Selective formation of a single atropisomer of meso-meso-linked Zn II diporphyrin through supramolecular self-assembly

Article abstract of DOI:10.1002/chem.200901475

One-way atropisomerism: ineso-(4-Butoxypyrid-3-yl)-substituted Zn ^II porphyrins self-assemble to form zigzagshaped cyclic hexamers, whereas the meso-meso-linked Zn^II diporphyrins 1 undergo self-sorting self-assembly to form different assemblies. Upon heating, entropically driven one-way atropisomerism to 1_in-in has been achieved through fragmentary reconstitution of supramolecular aggregates.

Full text of DOI:10.1002/chem.200901475

COMMUNICATION
DOI: 10.1002/chem.200901475
Selective Formation of a Single Atropisomer of meso–meso-Linked ZnII
Diporphyrin through Supramolecular Self-Assembly
Chihiro Maeda,[a] Taisuke Kamada,[a] Naoki Aratani,[a] Takahiro Sasamori,[b]
Norihiro Tokitoh,[b] and Atsuhiro Osuka*[a]
Atropisomerism is an intrinsic property of meso-aryl sub-
stituted porphyrins, which is thermally accessible and diffi-
cult to control, because there is usually no particular energy
gain for rotation of the meso-aryl group in either direction.[1]
But judicious control of por-
The 1H NMR spectrum of 4-butoxypyrid-3-yl-substituted
ZnII porphyrin 1 in CDCl3 exhibited a single set of signals
both for the pyridyl protons and the butoxy protons in a
shielded region (Figure 1), indicating the formation of a
phyrin atropisomerization could
be quite useful for the realiza-
tion of orientationally ordered,
functional molecular systems.[2]
Despite this expectation, only
limited examples have been
known for one-way atropisome-
rization, in which the atropiso-
merization control is driven by
favorable enthalpic interactions
between the host and guest.[3]
Figure 1. 1H NMR spectrum of 1 in CDCl3.
Metalloporphyrins that have
coordinating side arms can be
used for the construction of well-defined supramolecular ar-
chitectures.[4] By using this strategy, we have explored self-
sorting, self-assembly of meso-pyridine appended meso–
meso-linked ZnII diporphyrins.[5] Here we examined the self-
assembling behavior of meso-3-pyridyl-substituted ZnII por-
phyrin and meso–meso-linked ZnII diporphyrins. In this
study, a moderately bulky butoxy group was introduced at
the 4-pyridyl position to set a considerable rotational barrier
at room temperature. Herein, we report self-sorting, self-as-
sembling, and entropically driven one-way atropisomeriza-
tion.
symmetric and complementarily coordinating discrete as-
sembly. The aggregated structure has been determined by
single-crystal X-ray diffraction analysis to be a cyclic hex-
americ wheel (Figure 2)[6a] in which all the porphyrin units
are identical and connected through a complementary coor-
dination network. In accordance with the observed upfield
shifts, each butoxy group is located over each porphyrin ring
as a result of the pyridyl coordination. The dihedral angle
between neighboring porphyrin rings is 65.78 and the center-
to-center distances are 8.8 and 9.4 ꢀ for an adjacent pair
and a neighboring pair, respectively, to give a unique zigzag
wheel structure. This assembly can be also detected by ab-
sorption spectroscopy (Figure 3). At low concentration, the
absorption spectrum of 1 showed a typical feature of a mon-
omeric ZnII porphyrin with a Soret band peak at 413 nm,
whereas the Soret band is split into two peaks at 413 and
422 nm at high concentration, indicating notable excitation
interactions between constituent porphyrins. In contrast, the
1H NMR and absorption spectra of 1 in pyridine indicate
that 1 exists as a monomer (see the Supporting Informa-
tion). This formation of a hexameric structure is different
[a] C. Maeda, T. Kamada, Dr. N. Aratani, 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
[b] Dr. T. Sasamori, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901475.
Chem. Eur. J. 2009, 15, 9681 – 9684
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9681

likely to be the possible smallest structure formed from
meso-3-pyridyl substituted ZnII porphyrin. Note that the 4-
butoxy-substituent introduced at the pyridyl group prevents
the formation of such a tetrameric assembly by steric hin-
drance and drives 1 to form the hexameric assembly.
In the next step, ZnII porphyrin 1 was coupled with AgPF6
in CHCl3 at reflux to provide the meso–meso-linked ZnII di-
porphyrins 3 (Scheme 1).[8] After demetallation, the free
Scheme 1. Synthesis of meso–meso-linked diporphyrins.
Figure 2. X-ray crystal structure of (1)6. a) Top view and b) side view. The
meso-aryl groups and hydrogen atoms are omitted for clarity. The ther-
mal ellipsoids were at the 50% probability level.
base diporphyrin 2 was obtained by gel permeation chroma-
tography (GPC) analysis in 41% yield, which consisted of
three atropisomers 2outÀout, 2inÀout, and 2inÀin. These atro-
pisomers, which are different in the orientation of the pyrid-
yl substitutents with respect to the meso–meso connection,
were separated over a silica-gel column.[9] But these isomers
underwent slow atropisomerism to reach a 1:2:1 mixture at
room temperature (see Figure S10 in the Supporting Infor-
mation). Respective ZnII metallation provided the ZnII com-
plexes 3outÀout, 3inÀout, and 3inÀin, respectively, which exhibited
1H NMR spectra in [D5]pyridine that was indicative of their
dissociation as a monomeric diporphyrin (see the Support-
ing Information).[10] These complexes showed similar atropi-
somerism to their free-base counterparts in pyridine.
In noncoordinating CHCl3, however, 3outÀout, 3inÀout, and
3inÀin self-assembled to form different aggregates A, B, and
C, respectively, which showed no interconversions at room
temperature, and ZnII metallation of the pristine mixture of
2 led to a roughly 1:2:1 mixture of A, B, C, indicating high
self-sorting self-assembly.[5] As found from the GPC reten-
tion times (Figure 4 a), the size of the aggregates becomes
larger in the order of C (22.6 min; 2-mer)<A (19.7 min; 10–
12-mer)<B (18.2 min; 24–28-mer).[11] Although the
1H NMR spectra of A and B are considerably broad owing
to their large sizes, the smallest aggregate C shows a rela-
Figure 3. UV/Vis absorption spectra of 1 at 10À4 ~10À5 m in CHCl3. Inset
shows the concentration-dependent absorption coefficient change at
542 nm and a theoretical curve fitting analysis for hexamer formation
with k6 =2.0ꢂ1024 mÀ5
.
from the previously reported tetrametric zigzag-shaped as-
semblies formed from ZnII porphyrins that contain a meso-
3-pyridyl substituent.[7] The tetrameric zigzag assembly is
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meso–meso-Linked ZnII Diporphyrins
COMMUNICATION
A, B, and C, but the aggregates
A and B are thermally more
labile and tend to be fragment-
ed, contributing to the accumu-
lation of C. As a consequence,
one-way atropisomerism to
3inÀin is entropically driven.
Note that the heating of 3outÀout
,
3
inÀout, and 3inÀin in pyridine at
808C for 5 h led to the nonse-
lective statistical population of
3
outÀout, 3inÀout, and 3inÀin in a
1:2:1 ratio. These results indi-
cate a relatively low rotational
barrier for a meso-butoxy-3-pyr-
idyl substituent in nonaggregat-
ed dimers, whereas such barri-
Figure 4. a) Analytical GPC-HPLC chromatograms (eluent: CHCl3; c=the pristine mixture of 3; g=2; ers become higher in the aggre-
gates.[13] More importantly, the
one-way atropisomerism to
c=A, B, and, C. The band intensities of A, B, and C were adjusted to simulate the pristine mixture).
b) Thermal conversion of the pristine mixture to C in toluene at 1108C for 24 h.
1
tively simple and sharp H NMR spectrum in which the sig-
nals due to the Ha, Hb, and Hc protons are observed at 1.93,
3.14, and 5.53 ppm, respectively, indicating the coordination
of pyridyl groups to zinc ions.
The aggregates A, B, and C are considered to be all race-
mic. Although the optical resolutions of A and B were un-
successful, C was actually separated through a chiral HPLC
column (SUMICHIRAL OA-3100) into two fractions C1
and C2 in the order of elution. These two fractions are
mirror images of each other in the circular dichromism
(CD) spectra (see Figure S12 in the Supporting Informa-
tion). The structure of C1 was determined by single-crystal
X-ray diffraction analysis (Figure 5),[6b] indicating that C
consists of two molecules of 3inÀin that are complementarily
coordinated in a face-to-face fashion. The meso-substituted
pyridine rings are inclined by 17.88 towards the coordinating
direction, the central zinc atoms are displaced by 0.4 ꢀ from
the porphyrin mean plane, the dihedral angle of the meso–
meso-linked diporphyrin is 83.48, and the interplanar dis-
tance of the two porphyrin planes is 4.8 ꢀ. This X-ray struc-
ture revealed that C1 consists of two R isomers.
Figure 5. X-ray crystal structure of C1. The meso-aryl groups and hydro-
gen atoms are omitted for clarity. The thermal ellipsoids were at the
50% probability level.
The aggregates, A, B, and C were stable at room tempera-
ture, but upon heating at 1108C in toluene, the large aggre-
gates A and B were converted into the small aggregate C as
monitored by analytical GPC–HPLC (Figure 4 b) and
1H NMR spectroscopic measurements (see the Supporting
Information), whereas C was quite stable under the heating
conditions.[12] This thermal conversion to C can be rational-
ized in terms of the entropy increase associated with the
fragmentary reconstitution of the larger aggregates A and B
to the small one C. Considering the dynamic supramolecular
nature of these aggregates, the most probable mechanism is
the dissociation of the aggregates to the respective mono-
mers and subsequent random atropisomerization to give
3inÀin has been achieved only through thermal fragmentary
reconstitutions of A and B to C. Another requirement for
this process is that the aggregate C is thermodynamically
more stable than its fragments at 1108C. Finally, the synthet-
ic merit of this process has been shown by isolation of 3inÀin
in more than 90% yield after full one-way atropisomerism.
In summary, meso-(4-butoxypyrid-3-yl)-substituted ZnII
porphyrins quantitatively assemble to form a discrete hex-
americ macroring and the meso–meso-linked ZnII diporphyr-
ins undergo self-sorting self-assembly to form the respective
discrete aggregates dictated by the orientation of meso-
pyrid-3-yl substituents with respect to meso–meso connec-
3
outÀout, 3inÀout, and 3inÀin, which all form respective aggregates
Chem. Eur. J. 2009, 15, 9681 – 9684
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www.chemeurj.org
9683

A. Osuka et al.
[6] Crystallographic data for (1)6: formula C258H258N30O6Zn6; Mr =
tion. One-way atropisomerism has been achieved through
the fragmentary reconstitution of the supramolecular aggre-
gates to form the smallest assembly.
¯
4267.16; tricrinic; space group P1; a=20.531(3), b=20.913(3), c=
22.602(4) ꢀ; a=65.044(2), b=65.706(2), g=64.760(2)8; V=
7634(2) ꢀ3; Z=1; 1calcd =1.217 gcmÀ3; T=À1838C; 38538 measured
reflections; 26158 unique reflections (Rint =0.0545); R1 =0.0759 (I>
2s(I)); wR2 =0.2040 (all data); GOF=0.875. Crystallographic data
for (3inÀin)2: formula C172H168N20O4Zn4; Mr =2840.74; tetragonal;
space group P43212; a=25.440(4), b=25.440(4), c=62.575(13) ꢀ;
V=40497(11) ꢀ3; Z=2; 1calcd =0.982 gcmÀ3; T=À1708C; 346836
measured reflections; 37056 unique reflections (Rint =0.1103); R1 =
0.0807 (I>2s(I)); wR2 =0.2332 (all data); GOF=1.028. The contri-
bution to the scattering arising from the presence of the disordered
solvents in the crystals was removed by use of the utility SQUEEZE
in the PLATON software package.[14] CCDC-668107 (1)6 and 668108
Acknowledgements
This work was supported by Grants-in-Aid (No. 19205006 (A) and
20108001 “pi-Space“) from MEXT, Japan. C.M. thanks the JSPS fellow-
ship for young scientists.
Keywords: atropisomerism
self-assembly · self-sorting
· entropy · porphyrinoids ·
(3inÀin
) contain the supplementary crystallographic data for this
2
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
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Received: June 2, 2009
Published online: August 18, 2009
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Chem. Eur. J. 2009, 15, 9681 – 9684