Curved aromaticity of a corannulene-based neutral radical: Crystal structure and 3D unbalanced delocalization of spin

Article abstract of DOI:10.1002/anie.200704752

(Figure Presented) A well-rounded radical: A stable corannulene-based neutral radical with curved p conjugation was synthesized, and the crystal structure was determined. Unbalanced delocalized electronic spin on the three-dimensional p surface of the corannulene moiety (see spin-density distribution) demonstrates a sizable intermolecular exchange interaction and curved aromaticity.

Full text of DOI:10.1002/anie.200704752

Angewandte
Chemie
DOI: 10.1002/anie.200704752
Corannulene-Based Radicals
Curved Aromaticity of a Corannulene-Based Neutral Radical: Crystal
Structure and 3D Unbalanced Delocalization of Spin**
Yasushi Morita,* Akira Ueda, Shinsuke Nishida, Kozo Fukui, Tomoaki Ise, Daisuke Shiomi,
Kazunobu Sato, Takeji Takui,* and Kazuhiro Nakasuji*
Bowl-shaped polycyclic aromatic hydrocarbons such as cor-
annulene, which shares a fullerene substructure itself with a
non-alternant p conjugation, have been studied extensively in
recent years.[1] Such studies have emphasized the description
of their aromaticity and electron interactions on curved
surfaces or between a curved surface and a metal ion from
both the experimental and theoretical perspectives. In con-
trast to the closed-shell systems studied so far, neutral open-
shell molecules with curved p conjugation have been studied
only in degassed solution owing to their low stability in air.[2,3]
Spin delocalization on a curved p-conjugated system is
intrinsically three-dimensional, and thus elucidation of the
crystal/electronic-spin structures and intra- and intermolecu-
lar exchange interactions in the crystalline state is a current
issue for studying the 3D interelectronic exchange and dipolar
interactions in curved p systems such as fullerenes.[4]
emphasizing the occurrence of unbalanced delocalization of
spin within the corannulene moiety.
A synthetic route for 1 is depicted in Scheme 1. The
radical precursor 2 was obtained as colorless plates by Suzuki
coupling of a pinacol boronate derivative 3[6] with methoxy-
Scheme 1. Synthesis of neutral radical 1: a) [Pd(PPh3)4], K2CO3, DMF,
1108C, 39%; b) 2m HCl, AcOH, room temperature, 95%; c) PbO2,
CH2Cl2, room temperature, 99%.
In this study, we have synthesized a corannulene-based
neutral radical with a phenoxyl moiety (1) and determined for
the first time the crystal structure of a neutral radical
derivative with curved p conjugation. The high stability and
extensively spin-delocalized nature of the corannulene
moiety 1 have enabled us to investigate curved aromaticity
and intermolecular interactions of this class of curved neutral
radical systems with a non-alternant p conjugation,[5] thus
methyl-protected (MOM-protected) bromophenol 4 and
subsequent deprotection. Treatment of 2 with an excess of
PbO2 and recrystallization gave 1 as black plates. The radical 1
in the crystal is stable in air at À308C for a few weeks, and is
extremely stable also in degassed solution.
We have succeeded in the crystal structure analysis of
radical 1 (Figure 1a).[7] The structural features of 1 were
revealed by comparison with the molecular structure of the
[*] Prof. Dr. Y. Morita, A. Ueda, Prof. Dr. K. Nakasuji
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
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
Figure 1. a) Molecular structure of 1; hydrogen atoms are omitted for
clarity. b) Major changes of bond lengths in 1 from 2. Red and blue
bonds represent shorter and longer bonds, respectively, in 1 as
compared with corresponding bonds in 2.
E-mail: takui@sci.osaka-cu.ac.jp
phenol 2.[8] While the bowl depth and POAV (p-orbital axis
vector) angles[9,10] of 1 are similar to those of 2, significant
Prof. Dr. Y. Morita, Dr. K. Fukui
PRESTO
Japan Science and Technology Agency (JST)
À
changes in bond lengths of the phenoxyl moiety and C7 C8
[**] This work was supported by PRESTO-JST, Grant-in-Aid for Scientific
Research (No. 18655058) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan), and the Global COE
Program “Global Education and Research Center for Bio-Environ-
mental Chemistry” of Osaka University.
À
and C8 C17 of the corannulene moiety were observed
À
(Figure 1b, Table 1). Particularly, the O C1 bond of 1
=
(1.250(2) Š) is closer to the C O bond length of p-benzoqui-
none (1.222 Š)[11a] and p-terphenoquinone (1.231 Š),[11b]
À
indicating that the C O bond of 1 has a substantial double-
Supporting information for this article, including detailed synthetic
procedures for 1 and 2, is available on the WWW under http://
www.angewandte.org or from the author.
bond character. IR measurements of 1 and 2[12] also demon-
À
strated the double-bond character of the O C1 bond of 1. In
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Table 1: Bowl depths, POAV angles, selected bond lengths, and dihedral
angles of 1 and 2.
substituted position in the corannulene moiety, that is, the
positive sign of the spin density at the carbon atom attached
to the corannulene skeleton (C4 in Figure 1 and Figure 2).[18]
Notably, the highly delocalized unpaired electron on the
corannulene moiety in 1 effects an unbalanced delocalization
of spin, that is, the uneven spin distribution over spin-rich
(rings A and B) and spin-poor regions (rings D and E) within
the corannulene moiety. This differential spin distribution
attributable to the topological effect of the corannulene
p conjugation can also be interpreted in terms of classical
canonical resonance structures.[14]
Compound
1
2
Bowl depth [Š][a]
0.91
8.5
0.92
8.4
POAV angle [8][b]
Bond lengths [Š]
À
O C1
À
C1 C2
1.250(2)
1.468(2)
1.362(3)
1.414(2)
1.471(3)
1.409(2)
1.414(3)
1.385(5)
1.409(5)
1.382(5)
1.394(5)
1.494(5)
1.394(5)
1.441(6)
À
C2 C3
À
C3 C4
This unique spin delocalization on the corannulene
moiety affects the packing structure of 1 in the solid state.
As shown in Figure 3, radical 1 forms a dimeric pair with an
À
C4 C7
À
C7 C8
À
C8 C17
Dihedral angles [8]
C3-C4-C7-C8
C5-C4-C7-C21
35.9(2)
41.5(2)
38.6(5)
43.7(5)
[a] Bowl depths were measured from the plane containing the five-
membered ring to the plane containing the peripheral aromatic carbon
atoms. [b] Average of carbon atoms of the five-membered ring.
the radical 1, the O H stretching found in 2 (3636 cmÀ1 in the
À
KBr, 3631 cmÀ1 in CH2Cl2 solution) disappeared, and a new
sharp absorption appeared at 1565 cmÀ1 in the solid and at
1567 cmÀ1 in solution. These new absorptions are similar to
=
the C O vibration frequency of p-terphenoquinone
(1575 cmÀ1 in the solid state).[11b,13] Table 1 also shows that
the dihedral angles between the corannulene and phenoxyl
moiety of 1 (C3-C4-C7-C8 35.9(2)8, C5-C4-C7-C21 41.5(2)8)
are slightly decreased from those of 2 (C3-C4-C7-C8 38.6(5)8,
C5-C4-C7-C21 43.7(5)8). These changes are reasonably
interpreted by considering a quinoidal structural contribution
in the classical canonical resonance structures of 1.[14] All the
experimental results suggest extensive spin delocalization
onto the corannulene moiety from the phenoxyl moiety in the
solid state.
Figure 3. Crystal structure of 1. The dashed line represents the
intermolecular short contact (3.791 Š). Hydrogen atoms and the tert-
butyl groups are omitted for clarity. The thermal ellipsoids are shown
at the 50% probability level.
DFT calculations of 1 based on the crystal structure also
showed extensive spin delocalization onto the corannulene
moiety from the phenoxyl moiety (Figure 2),[15] as revealed by
comparing the sum of the absolute spin density on the
corannulene moiety of 1 (0.553)[16] with those of verdazyl
(0.241)[17] and iminonitroxide (0.175)[17] derivatives of coran-
nulene. This trend of the spin delocalization depending on
radical moieties is attributable to the topological nature of the
intermolecular separation of 3.791 Š between carbon atoms
(C8···C8i) of the corannulene moieties (symmetry operation i:
Àx, Ày + 1, Àz + 2). Because the DFT calculations indicate
that these carbon atoms have relatively large amounts of spin
density (Figure 2), a sizable intermolecular exchange inter-
action between the corannulene moieties was expected. To
evaluate this interaction and characterize the bulk magnetic
properties of the crystalline state of 1, the magnetic suscept-
ibility cp of a polycrystalline sample was measured in the
range from 1.9 to 300 K in a static magnetic field of 0.1 T.[19]
The result showed a ground-state spin-singlet formation with
an antiferromagnetic intermolecular interaction (J/kB =
À22.5 Æ 0.2 K) owing to the intermolecular exchange inter-
action of 1 in the crystal. This experimental J value is in good
agreement with the calculated one (J/kB = À27.0 K).[20]
To elucidate the electronic-spin structure of 1 in solution,
1
we carried out liquid-phase ESR and H ENDOR/TRIPLE
measurements. An ESR spectrum of 1 in a degassed toluene
solution (4.4 ” 10À4 m) shows five broad hyperfine splittings
(g = 2.0046).[21] Hyperfine coupling constants (hfccs) and their
relative signs of the protons were determined by 1H ENDOR/
TRIPLE spectroscopy (Figure 4).[21] These hfccs were suc-
Figure 2. Spin-density distribution of 1 calculated at the UB3LYP/6-
31G(d,p) level. The molecular geometry was taken from the X-ray
crystal structure. The red and blue colors denote positive and negative
spin densities, respectively.
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all ring systems on the corannulene moiety, especially for
rings A and B (Figure 5b), which is in agreement with the
positions having a large amount of spin density (Figure 2).
Furthermore, the positive NICS(0) value (+ 1.3) in ring G
indicates a local antiaromaticity of this (phenyl) ring system.
These findings demonstrate that the local aromaticity of the
ring system having a sizable spin density decreases signifi-
cantly in the curved p conjugation of corannulene, which is
consistent with the case of a planar odd-alternant p-radical
system.[23c]
In conclusion, the stable corannulene-based neutral
radical 1 with a phenoxyl moiety was synthesized, and its
electronic-spin structure was elucidated experimentally on
the basis of the crystal structure analysis with the help of
resonance-structure studies, DFT calculations, and magnetic
and ESR/ENDOR measurements. We have revealed, for the
first time, the three-dimensional spin delocalization on a
corannulene-based neutral radical. This study was inspired by
the high stability and highly spin-delocalized nature in the
corannulene moiety,[24] as well as the unique geometric
relationship between the planar p radical[25] and tetrahedral
s radical, whereby we have focused on studying the inter- and
intramolecular exchange interactions through the corannu-
lene p surface.[26] Thus, such bowl-shaped neutral radicals
with non-alternant p conjugation are useful for exploring new
aspects of spin chemistry for applications in molecule-based
functional materials. They also serve as a basis for both
experimental and theoretical investigation of three-dimen-
sional intra- and intermolecular exchange interactions within/
between curved p-conjugated systems as well as the dynamic
behavior of electronic spin and molecular structure as a
function of bowl-to-bowl inversion.[27]
Figure 4. a) 1H ENDOR and b) 1H TRIPLE spectra (pump frequency
10.27 MHz) at 240 K of 1 in a degassed toluene solution (4.4”10À4 m).
cessfully assigned with the help of the results of the DFT
calculations based on the crystal structure (see above;
Table 2), and a spectral simulation well reproduced the
Table 2: Observed and calculated hyperfine coupling constants (hfccs, in
mT) of 1.
H1, H2
H3
H4
H5 or H7
H(tBu)
Obsd[a]
Calcd[b]
+0.165
À0.295
À0.306
Æ0.043
À0.046
Æ0.025
Æ0.007
+0.217[c]
+0.032 or
À0.045
+0.005[d]
[a] Values and relative signs of hfccs were determined from 1H ENDOR/
TRIPLE spectra. [b] Values were calculated at the UB3LYP/6-31G(d,p)
level based on the X-ray crystal structure. [c] Average of H1 and H2.
[d] Average of all tert-butyl protons.
observed spectrum.[21] Therefore, in solution, radical 1 main-
tains the unbalanced delocalization of spin.
For further evaluation of the electronic structures, we
have invoked the nucleus-independent chemical shift (NICS)
method for both 1 and 2 using their crystal structures
(Figure 5).[22] This method is known as a facile and efficient
Experimental Section
Crystal data for 1: C34H29O: Mr = 453.60, monoclinic, space group
P21/a (no. 14), a = 8.457(8), b = 23.85(2), c = 12.140(12) Š, b =
94.727(3)8, V= 2440(4) Š3, 1calcd = 1.234 gcmÀ3, Z = 4, m(MoKa) =
0.723 cmÀ1, 2qmax = 55.48, 18575 reflections, 5476 of which were
unique (Rint = 0.074). R1 = 0.0785, wR2 = 0.1035, GOF = 1.058. Data
were collected on a Rigaku Mercury CCD diffractometer (MoKa
radiation, l = 0.71073 Š) at À738C. The structure was solved by
direct methods and refined with full-matrix least-squares techniques
(CrystalStructure 3.7.0: Crystal Structure Analysis Package, Rigaku
and Rigaku/MSC, The Woodlands, USA, 2000–2005).
Crystal data for 2: C34H30O: Mr = 454.61, monoclinic, space group
P21/a (no. 14), a = 8.487(3), b = 24.152(8), c = 12.161(4) Š, b =
95.244(11)8,
V= 2482.1(14) Š3,
1calcd = 1.216 gcmÀ3
,
Z = 4,
Figure 5. NICS(0) values (ppm) of a) 2 and b) 1 calculated at the
UB3LYP/6-31G(d,p)//UB3LYP/6-31G level using the crystal structures
as initial structures.
m(MoKa) = 0.712 cmÀ1
,
2qmax = 55.08, 23496 reflections, 5661 of
which were unique (Rint = 0.047). R1 = 0.0935, wR2 = 0.1764, GOF =
1.00. Data were collected on a Rigaku RAXIS-RAPID Imaging Plate
(MoKa radiation, l = 0.71073 Š) at À738C. The structure was solved
by direct methods and refined with full-matrix least-squares techni-
ques (CrystalStructure 3.8., 2000–2006).
CCDC-663934 for 1 and 663935 for 2 contain the supplementary
crystallographic 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
probe for evaluating aromaticity even for open-shell sys-
tems.[23] Negative NICS values indicate the presence of
induced diatropic ring currents and “aromaticity”, whereas
positive values denote paratropic ring currents and “antiar-
omaticity”. In phenol 2, negative NICS(0) values were
obtained in all six-membered rings (Figure 5a). In sharp
contrast, in radical 1, more-positive values were obtained in
Received: October 14, 2007
Published online: February 5, 2008
Angew. Chem. Int. Ed. 2008, 47, 2035 –2038
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www.angewandte.org
2037

Communications
[14] For details of resonance structures, see the Supporting
Information.
Keywords: aromaticity · density functional calculations ·
EPR spectroscopy · radicals · X-ray diffraction
.
[15] All DFT calculations were performed with the Gaussian03
program (revision B.05) Gaussian, Inc., Wallingford CT, 2004;
the full reference is listed in the Supporting Information.
[16] Details of the calculated hfccs and spin densities of 1 are found in
the Supporting Information.
[17] K. Fukui, Y. Morita, S. Nishida, T. Kobayashi, K. Sato, D.
Shiomi, T. Takui, K. Nakasuji, Polyhedron 2005, 24, 2326 – 2329.
[18] Verdazyl and iminonitroxide derivatives have negative spin
densities on this position.
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[19] For details of the magnetic measurements, see the Supporting
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[20] Calculations were performed at the UB3LYP/6-31G(d,p) level
using the crystal structure of 1; for details, see the Supporting
Information.
[21] For details of the ESR/1H ENDOR spectra, see the Supporting
Information.
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[7] To our knowledge, this is not only the first crystal structure of a
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[8] For details of the crystal structure of 2, see the Supporting
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[26] Syntheses of corannulene derivatives with two phenoxyl moi-
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[27] Bowl-to-bowl inversions of corannulene derivatives with closed-
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and references therein.
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Angew. Chem. Int. Ed. 2008, 47, 2035 –2038