Three-dimensional intramolecular exchange interaction in a curved and nonalternant π-conjugated system: Corannulene with two phenoxyl radicals

Article abstract of DOI:10.1002/anie.200906666

Chemical Equation Presented A neutral diradical based on corannulene with curved and nonalternant π conjugation has been synthesized and isolated as air-stable crystals. Sizable spin derealization and strong intramolecular exchange interaction in the intrinsically three-dimensional π network (see picture), as well as significant diradical contributions in the ground state, were experimentally disclosed.

Full text of DOI:10.1002/anie.200906666

Communications
DOI: 10.1002/anie.200906666
Curved p Diradical
Three-Dimensional Intramolecular Exchange Interaction in a Curved
and Nonalternant p-Conjugated System: Corannulene with Two
Phenoxyl Radicals**
Akira Ueda, Shinsuke Nishida, Kozo Fukui, Tomoaki Ise, Daisuke Shiomi, Kazunobu Sato,
Takeji Takui,* Kazuhiro Nakasuji, and Yasushi Morita*
In recent years, curved p-conjugated molecules such as
fullerenes and carbon nanotubes have attracted much atten-
tion not only in chemistry but also in materials science.[1]
Their intra/intermolecular interactions within/between three-
dimensional (3D) curved p-electron networks play intrinsi-
cally vital roles in their unique properties and functionalities.
Among them, intramolecular magnetic interaction between
electronic spins on a curved p surface was extensively studied
nitroxide radicals.[4] However, their intramolecular exchange
interaction J through the C60 skeleton was very weak (j JkBꢀ1
j
< 0.1 K)[4b] because of the small spin delocalization onto the
C
60 p network from the nitroxide radicals with spin-localized
nature on the NO moieties. Therefore, in order to evaluate an
intramolecular exchange interaction in a curved p-conjugated
system in a quantitative manner, synthesis and isolation of a
stable neutral diradical derivative with extensively spin-
delocalized nature on its curved p-conjugated system have
been the focus of current attention in molecular magnetism
and open-shell chemistry.[2,5]
2ꢀ
3ꢀ [2]
for ionic species of C60 such as C60 and C60
.
Their
electronic structures are greatly influenced by not only the
dynamic spin polarization of electrons but also the negative
charges on the spherical p-conjugated system and the
countercation.
Thus, we have focused on neutral diradical systems, which
are known to be the most useful probes for studying
intramolecular magnetic interactions in organic molecules.[3]
While many neutral diradical derivatives relevant to planar p-
conjugated systems have so far been investigated, studies on
curved p-conjugated neutral diradicals are limited to a single
C60-based system in which [60]fullerene is linked to two
Recently, we studied corannulene[6]-based stable neutral
monoradical systems,[7] such as a phenoxyl radical derivative
1
[7d] with highly spin-delocalized nature on the intrinsically 3D
bowl-shaped and nonalternant p-conjugated network. These
[*] A. Ueda, Prof. Dr. K. Nakasuji, Prof. Dr. Y. Morita
Department of Chemistry
Graduate School of Science, Osaka University
Toyonaka, Osaka 560-0043 (Japan)
Fax: (+81)6-6850-5395
E-mail: morita@chem.sci.osaka-u.ac.jp
Homepage: http://www.chem.sci.osaka-u.ac.jp/lab/nakasuji/
morita/index_eng.html
Dr. S. Nishida, Dr. T. Ise, Prof. Dr. D. Shiomi, Prof. Dr. K. Sato,
Prof. Dr. T. Takui
Departments of Chemistry and Materials Science
Graduate School of Science, Osaka City University
Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax: (+81)6-6605-2522
E-mail: takui@sci.osaka-cu.ac.jp
Homepage: http://www.sci.osaka-cu.ac.jp/chem/phy2/indexe.html
studies inspired us to propose a 3D intramolecular exchange
interaction in this class of curved p radicals: in terms of
geometry, they are intermediate between a planar p radical
such as a phenalenyl system[8] and a tetrahedral s radical. We
have now, for the first time, synthesized and isolated a
corannulene-based neutral diradical, namely, 2, which has two
phenoxyl radical moieties, as air-stable crystals. Due to its
highly spin-delocalized nature, 2 shows strong intramolecular
exchange interaction (JkꢀB1 = ꢀ405 ꢁ 2 K) through the 3D
corannulene p-electron network. Bond-length analyses and
DFT calculations showed that 2 has contributions from
diradical canonical forms a–g and closed Kekulꢀ structure d
(Scheme 1). Furthermore, in the crystalline state the presence
of three crystallographically independent diradical molecules
with different curvature enabled us to study the relationship
between the curvature of the corannulene p skeleton and the
Dr. K. Fukui, Prof. Dr. Y. Morita
PRESTO (Japan) Science and Technology Agency (JST) (Japan)
[**] We thank Dr. Motoo Shiro (Rigaku Corporation) for X-ray analyses.
This work was partially supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (No. 20110006) from the Ministry of
Education, Culture, Sports, Science and Technology (Japan), and the
Global COE Program “Global Education and Research Center for
Bio-Environmental Chemistry” of Osaka University. A.U. is a
recipient of a Japan Society for the Promotion of Science (JSPS)
research fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906666.
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(Figure 1b). Thus, 2 has much larger
contributions of the diradical structures
with quinoidal character such as b and
g[18] than closed Kekulꢀ structure d
(Scheme 1). These significant diradical
contributions are strongly supported by
natural orbital occupation number
(NOON) analysis[19] (see below) and
comparison between the observed and
calculated bond lengths.[20] Further-
more, the dihedral angles between the
corannulene skeleton and the phenoxyl
or phenol moieties are significantly
decreased (by 11–198) in 2 from 3.[9,11]
These changes can be interpreted as a
Scheme 1. Canonical resonance structures of 2.
ꢀ
result of the shortened C4 C13 and
ꢀ
C10 C18 bond lengths in 2 and their
magnitude of the intramolecular exchange interaction or
diradical character.
increased double-bond character. These bond-length and
dihedral-angle analyses experimentally illustrate that two
electronic spins of 2 are delocalized onto the corannulene
skeleton with retention of diradical character in the crystal.
A synthetic route for 2 is depicted in Scheme 2. The
diradical precursor, bis-phenol derivative 3, was obtained as
yellow blocks[9] by Suzuki coupling reaction of dibromo
derivative 4[10] with boronic acid 5, followed by deprotection
of the methoxymethyl (MOM) groups. Oxidation of 3 with an
excess of PbO2 and subsequent recrystallization gave single
crystals of 2 suitable for X-ray crystal structure analysis. In the
crystalline state, most of 2 survives in air at ꢀ308C for a few
weeks. Diradical 2 is also stable in degassed solution.
The crystal structure of 2 is illustrated in Figure 1. This is
the first X-ray structure analysis of neutral diradical deriva-
tives with a curved p-conjugated system. The unit cell
contains three crystallographically independent molecules
2A, 2B, and 2C (Figure 1c).[11] They stack in a convex–
concave fashion.[12] Judging from their bowl depths[13] and p-
orbital axis vector (POAV) angles[14,15] of the corannulene
skeleton (2A: 0.84 ꢁ, 7.88; 2B: 0.80 ꢁ, 7.78; 2C: 0.85 ꢁ, 8.18;
3: 0.88 ꢁ, 8.38), the curvature decreases in the order 3 > 2C >
Scheme 2. Synthesis of 2. a) [Pd(PPh3)4], Na2CO3, toluene/EtOH/H2O,
1008C; b) 2m HCl, AcOH, 408C, 73% in two steps; c) PbO2, CH2Cl2,
room temperature, 100%.
2A > 2B. As shown in Figure 1b,[11] the O1 C1 and O2 C7
bond lengths of 2 (1.282 ꢁ) are
ꢀ
ꢀ
shorter than those of
3
(1.385 ꢁ) and are close to the
ꢀ
corresponding
C O
bond
length of 1 (1.250 ꢁ)[7d] and
=
the C O bond length of p-
terphenoquinone (1.231 ꢁ).[16]
ꢀ
This indicates that the O1 C1
ꢀ
and O2 C7 bonds of 2 have a
ꢀ
certain degree of C O double-
bond character.[17] In addition,
we found that significant
changes of bond lengths arise
in the two six-membered rings
(C1–C6 and C7–C12) and the
ꢀ
ꢀ
C4 C13 and C10 C18 bonds.
ꢀ
On the other hand, the C23
ꢀ
ꢀ
C28, C24 C29, and C28 C29
bond lengths in the corannulene
skeleton of 2 remain almost
Figure 1. a) Molecular structure 2A and c) packing structure of 2 along the a axis. # denotes the
symmetry operation x+1,y,z. The thermal ellipsoids are scaled to the 50% probability level. Hydrogen
atoms are omitted for clarity. b) Major changes of bond lengths in 2 relative to 3. Red bonds are shorter
and blue bonds are longer than the corresponding bonds in 3.
unchanged from those of
3
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To evaluate the intramolecular magnetic interaction
extensive spin delocalization onto the corannulene moiety
(Figure 3, see below) results in significant electronic-spin
communication via the curved and nonalternant p-surface of
through the corannulene p-conjugated network, we carried
out temperature-dependent magnetic susceptibility measure-
ments on a polycrystalline sample of 2 in the range of 1.9–
298 K.[21] Because of the lack of effective intermolecular
magnetic contacts,[12] we concluded that the very strong
intramolecular antiferromagnetic interaction (JkꢀB1 = ꢀ405 ꢁ
2 K) occurs through the curved and nonalternant 3D p con-
jugation of corannulene in the crystalline state. Thus, diradical
2 has a singlet (S = 0) ground state and a thermally accessible
excited triplet (S = 1) state. The singlet–triplet energy gap
2JkꢀB1 is estimated to be ꢀ810 K. In addition, we experimen-
tally determined the magnitude and relative sign of hyperfine
coupling constants (hfccs) of the triplet species by solution-
phase ESR and 1H ENDOR/TRIPLE spectroscopy (Fig-
ure 2a,b).[22] Glass-phase ESR measurements (Figure 2c)
gave the characteristic fine structure (DMs = ꢁ 1) and for-
bidden transition (DMs = ꢁ 2). The zero-field splitting param-
eters (D, E) and principal g values were determined by
spectral simulation.[22] Estimating the spin–spin distance
suggested considerable contributions of canonical resonance
structures b and g (Scheme 1) in the triplet state. In this
estimation, we carefully noted that a nonvanishing E value is
consistent with significant contributions of the structures b
and g. All these experimental results demonstrate that the
Figure 3. Calculated spin-density distributions of a) BS singlet state
and b) triplet state for 2A at the UB3LYP/6-31G(d,p) level. Red:
positive, blue: negative spin densities.
corannulene.[23] Furthermore, in terms of the curved p-
conjugated structure, we believe that diradical 2 has its own
place as an intermediate case between thus-far reported
planar p-conjugated systems and completely 3D p-conjugated
species such as neutral C60 in the triplet state, noting that spin–
orbit contributions are relevant to the particular canonical
resonance structures (see Figure S12 in the Supporting
Information).[2d,f]
Calculated spin-density distributions obtained by DFT
methods[24] indicate an extensive spin-delocalized nature of 2
in both broken-symmetry (BS) singlet and triplet states
(Figure 3).[25] Especially in the BS singlet state (Figure 3a),
rings A, B, and C of corannulene have a large amount of spin
density, and rings D and E a relatively small amount. Thus, the
unbalanced spin-delocalized nature, that is, spin-rich
(rings A–C) and spin-poor regions (rings D and E) on the
curved p-conjugated system of corannulene, is generated in
the diradical system 2 as well as in the monoradical system
1.[7c,d] The origin of this unique nature is the topological effect
arising from the nonalternant p conjugation of the corannu-
lene system, which is illustrated by the canonical resonance
structures.[26] Furthermore, the differences in spin-density
distributions between the two spin states (see Figure S13 in
the Supporting Information) affect sensitively their curved
aromaticity, as suggested by the nucleus-independent chem-
ical shift (NICS)[27] method.[28]
Figure 2. a) 1H ENDOR (290 K) and b) TRIPLE (290 K, pump frequency
18.674 MHz) spectra of 2 in a degassed toluene solution
(1.3ꢀ10ꢀ3 m). The vertical arrows in (b) denote increase or decrease in
intensity when the outermost ENDOR line at 18.674 MHz is pumped.
The intensity of the pumped line decreased. c) Fine-structure ESR
spectra of 2 (DMs =ꢁ1: microwave frequency 9.40792 GHz,
DMs =ꢁ2 (inset): microwave frequency 9.40799 GHz) in a degassed
frozen toluene glass (1.3ꢀ10ꢀ3 m) at 158 K. The black and red lines
indicate observed and simulated spectra, respectively.
Furthermore, we have found that the calculated intra-
molecular exchange interactions J[29] and singlet diradical
characters of the three crystallographically independent
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diradical molecules 2A–2C with different curvature provide
very intriguing findings for this class of curved diradical
systems. Diradicals 2A–2C all show large negative J values
(2A: ꢀ425.0 K, 2B: ꢀ525.4 K, 2C: ꢀ339.5 K), which means
the occurrence of strong antiferromagnetic intramolecular
exchange interaction. Their average value (JkꢀB1 = ꢀ430.0 K)
assemblies composed of bowl-shaped p-conjugated open-
and/or closed-shell molecules with unique geometrical and
topological features.[6,7,33]
Received: November 26, 2009
Published online: January 27, 2010
reproduces well the experimentally obtained one (JkBꢀ1
=
Keywords: conjugation · density functional calculations ·
ꢀ405 ꢁ 2 K). In addition, we are interested in the consider-
able differences between the calculated J values of 2A–2C.
Detailed studies on their structural differences related to such
differences in J value strongly suggest that the magnitude of
the J value becomes larger when the curvature of the
corannulene p-conjugated system decreases.[30] It is notable
that such small differences in curvature of 2A–2C (0.05 ꢁ in
bowl depth) give rise to very large differences in the J values
(186 K).[31] This is due to the enhancement of the p conjuga-
tion between the corannulene skeleton and the radical
moiety. An NOON analysis[19] also indicates that 2A–2C in
the singlet ground state have much larger contributions of
diradical structures [ca. 83% (2A: 83.7%, 2B: 80.1%, 2C:
84.6%)] than of the closed Kekulꢀ structure (ca. 17%; see
also Table S4 in the Supporting Information). This pro-
nounced diradical character is probably attributable to
aromatic stabilization of the corannulene p system, which
prevents 2 from forming Kekulꢀ structure d with fewer 6p
benzene-structure contributions than b and g (Scheme 1).[26]
Importantly, the difference in curvature also correlates with
the magnitude of the diradical character of 2 as well as the
intramolecular exchange interaction J. These results suggest
that the singlet diradical character of 2 decreases with
increasing magnitude of the antiferromagnetic intramolecular
exchange interaction J. In this context, a theoretical analysis
of the zero-field splitting tensor for 2 in the thermally
accessible triplet state gives a clue to understanding the three-
dimensional electronic-spin structure in a straightforward
manner (see the Supporting Information).
.
EPR spectroscopy · exchange interactions · radicals
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In summary, corannulene-based stable neutral diradical 2
bearing two phenoxyl radical moieties has been synthesized
and isolated as crystals stable in air. Thanks to the high
stability and the sizable spin delocalization onto the coran-
nulene skeleton from the radical moieties, we have exper-
imentally revealed, for the first time, the occurrence of 3D
intramolecular exchange interaction via the curved and
nonalternant p-conjugated system of corannulene. Further-
more, we have successfully illustrated that the magnitude of
the intramolecular exchange interaction is enhanced by
decreasing the curvature of the corannulene skeleton. In
addition to this geometrical effect, we have illustrated the
topological effect of the nonalternant p-conjugated network,
which influences the spin-delocalized nature, diradical char-
acter, and curved aromaticity of 2. These findings demon-
strate the intriguing aspects of a 3D intramolecular exchange
interaction of neutral diradical systems having curved and
nonalternant p-conjugated networks. Thus, we believe that
our present study will contribute not only to opening up a new
field of open-shell chemistry and molecular magnetism with
dynamic electronic-spin behaviors arising from bowl-to-bowl
inversion behavior,[7e,32] but also to developing functionalities
based on 3D intra/intermolecular interactions of molecular
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Communications
7277; b) S. Suzuki, Y. Morita, K. Fukui, K. Sato, D. Shiomi, T.
[22] The solution-phase ESR spectrum, hfccs, zero-field splitting
parameters, and g values are given in the Supporting Informa-
tion.
[23] UV/vis measurements for 2, 3, and 1 demonstrate the occurrence
of a sizable degree of p conjugation between the corannulene
and two phenoxyl moieties. For details, see the Supporting
Information.
[24] All DFT calculations were performed with Gaussian03 (revi-
sion B.05), Gaussian, Inc., Wallingford CT, 2004; the full
reference is listed in the Supporting Information.
[25] All of the spin density distributions of BS singlet and triplet
states of 2A–2C are given in the Supporting Information.
[26] For details of the canonical resonance structure studies on 2, see
the Supporting Information.
[27] a) P. von R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R.
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[28] For details of the curved aromaticity of 2 and 3, which is the
aromaticity in six- and five-membered rings of the curved p-
conjugated system, see the Supporting Information.
[29] Their J values were obtained by using calculated spin expect-
Takui, K. Nakasuji, J. Am. Chem. Soc. 2006, 128, 2530; c) Y.
Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H.
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[11] Molecular structure, bond lengths, and dihedral angles of 2A–
2C are shown in the Supporting Information.
[12] There are no intermolecular magnetic contacts between carbon
atoms with a large amount of spin density. However, p–p
concave–convex interactions occur between the corannulene
skeletons with short distances close to the sum of the van der
Waals radii of two carbon atoms (3.40 ꢁ). For details, see the
Supporting Information.
[13] Bowl depths are the average of the perpendicular distances
between the plane containing the central five-membered ring
composed of C28–C32 and the peripheral ten carbon atoms
(C13–C22) of the corannulene skeleton.
[14] a) R. C. Haddon, L. T. Scott, Pure Appl. Chem. 1986, 58, 137;
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in the five-membered ring (C28–C32). POAV values of all
carbon atoms in the five-membered rings of 2 and 3 are given in
the Supporting Information.
ation values and total energies for the BS singlet (hS2iBS and EBS
)
and triplet states (hS2iT and ET). These parameters are listed in
the Supporting Information. See S. Yamanaka, T. Kawakami, H.
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between the corannulene and the phenoxyl moiety. However,
this indicates that the differences of the dihedral angles of 2A–
2C are not attributable to the large differences in the J values.
For details, see the Supporting Information.
[31] Siegel et al. report that small differences in the bowl depth of
corannulene derivatives greatly affect the bowl-to-bowl inver-
sion barrier. For example, the inversion barrier of 2,3-dimethyl-
corannulene (7.7 kcalmolꢀ1) with a bowl depth of 0.83 ꢁ is
significantly smaller than that of corannulene (9.2 kcalmolꢀ1),
which has a bowl depth of 0.88 ꢁ. See T. J. Seiders, K. K.
Baldridge, G. H. Grube, J. S. Siegel, J. Am. Chem. Soc. 2001, 123,
517.
[32] The bowl-to-bowl inversion barriers of radical ion species of
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Jackson, L. T. Scott, J. Phys. Chem. B 2009, 113, 1954. See also
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[33] Based on the experimental results, Nishihara et al. propose that
zeolite-templated carbon, which has remarkable potential for
various applications, is made up of buckybowl-like nanogra-
phene assembled into a three dimensionally regular network: H.
Nishihara, Q.-H. Yang, P.-X. Hou, M. Unno, S. Yamauchi, R.
Saito, J. I. Paredes, A. Martꢃnez-Alonso, J. M. D. Tascꢄn, Y. Sato,
M. Terauchi, T. Kyotani, Carbon 2009, 47, 1220.
[16] The bond-length data of p-terphenoquinone (R. West, J. A.
Jorgensen, K. L. Stearley, J. C. Calabrase, J. Chem. Soc. Chem.
Commun. 1991, 1234.) are described in the Supporting Informa-
tion.
ꢀ
[17] The C O double-bond character of these bonds was also
disclosed by IR measurements; see the Supporting Information.
ꢀ
ꢀ
[18] The C13 C14 and C17 C18 bonds of 2 (1.398 ꢁ) are slightly
longer than those of 3 (1.391 ꢁ), due to the contribution of the g
structure.
[19] a) D. Dꢂhnert, J. Kouteck’y, J. Am. Chem. Soc. 1980, 102, 1789;
b) Y. Jung, M. Head-Gordon, ChemPhysChem 2003, 4, 522, and
references therein.
[20] The experimentally obtained bond lengths were significantly
closer to those calculated for the singlet diradical structure than
those of the Kekulꢀ structure. For details, see the Supporting
Information.
[21] The results of magnetic susceptibility measurements and its
fitting data are given in the Supporting Information.
1682
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Angew. Chem. Int. Ed. 2010, 49, 1678 –1682