Synthesis of Silicon and Germanium-Containing Heterosumanenes via Rhodium-Catalyzed Cyclodehydrogenation of Silicon/Germanium-Hydrogen and Carbon-Hydrogen Bonds

Article abstract of DOI:10.1021/acs.orglett.7b02254

A three-step synthesis of C₃-symmetric trisilasumanene and trigermasumanene, heteroanalogues of the π-bowl sumanene, was achieved using a threefold rhodium-catalyzed cyclodehydrogenation of Si/Ge-H and C-H bonds as the key step. Trigermasumanene was proven to adopt a planar geometry by single crystal X-ray diffraction for the first time. The optical properties were also investigated by UV-vis and fluorescence spectroscopy.

Full text of DOI:10.1021/acs.orglett.7b02254

Letter
pubs.acs.org/OrgLett
Synthesis of Silicon and Germanium-Containing Heterosumanenes
via Rhodium-Catalyzed Cyclodehydrogenation of Silicon/
Germanium−Hydrogen and Carbon−Hydrogen Bonds
Dandan Zhou,† Ya Gao,† Bingxin Liu,† Qitao Tan,*,† and Bin Xu*,†,‡,§
†Department of Chemistry, Innovative Drug Research Center, Shanghai University, Shanghai 200444, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
§Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, China
S
* Supporting Information
ABSTRACT: A three-step synthesis of C3-symmetric trisila-
sumanene and trigermasumanene, heteroanalogues of the π-
bowl sumanene, was achieved using a threefold rhodium-
catalyzed cyclodehydrogenation of Si/Ge−H and C−H bonds
as the key step. Trigermasumanene was proven to adopt a
planar geometry by single crystal X-ray diffraction for the first
time. The optical properties were also investigated by UV−vis and fluorescence spectroscopy.
tron mobility,16 and bowl chirality,17 and it is also a precursor
iloles (silacyclopentadienes) are silicon-based π-conjugated
Smolecules that possess intriguing photophysical and or template for the synthesis of large buckybowls.18
Doping of heteroatoms to the aromatic frameworks of
sumanene can drastically modulate their physical and chemical
properties.19 In 2012, we reported triazasumanene by
replacement of three sp2 carbons on the rim benzene rings
with nitrogen atoms (Figure 1a), which shows very stable
chirality ascribable to the higher bowl inversion energy (42.2
kcal mol−1) than that of sumanene (20.3 kcal mol−1).20
Sumanene has three benzylic carbons which have been replaced
by various atoms such as S,21 Se,21b Te22 and P,23 leading to
various heterosumanenes. Group 14 elements including Si, Ge,
and Sn have also been incorporated into sumanene (Figure
1b).24 In 2010, Kawashima et al. reported the first synthesis of
silasumanene from 2,3,6,7,10,11-hexabutoxytriphenylene
through bromination followed by repeated lithiation/silyla-
tion/sila-Friedel−Crafts reaction (Figure 1b).24a This pioneer-
ing synthesis provided the first silasumanene but suffered from
a tedious route and low total yield (∼0.7%). Furthermore, only
trisilasumanene containing alkoxyl groups could be prepared by
this method. Later, Saito et al. reported the synthesis of pristine
silasumanene and germasumanene by repeated lithiation and
silylation of the bay regions of triphenylene.24d,e However, only
n-Bu-containing trisilasumanene could be prepared due to the
methyl/butyl exchange under the harsh conditions (n-BuLi/
TMEDA, hexane, 60 °C). This method also suffered from low
yield and tedious procedures. Therefore, the exploration of
efficient and general synthesis of Si or Ge-containing
sumanenes is the prerequisite for potential applications as
electronic properties owing to their low-lying LUMO derived
from σ*−π* conjugation between the σ* orbital of the two
exocyclic Si−C bonds with the π* orbital.1 9-Silafluorene, a
silole embedded in a biphenyl framework, has recently received
much attention due to its great potentials as organic materials.2
Germanium locates below silicon in the periodic table, but due
to the d-block contraction,3 it has similar covalent radii with
silicon (1.22 Å versus 1.17 Å) which has subtle effects on the
molecular packing and morphology compared to Si.4 As a result
of the same d-block contraction effect, the electronegativity of
Ge is much closer to C than that of Si which makes
arylgermanes much more stable to bases and nucleophiles
than the corresponding arylsilanes.5 Therefore, it is interesting
to dope Si or Ge atoms to the framework of other π-extended
aromatic compounds.
Buckybowls are bowl-shaped aromatic hydrocarbons resem-
bling the fragments of fullerenes or the cap of single-walled
carbon nanotubes, which possess both concave and convex π-
surfaces.6 Besides their unique geometry and properties,
buckybowls are intriguing precursors for bottom-up synthesis
of fullerene,7 uniform carbon nanotubes (CNTs),8 and warped
nanographenes.9 Recent advances indicate buckybowls are
promising carbon-rich materials.10 Sumanene 1, named after
the Hindi word “suman” for a type of flower by Mehta et al.,11
is a C3v-symmetric buckybowl (Figure 1a), which represents a
basic unit of C60. It posed a great challenge to synthetic
chemists due to its deep bowl depth. In 2003, the synthesis of
sumanene was achieved by Sakurai et al.12 Since then, many
interesting properties have been studied, such as unique crystal
packing,13 bowl-to-bowl inversion,14 metal complexes,15 elec-
Received: July 22, 2017
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Trisilasumanene 5 and
Trigermasumanene 6
(100 °C) afforded germasumanene 6 in 80% yield. Moreover,
treatment of 5a with excess n-BuLi afforded all-butyl
substituted silasumanene 5c in 72% yield through Me/n-Bu
exchange.
Figure 1. Sumanene and heterosumanenes.
The single crystal of 6 suitable for X-ray crystallographic
analysis was obtained by slow evaporation of its solution in
hexane. It was the first germanium-containing sumanene that
has been resolved by X-ray diffraction. Similar to previously
reported trisilasumanene, trigermasumanene 6 has a planar
framework (Figure 2a and 2b), as a result of the replacement of
the benzylic carbons by larger-sized Ge atoms. The C−C bonds
in the hub benzene ring alter from 1.395 to 1.460 Å, while there
is no remarkable alternation of the C−C bonds in the three
external benzenes, indicating that the higher aromaticity of the
external benzenes than the central benzene ring. The bond
materials. We herein report the first synthesis of pristine all-
methyl substituted sila- and germasumanenes from 1,5,9-
triiodotriphenylene using a threefold rhodium-catalyzed cyclo-
dehydrogenation of silicon/germanium−hydrogen and car-
bon−hydrogen bonds as the key step (Figure 1c). The
geometry of trigermasumanene was investigated by single
crystal X-ray crystallographic analysis.
The synthesis of trisilasumanene and trigermasumanene is
demonstrated in Scheme 1. The synthesis started from 1,5,9-
triaminotriphenylene 1, which was readily prepared on
decagram scale from inexpensive starting materials through
two operationally simple steps.25 Compound 1 was converted
to 1,5,9-triiodotriphenylene 2 under classic Sandmeyer reaction
conditions. Then, lithiation of 2 followed by addition of
chlorodimethylsilane or chlorodiisobutylsilane afforded 3a and
3b in moderate yields. Similarly, treatment of 2 with n-BuLi and
dichlorodimethylgermane followed by reduction with LiAlH4
provided compound 4 in 38% yield.
Recently, several elegant examples on the synthesis of 9-
silafluorenes from arylhydrosilanes through C−H and Si−H
activation have been reported.24a,26 In 2010, Takai et al.
reported an elegant rhodium-catalyzed synthesis of silafluorene
derivatives via cleavage of silicon−hydrogen and carbon−
hydrogen bonds,26a which has been utilized in the synthesis
various silicon-containing compounds.26b−d The treatment of
3a with a catalytic amount of Willkinson’s catalyst, RhCl-
(PPh3)3, and excess 3,3-dimethyl-1-butene as a hydrogen
acceptor afforded the desired all-methyl substituted trisilasu-
manene 5a in 45% yield via a threefold cyclodehydrogenation
of Si−H and C−H bonds. Similarly, trisilasumanene 5b was
obtained from 3b in 52% yield. In a similar manner, the
cyclodehydrogenation of 4 under relatively mild conditions
Figure 2. X-ray crystallographic analysis of germasumanene 6 (pink is
the Ge atoms). (a) Top view; (b) side view; (c) packing model (space-
filled); (d) packing with C−H···π distances. Selected bond lengths
[Å]: C(1)−C(2) 1.399, C(2)−C(3) 1.398, C(3)−C(4) 1.383, C(4)−
C(5) 1.409, C(5)−C(6) 1.402, C(1)−C(6) 1.403, C(6)−C(7) 1.460,
C(7)−C(8) 1.395, C(8)−C(9) 1.448, C(9)−C(10) 1.404, C(5)−
C(10) 1.447, C(1)−Ge(1) 1.951, C(11)−Ge(1) 1.980, C(12)−Ge(1)
1.939, C(13)−Ge(1) 1.932.
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lengths of internal C−Ge bonds range from 1.95 to 1.98 Å,
while the external C−Ge bonds are significantly shorter (1.932
and 1.939 Å). The C−Ge−C bond angle is almost rectangular
(89.8°). Germasumanene 6 adopts a herringbone packing
mainly due to the C−H···π interactions (Figure 2c and 2d).27
The optical properties of 5a and 6 were investigated as
shown in Figure 3. Compounds 5a and 6 have very similar
observation that the incorporation of silicon atoms leads to
lower LUMO energy levels.1 The calculated HOMO levels
match the experimental results through cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) measurements
(Figure S1 and S2, see Supporting Information). This
observation also indicates that Ge-doped materials are
electronically more similar to the corresponding carbon
analogues than Si-doped ones, probably as a result of the d-
block contraction effect.
In conclusion, we have succeeded in the synthesis of
trisilasumanene and trigermasumanene using a threefold Rh-
catalyzed cyclodehydrogenation of Si−H/Ge−H and C−H
bonds as the key step. This three-step procedure from readily
available starting materials provides a shortcut to homogeneous
methyl or isobutyl substituted Si or Ge containing sumanenes
without substituents on their peripheral carbons, which are
difficult targets through known methods. X-ray crystallographic
analysis demonstrates that the germasumanene core adopts a
planar geometry, and it stacks in a herringbone model mainly
due to the strong intermolecular C−H···π interactions. The
doping of silicon and germanium atoms significantly stabilizes
the LUMOs of sumanene, while having little effect on the
HOMO energy levels. This work demonstrates the power of
the transition-metal-catalyzed cyclodehydrogenative reactions
for the synthesis of polycyclic aromatic hydrocarbons and their
heteroanalogues. Further studies of the trisilasumanene and
trigermasumanene as organic materials or for the synthesis of
nanographenes are in progress.29
Figure 3. Absorption (solid line) and emission spectra (dashed line)
of 5a (blue) and 6 (red) in CH2Cl2.
absorption profiles with the highest absorption peak at 270 nm
(ε = 1.4 × 105 M−1 cm−1 for 5a, 9.9 × 104 M−1 cm−1 for 6) and
two shoulder peaks at 260 and 293 nm. The intense absorption
bands of 5a and 6 at 270 nm are slightly blue-shifted from that
of sumanene (278 nm).28 Both 5a and 6 in CH2Cl2 solution
emit in the violet/blue regions.
To further understand the effect of heteroatom doping on
the electronic properties, we performed density functional
theory (DFT) calculations on 5a, 6, and sumanene. Calculation
demonstrates that both 5a and 6 are planar, as in the X-ray
crystal structure of 6, while sumanene is bowl-shaped
expectedly. Figure 4 shows their frontier orbitals and energy
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02254.
Experimental details, characterization data, NMR, MS,
FT-IR spectra for all new compounds (PDF)
X-ray crystallographic data for compound 6 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: qttan@shu.edu.cn.
*E-mail: xubin@shu.edu.cn.
ORCID
Qitao Tan: 0000-0002-6220-651X
Bin Xu: 0000-0002-9251-6930
Figure 4. DFT calculated frontier molecular orbitals and energy levels
of silasumanene 5a, germasumanene 6, and sumanene at the B3LYP/
6-311+G(d,p) level of theory.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
levels. The HOMO and LUMO orbitals of sumanene are
mainly delocalized over the triphenylene core. In comparison,
the HOMOs of silasumanene 5a and germasumanene 6 are
delocalized over the triphenylene core, while their LUMOs are
delocalized over the whole sumanene skeleton indicating the
σ*−π* conjugations of the siloles. As a result, the HOMO
energy levels of 5a, 6, and sumanene are quite similar, while the
LUMO energy levels decrease significantly in the order
sumanene > 6 > 5a. This result is consistent with the
We thank the National Natural Science Foundation of China
(Nos. 21672140, 21672136, and 21302123) for financial
support. The authors thank Prof. Hongmei Deng (Laboratory
for Microstructures, SHU) for NMR spectroscopic measure-
ments and Prof. Xiang He (Department of Chemistry, SHU)
for X-ray crystallographic analysis. The calculations were
supported by High Performance Computing Center, Shanghai
University (zq4000).
C
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