Moebius antiaromatic bisphosphorus complexes of [30]hexaphyrins

Article abstract of DOI:10.1002/anie.201001765

Aromaticity with a twist: The reversal of Moebius aromaticity occurs by changing the number of π electrons between [4n + 2] and [4n] in monophosphorus complexes of [28]hexaphyrins and bisphosphorus complexes of [30]hexaphyrins. The 30π Moebius antiaromatic bisphosporus[30]hexaphyrins (see picture) were determined to be the first structurally characterized Moebius antiaromatic systems, which are rigid, neutral, and stable. (Figure Presented)

Full text of DOI:10.1002/anie.201001765

Communications
DOI: 10.1002/anie.201001765
Aromaticity
Mꢀbius Antiaromatic Bisphosphorus Complexes of [30]Hexaphyrins**
Tomohiro Higashino, Jong Min Lim, Takahiro Miura, Shohei Saito, Jae-Yoon Shin,
Dongho Kim,* and Atsuhiro Osuka*
Aromaticity is a fundamental concept in organic chemistry
and is important in understanding, for instance, that the
energetic stability of benzene originates from cyclic p-
electron delocalization. The Hꢀckel rule is very useful, and
helps us to predict that cyclic [4n+2]p- and [4n]p-conjugated
systems should be aromatic and antiaromatic, respectively,
given that their p systems lie on two-sided normal planes. On
the other hand, Mꢁbius aromaticity, which complements the
Hꢀckel aromaticity, predicts that the above [4n+2] and
[4n] Hꢀckel rule should be reversed for those p systems that
lie on a singly twisted Mꢁbius topology.[1] The concept of
Mꢁbius aromaticity, first proposed by Heilbronner in 1964,[2]
is simple and elegant, and has considerably stimulated both
experimental and theoretical approaches toward Mꢁbius
aromatic molecules.[1b,c,3] However, the realization of
Mꢁbius aromatic systems is difficult, since the implementa-
tion of two conflicting structural features, that is, cyclic full
p conjugation and a singly twisted topology, within a single
macrocycle is not easy. Despite this difficulty, a seminal report
by Herges and co-workers described the synthesis and
moderate aromaticity of a twisted [16]annulene molecule.[4]
Importantly, this work revitalized the interest in Mꢁbius
aromaticity. In recent years, meso-aryl-substituted expanded
porphyrins have emerged as a remarkably effective platform
to realize stable Mꢁbius aromatic systems. The attributes of
these porphyrins include conformational flexibility, which
allows flipping of the constitutional pyrrole rings, and the
availability of the neutral states for two-electron oxidation
and reduction by releasing two protons from aminopyrrole
parts and accepting two hydride ions at the iminopyrrolic
parts, respectively.[5]
However, despite the increasing number of [4n]p Mꢁbius
aromatic molecules, [4n+2]p Mꢁbius antiaromatic species
still remain rather elusive. As a rare example of these species,
´
Latos-Graz˙ynski and co-workers reported that a cationic
palladium(II) vacataporphyrin displayed a weak paratropic
ring current (Dd = 1.63–2.67 ppm, nucleus-independent
chemical shift (NICS): + 4.0–6.0 ppm), which was ascribed
to the 18p antiaromatic character based on the calculated
Mꢁbius structure, but without crystal structure evidence.[6]
Thus, it can be said that structurally well-characterized
Mꢁbius antiaromatic species with a distinct paratropic ring
current have not been reported to date.[7]
In general, the evaluation of aromaticity relies on
energetic, geometric, and magnetic parameters. The most
sensitive and widely applicable techniques for studying the
1
magnetic properties are H NMR chemical shifts, NICS,[8a]
and anisotropy of the induced current density (AICD).[8b] In
addition, our recent studies have demonstrated common
photophysical properties for aromatic and antiaromatic
porphyrinoids.[9] These antiaromatic expanded porphyrins
exhibit several unique features that are not observed for their
aromatic congeners: 1) broad and ill-defined absorption
spectra without Q-like bands in the near-IR region, 2) weak
or practically no fluorescence, 3) small two-photon absorp-
tion (TPA) cross-section values, and 4) very short excited-
state lifetimes. These features are attributable to electronic
features such as the relatively narrow HOMO–LUMO gap
(HOMO = highest occupied molecular orbital, LUMO =
lowest unoccupied molecular orbital) with perturbed degen-
eracy of HOMO/HOMO-1 and LUMO/LUMO + 1, and the
presence of an optically forbidden S1 state. These features are
not observed for aromatic expanded porphyrins. Herein, we
report a bisphosphorus complex of [30]hexaphyrin as the first
example of a structurally well-characterized and stable
[4n+2]p Mꢁbius antiaromatic molecule, in which the two
incorporated phosphoramide moieties play important roles in
rigidifying a singly twisted Mꢁbius conformation and render a
highly reduced [30]hexaphyrin stable.
[*] J. M. Lim, J.-Y. Shin, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-364-7050
E-mail: dongho@yonsei.ac.kr
T. Higashino, T. Miura, S. Saito, 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
[**] This work was supported by Grants-in-Aid (A) (No. 19205006 (A)
and 20108001 “pi-Space”) for Scientific Research from MEXT. The
work at Yonsei University was supported by the Star Faculty and
World Class University (R32-10217) programs from the Ministry of
Education, Science, and Technology (MEST) of Korea and the
AFSOR/AOARD grant (FA2386-09-1-4092). The AICD calculations
were performed by using the supercomputing resource of the Korea
Institute of Science and Technology Information (KISTI). S.S.
acknowledges a JSPS Fellowship for Young Scientists. J.M.L
acknowledges the Seoul Science Fellowship.
Phosphorus insertion into porphyrinoids has proved a
useful means to realize isophlorin-type fully reduced annu-
lenic p conjugation.[10] Encouraged by these results, we
attempted the reaction of meso-pentafluorophenyl [28]hexa-
phyrin(1.1.1.1.1.1) (1a) with POCl3 (100 equiv) in the pres-
ence of triethylamine at room temperature for 24 h, which
gave monophosphorus [28]hexaphyrin 2a in 65% yield after
aqueous workup. We also found that a further treatment of 2a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001765.
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with PCl3 (50 equiv) in the presence of triethylamine at 08C
for 1 h gave bisphosphorus [30]hexaphyrin 3a in 46% yield.
In the same manner, phosphorus complexes 2b and 3b were
obtained from meso-(2,4,6-trifluorophenyl) substituted
[28]hexaphyrin 1b in 24 and 10% yield, respectively
(Scheme 1).
Single-crystal X-ray diffraction analysis revealed a twisted
=
Mꢁbius structure of 2a, in which a P O moiety was bound to
the b-carbon atom of the pyrrole unit C and the two nitrogen
atoms of the pyrrole units A and B (Figure 1a).[11a] The
1H NMR spectrum showed ten signals that correspond to the
outer b protons in the range d = 7.75–6.53 ppm and a doublet
at d = 2.99 ppm that corresponds to the inner b proton of the
pyrrole unit C that was coupled to a phosphorus atom (J =
6.9 Hz; Figure 2a and the Supporting Information). The
assignment was confirmed by 1H–1H and 1H–31P NMR
correlation measurements, and by the addition of D2O (see
the Supporting Information). Accordingly, the difference
between the chemical shifts (Dd) of the most shielded and
deshielded b protons was d = 4.76 ppm, thus indicating a
moderate diatropic ring current despite the large dihedral
angle (658) along its p conjugation. The structural rigidity of
2a in solution was indicated by virtually temperature-
independent NMR spectra and only weak solvent-effects in
the absorption spectra (see the Supporting Information),
whereas 1a exhibited dramatic temperature-dependent spec-
tral changes.[5e] Accordingly, it can be concluded that 2a is a
normal Mꢁbius aromatic molecule. The complex 2b exhibited
properties analogous to those of 2a.
Figure 1. X-ray crystal structures (left) and schematic representations
of molecular topologies (right) of a) 2a and b) 3a. Thermal ellipsoids
represent 50% probability and meso-aryl substituents are omitted for
clarity. Dihedral angles at the most distorted points are given.
The structure of bisphosphorus complex 3a was unam-
biguously determined by X-ray diffraction analysis to be a
singly twisted Mꢁbius conformation, in which the additionally
=
embedded P O moiety was bound to the three nitrogen
Figure 2. 1H NMR spectra of a) 2a in CDCl3 and b) 3a in CD2Cl2 at
258C. Peaks marked with a * arise from residual solvents and
impurities.
atoms of the pyrrole units D, E, and F (Figure 1b).[11b] In
addition, high-resolution ESI-TOF-MS and charge-balance
considerations showed the complex 3a was a reduced
[30]hexaphyrin. This highly reduced molecule is probably
stabilized by two strongly electron-withdrawing phosphor-
1
amide moieties. Interestingly, the H NMR spectrum of 3a
be ascribed to a large dihedral angle (66.88) at the most
distorted position. Complex 3b displayed properties that are
analogous to those of 3a. The 1H NMR analysis also indicated
a moderate but distinct 30p Mꢁbius antiaromaticity of 3b.
The most intriguing point here is that the shielding and
deshielding effects of 2 and 3 are inverted only by changing
the number of p electrons from [4n] to [4n+2], although their
Mꢁbius-type structures remain nearly the same. This obser-
vation suggests that the concept of Mꢁbius aromaticity, which
is a reversal of the Hꢀckel rule, is valid for the expanded
porphyrin systems.
revealed a remarkably deshielded doublet at d = 11.14 ppm
that corresponds to the inner b proton of the pyrrole unit C,
which was coupled with Pa (J = 5.0 Hz), ten signals that
correspond to the outer b protons in a range of d = 6.72–
5.54 ppm, and a relatively shielded broad singlet at d =
7.15 ppm that correspond to the outer NH proton (Figure 2b
and the Supporting Information). The observed apparent
deshielding of the inner b proton and significant shielding of
the outer b protons indicated a distinct paratropic ring current
with Dd = 5.60 ppm. The observed moderate ring current may
Scheme 1. Stepwise insertion of phosphorus into [28]hexaphyrins 1a and 1b.
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In general, the absorption spectra of aromatic expanded
GSB and large ESA signals, and are quite different from
those of 2a (Figure 4b). These spectral features, together with
faster decay dynamics of 3a than 2a (Figure 4), are unique for
Hꢀckel antiaromatic expanded porphyrins.[9a,b] The difference
in excited-state dynamics between aromatic and antiaromatic
expanded porphyrins could be mainly attributed to the
different electronic structures of these two classes of mole-
cules. The perturbed degeneracy of HOMO/HOMO-1 and
LUMO/LUMO + 1 causes the interruption of two additional
frontier MOs, thus resulting in a change in configuration
interactions. As a consequence, the electronic transitions or
states that affect the steady- and excited-state dynamics are
perturbed. In fact, it was reported that a trans-vinylene-
bridged antiaromatic hexaphyrin with MO structures similar
to 3a reveals the six frontier MOs mixing in time-dependent
DFT (TD-DFT) calculations along with the same trend in the
steady- and excited-state dynamics of 3a, and the presence of
the optical dark state governs its photophysical behavior.[9b]
Based on these results, the fast decay of 3a can be ascribed to
the NIR dark state that acts as a ladder in the deactivation
processes. The two-photon absorption (TPA) cross-section
values of phosphorus hexaphyrins also exemplify the aroma-
ticity difference between 2a and 3a. The aromatic 2a shows a
relatively large TPA cross-section value (3950 GM) compared
to that of the antiaromatic 3a (2400 GM; see the Supporting
Information), which is also consistent with previous reports of
the approximate proportionality between the degree of
aromaticity and TPA values.[9c]
porphyrins confirm a porphyrinlike structure, which is
predicted by Goutermanꢂs four orbital model.[12] As in the
porphyrinoids, the four frontier molecular orbitals (MOs) of
the aromatic expanded porphyrins, which consist of nearly
degenerated HOMO/HOMO-1 and LUMO/LUMO + 1, par-
ticipate in configuration interactions. The UV/Vis/NIR
absorption spectrum of 2a exhibits an intense B-like band
at 588 nm and broad Q-like bands at 876 and 1055 nm
(Figure 3). DFT calculations for 2a show that the HOMO and
Figure 3. UV/Vis absorption spectra of 2a (gray) and 3a (black) in
CH2Cl2. The fluorescence emission spectrum of 2a is shown by a
dotted line.
HOMO-1 are nearly degenerate, which is consistent with the
absorption spectrum (see the Supporting Information). More-
over, 2a exhibits a fluorescence band at 1090 nm, which is a
mirror image of the corresponding absorption spectrum
(Figure 3). In contrast, the UV/Vis/NIR absorption spectrum
of 3a shows ill-defined Soret-like bands and broad, weak NIR
bands. Furthermore, 3a shows no fluorescence emission
(Figure 3). It should be
We performed DFT calculations (B3LYP/6-311G(d,p))
for 2a and 3a on the basis of both the crystal structure and the
noted that the absorption
and fluorescence behavior
of 3a is similar to those of
typical Hꢀckel antiaromatic
expanded porphyrins.[9a,b]
The
aromaticity
of
expanded porphyrins is
prominently distinguished
in time-resolved and nonlin-
ear optical (NLO) spectro-
scopic measurements. The
Mꢁbius aromatic molecule
2a shows the distinct
excited-state properties of
aromatic expanded porphyr-
ins. Typically, we have
observed that the differen-
tial absorption spectra of
aromatic expanded porphyr-
ins show strong ground-state
bleaching (GSB) signals
with
relatively
weak
excited-state
absorption
(ESA) signals. These signals
are also seen in the transient
absorption spectra of 2a
(Figure 4a). In contrast, the
Figure 4. Femtosecond transient absorption spectra and decay profiles of a) 2a and b) 3a. The pump
excitation is 585 nm for 2a and 535 nm for 3a. The decay profiles are observed at each maximum wavelength
spectra of 3a show small of bleaching and ESA signals.
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optimized structures (see the Supporting Information).[13]
Large negative NICS values were calculated at the center of
2a (d = À8.25 ppm for the crystal structure and À11.99 ppm
for the optimized structure) but small positive NICS values
were calculated for 3a (d = + 3.65 ppm for the crystal
structure and d = + 2.05 ppm for the optimized structure);
these values are consistent with the assignments of aromatic
and antiaromatic character. In addition, we also attempted
the direct visualization of the induced ring current.[8b] The
AICD method is a powerful tool for the determination of the
degree of aromaticity or antiaromaticity because it represents
the 3D image of delocalized electron densities with a scalar
field,[14] and is particularly useful for nonplanar p-conjugated
systems such as 2a and 3a.Since the AICD plot illustrates the
paramagnetic term of the induced current density, the
aromatic molecules show clockwise current density and the
antiaromatic species show counter-clockwise current density.
The AICD plot of 2a reveals clear clockwise current-density
vectors, thus indicating a diamagnetic ring current (see the
Supporting Information). On the contrary, the AICD plot of
3a shows relatively weak counter-clockwise current flow,
which implies an induced paratropic ring current. Further-
more, the AICD plots of 2a and 3a show a prominent
difference upon changing the isosurface values (see the
Supporting Information). Although 2a shows a continuous
boundary surface that encloses the delocalized electrons even
at the isosurface value of 0.05, 3a does not show a continuous
current density above the isosurface value of 0.045. This
difference between 2a and 3a indicates that p-electron
conjugation is more facile in 2a, which has a 28p-electronic
distorted Mꢁbius topology, compared with 3a, which has a
30p-electronic circuit. These results demonstrate that 2a has a
Mꢁbius aromatic character with continuous and diamagnetic
current density in its distorted conformation and that, more
importantly, the Mꢁbius antiaromaticity of 3a is confirmed by
a 3D paratropic ring current in spite of a weak density of
delocalized electrons.
The electrochemical properties were studied by cyclic
voltammetry in CH2Cl2 versus ferrocene/ferrocenium (Fc/
Fc+) with tetrabutylammonium hexafluorophosphate as elec-
trolyte. Complex 2a underwent two reversible oxidations at
0.46 and 0.84 V and two reversible reductions at À0.73 and
À1.05 V, while 3a showed two reversible oxidations at 0.12
and 0.27 V and two irreversible reductions around À1.41 and
À1.86 V. The highly reduced hexaphyrin 3a exhibited an
elevated HOMO and resistance toward reduction.
In summary, the monophosphorus complexes of [28]hexa-
phyrins and bisphosphorus complexes of [30]hexaphyrins
have been synthesized and characterized as [4n]p Mꢁbius
aromatic and [4n+2]p Mꢁbius antiaromatic compounds,
respectively. The twisted Mꢁbius structures were confirmed
by single-crystal X-ray diffraction analysis. The aromaticity
and antiaromaticity was investigated by examining magnetic
environments (1H NMR chemical shifts, NICS, and induced
current density maps) and the photophysical properties (UV/
Vis/NIR absorption, singlet excited-state dynamics, NLO
properties, and quantum mechanical calculations). Bisphos-
phorus [30]hexaphyrins have been determined to be the first
structurally characterized Mꢁbius antiaromatic systems,
which are stable, neutral, and rigid. In this system, the
phosphoramide moieties help maintain the distorted Mꢁbius
structure and, at the same time, stabilize the highly reduced
30p-electron state by their inductive electron-withdrawing
effect. In this work, the aromaticity reversal in the Hꢀckel
rule upon changing the number of p electrons between
[4n+2] and [4n] has been validated also for cyclic conjugated
molecules with a twisted Mꢁbius topology.
Received: March 25, 2010
Published online: June 2, 2010
Keywords: aromaticity · hexaphyrins · Mꢀbius aromaticity ·
.
phosphorus · porphyrinoids
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