Internally 1,4-phenylene-bridged meso aryl-substituted expanded porphyrins: The decaphyrin and octaphyrin cases

Article abstract of DOI:10.1002/anie.200502769

Current affairs: Incorporation of a 1,4-phenylene bridge into meso aryl-substituted decaphyrin and octaphyrin has a profound impact on the structural and electronic properties of the macrocycles. Bridged decaphyrin has two oxidation states, one of which s

Full text of DOI:10.1002/anie.200502769

Communications
is realized in expanded porphyrins such as sapphyrins,[3]
rubyrins,[1d,4] and hexaphyrins.[5] However, such aromaticity
becomes increasingly difficult to obtain with larger expanded
porphyrins, particularly for systems larger than octapyrroles,
mainly because of intrinsic conformational distortion from
planarity.[2c,d,6] Rare examples are the 34p-electron core-
modified octaphyrin[1d,7] and 30p-electron cyclo[8]pyrrole,[8]
both of which exhibit planar conformations and strong
aromaticity, thus underscoring the influence of the confor-
mation of the expanded porphyrins upon their electronic
structures.
Several years ago we found that a series of meso aryl-
substituted expanded porphyrins could be prepared by a
simple one-pot reaction by using modified Rothemund–
Lindsey conditions.[5c,9] These meso aryl expanded porphyrins
can be regarded as real homologues of porphyrins in terms of
regular alternate arrangements of pyrrole rings and methine
carbon atoms. These molecules become more twisted with
increasing number of pyrrolic subunits. It then occurred to us
that suppression of this intrinsic twisting attribute may be a
promising means to tune their conformational and electronic
properties to generate large aromatic ring systems. We now
report the influence of an internal 1,4-phenylene bridge that
bisects large meso aryl-substituted expanded porphyrins—
particularly the cases of decaphyrin and octaphyrin—on their
structural and electronic properties.
A solution of one equivalent of 1,4-phenylene-bridged
bis(dipyrromethane) 1a[10] and two equivalents of tripyrrane
diol 2 in dichloromethane was stirred in the presence of p-
toluenesulfonic acid for 1 h, followed by oxidation with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (2.6 equiva-
lents relative to 1a) for an additional 1 h. Separation of the
products by column chromatography on silica gel gave
internally 1,4-phenylene-bridged [46]decaphyrin 4a as a
major product with a green metallic luster in 7% yield.
Neither the 1,4-phenylene-bridged pentaphyrin dimer nor its
derived products[11] were detected in the reaction mixture.
Treatment of 4a with 10 equivalents of DDQ resulted in its
quantitative conversion into [44]decaphyrin 5a, with a color
change from dark red to dark green. [44]Decaphyrin 5a was
in turn quantitatively reduced to 4a with NaBH4, thus
indicating a reversible redox interconversion. Electrospray
ionization mass spectrometry revealed a parent ion peak for
4a at m/z 2179.2163(calcd for C 104H31F40N10 [M+H]+, m/z
2179.2100) and that for 5a at m/z 2177.2032 (calcd for
Porphyrins
DOI: 10.1002/anie.200502769
Internally 1,4-Phenylene-Bridged meso Aryl-
Substituted Expanded Porphyrins: The
Decaphyrin and Octaphyrin Cases**
Venkataramanarao G. Anand, Shohei Saito,
Soji Shimizu, and Atsuhiro Osuka*
The chemistry of expanded porphyrins is the current focus of
intensive research because of their unique properties.[1,2]
Among their many expected properties, extensive aromaticity
C
104H29F40N10 [M+H]+, m/z 2177.1943).
[*] Dr. V. G. Anand, S. Saito, S. Shimizu, Prof. Dr. A. Osuka
Department of Chemistry
The structures of both 4a and 5a have been confirmed by
X-ray diffraction analyses. Decaphyrin 4a exhibits a C2-
symmetric nonplanar, but not a “figure-eight”, structure
(Figure 1),[12a] in which the two twisted pentapyrrolic subunits
are interconnected by the central 1,4-phenylene bridge. Each
pentapyrrolic arm consists of a dipyrromethene unit (pyrrole
rings A and B or F and G) and a tripyrrodimethene unit
(pyrrole rings C, D, and E or H, I, and J), both of which are
roughly planar but almost opposite each other. The central
1,4-phenylene unit (ring K) is tilted relative to the neighbor-
ing pyrrole rings A and J as well as E and F, with dihedral
angles of 60.18 and 59.88, respectively, hence allowing its
deconjugation from the rest of the conjugated network.
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
[**] This work was supported by Core Research for Evolutional Science
and Technology (CREST), Japan Science and Technology Agency
(JST). We thank Shinetsu Igarashi and Kenji Yoza, Bruker Japan, for
X-ray data. V.G.A. and S.S. thank the JSPS for a postdoctoral
fellowship and for a Research Fellowship for Young Scientists,
respectively.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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singlet at d = 3.78 ppm. These data
support a diatropic ring current in 4.
On the other hand, [44]decaphyrin
5a shows a D2-symmetric framework
that is distinctly different from that of
4a (Figure 2).[12b] Pairs of neighboring
pyrrole rings (A and B, D and E, Fand
G, and I and J) constitute planar
dipyrromethene units, and the pyrrole
rings C and
H
are completely
inverted, with the pyrrolic nitrogen
atoms pointing outward. Each penta-
pyrrolic arm adopts a twisted but
relatively planar structure, which con-
stitutes a so-called benzihexaphyrin
macrocycle. The arms are connected
by the central 1,4-phenylene spacer in
an almost perpendicular arrange-
ment. Decaphyrin 5a exhibits a well-
1
resolved H NMR spectrum at room
temperature, with the inner four NH
protons as a singlet at d = 13.6 ppm;
the outer b protons as doublets at d =
6.82 and 6.30 ppm and a multiplet at
d = 6.36 ppm; the inverted pyrrole
b protons as
a
singlet at d =
7.78 ppm, and the bridging 1,4-phen-
ylene protons as a singlet at d =
8.98 ppm. The protons of the 1,4-
phenylene bridge were assigned by
comparison with the 1H NMR spec-
trum of 5b. There is no distinct differ-
ence between the chemical shifts of
the inner and outer protons in 5a,
which is indicative of its non-aromatic
character, and in line with its 44p elec-
tronic circuit and solid-state structure.
The absorption spectrum of 4a shows
a Soret-like band at 505 nm and
strong Q-like split bands at 800 and
856 nm, while that of 5a exhibits
broad bands at 454 and 619 nm
(Figure 3). Interestingly, protonation
À
Interestingly, the two C(pyrrole-a) C(meso) bond lengths at
of 4a with trifluoroacetic acid (TFA) led to an intense and
sharp Soret-like band at 846 nm and a Q-like band at
1221 nm.
the bridging positions are quite similar, thus suggesting
effective conjugation over the overall macrocycle and in
line with the 46p electronic circuit. The 1H NMR spectrum of
4a was quite broad at room temperature, indicative of fast
conformational dynamics in solution relative to the NMR
time scale. However, the spectrum became well resolved at
233 K with distinct differences in chemical shifts evident
between the outer and inner protons of the macrocycle: the
outer two NH protons at d = 9.01 ppm versus the inner four
NH protons at d = 5.84 ppm; the outer b protons as two sets
of mutually coupled doublets at d = 7.36 and 6.84 ppm and
d = 7.31 and 7.19 ppm versus the inner b protons as a singlet
at d = 1.2 ppm. The protons of the 1,4-phenylene bridge,
assigned by comparison of the spectrum with that of the
corresponding deuterated decaphyrin 4b, appear as a sharp
We then examined the reaction of 1a with dipyrro-
methane diol 3. A solution of 1a and 3 in dichloromethane
and THF (3:1) was treated with methanesulfonic acid for 1 h
and the resulting solution was oxidized with p-chloranil for
2 h. Together with the expected 1,4-phenylene-linked por-
phyrin dimer[14] (1–2% yield) which appeared as a red band
by TLC analysis of the reaction mixture, two violet bands
were observed and the products were isolated in yields of 0.6
and 0.5%. Despite the low yields, the formation of these
products is quite reproducible when p-chloranil is used as the
oxidant and the reaction time of the secondary oxidation step
is maintained at 2 h. High-resolution electrospray ionization
mass spectrometry revealed the parent ion peak at m/z
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Figure 2. X-ray crystal structure of 5a: a) top view and b) side view.
Solvent molecules and meso-pentafluorophenyl groups are omitted for
clarity.
Figure 1. X-ray crystal structure of 4a: a) top view and b) side view.
Solvent molecules and meso-pentafluorophenyl groups are omitted for
clarity.
Figure 3. Absorption spectra of 4a, 4a + TFA, and 5a in dichloro-
methane.
octaphyrin with two sp3 meso carbon atoms at the diagonal
positions and overall C2h symmetry (Figure 4).[13] The bicyclic
macrocycle adopts a saddle conformation consisting of four
planar dipyrromethene units rather than a non-figure-eight
structure. The quinodimethane structure is evident from a
1693.1920 for 6a (calcd for C82H27F30N8 [M+H]+, m/z
1693.1874) and 1693.1915 for 7a (calcd for C82H27F30N8
[M+H]+, m/z 1693.1874). The absorption spectra of 6a and
7a are also quite similar to each other, with bands at 394, 593,
and 895 nm, and 392, 593, and 898 nm, respectively. The
structure of 6a has been unambiguously revealed by X-ray
diffraction analysis to be a p-quinodimethane-bridged calix-
À
distinctly shorter C(22) C(23) bond (1.367(4) Š) relative to
À
that of C(21) C(22) (1.430(4) Š). The angle C(23)*-C(21)-
C(22) is 115.6(2)8, which is distinctly smaller than that of
C(21)-C(22)-C(23) (122.7(3)8) and C(22)-C(23)-C(21)*
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The reaction of 1 with dipyrromethane diol 8 afforded
calixoctaphyrin 9 as a single major product as a violet solid in
3% yield. In the 1H NMR spectrum of 9 the internal p-
quinodimethane protons appear as a singlet at d = 4.65 ppm
and the gem-dimethyl protons as two singlets at d = 1.39 and
À1.03ppm. The high chemical shifts of one of the dimethyl
groups can be ascribed to its positioning just above the central
bridge. Therefore, a p-quinodimethane structure may be
considered to be a general concequence when a 1,4-phenylene
bridge is incorporated in meso aryl-substituted calixoctaphyr-
ins. In sharp contrast to 6a and 7a the calixoctaphyrin 9 is
quite stable under oxidizing conditions.
This study shows that the incorporation of a 1,4-phenylene
bridge into meso aryl-substituted expanded porphyrins has a
profound impact on the structural and electronic properties of
the macrocycles. Such modifications lead to suppression of
the intrinsic property to take a twisting conformation and
result in planar conformations. The structural modification
result in diatropic ring current in the decaphyrin case, and
forces a macrocycle to become calixoctaphyrin with an
internal p-quinodimethane bridge in the octaphyrin case.
Lastly, it is interesting to note that [46]decaphyrin is, to the
best of our knowledge, the largest macrocycle that exhibits a
diatropic ring current.
Received: August 5, 2005
Published online: October 17, 2005
Keywords: aromaticity · macrocycles · porphyrins ·
.
structure elucidation
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Solvent molecules are omitted for clarity.
(121.7(3)8). The exo-methylene moieties in the quinodimeth-
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structure of 7a is in good agreement with the NMR data,
with the central quinodimethane protons appearing as two
singlets at d = 5.21 and 4.85 ppm and the meso protons
appearing as two singlets at d = 6.42 and 2.45 ppm.
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[12] a) Crystallographic data for 4a:
C113H50Cl4F40N10O6, Mr =
¯
2545.43, triclinic, space group P1 (No. 2), a = 17.897(2), b =
18.824(2), c = 19.929(3) Š, a = 62.486(3), b = 78.326(3), g =
62.418(3)8, V= 5277.3(11) Š3, 1calcd = 1.602 mgmmÀ3
, Z = 2,
R1 = 0.1069, Rw (all data) = 0.3199, GOF = 1.044 (I > 2.0s(I));
b) crystallographic data for 5a: C254H160Cl8F80N20, Mr = 5343.62,
monoclinic, space group Cc (No. 9), a = 32.0109(8), b =
21.9539(5),
c = 37.7303(10) Š,
b = 111.0570(10)8,
V=
24744.9(11) Š3, 1calcd = 1.434 mgmmÀ3, Z = 4, R1 = 0.0956, Rw
(all data) = 0.2966, GOF = 1.028 (I > 2.0s(I)).
[13] Crystallographic data for 6a: C86H26F30N10, Mr = 1769.17, tri-
¯
clinic, space group P1 (No. 2), a = 11.002(5), b = 11.829(5), c =
16.816(10) Š, a = 77.742(18), b = 80.313(19), g = 62.352(14)8,
V= 1888.3(16) Š3, 1calcd = 1.556 mgmmÀ3
, Z = 1, R1 = 0.0678,
Rw (all data) = 0.1726, GOF = 1.063( I > 2.0s(I)). CCDC-
280276–280278 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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