) digermyne 2b (atomic displacement parameters set at 50%
probability; hydrogen atoms omitted for clarity). Selected bond
lengths (Å) and angles (º) for (a): Ge1–Ge1*, 2.4064(8); Ge1–
Br1, 2.3655(6); Ge2–Ge2*, 2.3969(8); Ge2–Br2, 2.3636(6).
Selected bond lengths (Å) and angles (º) for (b): Ge1–Ge1*,
2.2410(9); Ge2–Ge2*, 2.2221(9); C–Ge1–Ge1*, 130.46(11);
C–Ge2–Ge2*, 130.69(11).
16
(–89.6)
INT-3
(–63.1)
Tbb
Tbb
Tbb
[4+2]
Ge Ge
Ge
Ge
Tbb
14 (–130.4)
TS3
(–77.3)
TS2
(–55.4)
13 (–97.2)
Scheme 4. Calculated reaction mechanism for the reaction of
2b with acetylene. Relative ZPE-corrected energies (kcal/mol)
were
calculated
at
the
B3PW91/6-311G(3d,p)//
When a hexane solution of 2b was exposed to 1 atm of
acetylene at room temperature, its deep red color changed
immediately to pale yellow. After removing the solvent, signals
for both 1,2-digermabenzene 13 and 1,4-digermabarrelene 14
were observed in the NMR spectra of the crude mixture,15 and
B3PW91/3-21G(d) level of theory, and are shown in
parentheses. (a) We could not locate TS0, but instead, we
obtained a reactant complex (re-complex with 8.2 kcal/mol
higher in energy than reactants), which is very close to TS0.
That means the barrier from re-complex to TS0 is almost zero.
1
judging from their H NMR spectra, yields of 61% (13) and
22% (14) were estimated (Scheme 3). Recrystallization of the
crude mixture from benzene at room temperature afforded
yellow crystals of 13 in 25% yield. Subsequently, the filtrate
was exposed to air, and recrystallization of the thus obtained
residue from hexane at room temperature afforded 14 as
colorless crystals in 20% yield. Both 13 and 14 were fully
characterized by spectroscopic and single-crystal X-ray
diffraction techniques.19
Potential energy surface
Reaction coordinate
Reaction mechanism.
Figure 2. Energy profile for the reaction of 2b with acetylene.
It seems feasible to conclude that 1,2-digermabenzene 13
should be the major product from the reaction of digermyne 2b
with acetylene, whereas 1,4-digemabarrelene 14 should be a
by-product. The formation of 14 should probably be interpreted
Ph
Ph
Ph
Ph
Ar
Ph
Ph
Ge
Ge
Ar
Ge Ge
Ge Ge
as
a
result of the presence of the intermediate
Ar
Ar
Ar
Ar
ref.21
1,4-digermabenzene 16, which could readily undergo a [4+2]
cycloaddition with one molecule of acetylene to give 14. In
order to elucidate the underlying reaction mechanism, detailed
theoretical calculations for the reaction of 2b with acetylene to
generate 13 and 14 were carried out at the
B3PW91/6-311G(3d,p)//B3PW91/3-21G(d) level of theory
(Scheme 4). The key intermediate in this reaction is
digermacyclobutadiene INT-2, which is the common
intermediate for both 13 and 14. The pathway from 2b to
INT-2 proceeds via the formation of a reactant complex (a van
der Waals complex), which is formed by the interaction of the
out-of-plane π*-orbital of 2b with the π-orbital of acetylene.20
3: Ar = ArDip
2b: Ar = Tbb
17A: Ar = ArDip
18A: Ar = Tbb
17B
18B
Ge Ge
Ge Ge
R
R
R
R
19A: R = H
INT-2A: R = Tbb
19B
INT-2B
Scheme 5. Reaction of digermynes with tolan and the
canonical structures for the resulting
1,2-digermacyclobutadienes.

leads to the energy increase. Nevertheless, this intermediate
should be formed due to the existence of stabilizing Ge···HC
interactions. This interaction seems to be a requisite for the
reaction to occur and accordingly the reason why no further
reaction proceeded in the reaction of 2b with tolan, which does
Ph
Ph
1.362(5) Å
C
C*
C
105.06(7)º
C*
not
contain
HC≡
bonds.
Subsequently,
2.022(2) Å
Ge
Ge*
Dewar-1,2-digermabenzene INT-3 would be generated from
INT-2’ through a [2+2] cycloaddition of INT-2’ with acetylene
via TS1, which includes an activation barrier of 3.8 kcal/mol.26
The last step, i.e. the dissociation of a Ge–C bond in INT-3 to
form 1,2-digermabenzene 13, involves an activation barrier of
7.7 kcal/mol.
74.87(7)º
Ge
Ge*
2.4160(5) Å
Tbb
Tbb
Figure 3. Molecular structure of 1,2-digermacyclobutadiene 18
(left; atomic displacement parameters set at 50% probability),
and selected structural parameters for the [Ge2C2] ring (right).
INT-2 + HCCH
INT-2'
INT-2"
0.0 kcal/mol
5.6 kcal/mol
9.7 kcal/mol
1.348
1.359
1.359
H
H
H
H
H
H
C2
C1
2.032
Ge1
2.459
C2
C1
C2
C1
The formation of 1,2-digermacyclobutadiene (17) from
the reaction of digermyne 3 with tolan (PhC≡CPh) via formal
[2+2]cycloaddition has previously been reported (Scheme
5).11,21 Similarly, the reaction of 2b with tolan in C6D6 at room
2.028
1.956
2.064
Tbb
1.967
Tbb
2.019
Ge2
Tbb
Ge2
Tbb
Ge1
2.956
Ge2 Ge1
Tbb
Tbb
H
H
C3 C4 H
1.225
2.856
C4
2.558
temperature
resulted
in
the
formation
of
1.207
1,2-digerma-3,4-diphenylcyclobutadiene 18 as a stable, red,
crystalline compound. The structural parameters of 18,
determined by single-crystal X-ray diffraction, are similar to
those of 17,22 exhibiting an almost planar [Ge2C2] ring (Figure
3). Moreover, 18 contains an intramolecular crystallographic
C2 axis through the center of the Ge–Ge and C–C bonds.
Considering the lengths of the Ge–Ge [2.4160(5) Å], Ge–C
[2.022(2) Å], and C–C [1.362(5) Å] bonds, it seems feasible to
assume that the predominant contribution to the observed
structure arises from canonical structure 18A, which exhibits a
Ge=Ge double bond character (Scheme 5). However, the
results of detailed theoretical calculations on the parent
1,2-digermacyclobutadiene (19), including WBI, AIM, and
C3
H
Scheme 6. Optimized structures for INT-2, INT-2’, and INT-2”
(bond distances in Å).
Starting from INT-2’, the reaction pathway to
1,4-digermabarrelene 14 contains INT-2” as another important
intermediate (Scheme 6). While the Ge–Ge bond length
increases significantly from INT-2’ (2.558 Å) to INT-2’’
(2.856 Å), the C–C bond length in the coordinated acetylene
(1.225 Å) is just slightly longer than that calculated for free
C2H2 (1.205 Å). These conformational changes are probably
due to a strong charge transfer (CT) between the Ge atom and
the second C2H2 moiety.27 The main contribution to this CT
should arise from the overlapping of the π-orbitals of the
C3-C4 bond with the unoccupied π*-orbital located at the Ge2
atom. On the basis of NBO calculations (second order
perturbation theory), a stabilization energy of 2.4 kcal/mol was
calculated for the CT.24 However, INT-2’’ is still 4.1 kcal/mol
less stable than INT-2’ and 9.7 kcal/mol less stable than INT-2.
This may be attributed to the energetically demanding
conformational changes, even after considering the CT effect.
Accordingly, the Ge–Ge bond weakens substantially upon
coordination of the second molecule of acetylene in INT-2”,
and INT-4 containing a three-membered [C2Ge] ring is readily
formed given its very small activation barrier. Subsequently,
INT-4 can readily undergo isomerization to afford
1,4-digermabenzene 16, which exhibits a planar geometry at a
singlet closed-shell state confirmed by stable wave functions.28
The other product, 1,4-digermabarrelene 14, can be formed via
a [4+2] cycloaddition between 16 and a third molecule of
acetylene, which includes an activation barrier of 12.3 kcal/mol.
Overall, both pathways in Scheme 4 show highly exothermic
processes. Although the pathway toward 14 is by 33.2 kcal/mol
more exothermic than that furnishing 13, the main product was
13 and not 14. The theoretically calculated pathways showed
that the activation energy from key intermediate INT-2’ to
INT-3 (3.8 kcal/mol), which is involved in the formation of 13,
should be slightly smaller (0.3 kcal/mol) relative to that from
INT-2’ to INT-2’’. As a result of these theoretical insights, we
examined the product ratios obtained at higher and lower
reaction temperature. A 13:14 product ratio of 2:1 and 7:1 was
observed at 50 ºC and –78 ºC, respectively, thus supporting the
calculated reaction mechanism.
NBO analyses, suggested
a dominant contribution from
canonical structure 19B.23 The long Ge–Ge bond (2.600 Å;
WBI 1.086), the relatively short C–C bond (1.426 Å; WBI
1.349), in combination with the elongated Ge=C bond (1.854
Å; WBI 1.419) clearly support this notion and the
corresponding description as a 1,4-digerma-1,3-cyclobutadiene
with a loose Ge-Ge bond. Especially the NBO calculations
showed only one σ-bond between the Ge atoms, as well as σ-
and π-bonds between the Ge and carbon atoms.24 Conversely,
NBO and AIM calculations on 18 and INT-2 suggested the
presence of Ge=Ge and C=C double bonds, as well as of Ge–C
single bonds.24,25 In their entirety, these theoretical results
indicate that, on account of the steric and/or electronic effect of
the bulky Tbb groups, INT-2 should consist mainly of INT-2A
with
a
–Ge=Ge–C=C–
character.
The
parent
1,2-digermacyclobutadiene (19) exhibits a different electronic
distribution, mostly based on 19B with a –Ge=C–C=Ge–
character. The combined consideration of these theoretical
results on 18 and 19 point to an increased C=C bond character
in the case of the bis-C-phenyl substituted 18, in order to allow
for a dominant stabilization effect from the conjugation
between these aromatic rings, which in turn favors structure
type A. Therefore, INT-2 should exhibit a Ge=C bond
character (as in INT-2B), which is to some extent similar to
19B.
Once INT-2 is formed via an exothermic step (27.3
kcal/mol) from 2b, the second molecule of acetylene should
approach giving rise to the intermediate complex INT-2’ due to
charge transfer, which is energetically more unfavorable (5.6
kcal/mol) than INT-2 (Scheme 6). The calculated Ge1···HC4
(2.956 Å) and Ge1···C4 (4.020 Å) distances in INT-2’ are
relatively long, probably due to the steric repulsion that also

order to compare their aromaticity, calculated NICS(r) and
NICSzz(r) values33 were plotted for parent models of a
Molecular Structures of 1,2-digermabenzene 13 and
1,4-digermabarrelene 14.
1,2-digermabenzene (20; non-planar C2 symmetry),
1,2-disilabenzene (21; planar C2v symmetry), and for benzene
[GIAO-B3PW91/6-311G(3d,p)// B3PW91/6-311G(3d,p)]
a
The structural parameters of 1,2-digermabenzene 13 and
1,4-digermabarrelene 14 were determined unambiguously by
single-crystal X-ray diffraction analysis.19 Single crystals of 13
were obtained by recrystallization from benzene. The structure
of 13 (Figure 4) exhibits a crystallographic C2 axis that bisects
the Ge–Ge* and C–C* bonds.29 The Ge=Ge distance in 13 is
2.3117(6) Å, which is slightly longer than those in previously
reported digermenes (e.g., 2.2856(8) Å for Mes2Ge=GeMes2;
Mes = mesityl30), showing a distinct, albeit weakened, Ge=Ge
π-bond character. Within the central [Ge2C4] ring, a Ge–C bond
length of 1.897(3) Å was observed, which falls between
previously reported average values for Ge–C (~ 1.95 Å) and
Ge=C bonds (~ 1.83 Å).31 In the central [Ge2C4] ring, C–C
bond lengths of 1.359(5) Å (C1–C2) and 1.417(7) Å (C2–C2*)
were observed, which are similar to those in benzene
(1.39-1.40 Å), suggesting a delocalization of π-electron density
over the [Ge2C4] moiety. It should also be noted that the central
[Ge2C4] ring in 1,2-digermabenzene 13 exhibits a non-planar
geometry, wherein the Ge–Ge axis comprises an angle of 8.6°
relative to the [C4] plane (Figure 4). This result stands in sharp
contrast to stable 1,2-disilabenzenes,14 which contain a virtually
planar [Si2C4] ring.
(Figure 5).34 The NICS(r) and NICSzz(r) profiles for benzene
were almost identical to those previously reported.34 The
NICSzz(r) profile for benzene exhibits the highest absolute
value (–30.0) at r = 1 Å above the center of the C6 plane, while
the highest absolute value (–10.8) for the corresponding
NICS(r) profile is observed at r = 0.8 Å. The profiles of
1,2-digermabenzene 20 are similar, albeit that the absolute
values are slightly smaller relative to those of 1,2-disilabenzene
21. The NICSzz(r) profiles for 20 and 21 exhibit highest values
at r = 1.3 and 1.1 Å, respectively, which is slightly further away
from the central ring than in the case of benzene, reflecting the
larger π-orbitals of Si and Ge relative to C. Even though the
highest absolute NICS(r) and NICSzz(r) values for 20 and 21
are smaller than those of benzene, they still suggest
considerable
aromaticity
for
1,2-disilabenzene
and
1,2-digermabenzene. Moreover, the NICS(r) values of 20 and
21 are comparable and slightly higher than that of benzene. The
most negative NICS values for 20 and 21 were found at r = 0.8
and 0.7 Å, which is similar to that of benzene (r = 0.8 Å). On
the basis of the highest absolute NICS(r) and NICSzz(r) values,
it can be concluded that the aromaticity in the
1,2-digermabenzene and the 1,2-disilabenzene is considerable,
albeit lower relative to benzene.
C2axis
(b)
(a)
Tbb
Tbb
2.3117(6)
Ge Ge*
Solid-state Raman spectra of 13 were recorded under
excitation from a He/Ne laser at 633 nm. The broadened
Raman shift observed at ~ 320-340 cm–1 should be assigned to
Ge–Ge vibrational frequencies, given that it is comparable to
those of previously reported digermenes (e.g., 404 cm–1 for
Me2Ge=GeMe2).35 Moreover, the results of calculations for 13
at the B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p) level of
theory predict Raman shifts at 304 and 324 cm–1 (Figure 6).
The Raman shifts observed at 598 and 1469 cm–1 were
assigned to the skeletal vibrations that are predominantly due to
the [C4] moiety in the 1,2-digermabenzene, and for which
values of 602 and 1503 cm–1 were calculated.
Ge
Ge*
1.897(3)
101.35(10)°
129.3(2)°
C1
C1*
C1
C1*
128.07(19)°
1.359(5)
C2
C2*
C2
C1*
C2*
(c)
Ge*
C2
C1
Tbb
C2*
Ge
Tbb
Figure 4. (a) Molecular structure of 13 (atomic displacement
parameters set at 50% probability; hydrogen atoms omitted for
clarity); (b) selected metric parameters for the [Ge2C4] core in
13; (c) side view of the [Ge2C4] core in 13.
–5
NICS(r)
–10
NICSzz(r)
The 1H NMR spectrum of 13 in C6D6 suggested a
C2-symmetric structure in solution on the basis of the
diastereomeric non-equivalency of the SiMe3 groups, which is
consistent with the C2-structure with a non-planar geometry at
Ge atoms as observed in the solid state.29,32 For the parent
1,2-digermabenzene C4Ge2H6 (20), theoretical calculations at
the B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p) level of
theory suggested a planar geometry (C2v) for the transition state,
which was ca. 0.36 kcal/mol more unstable relative to the
non-planar structure (C2) at the minimum. Conversely,
disilabenzene C4Si2H6, exhibited a C2v symmetric structure at
the energetic minimum. Thus, it can be concluded that the
non-planar structure of 13 should not be due to steric reasons
and/or crystal packing, but arises from the intrinsic nature of
the 1,2-digermabenzene ring system. The considerable
aromaticity of the [Ge2C4] ring in 13 is reflected experimentally
in its characteristic NMR spectral features, i.e. low-field-shifted
1H NMR chemical shifts (δH = 7.89 and 8.41) of the protons in
the [Ge2C4] ring, which arise from the ring current effect of the
6π-electrons. It is moreover reflected theoretically in the
calculated NICS values and the ideal hydrogenation heat.29 In
–15
–20
–25
–30
–35
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
r/Å
Lowest values:
Benzene: NICS(0.8) = –10.8, NICSzz(1) = –30.0
21 (Si): NICS(0.7) = –10.0, NICSzz(1.1) = –23.2
20 (Ge): NICS(0.8) = –8.7, NICSzz(1.3) = –20.8
H
Si Si
H
H
H
NICS(r)
NICS(r)
NICS(r)
Ge Ge
NICSzz(r)
NICSzz(r)
NICSzz(r)
21
20
Figure
5.
NICS(r)
and
NICSzz(r)
values
for
1,2-digermabenzene (20), 1,2-disilabenzene (21), and benzene
calculated at the GIAO-B3PW91/6-311G(3d,p)//
B3PW91/6-311G(3d,p) level of theory.

angles (º): α(C1–Ge1–C3), 96.65(18); α(C1–Ge1–C5),
102.28(18); α(C3–Ge1–C5), 101.13(18); α(C2–Ge2–C4),
96.75(18); α(C2–Ge2–C6), 102.73(18); α(C4–Ge2–C6),
100.93(17); β(Ge1–C1–C2), 117.1(3); β(Ge1–C3–C4),
117.7(3); β(Ge1–C5–C6), 118.1(3); β(Ge2–C2–C1), 118.4(3);
β(Ge2–C4–C3), 117.4(3); β(Ge2–C6–C5), 117.6(3).
Tbb
Tbb Tbb
Tbb
Ge
C
Ge
Ge
Ge
C
C
C
C
C
C
C
One of the most intriguing intrinsic properties of the
1,4-digermabarrelene is its low-lying degenerated LUMOs.
Their energy levels are much lower than those of the parent
carbon analog, i.e., barrelene 23,38 and comparable to those of
silicon analogue 2439 (Figure 8). One of the LUMOs is
composed of π*-orbitals of the C=C moieties, while the other
includes the two C=C π*-orbitals and the Ge–C σ*-orbitals.
Moreover, the HOMO of 22, which comprises the three
π-orbitals of the C=C bonds, lies also below that of 23.
Currently, investigations into the redox properties of 14 are in
progress in our laboratories.
304 cm–1
602 cm–1
324 cm–1
1503 cm–1
Tbb
Tbb
Tbb
Tbb
Ge
Ge
Ge
C
Ge
C
C
C
C
C
C
C
C
Si
Ge
0.73
1.26
0.58
–0.16
Figure 6. Vibrational modes for the 1,2-digermabenzene
skeleton in 13 calculated at the B3PW91/6-311G(3d,p) level of
theory.
–1.33
–0.97
LUMO1
HOMO
LUMO2
–5.82
–7.35
So far, examples and structural analyses of isolable
1,4-digermabarrelenes still remain elusive. Conversely, the
benzo-fused analogs, 1,4-dimetallatriptycenes, have already
been structurally characterized.36 Single crystals of
1,4-digermabarrelene 14, suitable for single-crystal X-ray
diffraction, were obtained by recrystallization from benzene,
and its molecular structure, as well as selected structural
parameters are shown in Figure 7. The [Ge2C6] core in 14
–6.81
–7.67
–6.96
–7.38
H
H
H
C
Si
Ge
Ge
C
Si
exhibits
the
characteristic
structure
of
a
H
H
H
9,10-dimetallatriptycene containing heavier group 14
elements.36 For example, the average internal angles at the
bridging parts β(Ge–C–C; see Fig. 7) are slightly smaller than
120°, while the average angles α(C–Ge–C) are substantially
smaller than 109.5°. The averages lengths of the Ge–C (~1.97
Å) and C–C bonds (~1.33 Å) are consistent with typical single
Ge–C and double C=C bond values. Although the two
germanium atoms were observed to be relatively close
[Ge1···Ge2 = 3.1589(5) Å] compared to the sum of their van
der Waals radii (~4.5 Å),37 the theoretical calculations on the
H-substituted parent model for the 1,4-digermabarrelene (22)
did not indicate any favorable electronic interactions.
23
24
22
Figure 8. Frontier orbital diagrams for barrelene 23,
1,4-disilabarrelene 24, and 1,4-digermabarrelene 22, calculated
at the B3PW91/6-311G(3d,p)//B3PW91/6-311G(3d,p) level of
theory (left), together with depictions of the HOMO and
LUMOs of 22 (right).
3. Conclusion
The reaction of digermyne 2b with acetylene afforded
stable
1,2-digermabenzene
13
together
with
1,4-digermabarrelene 14. The reaction mechanism for the
formation of 13 and 14 was investigated by detailed theoretical
calculations. The results allow drawing the conclusion that
1,2-digermacyclobutadiene INT-2 is the key intermediate for
both 13 and 14, whereby 13 represents the kinetically slightly
favored product. Both 13 and 14 were structurally characterized
and the aromaticity of the 1,2-digermabenzene was evaluated
by theoretical calculations.
(b)
(a)
1.326(6)
C1
C2
3.1589(5)
α
Tbb
Tbb
Ge1
Ge2
C6
Ge2
β
C5
C2
C1
C5
1.339(6)
C6
C3
1.330(6)
C4
C4
4. Experimental
Ge1
C3
α(avg.) = 100.1º
β(avg.) = 117.7º
General Remarks.
All manipulations were carried out under an argon
atmosphere using either Schlenk line techniques or glove boxes.
All solvents were purified by standard methods and/or the
Ultimate Solvent System, Glass Contour Company. Remaining
trace amounts of water and oxygen in the solvents were
Figure 7. (a) Molecular structure of 14 (atomic displacement
parameters set at 50% probability; hydrogen atoms and one
molecule of benzene omitted for clarity); (b) selected metric
parameters for the digermabarrelene core in 14. Selected bond
thoroughly removed by bulb-to-bulb distillation from
a
potassium mirror prior to use. 1H (300 MHz) and 13C (75 MHz)

NMR spectra were recorded on
a
JEOL JNM AL-300
Monitoring of the reaction of 1,2-Tbb2-digermyne 2b with an
excess of acetylene at –78 ºC.
spectrometer. High-resolution mass spectra (HRMS) were
obtained on a Bruker micrOTOF focus-Kci mass spectrometer
(DART) or a JEOL JMS-700 MStation (FAB). Raman spectra
were measured 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 (633
nm) was used for Raman excitation. All melting points were
determined on a Büchi Melting Point Apparatus M-565 and are
uncorrected. Elemental analyses were carried out at the
Microanalytical Laboratory (Institute for Chemical Research)
of Kyoto University. Digermyne 2b was prepared according to
literature procedures.15
A solution of 2b (31.6 mg, 28.8 µmol) in n-hexane (0.5
mL) was degassed in a J-Young NMR tube, before being
exposed to an excess of acetylene (ca. 1 atm.) at –78ºC for 3
min. Subsequently, the solution was warmed to room
temperature and all volatiles were removed under reduced
1
pressure. The yield of 13 and 14 was estimated by H NMR
spectroscopy (13:14 = 7:1).
Monitoring of the reaction of 1,2-Tbb2-digermyne 2b with an
excess of acetylene at 50 ºC.
A solution of 2b (31.6 mg, 28.8 µmol) in toluene (0.5 mL)
was degassed in a J-Young NMR tube, before being exposed to
acetylene. The solution was treated with an excess of acetylene
(ca. 1 atm.) at 50 ºC for 10 min. Subsequently, the solution was
warmed to room temperature, before all volatiles were removed
under reduced pressure. The yield of 13 and 14 was estimated
by 1H NMR spectroscopy (13:14 = 2:1).
Reaction of 1,2-Tbb2-digermyne 2b with an excess of
acetylene in hexane.
In a Schlenk tube with a J-Young tap, degassed water
was added to CaC2 at r.t. under an argon atmosphere to
generate acetylene, which was dried by passing through a
column of P2O5. In an NMR tube with a J-Young tap, an
n-hexane solution (0.3 mL) of 2b (19.6 mg, 18.8µmol) was
degassed via freeze-pump-thaw cycles. Then, the solution was
exposed to acetylene gas (1 atm), generated as described above,
and the tube was shaken at room temperature. The color of
the solution changed from dark red to pale yellow after 10 min
of shaking. All volatiles were removed under reduced
pressure, and the pale yellow residue was recrystallized from
benzene to afford 1,2-digermabenzene 13 (5.2 mg, 4.7 µmol,
25%, NMR yield 61%) and 1,4-digermabarrelene 14 (4.2 mg,
3.8 µmol, 20%, NMR yield 22%). 13: yellow crystals, mp
198.1-199.0 °C (dec); 1H NMR (300 MHz, C6D6, 298 K) δ
0.204 (s, 36H), 0.215 (s, 36H), 1.34 (s, 18 H), 2.58 (s, 4H),
7.04 (s, 4H), 7.89 (AA’BB’, J = 3.9, 13.5 Hz, 2H), 8.41
(AA’BB’, J = 3.9, 13.5, Hz, 2H); 13C NMR (75 MHz, C6D6,
298 K) δ 1.37(q), 1.89 (q), 31.33 (q), 34.57 (d), 34.81 (s),
122.10 (d), 137.04 (d), 144.24 (s), 149.54 (s), 151.29 (s),
157.40 (d). UV/vis (hexane) λmax (nm, ε) = 383 (9,600). Anal.
Calcd for C52H102Ge2Si8: C, 56.92; H, 9.37. Found: C, 56.97; H,
9.36. Anal. MS (DART-TOF, positive mode): m/z calcd for
C52H102 74Ge2Si8 1098.4559 ([M]+), found 1098.4599 ([M]+).
Computational calculations.
The level of theory and the basis sets used for the
structural optimization are contained within the references of
the main text. Frequency calculations confirmed minimum
energies for all optimized structures. All calculations were
carried out using the Gaussian 09 program package40 except
the structural and electronic analysis of the parent
1,2-digermacyclobutadiene (19) that was conducted with
ORCA.41
X-ray crystallographic analysis.
Single crystals of [2b·C6H6], [13·C6H6], [14·C6H6],
[15·C6H6], and 18 were obtained from recrystallization from
benzene. Intensity data were collected on a RIGAKU Saturn70
CCD system with VariMax Mo Optics using MoKα radiation
(λ = 0.71075 Å). Crystal data are shown in the references. The
structures were solved by a direct method (SIR200442) and
refined by a full-matrix least square method on F2 for all
reflections (SHELXL-9743). All hydrogen atoms were placed
using AFIX instructions, while all other atoms were refined
anisotropically. Supplementary crystallographic data were
deposited at the Cambridge Crystallographic Data Centre
(CCDC; under reference numbers: CCDC1035078, 1035079,
1485559, 1035077, 1485558 for [2b·C6H6], [13·C6H6],
[14·C6H6], [15·C6H6], and 18, respectively) and can be
1
14: colorless crystals; m.p. 79.7-81.3 °C; H NMR (300 MHz,
C6D6, 298 K) δ 0.194 (s, 72H), 1.34 (s, 18 H), 2.54 (s, 4H),
7.01 (s, 4H), 7.96 (s, 6H); 13C NMR (75 MHz, C6D6, 298 K) δ
0.838 (q), 29.89 (q), 31.26 (d), 34.40 (s), 121.80 (d), 129.01 (d),
149.56 (d), 150.80 (s), 151.43 (s). UV/vis (hexane) λmax (nm, ε)
= 287 (4,100), 296 (4,600). MS (DART-TOF, positive mode):
m/z calcd for C54H105 74Ge2Si8 1125.4794 ([M+H]+), found
1125.4810 ([M+H]+).
obtained
free
of
charge
via
www.ccdc.cam.ac.uk/data_request.cif.
Reaction of digermyne 2b with diphenylacetylene.
Acknowledgement
In a J-Young NMR tube, a solution of 2b (30.3 mg, 29.2
µmol) in C6D6 (0.5 mL) was treated with diphenylacetylene
(6.0 mg, 0.034 mmol, 1.1 equiv.). After 10 min, the
quantitative formation of 1,2-digermacyclobutadiene 18 was
confirmed by 1H NMR spectroscopy. The mixture was
recrystallized from hexane to afford 18 as stable dark red
crystals (13.0 mg, 10.7 µmol, 37%). 18: dark red crystals, mp
This work was partially supported by the following
grants: Grant-in-Aid for Scientific Research (B) (JSPS
KAKENHI Grant Numbers JP22350017 and JP15H03777),
Grant-in-Aid for Challenging Exploratory Research (JSPS
KAKENHI Grant Number JP15K13640), Grant-in-Aid for
JSPS Fellows (JSPS KAKENHI Grant Number JP16J05501),
Scientific Research on Innovative Areas, “New Polymeric
Materials Based on Element-Blocks” [#2401] (JSPS
KAKENHI Grant Numbers JP25102519, JP15H00738),
“Stimuli-Responsive Chemical Species for the Creation of
Functional Molecules” [#2408] (JSPS KAKENHI Grant
Number JP24109013), the Integrated Research on Chemical
Synthesis project of the Japanese Ministry of Education,
Culture, Sports, Science, and Technology (MEXT), as well as
by the “Molecular Systems Research” project of the RIKEN
Advanced Science Institute and the Collaborative Research
1
68.6-68.7 °C; H NMR (300 MHz, C6D6, r.t.): δ 0.23 (s, 72H),
1.37 (s, 9H), 2.86 (s, 4H), 6.85-7.02 (m, 10H), 7.18-7.21 (m,
4H); 13C NMR (75 MHz, C6D6): δ 1.65 (q), 31.32 (q), 32.76 (d),
34.47 (s), 122.67 (d), 128.68 (d), 129.12 (d), 129.90 (d), 139.93
(s), 146.92 (s), 149.12 (s), 151.52 (s), 184.18 (s). UV/vis
(hexane) λmax (nm, ε)
= 369 (sh, 12,000). Anal. MS
(DART-TOF, positive mode):ꢀ m/z calcd. for C62H10874Ge2Si8:
1224.5029 ([M+H]+); found: 1224.5045 ([M+H]+).

Program of the Institute for Chemical Research (ICR), Kyoto
University. The Preliminary X-ray diffraction data of 18 were
collected at the BL40XU in Spring-8 (JASRI, 2015B1074).
Computation time was partially provided by the Supercomputer
Laboratory, Institute for Chemical Research, Kyoto University.
We would like to thank Mr. Toshiaki Noda and Ms. Hideko
Natsume at Nagoya University for the expert manufacturing of
custom-tailored glassware. The manuscript was partially
written at the Rheinische Friedrich-Wilhelms-Universität Bonn
during the tenure of a Friedrich Wilhelm Bessel Research
Award (TS). TS would like to express his gratitude to Prof.
Rainer Streubel and his co-workers for their kind hospitality, as
well as to the Alexander von Humboldt Stiftung for their
generosity. One of the authors (AE) is grateful to the Kyoto
University Research Coordination Alliance for their support of
the collaboration and to "Servicio de Cálculo Científico" at the
University of Murcia for computational resources and technical
support.
Am. Chem. Soc. 2000, 122, 3524.
8.
9.
For reviews, see: a) D. Troegel, J. Stohrer, Coord. Chem.
Rev. 2011, 255, 1440. b) H. C. Brown, B. Singaram, Acc.
Chem. Res. 1988, 21, 287. c) V. Busico, R. Cipullo, Prog.
Polym. Sci. 2001, 26, 443.
For reviews, see: a) P. P. Power, Nature 2010, 463, 171.
b) P. P. Power, Acc. Chem. Res. 2011, 44, 627. c) C. M.
Cui, M. M. Olmstead, J. C. Fettinger, G. H. Spikes, P. P.
Power, J. Am. Chem. Soc. 2005, 127, 17530.
10. Y. Peng, B. D. Ellis, X. P. Wang, J. C. Fettinger, P. P.
Power, Science 2009, 325, 1668.
11. R. Kinjo, M. Ichinohe, A. Sekiguchi, N. Takagi, M.
Sumimoto, S. Nagase, J. Am. Chem. Soc. 2007, 129,
7766.
12. J. S. Han, T. Sasamori, Y. Mizuhata, N. Tokitoh, J. Am.
Chem. Soc. 2010, 132, 2546.
13. T. Sasamori, T. Sugahara, T. Agou, K. Sugamata, J.-D.
Guo, S. Nagase, N. Tokitoh, Chem. Sci. 2015, 6, 5526.
14. J. S. Han, T. Sasamori, Y. Mizuhata, N. Tokitoh, Dalton
Trans. 2010, 39, 9238.
References
#Dedicated to Emeritus Professor Takayuki Kawashima on the
occasion of his 70th birthday.
15. T. Sasamori, T. Sugahara, T. Agou, J.-D. Guo, S. Nagase,
R. Streubel, N. Tokitoh, Organometallics 2015, 11, 2106.
16. K. Nagata, T. Agou, N. Tokitoh, Angew. Chem., Int. Ed.
2014, 53, 3881.
1.
B. Solouki, P. Rosmus, H. Bock, G. Maier, Angew.
Chem., Int. Ed. Engl. 1980, 19, 51. b) G. Maier, G. Mihm,
H. P. Reisenauer, Angew. Chem., Int. Ed. Engl. 1980, 19,
52. c) G. Maier, G. Mihm, H. P. Reisenauer, Chem. Ber.
1982, 115, 801. d) C. L. Kreil, O. L. Chapman, G. T.
Burns, T. J. Barton, J. Am. Chem. Soc. 1980, 102, 841. e)
G. Maier, K. Schöttler, H. P. Reisenauer, Tetrahedron
Lett. 1985, 26, 4079. f) G. Märkl, P. Hofmeister, Angew.
Chem., Int. Ed. Engl. 1979, 18, 789.
17. X-ray
crystallographic
data
for
[15·C6H6]:15
C54H104Br2Ge2Si8, FW = 1283.09, T = −170 °C, triclinic,
P–1 (no. 2), a = 9.5903(2) Å, b = 12.1744(3) Å, c =
30.5707(9) Å, α = 82.3768(19)º, β = 86.935(2)º, γ =
75.4857(13)º, V = 3424.10(15) Å3, Z = 2, Dcalcd = 1.224
g/cm3, µ = 2.215 mm–1, λ = 0.71075 Å, 2 θmax = 50.0°,
27891/11871 measured/independent reflections (Rint
0.0498), 625 refined parameters, GOF = 1.021, R1
=
=
2.
For reviews, see: a) P. P. Power, Chem. Commun. 2003,
2091. b) M. Weidenbruch, Angew. Chem., Int. Ed. 2005,
44, 514. c) A. Sekiguchi, M. Ichinohe, R. Kinjo, Bull.
Chem. Soc. Jpn. 2006, 79, 825. d) E. Rivard, P. P. Power,
Inorg. Chem. 2007, 46, 10047. e) A. Sekiguchi, Pure
Appl. Chem. 2008, 80, 447. f) M. Asay, A. Sekiguchi,
Bull. Chem. Soc. Jpn. 2012, 85, 1245. g) T. Sasamori, N.
Tokitoh, Bull. Chem. Soc. Jpn. 2013, 86, 1005.
0.0449 (I > 2 σ(I)), wR2 = 0.1256 (all data), largest
difference peak and hole 1.841 and −1.098 e Å–3. X-ray
crystallographic data for [2b·C6H6]:15 C54H104Ge2Si8, FW
= 1123.27, T = −170 °C, triclinic, P–1 (no. 2), a =
13.2964(4) Å, b = 16.4120(10) Å, c = 17.2864(8) Å, α =
87.040(2)º, β = 73.057(2)º, γ = 69.160(3)º, V = 3366.7(3)
Å3, Z = 2, Dcalcd = 1.108 g/cm3, µ = 1.065 mm–1, λ =
0.71075
Å,
2θmax
=
51.0°,
29799/12418
3.
For examples, see: a) M. Stender, A. D. Phillips, R. J.
Wright, P. P. Power, Angew. Chem., Int. Ed. 2002, 41,
1785. b) A. D. Phillips, R. J. Wright, M. M. Olmstead, P.
P. Power, J. Am. Chem. Soc. 2002, 124, 5930. c) R. C.
Fischer, L. Pu, J. C. Fettinger, M. A. Brynda, P. P. Power,
J. Am. Chem. Soc. 2006, 128, 11366. d) L. Pu, B.
Twamley, P. P. Power, J. Am. Chem. Soc. 2000, 122,
3524.
measured/independent reflections (Rint = 0.0501), 602
refined parameters, GOF = 1.016, R1 = 0.0604 (I > 2 σ(I)),
wR2 = 0.1718 (all data), largest difference peak and hole
1.255 and −0.505 e Å–3.
18. a) S. Nagase, K. Kobayashi, N. Takagi, J. Organomet.
Chem. 2000, 611, 264. b) N. Takagi, S. Nagase,
Organometallics 2001, 20, 5498. c) N. Takagi, S. Nagase,
J. Organomet. Chem. 2007, 692, 217.
4.
5.
a) A. Sekiguchi, R. Kinjo, M. Ichinohe, Science 2004,
305, 1755. b) N. Wiberg, S. K. Vasisht, G. Fischer, P. Z.
Mayer, Anorg. Allg. Chem. 2004, 630, 1823.
a) T. Sasamori, K. Hironaka, Y. Sugiyama, N. Takagi, S.
Nagase, Y. Hosoi, Y. Furukawa, N. Tokitoh, J. Am.
Chem. Soc. 2008, 130, 13856. b) T. Sasamori, J. S. Han,
K. Hironaka, N. Takagi, S. Nagase, N. Tokitoh, Pure
Appl. Chem. 2010, 82, 603.
a) T. Sasamori, Y. Sugiyama, N. Takeda, N. Tokitoh,
Organometallics 2005, 24, 3309. b) Y. Sugiyama, T.
Sasamori, Y. Hosoi, Y. Furukawa, N. Takagi, S. Nagase,
N. Tokitoh, J. Am. Chem. Soc. 2006, 128, 1023.
a) M. Stender, A. D. Phillips, R. J. Wright, P. P. Power,
Angew. Chem., Int. Ed. 2002, 41, 1785. b) A. D. Phillips,
R. J. Wright, M. M. Olmstead, P. P. Power, J. Am. Chem.
Soc. 2002, 124, 5930. c) R. C. Fischer, L. Pu, J. C.
Fettinger, M. A. Brynda, P. P. Power, J. Am. Chem. Soc.
2006, 128, 11366. d) L. Pu, B. Twamley, P. P. Power, J.
19. X-ray crystallographic data for [13·C6H6]: C58H108Ge2Si8,
FW = 1175.34, T = −170 °C, orthorhombic, Pnna (no. 52),
a = 18.2435(3) Å, b = 30.6273(4) Å, c = 12.6638(2) Å, V
= 7075.89(19) Å3, Z = 4, Dcalcd = 1.103 g/cm3, µ = 1.016
mm–1, λ = 0.71075 Å, 2 θmax = 51.0°, 59699/6580
measured/independent reflections (Rint = 0.0791), 323
refined parameters, GOF = 1.034, R1 = 0.0405 (I > 2 σ(I)),
wR2 = 0.1128 (all data), largest difference peak and hole
1.145 and −0.523 e Å–3. X-ray crystallographic data for
[14·C6H6]: C60H110Ge2Si8, FW = 1201.38, T = −170 °C,
triclinic, P–1 (no. 2), a = 12.2815(2) Å, b = 17.1720(5) Å,
c = 18.6944(3) Å, α = 93.8158(13)°, β = 106.5432(10)°, γ
6.
7.
= 105.0468(10)°, V = 3607.56(13) Å3, Z = 2, Dcalcd
=
=
1.106 g/cm3, µ = 0.998 mm–1, λ = 0.71075 Å, 2 θmax
52.0°, 43493/13645 measured/independent reflections
(Rint = 0.0611), 683 refined parameters, GOF = 1.162, R1
= 0.0839 (I > 2 σ(I)), wR2 = 0.1952 (all data), largest
difference peak and hole 3.018 and −0.667 e Å–3.

20. The geometry of the re-complex revealed that the Ge1-C1
distance (2.788 Å) is much longer than the Ge1-C2
distance (2.385 Å). This may be explained by a canonical
molecular orbital analysis (for details, see SI). The
HOMO and HOMO-1 of the re-complex exhibited
interactions between both of the in-plane π-orbitals and
the out-of-plane π-orbital of 2b and the π*-orbitals of
acetylene.
21. (a) C. Cui, M. M. Olmstead, P. P. Power, J. Am. Chem.
Soc. 2004, 126, 5062. (b) N. Y. Tashkandi, L. C. Pavelka,
C. A. Caputo, P. D. Boyle, P. P. Power, K. M. Baines,
Dalton Trans. 2016, 45, 7226. (c) L. Zhao, C. Jones, G.
Frenking, Chem. Eur. J. 2015, 21, 12405.
35. a) P. Bleckmann, R. Minkwits, W. P. Neumann, M.
Schriewer, M. Thibud, B. Watta, Tetrahedron Lett. 1984,
25, 2467. b) P. J. Davidson, D. H. Harris, M. M. F.
Lappert, J. Chem. Soc., Dalton Trans. 1976, 2268.
36. a) M. A. Dam, F. J. J. de Kanter, F. Bickelhaupt, W. J. J.
Smeets, A. L. Spek, J. Fornies-Camer, J. C. Cardin, J.
Organomet. Chem. 1998, 550, 347. b) M. A. Dam, W. J.
Hoogervorst, F. J. J. de Kanter, F. Bickelhaupt, A. L.
Spek, Organometallics 1998, 17, 1762.
37. M. Mantina, A. C. Chamberlin, R. Valero, C. J. Cramer,
D. G. Truhlar, J. Phys. Chem. A 2009, 113, 5806.
38. Some stable barrelene derivatives have been structurally
characterized,
but
most
contain
strongly
22. X-ray crystallographic data for 18: C62H108Ge2Si8: FW =
1223.38, T = −170 °C, orthorhombic, P41/a (no. 88), a =
22.7759(2) Å, b = 22.7759(2) Å, c = 28.2418(3) Å, V =
14650.2(2) Å3, Z = 8, Dcalcd = 1.109 g/cm3, µ = 0.984
mm–1, λ = 0.71075 Å, 2 θmax = 54.0°, 128111/7997
measured/independent reflections (Rint = 0.0620), 341
refined parameters, GOF = 1.012, R1 = 0.0442 (I > 2σ(I)),
wR2 = 0.1295 (all data), largest difference peak and hole
0.749 and −0.649 e Å–3.
23. The results for the detailed theoretical calculations on 19
are shown in the Supporting Information.
24. NBO 6.0 program, NBO 6.0: Natural bond orbital
analysis program. E. D. Glendening, C. R. Landis, F.
Weinhold, J. Comp. Chem. 2013, 34, 1429.
electron-withdrawing groups such as cyanides, which
perturb the electronic properties. For examples, see: a) H.
Irngartinger, T. Oeser, R. Jahn, D. Kallfass, Chem. Ber.
1992, 125, 2067. b) A. Sygula, R. Sygula, A. Ellern, P. W.
Rabideau, Org. Lett. 2003, 5, 2595. c) W.
Matsuda-Sentou, T. Shinmyozu, Tetrahedron: Asymm.
2001, 12, 839.
39. So far, only one stable 1,4-disilabarrelene has been
structurally characterized; see: A. Sekiguchi, G. R.
Gillette, R. West, Organometallics 1988, 7, 1226.
40. Gaussian 09, Revision E.01, full reference is shown in SI.
41. Orca, version 3.0.3, full reference is shown in SI.
42. M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G.
L. Cascarano, L. De Caro, C. Giacovazzo, G. Polidori, R.
Spagna, J. Appl. Cryst. 2005, 38, 381.
25. AIM2000, Version 2.0, Copyright 2002. F. Biegler-König,
J. Schönbohm, J. Comput. Chem. 2001, 22, 545.
43. G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112.
26. The related transition state (TS1) shows that the π-orbital
of the second molecule of acetylene acts as an electron
donor, which is consistent with its charge distribution:
+0.02 (Mulliken) / +0.04 (NBO).
27. Total charge (NBO charge) of the HCCH part in INT-2’
is –0.012 and that in INT-2’’ is +0.016.
28. The assigned aromaticity of 16 is supported by the
geometry parameters (C=C: 1.392 Å and Ge–C: 1.869 Å)
and the NICS values [NICS(0): –5.0 ppm and NICS(1): –
5.8 ppm] for the ring. Moreover, its triplet state is by 25.1
kcal/mol less stable (without ZPE correction) than the
singlet close shell state.
29. Detailed
discussions
on
the
structure
of
1,2-digermabenzene 13 can be found in ref. 15.
30. K. L. Hurni, P. A. Rupar, N. C. Payne, K. M. Baines,
Organometallics 2007, 26, 5569.
31. a) H. Meyer, G. Baum, W. Massa, A. Berndt, Angew.
Chem., Int. Ed. Engl. 1987, 26, 798. b) C. Couret, J.
Escudié, J. Satgé, M. Lazraq, J. Am. Chem. Soc. 1987,
109, 4411. c) N. Tokitoh, K. Kishikawa, R. Okazaki, J.
Chem. Soc., Chem. Commun. 1995, 1425.
32. It is not clear whether [Ge2C4] plane is planar or
non-planar in solution. The non-equivalency of the SiMe3
signals in NMR spectra suggests the pyramidal geometry
of the Ge atoms.
33. Calculated
at
the
GIAO-HF/6-311G(3df,p)//
M062x/6-311G(3df,p) level of theory. On account of the
non-planar structures of the 1,2-digermabenzene, it is
unfortunately difficult to isolate the contribution from the
π-electrons to the NICS values. Therefore, the mean
planes of the H6C4E2 (E = C, E = Si for 21, E = Ge for
20) models were put on the x-y plane, and the “bq”s
(probe points) were added with z-coordinates. NICSzz(r)
values were adopted to evaluate the out-of-plane
π-electron contribution of the corresponding NICS(r)
values.
34. A. Stanger, J. Org. Chem. 2006, 71, 883.

Graphical Abstract
<Title>
Reaction of a Stable Digermyne with Acetylenes: Synthesis of a 1,2-Digermabenzene and a 1,4-Digermabarrelene
<Authors' names>
Tomohiro Sugahara, Jing-Dong Guo, Takahiro Sasamori, Yusaku Karatsu, Yukio Furukawa, Arturo Espinosa Ferao,
Shigeru Nagase, and Norihiro Tokitoh
<Summary>
Reactions between acetylenes and a stable digermyne afforded the corresponding stable 1,2-digermabenzenes together with the
respective 1,4-digermabarrelenes. The properties of the obtained products and the reaction mechanism are discussed on the basis of
experimental and theoretical results.
<Diagram>
Tbb
Tbb
Ge Ge
Tbb
Tbb
Tbb
6π-system
Tbb
Ge
Ge
Ge
Tbb
Ge Ge
4π-system
Ge
Tbb
Generation of 1,2-digermabenzene
and 1,4-digermabarrelene
Me3Si
Me3Si
SiMe3
SiMe3
Tbb
Tbb =
Ge
Ge
Tbb