Nitrogen-Centered Concave Molecules with Double Fused Pentagons

Article abstract of DOI:10.1021/acs.orglett.9b01861

Distinctive concave compounds bearing a nitrogen core and double fused pentagons were synthesized with a palladium-catalyzed intramolecular coupling of 1-chloro-8H-indolo[3,2,1-de]acridine as the key step. Structural analysis confirmed the formation of bo

Full text of DOI:10.1021/acs.orglett.9b01861

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Nitrogen-Centered Concave Molecules with Double Fused
Pentagons
Niping Deng and Gang Zhang*
Co-Innovation Center for Efficient Processing and Utilization of Forest Products, College of Chemical Engineering, Nanjing
Forestry University, Nanjing, 210037, P. R. China
*
S Supporting Information
ABSTRACT: Distinctive concave compounds bearing a nitrogen core and double
fused pentagons were synthesized with a palladium-catalyzed intramolecular coupling
of 1-chloro-8H-indolo[3,2,1-de]acridine as the key step. Structural analysis confirmed
the formation of bowl-shaped molecules. Experimental characterizations and theoretical
calculations reveal the incorporation of nitrogen is crucial to alleviate the pentagon
fusing strain, as well as in property regulation.
olycyclic aromatic concave molecules, the so-called
P
buckybowls, are partial structures of fullerenes and tips
1
of carbon nanotubes. They have attracted considerable
attention not only for the structure-related properties but
also for the synthetic challenges due to the high strain
2
originated from the curved structure. One way to form bowl-
shaped molecules is to reduce the size of the central rings of
polycyclic compounds by the introduction of smaller rings,
such as a pentagon and tetragon, with corannulene, sumanene,
and quadrannulene as representatives. Another way is to use
multiple heteroatoms to cause different bond lengths and as a
3
Figure 1. Structures of nitrogen-centered planar and concave
molecules with different types of rings.
concave vertex for the favored tetrahedron geometry deriving
from the lone pair electrons. The heteroatoms can modulate
4
The synthetic route started with 2-chlorocarbazole 5, which
reacted with methyl 2-iodobenzoate via copper-catalyzed
Ullmann amination to give compound 6 in 82% yield (Scheme
the properties of the buckybowls, some of which are
5
unreachable by pure carbon ones.
1
). The ester was then hydrolyzed to generate the carboxylic
Most of the reported bowl-shaped molecules have one thing
in common: the pentagon is isolated from each other by
hexagons. The synthesis of concave molecules with fused
pentagons is quite limited. Inspired by the structure of N-
acid, which was directly cyclized in polyphosphoric acid to
afford the acridone−carbazole hybrid 7 in 73% yield in two
steps. Attempts to close the fjord in compound 7 via
palladium-catalyzed C−C bond formation was not successful;
only a dechlorinated compound was obtained. The difficulty in
the formation of a concave structure is to overcome the
stiffness of the coplanar acridone and carbazole backbone. To
lessen the rigidity, the carbonyl in compound 7 was reduced to
methylene by borane nearly quantitatively. Indeed, the slightly
flexible compound 8 could undergo the intramolecular
cyclization readily. The bowl-shaped amine 4 was acquired in
6
heterotriangulene 1, herein we report the synthesis of
nitrogen-centered bowl-shaped molecules with double fused
pentagons by a strategy of introducing a pentagon and
heteroatom. Compound 1 is a typical structure of nitrogen-
centered polycyclic compounds, which can be viewed as a
triphenylamine that is bridged with three carbonyl moieties at
the rim to form a planar conformation (Figure 1). Once a
carbonyl in 1 is replaced by a C−C single bond, the resulted
ketone 2 is still in planarity, as confirmed by the density
functional theory (DFT) calculation. However, switching
another carbonyl in 2 to a C−C single bond will generate
the ketone 3, which possesses two fused pentagons with higher
strain, thus leading to the formation of a curved shape. If the
carbonyl was reduced to methylene, the gained compound 4 is
still bowl-shaped in principle. Considering the nitrogen-
centered tricyclic moiety, compounds 1−4 also can be viewed
as a π-extended cyclazine, and the latter, as chromophores have
72% yield via the palladium-catalyzed C−Cl/C−H coupling
reaction when 4 equiv of potassium carbonate was used. To
render the conjugation at the rim, the oxidation of methylene
to carbonyl by sodium dichromate supplied compound 3 in
95% yield.
Compounds 3 and 4 are stable at ambient condition and
show good solubility in common solvents, similar to the stable
indolo[3,2,1-jk]carbazole if the carbonyl or methylene is
Received: May 29, 2019
7
shown interesting photoelectronic properties.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01861
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Synthetic Route to Bowl-Shaped Molecules 3 and
4
Figure 2. Structural information on 3 and 4. (a) Top view of the
crystal structure of 3 and POAV angles; (b) molecular stacking in
crystalline state and the distances of intermolecular interactions; (c)
side view of the crystal structure of 3 and bowl depth; (d) top view of
the optimized structure of 4 and POAV angles; (e) side view of the
optimized structure of 4 and bowl depth.
calculation was carried out to optimize its structure at the
B3LYP/6-31G(d,p) level of theory, which has been proved to
match well with the crystal data of bowl-shaped molecules, as
8
removed. The structures of 3 and 4 were confirmed by NMR,
1
13
IR, and HRMS. The H NMR spectrum of 4 shows simple
three doublet, two triplet peaks and a methylene peak at δ =
well as in the case of 3. The calculated structure of 4 is also
bowl-shaped with a depth of 1.54 Å, a little deeper than 3. The
more flexible methylene group sinks a little bit toward the
bottom direction, which in turn tilts the vicinal benzene rings,
thus making the bowl deeper.
4
.33 ppm due to the presence of a symmetry plane in the
molecule. After the oxidation, the methylene peak disappears
and the protons of the benzene rings shift to low field in the
spectrum of 3 as a result of the electron-withdrawing carbonyl
group. The molecular structure of 3 was further confirmed by
the X-ray diffraction analysis of single crystals grown by slow
evaporation of the dichloromethane/isopropanol solution.
The difference in bowl depth can also be evaluated by the π-
orbital axis vector (POAV) angle, which is a meaningful index
14
for nonplanar conjugated molecules. In bowl 3, the POAV
angles are 4.8°−9.0° for the inner carbon atoms and 15.9° for
the nitrogen atom. For compound 4, the POAV angles are
4.7°−9.3° for the inner carbon atoms and 16.5° for the
nitrogen atom, which are larger than those of bowl 3, reflecting
a more curved structure. The carbon POAV angles are smaller
Compound 3 crystallized in the P2 /c group with eight
1
molecules in the unit cell. The crystal structure reveals that 3 is
a bowl-shaped molecule and forms a concave−convex dimer
by π−π interaction with the shortest distance of 3.24 Å, which
is shorter than that of the classical buckybowls sumanene (3.86
9
10
15
16
Å) and corannulene (3.32 Å), a hint of strong π−π
interactions between the bowls (Figure 2). Referring to the
distance of the central nitrogen atom and the plane of the
outer three carbon atoms on the rim, the bowl depth is 1.48 Å,
which is deeper than that of corannulene (0.87 Å) and
sumanene (1.11 Å). Due to the compression from the outer
rings, the three N−C bond lengths are in the range 1.36−1.37
Å, which are slightly shorter than those in the analogous
than that of corannulene (8.4°) and sumanene (8.7°) with
the exemption of C20 (9.3°) in 3 and C13 (9.0°) in 4. The
nitrogen POAV angles in 3 and 4 are much larger than those of
5
e,f,i
single nitrogen-centered buckybowls (7.55°−9.23°),
in-
dicating further deviation of the nitrogen atom from the planes
of pyrrole and pyridone in 3 and 4.
An interesting feature of the bowl-shaped molecule is the
bowl-to-bowl inversion. DFT calculations of the single-point
energy at the B3LYP/6-311+G(2d,p) level of theory was
11
structures, such as N-phenylcarbazole (1.39 Å) and N-
12
16
phenylacridone (1.39 Å). The three C−N−C bond angles
are 121.6°, 107.8°, and 108.7°, respectively, which are quite
close to that at the corresponding positions in N-phenyl-
carbazole (108.3°) and N-phenylacridone (121.5°). The
molecules are stacked in the slipped concave−convex manner
with the bowls aligned in one direction to form a column. The
arrangement of neighboring columns is oriented as opposed to
each other.
performed to determine the inversion barrier. The calculated
bowl inversion energy of compounds 3 and 4 are 8.1 and 11.6
kcal/mol, respectively. These values are lower than that of
16
sumanene (19 kcal/mol), but quite close to corannulene
(10.6 kcal/mol) calculated at the same level.
DFT calculations in vacuo revealed the π-delocalization of
the concave molecules 3 and 4 (Figure 3). Due to the bowl-
shaped structure, the orbital coefficients were not symmetric at
the concave and convex sides. LUMOs are mainly located on
the acridone moiety in 3, but are distributed over the whole
Although single crystals of 4 were obtained, the collected
data were too bad to be used for the structural analysis. DFT
B
DOI: 10.1021/acs.orglett.9b01861
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 4. UV/vis absorption (solid line) and emission (dot line)
spectra of 3 and 4 in dichloromethane.
Figure 3. DFT calculations of molecular orbitals viewed at the
concave side and energy levels at the B3LYP/6-311+G(d,p) level of
theory.
two oxidation waves at 1.07 and 1.45 V and the calculated
HOMO and LUMO energy levels are −5.87 eV and −2.58 eV,
respectively, considering the optical energy gap. The variation
trends of energy levels are in good agreement with the DFT
calculations. In comparison to 3, amine 4 is more likely to be
oxidized, attributable to the absence of the electron-with-
drawing carbonyl group.
molecule in 4. The lone pair electrons of the nitrogen atom
contributed to the HOMOs, and the latter delocalized over the
whole backbones of 3 and 4. The distributions of the HOMO
and LUMO orbitals in 3 revealed an intramolecular charge
transfer process between the nitrogen atom and carbonyl
group. The HOMO and LUMO energy levels of 4 are −5.97
eV and −1.63 eV (Table 1), respectively, which decline to
The aromaticity of 3 and 4 was studied by nucleus-
17
independent chemical shift (NICS) calculations (Table 2).
−6.45 eV and −2.38 eV in 3 ascribed to the π-extension and
Table 2. NICS Values of 3 and 4
zz
the influence of the electron-withdrawing carbonyl group.
The photophysical properties of 3 and 4 in dichloromethane
are evaluated by UV/vis absorption and emission spectros-
copies (Figure 4). The UV/vis spectrum of 4 shows three
absorption bands at 246, 297, and 346 nm. These bands are
red-shifted in the spectrum of 3 due to the intramolecular
charge transfer, which is consistent with the theoretical
calculations. The optical energy gaps of 3 and 4 are 3.10 and
,
ab
compd
NICSzz
A
B
C
D
3
NICS(1)zz
NICS(−1)zz
NICS(1)zz
NICS(−1)zz
−22.1
−26.1
−22.3
−27.5
5.2
5.3
4.9
0.9
−1.5
−4.2
−5.6
−8.5
−19.0
−27.3
−22.1
−29.8
3
.29 eV, respectively, estimated from the onset of absorption.
Both 3 and 4 demonstrate weak blue fluorescence in
4
dichloromethane with the emission maxima at 417 nm (Φ
F
=
0.02, τ = 1.98 ns) and 405 nm (Φ = 0.08, τ = 1.04 ns),
a
F
Calculated at the GIAO/DFT/B3LYP/6-311+G(d) level of theory
of the DFT/B3LYP/6-31G(d,p) optimized geometries. 1 and −1
denote the points at convex and concave side, respectively.
respectively.
b
The redox properties of 3 and 4 were studied by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
measurements in dichloromethane. Both 3 and 4 showed
irreversible redox waves in the CV spectra due to the
interference of electrochemical polymerization at the para-
position of amine. The DPV measurements clarified a further
redox property. 3 exhibits an oxidation wave at 1.34 V and a
reduction wave at −2.05 V. The HOMO and LUMO energy
levels calculated from the oxidation and reduction waves are
The NICS values at the same positions are always larger in 3
than those in 4 owing to the influence of the electron-
withdrawing carbonyl group. Due to the curved structures, the
NICS values of the peripheral benzene rings at the concave
side are smaller than those at the convex side. Compared to 3,
the values of pyrrole ring in 4 are smaller, indicating more
aromatic character. The NICS values of pyrrole moieties in 3
and 4 are larger than those of the other azabuckybowls
zz
zz
−
6.14 eV and −2.75 eV, respectively, with an energy gap of
3
.39 eV, close to the optical energy gap. Meanwhile, 4 affords
Table 1. Photophysical and Electrochemical Properties of 3 and 4
compd λab , (nm) λem (nm) ΦF (%)
a b
a
a
τ
a
(ns) Eox1 (V) Ered1 (V)
a
,
c
a
,
c
Eg (eV)
d
EHOMO (eV) ELUMO (eV) EHOMO (eV) ELUMO (eV)
e
e
f
f
h
3
4
371
346
417
405
2
8
1.98
1.04
1.34
1.07
−2.05
3.10 (3.39)
3.29
−6.14
−5.87
−2.75
−2.58
−6.45
−5.97
−2.38
−1.63
g
i
−
a
b
c
Measured in dichloromethane at room temperature. The absorption maxima at the longest wavelength. Bu NPF (0.1 M) as the supporting
4
6
d
e
f
electrolyte and ferrocene as internal reference. Estimated from the onset of absorption. E
B3LYP/6-311+G(d,p) level of theory. Not observed. Estimated from the electrochemical analysis. E
= −(4.8 + E(ox1/red1)) eV. Calculated at the
LUMO
HOMO/LUMO
g
h
i
= EHOMO + Eg.
C
DOI: 10.1021/acs.orglett.9b01861
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
5f,i
containing a single pyrrole ring in the center (−16.6, −17.6).
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trigonal nitrogen atom, which is verified by the analysis of
nitrogen POAV angles. The pyridone ring is slightly
antiaromatic, while the dihydropyridine ring is nonaromatic.
In summary, we have developed a synthetic protocol for the
preparation of nitrogen-centered bowl-shaped compounds with
two fused pentagons. The centric nitrogen atom plays a
significant role not only in releasing the strain of double fused
pentagons but also in the modulation of the molecular
property in combination with the carbonyl group. These bowl-
shaped molecules provide novel insight into heteroatom
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ASSOCIATED CONTENT
Supporting Information
■
*
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The Supporting Information is available free of charge on the
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glett.9b01861.
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ORCID
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Gang Zhang: 0000-0002-4674-7252
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We are grateful to the Jiangsu Specially Appointed Professor
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DOI: 10.1021/acs.orglett.9b01861
Org. Lett. XXXX, XXX, XXX−XXX