Redox-induced palladium migrations that allow reversible topological changes between palladium(II) complexes of Moebius aromatic [28]Hexaphyrin and Hueckel aromatic [26]hexaphyrin

Article abstract of DOI:10.1002/anie.201005334

Plane twisted: A twisted Moebius aromatic [28]hexaphyrin/Pd ^II complex and a planar Hueckel aromatic [26]hexaphyrin/Pd ^II complex are reversibly interconvertible by an unprecedented palladium migration through oxidation and reduction

Full text of DOI:10.1002/anie.201005334

Communications
DOI: 10.1002/anie.201005334
Aromaticity
Redox-Induced Palladium Migrations that Allow Reversible
Topological Changes between Palladium(II) Complexes of Mꢀbius
Aromatic [28]Hexaphyrin and Hꢁckel Aromatic [26]Hexaphyrin**
Mitsunori Inoue and Atsuhiro Osuka*
Mꢀbius aromaticity is the concept that complements the
Hꢁckel aromaticity by predicting aromatic and antiaromatic
characters for [4np] and [(4n+2)p] annulenes having a
Mꢀbius topology.[1] Since its first proposal by Heilbronner in
1964,[2a] this concept has encouraged both experimental and
theoretical studies. Importantly, the seminal papers by Herges
and co-workers[3] have regenerated interest in this concept
leading to reports on a [16]annulene that possesses a twisted
conformation and a moderate aromatic character.[4] Recently,
expanded porphyrins[5] have emerged as a nice platform to
realize various Mꢀbius aromatic molecules.[6–8] Although
these molecules are important and interesting, displaying
singly twisted conformations and diatropic ring currents
originating from their aromatic characters, their chemical
reactivity has been hardly unexplored thus far. Herein we
report the novel reactivity of the Mꢀbius aromatic [28]hexa-
phyrin/palladium(II) complex 3 (Figure 1), which undergoes a
molecular topology change to give Hꢁckel aromatic [26]hex-
aphyrin/palladium(II) complex 4 upon oxidation with tris(4-
bromophenyl)aminium hexachloroantimonate (TBAH).
Interestingly, the reverse transformation from 4 into 3 can
Figure 1. Hexaphyrins and their metal complexes in this paper. Bold
bonds represent p-conjugated circuits.
be effected by reduction with NaCNBH3.
In the course of our studies on Mꢀbius aromatic mole-
cules, we have revealed that although [26]hexaphyrin (2) is a
Hꢁckel aromatic molecule with certain structural rigidity, and
its two-electron reduced congener [28]hexaphyrin (1) has
been shown to exist in solution at 258C largely as an
equilibrium of several rapidly interconverting twisted
Mꢀbius conformations having distinct aromaticities.[8a] In
contrast, the rigid conformation of the [28]hexaphyrin/
palladium(II) complex 3 was determined on the basis of its
such Mꢀbius antiaromatic molecules have been quite scarce.[9]
We thus attempted the oxidation of 3 with various oxidants.
Whereas 3 was found to be inert towards oxidants such as
P2O5, NaIO4, and NaBrO3, it was readily decomposed upon
treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), para-chloranil, MnO2, meta-chloroperbezoinc acid
(MCPBA), and ceric ammonium nitrate (CAN). Fortunately,
we found that clean oxidation of 3 proceeded with TBAH in
acetonitrile. An acetonitrile solution of 3 was stirred in the
presence of TBAH for 10 minutes at room temperature, and
then the reaction mixture was quenched by the addition of
methanol. After the usual work-up, a single product was
isolated in 71% yield, and has been identified as the planar
and rectangular macrocycle 4 on the basis of single-crystal
X-ray diffraction analysis (Figure 2).[10] Interestingly, NNNC
PdII coordination in 3 is changed to NNCC PdII coordination
in 4 with a molecular topology change from a twisted Mꢀbius
conformation to a planar Hꢁckel conformation. In 4, the
representative bond distances are 2.105(19) ꢂ for Pd-N(2),
2.105(2) ꢂ for Pd-N(3), 1.996(2) ꢂ for Pd-C(3), and
1.990(2) ꢂ for Pd-C(17), and the pyrrolic interior angles of
C-N(4)-C and C-N(1)-C are 109.1(18) and 105.7(2)8, indicat-
ing the location of amino-type and imino-type pyrroles,
1
temperature-independent H NMR spectrum.[7a] We thought
that the two-electron oxidation of 3 might give a Mꢀbius
antiaromatic [26]hexaphyrin with a twisted conformation.
Isolation of stable Mꢀbius antiaromatic molecules is quite
important to confirm that the [4np] and [(4n+2)p] rule is also
valid for conjugated macrocycles of Mꢀbius topology, but
[*] M. Inoue, 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 (No. 19205006 (A), and
20108001 “pi-Space“) from MEXT (Japan). M.I. thanks the JSPS for
Research Fellowship for Young Scientist.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005334.
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Figure 3. UV/Vis absorption spectra of 3 and 4 in CH2Cl2.
À
cleaved, and subsequent Pd N1 bond formation achieves the
twisted Mꢀbius aromatic [28]hexaphyrin. Therefore, it may be
concluded that the hexaphyrin/monopalladium(II)complex is
a unique platform, wherein the PdII metal rearranges between
N1 and C3 to achieve the topologically different aromatic
Figure 2. X-ray crystal structure of 4: a) top view and b) side view. In
the side view, meso-pentafluorophenyl substituents are omitted for
clarity. The thermal ellipsoids are drawn at the 50% probability
level.
respectively. The structure of 4 confirms the conjugated
26p circuit as shown in Figure 1. Importantly, the com-
plex 4 cannot be obtained directly by the metallation of
hexaphyrin 1 or 2 with palladium salts.[7a,11] It is likely that
the PdII metal migrates from N(1) to C(3) with C H
À
activation during the oxidation to avoid unfavorable
Mꢀbius topology with a 26p electronic circuit. Previously,
we reported that the metallation of 2 with NaAuCl4 in the
presence of sodium acetate led to the formation of
complexes 8 and 9, in which the AuIII ions are coordinated to
NNCC, similar to the PdII ion in 4.[12a] However, the structure
of 4 differs from that of 8 in terms of the presence of the outer
NH group, which balances the overall charge and causes the
nonsymmetric structure. The 1H NMR spectrum of 4 is
consistent with its nonsymmetric structure, exhibiting a
singlet at d = 11.8 ppm that results from the outer NH, eight
doublets in the range of d = 9.58–8.93 ppm that result from
the outer b protons, a singlet at d = À2.70 ppm that results
from the inner NH, and two singlets at d = À3.62 and
À3.73 ppm, respectively that result from the inner b protons.
The UV/Vis absorption spectrum of 4 shows a distinct Soret-
like band at 557 nm and Q-like bands at 941 nm which are
signatures of its aromatic nature (Figure 3). The spectrum of 4
is however considerably broader than that of 3.
Scheme 1. Redox interconversions between 3 and 4.
molecular systems, thus responding to the number of p elec-
trons involved in the conjugation circuit.
In the next step, the metallation of 4 in the preorganized
vacant CCNN cavity has been performed to create hetero-
bis(metal) complexes.[12] The reaction of 4 with Cu(OAc)2 in a
4:1 mixture of CH2Cl2 and methanol in the presence of
NaOAc for 3 hours at room temperature afforded the
[26]hexaphyrin/PdII/CuIII complex 5 in 90% yield. The high-
resolution electrospray-ionization time-of-flight mass spec-
troscopy (HR-ESI-TOF/MS) revealed m/z 1624.8671-
([MÀH]À) as the parent ion peak of
5 (calcd for
1
C66H8N6F30PdCu, m/z = 1624.8682). The H NMR spectrum
of 5 exhibits a singlet at d = 11.5 ppm that results from the
outer NH and eight doublets in the range of d = 9.28–
8.76 ppm that result from the outer b protons. The clean
1H NMR spectrum indicated the presence of CuIII ion in the
pocket, hence confirming that the NNCC core in 4 can serve
as a tridentate ligand to realize the high valent CuIII state in a
similar manner as the carbaporphyrins.[13] The UV/Vis
absorption spectrum of 5 exhibits a rather broad and split
Soret-like band and Q-like bands that extend up to 1200 nm.
Similarly, the metallation of 4 with AgOTf in acetonitrile
after 30 hours at room temperature provided a PdII/AgIII
To test the reversibility of the palladium(II) rearrange-
ment, the reduction of 4 was examined. Whereas the
reduction of 4 with NaBH4 caused immediate decomposition,
the reduction of 4 was successful when using NaCNBH3 in
THF, thus regenerating 3 in 74% yield (Scheme 1). Therefore,
the reverse palladium(II) rearrangement has been confirmed
through reduction. With NaCNBD3, C3-deuterated 3 was
À
obtained, suggesting that the Pd C3 bond is reductively
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complex of [26]hexaphyrin (6) in 93% yield. The structure of
6 was unambiguously confirmed by the single-crystal X-ray
diffraction analyses.[10] In 6, the PdII ion and AgIII ion are
coordinated to two pyrrolic nitrogen atoms and two b carbon
atoms with following bond lengths: 2.137(4) ꢂ for Pd-N(2),
2.121(4) ꢂ for Pd-N(3), 2.010(4) ꢂ for Pd-C(3), 1.979(5) ꢂ
for Pd-C(17); 2.157(5) ꢂ for Ag-N(5), 2.113(4) ꢂ for Ag-
N(6), 1.970(6) ꢂ for Ag-C(2), 2.041(5) ꢂ for Ag-C(18). The
pyrrole interior angles of C-N(4)-C and C-N(1)-C are 109.3(3)
and 105.2(3)8, indicating the location of amino-type and
imino-type pyrroles, respectively. Compared with 4, 6 is more
planar with the smaller mean plane deviation of 36 atoms (4,
1
0.425 ꢂ; 6, 0.349 ꢂ). H NMR and 19F NMR data as well as
UV/Vis absorption spectra of 6 are very similar to those of 5.
Refluxing a solution of 6 in CH2Cl2 containing methanol
for 48 hours in the presence of AgOTf and NaOAc led to
production of the bis(methoxy) adduct 7 in 86% yield. X-Ray
diffraction analysis revealed the structure of 7, in which two
methoxy groups are attached to the inner pyrrolic b positions
in an anti manner (Figure 4B).[10] The C–C bond distances of
the reacted pyrrolic unit are 1.553(5), 1.549(5), and
1.539(5) ꢂ, which are well within the typical single C–C
1
bond distance. The H NMR, 19F NMR, and HR-ESI-TOF
data are fully consistent with the solid-state structure. The
1H NMR spectrum exhibits two highly shielded singlets at d =
À2.41 and À2.75 ppm that result from the two methoxy
groups, indicating that a strong diatropic ring current is
preserved in 7. The UV/Vis absorption spectrum of 7 exhibits
a sharp and split Soret-like band and intensified Q-like bands,
which also supports the aromatic nature and expanded
chlorin-like structure of 7 (Figure 5).
In summary, the oxidation of the Mꢀbius aromatic
hexaphyrin 3 with TBAH induced a topological change
(from twisted to planar) to produce the Hꢁckel aromatic
hexaphyrin 4 with PdII migration from N1 to C3, and the
reverse topological change was effected by the reduction of 4
with NaCNBH3 to regenerate 3. The complex 4 has a
preorganized cavity that has been used as a binding site for
another metal ion to produce hetero-bis(metal) complexes
such as the PdII/CuIII complex 5 and PdII/AgIII complex 6.
Finally, the silver(I)-catalyzed addition reaction of methanol
Figure 4. X-ray crystal structures of 6 (a) and 7 (b); for each structure
a top and side view are shown. In 7, solvent molecules are omitted for
clarity. In the side view, meso-pentafluorophenyl substituents are
omitted for clarity. The thermal ellipsoids are drawn at the 50%
probability level.
Figure 5. UV/Vis absorption spectra of 4, 6, and 7 in CH2Cl2.
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Chemie
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Received: August 26, 2010
Published online: October 28, 2010
Keywords: aromaticity · hexaphyrins · metalation · palladium ·
.
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