Multiple conformational changes of β-tetraphenyl meso- hexakis(pentafluorophenyl) substituted [26] and [28]hexaphyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1039/b910167e

β-Tetraphenyl meso-hexakis(pentafluorophenyl) substituted [26]hexaphyrin 3 is conformationally flexible between rectangular and figure-of-eight shapes and its two-electron reduced [28]hexaphyrin 4 takes figure-of-eight conformations, which are changed, up

Full text of DOI:10.1039/b910167e

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Multiple conformational changes of b-tetraphenyl
meso-hexakis(pentafluorophenyl) substituted [26] and
[28]hexaphyrins(1.1.1.1.1.1)w
Taro Koide, Katsuyuki Youfu, Shohei Saito and Atsuhiro Osuka*
Received (in Cambridge, UK) 22nd May 2009, Accepted 29th July 2009
First published as an Advance Article on the web 21st August 2009
DOI: 10.1039/b910167e
b-Tetraphenyl meso-hexakis(pentafluorophenyl) substituted
[26]hexaphyrin 3 is conformationally flexible between rectangular
and figure-of-eight shapes and its two-electron reduced
[28]hexaphyrin 4 takes figure-of-eight conformations, which
are changed, upon protonation, to twisted conformations with
[26]Hexaphyrin 3 was prepared by the condensation reaction
of dipyrromethane-dicarbinol 58 and 3,4-diphenylpyrrole (6).9
An equimolar mixture of 5 and 6 was treated with methane-
sulfonic acid at room temperature for 1 h followed by the
oxidation with DDQ for 3 h. Chromatographic separation
over a silica gel column gave 3 in 20% yield (Scheme 2). The
crystal structure of 3 has been revealed by X-ray crystallo-
graphic analysis to be a planar rectangle similar to that of 1
(Fig. 1). Four b-phenyl groups are found at the diagonal
peripheral b-positions. Quite unexpectedly, however, the 1H
NMR spectrum of 3 is very broad at room temperature,
suggesting rapid conformational changes that are comparable
to the NMR measurement time scale. As the temperature was
lowered, the signals became sharper, featuring two sets of
signals due to different conformations at ꢀ60 1C. One set
includes a singlet at ꢀ0.53 ppm due to the N–H proton and
two sets of doublets at 9.02 and 8.67 ppm, and ꢀ0.45 and
ꢀ0.56 ppm due to the outer and inner b-protons, respectively.
This has been ascribed to a flat rectangular conformation 3A
that is observed for the crystal. The other set exhibits the NH
proton at 9.01 ppm and the b-protons as two sets of doublets
at 6.84 and 6.72 ppm, and 5.44 and 4.31 ppm. This set has been
assigned as a non-aromatic figure-of-eight conformation 3B
on the basis of the chemical shifts and the simplicity of the
spectrum. The moderately highfield shifted signals of the
b-protons may suggest a small diatropic ring current also for
3B.10 At ꢀ60 1C conformational exchange is slow enough to
see two separate sets of resonances. Here, it is to be noted that
the conformational flexibility of 3 is likely larger than that of 1,
whose rectangular conformation is rather stable in a wide
range of temperature.5 A possible explanation is that the newly
introduced four b-phenyl substituents endow additional
stabilization for the figure-of-eight conformations via p–p
interaction between the b-phenyl and meso-pentafluorophenyl
substituents.
¨
distinct Mobius aromaticity.
Increasing attention has been focused on expanded porphyrins
that are conjugated macrocycles consisting of more than
five pyrrolic subunits because of their diverse intriguing
electrochemical, optical, structural, and coordination properties.1
Among these, meso-hexakis(pentafluorophenyl) substituted
[26]hexaphyrin (1)2 serves as a benchmark molecule, exhibiting
a planar molecular shape, strong aromaticity arising from the
26p-electronic system, and versatile metalation behaviors.3
Curiously, the metalation of 1 with group 10 metals (Ni(II),
Pd(II), and Pt(II)) led to the formation of Mobius aromatic
¨
molecules with twisted topology.4,5 Another important
chemical reactivity is the reversible interconversion between
1 and [28]hexaphyrin 2 through reduction with NaBH4 and
oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ). Recently, [28]hexaphyrin 2 has been revealed to be a
unique macrocycle that exists as a dynamic mixture of
several rapidly interconverting Mobius aromatic and Huckel
¨
¨
antiaromatic conformers in solution at room temperature
(Scheme 1).6
Despite these attractive features, the chemistry of meso-aryl
[26] and [28]hexaphyrins has been rather limited to the
parent molecules 1 and 2, because of the lack of available
synthetic methods to fabricate the hexaphyrins. Particularly,
peripherally modified [26] and [28]hexaphyrins have been only
scarcely reported so far.7 It was thought that introduction of
substituents at the peripheral b-positions would be a useful
means to change the electronic and structural properties,
hence contributing to expand the chemistry of meso-aryl
expanded porphyrins. In this communication, we report the
synthesis of b-tetraphenyl substituted [26]- and [28]-hexaphyrins
Reduction of
3 with NaBH4 gave [28]hexaphyrin 4
quantitatively (Scheme 3). The crystal structure of 4 has been
determined to be figure-of-eight, which is likely stabilized by
favorable stacking interaction between the b-phenyl groups
3
and
4 and their intriguing multiple conformational
changes.z
Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
E-mail: osuka@kuchem.kyoto-u.ac.jp; Fax: +81 75-753-3970;
Tel: +81 75-753-4008
w Electronic supplementary information (ESI) available: Experimental
section, spectroscopic data and NMR spectra. CCDC 728271–728273.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b910167e
Scheme 1
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Chem. Commun., 2009, 6047–6049 | 6047

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Scheme 2
Fig. 1 X-Ray crystal structures of 3 (a) top view and (b) side view
and 4 (c) top view and (d) side view. Thermal ellipsoids are scaled to
50% probability level. In the side views, meso-pentafluorophenyl
substituents are omitted for clarity.
and meso-pentafluorophenyl groups. Hydrogen bonding
between the pyrrolic amino-NH and imino-N are also
supporting the figure-of-eight conformation (Fig. 1). At room
temperature the 1H NMR spectrum of 4 in CDCl3 showed two
sets of signals due to the N–H protons at 14.68 and 14.05 ppm
as sharp signals and at 16.12 and 13.15 ppm as broad signals in
a ratio of 1 : 0.72, suggesting that two major figure-of-eight
conformers are present in solution. In a region of 8.42–5.56 ppm,
a set of b- and phenyl-protons were observed as sharp signals,
and the other set was observed as broad signals, which became
sharper as the temperature was decreased.
Scheme 3
The UV/vis absorption spectrum of 3 in CH2Cl2 exhibits
split Soret-like bands at 580 and 619 nm and Q-like bands at
731, 796, 896, and 946 nm (Fig. 2), in line with its aromaticity.
The splitting of Soret-like bands reflects its molecular
symmetry, since two transition dipole moments along the
diagonal axes are not equivalent. The absorption spectrum
of 4 is ill-defined, being characteristic of non-aromatic or
antiaromatic porphyrinoids.10 The intensity of the Soret-like
bands of 3 is not so prominent as compared with that of 1,
probably as a consequence of non-negligible contribution of
non-aromatic figure-of-eight conformations.
Fig. 2 Absorption spectra of 3 (black solid) and 4 (blue solid) in
CH2Cl2 and spectral changes for titration experiments of 4 with TFA.
protonation step to form 4ꢂTFA1 up to [TFA] = 4.5 ꢃ 10ꢀ4 M.
Upon further addition of TFA, the Soret-like band was
slightly more red-shifted to 631 nm and the Q-like bands
appeared at 861 and 967 nm. These secondary spectral
changes, which strongly indicated the formation of 4ꢂTFA2,
occurred with different isosbestic points up to [TFA] = 4.2 ꢃ
10ꢀ3 M (Fig. 2, ESIw). The absorption spectra of 4ꢂTFA1 and
4ꢂTFA2 indicate their strong aromaticity. The protonation
steps were also examined by 1H NMR spectroscopy. The
In the next step, we examined the protonation-induced
conformational change of 4 using trifluoroacetic acid (TFA)
in CH2Cl2, since protonation has been shown to be effective in
inducing twisted conformations of Mobius aromaticity for
¨
meso-aryl substituted [32]heptaphyrins.11 Addition of TFA
indeed induced the absorption spectral changes including the
appearance of
a prominent sharp Soret-like band at
617 nm and Q-like bands at 828, 869, 928, and 988 nm with
isosbestic points at 365, 414, 569, 683, and 744 nm as the first
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6048 | Chem. Commun., 2009, 6047–6049

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These values have been obtained by removal of the solvent molecules
by using the utility SQUEEZE in PLATON software package.
SQUEEZE-PLATON: (a) ref. 12; (b) ref. 13. CCDC 728271.
Crystallographic
Mw = 1989.72, monoclinic, P21/c (no. 14), a = 13.6922(11) A,
47.412(4) A, 12.5608(10) A, 90.791(2)1,
V = 8153.4(11) A3, Dc = 1.621 g cmꢀ3, Z = 4, T = 90(2) K,
42 183 reflections measured, 14 315 unique (Rint 0.0662),
data
for
4:
C
90H32F30N6ꢂ2.62(CH2Cl2),
b
=
c
=
b =
=
R1 = 0.0975 (I 4 2.0s(I)), wR2 = 0.2117 (all data), GOF = 1.207
(I 4 2.0s(I)). CCDC 728273. Crystallographic data for 4-TFA2:
C
90H34F30N6ꢂ2(C2HF3O2)ꢂ2(C2F3O2)ꢂ3(CH2Cl2), Mw
= 2478.11,
monoclinic, P21/c (no. 14), a = 13.8248(14) A, b = 21.260(2) A,
c = 34.396(4) A, b = 100.059(2)1, V = 9953.8(18) A3, Dc = 1.654 g
cmꢀ3, Z = 4, T = 90(2) K, 60 223 reflections measured, 22 188 unique
(Rint = 0.0415), R1 = 0.0777 (I 4 2.0s(I)), wR2 = 0.2445 (all data),
GOF = 1.015 (I 4 2.0s(I)). CCDC 728272.
Fig. 3 X-Ray crystal structure of 4ꢂTFA2. Thermal ellipsoids are
scaled to 50% probability level. meso-Pentafluorophenyl groups,
b- and phenyl-protons, and solvent molecules are omitted for clarity.
1H NMR spectrum of 4ꢂTFA1 in CD2Cl2 was very broad at
room temperature, indicating rapid conformational inter-
conversion among several aromatic species (ESIw). The
spectrum became sharpened at ꢀ80 1C, showing two sets of
signals in a ratio of 1 : 0.95 due to the inner pyrrolic b-protons
in a shielded region. These data indicated the presence of two
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1H NMR spectrum displayed a different sharp spectrum that
contains two sets of signals due to the inner b-protons in a
shielded region in a ratio of 1 : 0.88 (ESIw). This spectrum
3 (a) S. Shimizu and A. Osuka, Eur. J. Inorg. Chem., 2006, 1319;
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indicates the presence of two twisted conformations of Mobius
¨
aromaticity for 4ꢂTFA2, which differ possibly in position of
b-phenyl groups. Upon neutralization with aqueous NaHCO3,
4ꢂTFA2 reverted to 4 with concurrent conformational changes.
Fortunately, crystals suitable for X-ray analysis were
obtained from slow diffusion of n-hexane into a solution of
4 in CH2Cl2–TFA. The structure of this crystal, possibly
4 (a) H. S. Rzepa, Chem. Rev., 2005, 105, 3697; (b) R. Herges, Chem.
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Chem., 2009, 1, 113; (d) D. Ajami, O. Oeckler, A. Simon and
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Grazynski, N. Sprutta, P. Chwalisz and L. Szterenberg, Angew.
4ꢂTFA , was revealed to be a twisted Mobius conformation
¨
2
´
Chem., Int. Ed., 2007, 46, 7869.
˙
held by hydrogen bonding between TFA molecules and N–H
protons in line with the above assignments (Fig. 3). Importantly,
the six pyrrolic segments in 4ꢂTFA2 are smoothly linked with
dihedral angles of less than 321 to constitute a 28p-electron
conjugated electronic circuit.
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H. Shinokubo, K. S. Kim, Z. S. Yoon, J.-Y. Shin, J. M. Lim,
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In summary, b-tetraphenyl substituted [26]hexaphyrin 3 was
prepared and has been shown to be conformationally flexible
among rectangular and figure-of-eight conformations. Its
two-electron reduced congener, [28]hexaphyrin 4, exists mainly
as a dynamic equilibrium of figure-of-eight conformations but
acquires Mobius aromaticity upon protonation. Attempts to
¨
use these multiple conformational changes for some signaling
or switching systems are actively in progress in our laboratory.
This work was supported by Grants-in-Aid (no. 19205006
(A) and 20108001 (p-Space)) for Scientific Research
from MEXT.
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Notes and references
z Crystallographic data for 3: C90H30F30N6ꢂ0.67(CHCl3), Mw
=
ꢀ
1844.78, triclinic, P1 (no. 2), a = 18.209(3) A, b = 18.259(3) A,
c = 19.016(3) A, a = 98.298(11)1, b = 105.095(10)1, g = 94.010(11)1,
V = 6002.0(17) A3, Dc = 1.531 g cmꢀ3, Z = 3, T = 123(2) K, 50 467
reflections measured, 18 798 unique (Rint = 0.0757), R1 = 0.0808
(I 4 2.0s(I)), wR2 = 0.2453 (all data), GOF = 0.800 (I 4 2.0s(I)).
12 A. L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 2005.
13 P. V. D. Sluis and A. L. Spek, Acta Crystallogr., Sect. A: Found.
Crystallogr., 1990, 46, 194.
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