Thiatriphyrin(2.1.1): A core-modified contracted porphyrin

Article abstract of DOI:10.1002/anie.201209678

Elusive in its free-base form, the core-modified contracted porphyrin thiatriphyrin(2.1.1) was prepared with p-tolyl substituents by intramolecular McMurry coupling and then converted into various alkoxy-substituted analogues (see scheme; DBU=1,8-diazabicyclo[5.4.0]undec-7-ene). In the presence of trifluoroacetic acid (TFA), each of these compounds was transformed into the protonated thiatriphyrin(2.1.1), which exhibited moderate aromaticity.

Full text of DOI:10.1002/anie.201209678

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DOI: 10.1002/anie.201209678
Contracted Porphyrins
Thiatriphyrin(2.1.1): A Core-Modified Contracted Porphyrin**
Daiki Kuzuhara, Yuka Sakakibara, Shigeki Mori, Tetsuo Okujima, Hidemitsu Uno, and
Hiroko Yamada*
Dedicated to the memory of Christian Claessens
Triphyrins are porphyrin analogues that contain three pyrrole
rings linked through meso sp2-hybridized carbon atoms. They
hold a unique position in porphyrin chemistry and are relative
newcomers. Inevitably, all subphthalocyanines 1[1] and sub-
porphyrins 2[2,3] have been boron complexes with nonplanar,
dome-shaped conformations (Scheme 1). Triphyrins have
demonstrated a variety of optoelectronic properties, such as
nonlinear optical absorption[4] and high emission quantum
yields,[4c,5] and have been applied in organic electronic
devices, such as organic light-emitting diodes[6] and organic
solar cells.[7] In 2008, we succeeded in preparing [14]triphyrin-
(2.1.1) (3) as a boron-free triphyrin with a near-planar
structure. The triphyrin was synthesized by the acid-catalyzed
condensation of a bicyclo[2.2.2]octadiene-fused pyrrole with
Scheme 1. Structures of subphthalocyanines 1, subporphyrins 2,
an aryl aldehyde,[8] or by the intramolecular McMurry
[14]triphyrin(2.1.1) (3), subpyriporphyrins 4, the thiatriphyrin(2.1.1)
coupling of diformyltripyrrane.[9] Recently, the condensation
of dipyrroethane and pentafluorobenzaldehyde to make
a meso-tetraaryltriphyrin was reported.[10] [14]Triphyrins-
(2.1.1) have a 14p-electron aromatic system composed
solely of pyrrole moieties and act as monovalent ligands. As
a result of their boron-free composition, they can be
converted into bowl-shaped MnI, ReI, and RuII complexes.[8b,9]
It is well-known that remarkable changes in the optical
and electrochemical properties and coordination abilities of
porphyrins can be induced by the core modification of
TTP (Ar=p-tolyl), and the protonated thiatriphyrin TTPH+ (Ar=
p-tolyl).
porphyrinoids.[11,12] It is therefore naturally expected that
the core modification of triphyrins will also lead to new
functionality. Subpyriporphyrin
4
(Ar1 = mesityl, Ar2 =
phenyl) has one pyridine ring in place of one of the three
pyrrole rings. Compound 4 and the corresponding subpyri-
porphyrin with a p-nitrophenyl group in place of the phenyl
group are the only metal-free triphyrin(1.1.1) analogues
reported to date.[13] The free-base form of this compound
exhibits no aromaticity; however, a 14p-electron ring current
is observed for its boron complex.
In this study, we attempted to synthesize a thiophene-
containing triphyrin, the [14]thiatriphyrin(2.1.1) TTP, which
should have a 14p-electron pathway within the macrocycle
without an inner NH group. We reveal that the core-modified
triphyrin is unstable in its neutral form and that thiatriphyrins
with a meso-alkoxy substituent (ORTTPs) were readily
formed. Interestingly, the treatment of these ORTTPs with
acid led to the elimination of the alkoxy group and generated
the protonated thiatriphyrin TTPH+. Herein, we discuss the
synthetic procedures as well as the unique reactivity, the
crystal structures, and the optical properties of these thia-
triphyrin derivatives.
[*] Dr. D. Kuzuhara, Y. Sakakibara, Prof. Dr. H. Yamada
Graduate School of Materials Science
Nara Institute of Science and Technology
Ikoma 630-0192 (Japan)
E-mail: hyamada@ms.naiat.jp
Prof. Dr. H. Yamada
CREST, JST, Chiyoda-ku, Tokyo 102-0075 (Japan)
Dr. S. Mori
Department of Molecular Science
Integrated Center for Sciences, Ehime University
Matsuyama 790-8577 (Japan)
Prof. Dr. T. Okujima, Prof. Dr. H. Uno
Department of Chemistry and Biology
Graduate School of Science and Engineering, Ehime University
Matsuyama 790-8577 (Japan)
[**] We thank S. Katao for the measurements of X-ray diffraction and
M. Yamamura and Y. Nishikawa for ESIMS measurements. This
research was partly supported by Grants-in-Aid (No. 24655034 to
H.Y. and D.K.) and the Green Photonics Project at NAISTsponsored
by the Ministry of Education, Culture, Sports, Science and
Technology, MEXT (Japan).
The synthetic route to TTP is shown in Scheme 2. Among
the reported synthetic methods for triphyrins, the intra-
molecular McMurry coupling turned out to be suitable for the
preparation of thiatriphyrins from diformylthiatripyrranes
such as 5.[14] The McMurry coupling reaction of 5 gave 5,10-
dihydro-5,10-di-p-tolylthiatriphyrin (6) in 50% yield. The
direct oxidation of 6 with of 2,3-dichloro-5,6-dicyano-1,4-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209678.
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TTP, the molecular orbitals of TTP were calculated at the
B3LYP/6-31G** level (see Figure S1 in the Supporting
Information).[16] Steric repulsion between the large sulfur
atom and the two lone electron pairs of the imine nitrogen
atoms in the small cavity make the TTP molecule nonplanar
and reduce the aromatic stability. Furthermore, the molecular
orbitals of the lone pairs of the inner nitrogen atoms were
calculated to be close to each other at the HOMO level. This
structure is unstable owing to the electronic repulsion of the
lone pairs,[17] and as a result, TTP is unstable and highly
reactive toward nucleophilic addition at the meso position. In
contrast, macrocycles 7a–c and 8 have a hydrogen bond
between the inner nitrogen atoms, as indicated by the N–N
distance and the orientation of the two pyrrole rings toward
one another. This hydrogen bond is very important in the
stabilization of these compounds (see Scheme S1 in the
Supporting Information).
Scheme 2. Synthesis of TTP and its derivatives.
The structure of 7a was characterized by 1H NMR
spectroscopy, high-resolution electrospray ionization time-
of-flight (HR ESI-TOF) mass spectrometry, and X-ray
crystal-structure analysis. The HR ESI-TOF mass spectrum
of 7a displays the parent-ion peak at m/z 489.1995 [M+H]+
(calcd for C32H29N2OS: 489.1922). The signals due to the
thienyl hydrogen atoms in the 1H NMR spectrum of 7a were
evident as two doublets at d = 7.52 and 6.95 ppm, and those of
the pyrrole hydrogen atoms appeared around d = 6.84–
6.02 ppm (Figure 2a). The signal for the inner NH hydrogen
atom was observed at d = 12.69 ppm owing to hydrogen
benzoquinone (DDQ; 2 equiv) in CHCl3 was initially
attempted and resulted in the formation of a small amount
of the thiatriphyrin 7a with an ethoxy group at the meso
position. We assume that the ethoxy group is derived from
ethanol present in the CHCl3 solvent. Next, we examined
a stepwise route. The oxidation of 6 with DDQ (1 equiv) in
CH2Cl2 gave the 5-hydrothiatriphyrin 8 in 50% yield. Com-
pound 8 is formally the partially oxidized triphyrinogen, and
1
its structure was confirmed by H NMR spectroscopy, mass
spectrometry, and X-ray diffraction analysis. The crystal
structure of 8 showed that the tolyl group at the C16 position
was oriented in the same direction as the sulfur atom
(Figure 1).[15]
The subsequent oxidation of 8 with DDQ (1 equiv) in the
absence of nucleophiles, such as ethanol, gave neither the
desired product TTP nor the starting material, but only
decomposition occurred. When the oxidation reaction was
performed in CH2Cl2 in the presence of methanol, however,
the methoxy-substituted product 7b was obtained in 35%
yield along with the corresponding isomeric product 7b-II in
very low yield. The addition of small amounts of either
ethanol or 2-propanol to the CH2Cl2 solvent afforded 7a and
7c in 41 and 20% yield, respectively. The corresponding
isomers 7a-II and 7c-II were not obtained. These results
indicate that TTP is highly reactive at the meso position
towards nucleophiles. To understand the high reactivity of
Figure 2. 1H NMR spectra of 7a a) in CDCl3 without TFA and b) in
CD2Cl2 with TFA (2.2 equiv) at À408C.
bonding with the neighboring nitrogen atom. These data
suggest that 7a has no continuous conjugation around the
three heterocyclic rings in its core structure. Single crystals of
7a suitable for X-ray diffraction analysis were obtained by the
slow evaporation of a solution in ethanol (Figure 3). The
ethoxy group at the C16 position is oriented in the same
direction as the sulfur atom relative to the plane of the
macrocycle. The bond lengths in the pyrrole moieties indicate
that one of the pyrrole units has the structure of an imine
À
(C9 N1: 1.320 ꢀ), whereas the other has an amine structure
Figure 1. Crystal structure of 8. Left: top view (hydrogen atoms except
À
(C12 N2: 1.409 ꢀ); this observation suggests a hydrogen-
À
À
for N H and C16 H are omitted for clarity); right: side view (hydro-
bonding interaction between the two inner nitrogen atoms.
The thiophene ring is significantly tilted out of the plane of
À
gen atoms except for C16 H are omitted for clarity). Thermal
ellipsoids represent 50% probability.
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Figure 3. Crystal structure of 7a. Left: top view (hydrogen atoms
À
except for N H are omitted for clarity); right: side view (hydrogen
atoms and aryl groups are omitted for clarity). Thermal ellipsoids
represent 50% probability.
Figure 4. Changes in the absorption spectrum of 7b in CH2Cl2 with
increasing amounts of TFA (0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.8,
3.5 equiv). Black line: 0 equivalents of TFA; red line: 3.5 equivalents of
TFA.
the macrocycle, with a dihedral angle between the thiophene
ring and pyrrole 1 (see Figure 3) of 69.538.
During thermogravimetric analysis, 7b exhibited an 8%
mass loss at 1628C (see Figure S2), which is in good agree-
ment with the predicted value of 7% expected for the
elimination of methanol from 7b to form TTP on heating. The
heating of 7b in a glass tube oven at 2008C, however,
produced only insoluble material rather than TTP. Interest-
ingly, when a solution of 7b in ethanol was heated at reflux for
12 h, 7a was obtained in quantitative yield. When we
attempted to obtain a single crystal of the structural isomer
7b-II from a solution in CH2Cl2/methanol, only 7b was
obtained. These results indicate that methanol was removed
to give TTP as an intermediate, but that the TTP reacted with
methanol immediately on account of its instability to revert
back to the substituted compound. According to DFT
calculations at the B3LYP/6-31G** level, the total energy of
7a is lower than that of its isomer 7a-II (see Figure S3).
The elimination of methoxy groups under acidic condi-
tions has been observed for subpyriporphyrins,[13a] an N-fused
porphyrin–boron complex,[18] dithiaethyneporphyrin,[19] and
a homoporphyrin–nickel complex.[20] We therefore checked
the reactivity of 7b for the production of protonated TTP
(TTPH+) under acidic conditions. The absorption spectrum of
7b showed two broad bands at 325 and 530 nm (see Fig-
ure S4). Upon the addition of trifluoroacetic acid (TFA) to
a solution of 7b in CH2Cl2, the absorption spectrum changed
according to the number of equivalents of TFA added, with
isosbestic points at 355, 492, and 570 nm (Figure 4). The broad
band at 530 nm decreased simultaneously with the increase in
the Soret-like band at 414 nm and the Q-like bands at 490,
525, and 574 nm. After the addition of 3.5 equivalents of TFA,
there were no further changes, and the spectrum was similar
to that reported for protonated triphyrins.[8a,10] These spectral
changes suggested that TTPH+ was generated from 7b in
response to the addition of the acid. Moreover, the treatment
of TTPH+ with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in
the presence of methanol in CH2Cl2 regenerated 7b (see
Figure S5). We also acquired the 1H NMR spectrum of
TTPH+ formed from a mixture of 7a and TFA (2.2 equiv)
in CD2Cl2 at À408C (Figure 2b; see also Figure S6). A singlet
peak corresponding to the thiophene moiety was observed at
d = 8.70 ppm, and another due to the hydrogen atoms on the
ethenyl bridge was observed at d = 7.44 ppm. The pyrrole
signals were shifted downfield relative to those for 7a. The
NH signal was observed at d = 9.42 ppm, and was thus shifted
upfield relative to that for 7a. It disappeared upon the
addition of D2O (see Figure S6). In the H–H COSY spectrum,
cross-peaks between the NH and pyrrolic signals were
observed (see Figure S7). The chemical shifts of these hydro-
gen atoms reflect a diatropic ring current associated with the
TTPH+ macrocycle.
A single crystal of TTPH+ [(CF3COO)2H]À was obtained
from a mixture of toluene, TFA, and water (Figure 5; see also
Figure S9). The thiophene ring is again tilted out of the plane
Figure 5. Crystal structure of TTPH+ [(CF3COO)2H]À. Left: top view
À
(hydrogen atoms except for N H and solvent molecules are omitted
for clarity); right: side view (hydrogen atoms, aryl groups, and solvent
molecules are omitted for clarity). Thermal ellipsoids represent 30%
probability.
of the macrocycle; the dihedral angles between the thiophene
ring and the neighboring pyrrole rings are 50.28 and 48.618,
both of which are smaller than in 7a. The lengths of the bonds
between the meso carbon atoms and the a carbon atoms in
À
the pyrrole and thiophene rings were 1.434(4) ꢀ for C4 C5,
À
À
1.415(4) ꢀ for C5 C6, 1.401(4) ꢀ for C15 C16, and
À
1.456(3) ꢀ for C16 C1. These values indicate that there is
no bond alternation; the bonds are slightly longer than those
of previously reported 14p-electron aromatic triphyrins.[8–10]
We also estimated the nucleus-independent chemical shift
(NICS(0)) values[21] of TTPH+ at several points within the
molecular plane on the basis of an optimized structure from
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Easwaramoorthi, J. M. Lim, D. Kim, A. Osuka, Chem. Eur. J.
the X-ray diffraction data (see Figure S10). The NICS(0)
value of TTPH+ at point “a” in Figure 5 is d = À11.37 ppm,
which indicates moderate aromaticity. Taken together, the
changes in the UV and 1H NMR spectra upon the addition of
acid, the single-crystal X-ray analysis data, and the NICS(0)
values suggest that the protonated compound TTPH+ has
a distinct 14p-electron aromatic structure.
When 8 was treated with TFA in CH2Cl2, a gradual
decrease in the absorption band at 558 nm was accompanied
by an increase in an absorption band at 640 nm (see
Scheme S11). These results indicate that protonation of the
second nitrogen atom (N2) occurred to generate 8H+. It thus
appears that the alkoxy leaving groups play an important role
during the conversion of 7a, 7b, or 7c into TTPH+ in the
presence of an acid.
In summary, we have synthesized several alkoxy-group-
substituted thiatriphyrins. Under acidic conditions, these
compounds can be converted into protonated TTPH+,
which shows moderate aromaticity. TTPH+ is the first
reported example of the generation of aromaticity in a core-
modified contracted porphyrin upon the protonation of its
free-base form. It was also determined that these alkoxy-
substituted thiatriphyrins have a stereogenic C16 center and
a NNS coordination site. The results of studies concerning the
control of the chirality and coordination abilities of these
compounds will be reported in the near future.
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Keywords: aromaticity · core modification · porphyrinoids ·
protonation · triphyrins
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