Unambiguous identification of Moebius aromaticity for meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1021/ja801983d

meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by ¹H, ¹³C, and ¹⁹F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25°C exist largely as an equilibrium among several rapidly interconverting twisted Moebius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take either planar or Moebius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100°C in THF, these rapid interconversions among Moebius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Moebius conformation. Detailed analyses of the solid-state Moebius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic π-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Moebius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.

Full text of DOI:10.1021/ja801983d

Published on Web 09/23/2008
Unambiguous Identification of Mo¨bius Aromaticity for
meso-Aryl-Substituted [28]Hexaphyrins(1.1.1.1.1.1)
Jeyaraman Sankar,† Shigeki Mori,† Shohei Saito,† Harapriya Rath,†
Masaaki Suzuki,† Yasuhide Inokuma,† Hiroshi Shinokubo,† Kil Suk Kim,‡
Zin Seok Yoon,‡ Jae-Yoon Shin,‡ Jong Min Lim,‡ Yoichi Matsuzaki,|
Osamu Matsushita,§ Atsuya Muranaka,§ Nagao Kobayashi,*,§ Dongho Kim,*,‡ and
Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan, Center for Ultrafast Optical Characteristics Control and Department of
Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, Department of Chemistry, Graduate School
of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan, AdVanced Technology
Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu,
Chiba 293-8511, Japan
Received March 17, 2008; E-mail: nagaok@mail.tains.tohoku.ac.jp; dongho@yonsei.ac.kr; osuka@kuchem.kyoto-u.ac.jp
Abstract: meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by 1H, 13C, and 19F
NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and
single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1)
in solution at 25 °C exist largely as an equilibrium among several rapidly interconverting twisted Mo¨bius
conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation
with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take
either planar or Mo¨bius-twisted conformations, depending upon the meso-aryl substituents and crystallization
conditions, indicating a small energy difference between the two conformers. Importantly, when the
temperature is decreased to -100 °C in THF, these rapid interconversions among Mo¨bius conformations
are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Mo¨bius
conformation. Detailed analyses of the solid-state Mo¨bius structures of compounds 2b, 2c, and 2f showed
that singly twisted structures are achieved without serious strain and that cyclic π-conjugation is well-
preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model
for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively),
confirming the first demonstration of stable Mo¨bius aromatic systems consisting of free-base expanded
porphyrins without the assistance of metal coordination.
to the Walsh model for cyclopropane.6 These theoretical studies
stimulated the synthesis of actual Mo¨bius aromatic molecules,
Introduction
The Hu¨ckel rule1 that predicts aromaticity for [4n + 2]an-
nulenes and antiaromaticity for [4n]annulenes lying in a plane
has been the firm basis for understanding aromaticity. As a
topological complement of the Hu¨ckel rule, the concept of
Mo¨bius aromaticity predicts that aromatic characteristics will
appear for [4n]annulenes when they are lying on a twisted
Mo¨bius strip.2 This important concept was first implied by
Heilbronner3 in 1964 and later proposed for transition states4
and reactive intermediates5 and extensively discussed in relation
which had been elusive until the seminal work of Herges and
co-workers,7 who reported the synthesis of [16]annulene
molecules having a twisted topology as the first examples of
Mo¨bius aromatic molecules; the synthesis was achieved by a
strategic lateral combination of a normal planar conjugated
segment and a belt-shaped conjugated segment. The aromatic
characters of these macrocycles were argued with respect to
the large dihedral angle (-107.7°) and small nucleus-indepen-
dent chemical shift (NICS) value (-3.4 ppm).8 However, Herges
and co-workers7b showed that Mo¨bius-twisted annulenes are
† Kyoto University.
‡ Yonsei University.
(6) (a) The Walsh model for cyclopropane was first proposed in: Walsh,
A. D. Nature 1947, 159, 165. (b) It was organized in: Walsh, A. D.
Trans. Faraday Soc. 1949, 45, 179–190. (c) In this context, Zimmer-
man indicated that the p orbitals of the model could be treated as a
Mo¨bius circle, as well as those of barrelene and allene. See:
Zimmerman, H. E. Acc. Chem. Res. 1971, 4, 272–280, and references
therein.
| Nippon Steel Corporation.
§ Tohoku University.
(1) (a) Hu¨ckel, E. Z. Phys. 1931, 70, 204–286. (b) Hu¨ckel, E. Z. Phys.
1932, 76, 628–648.
(2) (a) Rzepa, H. S. Chem. ReV. 2005, 105, 3697–3715. (b) Herges, R.
Chem. ReV. 2006, 106, 4820–4842.
(3) Heilbronner, E. Tetrahedron Lett. 1964, 5, 1923–1926.
(4) Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564–1567.
(5) Mauksch, M.; Gogonea, V.; Jiao, H.; Schleyer, P. v. R. Angew. Chem.,
Int. Ed. 1998, 37, 2395–2398.
(7) (a) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426,
819–821. (b) Ajami, D.; Hess, K.; Ko¨hler, F.; Na¨ther, C.; Oeckler,
O.; Simon, A.; Yamamoto, C.; Okamoto, Y.; Herges, R. Chem.-Eur.
J. 2006, 12, 5434–5455.
9
13568 J. AM. CHEM. SOC. 2008, 130, 13568–13579
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2008 American Chemical Society

Unambiguous Identification of Mo¨bius Aromaticity
A R T I C L E S
Chart 1
lenic systems to mitigate distortion associated with a single
molecular twist. In such a situation, however, cis-trans isomer-
ization barriers would be very small, making it very difficult to
lock-in a twisted structure. A recent interesting example is a
di-p-benzihexaphyrin reported by Latos-Graz˙yn´ski and co-
workers9 that exhibits a Mo¨bius structure in the solid state but
shows a temperature-dependent Hu¨ckel-to-Mo¨bius topological
switch in solution.
Because of their unique chemical and optical properties as
well as their versatile coordination chemistry, increasing atten-
tion has recently been focused on expanded porphyrins that
consist of more than five pyrrolic subunits.10 Among these, we
reported the synthesis of a series of meso-aryl-expanded
porphyrins using a modified Rothemund-Lindsey protocol.11
As one of their unique and surprising properties, [36]octaphy-
rin(1.1.1.1.1.1.1.1), [32]heptaphyrin(1.1.1.1.1.1.1), and [26]hexa-
phyrins(1.1.1.1.1.1) have been demonstrated to spontaneously
provide stable Mo¨bius aromatic molecules upon metalation with
group-10 metals.12 These are the first twisted Mo¨bius molecules
with distinct aromaticities, as indicated by their large differences
between the chemical shifts of the outer and inner ꢀ-protons,
large negative NICS values, and large two-photon absorption
cross sections. Quite recently, we reported that a Mo¨bius
aromatic molecule was also formed from metalation of an
N-fused pentaphyrin with a Rh(I) salt as the smallest Mo¨bius
structure from expanded porphyrins.13In these two cases, metal
coordination plays a vital role in locking the structures into
stable Mo¨bius aromatic conformations. In contrast, none of
the free-base expanded porphyrins reported to date has
exhibited distinct Mo¨bius aromaticity without any assistance
from metal coordination.
Scheme 1
meso-Hexakis(pentafluorophenyl)-substituted [26]hexaphy-
rin(1.1.1.1.1.1) (1a) is a planar rectangular macrocycle with a
metallic luster in the solid state and a vivid purple color in
solution.11a,14This expanded porphyrin can be regarded as a
representative porphyrin homologue in terms of its planar
1
structure and strong aromaticity. The H NMR spectrum of
1a reveals a distinct diatropic ring current by displaying
strongly shielded signals due to the inner ꢀ- and NH-protons
and strongly deshielded signals due to the outer ꢀ-protons.
We recently reported that metalation of 1a with Au(III) ion
provided the almost-planar mono[Au(III)] and bis[Au(III)]
complexes 3 and 4 (Chart 1), which have planar, conjugated
structures with 26 π electrons and thus exhibit Hu¨ckel
aromaticity. Importantly, complexes 3 and 4 were reduced
with NaBH4 to the corresponding [28]hexaphyrins 5 and 6
more aromatic than nontwisted isomers through the use of an
extensive examination of all possible data following a principal-
component analysis. A main difficulty in the synthesis of Mo¨bius
aromatic molecules lies in the need to compromise two
conflicting structural elements, a twisting single-sided topology
and a overall macrocyclic conjugation, with a 4n π-electron
system. Mo¨bius aromatic compounds must contain large annu-
Figure 1. Topological models of [28]hexaphyrins: (a) planar rectangular conformation of Hu¨ckel topology; (b) twisted Mo¨bius topology.
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A R T I C L E S
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1
Figure 2. Variable-temperature H NMR spectra of 2a in THF-d8 from 25 to -100 °C. Peaks marked with * are due to residual solvents and impurities.
(Chart 1), both of which display strong paratropic ring
currents due to their planar, conjugated structures with 28 π
electrons.15 On the other hand, the structures and electronic
natures of the meso-aryl-substituted [28]hexaphyrin(1.1.1.1.1.1)
free bases 2have been puzzling so far, since the H NMR
spectrum of 2a seems to suggest that it has a symmetric
current, but such a planar conformation with 28 π electrons
should lead to a paratropic ring current according to the
Hu¨ckel rule (Figure 1a). This contradiction has been a big
mystery since the discovery of 2 in 1999.14
1
In this paper, we demonstrate that the meso-aryl-substituted
[28]hexaphyrin(1.1.1.1.1.1) free bases 2 are conformationally
rather floppy molecules that in solution at room temperature
planar rectangular conformation and a moderate diatropic ring
(8) Castro, C.; Chen, Z.; Wannere, C. S.; Jiao, H.; Karney, W. L.;
Mauksch, M.; Puchta, R.; Hommes, N. J. R. v. E.; Schleyer, P. v. R.
J. Am. Chem. Soc. 2005, 127, 2425–2432.
(9) Stepien´, M.; Latos-Graz˙yn´ski, L.; Sprutta, N.; Chwalisz, P.; Szteren-
berg, L. Angew. Chem., Int. Ed. 2007, 46, 7869–7873.
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A R T I C L E S
Scheme 2. Plausible Conformational Changes of 2 in Solution Among P (Having a Planar Hu¨ckel Topology) and M1-M4 (Having Twisted
Mo¨bius Topologies). For 2a-e, M1 is Identical to M4 and M2 is Identical to M3, and These Two Pairs are Enantiomeric. In the Case of 2f,
M1 and M4 are Enantiomers of Each Other, as are M2 and M3
exist largely as an equilibrium among rapidly interconverting
twisted Mo¨bius structures with distinct aromaticities (Figure 1b).
prediction, the 1H NMR spectrum of 2a shows no features
indicative of a paratropic ring current arising from antiaroma-
ticity and hence poses a question concerning the actual
conformation of 2a in solution.
Results and Discussion
One possible explanation for the strange 1H NMR spectrum
of 2a is the invocation of a rapid interconversion among twisted
Mo¨bius conformations that should be aromatic (given their
cyclic, conjugated, 28 π-electron structures) and thus display
diatropic ring currents. In addition, the observed moderate
diatropic ring current may indicate a contribution from another
conformer (such as a planar rectangular conformation with
antiaromatic character) that mitigates the effects of the diatropic
ring current from the Mo¨bius conformers (Scheme 2). When
these conformational interconversions are fast on the 1H NMR
NMR Measurements. [26]Hexaphyrins 1a-f were prepared
according to our reported methods11c and quantitatively reduced
to the corresponding [28]hexaphyrins 2a-f by NaBH4 reduction
(Scheme 1). Compounds 2a-f display characteristic blue colors
1
in solution. As shown in Figure 2a, the H NMR spectrum of
2a in THF-d8 at 25 °C is simple, exhibiting a singlet at 2.46
ppm for four ꢀ-protons (signal e), a broad signal at 4.11 ppm
for two NH protons (signal d), two doublets at 7.88 and 7.95
ppm, each for four ꢀ-protons (signals b and c), and a broad
signal at 9.55 ppm for two NH protons (signal a), suggesting
that it has a symmetric structure. The NH assignments of the
signals a and d were made on the basis of deuterium-exchange
1
time scale, we observe an averaged H NMR spectrum of a
mixture of such isomers. In the postulated twisted Mo¨bius
conformations, all of the pyrroles are expected to be nonequiva-
lent, and thus, the 12 pyrrolic ꢀ-protons, the 6 meso carbon
atoms, and the 30 fluorine atoms in the meso-pentafluorophenyl
1
experiments with D2O. At first glance, the H NMR spectral
pattern of 2a is similar to that of 1a, whose structure has been
unambiguously determined to be a planar rectangular shape,11,14
and hence might be interpreted in terms of a rectangular
conformation analogous to that of 1a by assigning signals e
and d as the inner ꢀ- and NH-protons, respectively, signals b
and c as the outer ꢀ-protons, and signal a as the outer NH-
protons. Given the planar rectangular conformation, this as-
signment leads to the interpretation that [28]hexaphyrin 2a
exhibits a moderate diatropic ring current, since the inner protons
are shielded and the outer ones are deshielded. This interpreta-
tion, however, is hard to accept, since the postulated rectangular
conformation with a 28 π-electron conjugated network must
lead to antiaromaticity according to the Hu¨ckel rule. Actually,
the conformationally rigid systems 5 and 6, in which one or
two Au(III) ions help to strongly maintain the planar rectangular
structures, feature large paratropic ring currents, as predicted
by the Hu¨ckel rule.15 In addition, the NICS value, a quantitative
measure of aromaticity that was proposed by Schleyer,16 was
calculated to be large and positive (+34.5 ppm) for the planar
rectangular conformation of 2a. In sharp contrast to this
1
substituents should be different in the H, 13C, and 19F NMR
spectra, respectively. In addition, the 12 ꢀ-protons should be
observed in either shielded or deshielded fields, depending upon
their locations (either inside or outside of the global ring current,
(11) (a) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. J. Am.
Chem. Soc. 2001, 123, 7190–7191. (b) Taniguchi, R.; Shimizu, S.;
Suzuki, M.; Shin, J.-Y.; Furuta, H.; Osuka, A. Tetrahedron Lett. 2003,
44, 2502–2507. (c) Suzuki, M.; Osuka, A. Org. Lett. 2003, 5, 3943–
3946.
(12) (a) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata,
N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; Park, J. K.;
Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 681–684. (b)
Jux, N. Angew. Chem., Int. Ed. 2008, 47, 2543–2546.
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J.-Y.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2008, 130, 1824–1825.
(14) Neves, M. G. P. M. S.; Martins, R. M.; Tome´, A. C.; Silvestre, A. J. D.;
Silva, A. M. S.; Fe´lix, V.; Drew, M. G. B.; Cavaleiro, J. A. S. Chem.
Commun. 1999, 385–386.
(15) (a) Mori, S.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8030–8031. (b)
Mori, S.; Kim, K. S.; Yoon, Z. S.; Noh, S. B.; Kim, D.; Osuka, A.
J. Am. Chem. Soc. 2007, 129, 11344–11345.
(10) (a) Jasat, A.; Dolphin, D. Chem. ReV. 1997, 97, 2267–2340. (b) Lash,
T. D. Angew. Chem., Int. Ed. 2000, 39, 1763–1767. (c) Furuta, H.;
Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795–1804. (d) Sessler,
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Chandrashekar, T. K.; Venkatraman, S. Acc. Chem. Res. 2003, 36,
676–691.
(16) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317–6318. (b) Schleyer,
P. v. R.; Jiao, H.; Hommes, N. J. R. v. E.; Malkin, V. G.; Malkina,
O. L. J. Am. Chem. Soc. 1997, 119, 12669–12670. (c) Chen, Z.;
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Figure 3. Variable-temperature 13C NMR spectra of meso-13C-enriched 2a in THF-d8 from 25 to -100 °C.
respectively), provided that such twisted Mo¨bius molecules
exhibit a measureable diatropic ring current.
two (e′) at -0.48 and -1.66 ppm. These spectral data thus show
a large difference in the chemical shifts of the inner and outer
pyrrolic ꢀ-protons, indicating a distinct diatropic ring current.
These features, taken together a 28 π-electron circuit, can be
explained in terms of Mo¨bius aromaticity by postulating a
twisted, one-sided macrocyclic structure.
We thus examined the variable-temperature 1H, 13C, and 19
F
NMR spectra of 1a and 2a. The NMR spectra of 1a were
practically temperature-independent (see the Supporting Infor-
mation). In sharp contrast, as the temperature decreased, the
1H NMR spectrum of 2a actually changed as shown in Figure
2. Initially, the peaks broadened, and this continued as the
temperature decreased to -80 °C; by -100 °C, the peaks
sharpened, exhibiting an interesting distribution of the ꢀ-proton
signals: eight (b′ and c′) appeared in a relatively deshielded
region (8.55-7.68 ppm), two (e′) at 5.08 and 4.93 ppm, and
The variable-temperature 13C NMR spectra of meso-13C-
enriched 2a were also consistent with the postulated dynamic
conformational interconversion. At 25 °C, only two peaks were
observed at 96.3 and 101.9 ppm (Figure 3a). This simplicity
might indicate a symmetric planar rectangular conformation but
instead should be interpreted in terms of the dynamic confor-
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Unambiguous Identification of Mo¨bius Aromaticity
A R T I C L E S
1
We then examined the variable-temperature H NMR and
19F NMR spectra of 2b-f and found that all of these [28]hexa-
phyrins exhibited temperature-dependent spectra similar to those
of 2a with coalescence temperatures of approximately -80 °C.
These data strongly support the view that all of these meso-
aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) rapidly interconvert
among Mo¨bius aromatic conformations at room temperature and
that such conformational dynamics are almost frozen at low
temperature. At -100 °C, [28]hexaphyrin molecules prefer to
avoid a planar conformation that gives rise to antiaromatic
characters and settle in a nonplanar twisted Mo¨bius geometry,
probably in order to achieve energetic stabilization. It should
be noted that this is the first Mo¨bius aromatic molecular system
consisting of free-base expanded porphyrins without the as-
sistance of metal coordination, although low temperatures (-100
°C) are needed to freeze such structures on the NMR time scale.
Figure 4. Variable-temperature UV-vis spectra of 2a in THF from 20 to
-100 °C.
Temperature-Dependent Absorption Spectra. The absorption
spectrum of 2a was examined as a function of temperature from
20 to -100 °C in THF (Figure 4) and from 25 to 100 °C in
toluene (Figure 5). At 20 °C, 2a exhibits a Soret-like absorption
band at 596 nm and Q-band-like features at 764, 847, 892, and
1006 nm in THF; in toluene, these bands appear at 591 nm and
at 763, 849, 896, and 1009 nm, respectively. The absorption
spectra of 2a, which are similar to that of 1a with a slight
broadening, seem to indicate aromatic character for 2a. As the
temperature was decreased (Figure 4), the absorption spectrum
of 2a changed gradually but distinctly: the Soret-like band
became slightly red-shifted and more intense, and a shoulder
appeared at 650 nm; the Q-band-like bands were also slightly
more intense. The effects of an increase in temperature were
also examined in toluene. As the temperature increased (Figure
5), the Soret band of 2a became slightly broadened and blue-
shifted and its intensity diminished. It is noteworthy that the
intensity of the Q-band-like feature of 2a also decreased with
increasing temperature. The other [28]hexaphyrins(1.1.1.1.1.1)
2b-e exhibited essentially the same spectral changes with an
increase in temperature. These spectral changes indicate that
decreasing the temperature caused an increase in aromatic
character.
Single-Crystal X-ray Diffraction Studies. The solid-state
structure of [26]hexaphyrin 1a is a planar rectangular shape,
and its aromaticity can be interpreted in terms of the Hu¨ckel
rule.14 It has been shown without exception to date that
[26]hexaphyrins bearing 2,6-disubstituted aryl meso substituents
all adopt essentially the same rectangular structures. Actually,
the planar rectangular structures of 1c and 1d have been
determined for the first time in the present work (Figures S4-1
and S4-3, respectively, in the Supporting Information). In sharp
contrast, the solid-state structure of [28]hexaphyrin 2a has
remained elusive so far. A rare related example is the planar
rectangular structure revealed for hexakis(4-isopropylamino-
2,3,5,6-tetrafluorophenyl)-substituted [28]hexaphyrin.17 In the
present work, extensive efforts to obtain single crystals of the
[28]hexaphyrins 2 were made in order to obtain more convincing
solid-state evidence for their observed aromatic characters in
solution. After repeated attempts, X-ray-quality single crystals
of 2a were first obtained by slow evaporation of a solution of
2a dissolved in a ternary solvent system of dimethyl sulfoxide
(DMSO), methanol, and water. Contrary to our conjecture that
2a has a twisted Mo¨bius topology, the single-crystal X-ray
Figure 5. Variable-temperature UV-vis spectra of 2a in toluene from 25
to 100 °C.
mational interconversion among possible twisted Mo¨bius con-
formations, since the spectrum became broadened as the
temperature decreased to -80 °C and then sharpened by -100
°C, displaying six peaks at 93.6, 94.1, 95.5, 97.8, 106.2, and
109.8 ppm (Figure 3). This spectrum is in accord with the
Mo¨bius structure, since such a structure has no symmetry and
thus all of the meso carbons should be nonequivalent. These
data again indicate that low-symmetry conformations are the
most stable for 2a. Variable-temperature 19F NMR measure-
ments on 2a exhibited a simple spectrum at 25 °C whose peaks
first broadened and then sharpened as the temperature decreased
to -100 °C (Figure S3-9 in the Supporting Information); these
results also support the conclusion that a dynamic conforma-
tional interconversion existing at 25 °C becomes frozen at low
1
temperature. The H, 13C, and 19F NMR spectral data consis-
tently indicated a coalescence temperature of approximately -80
°C for 2a.
We also examined the influence of an increase in temperature
1
on the H NMR spectrum of 2a in 1,1,2,2,-tetrachloroethane-
d2 (Figure S3-8 in the Supporting Information). The variable-
1
temperature H NMR spectral data in the range 25-140 °C
indicated a continuous decrease in the difference between the
chemical shifts of the outer and inner ꢀ-protons and hence a
continuous decrease in average ring current with increasing
temperature. These data can be explained by invoking an
increasing contribution from conformers that have nonaromatic
or antiaromatic characters. This will be discussed in detail below.
(17) Suzuki, M.; Shimizu, S.; Shin, J.-Y.; Osuka, A. Tetrahedron Lett. 2003,
44, 4597–4601.
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Figure 6. X-ray structure of 2a obtained from DMSO/methanol/H2O: (a) top view; (b) side view; (c) side view including the hydrogen-bonded DMSO
molecules. The thermal ellipsoids are scaled to the 50% probability level. In the side views, the meso-aryl substituents have been omitted for clarity.
Figure 7. X-ray structure of 2b obtained from CHCl3/n-heptane: (a) top view; (b) side view. The thermal ellipsoids are scaled to the 50% probability level.
In the side view, the meso-aryl substituents have been omitted for clarity.
diffraction analysis of these crystals revealed a planar rectangular
structure (Figure 6) roughly similar to that of 1a. The mean
plane deviation is quite small (0.127 Å); the two inverted pyrrole
rings are tilted by 26.7° with respect to the mean plane, and
their outward-pointing NH protons are hydrogen-bonded to two
solvent DMSO molecules, one above and one below the
macrocyclic plane. This planar solid-state structure seems to
arise from its favorable crystal packing, in which face-to-face,
parallel, offset dimeric units are arranged along one direction
and other dimeric units are arranged along a different direction
in an interpenetrating manner (Figure S4-7 in the Supporting
Information). Importantly, these results indicate that the planar
rectangular conformation is also a stable form that can exist to
some extent under certain conditions (such as in the solid state)
and thus possibly in solution. A similar planar rectangular
structure was found for single crystals of 2d that were obtained
by slow vapor diffusion of n-heptane into a CH2Cl2 solution of
2d (Figure S4-4 in the Supporting Information).
In the meantime, we found that [28]hexaphyrin 2b provided two
different crystal structures, a planar rectangular conformation and
a twisted conformation, depending on the solvents used for
crystallization. Slow vapor diffusion of n-heptane into a solution
of 2b in CHCl3 yielded single crystals that provided a planar
rectangular structure with a mean plane deviation of 0.203 Å
(Figure 7), similar to those of 2a and 2d. On the contrary, slow
vapor diffusion of ethanol into a solution of 2b in 1,2-dichloroet-
hane gave different single crystals that were found to have a
distorted structure of Mo¨bius topology (Figure 8) in which two
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13574 J. AM. CHEM. SOC. VOL. 130, NO. 41, 2008

Unambiguous Identification of Mo¨bius Aromaticity
A R T I C L E S
Figure 8. X-ray structure of 2b obtained from 1,2-dichloroethane/ethanol: (a) top view; (b) side view; (c) side view including the hydrogen-bonded ethanol
molecules. The thermal ellipsoids are scaled to the 50% probability level. In the side views, the meso-aryl substituents have been omitted for clarity.
sumed a similar twisted Mo¨bius structure for 2a, it became
possible to assign the e′ signals at -2.25 and -1.47 ppm in the
1H NMR spectrum recorded at -100 °C (Figure 2h) to the
ꢀ-protons of inverted pyrrole B and the e′ signals at 4.90 and
4.82 ppm to those of pyrrole E (for ring labels, see Figure 8).
As another examples, we obtained single crystals of 2c and
2f by slow vapor diffusion of hexane into a CHCl3 solution
and n-heptane into a THF solution, respectively. The crystal
structures of 2c and 2f (Figures S4-2 and S4-5, respectively, in
the Supporting Information) were found to be Mo¨bius structures
similar to that of 2b. The large strain associated with the
molecular twist is dissipated nicely across the whole molecule,
assuring effective π conjugation over the macrocycle. In the
case of 2f, the crystal structure indicated a single regioisomer,
1
but the H NMR spectrum at -100 °C (Figure S3-21h in the
Supporting Information) revealed the presence of a minor
regioisomer (∼20%), reflecting the alternating meso substitution
Figure 9. Detailed structural data for the Mo¨bius structure of 2b from
Figure 8. The conjugated 28 π-electron circuit is indicated in green, along
which bond lengths (Å) are indicated in red and dihedral angles (deg) in
blue.
pattern. These results indicate that 2c and 2f settle at their most
stable twisted Mo¨bius conformations at -100 °C, at which the
possible conformational changes are practically frozen.
The Mo¨bius solid-state structures observed for 2b, 2c, and
2f are significant because they confirm that the aromatic
characters actually arise from their 28 π-electron circuits lying
on single-sided Mo¨bius strips. The experimental fact that two
different crystal structures were obtained for 2b is also
important, as it indicates not only that the energy difference
between these two structures is small but also that differences
in solvents such as polarity or hydrogen-bonding ability can
affect the crystal structure, which should also depend on the
crystal packing. Therefore, it is plausible to conclude that there
is some contribution from the planar rectangular conformer or
even from other conformers of [28]hexaphyrins in solution.
Importantly, however, it is also reasonable to conclude that the
Mo¨bius conformations are the most energetically stable, since
they are predominant at low temperatures.
ethanol molecules are hydrogen-bonded with the outward-pointing
NH protons of pyrroles B and E. Detailed structural data, including
bond distances and dihedral angles, are summarized in Figure 9.
The overall electronic system can be delineated as a conjugated
cyclic network containing a 28 π-electron periphery. Significantly,
even at the most distorted position along the conjugated chain,
C(25)-C(26), the dihedral angle is only 26.5°, which is still
favorable for the overall conjugation. This feature is different from
those observed for Pd(II) complexes of expanded porphyrins,12a
in which the metal coordination helps rigidify the twisted
structure but enforces relatively planar local coordination
structures [square planar in the case of Pd(II)], resulting in a
considerably large dihedral angle. In the Mo¨bius structure of
2b, the large strain arising from the molecular twist is dissipated
across the whole macrocyclic ring system, probably as a
consequence of the conformational flexibility. When we as-
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A R T I C L E S
Sankar et al.
MCD Spectroscopy. Magnetic circular dichroism (MCD)
spectroscopy has long proven to be an important technique for
determining the electronic and magnetic properties and coor-
dination structures of organic and inorganic systems.19 Analysis
of MCD spectra often facilitates identification of the main
electronic transitions in instances where such identification is
not possible on the basis of an analysis of the UV-visible
absorption spectra alone. In the case of porphyrinoid systems
such as porphyrins,20 chlorins,21 azaporphyrins,22 and por-
phycenes, 23 MCD spectroscopy has provided the key informa-
tion required for definitive band assignment and characterization
of the electronic excited states. In recent years, the electronic
24
structures of expanded porphyrins such as texaphyrins,
isosmaragdyrins,25 sapphyrins,26 and cyclo[n]pyrroles27 have
also been analyzed using MCD spectroscopy. We measured the
MCD spectra of a range of different hexaphyrins in order to
determine the relationship between Hu¨ckel- and Mo¨bius-type
aromaticities and the electronic structures.
Figure 10a shows the electronic absorption and MCD spectra
of [26]hexaphyrin 1a measured in CH2Cl2 at room temperature.
1a exhibits four weak absorption bands (1022, 905, 769, and
712 nm) and an intense absorption band (567 nm) with a
shoulder on the longer-wavelength side (∼590 nm). In the MCD
spectrum of 1a, four Faraday B terms having a -,-,+,+ sign
sequence with increasing energy were observed for the four
weak absorption bands. Intense positive and negative MCD
signals corresponding to the 567 nm absorption and its shoulder,
respectively, were detected, indicating that there are at least two
different electronic transitions in the 550-600 nm region. The
overall MCD pattern of 1a appears to be essentially similar to
those of conventional free-base porphyrins.28 We calculated the
absorption spectrum of Hu¨ckel-type 1a using the ZINDO/S
method in order to make definitive band assignments. As shown
in Figure 10b, the calculated results agreed well with the
experimental observations. Two closely lying transitions were
predicted at 507 and 535 nm, corresponding to the observed
567 nm absorption band and 590 nm shoulder band, respectively.
These transitions are polarized along the long and short
molecular axes, respectively. This can account for the strongly
coupled Faraday B terms observed experimentally, since ac-
cording to theory, mutually perpendicular transitions in a narrow
energy region give strongly coupled Faraday B terms.19 With
respect to the longer-wavelength region, two very weak
electronic transitions were calculated at 949 and 1018 nm. Since
1a has four weak absorption bands in the near-IR region, it was
reasonable to assign the observed 769 and 1022 nm transitions
to an electronic origin and the 712 and 905 nm bands as vibronic
bands, as is the case of normal free-base porphyrins.28 It is noted
that these four excitations consist of electronic transitions
involving the four π frontier molecular orbitals (MOs) HO-
MO-1, HOMO, LUMO, and LUMO+1 (see Table 1). In Platt
Figure 10. (a) Experimental MCD and absorption spectra of 1a measured
in CH2Cl2 at room temperature. (b) Calculated absorption spectrum of
Hu¨ckel-type 1a obtained using the ZINDO/S method. The inset shows the
B3LYP-optimized structure used in the ZINDO/S calculations.
The harmonic-oscillator model for aromaticity (HOMA) values
obtained from the X-ray diffraction data provide important
information about the extent of effective electronic delocalization.18
The solid-state planar structure of 2a gives a HOMA value of 0.450,
which is smaller than the value of 0.647 for the [26]hexaphyrin
1a, indicating less effective conjugation in the planar form of 2a.
On the other hand, significantly large HOMA values were
commonly observed for the Mo¨bius structures of 2b (0.85), 2c
(0.69), and 2f (0.71). Therefore, it can be concluded that the overlap
of π orbitals in [28]hexaphyrin is more effective in a twisted
Mo¨bius topology than in a planar rectangular shape.
Table 1. Calculated Excitation Wavelengths (λ), Oscillator Strengths (f), and Compositions of Electronic Transitions in Hu¨ckel-Type 1a and
Mo¨bius-Type 2a Obtained Using ZINDO/S Calculations
compound
λ(nm)
assign.
f
composition (%)a
1a (Hu¨ckel)
1018
949
535
507
1103
957
605
549
LI
0.006
0.003
2.319
4.112
0.019
0.009
1.169
2.892
H f L (37.5); H-1 f L+1 (26.1); H-1 f L (16.1); H f L +1(15.6)
H f L+1 (32.4); H-1 f L (30.3); H f L (17.1); H-1 f L+1 (14.0)
H-1 f L+1 (38.7); H f L (28.3); H f L+1 (11.1); H-1 f L (10.0)
H-1 f L (31.8); H f L+1 (30.1); H-1f L+1 (11.9); H f L (9.3)
H f L (55.1); H-1 f L+1 (39.2)
LII
BI
BII
LI
LII
BI
BII
2a (Mo¨bius)
H-1 f L (57.8); H f L+1 (35.2)
H f L+1 (48.7); H-1 f L (27.1)
H-1 f L+1 (48.4); H f L (36.0)
a H ) HOMO, L ) LUMO.
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Unambiguous Identification of Mo¨bius Aromaticity
A R T I C L E S
nomenclature,29 the two lower-energy quasi-allowed transitions
and the two higher-energy strongly allowed transitions are labeled
as the L and B bands, respectively. Since it is customary to refer
to the two L bands of porphyrins as the Q bands and the two B
bands as the Soret bands, we can also use the labels Q and Soret,
respectively, to refer to the L and B transitions of 1a.
The electronic absorption and MCD spectra of [28]hexaphyrin
2a are shown in Figure 11a. As mentioned earlier, 2a exhibits
a red-shifted visible band at 591 nm with a shoulder at ∼620
nm, and the long-wavelength absorption bands observed be-
tween 700 and 1100 nm are weak and broad. We found that
the MCD sign pattern for the intense visible band of 2a was
inverted compared with that of 1a. Thus, an intense MCD peak
and trough were observed at 625 and 595 nm, respectively. In
the near-IR region, four MCD signals (1023, 912, 835, and 767
nm) were clearly resolved, indicating the presence of four
different electronic transitions. The MCD sign corresponding
to the lowest-energy transition was positive, in contrast to the
negative MCD for the corresponding peak of 1a. Figure 11b
gives the calculated absorption spectra of two types of 2a. One
can see clearly that the observed spectral patterns were well-
reproduced by the Mo¨bius structure. The results calculated on
the basis of the Hu¨ckel structure did not predict the red-shifted
visible band or the four near-IR bands. Two weak transitions
in the near-IR region (957 and 1103 nm) were predicted for
the Mo¨bius conformation, so the MCD signals at 835 and 1023
nm were assigned to have a purely electronic origin while the
767 and 912 nm signals are vibronic bands. As shown in Table
1, the four electronic absorption bands could be described in
terms of the four key π frontier MOs of Mo¨bius 2a. We
therefore applied the Platt nomenclature29 to assign the absorp-
tion bands of Mo¨bius 2a. The polarization directions of the 549
and 605 nm transitions for Mo¨bius 2a are almost parallel to the
long and short molecular axes, respectively.
Figure 12 illustrates the MOs of Mo¨bius [28]hexaphyrin,
Hu¨ckel [26]hexaphyrin, and Hu¨ckel [28]hexaphyrin calculated
using density functional theory (DFT) at the B3LYP/6-31G*
(18) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70–78, HOMA
values are a quantitative measure of bond-length equalization for
π-electron systems. HOMA ) 0 for the Kekule´ structure of an aromatic
system, and HOMA ) 1 for the system with all bonds equal to the
optimal value, such as in benzene.
(19) (a) Mason, W. R. A Practical Guide to Magnetic Circular Dichroism
Spectroscopy; John Wiley & Sons Inc.: Hoboken, NJ, 2007. (b)
Rodger, A.; Norde´n, B. Circular Dichroism and Linear Dichroism;
Oxford University Press: Oxford, U.K., 1997.
(20) (a) Michl, J. J. Am. Chem. Soc. 1978, 100, 6801–6811. (b) Michl, J.
J. Am. Chem. Soc. 1978, 100, 6812–6818. (c) Mack, J.; Asano, Y.;
Kobayashi, N.; Stillman, M. J. J. Am. Chem. Soc. 2005, 127, 17697–
17711.
Figure 11. (a) Experimental MCD and absorption spectra of 2a measured
in CH2Cl2 at room temperature. (b) Calculated absorption spectra of (top)
Mo¨bius-type 2a and (bottom) Hu¨ckel-type 2a obtained using the ZINDO/S
method. The insets show the B3LYP-optimized structures used in the
ZINDO/S calculations.
level.30 At first glance, a similarity is seen between Mo¨bius
[28]hexaphyrin and Hu¨ckel [26]hexaphyrin with respect to the
near degeneracy of their MOs, which is characteristic of general
aromatic compounds. The degenerate MOs account well for the
porphyrin-like absorption spectra of 1 and 2, which consist of
Soret-like and the Q-band-like bands. In sharp contrast, no
degeneracy was observed in the case of Hu¨ckel [28]hexaphyrin.
(21) (a) Keegan, J. D.; Stolzenberg, A. M.; Lu, Y.-C.; Linder, R. E.; Barth,
G.; Moscowitz, A.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. Soc.
1982, 104, 4305–4317. (b) Keegan, J. D.; Stolzenberg, A. M.; Lu,
Y.-C.; Linder, R. E.; Barth, G.; Moscowitz, A.; Bunnenberg, E.;
Djerassi, C. J. Am. Chem. Soc. 1982, 104, 4317–4329.
(22) Mack, J.; Stillman, M. J. Coord. Chem. ReV. 2001, 219-221, 993–
1032.
(23) Waluk, J.; Mu¨ller, M.; Swiderek, P.; Ko¨cher, M.; Vogel, E.; Hohl-
neicher, G.; Michl, J. J. Am. Chem. Soc. 1991, 113, 5511–5527.
(24) Waluk, J.; Hemmi, G.; Sessler, J. L.; Michl, J. J. Org. Chem. 1991,
56, 2735–2742.
(25) Gorski, A.; Lament, B.; Davis, J. M.; Sessler, J. L; Waluk, J. J. Phys.
Chem. A 2001, 105, 4992–4999.
(26) Lament, B.; Rachlewicz, K.; Latos-Graz˙yn´ski, L.; Waluk, J. ChemP-
hysChem 2002, 3, 849–855.
(27) Gorski, A.; Ko¨hler, T.; Seidel, D.; Lee, J. T.; Orzanowska, G.; Sessler,
J. L.; Waluk, J. Chem.-Eur. J. 2005, 11, 4179–4184.
(28) Gouterman, M. In The Porphyrins, Vol. III: Physical Chemistry, Part
A; Dolphin, D., Ed.;Academic Press: New York, 1978.
(29) Platt, J. R. J. Chem. Phys. 1949, 17, 484–495.
(30) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (c) Theoretical
calculations were performed with the Gaussian 03 package: Frisch,
M. J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford,
CT, 2004.
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A R T I C L E S
Sankar et al.
Figure 12. Molecular orbitals (B3LYP/6-31G*) of (a) Mo¨bius [28]hexaphyrin, (b) Hu¨ckel [26]hexaphyrin, and (c) Hu¨ckel [28]hexaphyrin.
The origin of the observed MCD properties of 1a and 2a is
now further discussed in terms of an examination of their
electronic structures. In the case of Hu¨ckel aromatic molecules
derived from a [4n + 2]-electron perimeter, the absolute MCD
signs associated with the two L and two B transitions can be
deduced from the properties of the frontier MOs.20 In particular,
in the case of low-symmetry aromatic molecules whose four
transitions are described within a four-orbital π-electron frame-
work, the MCD signs are predicted by the relative sizes of the
two MO energy differences ∆HOMO and ∆LUMO, which
represent the differences between the absolute values of the
energy of the two highest occupied MOs and between the two
lowest unoccupied orbitals, respectively.20b,21,23–26 When ∆HOMO
) ∆LUMO * 0, the MCD signs derive from µ- contributions,
and the MCD having a purely electronic origin is likely to be
generally quite weak. The MCD sign pattern for this kind of
system would be -,-,-,+ with increasing energy. When
∆LUMO - ∆HOMO is relatively large, µ+ contributions
dominate, and the MCD sign pattern is governed by the sign of
∆LUMO - ∆HOMO: when this quantity is negative (i.e., when
∆HOMO > ∆LUMO), a -,+,-,+ sign sequence in order of
increasing energy is predicted, whereas the inverted sequence
+,-,+,- is anticipated when ∆HOMO < ∆LUMO. We
calculated the orbital-energy differences for Hu¨ckel 1a and its
unsubstituted derivative 1H using the DFT, Hartree-Fock (HF),
and ZINDO/S methods. As shown in Table S3 in the Supporting
Information, ∆HOMO > ∆LUMO for 1a regardless of the
computational level. This led to the prediction of the -,+,-,+
sign pattern for the two L and two B bands, matching the
experimentally observed pattern. In the case of the parent 1H,
∆HOMO ≈ ∆LUMO, so the hexaphyrin skeleton itself can be
20b
classified into the soft-chromophore case.
The electronic structures of Mo¨bius-type 2a and the unsubstituted
derivative 2H were found to be considerably different from those
for Hu¨ckel-type 1a and 1H. As shown in Table S3 and Figure S6
in the Supporting Information, ∆HOMO < ∆LUMO relationships
were predicted for 2a and 2H. Although no theory for the absolute
MCD sign of Mo¨bius aromatic molecules has been reported to
date, the sign inversion of the ∆LUMO-∆HOMO values of
Mo¨bius-type 2a and 2H compared with those of 1a may be
correlated with the inversion of the MCD pattern observed for 2a,
since even in Mo¨bius aromatic systems, the orbital-energy differ-
ence depends on whether the transition is an electron-dominated
excitation or a hole-dominated excitation.
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Unambiguous Identification of Mo¨bius Aromaticity
A R T I C L E S
Figure 13. NICS values for (top) Hu¨ckel and (bottom) Mo¨bius [28]hexaphyrin calculated at the GIAO-B3LYP/6-31G* level.
Theoretical Calculations and Analysis of Aromaticity on
the Basis of NICS Values. To evaluate aromaticity and antiaro-
maticity of hexaphyrins both in Mo¨bius and Hu¨ckel topologies,
we carried out DFT calculations at the B3LYP/6-31G* level based
on the optimized structures. Initial geometries of the models were
obtained from X-ray structures, but all of the aromatic substituents
were replaced by hydrogens. NICS values of the optimized
structures were obtained as a qualitative measure of aromaticity
and antiaromaticity. For each model, the NICS values at the global
ring center of the macrocycle as well as at the center of each pyrrole
unit and each formal ring structure between adjacent pyrroles were
calculated, as shown in Figure 13. Clearly, [28]hexaphyrin in
the Mo¨bius topology, for which the NICS value was calculated to
be -15.2 ppm at the center of the molecule, is strongly aromatic.
This value is comparable to those of [26]hexaphyrin (-16.7 ppm)
and the bis[Au(III)] complex of [26]hexaphyrin (-12.5 ppm), both
of which have been shown to be strongly aromatic.15a Conse-
quently, the DFT calculations clearly suggested the strong aroma-
ticity of the macrocycle, which was also obvious from the 1H NMR
measurements and X-ray analysis. In addition, the HOMA value
for the optimized structure was calculated to be 0.71, again
indicating the aromaticity of Mo¨bius [28]hexaphyrin. This value
is in good agreement with the experimental value, demonstrating
the validity of the calculations. On the other hand, [28]hexaphyrin
in the Hu¨ckel topology was calculated to have a considerably
positive NICS value (+34.5 ppm) at the molecular center and a
HOMA value of 0.51; comparison of this NICS value with that of
the antiaromatic bis[Au(III)] complex of [28]hexaphyrin (37.3 ppm)
reveals the strong antiaromaticity of Hu¨ckel [28]hexaphyrin.
equilibrium of several rapidly interconverting twisted Mo¨bius
conformations with distinct aromaticities, with a small contribu-
tion from a rectangular conformation with antiaromatic char-
acter. When the temperature was decreased to -100 °C in THF,
these rapid interconversions were frozen, leading to the pre-
dominant existence of a single Mo¨bius conformation with
distinct aromaticity, as revealed by the NMR measurements.
The MCD data were also consistent with the Mo¨bius structure.
Furthermore, the Mo¨bius-twisted conformations were actually
confirmed by the single-crystal diffraction studies. Hence, this
work has demonstrated the first example of Mo¨bius aromatic
molecules consisting of free-base expanded porphyrins without
any assistance of metal chelation. These Mo¨bius aromatic free-
base expanded porphyrins constitute a nice platform for
examination of various properties of Mo¨bius aromaticity.
Acknowledgment. The work was partly supported by Grant-
in-Aid (A) (19205006) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan and the Star Faculty and
BK21 Programs of the Ministry of Education and Human Resources
of Korea (D.K.). J.S., S.M., S.S., M.S., Y.I., and H.R. are grateful
for JSPS fellowships. K.S.K., J.-Y.S., J. M.L., and Z.S.Y. acknowl-
edge BK21 fellowships.
Supporting Information Available: Experimental procedures,
UV-vis spectra, HR-ESI-TOF mass spectra, NMR spectra,
X-ray crystal structures, results of MO calculations related to
the MCS spectra, complete ref 30c (as ref S3), and CIF files
for 1c, 1d, 2a-d, and 2f. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusion
In summary, meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1)
have been shown to exist in solution at 25 °C largely as an
JA801983D
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