Synthesis of Air- and moisture-stable dibenzogallepins: Control of planarity of seven-membered rings in solid states by coordination to gallium atoms

Article abstract of DOI:10.1021/acs.orglett.5b00489

Air- and moisture-stable dibenzo[b,f]gallepins with planar seven-membered rings have been designed and synthesized. From X-ray single-crystal analyses, the effects of the coordination numbers were evaluated at the gallium atom on the planarity of the seven-membered rings with a series of the synthesized complexes. Accordingly, it was shown that the seven-membered ring of dibenzogallepin involving a three-coordinated gallium atom presented the highest planarity compared to other dibenzogallepins with four- or five-coordinated gallium atoms. In addition, from theoretical calculations, it was proposed that the electronic states of the gallium atom in the complexes contributed to the energy levels of LUMOs+1.

Full text of DOI:10.1021/acs.orglett.5b00489

Letter
pubs.acs.org/OrgLett
Synthesis of Air- and Moisture-Stable Dibenzogallepins: Control of
Planarity of Seven-Membered Rings in Solid States by Coordination
to Gallium Atoms
Takuya Matsumoto,† Hirofumi Takamine,† Kazuo Tanaka,*,† and Yoshiki Chujo*,†
†Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Air- and moisture-stable dibenzo[b,f ]gallepins with planar
seven-membered rings have been designed and synthesized. From X-ray
single-crystal analyses, the effects of the coordination numbers were evaluated
at the gallium atom on the planarity of the seven-membered rings with a series
of the synthesized complexes. Accordingly, it was shown that the seven-
membered ring of dibenzogallepin involving a three-coordinated gallium atom
presented the highest planarity compared to other dibenzogallepins with four-
or five-coordinated gallium atoms. In addition, from theoretical calculations, it was proposed that the electronic states of the
gallium atom in the complexes contributed to the energy levels of LUMOs+1.
even-membered cycloheptatrienes have attracted attention
their electronic states should be significant for further
comprehension of electronic structures of heavier atom-
containing π-conjugated molecules. Thus, the next challenge
is to establish the synthesis of these molecules and to clarify
their conformations and electronic properties. Herein, we
demonstrate the synthesis of dibenzo[b,f]gallepins involving a
series of three-, four-, and five-coordinated gallium atoms and
compare their conformations. We obtained the planar
dibenzogallepins, which are promised to be building units for
constructing heteroatom-containing conjugation systems.
We selected Mes* (2,4,6-tri-tert-butylphenyl),7b,e,f,h Mamx
(2,4-di-tert-butyl-6-[(dimethylamino)methyl]phenyl),11 and
NCN (2,6-bis[(dimethylamino)methyl]phenyl)12 as protecting
groups of gallium atoms to prevent the coordination of Lewis
bases or solvents and improve the durability toward water. As
described in Scheme 1, we synthesized three dibenzogallepins
(3-Gapin, 4-Gapin, and 5-Gapin) with a method similar to
that of the previous report by Robinson.10 After formation of
the dibenzogallepin frameworks in reactions with gallium
chloride, the protecting groups were introduced. All of the
synthesized dibenzogallepins had high stabilities to air and
moisture. Therefore, general purification operations such as
extraction with water and recrystallization under ambient
atmosphere were applicable. The dibenzogallepins storable
under air for at least 2 months without any decomposition. The
bulky tert-butyl groups and the coordination of amine groups
should be responsible for improvement of their stabilities.11a,b
In particular, it is known that the compounds containing three-
coordinated gallium atoms are sensitive to water.7e−h The
formation of a 6π-electron aromatic system could also
contribute to enhancement of the durability of 3-Gapin.
S
as flexible π-conjugated building units1 and as precursors to
the tropylium ion.2 Neutral cycloheptatriene usually forms a
boatlike conformation in which flipping frequently occurs.1a,b In
contrast, the one-electron oxidized form of cycloheptatriene
such as the tropylium ion has a planar structure due to
stabilization by the formation of a 6π-electron aromatic system
on the seven-membered ring.2a,b Heteropins are a class of
cycloheptatrienes containing a heteroatom instead of an sp3
carbon. The synthesis of heteropins involving group 141c,d,2c,3
and 153e,4 elements has been already established. In particular,
the structures and optical and electrochemical properties of
dibenzo[b,f ]silepins have been investigated in detail.1c,d,2c,3a,b
Heteropins containing group 13 elements can be regarded as
neutral analogues of the tropylium ion. The synthesis of various
borepins has been achieved, and their structure and material
properties have been examined.5,6 In particular, the aromaticity
in the seven-membered rings was observed in the tropylium
ion. In addition, it was shown that dibenzo[b,f ]borepins
possess the planar aromatic seven-membered ring and electron-
accepting property.6
Compared to the number of reports on organoboron-
conjugated heterocycles, those on heteroles,7 heterorbenzenes,8
and heteropins9,10 containing heavier group 13 elements are
still limited. The first synthesis of dibenzoaluminepin was
reported by Uchiyama et al.,9 and that of dibenzogallepin was
accomplished by Robinson et al.10 These reports show that
aluminepin and gallepin accepted coordination with the Lewis
base and solvent molecules and had boatlike seven-membered
rings. These conformations were attributed to the four-
coordinated structure of the group 13 elements.
The planar conformations of dibenzoheteropins involving
the heavier group 13 atoms were predicted theoretically.9,10
This means these compounds could be a building block for
obtaining robust π-conjugations. Furthermore, information on
Received: February 15, 2015
Published: March 4, 2015
© 2015 American Chemical Society
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Letter
planarity was comparable to reported planar dibenzoborepin-
s.3a,6a−e The Ga−N bond length of 4-Gapin (2.120 Å) was
within a typical coordination bond length between Ga and N
(reported Ga−N bond lengths: 2.013−2.192 Å10,11a,b,d,13). The
coordination with the Mamx group induced a bent con-
formation which was observed in the seven-membered rings
involving four-coordinated gallium atoms.10 The bent angles of
4-Gapin were 11.63° for θ1 and 24.70° for θ2, respectively. The
values of these angles were larger than those of 3-Gapin and
smaller than those of the reported gallepin.10 The higher
planarity of 4-Gapin compared to the reported gallepin10 could
be attributed to the quasi-trigonal planar structures coordinated
by nitrogen from the upper position.11a,b,d
Scheme 1. Synthesis of Dibenzo[b,f ]gallepins 3-Gapin, 4-
Gapin, and 5-Gapin
In a single crystal of 5-Gapin, it was found that two
geometries A and B were involved. Two distances between Ga
and N in A were obtained as 2.275 and 2.735 Å. The
conformation of the gallium atom of A was similar to the four-
coordinated structure of 4-Gapin including the bent geometry
at the seven-membered ring. In contrast, the B conformation
had more equivalent Ga−N bonds compared to A, and the
bond lengths were determined as 2.371 and 2.573 Å. These
mean that the gallium atom in B had a five-coordinated trigonal
bipyramidal structure. The trigonal bipyramidal geometry of B
showed a higher percentage of trigonal bipyramidal as 96%
TBPa and 91% TBPe compared to A (88% TBPa and 90%
TBPe), respectively.14 The three equatorial carbons and gallium
atoms of B revealed high symmetry, resulting in the higher
planar seven-membered ring. Therefore, the smaller bent
angles, θ1 and θ2, than those of 4-Gapin and A of 5-Gapin
were obtained as 5.06° and 13.93°, respectively. For
investigation of the Ga−N bonds of the synthesized gallepins,
we calculated WBI (Wiberg bond index) bond orders of the
Ga−N bonds of the dibenzogallepins with density functional
theory (DFT) method by using the geometries obtained by the
X-ray crystallographic analyses (Table 1). The bond orders of
two Ga−N bonds of A were 0.1825 and 0.1040. The bond
order of the shorter one was similar to that of 4-Gapin
(0.2304), while the longer one was lower than that of the
shorter one. In contrast, similar bond orders of the two Ga−N
bonds of B were observed (0.1629 and 0.1315). These results
support the four-coordinated-like structure of A and the five-
coordinated-like structure of B.
Figure 1. ORTEP drawings of (a) 3-Gapin, (b) 4-Gapin, (c) A of 5-
Gapin and (d) B of 5-Gapin (50% probability for thermal ellipsoids).
Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3-
Gapin, 4-Gapin, and 5-Gapin
5-Gapin
lengths (Å) and angles (deg) 3-Gapin 4-Gapin
A
B
Ga−N
2.120
2.275
2.735
2.371
2.573
∑ C−Ga−C
∑ inner angles
359.98
899.14
5.15
356.01
890.48
11.63
359.50
894.42
10.11
359.97
896.70
5.06
a
θ1
θ2
5.05
24.70
19.14
13.93
b
Compared to the reported gallepin,10 silepins,1d,3a and
aluminepin,9 all of the synthesized dibenzogallepins include
high planar seven-membered rings, as shown in Table S5
(Supporting Information).
Ga−N bond order
0.2304
0.1825
0.1040
0.1629
0.1315
a
b
Sum of inner angles of seven-membered rings. Calculated at the
B3LYP/6-31G(d,p) level by using the geometries of single crystals.
Figure 3 illustrates UV−vis absorption and photolumines-
cence spectra of the synthesized dibenzogallepins and
dibenzosilepin Sipin. Their optical data are listed in Table S6
(Supporting Information). All the synthesized dibenzogallepins
presented similar shapes of their absorption spectra. Absorption
bands with the peaks at 302, 316, 332 nm attributed to
vibrational structures were observed. In contrast, a broad
absorption band with the peak at 293 nm without vibrational
structures was observed from Sipin in the shorter wavelength
region compared to the synthesized dibenzogallepins. These
facts suggest that the conjugation system could be extended in
dibenzogallepins because of their higher planar structures even
in solution states, whereas Sipin vigorously flipped, resulting in
the weak conjugation on the bent conformation. The
synthesized dibenzogallepins provided photoluminescent spec-
tra with the peaks at around 360 nm similar to Sipin. It is
Figure 2. Bent angles θ1 and θ2 of the seven-membered rings.
In Figure 1 and Table 1, we show results of X-ray
crystallographic analyses. A gallium center of 3-Gapin was
protected from coordination with any Lewis bases or solvents.
The sum of internal angles of the seven-membered ring of 3-
Gapin was 899.14°. As presented in Figure 2, two angles of θ1
(5.15°) and θ2 (5.05°) were bent angles of the seven-
membered rings on the gallium sides and the CC double
bond sides, respectively. These facts indicate that the seven-
membered ring of 3-Gapin has high planar conformation. The
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Letter
moisture. The three-, four-, and five-coordinated structures
were precisely determined with single crystals. Accordingly, the
seven-membered rings in all dibenzogallepins showed higher
planarity compared to other heteropins such as silepin. In
particular, 3-Gapin had the highest planar heteropin rings of
the synthesized dibenzogallepins. Two geometries of 5-Gapin
originating from four- and five-coordinated-like structures were
observed. Dibenzogallepins presented absorption bands owing
to their high planar structures. Their frontier orbitals localized
at cis-stilbene units. From the computer calculation, it was
proposed that the conformations of gallium atoms significantly
influenced LOMOs+1. We accomplished the construction of a
planar structure based on heteropin rings by modulating the
coordination geometries of gallium atoms.
Figure 3. UV−vis absorption (solid lines) and photoluminescence
(dashed lines, excited at λmax,abs) spectra of 1.0 × 10−4 M 3-Gapin
(red), 4-Gapin (blue), 5-Gapin (green), and Sipin (black) in CHCl3.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental, measurement, and computational details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: kazuo123@chujo.synchem.kyoto-u.ac.jp.
*E-mail: chujo@chujo.synchem.kyoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the “Adaptable and
Seamless Technology Transfer Program” through target-driven
R&D, Japan Science and Technology Agency (JST), and a
Grant-in-Aid for Scientific Research on Innovative Areas “New
Polymeric Materials Based on Element-Blocks (No. 2401)”
(24102013) of The Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
Figure 4. Energy diagram and selected molecular orbitals of 3-Gapin,
4-Gapin, and 5-Gapin calculated by the DFT method. Hydrogen
atoms are omitted for clarity.
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