Preparation and characterization of N-Anisyl-substituted Hexaaza[1 6]paracyclophane

Article abstract of DOI:10.1002/anie.201002165

Localized or delocalized? For the first time, a cyclic oligoaniline, hexaaza[1₆]paracyclophane, has been prepared (see picture; carbon: gray; nitrogen: blue; hydrogen: light gray). The macrocycle exhibits a high electron-donating ability, and furthermore, the spin of the corresponding radical cation was delocalized over the macrocyclic molecular backbone.

Full text of DOI:10.1002/anie.201002165

Angewandte
Chemie
DOI: 10.1002/anie.201002165
Cyclophanes
Preparation and Characterization of N-Anisyl-Substituted
Hexaaza[16]paracyclophane**
Akihiro Ito,* Yuichiro Yokoyama, Ryosuke Aihara, Koji Fukui, Shoko Eguchi, Katsuyuki Shizu,
Tohru Sato, and Kazuyoshi Tanaka*
As opposed to the thoroughly studied carbon-bridged calix-
arenes,[1] heteroatom-bridged [1n]metacyclophanes and their
derivatives have recently attracted much attention mainly
because of novel structure-property relationships originating
from the replacement of methylene bridges with heteroatom
bridges.[2] On the other hand, only a few reports are known for
[1n]paracyclophanes, probably as a result of the difficulty in
synthesizing them (Scheme 1).[3,4]
Scheme 2. Schematic views of HOMOs of a) linear and b) cyclic
hexamers of polyaniline.
Scheme 1. Reported heteroatom-bridged [1n]paracyclophanes and hexa-
aza[16]paracyclophane 1 and 1’.
and therefore, spin delocalization took place only to a small
Aza[1n]paracyclophanes can be viewed as macrocyclic
oligomers of polyaniline, which is one of the well-known
organic conducting polymers.[5] As is apparent from the
simple Hꢀckel MO calculations, the HOMOs for linear and
cyclic oligoanilines assume different aspects, as exemplified in
Scheme 2. As for the linear hexamer, the large MO coeffi-
cients are concentrated on the central para-phenylenedi-
amine (PD) moiety. Owing to the localized HOMO character,
the ESR studies on the radical cations of linear oligoanilines
showed that the spin was confined to the central PD moiety,
extent.[6] In contrast, the HOMO coefficients of the cyclic
hexamer is mainly distributed equally on six nitrogen nuclei.
In fact, according to the DFT calculations, the HOMO
coefficients of all-N-phenyl-substituted hexaaza[16]para-
cyclophane (1’) is delocalized over the macrocyclic molecular
backbone, and therefore, the DFT-predicted spin density
distribution in the corresponding radical cation discerns the
delocalized character (Figure 1). In this respect, it is interest-
ing to examine the spin distribution of the radical cation of
cyclic oligoanilines in conjunction with the question: to what
extent is an unpaired electron delocalized within the poly-
aniline backbone?
However, the preparation of aza[1n]paracyclophanes are
hitherto unknown, and the elucidation of their electronic
properties is in demand from the viewpoint of materials
chemistry. This communication reports the synthesis and
characterization of electronic structures of all-N-anisyl-sub-
stituted hexaaza[16]paracyclophane (1) and its oxidized
species.
[*] Dr. A. Ito, Y. Yokoyama, R. Aihara, K. Fukui, S. Eguchi, K. Shizu,
Dr. T. Sato, Prof. Dr. K. Tanaka
Department of Molecular Engineering
Graduate School of Engineering
Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: aito@scl.kyoto-u.ac.jp
ktanaka@moleng.kyoto-u.ac.jp
Dr. T. Sato
Fukui Institute for Fundamental Chemistry, Kyoto University
Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103 (Japan)
The title compound (1) was not obtained by the simple
single-step coupling reaction from para-halogenated secon-
dary arylamines. In addition, convergent fragment coupling
synthesis[1] from the combinations of simple secondary aryl-
amines and aryl halides was also unsatisfactory to us. For
example, an equimolar coupling reaction of N,N’-bis(4-
[**] This work was supported by a Grant-in-Aid for Scientific Research
(B; 20350065) from the Japan Society for the Promotion of Science
(JSPS).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002165.
Angew. Chem. Int. Ed. 2010, 49, 8205 –8208
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8205

Communications
phenylene rings, AA’XX’ signals for six N-substituted anisyl
groups, and a singlet signal for six methoxy groups.
Although we could not obtain single crystals suitable for
the X-ray structural analysis of 1, the B3LYP/6-31G*
optimized structure of 1’ showed that the six nitrogen atoms
are predicted to be coplanar, whereas each triphenylamine
moiety adopts a propeller-like conformation, in which the
torsion angles of the phenyl rings range from 39.68 to 43.18
(Figure 1 and Figure S3 in the Supporting Information).
Cyclic voltammetry and differential pulse voltammetry
À
(DPV) of
1
in dichloromethane (0.1m nBu4N+BF4
,
100 mVsÀ1) at 298 K showed five oxidation processes as
shown in Figure S1 in the Supporting Information. The first
four oxidation processes were chemically reversible after
repeated potential cycling in dichloromethane. The oxidation
potentials [Eox vs. Fc0/+ (ne)] of 1 were determined to be À0.28
(1e), À0.17 (1e), + 0.20 (1e), + 0.45 (1e), and + 0.72 V (peak
potential, 2e), and therefore, 1 was oxidizable up to a
hexacation species according to six redox-active triarylamine
centers. Notably, the first and second oxidation potentials of
the macrocyclic oligmer 1 are much lower than the first
oxidation potentials of the related linear oligomers [À0.13 V
for the dimer 4 and À0.11 V for the trimer 5],[10] which were
measured under the same conditions. Furthermore, the
increase of electron-donating ability by macrocyclization of
oligo-p-arylamines is remarkable as compared with that by
macrocyclization of oligo-m-arylamines.[11]
Figure 1. a) HOMO (B3LYP/6-31G*[7]) of 1’ calculated by DFT and
b) the spin density distribution (black: positive spin, white: negative
spin; spin isosurface value=0.0003 electron/au3; UB3LYP/EPR-III//
ROB3LYP/6-31G*[7]) of 1’C+.
+
The ESR spectrum of 1C generated by chemical oxidation
of 1 by one equivalent of tris(4-bromophenyl)aminium
hexachroloantimonate in dichloromethane at 195 K showed
a multiplet hyperfine structure, and the observed g value is
2.0028 (Figure 2a). The splitting pattern can be explained by
the presence of six equivalent nitrogen nuclei and 24 hydro-
gen nuclei, although the broadness of the signals suggests
additional small hyperfine coupling constant (Figure 2b). The
optimum simulation of the observed spectrum gave the
following values: aN = 0.205 mT (6N) and aH(ortho) = 0.021 mT
(24H), and the contributions from the negligible hydrogen
nuclei were incorporated in the line width of the spectral
simulation (0.10 mT). Moreover, the hyperfine structure did
not change in the temperature range from À60 to 208C
(Figure S2 in the Supporting Information), and hence, this
result clearly demonstrates that the generated spin is delo-
calized over the macrocyclic molecular backbone.
anisyl)-1,4-phenylenediamine and N-anisyl-4,4’-dibromodi-
phenylamine in the presence of the phosphine-ligated palla-
dium catalyst (Buchwald–Hartwig aryl amination reaction[8])
did not afford the cyclic product, even after 5 days reaction.
During our synthetic attempts, however, we noticed that the
hexaaza[16]paracyclophane structure (2) can be directly
prepared at room temperature from N,N’-di(4-anisyl)-1,4-
benzenediamine and Boc-protected N,N-bis(4-bromophe-
nyl)amine in the presence of [Pd(dba)2] and P(tBu)3 as
catalyst (Scheme 3).[9] After removal of the Boc groups with
TFA, the palladium-catalyzed coupling of the macrocycle (2)
and p-bromoanisole afforded 1 as a pale yellow solid. The
observed simple 1H NMR spectrum of 1 is diagnostically
consistent with the macrocyclic structure: singlet signal for six
The UV/Vis/NIR spectroscopic change upon electro-
+
chemical oxidation from 1 through 1C to 12+ was recorded
8206 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 8205 –8208

Angewandte
Chemie
Scheme 3. Synthetic route for 1. Boc=tert-butoxycarbonyl, dba=trans,trans-dibenzylideneacetone, TFA=trifluoroacetic acid.
Figure 2. ESR spectra of 1C+: a) in CH2Cl2 at 233 K; b) simulated.
Figure 3. UV/Vis/NIR spectra of the stepwiÀse electrochemical oxida-
tion of 1 to 12+ in CH2Cl2/0.1m nBu4N+BF4 at room temperature.
by using an optically transparent thin-layer electrochemical
cell (Figure 3). Upon oxidation of 1, two absorption bands
appeared at 0.76 and ꢀ 2.7 eV and the initial absorption band
(3.56 eV) decreased, which can be ascribed to formation of
radical cation 1C . Judging from the ESR analysis of 1C , the
lowest energy band is attributable to the charge resonance
(CR) band originating from spin delocalization over the
entire molecule. Further oxidation to dication 12+, the lowest
energy band showed a hypsochromic shift to 0.83 eV, as was
often observed in the related macrocyclic oligoaryl-
amines.[10b,12]
Prompted by the finding of good electron-donating ability
of 1, we have checked the possibility of formation of charge-
transfer (CT) complexes with acceptor molecules. For exam-
ple, combining solutions of 1 and 7,7,8,8-tetracyano-p-quino-
dimethane (TCNQ) as a typical acceptor molecule in
dichloromethane showed a superposition of the absorptions
+
+
+
of 1C (0.78 eV) and TCNQC·À ( ꢀ 1.5 eV[13]), as shown in
Figure 4. Furthermore, the IR spectrum of the isolated
powder sample displayed a characteristic decrease in the
Angew. Chem. Int. Ed. 2010, 49, 8205 –8208
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8207

Communications
5291; d) H. Tsue, K. Ishibashi, R. Tamura, Top. Heterocycl.
Chem. 2008, 17, 73.
[3] a) Y. Miyahara, T. Inazu, T. Yoshino, Tetrahedron Lett. 1983, 24,
5277; b) G. W. Gribble, C. F. Nutaitis, Tetrahedron Lett. 1985, 26,
6023; c) T. Ogoshi, S. Kanai, S. Fujinami, T. Yamagishi, Y.
Nakamoto, J. Am. Chem. Soc. 2008, 130, 5022; d) T. Ogoshi, K.
Kitajima, T. Yamagishi, Y. Nakamoto, Org. Lett. 2010, 12, 636;
e) B. Kꢁnig, M. Rꢁdel, P. Bubenitschek, P. G. Jones, Angew.
Chem. 1995, 107, 752; Angew. Chem. Int. Ed. 1995, 34, 661; f) B.
Kꢁnig, M. Rꢁdel, P. Bubenitschek, P. G. Jones, I. Thondorf, J.
Org. Chem. 1995, 60, 7406; g) D. Takeuchi, I. Asano, K.
Osakada, J. Org. Chem. 2006, 71, 8614; h) M. L. Kaplan, W. D.
Reents, Jr., Tetrahedron Lett. 1982, 23, 373; i) J. Franke, F.
Vꢁgtle, Tetrahedron Lett. 1984, 25, 3445; j) I. Baxter, H. M.
Colquhoun, P. Hodge, F. H. Kohnke, D. J. Williams, Chem.
Commun. 1988, 1991.
Figure 4. Vis/NIR absorption spectrum of the combined solution of 1
and TCNQ in CH2Cl2 at room temperature.
[4] H. Takemura, Curr. Org. Chem. 2009, 13, 1633.
[5] E. M. Geniꢃs, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met.
1990, 36, 139.
wavenumber (vCN) corresponding to the CN bond stretching
vibration of TCNQ [observed vCN = 2180 cmÀ1; 2223 cmÀ1 for
neutral TCNQ and 2183 cmÀ1 for TCNQCÀ], as is often
utilized as a measure of charge transfer in the CT complex
with TCNQ.[14] Thus, this observation indicates a complete
charge transfer has occur from 1 to TCNQ.
[6] B. Grossmann, J. Heinze, T. Moll, C. Palivan, S. Ivan, G.
Gescheidt, J. Phys. Chem. B 2004, 108, 4669.
[7] The full geometry optimizations of 1’ and 1’C+ in C2h symmetry
were carried out at the hybrid HF/DF (B3LYP)/6-31G* level of
theory by using the Gaussian 03 program package: Gaussian 03
(Revision D.02), M. J. Frish et al. (see the Supporting Informa-
tion for the full citation).
In summary, we succeeded in synthesizing an
aza[1n]paracyclophane 1. The ESR analysis resulted in the
delocalized spin distribution for the radical cation of the cyclic
oligoaniline, in contrast to the spin confinement for the
radical cation of linear oligoanilines. Furthermore, the
electrochemical studies revealed that 1 can be regarded as a
good electron donor, so that the CT complex can be easily
obtained with electron acceptors such as TCNQ. In addition,
electronic structures for the higher oxidation states including
12+, should be intriguing from the viewpoints of aromaticity in
macrocyclic conjugated system, molecular magnetism in
toroidal molecular spin system, and so forth.[15] Studies on
these topics are currently underway.
[8] a) J. P. Wolfe, S. Wagaw, J. F. Marcoux, S. L. Buchwald, Acc.
Chem. Res. 1998, 31, 805; b) J. F. Hartwig, Acc. Chem. Res. 1998,
31, 852; c) J. F. Hartwig, Angew. Chem. 1998, 110, 2154; Angew.
Chem. Int. Ed. 1998, 37, 2046; d) A. R. Muci, S. L. Buchwald,
Top. Curr. Chem. 2002, 219, 133.
[9] In this palladium-catalyzed macrocyclization reaction, we iso-
lated the Boc-protected aza[19]paracyclophane in 8.7% yield. In
addition, we detected the formation of small quantity of the Boc-
protected aza[112]paracyclophane from the FABMS study, but
we could not isolate it. The remainder was mainly polymeric
material.
[10] a) A. Ito, S. Inoue, Y. Hirao, K. Furukawa, T. Kato, K. Tanaka,
Chem. Commun. 2008, 3242; b) A. Ito, Y. Yamagishi, K. Fukui,
S. Inoue, Y. Hirao, K. Furukawa, T. Kato, K. Tanaka, Chem.
Commun. 2008, 6573.
[11] K. Ishibashi, H. Tsue, N. Sakai, S. Tokita, K. Matsui, J. Yamauchi,
R. Tamura, Chem. Commun. 2008, 2812.
[12] I. Kulszewicz-Bajer, V. Maurel, S. Gambarelli, I. Wielgus, D.
Djurado, Phys. Chem. Chem. Phys. 2009, 11, 1362.
Received: April 13, 2010
Revised: July 22, 2010
Published online: September 21, 2010
[13] Y. Iida, Bull. Chem. Soc. Jpn. 1969, 42, 71.
Keywords: arylamines · cyclophanes · electrochemistry ·
EPR spectroscopy · radical ions
[14] J. S. Chappell, A. N. Bloch, W. A. Bryden, M. Maxfield, T. O.
Poehler, D. O. Cowan, J. Am. Chem. Soc. 1981, 103, 2442.
[15] As pointed out by one of the reviewers, there exists the
possibility that the aromaticity in 12+ is observed through the
NMR studies. In fact, upon oxidation of 1, the semiquinoidal
structural changes in macrocyclic skeleton were expected from
the B3LYP/6-31G* calculations for the higher oxidation states of
1’ (Figure S3b in the Supporting Information), and furthermore,
the nucleus-independent chemical shift (NICS) value for 1’2+
calculated at the center of the plane defined by the six nitrogen
atoms was comparable to that of benzene at the same level of
calculations (GIAO/B3LYP/6-311G*//B3LYP/6-31G*; Figure S4
in the Supporting Information). Our preliminary experiments to
record the NMR spectrum of 12+ were unsuccessful owing to
unavoidable paramagnetic impurities.
.
[1] a) C. D. Gutche in Monographs in Supramolecular Chemistry:
Calixarenes (Ed.: J. F. Stoddart), The Royal Society of Chemis-
try, Cambridge, 1989; b) Calixarenes: a Versatile Class of Macro-
cyclic Compounds (Eds.: J. Vicens, V. Bꢁhmer), Kluwer,
Dordrecht, 1991; c) C. D. Gutche in Monographs in Supra-
molecular Chemistry: Calixarenes Revisited (Ed.: J. F. Stoddart),
The Royal Society of Chemistry, Cambridge, 1998; d) Calixar-
enes (Eds.: Z. Asfari, V. Bꢁhmer, J. Harrowfield, J. Vicens, M.
Saadioui), Kluwer, Dordrecht, 2001.
[2] a) B. Kꢁnig, M. H. Fonseca, Eur. J. Inorg. Chem. 2000, 2303; b) P.
Lhotꢂk, Eur. J. Org. Chem. 2004, 1675; c) N. Morohashi, F.
Narumi, N. Iki, T. Hattori, S. Miyano, Chem. Rev. 2006, 106,
8208 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 8205 –8208