Quadruply Twisted Hückel-Aromatic Dodecaphyrin

Article abstract of DOI:10.1002/anie.201811433

Molecular topology of π-conjugated circuits becomes increasingly important in the chemistry of aromatic and antiaromatic compounds. meso-Pentafluorophenyl-substituted 5,35-(1,4-phenylene)bridged [56]dodecaphyrin was synthesized by condensation of 1,4-phen

Full text of DOI:10.1002/anie.201811433

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Accepted Article
Title: Quadruply Twisted Hückel Aromatic Dodecaphyrin
Authors: Atsuhiro Osuka, Takanori Soya, and Hirotaka Mori
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811433
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Quadruply Twisted Hückel Aromatic Dodecaphyrin
Takanori Soya, Hirotaka Mori, and Atsuhiro Osuka*
Abstract: Molecular topology of p-conjugated circuits becomes
increasingly important in the chemistry of aromatic and antiaromatic
In the last two decades, meso-aryl substituted expanded
porphyrins consisting of more than four pyrrole units have been
demonstrated to serve as effective platforms to realize versatile
electronic states including Hückel aromatic and antiaromatic,
Möbius aromatic and antiaromatic, and stable radical species.[9]
Among these, meso-trifluoromethyl substituted [56]dodecaphyrin
2 consisting of twelve pyrrolic units occupies a unique position, in
that it has been revealed to take a quadruply twisted topology (|Lk|
= 4) with a helical conformation in the solid state. But
dodecaphyrin 2 has been shown to exist as a mixture of many
conformers in solution, hampering the detailed analysis.[10a] It
occurred to us that an appropriate structural fixation of such large
p-conjugated macrocycles would lead to realization of triply or
more twisted topologies with aromatic or antiaromatic characters.
To test this idea, internally 5,35-(1,4-phenylene)bridged
dodecaphyrin 3 was designed as a highly twisted p-conjugated
system.[11] Herein, we report the synthesis and PdII-metalation of
3. Importantly, one of the bis-PdII complexes has been identified
as the first example of quadruply twisted Hückel aromatic
molecule.
compounds.
meso-Pentafluorophenyl-substituted
5,35-(1,4-
by
phenylene)bridged
[56]dodecaphyrin was synthesized
condensation of 1,4-phenylene-bridged dicarbinol dimer and 5,10,15-
tris-(pentafluorophenyl)tetrapyrrane followed by oxidation with DDQ
and was oxidized to its [54]- and [52]congeners in a stepwise manner.
Metalation of the [52]dodecaphyrin with Pd2(dba)3 gave two bis-PdII
complexes that are isomers of metalation sites, anti and syn with
regard to the 1,4-phenylene bridge. The anti-isomer was easily
oxidized to its N-fused form, a quadruply twisted non-aromatic or
weakly aromatic macrocycle. On the other hand, the syn-isomer was
revealed to be the first example of Hückel aromatic molecule with a
quadruply twisted structure.
Aromaticity is one of the most fundamental concepts in organic
chemistry, since it is important in understanding of energetic,
structural, and magnetic properties of p-conjugated cyclic
molecules.[1] Recent progress in this field has revealed in-plane[2]
and stacked aromatic[3] molecules, demonstrating that aromatic
molecules are diverse, not limited to the conventional planar ones.
In relation to these, the relationship between the topology of p-
conjugated macrocycles and their aromaticities has become
increasingly important since the first synthesis of a singly twisted
Möbius aromatic molecule by Herges et al. in 2003.[4–6]
Topology of p-conjugated macrocycles is defined by the
absolute value of linking number |Lk| that corresponds to the
number of half-twist (180°) formally applied to a ribbon, which can
experimentally be confirmed by cutting a closed ribbon along the
middle. Namely, closed ribbons with |Lk| = 0, 1, 2, 3, and 4 should
become two separated rings, a ring, a catenane, a trefoil knot, and
a
Solomon's link, respectively, after the middle cut
Figure 1. Representative examples of triply twisted (|Lk| = 3) and quadruply
twisted (|Lk| = 4) p-conjugated macrocycles. R = CF3.
procedure.[4c,8a,9b] When |Lk| is even (including zero), p-conjugated
macrocycles are aromatic with [4n+2]p electrons and antiaromatic
with 4np electrons. In contrast, those with odd values of |Lk| are
aromatic with 4np electrons and antiaromatic with [4n+2]p
electrons.[7] However, despite the extensive synthetic efforts,
there has been no report on aromatic nor antiaromatic molecules
possessing more than triply twisted (|Lk| ≥ 3) topology.[4,9b–e] As a
rare example, [24]annulene 1 was rationally designed and
synthesized as the first example of triply twisted (|Lk| = 3)
annulene by Herges et al. in 2014, but this molecule did not show
any aromatic character (Figure 1).[8a] Therefore, synthesis of
aromatic compounds with more than three molecular twists still
remains a big challenge, since it is difficult to accommodate
macrocyclic conjugation in such twisted systems.
1,4-Phenylene-bridged
synthesized via acid catalyzed condensation of a 1:2 mixture of
1,4-phenylene-bridged dicarbinol dimer and 5,10,15-tris-
[56]dodecaphyrin
3
was
8
(pentafluorophenyl)tetrapyrrane 9 followed by oxidation with DDQ
in 8.3% yield. High-resolution electrospray-ionization time-of-flight
mass-spectrometry (HR-ESI-TOF-MS) revealed the parent ion
peak at m/z = 2665.2234 ([M–H]–; calcd for C126H35N12F50, m/z =
2665.2315). Fortunately, single crystals of 3 suitable for X-ray
analysis were obtained by vapor diffusion of n-heptane into a
benzene solution of 3. The solid state structure of 3 has been
revealed to be a butterfly-like conformation (Figure 2a).[12a] Non-
twisted nature of 3 (|Lk| = 0) has been confirmed by the middle cut
procedure, which gives two separated rings (see Supplemental
1
Movie 1). The H NMR spectrum of 3 shows four singlets in the
[a]
T. Soya, Dr. H. Mori, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto, 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
range of d = 14.03–9.87 ppm due to the eight NHs, a singlet at d
= 7.13 ppm due to the four phenylene protons, and eleven signals
in the range of d = 7.73–6.04 ppm due to the twenty-four pyrrolic-
b protons, indicating Ci symmetry and the absence of macrocyclic
ring current effects. In addition to these major signals, there was
a set of minor signals, suggesting the existence of minor
Supporting information for this article is given via a link at the end of
the document.
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conformers in solution (Figure S5-1 in Supporting Information
(SI)).
current effects and rigid structure even in solution (Figure S5-3 in
SI). In contrast, the 1H NMR spectrum of 4 is complicated,
suggesting that 4 exists as a mixture of several conformers in
solution similar to 3 (Figure S5-2 in SI). These results imply that
the introduction of a 1,4-phenylene bridge is not sufficient to fix
the conformations of the dodecaphyrin.
Figure 2. X-Ray crystal structures of (a) 3 and (b) 5. The thermal ellipsoids are
represented at 50% probability level. Solvent molecules, pentafluorophenyl
groups, and hydrogen atoms except for NHs are omitted for clarity.
Representations of the p-conjugated circuits of (a) 3 and (b) 5.
In order to further rigidify the structure, we attempted
metalations of 5. After many attempts (Table S2-1 in SI), we found
that treatment of 5 with Pd2dba3 (5.0 eq.) in the presence of
NaOAc (10 eq.) in CH2Cl2 and MeOH gave two bis-PdII complexes
6 and 7 in 15 and 18% yields, respectively, after separation
through silica gel column chromatography (Scheme 2).[13] HR-
ESI-TOF-MS showed the parent ion peaks of 6 and 7 at m/z =
2870.9837 and 2870.9857 ([M–H]–; calcd for C126H29N12F50Pd2,
m/z = 2870.9962). The isomers 6 and 7 have been assigned,
respectively, as anti and syn isomers with regard to the 1,4-
phenylene-bridge as revealed by the discussion below. The 1H
NMR spectrum of 6 displays a singlet at d = 16.41 ppm due to the
NHs, twelve signals in the range of d = 14.05–3.32 ppm due to the
pyrrolic-b protons, and two doublets at d = 10.33 and 7.55 ppm
due to the bridging phenylene moiety, suggesting its C2 symmetric
structure (Figure 3a). Regrettably, the crystal structure of 6 has
not been obtained due to its facile fusion reaction to give 6-fused
in solution. Indeed, 6-fused was obtained in 82% yield upon
oxidation of 6 with DDQ. The structure of 6-fused was revealed
by single crystal X-ray diffraction analysis to take a plectonemic
conformation (Figure 4a).[12c] The N-fusion reaction occurred at
the closely located pyrrolic nitrogen atom and 1,4-phenylene
bridge. To our surprise, the quadruply twisted p-conjugated circuit
(|Lk| = 4) has been confirmed by the middle cutting procedure that
gives a Solomon's Links. The 1H NMR spectrum of 6-fused
showed a singlet at d = 13.14 ppm due to the NH, twenty-two
Scheme 1. Chemical structures of 3, 4, 5, 8, and 9 and the interconversion
between 3, 4, and 5. Pentafluorophenyl groups in 3, 4, and 5 are omitted for
clarity.
While 3 was not reduced with an excess amount of NaBH4, 3
was smoothly oxidized to [54]dodecaphyrin 4 upon treatment with
p-chloranil (1.0 eq.) in CH2Cl2, which was isolated as violet solids
in 29% yield. Further oxidation was accomplished with an excess
amount of MnO2 (10 eq.) to afford [52]dodecaphyrin 5 as red
solids in 70% yield. Reductions of 4 and 5 with NaBH4
regenerated 3 quantitatively. We could not get single crystals of 4
because of its easy reduction to 3 under recrystallization
conditions, but we obtained single crystals of 5 by vapor diffusion
of methanol into a CHCl3 solution of 5.[12b] In the solid state, 5
takes a crescent-like conformation (Figure 2b). The doubly
twisted topology (|Lk| = 2) has been confirmed by the middle cut
procedure that gives a catenane (Supplemental Movie 2). The 1H
NMR spectrum of 5 shows a set of sharp signals including two
singlets at d = 13.19 and 12.46 ppm due to the two NHs, two
signals at d = 8.69 and 7.35 ppm due to the four phenylene
protons, and twelve signals in the range of d = 8.17–6.08 ppm due
to the twenty-four pyrrolic-b protons, indicating the absence of ring
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signals in the range of d = 13.07–4.84 ppm due to the twenty-four
pyrrolic-b protons, and two singlets at d = 11.04 and 6.90 ppm due
to the 1,4-phenylene bridge, suggesting the absence of diatropic
ring current (Figure 3b).
moiety located at the inner part of the macrocycle, indicating a
substantial diatropic ring current (Figure 3c). The solid state
structure of 7 has been revealed by X-ray crystallographic
analysis (Figure 4b).[12d] Similarly to 6-fused, 7 displays quadruply
twisted topology (|Lk| = 4), which gives Solomon's Link by the
middle cut procedure (Figure 4b and Supplemental Movie 3). The
nucleus independent chemical shift (NICS)[14] values at the
selected inner positions of 7 were calculated to be –10.72 and –
11.50 ppm, also supporting the aromaticity of 7 (positions 29 and
30 in Figure S8-8 in the SI). In addition, the anisotropy of the
induced current density (ACID) of 7 was calculated to show a
diatropic ring current (Figure S8-14 in the SI).[15] In order to realize
unprecedented quadruply twisted Hückel antiaromatic molecules,
reductions and oxidations of 6-fused and 7 were attempted but
all these attempts failed, merely ending as the recovery or
decomposition of the starting materials.
Scheme 2. PdII metalation of 5 and fusion reaction of 6. Pentafluorophenyl
groups are omitted for clarity. A plausible structure is shown for 6.
Figure 4. Left side: X-Ray crystal structures of (a) 6-fused and (b) 7. One of
two independent molecules of 6-fused in the unit cell is shown here. The
thermal ellipsoids are represented at 50% probability. Pentafluorophenyl groups,
solvent molecules, and hydrogen atoms except for NHs were omitted for clarity.
Right side: Representations of the p-conjugated circuits of (a) 6-fused and (b)
7.
Figure 3. 1H NMR spectra of (a) 6, (b) 6-fused, and (c) 7 in CDCl3 at 25 °C.
Peaks marked with * are due to residual solvents and impurities.
Figure 5. UV/Vis/NIR absorption spectra of 6, 6-fused, and 7 in CH2Cl2 at 25 °C.
* indicates absorption due to solvents.
On the other hand, the 1H NMR spectrum of the syn-isomer 7
shows a singlet at d = 9.67 ppm due to the NHs, twelve signals in
the range of d = 8.59 – 4.22 ppm due to the pyrrolic-b protons,
and two doublets at d = 4.46 and 3.81 ppm due to the phenylene
The UV/Vis/NIR absorption spectra showed the absorption
maxima at 331, 425, 674, and 1215 nm for 6 and 326, 420, 667,
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Sung, I. D. Williams, X. Huang, R. Herges, B. Z. Tang, Chem. Eur. J.
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and 1118 nm for 6-fused without any absorption maxima around
lower energy region. In contrast, the UV/Vis/NIR absorption
spectrum of 7 showed the Soret-like band at 1147 nm and the Q-
like bands reaching to the region over 2500 nm, which is in line
with its aromatic character (Figure 5).
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Finally, the electrochemical properties of the dodecaphyrins
were measured by cyclic voltammetry (Figures S6-1 and S6-2 in
the Supporting Information). While 3 and 4 showed five and six
reversible redox waves respectively, 5 showed seven reversible
redox waves. Furthermore, the PdII complexes 6, 6-fused, and 7
showed eight reversible redox waves. The observed multiply
reversible redox behaviors can be ascribed to the large p-
conjugated networks and the rigid structures.
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In summary, fully p-conjugated macrocycles possessing
quadruply twisted topology (|Lk| = 4) 6-fused and 7 were realized
by fixing the conformation of a [54]dodecaphyrin with 1,4-
phenylene bridge and PdII ions. While 6-fused showed no
aromatic character, 7 has been identified as a 54p Hückel
aromatic species by 1H NMR and UV/Vis/NIR absorption spectra
and NICS and ACID calculations. To the best of our knowledge,
7 is the first quadruply twisted Hückel aromatic molecule.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
25220802, 18H03910, and 18K19074. T.S. and H.M.
acknowledge a JSPS Fellowship for Young Scientists.
[12] a) Crystallographic data for 3: C126H36F50N12·8(C6H6)·2(C6), Mr
=
3436.66; triclinic; space group P-1 (No.2), a = 14.177(2), b = 16.712(4),
c = 17.278(3) Å; a =106.043(5), b = 104.1545(3), γ = 97.629(7)°; V =
3725.5(12) Å3; rcalcd = 1.174 g·cm–3; Z = 1; R1 = 0.0531 [I > 2.0s(I)], wR2
= 0.1666 (all data), GOF = 1.052; b) Crystallographic data for 5:
C126H32F50N12·4(CHCl3), Mr = 3104.82; monoclinic; space group C2/c
Keywords: Topology • Quadruple Twist • Solomon's Link •
Aromaticity • Porphyrinoids
(No.15),
a = 63.06(3), b = 20.5623(10), c = 23.476(10) Å; b =
91.853(12)°; V = 30424(19) Å3; rcalcd = 1.356 g·cm–3; Z = 8; R1 = 0.0865
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data for 6-fused: 2(C126H28F50N12Pd2)·6.822(C6H6)·2(C2O), Mr
6379.24; triclinic; space group P-1 (No.2), 19.2854(13), b
=
=
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a
=
28.8466(12), c = 30.6271(18) Å; a = 108.8038(10), b = 108.136(14), γ =
103.865(15)°; V = 14191(2) Å3; rcalcd = 1.493 g·cm–3; Z = 2; R1 = 0.0703
[I > 2.0s(I)], wR2 = 0.2190 (all data), GOF = 1.009; d) Crystallographic
data for 7: C126H30F50N12Pd2·3(C6O)·6(CCl4), Mr = 4061.46; monoclinic;
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a = 26.6753(4), b = 29.1994(4), c =
42.9903(6) Å; b = 105.940(2)°; V = 32197.7(9) Å3; rcalcd = 1.676 g·cm–3
;
Z = 8; R1 = 0.1759 [I > 2.0s(I)], wR2 = 0.4388 (all data), GOF = 1.081.
CCDC- 1868904 (3), 1868905 (5), 1868906 (6-fused), and 1868907 (7)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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disordered solvents in the crystals of 5, 6-fused, and 7 were removed by
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[15] a) R. Herges, D. Geuenich, J. Phys. Chem. A 2001, 105, 3214; b) D.
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p-Conjugated macrocycles
possessing quadruply twisted
topology were realized by
conformational fixation by
insertion of 1,4-phenylene
bridge and PdII metalation.
Syn-isomer of bis-PdII
T. Soya, H. Mori, A. Osuka*
Page No. – Page No.
Quadruply Twisted Hückel
Aromatic Dodecaphyrin
complexes displays aromatic
characters, thus being
identified as the first example
of quadruply twisted Hückel
aromatic molecule.
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