A stable radical species from facile oxygenation of meso-free 5,10,20,25-tetrakis(pentafluorophenyl)-substituted [26]hexaphyrin(1.1.1.1.1.1)

Article abstract of DOI:10.1002/anie.200804570

(Figure Presented) A stable radical: The title compound 1 has been synthesized as the first example of a meso-free variant and characterized as a strongly aromatic macrocycle with a spectacles-like planar conformation. The incorporation of free meso positions is promising for the exploration of novel structural and electronic properties.

Full text of DOI:10.1002/anie.200804570

Angewandte
Chemie
DOI: 10.1002/anie.200804570
Porphyrinoids
A Stable Radical Species from Facile Oxygenation of meso-Free
5,10,20,25-Tetrakis(pentafluorophenyl)-Substituted
[26]Hexaphyrin(1.1.1.1.1.1)**
Taro Koide, Gengo Kashiwazaki, Masaaki Suzuki, Ko Furukawa, Min-Chul Yoon, Sung Cho,
Dongho Kim,* and Atsuhiro Osuka*
In recent years, the potential of expanded porphyrins (con-
jugated pyrrolic macrocycles larger than porphyrins) has been
increasingly recognized owing to their unique chemical,
optical, and electrochemical properties as well as rich
coordination chemistry.[1] Besides their earlier uses as scaf-
folds for anion binding[2] and their unique metal–metal
interactions,[3] recent interesting findings have been reported,
including 1) a smooth metathesis-type splitting reaction of the
bis(CuII) complex of [36]octaphyrin(1.1.1.1.1.1.1.1) into two
molecules of CuII porphyrin,[4] 2) transannular annulation
reactions between two ethynyl groups or between ethynyl and
phenyl groups of meso-substituted hexaphyrins to provide
unique rearranged products,[5] and 3) the facile formation of
stable antiaromatic or twisted Mꢀbius aromatic systems.[6]
These results indicate that expanded porphyrins can fulfill
roles that are not only difficult for porphyrins but are novel
and important in their own right. Herein, we report another
unexpected function of expanded porphyrins, namely of
[26]hexaphyrin, to stabilize a radical species to the extent
that it can be manipulated as a normal closed-shell organic
molecule in the presence of air and moisture. To the best of
our knowledge, this finding is without precedent in the
chemistry of porphyrins.
aromaticity, as well as being more or less analogous to
porphyrins. The chemical stability of these hexaphyrins is
crucially dependant on the meso-aryl substituents. Namely,
hexakis(pentafluorophenyl)-substituted hexaphyrin 1 is a
stable aromatic molecule with type II conformation (flat
rectangular shape; Scheme 1),[7,8] whereas the incorporation
Scheme 1. Structures of [26]hexaphyrins(1.1.1.1.1.1) 1 and 3 that
correspond to type II and type I conformations, respectively.
of phenyl or 2-tolyl substituents at the meso position causes a
significant drop in the chemical stability of hexaphyrin.[9,10]
According to DFT calculation (B3LYP/6-31G*) for the
unsubstituted hexaphyrin, the type I conformation (flat
spectacles-like shape; Scheme 1) is more stable than the
type II conformation by 9.9 kcalmolÀ1; this stability is due to
more effective intramolecular hydrogen-bonding interactions
in the former. Hence, the predominant type II conformation
observed for 1 can be attributed to the severe steric repulsion
between two inward-pointing meso-pentafluorophenyl sub-
stituents in its putative type I conformation. With this in mind,
we embarked on the synthesis of 5,10,20,25-tetrakis(penta-
fluorophenyl)-substituted hexaphyrin 3 as the first example of
a meso-free variant, with an expectation that 3 should take
predominantly the type I conformation. The synthesis of 3
was also motivated by its structural resemblance to meso-free
porphyrins, which have intriguing meso–meso coupling reac-
tivity upon oxidation.[11]
Hexaphyrin 3 was prepared by the condensation of 5,10-
bis(pentafluorophenyl)tripyrrane (2)[12] with an excess
amount of trimethyl orthoformate (22 equiv) in CH2Cl2 in
the presence of methanesulfonic acid under argon, and the
reaction was carried out in the dark at 08C for 7 h
(Scheme 2).[13] The resulting solution was oxidized using 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) for 1 h at
room temperature. The usual workup was followed by quick
separation by standard column chromatography on silica gel
In the area of expanded porphyrins, meso-aryl-substituted
[26]hexaphyrins(1.1.1.1.1.1) have served as benchmark mol-
ecules in view of their flat molecular shapes and distinct
[*] M.-C. Yoon, Dr. S. Cho, Prof. Dr. D. Kim
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Homepage: http://chem.yonsei.ac.kr/~CUOCC
T. Koide, G. Kashiwazaki, Dr. M. Suzuki, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Homepage: http://kuchem.kyoto-u.ac.jp/shuyu/index.html
Dr. K. Furukawa
Institute for Molecular Science, Myodaiji, Okazaki 444-8585 (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research
(A) from MEXT (Grant No. 19205006) of Japan and the Star Faculty
Program of MEST (Korea). M.C.Y. and S.C. are thankful for
fellowships of the BK 21 program from MEST (Korea). We thank
Prof. Hiromitsu Maeda and Takashi Hashimoto (Ritsumeikan
University) for MALDI-TOF MS measurement.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804570.
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Scheme 2. Synthesis of 3 and 4.
and gave hexaphyrin 3 (17%) and meso-oxygenated hexa-
phyrin 4 (15%). The yields of isolated 3 and 4 varied
depending upon how quickly the workup and separation were
carried out; this is because 3 gradually converted into 4 by
oxygenation in aerated solution.
High-resolution electrospray ionization time-of-flight
(HR ESI-TOF) mass spectrometry revealed the parent ion
peak of 3 at m/z 1129.1178 (calcd for C54H17F20N6 ([M+H]+):
1
1129.1190). The H NMR spectrum of 3 was quite simple,
Figure 1. X-ray crystal structures of a) 3, b) 4, and c) 5; top views (left)
and side views (right). Solvent molecules and TFA counter anions in
top view of 5 were omitted for clarity. The thermal ellipsoids are drawn
at the 50% probability level.
featuring a singlet ascribed to the inner meso protons at d =
À4.97 ppm, a singlet corresponding to the inner NH protons
at d = À0.37 ppm, a pair of doublets from the outer b protons
at d = 9.03 and 10.49 ppm, and a singlet resulting from the
outer b protons at d = 9.08 ppm.[15] These signals clearly
indicate a type I conformation and a strong diatropic ring
current arising from its 26p-electronic conjugation circuit.
The structure of 3 was confirmed by X-ray crystallographic
analysis (Figure 1a).[14] All the pyrrole rings are pointing
inward, among which the two pyrrole rings at the short side
were assigned as being of the amino type and the remaining
four pyrrole rings as being of the imino type, based on the Ca-
N-Ca angles of the pyrrole ring.[15] As expected, the two
inward-pointing meso hydrogen atoms do not cause severe
steric repulsion, so that 3 takes on a flat shape as charac-
terized by a small mean plane deviation (0.109 ꢁ), which is
much smaller than that of 1 (0.536 ꢁ).[7] The harmonic
oscillator model of aromaticity (HOMA) value, a measure of
bond-length alternation and therefore of aromaticity,[16] has
been calculated to be 0.783 ꢁ for 3, thus indicating it
possesses a well delocalized macrocycle. The nucleus-inde-
pendent chemical shift (NICS) value[17] at the center of the
molecule has been calculated to be d = À15.3 ppm and is in
line with its strong aromaticity. The strong aromaticity of 3 is
also clearly indicated by its UV/Vis absorption spectrum. The
Soret-like transition is observed at 549 nm (e =
380000mÀ1 cmÀ1) as a distinctly sharper band than that of 1,
and Q-like bands are observed in the low-energy region with a
clear vibronic structure as a signature for the aromatic
expanded porphyrins (Figure 2). The fluorescence peak of 3
is observed at 1089 nm with a very small Stokes shift
(76 cmÀ1), thus suggesting similar geometries between the S1
and S0 states.[15] Although oxygenation at the free meso po-
sitions proceeds gradually in solution, 3 can be stored as a
Figure 2. UV/Vis absorption spectra of 1, 3, 4, and 6 in CH2Cl2.
solid under inert conditions for at least several months
without any deterioration, hence constituting the first stable
meso-free hexaphyrin.
To our surprise, meso-oxygenated hexaphyrin 4, which
was formed as a secondary side product of 3, was identified as
an extremely stable radical species. Also, 4 was quite
unreactive to air and water at ambient temperature and
thus could be manipulated like usual stable organic com-
pounds. The HR MALDI-TOF mass spectrum of 4 exhibits
the parent ion peak at m/z 1145.1252 and 1145.1266 under
positive and negative ionization conditions, respectively. The
elemental analysis shows its composition to be C 56.52%, H
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1.42%, and N 7.30% in agreement with the molecular
formula of C54H17F20N6O (calcd: C 56.61%, H 1.50%, N
7.34%), thus indicating no counterion or solvent molecule to
obtained, which shows the parent ion peak at m/z 1145.1135
1
(calcd for C54H17F20N6O ([M+H]+): 1145.1139). Its H NMR
spectrum in CD2Cl2 exhibits twelve signals in the range d =
8.95–7.30 ppm ascribed to the b protons, three broad signals
1
be present in the solid form of 4. Importantly, its H NMR
À
À
spectrum exhibits no signal. The radical character of 4 was
confirmed by ESR and magnetic susceptibility measurements.
The mean g value was determined to be 2.0089 by ESR
spectroscopy in the solid state at room temperature. The
temperature-dependent magnetic susceptibility c value was
measured over the range 2–300 K. In agreement with its
monoradical character, 4 exhibits cT= 0.364 emuKmolÀ1 at
300 K, which is slightly smaller than the expected value of
0.375 emuKmolÀ1.[15] The observed temperature dependence
of the c value was reproduced by the Curie–Weiss model with
the Weiss temperature q = À8.0 K. A negative q value
indicates an intermolecular antiferromagnetic interaction.[15]
The structure of 4 was determined by X-ray crystallographic
analysis (Figure 1b).[14] While the type I conformation of the
macrocycle is preserved, one of the meso positions is actually
of N H and O H protons around d = 7.6, 6.05, and 4.86 ppm,
and a singlet corresponding to a meso proton at d =
1.69 ppm.[14] These data indicate a diatropic ring current for
6. Alternatively, 6 can be quantitatively prepared by the
oxidation of 4 with MnO2.
The electrochemical characteristics of 3 and 4 were
studied by cyclic voltammetry (Figure 3). As the oxidation
processes were complicated, here we will only focus on the
À
oxygenated to form an inward-oriented C O bond with a
bond length of 1.281 ꢁ. This bond length is between those of
C O single bonds and C O double bonds.[18] The mean plane
deviation is somewhat large (0.211 ꢁ), although the HOMA
value is considerably large (0.744), thus emphasizing the well
delocalized nature of p electrons on the macrocycle, and
which may be responsible for the unique stable radical
character of 4. There are diverse stable organic radicals
reported in the literature,[19] but 4 provides a rare example
where the delocalization of the radical over a pyrrolic
macrocycle leads to a special stabilization.
À
À
Figure 3. Cyclic voltammograms of 3 (top) and 4 (bottom) in CH3CN
containing 0.1m of TBAPF6. Scan rate 0.1 VsÀ1. TBA=tetra-n-butyl-
ammonium.
Radical 4 was considerably stable under ambient con-
ditions but changed to a diamagnetic species upon dissolution
in deuterated trifluoroacetic acid ([D]TFA), as indicated by
reduction steps. Hexaphyrin 1 undergoes reversible one-
electron reduction at À0.52 and À0.85 V versus the ferrocene/
ferrocenium ion (Fc/Fc+) couple,[6c] whereas 3 and 4 show
three reversible reduction potentials at À0.63, À0.93, À1.65 V,
and at À0.62, À1.28, À1.78 V, respectively. Only the radical 4
displays a reversible wave at À0.14 V, which was assigned as
the first oxidation step because the absorption spectrum of
state (b) is identical with that of 4. Meanwhile, the absorption
spectrum of state (a) exhibits a new feature that includes a
Soret-like band at 538 nm and a broad band around 800–
900 nm,[15] which has been assigned to a one-electron oxidized
species of 4. In a separate experiment, titration of 6 against
TFAwas observed to proceed in a stepwise manner to provide
monoprotonated and diprotonated species. Comparison of
the absorption spectra indicates that the one-electron-oxi-
dized species of 4 corresponds to the monoprotonated 6 and
the keto-dication 5 corresponds to the diprotonated 6.
Accordingly, an electrochemical gap
1
its sharp H NMR spectrum. The spectrum displays twelve
different signals in the range d = 7.69–6.57 ppm ascribed to
the pyrrolic b protons and a singlet at d = 7.99 ppm corre-
sponding to the inner meso proton, thus indicating a non-
symmetric and nonaromatic structure. Based on these data,
the diamagnetic species was assigned to be keto-hexaphyrin
dication 5, whose structure was confirmed by the X-ray
analysis of crystals grown from a [D]TFA solution (Figure 1c
and Scheme 3).[14] The C O bond length is 1.242 ꢁ, which is
À
À
slightly shorter than that of 4. The C C bonds adjacent to the
À
C O bond are 1.456 and 1.455 ꢁ, respectively, and are longer
than those of 4 by about 0.04 ꢁ. Two trifluoroacetate ions are
included in the crystal structure and are required for charge
balance. The nonsymmetric protonation of 5 is consistent with
1
its H NMR spectrum. After neutralization of this solution
with aqueous Na2CO3, meso-hydroxy hexaphyrin 6 was
between the first oxidation and reduc-
tion potential of 4 is only 0.48 eV, in line
with the nonbonding character of the
frontier orbital.
The singlet excited-state lifetimes
of 3 and 4 in toluene were determined
by femtosecond transient absorption
spectroscopy, and they show a dramatic
Scheme 3. Transformation of 4 into 5 and then 6.
change in decay times (Figure 4). The
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Communications
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Figure 4. The time profiles showing depopulation of the lowest excited
state of 3 and 4 as measured by transient absorption spectroscopy.
observed S1-state lifetime of 3 was found to be 138 Æ 5 ps,
which is slightly longer than that of 1 (98 ps),[20] this is due to
the more planar and rigid geometry of 3 as revealed by the X-
ray structure (Figure 1a). Interestingly, the population of the
lowest excited D1 state of 4 decays rapidly with a time
constant of 13.9 Æ 1 ps, which is about ten times faster than
that of 3. The stable radical character of 4 probably reflects
the greater density of states resulting from the presence of
unoccupied orbitals—a feature that distinguishes this species
from 3. Thus, the overall excited-state dynamics of 4 is
governed by the ultrafast internal conversion processes, which
may be accelerated by its radical character. The particularly
short excited-state lifetime of 4 is in good agreement with its
lack of near-infrared fluorescence.[15] Here, it should be noted
that the photophysical properties of the radical species have
only been investigated in a limited manner because of their
intrinsic instability. All the same, this is the first report on such
species of expanded porphyrins.[21]
In summary, we have synthesized 3 as the first example of
meso-unsubstituted [26]hexaphyrin(1.1.1.1.1.1) and con-
firmed its type I spectacles-like conformation with strong
aromaticity. In aerated solution, 3 was oxygenated to 4, which
has been identified as an exceptionally stable radical species.
These results demonstrate that large structural and electronic
changes in [26]hexaphyrin can be caused by the simple
removal of two meso-aryl substituents. Also, [26]hexaphyrins
offer a nice platform to confer a particular stability to a
monoradical probably owing to its delocalized electronic
nature. Exploration of expanded porphyrins bearing free
meso positions is actively in progress.
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[14] Crystal data for 3: C54H16F20N6 (Mr = 1129), monoclinic, space
group P21/c (No. 14), a = 13.653(5), b = 10.545(3), c =
18.317(7) ꢁ, b = 106.974(15)8, V= 2522.3(14) ꢁ3, Z = 2, 1calcd
=
1.486 gcmÀ3, T= 123(2) K, R1 = 0.0459 (I > 2s(I)), RW = 0.1534
(all data), GOF = 1.119. In this crystal structure there are
disordered solvent molecules, and their contribution to the
scattering values have been removed by using the PLATON
SQUEEZE program.[22] Crystal data for 4: C54H17F20N6O-
(C7H16)(C6H5CH3)
group Pbcn (No. 60), a = 10.8232(10), b = 35.604(3), c =
31.278(3) ꢁ, V= 12053.0(19) ꢁ3, Z = 8, 1calcd = 1.513 gcmÀ3
T= 90(2) K, R1 = 0.0783 (I > 2s(I)), RW = 0.2492 (all data),
GOF = 1.027. Crystal data for 5: C63.87H18F34.80N6O11.16 (Mr =
(Mr = 1374),
orthorhombic,
space
,
¯
1709), triclinic, space group P1 (No. 2), a = 12.647(5), b =
Received: September 17, 2008
Published online: November 10, 2008
14.656(7), c = 19.176(7) ꢁ, a = 69.389(14), b = 76.860(15), g =
77.937(17)8, V= 3207(2) ꢁ3, Z = 2, 1calcd. = 1.770 gcmÀ3
, T=
123(2) K, R1 = 0.0995 (I > 2s(I)), RW = 0.3290 (all data),
GOF = 1.218. Since the crystals of 5 were grown from [D]TFA,
Keywords: aromaticity · hexaphyrins · porphyrinoids · radicals
.
À
N H protons in the crystal structure were replaced by deuterium
atoms. CCDC 693211 (3), 693212 (4), and 693213 (5) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Rebouꢄas¸ I. Spasojevic, I. B. -Haberle, J. S. Lindsey, Bioorg.
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www.angewandte.org 9665