Reversible caterpillar-motion like isomerization in a N,N′-dimethyl hexaphyrin(1.1.1.1.1.1) induced by two-electron oxidation or reduction

Article abstract of DOI:10.1039/b506586k

Caterpillar-motion like rotational isomerization of meso-aryl substituted [26]- and [28]hexaphyrins has been revealed for the first time. Two-electron oxidation and reduction induce reversible interconversion between N,N′-dimethyl [26]- and [28]hexaphyrin

Full text of DOI:10.1039/b506586k

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Reversible caterpillar-motion like isomerization in a N,N9-dimethyl
hexaphyrin(1.1.1.1.1.1) induced by two-electron oxidation or reduction{
Masaaki Suzuki and Atsuhiro Osuka*
Received (in Cambridge, UK) 11th May 2005, Accepted 1st June 2005
First published as an Advance Article on the web 23rd June 2005
DOI: 10.1039/b506586k
Initially, we examined the possibility of such a structural
isomerization using [26]hexaphyrin 3 and 4 that were prepared
from the reaction of 5,10-bis(pentafluorophenyl)tripyrrane with
2,4,6-trifluorobenzaldehyde in 1:1 ratio.3e,4 These hexaphyrins do
not isomerize at 0 uC but gradually isomerize at 40 uC toward the
equilibrated mixture consisting of 3 and 4 in a 2.5:1 ratio. Thus the
isomer 3 is more stable than 4 in spite of the same steric hindrance
at the ortho-positions of the meso-substituted aryl groups. The
isomer 3 is less polar and was separated from 4 by silica gel column
chromatography while separation of 4 was more difficult due to
Caterpillar-motion like rotational isomerization of meso-aryl
substituted [26]- and [28]hexaphyrins has been revealed for the
first time. Two-electron oxidation and reduction induce
reversible interconversion between N,N9-dimethyl [26]- and
[28]hexaphyrins with a rotational isomerization, in which the
N-methylated pyrroles move from the corner in the former to
the centre in the latter.
Expanded porphyrins with conjugated pyrrolic macrocycles larger
than porphyrin are an interesting class of molecules that exhibit
unique physical and chemical properties that are difficult to find in
porphyrins.1,2 Among these, meso-hexakis(pentafluorophenyl)
substituted hexaphyrin (1) with a 26p-electronic circuit can be
regarded as a real homolog of porphyrin with an 18p-electronic
1
the tailing nature of 3. Variable temperature H NMR spectros-
copy on pure 3 has determined the activation energy of the
rotational isomerization to be only 1.4 kJ mol21. The rotational
isomerization has been also examined for [28]hexaphyrins 5 and 6
that were prepared by NaBH4 reduction of 3 or 4. Reduction of
the pure 3 with NaBH4 readily gave a mixture of 5 and 6 in a ratio
of 1:6.7. Extensive heating of this mixture did not cause any
changes, hence indicating this to be an equilibrated mixture.
Interestingly, the similar reduction of a mixture that contained
mainly 4 at room temperature provided essentially the same
mixture of 5 and 6, suggesting an even smaller activation barrier
for the interconversion between 5 and 6.
1
circuit in respect of strong aromaticity.3 In fact, the H NMR
spectrum of 1 exhibits doublets at 9.42 and 9.09 ppm due to
the outer b-protons and a singlet at 22.43 ppm due to the inner
b-protons. A notable difference in the chemical reactivity of 1 from
those of porphyrins is its easy and quantitative reduction to stable
[28]hexaphyrin 2. Another key structural difference is its
rectangular shape consisting of two different pyrroles, four corner
pyrroles and two inverted pyrroles at the centre, which contrasts
with the square shape of porphyrin consisting of four identical
pyrroles except the presence or absence of NH proton. This
inequality may lead to a structural isomerization shown in
Scheme 1, where a caterpillar-motion like macrocyclic ring
rotation changes the position of meso-aryl substituents. Such a
structural isomerization, which can be detected for hexaphyrins
that have a meso-substituent pattern like A and B has, to the best
of our knowledge, never been reported before.
In the course of our studies on meso-aryl substituted expanded
porphyrins, we examined N-methylation of 1.5 Initially, the
deprotonation of 1 was attempted with tetrabutylammonium
fluoride (TBAF). Addition of TBAF to a CH2Cl2 solution of 1
caused changes in absorption pattern as shown in Fig. 2, in which
the Soret-like band at 568 nm of 1 was shifted to 601 nm and
sharpened with a shoulder at 617 nm. These spectral changes have
been assigned due to formation of a doubly deprotronated
hexaphyrin. Such a species was also detected in the 1H NMR
spectrum, which exhibited a singlet at 24.33 ppm due to the inner
pyrrolic b-protons and doublets at 9.21 and 9.51 ppm due to the
outer pyrrolic b-protons. These data indicate an enhanced
diatropic ring current for the dianion compared with 1. Crystals
of the dianion suitable for X-ray diffraction analysis were grown
by vapor diffusion of hexane to its chloroform solution. In the
solid state, the dianion shows a more planar macrocycle with the
˚
mean plane deviation of 0.155 A compared with the parent
˚
[26]hexaphyrin 1 that shows the mean plane deviation of 0.410 A.
Scheme 1 A possible rotational isomerization between hexaphyrins A
and B.
3a
In addition, the dianion displays almost no bond length alternation
for the two C–C bonds at the methene bridges in line with the
enhanced diatropic ring current (Supporting Information{). Two
tetrabutylammonium cations are found just above and below the
negatively charged nitrogen atoms of the dianion of 1 with ca.
Department of Chemistry, Graduate School of Science, Kyoto
University, and Core Research for Evolutional Science and Technology
(CREST), Japan Science and Technology Agency, Sakyo-ku, Kyoto,
606-8502, Japan. E-mail: osuka@kuchem.kyoto-u.ac.jp;
Fax: +81 75 753 3970; Tel: +81 75 753 4008
˚
4.14 A separations, plausibly for the charge compensation.
{ Electronic supplementary information (ESI) available: experimental
details. See http://dx.doi.org/10.1039/b506586k
Collectively, these data support the enhanced aromaticity for the
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Fig. 1 Crystal structures of dianion of 1 (left), 8 (centre), and 9 (right). Upper: top views; lower: side views. In side views, meso-aryl groups are omitted for
clarity.{
dianion of 1, probably because of favourable p-electronic
delocalization.
Addition of excess amounts of methyl iodide to a solution of the
dianion and subsequent refluxing led to formation of mono
N-methyl [26]hexaphyrin 7 and N,N9-dimethyl [26]hexaphyrin 8
both as green solids in varying amounts that depended on the
amount of methyl iodide and reaction time. The N-methyl protons
of 7 appear at 23.31 ppm and those of 8 appear at 22.94 ppm,
indicating the preservation of strong diatropic ring current for
these [26]hexaphyrins. The structure of 8 has been determined by
X-ray diffraction analysis, which shows a ruffled structure with the
˚
mean plane deviation of 0.326 A and N-methylations at the
diagonal corner pyrroles with a syn-arrangement of the N-methyl
groups (Fig. 1). The N–C bond lengths in the pyrroles A and D are
˚
1.46(1) and 1.49(1) A, respectively, being similar to those of
N-methylated porphyrins.6 The absorption spectra of 7 and 8 are
similar to that of 1 in respect of the presence of a Soret-like band
and Q-band-like bands, but the Soret-like bands are red-shifted
and broadened in the order of 1 (568 nm) , 7 (570 nm) , 8
(578 nm). Other alkylating agents such as ethyl iodide or benzyl
bromide did not give the corresponding N-alkylated compounds,
probably due to steric reasons.
In a next step, [26]hexaphyrin 8 was reduced with NaBH4 to
give N,N9-dimethyl [28]hexaphyrin 9 quantitatively. The 1H NMR
spectrum of 9 showed a singlet at 2.42 ppm due to the N-methyl
groups that was not observed in the 1H NMR spectrum of N,N9-
di-CD3 labelled compound prepared from the reaction of the
dianion of 1 and methyl iodide-d3. The inner and outer pyrrolic
b-protons appeared at 4.37 and 7.19 ppm, respectively, indicating
that the diatropic ring current was largely lost but somewhat
retained as in the case of [28]hexaphyrin 2. The structure of 9 was
solved by single crystal X-ray diffraction analysis (Fig. 2).
Curiously, the N-methylated pyrrole rings, which are located at
the corner in 8, are moved to the centre of long side with inversion
in 9, indicating the occurrence of a caterpillar-motion like
macrocyclic ring rotation during the reduction. The [28]hexaphyrin
9 shows a distorted ruffled conformation with the mean plane
Chart 1 Molecular structures of hexaphyrins.
˚
deviation of 0.444 A. Comparison of the skeletal deviations of the
macrocycle atoms from the 36 atoms mean plane reveals more
Fig. 2 UV-visible absorption spectra of 1, dianion of 1, 7, and 8 in
CH2Cl2.
distortion in 9 than 8 (Supporting Information{). In 9, the
3686 | Chem. Commun., 2005, 3685–3687
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This work was partly supported by Grant-in-Aid from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (No. 15350022 and 17350017).
Notes and references
{ Crystal data of dianion of 1: C66H12F30N6?2(C16H36N)?(C6H14) 5 2030,
monoclinic, space group C2/c (No.15), a 5 28.13 (3), b 5 19.70 (3),
3
˚
˚
c 5 18.13 (3) A, b 5 103.83 (6)u, V 5 9757 (23) A , Z 5 4, Dcalcd. 5
1.742 g cm23, T 5 2150 uC, R1 5 0.065 (I . 2s(I)), RW 5 0.188 (all data),
GOF 5 0.864. Single crystal of dianion was obtained by slow evaporation
of 1 with tetrabutylammonium triphenyldifluorosilicate (TBAT) in
CH2Cl2–hexane mixed solvent. CCDC: 270832. Crystal data of 8:
C68H18F30N6?2(CHCl3) 5 1728, monoclinic, space group P21/c (No. 14),
Scheme 2 A reversible interconversion between 8 and 9 mediated by
two-electron oxidation or reduction.
N-methyl groups on the pyrroles A and D are orienting in the
˚
same direction with the N–C bond length of 1.470(4) A and such
˚
a 5 13.73 (8), b 5 20.42 (1), c 5 22.93 (1) A, b 5 90.13 (5)u, V 5
6429 (6) A , Z 5 4, Dcalcd. 5 1.785 g cm23, T 5 2150 uC, R1 5 0.098 (I .
3
pyrroles are nearly perpendicular to the mean plane of the
macrocycle with tilting angles of 43.7u. The absorption spectrum of
9 is similar to that of 2 (Supporting Information{).
˚
3s(I)), RW 5 0.132 (I . 3s(I)), GOF 5 1.096. CCDC: 270833. Crystal
data of 9: C68H18F30N6?2(CH2Cl2) 5 1661, monoclinic, space group C2
˚
(No. 5), a 5 34.11 (5), b 5 6.491 (9), c 5 14.94 (3) A, b 5 106.88.13 (6)u,
Importantly, the oxidation of 9 with DDQ regenerated 8
quantitatively, thus permitting the reversible interconversion
between 8 and 9 with concurrent caterpillar-motion like isomer-
ization.7 In contrast to 3–6, the conformations of 8 and 9 were
found to be rather rigid and related rotational isomerization was
severely prohibited, as revealed by variable temperature 1H NMR
measurements that did not detect any spectral changes even at
140 uC. The explanation for the observed conformational
preferences for 8 and 9 are not clear at the present stage. One
possible reason may be as follows. The N-methyl groups at the
pyrroles A and D as found for 9 may suffer from more steric
hindrance through the interactions with the flanking meso-
substituted pentafluorophenyl groups nearby. Thus, the preferred
conformation of 9 may be accounted for in terms of stabilization
due to the hydrogen bonding interaction between the pyrroles B
and C and E and F that exceeds the steric hindrance of the
N-methyl groups. In the [26]hexaphyrin 8, such hydrogen bonding
interaction is absent, which guides the N-methyl pyrroles to take a
corner position with less steric interaction.
V 5 3165 (8) A , Z 5 2, Dcalcd. 5 1.742 g cm23, T 5 2150 uC, R1 5 0.035
3
˚
(I . 3s(I)), RW 5 0.033 (I . 3s (I)), GOF 5 0.943. CCDC: 270834. See
http://dx.doi.org/10.1039/b506586k for crystallographic data in CIF or
other electronic format.
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Handbook; K. M. Kadish, K. M. Smith and R. Guilard, Eds; Academic
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1918.
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A. M. S. Silva, V. Felix, M. G. B. Drew and J. A. S. Cavaleiro, Chem.
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7 Similar rotational isomerization was reported to be induced by two-
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´
N. Sprutta and L. Latos-Graz˙ynski, Chem. Eur. J., 2001, 7,
5099.
In summary, we have revealed the caterpillar-motion like
rotational isomerization of meso-aryl substituted hexaphyrins for
the first time. Such an isomerization is prohibited both in N,N9-
dimethylated [26]hexaphyrin 8 and its [28]hexaphyrin 9. However
these two hexaphyrins 8 and 9 are quantitatively interconvertible
via two-electron reduction and oxidation processes, which are
accompanied by the rotational isomerization that moves the
N-methylated pyrroles at the corner in 8 to the centre of the long
side in 9. Application of this interesting isomerization is an
attractive next project worthy of further investigation.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3685–3687 | 3687