Diprotonated [28]hexaphyrins(1.1.1.1.1.1): Triangular antiaromatic macrocycles

Article abstract of DOI:10.1002/anie.201400301

Protonation of meso-aryl [28]hexaphyrins(1.1.1.1.1.1) triggered conformational changes. Whereas protonation with trifluoroacetic acid led to the formation of monoprotonated Moebius aromatic species, protonation with methanesulfonic acid led to the formati

Full text of DOI:10.1002/anie.201400301

Angewandte
Chemie
DOI: 10.1002/anie.201400301
Porphyrinoids
Diprotonated [28]Hexaphyrins(1.1.1.1.1.1): Triangular Antiaromatic
Macrocycles**
Shin-ichiro Ishida, Tomohiro Higashino, Shigeki Mori, Hirotaka Mori, Naoki Aratani,
Takayuki Tanaka, Jong Min Lim, Dongho Kim,* and Atsuhiro Osuka*
Abstract:
Protonation of
meso-aryl
[28]hexaphyr-
species in the presence of methanesulfonic acid (MSA). The
latter process constitutes a rare, but reliable method for the
ins(1.1.1.1.1.1) triggered conformational changes. Whereas
protonation with trifluoroacetic acid led to the formation of
monoprotonated Mçbius aromatic species, protonation with
methanesulfonic acid led to the formation of diprotonated
triangular antiaromatic species. A peripherally hexaphenylated
[28]hexaphyrin was rationally designed and prepared to
undergo diprotonation to favorably afford a triangular-
shaped antiaromatic species.
synthesis
of
triangular
antiaromatic
hexaphyrins
(Figure 1).[7,8]
E
xpanded porphyrins are often structurally flexible and
display diverse molecular shapes, which often dictate their
electronic properties.[1] Regular hexaphyrins(1.1.1.1.1.1) that
consist of six pyrrole rings arranged in alternate orientations
separated by the meso carbon atoms have been shown to
adopt various conformations, such as rectangular,[2] dumb-
bell,[3] figure-of-eight,[4] and twisted Mçbius strip-like
shapes,[5] depending on the meso and peripheral substituents,
intramolecular hydrogen bonding, stabilization induced by
aromaticity, and the nature of the coordinated metal. A
triangular shape is also a possible conformation for hexaphyr-
ins, but has only been observed for a protonated meso-
hexaphenyl [26]hexaphyrin(1.1.1.1.1.1).[6] Intriguingly, this
hexaphyrin is extremely unstable in its free base form because
of rapid oxidative decomposition. Herein, we report proto-
nation-triggered conformational changes of [28]hexaphyr-
ins(1.1.1.1.1.1) that provide a mono-protonated Mçbius
aromatic species upon treatment with trifluoroacetic acid
(TFA) and a diprotonated triangular Hꢀckel antiaromatic
Figure 1. Structures of hexakis(pentafluorophenyl) [26]hexaphyrin 1, its
[28]hexaphyrin congener 2, and monoprotonated and diprotonated
[28]hexaphyrins 3 and 4.
[28]Hexaphyrin 2, which is prepared by the reduction of
[26]hexaphyrin 1 with NaBH4, is known to exist as a dynamic
conformational mixture of twisted Mçbius aromatic and
planar Hꢀckel antiaromatic species at room temperature.[5c]
Encouraged by the recently described protonation-triggered
formation of Mçbius aromatic species from [32]heptaphyrins
and [36]octaphyrin,[9] we examined the protonation of 2. The
absorption spectrum of neutral 2 in CH2Cl2 exhibits a Soret
band at 591 nm and a Q band at 762 nm, reflecting a predom-
inance of the Mçbius conformers in the conformational
mixture. Addition of TFA to this solution caused a red shift of
the Soret-like band from 591 nm to 621 nm with clear
intensification and red shifts of the Q-like bands to 847 and
945 nm (Figure 2a). These spectral changes can be inter-
preted in terms of a shift from the above-mentioned dynamic
conformational mixture to a distribution that is dominated by
the monoprotonated Mçbius aromatic species 3. The
1H NMR spectrum of 3 in CDCl3 exhibits signals at d = 8.28
and 7.80 ppm, which correspond to the outer b protons, and
a signal at d = 0.02 ppm, which is due to the inner b protons, at
room temperature (see the Supporting Information). These
spectral patterns were interpreted in terms of a fast con-
formational exchange between the Mçbius aromatic species
and the rectangular Hꢀckel antiaromatic species, which leads
[*] S. Ishida, T. Higashino, Dr. S. Mori, H. Mori, Dr. N. Aratani,
Dr. T. Tanaka, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto, 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. J. M. Lim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry
Yonsei University
Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
[**] The work at Kyoto was supported by a Grant-in-Aid (25220802 (S))
for Scientific Research from MEXT of Japan. T.H. and H.M.
acknowledge JSPS Fellowships for Young Scientists. The work at
Yonsei was supported by the Global Frontier R&D Program of the
Center for Multiscale Energy System (2012-8-2081) of the National
Research Foundation (NRF), which is funded by MESTof Korea, and
AFSOR/AOARD (FA2386-09-4092).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400301.
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Figure 2. a,b) UV/Vis absorption spectral changes during titration of 2
with TFA (a) and MSA (b) in CH2Cl2.
1
to a symmetric H NMR spectral pattern that is due to the
averaged conformational equilibrium and similar to that of
2.[5c] The observed increase in diatropic ring current indicates
a predominance of the Mçbius aromatic conformers for 3. In
line with this interpretation, the 1H NMR spectrum of 3 at low
temperature clearly indicated that only a single Mçbius
aromatic species is present (see the Supporting Information).
Finally, the structure of 3 was revealed to be a twisted Mçbius
structure by X-ray diffraction analysis; the macrocyclic
conjugation is connected through the nearly perpendicular
pyrrole E and the inverted pyrrole F (Figure 3a). Impor-
tantly, further protonation of 3 could not been realized, even
upon addition of a large excess of TFA.
Following these preliminary investigations, we examined
the protonation of 2 with MSA, which is a stronger acid than
TFA. Upon addition of up to 0.5 equivalents of MSA to
a solution of 2 in CH2Cl2, formation of 3 was indicated by the
appearance of a band at 612 nm in the absorption spectrum,
but, upon further addition of MSA, a different species
evolved as confirmed by a blue shift and further enhancement
of the Soret-like band to 574 nm and replacement of the well-
structured Q-like bands by a very broad absorption tail at up
to 1200 nm (Figure 2b). These observations may simply be
explained by considering further protonation of 3, namely
formation of diprotonated species 4. Eventually, we obtained
crystals of 4 by the slow diffusion of n-heptane into a solution
of 2 in a mixture of CHCl3 and methanol in the presence of
MSA. X-Ray analysis revealed that the structure of 4 is
a triangle that consists of three corner pyrroles pointing
inwards (A, C, and E) and three side pyrroles pointing
outwards (B, D, and F), with a mean plane deviation of
Figure 3. a,b) X-Ray crystal structures of 3 (a) and 4 (b). Thermal
ellipsoids set at 30% probability. Counter anions and hydrogen atoms,
except for those attached to nitrogen atoms, are omitted for clarity.
approximately 0.3 ꢁ (Figure 3b). Therefore, hexaphyrin 4
was assigned as a Hꢀckel antiaromatic diprotonated molecule
owing to its planar structure and the p-conjugated system with
28 electrons. The disappearance of the Q-like bands and the
broad absorption tail support its antiaromaticity.[1e,f,4c,10] The
enhanced Soret-like band of 4 may be due to its high
molecular symmetry (ca. D3h). The extended triangular
conformation of 4 is likely favoured because of Coulombic
repulsion between the two positive charges in the molecule.
Then, it occurred to us that rational peripheral modifica-
tion of [28]hexaphyrins may render them more prone to
adopting triangular geometries. Therefore, we designed
2,3,12,13,22,23-hexaphenyl [28]hexaphyrin 8, which has an
alternate arrangement of unsubstituted pyrroles and 3,4-
diphenylpyrroles and may favor a C3-symmetric triangular
shape because of steric repulsion between the introduced
phenyl groups. Synthesis of 8 was accomplished by self-
condensation of monocarbinol 6. Aroyl dipyrromethane pre-
cursor 5 was reduced with NaBH4 to provide 6, which was
then condensed in the presence of para-toluenesulfonic acid
(p-TsOH) in CH2Cl2, followed by oxidation with 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ). Purification by
column chromatography on silica gel gave hexaphyrin 8 in
3% yield along with porphyrin 7[11] in 11% yield (Scheme 1).
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Scheme 1. Synthesis of [28]hexaphyrin 8 and [26]hexaphyrin 9.
The structure of 8 was confirmed by X-ray diffraction analysis
to be a figure-of-eight structure (see the Supporting Informa-
1
tion). On the basis of the H NMR spectral data, [28]hex-
aphyrin 8 was assigned as a weakly antiaromatic species. In
line with this, the absorption spectrum of 8 in CH2Cl2 showed
a broad band at 540 nm and very weak absorption in the near
infrared region, which are characteristic signatures of anti-
aromatic porphyrinoids (see the Supporting Information).
[28]Hexaphyrin 8 was quantitatively oxidized with MnO2 to
Figure 4. a,b) UV/Vis absorption spectral changes during titration of 8
with TFA (a) and MSA (b) in CH2Cl2.
1
afford [26]hexaphyrin 9, the H NMR spectrum of which at
À608C highlighted its weak but distinct aromaticity. The
aromaticity of 9 was corroborated by its absorption spectrum
in CH2Cl2, which displayed a sharp Soret band at 629 nm and
Q-like bands at 800 and 902 nm (see the Supporting
Information). [26]Hexaphyrin 9 could be easily reduced
back to 8 under ambient conditions.
Protonation-induced conformational changes of 8 were
examined by using TFA or MSA in CH2Cl2. The addition of
TFA induced absorption spectral changes, such as the
appearance of a remarkably sharp Soret band at 648 nm
and Q-like bands at 861 and 981 nm (Figure 4a), which are
quite similar to those observed for the titration of 2 with TFA;
therefore, these changes were attributed to the formation of
the monoprotonated Mçbius aromatic species 10. The
1H NMR spectrum of 10 in CDCl3 at À108C shows six signals
that correspond to the pyrrolic b protons at d = 7.98, 7.69,
3.61, 2.95, 0.30, and À0.37 ppm (Figure 5). These signals
indicate that a single Mçbius aromatic conformer is formed,
and that the conformational dynamics are still slower than the
1H NMR time scale even at room temperature (see the
Supporting Information), which is probably due to the steric
congestion that is exerted by the peripheral phenyl substitu-
ents.[12] Titration of 8 with MSA initially produced 10, as
indicated by the appearance of a peak at 639 nm, but soon
gave rise to diprotonated species 11 at the expense of 10
(Figure 4b). The absorption spectrum of the diprotonated
species 11 shows a sharp peak at 619 nm, which is blue-shifted
by 29 nm from that of 10, and a weak long tail extended to
approximately 1250 nm, which is similar to that observed for
4. Finally, the structure of 11 was determined by single-crystal
X-ray diffraction analysis (Figure 6). As expected, the pyr-
Figure 5. Monoprotonated [28]hexaphyrin 10 and diprotonated [28]hex-
aphyrin 11.
roles and diphenylpyrroles are pointing outwards and
inwards, respectively, to form a triangular conformation, in
which steric congestion is apparently minimized. It is worthy
to note that the amount of MSA needed for complete
diprotonation of 8 is approximately 1000 equivalents, which is
markedly smaller than the amount required for diprotonation
of 2 (ca. 20000 equiv).
The excited-state dynamics of expanded porphyrins are
sensitive to their molecular conformation and aromatic
nature.[13] Thus, we examined the excited-state dynamics of
[28]hexaphyrins by using femtosecond transient absorption
spectroscopy. The singlet excited state of 8 shows a double
exponential decay with ultrafast (0.4 ps, 75%) and relatively
long (5.1 ps, 25%) time components. According to previous
observations for highly distorted expanded porphyrins,[9]
these very fast excited-state dynamics are mainly attributable
to the acceleration of internal conversion processes in its
figure-of-eight conformation. In contrast, the decay profiles
of the ground-state bleaching recovery and the excited-state
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Figure 6. X-Ray crystal structure of 11 with counter anions. Thermal
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absorption signals of monoprotonated [28]hexaphyrin 10
exhibited a relatively long decay time constant of 64 ps in
THF in femtosecond transient absorption measurements. This
feature is in good agreement with the Mçbius aromatic nature
of 10. On the other hand, excited-state dynamics of diproto-
nated [28]hexaphyrin 11 revealed double-exponential decay
profiles with ultrafast (0.7 ps, 80%) and relatively long (8.9 ps,
20%) time components. This spectroscopic feature is also
characteristic of antiaromatic expanded porphyrins.[7,8] Fur-
thermore, we observed fluorescence emission of the monop-
rotonated [28]hexaphyrin, whereas its neutral and diproto-
nated congeners are nonfluorescent. Therefore, the spectro-
scopic signatures that were observed for 8, 10, and 11 are all
consistent with their assigned structures.
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monoprotonated by TFA to afford the twisted Mçbius
aromatic species 3, and that 2 is sequentially mono- and
diprotonated by MSA to form 3 and the Hꢀckel antiaromatic
species 4 in a fully reversible fashion. 2,3,12,13,22,23-Hex-
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aphenylated [28]hexaphyrin(1.1.1.1.1.1)
8 was rationally
designed and prepared to undergo diprotonation to favorably
afford a triangular-shaped antiaromatic species. For the
diprotonated [28]hexaphyrins 4 and 11, Coulombic repulsion
between the two positive charges is most likely a key factor
that encourages the triangular conformation with an elec-
tronically unfavorable antiaromatic character.[14] This work
underlines the conformational flexibility of [28]hexaphyrins;
conformational changes can be triggered by protonation.
Importantly, this protonation strategy constitutes a reliable
means to generate Mçbius aromatic and antiaromatic
expanded porphyrins.
Received: January 11, 2014
Published online: February 24, 2014
[10] J. M. Lim, Z. S. Yoon, J.-Y. Shin, K. S. Kim, M.-C. Yoon, D. Kim,
Chem. Commun. 2009, 261.
[11] opp-Tetraphenyl TPP-type porphyrins were already reported;
see: a) J. Takeda, M. Sato, Tetrahedron Lett. 1994, 35, 3565;
b) K. S. Chan, X. Zhou, M. T. Au, C. Y. Tam, Tetrahedron 1995,
Keywords: antiaromaticity · hexaphyrin · Mçbius aromaticity ·
protonation · triangular shape
.
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51, 3129; c) T. Nakanishi, K. Ohkubo, T. Kojima, S. Fukuzumi, J.
Am. Chem. Soc. 2009, 131, 577.
[14] One of the reviewers hypothesized that the methanesulfonate
anion could act as a template to interact with the inward-
pointing pyrrolic b protons. The distances between the oxygen
atom of the methanesulfonate and the b protons are in the range
of 2.58–2.76 ꢁ for 4 and 2.67 ꢁ for 11 (see the Supporting
Information), suggesting favorable interactions in the triangular
conformation.
[12] During our attempts to obtain crystals of 10, we isolated crystals
with a triangular structure (see the Supporting Information). It is
likely that the planar triangular hexaphyrins are more prone to
stacking because of their planar structure, although the Mçbius
aromatic species is predominant in solution.
[13] J.-Y. Shin, K. S. Kim, M.-C. Yoon, J. M. Lim, Z. S. Yoon, A.
Osuka, D. Kim, Chem. Soc. Rev. 2010, 39, 2751.
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