Fully μ-conjugated helices from oxidative cleavage of meso-aryl-substituted expanded porphyrins

Full text of DOI:10.1021/ja909929s

Published on Web 01/28/2010
Fully π-Conjugated Helices from Oxidative Cleavage of meso-Aryl-Substituted
Expanded Porphyrins
Shohei Saito,† Ko Furukawa,‡ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan, and
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Received November 24, 2009; E-mail: osuka@kuchem.kyoto-u.ac.jp
Helical molecules possess a unique position in current chemistry
as a result of their intriguing structural and electronic properties as
well as their prevalent occurrence as a structural motif in nature.
Various helical molecules have thus been extensively studied to
date, including helicene,1 helicates,2 foldamers,3 artificial DNA,4
(C2S)n helices,5 salt-bridged strands,6 and so on. However, to the
best of our knowledge, fully π-conjugated helices have remained
unexplored, despite their potential importance in basic and applied
chemistry. A promising possibility may be molecular solenoids
working on a conducting property.7 Ineffective π delocalization in
helical molecules is mainly the result of interruption of π conjuga-
tion by cross-conjugation8 and/or π-electron localization on aromatic
segments,9 which can be seen in their absorption spectra that display
absorption maxima at <450 nm.
long Cu-Cu distance of 6.28 Å and an irregular helix pitch of
5-8 Å. Cu(1) is bound to the four nitrogen atoms of the pyrrole
rings A, B, C, and D, while Cu(2) is bound to the four nitrogen
atoms of the pyrrole rings E, F, G, and H. This structure indicated
that the oxidative cleavage proceeded at the meso-R bond of the
octaphyrin macrocycle.10 The oxygen source for 4 was assigned
to be molecular oxygen, since 4 was not formed under anaerobic
conditions even in the presence of water. In addition, the experi-
mental facts that 1 was not cleaved without Cu(OAc)2 and that 4
was not formed upon treatment of isolated 2 with Cu(OAc)2 have
led us to consider that the oxidative ring opening may occur during
the Cu(II) metalation of 1. The importance of Cu(II) ion in the
oxidation process can be seen in other related examples.10h,14
Here we report that fully π-conjugated octa- and heptapyrrolic
helices were obtained as bis-Cu(II) complexes from oxidative
cleavage of octaphyrin and heptaphyrin, respectively. Although
similar oxidative cleavages are known for cyclic tetrapyrrolic
pigments,10,11 such long coiling oligopyrromethene helices are
unknown.
Scheme 1. Copper Metalation of 1 under O2 Bubbling Conditions
Figure 1. (a) X-ray crystal structure of 4 (50% probability thermal ellipsoid
plot; only the inserted meso-C6F5 substituent is shown). (b) Temperature-
dependent magnetic susceptibilities of 4 and 6.
Figure 2. UV-vis-NIR absorption spectra of 4 and 6 in CH2Cl2. The *
marks vibrational overtones of the solvent.
Recently, we reported that Cu(II) metalation of the figure-eight
macrocycle [36]octaphyrin(1.1.1.1.1.1.1.1) (1) gave bis-Cu(II)
complex 2, which underwent an unprecedented splitting reaction
to give two Cu(II) porphyrins 3.12 Here we found that the new
reddish-brown product 4 was obtained in 15% yield along with 2
(55%) and 3 (22%) when the Cu(II) metalation of 1 was conducted
under continuous O2 bubbling at room temperature for 8 h (Scheme
1). X-ray diffraction (XRD) analysis revealed that the structure of
4 is an octapyrrolic helix winding around two copper ions (Figure
1a)13 in which one of the meso-pentafluorophenyl substituents is
inserted between the two turns of the helical backbone, causing a
The UV-vis-near-IR (NIR) absorption spectrum of 4 in CH2Cl2
(Figure 2) displays a broad band at λmax ) 1248 nm with ε ) 25 000
M-1 cm-1 that serves as evidence of the effective delocalization of
the 38 π electrons over the whole helical structure. The structure
of 4 is conformationally rigid, as shown by the fact that variable-
temperature 19F NMR measurements recorded the presence of a
single conformer from 213 to 383 K in toluene-d8 [see the
Supporting Information (SI)]. Variable-temperature magnetic sus-
ceptibility measurements indicated that the oxidation state of both
copper ions is +2 and that these Cu(II) ions interact through a very
weak antiferromagnetic coupling with J/kB ) -0.88 K (Figure 1b).
Electron spin resonance (ESR) measurements at 5 K showed a small
half-field resonance signal (see the SI). The observed spin-spin
† Kyoto University.
‡ Institute for Molecular Science.
9
2128 J. AM. CHEM. SOC. 2010, 132, 2128–2129
10.1021/ja909929s  2010 American Chemical Society

C O M M U N I C A T I O N S
interaction probably operates through effective spin delocalization
ibility of 6 is noteworthy, since 6 gradually reaches an equilibrium
between two conformers in solution, as revealed by TLC analysis,
UV-vis-NIR absorption, and 19F NMR spectra. According to the
19F NMR analysis, the ratio of the helical conformer to the other
conformer is ∼2:1 (see the SI).
over the helical π conjugation.15
Scheme 2. Oxidative Cleavage of Biscopper Heptaphyrin 5
In summary, oxidative cleavage of meso-aryl-substituted [36]oc-
taphyrin and [32]heptaphyrin provided fully π-conjugated helices
as bis-Cu(II) complexes with smoothly π-conjugated networks.
These helical molecules exhibited NIR absorption bands with large
absorbance as evidence of full π conjugation, hence showing that
linear oligopyrromethenes are particularly suitable for the formation
of fully π-conjugated helices upon appropriate metal coordination.
These transformations constitute a new synthetic route to linear
oligopyrromethenes via a topological change from a figure-eight
macrocycle to a coiling helix. Chiral resolution and further
elongation of such π-conjugated helices are actively being pursued
in our laboratory.
We also found that figure-eight bis-Cu(II) heptaphyrin 5 with
unique T-shaped coordination16 gradually changed in air over 3
weeks to give brown product 6 in 45% yield with the recovery of
5 (52%) (Scheme 2). Here again, the oxygen source for 6 was
determined to be molecular oxygen, since 5 was stable under inert
gas. The structure of 6 was determined by XRD analysis to be a
genuine helix with a constant helix pitch of ∼4.0 Å and a Cu-Cu
distance of 3.25 Å (Figure 3).13 Cu(1) is bound to the four nitrogen
atoms of pyrrole rings A, B, C, and D with distances of 1.95, 1.92,
1.95, and 2.02 Å, respectively, while Cu(2) is bound to the three
nitrogen atoms of pyrrole ring E, F, and G with distances of 1.92,
1.94, and 1.88 Å, respectively, and to the terminal benzoyl oxygen
atom with a distance of 2.20 Å. In addition, a Cu(1)-N(4)-Cu(2)
network with distances of 2.02 and 2.61 Å for Cu(1)-N(4) and
Cu(2)-N(4), respectively, and an angle of 88.4° for Cu(1)-N(4)-
Cu(2) was observed. Importantly, the helical 34 π electron network
is preserved smoothly, with small dihedral angles (<20.3°) to allow
the full conjugation. It is considered that the formation of 6 is
triggered by regioselective cleavage at the meso-R bond next to
pyrrole ring A and subsequent bond rotation between pyrrole rings
D and E (Scheme 2).
Acknowledgment. This work was supported by a Grant-in-Aid
(A) [19205006 (A) and 20108001 “pi-Space”] for Scientific
Research from MEXT. S.S. acknowledges a JSPS Fellowship for
Young Scientists.
Supporting Information Available: Sample preparation, charac-
terization, DFT single-point calculations, and X-ray crystallographic
details for 4 (CCDC-751202) and 6 (CCDC-751203). This material is
available free of charge via the Internet at http://pubs.acs.org.
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