Macrocyclic high-spin (S = 2) molecule: Spin identification of a sterically rigid metacyclophane-based nitroxide tetraradical by two-dimensional electron spin transient nutation spectroscopy

Article abstract of DOI:10.1002/anie.200705583

A spin quintet: The electronic and molecular structure of the metacyclophane-based spin-quintet tetraradical 1 with a sterically controlled rigid structure and small fine-structure parameters is characterized. Pulse-ESR based 2D electron spin nutation spectroscopy allows the spin multiplicity of 1 to be unequivocally identified, providing its fine-structure parameters and g-values. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200705583

Communications
DOI: 10.1002/anie.200705583
Macrocyclic High-Spin Molecule
Macrocyclic High-Spin (S = 2) Molecule: Spin Identification of a
Sterically Rigid Metacyclophane-Based Nitroxide Tetraradical by Two-
Dimensional Electron Spin Transient Nutation Spectroscopy**
Takatoshi Sawai, Kazunobu Sato,* Tomoaki Ise, Daisuke Shiomi, Kazuo Toyota, Yasushi Morita,
and Takeji Takui*
Macrocyclic molecules, such as cyclophanes and calixarenes,
have attracted much interest because of their ability to
include various guests and the possibility of forming supra-
molecular assemblies.[1] Calixarenes functionalized by nitro-
xide radicals, which are widely used as stable spin sources,
have been documented,[2–5] revealing their peculiar proper-
ties; especially, remarkable changes in continuous-wave (cw)-
ESR spectra with and without guests were observed.[3]
Calixarene-based high-spin entities, nevertheless, have
rarely been well-characterized, particularly in terms of their
electronic-spin properties versus their molecular structure.
The design and syntheses of well-characterized macrocyclic
high-spin systems are an important issue not only for exotic
molecular magnetic materials formed by supramolecular
assembly,[4] but also for magnetic chemical sensors with
multi-sensing sites. The difficulty in studying macrocyclic
high-spin systems results from the lack of well-established
experimental methods which allow the apparently small
exchange or spin–spin interactions to be characterized in
supramolecular species which have sizable inclusion pockets.
In addition, synthetic macrocyclic systems are frequently
contaminated by lower spin species having similar molecular
frames. Rajca et al. reported pioneering work on a calix[4]-
arene derivative having four nitroxide radicals in the upper
(wider) rim of the calixarene skeleton.[5] The X-ray crystal
structure of that calixarene derivative revealed that the two
diagonally arranged radical sites formed an intramolecular
dimeric structure undergoing a strong antiferromagnetic
interaction. The spin state of the derivative was only
characterized from magnetic susceptibility measurements,
which did not give clear and straightforward evidence for the
molecular-structure-related microscopic magnetic properties
of calixarene-based exchange-coupled systems.
Herein, we show for the first time that nitroxide tetrar-
adical 1 (Scheme 1) with four nitroxide radical sites in the
Scheme 1. Molecular structures of nitroxide radicals, 1 and 2.
upper rim of the calixarene skeleton is in a quintet state. We
have exploited the sterically rigid molecular structure of the
macrocyclic skeleton to control intramolecular through-space
exchange interactions.[6] Molecule 1 has a unique structure
with three methyl substituents in each benzene ring (mesityl
groups), giving rise to steric rigidity, and suppressing the
flexibility of the skeleton. To unequivocally identify the spin
multiplicity of 1 and possibly occurring lower-spin chemical
species in an organic glass, 2-dimensional pulse-based elec-
tron spin transient nutation (2D-ESTN) spectroscopy devel-
oped in our group has been applied. The 2D-ESTN technique
gives straightforward information on the spin multiplicity
which cannot be derived in terms of cw-ESR spectroscopy.[7]
We have prepared both tetraradical 1 and mesityl nitrox-
ide 2 (Scheme 1), which corresponds to a monoradical
component of 1. Compound 2 is used to check any subtle
effect of the calixarene structure on the spin structure.
Figure 1 shows the X-ray crystal structure of 1.[8] The structure
has a sterically hindered 1,3-alternate conformation giving
pseudo-tetrahedral symmetry. The structurally robust macro-
cyclic structure consists of the metacyclophane derivatives
with three methyl groups at the 2-,4-, and 6-positions in each
[*]T. Sawai, Prof. Dr. K. Sato, Dr. T. Ise, Prof. Dr. D. Shiomi, Dr. K. Toyota,
Prof. Dr. T. Takui
Departments of Chemistry and Materials Science
Graduate School of Science
Osaka City University
Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax: (+81)6-6605-2522
E-mail: sato@sci.osaka-cu.ac.jp
takui@sci.osaka-cu.ac.jp
Prof. Dr. Y. Morita
Department of Chemistry
Graduate School of Science, Osaka University
Toyonaka, Osaka 560-0043 (Japan)
[**]This work has been supported by Grants-in-Aids for Scientific
Research from the Ministry of Education, Sports, Culture, Science
and Technology (Japan). One of the authors (T.S.) acknowledges
Research Fellowships of the Japan Society for the Promotion of
Science for Young Scientists.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3988
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Chemie
Figure 1. ORTEP representation of 1 with thermal ellipsoids set at
50% probability. Solvents and hydrogen atoms are omitted for clarify.
Figure 3. Contour plot of the field-swept 2D nutation spectra of 1
(T=3.8 K, nMW =9.65684 GHz); see text for details.
p
p
p
benzene ring, as reported by C. Klein et al.[9,10] The NO
groups of 1 point away from the cavity due to the ortho-
methyl substituents. The four dihedral angles between the
benzene ring and the C-N-O plane of the (CH3)3CNO unit are
73.51, 87.39, 68.52, and 89.738, respectively. The distance
between the oxygen atoms in the nitroxide radical moieties
ranges from 10.38 to 11.43 Š. Thus, in this macrocyclic system,
unwanted robust through-space exchange interactions are
reasonably suppressed.
being consistent with w1: 2w1: 3w1:2w1: 6w1, as the corre-
sponding theoretical values according to the Equation (S1)
given in Supporting Information. The theoretical frequencies
2w1 and 6w1 correspond to the allowed transitions for the
quintet high-spin state. The signals of the other frequencies
p
p
p
with 3w1, 2w1, and w1 show that partially nitroxide-
deficient radicals, that is, tri, di, and mono radicals, were
identified as minorities in the sample (Table S1 in the
Supporting Information). In Figure 2, the cw-ESR spectrum
is compared with a simulated spectrum. Based on the spectral
transition assignments from the 2D-nutation spectra, it is
confirmed that more than three open-shell species contribute
to the experimentally observed spectrum. As shown in the
Supporting Information, Figure S4, the cw-ESR spectrum is
well explained by the superposition of both S = 2and S = 3/2
species whose transitions were dominantly observed in the
ESTN spectra (Supporting Information, Figure S3). The
results of the 2D-ESTN spectroscopy consistently demon-
strate that the mono and di radicals are minor species. It
should be noticed that the nutation signals arising from the
quintet state are observed at 3.8 K, showing the quintet state
is in the ground state or nearly degenerate with singlet ground
states. Other thermally accessible intermediate spin states
(S = 1) from 1 were not detected up to 20 K.[12]
The fine-structure parameters and g-values for the quintet
species (S = 2) derived from the spectral simulation method
are: j D j= 0.0014 cmꢁ1, j E j= 0.00017 cmꢁ1, gxx = 2.0062,
gyy = 2.0062, and gzz = 2.0056. To consider the molecular
structure of 1 in the rigid glass, D-value calculations using
the point dipole–dipole approximation were performed based
on a four-spin cluster model.[11] There are other possible
orientations of the NO moieties in the rigid glass in addition
to those in the crystal structure of 1. Considering distortion
from the macrocyclic skeleton of the crystal structure of 1, the
most reasonable and probable fine-structure parameters have
been calculated to be D = ꢁ0.00140 cmꢁ1 and j E j=
0.00014 cmꢁ1. The corresponding structure is as follows; the
three NO sites are inside the cavity and the remaining one
outside as shown in the Supporting Information, Figure S5.
The structure obtained for tetraradical 1 in the rigid glass is
different from the one determined by the X-ray analysis, as
given in Figure 1. Another dominant species identified is the
triradical molecule. The fine-structure parameters of trirad-
ical were determined by the spectral simulation method:[13]
Rigid-glass cw-ESR spectra of 1 in the DMS = ꢀ 1 and ꢀ 2
regions are shown in Figure 2. The signal of DMS = ꢀ 2is very
Figure 2. cw-ESR spectra of 1 observed at liquid helium temperatur-
es (upper traces). a) Forbidden transitions of DMs =ꢀ2 (T=2.7 K,
nMW =9.49407 GHz). b) Allowed transitions of DMs =ꢀ1 (T=3.4 K,
nMW =9.49409 GHz). The lower traces show simulated spectra gener-
ated by the superposition of both the spectrum arising from the
quintet and the one from the quartet species.
weak, and the forbidden transitions of neither DMS = ꢀ 3 nor
ꢀ 4 were observed, indicating that these transition probabil-
ities are extremely small as a result of a small D-value of the
high-spin states from 1. The probabilities of the forbidden
transitions were confirmed by the hybrid eigenfield method.
The ESR spectra were simulated with the help of exper-
imental results of the 2D-ESTN measurements described
below.
To identify the spin multiplicity of 1 in a straightforward
manner, two-dimensional field-swept 2D-ESTN spectroscopy
was applied to the rigid-glass sample (Figure 3). An echo-
detected field-swept fine-structure spectrum is given on the
right hand side in Figure 3. From the 2D-ESTN spectra, five
nutation frequency components, 16.9, 22.8, 26.8, 32.0, and
39.8 MHz are discriminated. They are denoted by a, b, c, d,
and e, respectively, as given in Figure 3. The relative ratios of
the observed frequency components are 1:1.35:1.59:1.89:2.36,
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j D j= 0.00155 cmꢁ1 and j E j < 0.0001 cmꢁ1. Those parame-
Angew. Chem. 1999, 111, 3387 – 3390; Angew. Chem. Int. Ed.
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Am. Chem. Soc. 2003, 125, 8534 – 8538.
[6] Recently, a similar approach was carried out by Raija et al. to
investigate magnetic properties of calix[4]arene-based nitroxide
radicals; A. Rajca, S. Mukherjee, M. Pink, S. Rajca, J. Am.
Chem. Soc. 2006, 128, 13497 – 13507.
[7] a) K. Sato, M. Yano, M. Furuichi, D. Shiomi, T. Takui, K. Abe, K.
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ters for triradical were also reasonably reproduced by the
spin-cluster model based on the molecular structure with one
NO-spin-deficient site, such as NOH. The D-value calcula-
tions of tetraradical 1 and the single NO-site deficient
triradical are given in the Supporting Information,
Table S2–4. It is indicated that tetraradical 1 has somewhat
flexible structure even in the sterically hindered macrocyclic
skeleton in the glass.
In conclusion, we have characterized the electronic-spin
and molecular structure of tetraradical 1 which has a steri-
cally-controlled molecular structure by using cw- and pulsed-
ESR spectroscopy. 2D-ESTN spectroscopy has been applied
to identify the spin multiplicity of 1 in a straightforward
manner, illustrating its spectroscopic usefulness for open-shell
entities featuring small fine-structure parameters arising from
small spin-dipolar interactions of multicentered high spins,
such as polyradicals. It was indicated that the steric hindrance
introduced by mesityl groups yields a relatively rigid macro-
cyclic molecule with four nitroxide radicals, generating the
high-spin quintet state. The systems based on flexible macro-
cycles, in which the sterically controlled conformation is
relevant to inclusion phenomena, are easily tunable, in which
the exchange interaction is modulated by external stimuli,
such as ligation or ionic capture.[12] The high-spin states of
macrocyclic compounds, combined with their inclusion prop-
erties, allow the exploration of spin-sensing multifunctional
materials.
MagneticProperties of OrganicMaterials
(Ed.: P. M. Lahti),
Marcel Dekker, New York, 1999, pp. 197 – 236.
[8] X-ray crystal structure analysis of 1: An orange-red block (0.5 ”
0.1 ” 0.15 mm) of 1 for the X-ray crystal structure analysis was
prepared by slow evaporation of a diethyl ether solution of 1.
Diffraction data were obtained with 2qmax = 55.08 at 248C.
¯
C56H80N4O4·C4H10O, Mr = 947.38, Triclinic, space group P1, a =
12.22(1), b = 15.62(1), c = 15.76(1) Š, a = 62.68(3), b = 84.65(4),
3
g = 83.16(4)8, V= 2651(4) Š and 1calcd = 1.144 gcmꢁ3 for Z = 2.
Anisotropic temperature factors were applied for non-hydrogen
atoms. Hydrogen atoms with isotropic thermal parameters were
refined as riding models. Refinement converged at R = 0.107 and
Rw = 0.159 for 5032observed reflections, with I > 3s (I) and 690
variables. CCDC 669668 contains the supplementary crystallo-
graphic 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.
Experimental Section
Experimental procedure: Detailed information on the syntheses,
ESR/2D-ESTN measurements, D-tensor calculations, and DFT
calculation, are given in Supporting Information.
[9] C. Klein, E. Graf, M. W. Hosseini, A. D. Cian, N. Kyritsakas-
Gruber, Eur. J. Org. Chem. 2002, 802– 809.
[10] This type of the skeleton having many methyl groups is rigid over
a wide temperature range; see reference [9].
Received: December 6, 2007
Published online: April 15, 2008
[11] A. Bencini, D. Gatteschi, Electron Paramagnetic Resonance of
Exchange Coupled Systems, Springer, Berlin, 1990, pp. 48 – 85.
[12] The location and fine-structure parameters of the thermally
accessible triplet states from tetraradical 1 were theoretically
calculated, and are in agreement with the experimental result.
[13] It is noted that minor contributions from quartet species with
slightly different fine-structure parameters to the observed
spectrum are not included in the present spectral simulation.
Keywords: electron spin transient nutation · EPR spectroscopy ·
high-spin systems · macrocycles · radicals
.
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