Metalation of expanded porphyrins: A chemical trigger used to produce molecular twisting and Moebius aromaticity

Article abstract of DOI:10.1002/anie.200704407

(Chemical Equation Presented) Round the twist: Metalation of [36]octaphyrin 1 provided the Moebius aromatic Pd₂ complex 3 as well as the Hueckel antiaromatic Pd₂ complex 2. This method can be applied to other expanded porphyrins and Group 10 metal ions. The aromatic/antiaromatic character was supported by NMR spectrscopy, NICS calculation, and two-photon absorption measurements.

Full text of DOI:10.1002/anie.200704407

Angewandte
Chemie
DOI: 10.1002/anie.200704407
Aromaticity
Metalation of Expanded Porphyrins: A Chemical Trigger Used To
Produce Molecular Twisting and Möbius Aromaticity**
Yasuo Tanaka, Shohei Saito, Shigeki Mori, Naoki Aratani, Hiroshi Shinokubo, Naoki Shibata,
Yoshiki Higuchi, Zin Seok Yoon, Kil Suk Kim, Su Bum Noh, Jong Kang Park, Dongho Kim,*
andAtsuhiro Osuka*
The concept of Möbius aromaticity, a postulate that states
that aromatic characteristics will be observed for [4n]annu-
lenes lying in a twisted Möbius strip, is a time-honored
concept that is fascinating from both the theoretical and
experiment standpoints,[1,2] since it provides a natural comple-
ment to the well-recognized idea of Hückel aromaticity that
governs the properties of planar [4n + 2]annulenes (Figure 1).
Although first suggested by Heilbronner for a ground-state
system early in 1964,[1] and later put forth as being relevant to
certain transition states[3] and reactive intermediates,[4] real
Möbius aromatic systems proved elusive until the recent
seminal report by Herges and co-workers,[5] wherein the first
example of a Möbius aromatic hydrocarbon was disclosed.
This system was obtained by the strategic lateral combination
of a normal, planar conjugated segment with a belt-shaped
conjugated segment. Despite the laborious synthetic
sequence required to obtain this particular product, the
aromatic character of this macrocycle is not very strong, a
finding consistent with the large dihedral angle (À107.78) and
small nucleus-independent chemical shift (NICS) value
(À3.4 ppm).[6,7] Recently, however, it has been suggested on
the basis of an extensive principal component analysis that
Möbius twisted [16]annulenes should be more aromatic than
the corresponding nontwisted isomers.[8] These studies have
served to rekindle theoretical interest in Möbius aromatic
molecules and underscore the need for further syntheses. In
this context, the recent finding that a di-p-benzihexa-
phyrin(1.1.1.1.1.1), which is a hexapyrrolic “expanded por-
phyrin”, exhibits a Möbius structure in the solid state, but
shows a dynamic Hückel–Möbius topological interconversion
in solution,[9] is interesting. Therefore, the need for a simple,
versatile approach to obtain real Möbius aromatic systems
with distinct aromatic characters remains.
Figure 1. Schematic representation of the topologies of p-conjugated
electron systems.
[*] Z. S. Yoon, K. S. Kim, S. B. Noh, J. K. Park, Prof. Dr. D. Kim
Department of Chemistryand
Center for Ultrafast Optical Characteristics Control
Yonsei University
The synthesis of Möbius aromatic molecules is made
inherently difficult by the need to accommodate two con-
flicting structural objectives, namely a twisted Möbius top-
ology and an overall conjugated 4n p-electron periphery. The
twisted nature of the Möbius topology necessitates the use of
large annulenic systems to mitigate the distortion associated
with molecular twists. Unfortunately, such systems often show
considerable conformational flexibility, which makes “locking
in” a twisted conformation difficult.
Seoul 120-749 (Korea)
Fax: (+82)2-364-7050
E-mail: dongho@yonsei-u.ac.kr
Y. Tanaka, S. Saito, S. Mori, Dr. N. Aratani, Prof. Dr. H. Shinokubo,
Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Recently, conjugated oligopyrrolic macrocycles, the so-
called expanded porphyrins, have emerged as a new class of
heteroannulene variants that display a range of interesting
electronic and conformational characteristics,[10–12] including
the formation of structures with figure-eight geometries.[13–15]
These features led us to consider that, compared with more
conventional annulenes, expanded porphyrins might have
distinct advantages in the construction of Möbius-type
aromatic systems. These include 1) an overall conformational
flexibility, 2) an ability to invert, or “flip out”, the constituent
pyrrolic subunits under certain conditions, 3) the capacity to
respond to two-electron oxidation and reduction through the
facile release or capture of a pair of pyrrolic NH hydrogen
atoms to balance the charge,[11] and 4) the possibility of
Prof. Dr. N. Shibata, Prof. Dr. Y. Higuchi
Graduate School of Life Science
Universityof Hyogo
3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan)
and
The RIKEN SPring-8 Center
1-1-1 Koto, Sayo-cho, Sato-gun, Hyogo 678-5248 (Japan)
[**] This work was supported bya grant-in-aid for Scientific Research (A)
from the Ministryof Education, Science, Sports, and Culture of
Japan (no. 19205006). Y.T., S.S., and S.M. thank the JSPS Research
Fellowship for Young Scientists. The work at Yonsei Universitywas
supported byBK21 and the Star FacultyPrograms from the Ministry
of Education and Human Resources Development, Korea.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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681

Communications
“locking in” conformations through metalation, especially in
those systems that permit the formation of both nitrogen–
metal and carbon–metal bonds.[16,17] In spite of this promise,
no general approach to Möbius-type aromatic systems
involving expanded porphyrins has been reported. However,
we have now found that Möbius aromatic molecules are
formed readily and spontaneously upon subjecting several
larger meso-substituted expanded porphyrins to metalation
with Group 10 metals. As detailed below, this approach,
which to the best of our knowledge is without precedent in the
literature, is both simple and can potentially be generalized.
[36]Octaphyrin(1.1.1.1.1.1.1.1) 1 (Scheme 1), which has 36
p electrons within its conjugation pathway, is an intriguing
expanded porphyrin that undergoes an unprecedented scis-
sion reaction to give two porphyrins upon metalation with two
copper(II) ions.[18] In a continuation of this study, which has
clear topological implications, we examined the metalation of
1 with palladium acetate. We found that heating this metal-
free species in methanol in the presence of palladium acetate
gave rise to the bispalladium(II) complexes 2 and 3 in yields
of 51 and 20%, respectively. A single-crystal X-ray analysis
revealed that 2 possesses a C2-symmetric figure-eight struc-
ture with two inverted pyrrole rings. The two palladium atoms
are each bound by three pyrrolic nitrogen atoms and a
pyrrolic b-carbon atom, thereby resulting in a square-planar
NNNC coordination environment, a mode of metal complex-
ation that is new for octaphyrins (Figure 2). This structure is
consistent with a 36-p-electron circuit within what can be
considered a twisted double-sided (orientable) Hückel top-
ology. As would be expected for such a formulation, protons
Ha and Hb (see Scheme 1) which are located near the crossing
point of the figure-eight structure resonate at d = 17.14 and
8.60 ppm, in the deshielded region. Such findings are con-
sistent with a paratropic ring current. The UV/Vis absorption
spectrum of 2 is ill-defined, with band features appearing at
Figure 2. X-raystructures (left) and schematic representations of
molecular topologies (right) of 2 (A), 3 (B), and 5 (C). The thermal
ellipsoids are scaled to the 50% probabilitylevel and meso-aryl
substituents are omitted for clarity. Dihedral angles at the most
distorted points are also indicated; dihedral angles for the three
molecules in crystal are shown for 3.[19]
413, 572, and 660 nm. We thus infer
that 2 is largely antiaromatic in
character, a conclusion that is sup-
ported by the finding of a very large
calculated NICS value (+ 38.4 ppm)
at the center of the molecule. By
way of comparison, compound 1, a
system considered to be non-aro-
matic on the basis of its UV/Vis and
1H NMR spectroscopic features, has
a
calculated NICS value of
À2.1 ppm at its center.
In contrast to 2, the structure of
3 was found to possess a twisted
single-sided Möbius topology. The
overall charge and bonding pattern
are consistent with a system con-
taining a 36-p-electron periphery
that is defined by the two central
Pd atoms coordinated in NNNC
and NNCC modes (Figure 2). The
greatest deviations from planarity
À
À
are seen at the C19 C20 and C39
C40 bonds, sites that serve to con-
Scheme 1. Expanded porphyrins and their Group 10 metal complexes.
682
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Chemie
nect the two relatively planar palladium-coordinating seg-
ments. However, even here the dihedral angles are rather
modest (Figure 2and Table 1), which allows for effective
with Möbius-type aromatic character (see Figures S4 and S16
in the Supporting Information). Metalation of 4 with palla-
dium acetate in dichloromethane provided monopalladium
complex 5 in 88% yield. X-ray diffraction analysis of 5
revealed a twisted Möbius strip type structure, in which the Pd
atom is bound to pyrrole rings A, B, C, and G in a square-
planar CNNN coordination mode, while the rest of the
pyrrole rings define a relatively planar segment. The dihedral
angles between these two segments are characterized by only
modest distortion (Table 1), as in the case of 3. Thus, the
organic framework present in 5 is best considered as a 32-p-
electron conjugated macrocycle (Figure 2). The aromatic
character of 5 is apparent from its 1H NMR spectrum
recorded at À608C, which is characterized by distinctly
shielded signals ascribed to the Ha and NH protons at d =
À2.20 ppm, and 0.07 and 3.84 ppm, respectively. Deshielded
signals corresponding to the outer b and NH protons are seen
in the range d = 7.3–8.0 ppm and at 9.44 ppm, respectively,
which leads to a large Dd value of d = 9.97 ppm (at room
temperature) and d = 10.21 ppm (at À608C). The UV/Vis
absorption spectrum of 5 exhibits a sharp Soret band at
663 nm, along with Q-like bands. In this case, the palladium
metalation serves to “lock in” the Möbius structure, for which
a NICS value of d = À12.2 ppm was calculated. Again, this is a
Table 1: Selected data.
Compound f [8][a]
Dd [ppm][b] NICS [ppm][c]
À2.1
s
(2) [GM]
(lex [nm])[d]
1
2
3
À164.23, À159.69 1.56
870 (1240)
690 (1240)
6400
150.82, 147.60
12.04
11.12
38.4
À40.62, 30.46
À14.6
(1440)
À39.67, 33.11
34.12, À32.98
–
4
5
7
8
9
–
À12.3
À12.2
À15.7
À13.3
À11.9
2600
(1240)
4200
(1300)
5500
(1200)
4000
(1200)
4600
(1200)
135.45, À157.27 9.97
À45.83, À138.74 5.90
À41.14, À134.13 7.05
À39.92, À133.47 6.37
[a] Dihedral angle at the two most distorted points; for 3, the values of the
three molecules in the crystal are shown. [b] Difference between the most
upfield-shifted and the most downfield-shifted chemical shifts for b-CH value that supports the assignment of aromaticity.
1
signals in the H NMR spectra recorded in CDCl3 at room temperature.
As a new quantitative measure of aromaticity, we
examined the two-photon absorption (TPA) cross-section
[c] Nucleus-independent chemical shift at the center of the macrocycles
(see Table S3 in the Supporting Information) calculated bythe GIAO
(s(2)) by the open-aperture Z-scan method.[21] Since the
method with the B3LYP functional and employing a basis set consisting
of SDD for Ni, Pd, and Pt, and 6-31G(d) for C, H, and N, on the optimized
structure models without aromatic substituents; NICS. [d] TPA cross-
section value and wavelength determined in CH2Cl2 bythe open-aperture
Z-scan method.
s
(2) value is largely determined by the degree of electronic
delocalization, large values for aromatic expanded porphyrins
are routinely recorded.[22–24] The measurements were carefully
performed in the near-infrared region (Table 1) to avoid any
contribution from one-photon absorptions. The maximum
s
(2) value of 3 was found to be 6400 GM, a value that is
conjugation of the macrocycle. A distinct diatropic ring
current is apparent in the 1H NMR spectrum of 3. Specifically,
the outer pyrrolic b-CH signals appear in the deshielded
region (d = 8.19–7.42ppm), while the inner pyrrolic H a, Hb,
and Hc signals appear at d = À1.77, À2.93, and 0.24 ppm, each
as a singlet. Thus, a large Dd value of 11.12ppm is observed,
as would be expected for an aromatic system. The aromatic
character proposed for 3 is also supported by its UV/Vis
absorption spectrum, which reveals an intense Soret-like band
at 735 nm and Q-like bands at 822, 1020, and 1143 nm. It is
distinctly larger than those of 1 (870 GM) and 2 (690 GM),
but comparable with those seen for Hückel aromatic mole-
cules.[23] The maximum s(2) values for 4 and 5 were found to be
2600 and 4200 GM, respectively, thus supporting the aromatic
characters proposed for these macrocycles. These are the first
TPA measurements of Möbius aromatic molecules and thus
represent an important demonstration that s(2) values may be
used to quantify the extent of aromaticity regardless of the
topology of the specific conjugated electronic circuit under
consideration.
The demonstration that both octaphyrin and heptaphyrin
can provide Möbius aromatic complexes led us to question
whether the smaller, and inherently less flexible, hexaphyrin
congener would also work in this capacity. This drove us to
reexamine the metal complexes of [28]hexaphyrins (7, 8, and
9), which we previously found to form in good yields upon
metalation of [26]hexaphyrin(1.1.1.1.1.1) 6 with Group 10
metals. Although we did not recognize the specific nature of
these structurally characterized complexes at the time of
initial publication,[17] we now appreciate that they all possess
distinct Möbius topologies. Moreover, they all contain 28-p-
electron peripheries and display diatropic ring currents, as
evidenced by the fact that the signals for the inner b protons
(Ha in Scheme 1) appear in a strongly shielded region at d =
1.70, 0.80, and 1.23 ppm, for 7, 8, and 9, respectively, while the
1
noteworthy that the H NMR spectra of 2 and 3 are both
somewhat independent of changes in either the temperature
or solvent, a finding we interpret in terms of their robust
structures which are made rigid by the two coordinated
palladium atoms. This finding stands in contrast to what is
seen in the case of 1, which displays features that are
considerably temperature and solvent dependent, as befits its
conformational flexibility. The calculated NICS value at the
center of macrocycle 3 is d = À14.6 ppm, thus providing
support for the above assignment.
1
The H NMR spectrum of [32]heptaphyrin(1.1.1.1.1.1.1)
4,[20] recorded at room temperature, revealed broad signals
consistent with conformational flexibility. However, an anal-
ogous analysis carried out at low temperature was best
interpreted in terms of the existence of several conformers
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display distinct aromaticity. The formation of such Möbius
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Received: September 25, 2007
Published online: November 6, 2007
[19] The crystallographic asymmetric unit of 3 contains three
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es S1-2and S1-3 in the Supporting Information).
Keywords: aromaticity· Mꢀbius aromaticity· porphyrinoids ·
two-photon absorption
.
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