Azacalix[4]arene cation radicals: Spin-delocalised doublet- and triplet-ground states observed in the macrocyclic m-phenylene system connected with nitrogen atoms

Article abstract of DOI:10.1039/b801127c

Electron paramagnetic resonance spectroscopy has unmasked for the first time the spin-delocalised doublet- and triplet-ground states of azacalix[4]arene cation radicals. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801127c

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Azacalix[4]arene cation radicals: spin-delocalised doublet- and
triplet-ground states observed in the macrocyclic m-phenylene system
connected with nitrogen atomsw
Koichi Ishibashi, Hirohito Tsue,* Naoko Sakai, Satoshi Tokita, Kazuhiro Matsui,
Jun Yamauchi and Rui Tamura
Received (in Cambridge) 21st January 2008, Accepted 11th March 2008
First published as an Advance Article on the web 11th April 2008
DOI: 10.1039/b801127c
Electron paramagnetic resonance spectroscopy has unmasked
for the first time the spin-delocalised doublet- and triplet-ground
states of azacalix[4]arene cation radicals.
Calixarenes have been attracting much attention not only in
supramolecular chemistry1 but also in the chemistry of mol-
ecular magnetism because the 1,3-phenylene moiety acts as a
ferromagnetic coupler.2 While a variety of calixarene-based
polyradicals in a high-spin ground state have thus far been
synthesised,3 the instability inherent in the carbon-based radi-
Chart 1
cals places an obstacle to greater access to organic magnetic
materials. Replacement of the bridging carbons of calixarenes
each of two bridging nitrogen atoms. This two-electron redox
with nitrogen atoms is of great interest because the bridging
process of 2 was almost reversible even after voltammetric
nitrogen atoms can offer more stable spin-bearing sites than
the electronically less negative carbon bridges.4 However, no
scans were repeated 10 times, as shown in Fig. S7 (ESIw).
Another oxidation wave, though irreversible, was observed at
precedent exists for the observation of high-spin states of
a higher potential up to +1.0 V (Fig. S8a, ESIw), illustrating
the instability of further oxidised species on the examined
nitrogen-bridged calixarene radicals, though Tanaka and co-
workers have theoretically predicted that deoxyazacalix[4]ar-
voltammetric time scale. Similarly, the reference compounds
ene 1 is accessible to a high-spin ground state upon oxidation.5
5–7 exhibited two oxidation waves with potentials given in
Recently, we independently reported the synthesis and mol-
Table 1, but only the first oxidation process was reversible in
ecular structure of azacalix[4]arene 2 with a 1,3-alternate
these compounds (Fig. S8b–d, ESIw). The oxidation behaviour
of 2 differs greatly from that of analogous deoxyazacalix[4]-
arene 4 which yields no stable oxidised species,5 suggesting
conformation in solution and in the solid state (Fig. S1,
ESIw).6 A DFT study indicates that model compound 3 has
four quasi-degenerate non-bonding molecular orbitals (Fig.
S3, ESIw). Therefore, it is conceivable that, if we remove
electrons from this molecule, the resultant radical species exists
in a high-spin ground state. To elucidate the feasibility of
applying azacalixarenes to high-spin materials, we have in-
vestigated the oxidation behaviour of azacalix[4]arene 2 and
the spin-state of the oxidised species. In this communication,
we report the first and successful observation of the spin-
delocalised doublet- and triplet-ground states of the cationic
radicals of 2 (Chart 1).
that introduction of sterically bulky tert-butyl and methoxy
groups contribute to the radical stability of 2ꢀ+ and 22( +).7 It
ꢀ
is interesting to note that Eo1x of 2 is more than 0.20 V lower
than those of acyclic compounds 5–7. The enhanced electron
donor ability of 2 results from its cyclic framework, by which
+
spin density of the monocationic species 2ꢀ is distributed
Electrochemical measurements of 2 and reference com-
pounds 5, 6 and 7 were carried out in CH2Cl2 containing
0.1 M Bu4NClO4 as supporting electrolyte. As shown in Fig. 1,
the cyclic voltammogram of 2 consisted of two reversible one-
electron oxidation waves at E = +0.14 and +0.45 V (vs.
Fc/Fc+), which correspond to the removal of an electron from
Graduate School of Human and Environmental Studies, Kyoto
University, Kyoto, 606-8501, Japan.
E-mail: tsue@ger.mbox.media.kyoto-u.ac.jp; Fax: +81 75 753 6722
w Electronic supplementary information (ESI) available: Full experi-
mental details for the syntheses of compounds 5–7, DFT calculations,
electrochemical measurements and variable-temperature EPR mea-
surements. See DOI: 10.1039/b801127c
Fig. 1 Cyclic voltammogram of azacalix[4]arene 2 (1 mM) in
CH2Cl2–Bu4NClO4 (0.1 M) at 298 K, scan rate 100 mV sꢁ1, potential
calibrated against Fc/Fc+
.
ꢂc
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Table 1 Oxidation potentials of azacalix[4]arene 2 and the reference
compounds 5–7a
Compound
Eo1x
Eo2x
2
4
5
6
7
0.14
0.31bc
0.50
0.37
0.36
0.45
—
0.85c
0.61c
0.58c
a
Conditions: [2] = [5] = [6] = [7] = 1 mM, 0.1 M Bu4NClO4 in
298 K, scan rate 100 mV
Cited from ref. 5. Anodic potential.
CH2Cl2, potentials versus Fc/Fc+
sꢁ1
,
b
c
.
over the molecular framework, as revealed by EPR study
described below.
Fig. 3 EPR spectrum of dication radical 22( +) observed at 100 K in
the CH2Cl2 glass containing 10% (v/v) trifluoroacetic acid. Inset: the
half-field resonance for the forbidden transition (DMs = ꢃ2).
ꢀ
EPR measurement was performed for the monocation
radical 2ꢀ+, which was generated in situ by electrochemical
oxidation of 2 (1 mM) at +0.30 V.z As shown in Fig. 2(a), the
EPR spectrum of 2ꢀ+ in CH2Cl2 containing 0.1 M Bu4NClO4
at 223 K exhibited a multiplet signal with around 15 lines at
g = 2.0034. The spectrum was reproduced by the simulation
analyses (Fig. 2(b) and S9, ESIw), demonstrating that the
unpaired spin was delocalised over the molecular framework
of 2ꢀ+. In other words, the spin was distributed throughout
the four bridging nitrogen atoms and the four aromatic rings
of 2ꢀ+, as supported by a DFT calculation on the model
compound 3ꢀ+ (Fig. S4, ESIw). The observed spin-delocalisa-
tion would be due to the intramolecular charge transfer
between the redox centres as in the case of the m,p-linked
polyaniline systems.8 Such spin-delocalisation over the cyclic
Dication radical 22( +) was also characterised by means of
ꢀ
EPR spectroscopy. Chemical oxidation of 2 (5 mM) with 2
equiv. of phenyliodine(III) bis(trifluoroacetate)9 at ambient
temperature for 5 min in CH2Cl2 containing 10% (v/v)
trifluoroacetic acid gave a dark-blue solution of 22( +), which
ꢀ
survived at this temperature for more than 10 min. Once ꢀthe
solution was frozen, no detectable decomposition of 22( +)
was observed at least during EPR measurement. Fig. 3 shows
the EPR spectrum of 22( +) in the solvent glass at 100 K.
ꢀ
There appeared four broad lines characteristic of zero-field
splitting with an axial symmetry. Half-field resonance for the
forbidden transition (DMs = ꢃ2) was also observed, clearly
indicating that dication radical 22( +) existed in a triplet state.
ꢀ
+
framework of 2ꢀ leads to a decrease in spin density per
The zero-field splitting parameter |D| was determined to be
6.9 mT, from which an average distance between the two
radical centres was estimated to be 7.4 A according to the
point-dipole approximation. This calculated distance is close
to the distance of 6.91 A between two diagonal nitrogen atoms
of neutral species 2 (Fig. S1b, ESIw). A fuꢀrther insight into the
structural and electronic properties of 22( +) wꢀas provided by
a DFT calculation on the model compound 32( +). Fig. 4 and
aromatic carbon ortho to the nitrogen bridges. Besides, the
bulky tert-butyl and methoxy groups effectively protect those
aromatic carbon atoms against an attack by solvent molecules
and/or other molecules eventually inducing undesirable de-
composition. Accordingly, the observed radical stability of
2ꢀ+ is ascribed to a combination of the spin-delocalisation and
the steric protection of the reactive sites.
+
Fig. 2 (a) EPR spectrum of azacalix[4]arene cation radical 2ꢀ
observed at 223 K in CH2Cl2 containing 0.1 M Bu4NClO4. (b)
Simulated EPR spectrum with the following parameters: AN
=
0.200 mT (for the four equivalent nitrogen atoms), ANCH = 0.255
3
mT (for the twelve equivalent N-methyl hydrogen atoms), AArH
=
Fig. 4 The optimised structure and the selected spin densities of
+)
in a triplet state calculated at a B3LYP/
model compound 32(
ꢀ
0.247 mT (for the eight equivalent aromatic hydrogen atoms), and
simulation line width = 0.18 mT.
6-31G**//B3LYP/6-31G** level.
ꢂc
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S5, ESIw displaꢀy the optimised structure and the selected spin
densities of 32( +). In this model, the spins are mainly dis-
tributed to the two diagonal nitrogens (N1 and N3) and four
phenyl rings (ring A, B, C and D), and the separation distance
of 6.91 A between N1 and N3 is close to the calculaꢀted
distance of 7.4 A between two radical centres of 22( +)
mentioned abꢀove. Interestingly, the other nitrogen atoms N2
and N4 of 22( +) retain almost no spin density, implying that
This paper is dedicated to Professor Yoshiteru Sakata on
the occasion of his 70th birthday. This work was supported by
the Grant-in-Aid for Scientific Research (C) No.19550037 (to
H. T.) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We are much indebted to
Dr Masayasu Taki of Graduate School of Human and En-
vironmental Studies, Kyoto University for his help in the
electrochemical measurements. We wish to express our grati-
tude to Mr Takayuki Suzuki of Japan Electron Optics La-
boratory Co., Ltd. (JEOL) for his help in the attempted
ENDOR measurements.
dication radical 22( +) can be viewed as a cyclic array of two
ꢀ
diphenylamine-type cation radicals10 connected with nitrogen
atoms N2 and N4.
To further explore whether the observed triplet state of
22( +) was a ground or thermally excited state, variable-
ꢀ
Notes and references
temperature EPR experiments were performed in the range
of 4.5 to 100 K. As shown in Fig. S10 (ESIw) the signal
intensity for the forbidden transition (DMs = ꢃ2) increased
with the decrease in temperature, indicating that dication
z Chemical oxidation of 2 (1 mM) with 0.9 equiv. of phenyliodine(III)
bis(trifluoroacetate)9 in CH2Cl2 afforded the identical EPR spectrum
with that of Fig. 2(a).
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¨
radical 22( +) existed either in a triplet ground state or in a
ꢀ
degenerate state of triplet and singlet states. In a DFT
ꢀ
calculation, the model compound 32( +) favors a triplet
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ꢀ
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and the average torsion angles +C(sp2)–N–C(sp2)–C(sp2)
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In summary, we have demonstrated that a high-spin ground
state is attainable in azacalix[4]arene system. The first and
successful observations of the triplet and doublet states of 2
were ensured by the radical stability arising from a combina-
tion of steric protection and spin-delocalisation. In addition, a
cyclic but non-planar framework of 2 is crucial to render the
bridging-nitrogen a ferromagnetic coupler rather than an
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applicable to its analogous systems with larger ring size and
thus with the promising potential for higher-spin states.
Investigations along this line are now in progress.
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ꢂc
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2814 | Chem. Commun., 2008, 2812–2814