Stable Non-fused [22]Pentaphyrins and A Fused [24]Pentaphyrin Displaying Crystal Polymorphism between Hückel and M?bius Structures

Article abstract of DOI:10.1002/asia.201601689

Partially β-substituted and meso-tetrakis(pentafluorophenyl)-substituted [22]pentaphyrins 11 and 12 were synthesized by acid-catalyzed condensation of a meso-C₆F₅-substituted tripyrrane dicarbinol with β-alkylated dipyrromethanes. Th

Full text of DOI:10.1002/asia.201601689

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Accepted Article
Title: Stable Non-fused [22]Pentaphyrins and A Fused [24]Pentaphyrin
Displaying Crystal Polymorphism between Hückel and Möbius
Structures
Authors: Tomoki Yoneda, Tyuji Hoshino, and Saburo Neya
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Stable Non-fused [22]Pentaphyrins and A Fused [24]Pentaphyrin
Displaying Crystal Polymorphism between Hückel and Möbius
Structures
Tomoki Yoneda*[a], Tyuji Hoshino[a] and Saburo Neya*[a]
Abstract:
Partially
-substituted
and
meso-
tetrakis(pentafluorophenyl)-substituted [22]pentaphyrins 11 and
12 were synthesized by acid-catalyzed condensation of a meso-
C6F5-substituted
tripyrrane
dicarbinol
with
-alkylated
dipyrromethanes. These [22]pentaphyrins are stable, allowing
their characterization by NMR and UV-vis spectroscopies, and X-
ray crystallography, and display strong aromaticity due to 22-
electronic circuits. In methanol, ,-diethoxycarbonylated
pentaphyrin 12 underwent an N-fusion reaction to give N-fused
pentaphyrin 13, which exhibits crystal polymorphism between
Hückel and Möbius structures, depending upon the
recrystallization solvent
Expanded porphyrins, porphyrinoids with more than four pyrrole
units, are attracting much attention because of the optical,
electrochemical, and coordination properties that arise from their
large -conjugated frameworks.[1,2] Pentaphyrins(1.1.1.1.1)
consisting of five pyrroles that are regularly connected through
meso-carbons exhibit characteristic structural and electronic
properties arising from their larger -conjugative systems as
compared with porphyrins. -Alkyl-substituted pentaphyrins 1 and
2 were synthesized as the first example, showing large diatropic
ring currents and thus aromaticity arising from their 22-electronic
circuits at their protonated form.[3] Later, meso-diphenyl- and -
tetraalkyl-substituted pentaphyrin 3 was synthesized to show
similar aromatic characters.[4] While the solid-state structures of
these pentaphyrins have still remained unknown, it is likely that
these pentaphyrins do not suffer from serious sterical distortion.
In the one-pot synthesis of meso-aryl-substituted expanded
porphyrins,
meso-aryl-substituted
[22]pentaphyrins
were
obtained as N-fused species, 4 and 5. The observed efficient N-
fusion reaction may be ascribed to large strain of non-fused meso-
aryl-substituted pentaphyrins (Scheme 2). Aromatic 22 N-fused
pentaphyrins 4 and 5 can be reduced to nonaromatic 24-
congeners 6 and 7.[5] On the other hand, meso-trifluoromethyl-
substituted N-fused pentaphyrin 8 is considered to be a 24
Möbius aromatic system and oxidation to its 22 counterpart did
not proceed (Scheme 1). [6,7]
Scheme 1. Reported meso- and -substituted pentaphyrins.
[a]
Dr. Tomoki Yoneda, Dr. Tyuji Hoshino, Prof. Dr. Saburo Neya
Department of Pharmaceutical Sciences, Chiba University
1-8-1, Inohana, Chuo-ku, Chiba, 260-8675, Japan
E-mail: yoneda@chiba-u.jp
sneya@faculty.chiba-u.jp
Supporting information for this article is given via a link at the end of
the document.
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Scheme 2. N-fusion reaction of meso-aryl pentaphyrin. (DDQ = 2,3-dichloro-
5,6-dichanobenzoquinone)
For the purpose of mitigating the steric constraint of
pentaphyrin 4, we prepared tetrakis(pentafluorophenyl)-
substituted [22]pentaphyrin 9 bearing one unsubstituted meso-
position and found that N-fusion reaction was actually suppressed.
[8]
Pentaphyrin 9 showed an inward-pointing free meso-position
and modest aromaticity but underwent oxidative decomposition
slowly. Chemical stability of bis(rhodium(I)) complex 10 was
somewhat improved, probably through better steric protection at
the free meso-position by the coordinated rhodium metals. The
structural characteristics of these pentaphyrins were also
theoretically studied by Alonso et al.[9]
With these in background, we report here the synthesis of
meso-free [22]pentaphyrins 11 and 12 (Scheme 3). It was
considered that ,-disubstituted pyrroles flanking the free meso-
position might protect it from oxidative damage, hence enhancing
the chemical stability.
Figure 1. X-Ray crystal structure of pentaphyrin 11. a) Top view and b) side
view. Solvent molecules and meso-pentafluorophenyl groups are omitted in the
side view. Thermal ellipsoids are displayed at 50% probability.
We first prepared 2,3,22,23-tetramethyl-5,10,15,20-
tetrakis(pentafluorophenyl)-substituted pentaphyrin 11. A solution
of meso-pentafluorophenyl-substituted tripyrrane dicarbinol and
2,3,7,8-tetramethyl dipyrromethane[10] in dichloromethane was
treated with p-toluenesulfonic acid under nitrogen atomsphere.
The resulting mixture was oxidized with DDQ to afford
pentaphyrin 11 in 6.7% yield (SI).Contrary to 9, compound 11 was
stable under ambient conditions and was isolated in a pure form
by silica-gel column chromatography and recrystalization with
pentane. High-resolution electrospray ionization time-of-flight
mass spectroscopy (HR-ESI-TOF MS) indicated the parent ion
peak of 11 at m/z = 1108.1552 (calcd for C53H22N5F20 [M+H]+
Scheme 3. Structure of the mono-meso-free pentaphyrins reported in this paper
1
1108.1550). The H NMR spectrum of 11 displays two doublets
due to the outer  protons at  = 9.04 and 8.74 ppm and singlets
due to the methyl groups at  = 3.74 and 2.90 ppm, a singlet due
to the inner NH protons at 0.55 ppm, a singlet due to the inner 
protons at  = -1.76 ppm, and a singlet due to the inner meso-
1
proton at  = -6.96 ppm. These H NMR data indicate a strong
diatropic ring current. The structure of 11 has been revealed by
X-ray crystallographic analysis as a non-fused and roughly planar
pentaphyrin possessing an inward-pointing meso-hydrogen atom
(Figure 1).[11] The mean plane deviation (MPD) of the pentaphyrin
skeleton was calculated to be 0.264 Å. The UV-vis absorption
spectrum of 11 was similar to that of 9, exhibiting a sharp Soret
band at 517 nm and Q bands at 649, 681, 749, and 837 nm. These
spectral features are conistent with the assignment of 11 as an
aromatic expanded porphyrin (Figure 2).
Although pentaphyrin 11 was stable enough for the isolation
and physical characterizations, it slowly decomposed in CH2Cl2
over a few weeks. Since the decomposition of meso-free
pentaphyrins may be ascribed to oxidative damages at the free
meso-position, we supposed that substitution of electron-
withdrawing groups at the periphery would suppress its
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decomposition. Therefore, we synthesized pentaphyrin 12 having
two electron-withdrawing ethoxycarbonyl groups. A solution of
tripyrrane dicarbinol and 3,7-bis(ethoxycarbonyl)-2,8-dimethyl
dipyrromethane[12] in dichloromethane was treated with p-
toluenesulfonic acid and subsequently oxidized with DDQ,
affording pentaphyrin 12. Even being stored in CH2Cl2 solution
under aerobic conditions, pentaphyrin 12 did not show any
decomposition, showing its much improved stability. HR-ESI-TOF
MS analysis indicated the parent ion peak of 11 at m/z =
1122.1495 (calcd for C54H24N5F20 [M–H]-: 1122.1509). The 1H
NMR spectrum of 12 displays two doublets due to the outer 
protons at  = 9.04 and 8.78 ppm, and a singlet due to the inner
 protons at  = -1.19 ppm, and a singlet due to the inner meso-
proton at  = -5.97 ppm. These data also indicate a strong
diatropic ring current. As revealed by X-ray crystallography, the
structure of 12 is similar to that of 11, showing a roughly planar
non-fused pentaphyrin with MPD of 0.221 Å (Figure 3).[13] The UV-
vis absorption spectrum of 12 also exhibits a strong Soret band at
523 nm and Q bands at 647, 696, and 769 nm (Figure 2).
25
20
15
10
5
Figure 3. X-Ray crystal structure of pentaphyrin 12. a) Top view and b) side
view. Solvent molecules and meso-pentafluorophenyl groups are omitted in the
side view. Thermal ellipsoids are shown at 50% probability.
0
300
500
700
900
1100
1300
Figure 2. UV-vis absorption spectra of 11 (black), 12 (red, dashed), and 13
(blue, dotted) in CH2Cl2.
Scheme 4. Non-oxidative N-fusion reaction of 12
As mentioned above, pentaphyrin 12 was quite stable in
CH2Cl2. However, we noticed that 12 was changed to a brown
compound in methanol (Scheme 4).[14] This transformation
proceeded smoothly at room temperature, giving 13 in 86% yield
after 4 h. Crystals of 13 suitable for X-ray diffraction analysis were
obtained by slow diffusion of n-nonane into its chlorobenzene
solution.[15] The structure of 13 has been revealed to be a N-fused
[24]pentaphyrin, in which the N1 atom on the pyrrole A is directly
bonded to the -position (C7) on the pyrrole B moiety (Figure 4).
In this crystal, 13 exhibits a highly skewed conformation around
the pyrrole E with a largest torsion angle of 62.5°. Despite the
largely skewed structure, 13 has been shown to possess a Hückel
topology.
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Figure 4. X-Ray crystal structure of pentaphyrin 13 formed from
a
chlorobenzene/nonane solution. a) Top view and b) side view. Solvent
molecules and meso-pentafluorophenyl groups are omitted in the side view.
Thermal ellipsoids are shown at 30% probability. One of two similar molecules
in the asymmetric unit is illustrated.
Figure 5. X-Ray crystal structure of pentaphyrin 13-Möbius afforded from a
toluene/cyclohexane solution. a) Top view and b) side view. Solvent molecules
and meso-pentafluorophenyl groups are omitted in the side view. Thermal
ellipsoids are shown at 50% probability. One of two similar molecules in the
asymmetric unit is illustrated.
Since the 24 skeleton of 13 is isoelectronic with the known
meso-aryl substituted N-fused [24]pentaphyrins 6 and 7, this
reaction suggests an important insight into the mechanism of the
N-fusion reactions of meso-aryl-substituted pentaphyrins. The N-
fusion reaction of 12 to 13 is a dehydrogenation process of the -
CH and NH protons with a concurrent reduction (from 22 to 24)
of the macrocyclic -electronic network. Therefore, the overall
change is a non-redox process and this fusion reaction required
no oxidant and proceeded almost spontaneously in methanol.
Such a process has not been predicted in previous theoretical
considerations of N-fusion reactions.[9]
In the meanwhile, we found crystal polymorphism[16] of 13.
Namely, single crystals of 13 obtained by slow diffusion of
cyclohexane into its toluene solution have been shown to display
a twisted Möbius structure (13-Möbius) with a largest torsional
angle of 60.3° (Figure 5).[17] This finding suggests that the energy
barrier of the interconversion between the Hückel and Möbius
conformers may be small, being influenced by packing forces
induced by different solvent systems (Scheme 5). To the best of
our knowledge, this is the first example of solvent-induced
conformational topology switching of pentaphyrins.[18,19] It is
plausible that the omittion of one pentafluorophenyl substituent
results in a reduced barrier of the topological inversion.
Nonaromatic or weakly antiaromatic character of 13 in
solution has been indicated by its 1H NMR spectrum that displays
four doublets at  = 6.37, 6.20, 6.16, and 6.03 ppm due to the -
protons, two singlets at  = 7.21 and 6.79 ppm due to the meso-
or -protons, and three signals at  = 12.8, 6.86, and 6.62 ppm
due to the NH protons. These spectral features were not
significantly changed even at low temperatures (−90 °C in CH2Cl2),
strongly suggesting that the Hückel structure is dominant in
solution. The UV/vis absorption spectrum of 13 is drastically
different from that of 12, exhibiting a weak Soret band at 484 nm
and a Q-like broad band at 767 nm that spreads to ~1200 nm.
(Figure 2), consistent with the assignment as a nonaromatic
species.
Scheme 5. Hückel-Möbius equilibrium of 13
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[9]
M. Alonso, P. Geerlings, F. D. Proft, J. Org. Chem. 2013, 78, 4419.
In order to gain more information, we performed DFT
calculations of 11, 12, 13, and 13-Möbius with B3LYP/6-31G(d)
level.[19] These calculations have revealed nearly degenerate
HOMO/HOMO-1 and LUMO/LUMO+1 for 11 and 12 as an
indication that these two pentaphyrins are both aromatic species.
Namely, the HOMO-LUMO gaps for 11 and 12 have been
calculated to be 2.05 and 2.01 eV, respectively. On the other hand,
the HOMO-LUMO gap of 13, 1.63 eV, has been calculated to be
relatively small, while that of 13-Möbius has been calculated to
be 1.90 eV. These calculations have indicated that 13 is more
stable than 12 by 13.1 kcal/mol, supporting the observed
spontaneous N-fusion reaction of 12, and 13 is slightly more
stable than 13-Möbius by 0.21 kcal/mol, in line with the observed
predominant distribution of 13 over 13-Möbius. The nucleus-
independent chemical shift (NICS) values[20] at the center of the
macrocycle have been calculated to be -15.54 and -15.31 ppm for
11 and 12, respectively. These results are in line with the strong
aromaticity of non-fused pentaphyrins. The calculated NICS(0)
value of -0.53 ppm for 13 suggests its nonaromatic character,
while that of 13-Möbius is -10.07 ppm, strongly supporting its
Möbius aromatic character (SI).
[10] a) P. S. Clezy, A. W. Nichol, Aust. J. Chem. 1965, 18, 1835; b) C. K.
Chang, J. Org. Chem. 1981, 46, 4610.
[11] Crystal data for 11, 2(C53H21F20N5)·(pentane) (Mr
= 2275.55),
orthorhombic, space group Pbcn (No. 60), a = 35.84(2), b = 10.112(6), c
= 27.180(17) Å, V = 9850(10) Å3, Z = 4, calcd = 1.535 gcm-3, T = 93(2) K,
R1 = 0.0769 (I >2(I)), wR2 = 0.2304 (all data),GOF = 0.987. CCDC =
1519168.
[12] J. B. Paine III, D. Dolphin, J. Org. Chem. 1988, 53, 2787.
[13] Crystal data for 12, C57H25F20N5O4·(pentane) (Mr =1295.97), monoclinic,
space group P21/c (No. 14), a = 19.9549(4), b = 14.3564(3), c =
20.0023(4) Å,  = 104.0506(7)° V = 5558.8(2) Å3, Z = 4, calcd = 1.269
gcm-3, T = 93(2) K, R1 = 0.0749 (I >2(I)), wR2 = 0.2360 (all data), GOF
= 1.046. CCDC = 1519169
[14] A similar color alteration was observed in the case of 11 in methanol.
However, much inseparable byproducts were also observed.
[15] Crystal data for 13, 2(C57H25F20N5O4)·0.5(nonane) ·1.5(chlorobenzene)
(Mr = 2670.51), triclinic, space group P-1 (No. 2), a = 14.024(4), b =
20.155(6), c = 22.672(4) Å, α = 64.022(19)  = 79.54(4)  = 88.32(3) V =
5656(3) Å3, Z = 4, calcd = 1.568 g cm-3, T = 93(2) K, R1 = 0.1162 (I >2(I)),
wR2 = 0.3468 (all data),GOF = 1.182. CCDC = 1519170. The high R1
value is derived from the weakness of diffraction spots and the disorders
of solvent molecules, but the Hückel structure of 13 is unambiguously
assigned.
In summary, we have synthesized chemically stable meso-
free pentaphyrins 11 and 12. While 12 was much more stable in
CH2Cl2, it underwent a smooth N-fusion reaction in methanol to
give N-fused pentaphyrin 13, which displayed crystal
polymorphism, taking either Hückel or Möbius conformation
depending upon recrystallization solvents. Exploration of new
hybrid expanded porphyrins bearing meso-aryl and -alkyl
substituents are now being investigated in our laboratory.
[16] Polymorphism; a) J. D. Dunitz, J. Bernstein, Acc. Chem. Res. 1995, 28,
193. b) J. Bernstein, R. J. Davey, J.-O. Henck, Angew. Chem. Int. Ed.
1999, 38, 3440; Angew. Chem.1999, 111, 3646. c) A. Nangia, Acc. Chem.
Res. 2008, 41, 595.
[17] Crystal data for 13-Möbius, 2(C57H25F20N5O4)·5(cyclohexane) (Mr =
2868.42), monoclinic, space group P21/c (No. 14), a = 23.178(4), b =
20.822(3), c = 28.691(5) Å,  = 96.365(5) V = 13763(4) Å3, Z = 4, calcd
=
1.384 gcm-3, T = 93(2) K, R1 = 0.0834 (I >2(I)), wR2 = 0.2632 (all data),
GOF = 1.012. CCDC = 1519171.
[18] Examples of solvent-dependent Hückel-Möbius conformational changes
of expanded porphyrins; a) J. Sankar, S.Mori, S. Saito, H. Rath, M.
Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon, J.-Y. Shin, J.
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J. M. Lim, K. S. Kim, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2008, 47,
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Lim, S. Saito, T. Yoneda, A. Osuka, D. Kim, Chem. Eur. J. 2011, 17,
6707; d) T. Yoneda, Y. M. Sung, J. M. Lim, D. Kim, A. Osuka, Angew.
Chem. Int. Ed. 2014, 53, 13169; Angew. Chem. 2014, 126, 13385.
[19] Topological switching of N-fused [24]pentaphyrins; a) S. Mori, J.-Y. Shin,
S. Shimizu, F. Ishikawa, H. Furuta, A. Osuka, Chem. Eur. J. 2005, 11,
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O. Kim, D. Kim, A. Osuka, Chem. Eur. J. 2016, 22, 16554.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
26460031 and 20249072 and the Program to Disseminate Tenure
Tracking System from MEXT of Japan. We thank Dr. Takayuki
Tanaka and Prof. Dr. Atsuhiro Osuka (Kyoto University) for X-ray
crystallography and cyclic voltammetry measurements.
Keywords: expanded porphyrin • pentaphyrin• aromaticity •
fusion reaction • crystal polymorphism
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T. Yoneda*, T. Hoshino, S. Neya*
Page No. – Page No.
Stable non-Fused [22]Pentaphyrins and A
N-fused [24]Pentaphyrin Displaying Crystal
Polymorphism between Hückel and Möbius
Structures
Stable meso-aryl- and -alkyl-substituted pentaphyrins with strong 22 aromaticity
have been synthesized and shown to be stable despite the presence of an
unsubstituted meso-position. One of the pentaphyrins underwent a smooth N-fusion
reaction in methanol to give an N-fused [24]pentaphyrin, which exhibited crystal
polymorphism between Hückel and Möbius structures, depending on the
recrystallization solvents.
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