Doubly N-Confused [36]Octaphyrin(1.1.1.1.1.1.1.1): Isomerization, Bis-Metal Coordination, and Topological Chirality

Article abstract of DOI:10.1002/anie.201708253

A novel [36]octaphyrin analogue embedding two N-confused pyrrole units demonstrated unique prototropy-coupled isomerization between the Figure-of-eight and dumbbell conformers. Upon bis-metal coordination, fixation of fully π-conjugated Figure-of-eight structures was achieved as referred from the X-ray crystal structure. Chirogenesis of the helical enantiomers was proved by intense circular dichroism (CD) response in the near infrared (NIR) region.

Full text of DOI:10.1002/anie.201708253

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Accepted Article
Title: Doubly N-Confused [36]Octaphyrin(1.1.1.1.1.1.1.1):
Isomerization, Bis-Metal Coordination, and Topological Chirality
Authors: Hiroyuki Furuta, Koki Mitsuno, Takafumi Yoshino, Iti Gupta,
Shigeki Mori, Satoru Karasawa, and Masatoshi Ishida
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708253
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Doubly N-Confused [36]Octaphyrin(1.1.1.1.1.1.1.1): Isomerization,
Bis-Metal Coordination, and Topological Chirality
Koki Mitsuno, Takafumi Yoshino, Iti Gupta, Shigeki Mori, Satoru Karasawa, Masatoshi Ishida,* and
Hiroyuki Furuta*
Abstract: A novel [36]octaphyrin analogue embedding two N-
confused pyrrole units demonstrated unique prototropy-coupled
isomerization between the figure-of-eight and dumbbell conformers.
Upon bis-metal coordination, fixation of fully p-conjugated figure-of-
eight structures were achieved as referred from the X-ray crystal
structure. Chirogenesis of the helical enantiomers was proved by
intense circular dichroism (CD) response in the near infrared (NIR)
region.
units into the hexapyrrolic macrocycles gave rise to the
energetic benefit on the geometry stabilization because of the
formation of the effective inner hydrogen bonding network.[9]
This strategy is also expected to function in the octapyrrolic
systems; the modified octaphyrin embedding two confused
pyrrole rings can adopt less sterically hindered structure with
figure-of-eight geometry than that of regular one. In a structural
sense, this can be regarded as skeletal fusion of N-confused
porphyrins (NCPs), yielding the N-confused octaphyrin analogue
(2) (Figure 1).[8] The intrinsic NH tautomerism-coupled
isomerization could assist in stabilizing the distinct
conformations.
Exploiting topology to control the shape and property of the
matter at
a molecular level is an appealing prospect in
chemistry.[1] The topological objects are commonly flexible and
nonplanar, and naturally seen in DNAs and proteins as knotted
forms, realizing unusual biochemical activities.[2] It is currently
well-recognized that the rational design of topological molecular
systems is associated with many fundamental and practical
goals, such as the quest for novel types of topological chirality[3]
and utilization as the building parts in molecular machinery.[4]
Among the various topologically compounds (e.g., knots,
catenanes) reported so far, octaphyrin (1, Scheme 1) is a large
and fully p-conjugated macrocycle with highly adoptable
structures like figure-of-eight, square planar, Möbius-twist, etc.[5]
Accordingly, specific features of these octaphyrin homologues
such as NIR optical response, facile multi-redox reaction,
multiple metal binding, twisted aromaticity and so on, have been
appearing. However, only few investigations on the chiroptical
properties have been conducted presumably due to the
intrinsically labile stereo-structures and/or unique metalation-
coupled reactivity.[6, 7]
Figure 1. Schematic illustration of the p-expansion strategies to invoke
versatile NH tautomerism of N-confused octaphyrin.
Herein we disclose the synthesis and characterization of a
novel figure-of-eight shaped octapyrrolic macrocycle wherein
two confused pyrrole rings are located at the hinge positions,
namely, doubly N-confused octaphyrin(1.1.1.1.1.1.1.1) (2,
Scheme 1). This macrocycle has
a highly flexible 36p
conjugated structure and takes two stable conformations: a
figure-of-eight conformer (2a: major) and a hitherto unknown
dumbbell-like conformer (2b: minor) in a prototropy-coupled
equilibrium. Moreover, octaphyrin 2 serves as a bis-metal
chelating ligand for stable bis-zinc(II) and bis-copper(II)
complexes without affording thermally split- or helically oxidized
products.[7] The p-conjugated circuit with the helical geometry of
2 allowed to generate fully p-conjugated topological chirality as
evident from the NIR CD response.
In order to control the topological geometries, we utilized an
N-confusion approach that can provide the thermodynamically
favorable conformation through the core modification of the
parent octaphyrin scaffold.[8] In analogy to the original scaffold,
the implementation of the confused (i.e., a,b’-linked) pyrrole
[*]
K. Mitsuno, T. Yoshino, Dr. M. Ishida, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry
Graduate School of Engineering and Center for Molecular Systems
Kyushu University, Fukuoka 819-0395 (Japan)
Fax: (+81) 92-802-2865
The preparation of octaphyrin 2 was achieved via a [5+3]-
condensation strategy using an N-confused tripyrrane bis-
carbinol (4) and an N-confused pentapyrrane precursor (5)
(Scheme 1). The synthetic attempt by either a [2+2+2+2]-type
E-mail: ishida.masatoshi.686@m.kyushu-u.ac.jp
hfuruta@cstf.kyushu-u.ac.jp
Prof. Dr. I. Gupta
coupling with an N-confused dipyrromethane biscarbinol (6)[10]
/
Indian Institute of Technology Gandhinagar, Village Palaj,
Simkheda, Gandhinagar, Gujarat-382355 (India)
Dr. S. Mori
Advanced Research Support Center
Ehime University, Matsuyama 790-8577 (Japan)
Prof. Dr. S. Karasawa
regular dipyrromethane, or [4+4]-type reaction with an N-
confused tetrapyrrane (7)[11] and the corresponding aldehyde,
resulted in the formation of tetrapyrrolic NCP derivative,
predominantly. To avoid this side product, the combination of
odd-numbered oligopyrrolic precursors was thus examined for
the acid-catalyzed macrocyclization; the pentapyrrane 4 and the
Division of Pharmaceutical Chemistry, Showa Pharmaceutical
University, Machida 194-8543 (Japan)
biscarbinol derivative
5 were mixed in CH2Cl2 at room
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.xxxxxxxx
temperature and allowed to stir after the addition of p-
toluenesulfonic acid catalyst, followed by 2,3-dichloro-5,6-
1
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Figure 2. Crystal structure of (a) 2a and (b) Cu2-2. The thermal ellipsoids are
scaled to the 50% probability. meso-Aryl substituents and solvent molecules
are omitted for clarity.
Scheme 1. Synthesis of octaphyrin 2 and its bis-metal complexes, M2-2; a, i)
p-toluenesulfonic acid (0.3 equiv) in CH2Cl2, ii) DDQ (3 equiv); b, Cu(OAc)2 or
Zn(OAc)2 in MeOH/CH2Cl2 (1:10).
dicyano-1,4-benzoquinone (DDQ) oxidation, afforded the
desired macrocycle 2. An Al2O3 column chromatographic
purification of the crude mixture gave a blue product of 2a in
13% yield. The detailed condition for the synthesis of 2a is given
in the supporting information.
1
The H NMR spectrum of 2a in CD2Cl2 showed two broad
NH signals in the low-field region and eight sp2-CH resonances
in a 5–9 ppm range (Figure S1). In terms of the chemical shift
values and the C2 symmetric splitting patterns in the spectrum,
the octaphyrin 2a can be regarded as a 36p nonaromatic
macrocycle. The explicit structure of 2a was finally elucidated by
X-ray crystallographic analysis (Figures 2a, S2, and Table S1).
The overall macrocyclic core of 2a adopts a figure-of-eight
structure stabilized by the effective inner hydrogen bonding
network within the porphyrin-like N4 sites. The confused pyrrole
segments are positioned at the hinge of the figure-of-eight core
in 2a, thereby configuring a stretched double twisted circuit
along with the smaller maximum torsional angles of ca. 40°
(Figure S2). The two meso-aryl groups connected to the b-
positions of the confused pyrroles are directing in the same side
with respect to the center axis. The presence of the correlation
peaks between the NH and b-CH of the regular pyrrole moieties
in the H-H COSY spectrum of 2a also supports the assumption
for the proposed structure in solution (Figure S3).
Figure 3. UV-vis-NIR absorption spectral change of 2a (black: 0 h; red: after
10 h) in CH2Cl2 at 298 K. The inset shows the time versus absorbance change
(DA) at 580 nm.
monitored by the absorption spectra (Figure 3). The spectral
change was completed after 10 h; a shoulder band at 580 nm
emerged gradually. The corresponding changes were also
1
observed in the H NMR spectra of 2a in CDCl3. Along with the
original peaks, sets of the similar resonance patterns (for 10
pyrrole CHs and 2NHs) gradually appeared in the spectrum
(Figure S4). Considering the distinct C2 symmetric structural
feature and the identical mass number (m/z) observed for the
resulting product, dynamic isomerization to the different type of
tautomer was predicted upon changing the conformation. The
detailed COSY spectral analysis of the product mixture indicates
Unexpectedly, a dynamic conformational change of 2a was
seen in solution (e.g., CH2Cl2) at 298 K. The solution color of 2a
gradually changed from blue to purplish blue on standing under
atmospheric conditions. The time course of this conversion was
2
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that the confused pyrrole moieties in the isomer possess amine-
nitrogen (NH) forms with C2-like symmetry (Figure S5). In
addition, the ROESY spectrum revealed the distinct cross peaks
originated from the proximity of the specific regular pyrrole NH
and b-pyrrolic CH (Figure S6). Based on these observations, we
corresponding bis-copper complex (Cu2-2) as
a
single
conformational product in nearly quantitative yield. Based on the
crystal structure of Cu2-2, copper cations are localized at the
inner N4 coordination environment with retaining the distinct
figure-of-eight structure as similar to the freebase 2a (Figures 2b,
S13, and Table S1). The atomic distance between the copper
proposed that transformation to
a dumbbell-shaped core
structure (i.e., 2b) with alternately twisted dipyrrin units through
flipping out the confused pyrrole rings of 2a.[13] At the equilibrium
(after 24 h), the final ratio of 2a and 2b was estimated to be 5:1
at 298 K, indicating the Gibbs free energy difference of ca. 1
kcal mol–1.
ions is calculated to be approximately 7.85 Å which is
significantly larger than the regular one (d = 5.42 Å).[12] This
larger copper-copper separation is reflected in the weak
intramolecular magnetic interaction JCu-Cu between the d9
copper(II) ions in Cu2-2, which was inferred by the temperature-
dependent magnetic susceptibility (SQUID) measurements
(Figure S14). The very weak antiferromagnetic interaction
between two copper(II) ions was estimated to be J = –0.7 K
upon fitting the plot with the Bleaney-Bowers model.[17]
Combined with the theoretical aspects with DFT calculations
(B3LYP/6-31G**), it is highly essential to consider the effective
hydrogen bonding network between the adjacent imino-nitrogen
atoms and amine protons of the coplanar dipyrrin segment for
stabilization of the corresponding model structures.[14] Among
the potential conformers of 2 (2a-j, Figure S7), the energies for
2a and 2b were considerably small (DE = –4.39 and –5.03 kcal
mol–1, respectively), compared to the regular octaphyrin 1. This
result indicates the introduction of two confused pyrrole moieties
into the parent framework in the proper way, increases the
stability of the molecules. The atomic distances between the
specific regular pyrrole NH and b-pyrrolic CH in 2b are close to
be 2.80 and 2.33 Å, respectively, which is well consistent with
the ROESY spectroscopic assignment. In addition, the time-
dependent (TD)-DFT calculations indicate the distinct transition
modes of 2a and 2b; the most intense band around 600 nm is
likely broaden due to the overlapped contributions from each
conformer (Figures S8 and Table S2).
Intriguingly, the lower-energy absorption beyond 1000 nm in
the UV-vis-NIR spectrum of Cu2-2 suggested the smooth
electron delocalization through the figure-of-eight 36p-
conjugated circuit (Figure S16). The most pronounce properties
of the complex Cu2-2 is the higher chemical stability even under
heating condition. No thermal metathesis-like splitting allows us
for further investigation of the chiroptical property.[18] The
racemic complex Cu2-2 was successfully separated to (P,P) and
(M,M) helical complexes by chiral phase HPLC technique
(DAICEL, Chiralpak IA, hexane/i-PrOH), respectively (Figure
S18). The CD spectrum of each fraction shows the clear cotton
bands up to 1000 nm with the mirror image structures due to the
fully p-conjugation nature of the scaffold (Figure 4). The strength
of chiroptical properties was determined by the anisotropy factor
(g value = Demax/e) of 0.019 at 900 nm, which is remarkably larger
despite the NIR region.[19] With the aid of TD-DFT calculations,
the first fraction was predicted to be (P,P) twisted enantiomer,
while the second one is to be opposite (M,M) helices (Table
S2c). The distinct stability of the enantiomeric structures of Cu2-
2 was proved under heating conditions (more than 100 °C), not
to be racemic.
This NIR-chiroptical property was well responsive to the redox
chemistry of the helical figure-of-eight complexes. Cyclic
voltammetry of Cu2-2 showed two oxidation waves at 0.14 and
0.53 V (vs. ferrocene/ferrocenium standard) in CH2Cl2 (Figure
S19). Upon bulk electrolysis applied at 0.2 V or addition of an
equivalent of magic blue oxidant, the distinct spectral change
occurred with the appearance of the broad NIR absorbance
around 1900 nm (Figures S20–S21). The EPR spectrum of the
oxidized product obtained by addition of one equivalent of magic
blue oxidant at 298 K revealed a characteristic spectrum of d9
copper like species (Figure S22). The antiferromagnetic coupling
interaction between the copper ion and the ligand p-cation
radical (35p species) was anticipated for the ground S = 1/2
electronic structure. Notably, the resulting p-radical delocalized
The thermal isomerization reaction was further analyzed by
the kinetic studies. From the rate constants measured at 313,
315.5, 318, 320.5, 323, and 325.5
K
in CH2Cl2, the
thermodynamic parameters (DG‡, DH‡, DS‡) from 2a to 2b at 298
K were determined to be 22.3 kcal mol–1, 14.8 kcal mol–1, and
23.6 cal (mol•K)–1, respectively (Figure S9). The positive entropy
of activation indicates that the flexibility of the macrocycle may
raise the transition energies. The slow interconversion could be
governed by the large conformational change. Taking advantage
of this large energy barrier, the 2b-enriched sample was
obtained by rapid chromatographic purification of the equilibrium
mixture, and subjected to the thermal isomerization from 2b to
2a. From the absorption spectral changes as expected, the
mutual conversion behavior was confirmed (Figure S10).
Considering the solvent-controlled NH-tautomerism for N-
confused porphyrins (regarded as a half skeletal unit of 2a), this
equilibrium behavior of 2a could be dependent on the choice of
solvents (Figure S11).[15] For example, upon dissolving in
acetonitrile, the purple solution was obtained and the blue-
shifted band peak of 579 nm was observed in the absorption
spectrum. The 1H NMR spectrum in CD3CN exhibited the
severely broaden peaks of 2a with the shift of the NH resonance, helical complex, [Cu2-2]+• is relatively stable under the ambient
which suggests that the acetonitrile molecule may interact with
the pyrrolic amine moieties via hydrogen bondings (Figure S12).
The NH-tautomeric nature in 2a could play a role in this mutual
conformational interconversion.[16]
In contrast to the dynamic behavior of the freebase 2a, the
bis-metal coordination with copper(II) cations switched to the
rigid figure-of-eight structure (Scheme 1). The treatment of the
mixture of 2 with Cu(OAc)2 in CH2Cl2/MeOH yielded the
conditions (e.g., t1/2 = 11.2 h at 298 K in CH2Cl2, Figures S23–
S24). In this way, the CD spectrum of the oxidized species,
[Cu2-2]+• exhibited the distinct spectral response with retaining
the cotton effect of the twisted complex, which suggested the
redox-responsive chiral information storage applications (Figure
S25).
3
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Figure 4. CD and absorption spectra of the enantiomeric forms of Cu2-2 in
CH2Cl2 at 298 K. The simulated TD-DFT stick spectra of P-twisted form of
Cu2-2 are given.
[11] K. Fujino, Y. Hirata, Y. Kawabe, T. Morimoto, A. Srinivasan, M.
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In summary, we have successfully synthesized a novel
[36]octaphyrin isomer 2 possessing two confused pyrrole rings,
as the largest N-confused families of expanded porphyrinoids.[8]
The intrinsic figure-of-eight geometry of 2 is gradually converted
to the dumbbell one in equilibrium with prototropy-coupling event
on the pyrrolic NHs. Upon bis-copper(II) complexation, a stable
helical twisted complex Cu2-2 was generated in the pursuit of
topological architectures. The pure enantiomers showed distinct
NIR-CD spectrum, being interesting in the perspective of their
potential applications as NIR optical chiral sensors through
molecular recognitions. The marked stabilization of the radical
spin delocalized onto the helical twisted octaphyrin scaffold may
also utilize the potential magneto-electronic device applications.
[12] J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi, A. Osuka, J. Am. Chem. Soc.
2001, 123, 7190–7191.
[13] The similar type of dumbbell-like octapyrrole structure was seen in the
confused calix[8]pyrrole boron difluoride complex. M. Ishida, T.
Omagari, R, Hirosawa, K. Jono, Y. M. Sung, Y. Yasutake, H. Uno, M.
Toganoh, H. Nakanotani, S. Fukatsu, D. Kim, H. Furuta, Angew. Chem.
Int. Ed. 2016, 55, 12045–12049; Angew. Chem. 2016, 128, 12224–
12228.
[14] The energetically similar tautomeric model, 2d with the identical figure-
eight configured geometry was ruled out since the Gibbs energy profile
for the NH tautomerism of typical porphyrins is known to be DG‡ ~15
kcal mol-1.
[15] H. Furuta, T. Ishizuka, A Osuka, H. Dejima, H. Nakagawa, Y. Ishikawa,
J. Am. Chem. Soc. 2001, 123, 6207–6208.
[16] The NH tautomerism-induced cis-trans isomerization of N-confused
porphyrin derivative was recently reported. R. Sakashita, Y. Oka, H.
Akimaru, P. E. Kesavan, M. Ishida, M. Toganoh, T. Ishizuka, S. Mori, H.
Furuta, J. Org. Chem. 2017, 82, 8686–8696.
Acknowledgments
The present work was supported by Grant-in-Aids (15K13646 to
H.F.; 16K05700 and 17H05377 to M.I.) from Japan Society for
the Promotion of Science (JSPS). The financial support from a
bilateral program between JSPS and the National Research
Foundation (NRF) of South Africa is also acknowledged.
[17] The EPR spectrum of Cu2-2 exhibited the typical feature of the isolated
square planar copper(II) species with the giso tensor value of 2.06 and
g// of 2.13 (A// = 80 mT) in the frozen toluene solution at 77 K (Figure
S15). This is well consistent with the values (i.e., giso = 2.09, g// = 2.12)
reported in the copper(II) porphyrin. See, O. Kahn, Molecular
Magnetism, Wiley-VCH, New York, 1993.
Keywords: octaphyrin • helix • chirality • Isomerization• bis-
metal coordination
[18] Upon heating the solution of Cu2-2 up to 120 °C, none of the
methathesis-like products were obtained. See, refs. 6 and 9.
[19] E. L. Eliel, S. H. Wilen, Stereochemistry of Organic Compounds; Wiley,
New York, 1994.
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4
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Eight-Figured Chirality:
Koki Mitsuno, Takafumi Yoshino, Iti
Gupta, Shigeki Mori, Satoru Karasawa,
Masatoshi Ishida*, and Hiroyuki
Furuta*
A novel [36]octaphyrin analogue
embedding
two
N-confused
pyrrole units demonstrated unique
prototropy-coupled isomerization.
Upon bis-metal complexation, the
fully p-conjugated, helical figure-
of-eight structures were realized.
Chirogenesis of the helical
enantiomers was proved by
distinct CD response in the NIR
region.
Page No. – Page No.
Doubly N-Confused
[36]Octaphyrin(1.1.1.1.1.1.1.1):
Isomerization, Bis-Metal
Coordination, and Topological
Chirality
5
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