Aromaticity and π-bond covalency: Prominent intermolecular covalent bonding interaction of a Kekule hydrocarbon with very significant singlet biradical character

Article abstract of DOI:10.1039/c2cc31955a

An anthracene-linked bisphenalenyl Kekule molecule with very significant singlet biradical character has shown a prominent covalent bonding interaction between molecules in a molecular aggregate. High aromatic stabilization energy in the anthracene linker is responsible for the significant singlet biradical character.

Full text of DOI:10.1039/c2cc31955a

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COMMUNICATION
Aromaticity and p-bond covalency: prominent intermolecular covalent
´
bonding interaction of a Kekule hydrocarbon with very significant
singlet biradical characterwzy
Akihiro Shimizu,a Yasukazu Hirao,a Kouzou Matsumoto,a Hiroyuki Kurata,a
Takashi Kubo,*a Mikio Uruichib and Kyuya Yakushib
Received 17th March 2012, Accepted 3rd April 2012
DOI: 10.1039/c2cc31955a
´
An anthracene-linked bisphenalenyl Kekule molecule with very
significant singlet biradical character has shown a prominent
covalent bonding interaction between molecules in a molecular
aggregate. High aromatic stabilization energy in the anthracene
linker is responsible for the significant singlet biradical character.
Aromaticity, which is defined in an energetic criterion by
special thermodynamic stability originating from the cyclic
delocalization of 4n + 2 p electrons, is associated with
relatively low reactivity, bond length equalization, diamagnetic
ring current and characteristic UV absorption of aromatic
compounds.1,2 Recently, we have demonstrated that aromaticity is
also closely related to p-bond covalency of conjugated molecules
by investigating singlet biradical character of bisphenalenyl Kekule
´
molecules with benzene (1a, b) and naphthalene (2a, b) linkers.3
The prominent feature of the bisphenalenyl molecules is a covalent
bonding interaction between molecules in a molecular aggregate
Scheme 1 Synthetic route for 3a and 3b: (i) 1,2,4,5-tetrabromobenzene,
BuLi, toluene, rt, then p-chloranil, 90 1C, 40%. (ii) (a) Pb3O4, AcOH,
benzene, rt; (b) LiAlH4, THF, rt, 49% (2 steps). (iii) TsOHꢀH2O, toluene,
90 1C, 41%. (iv) Pb3O4, AcOH, benzene, rt, 27%. (v) (a) TsOHꢀH2O,
toluene, 90 1C, 87%; (b) p-benzoquinone, toluene, 90 1C, 55%.
although they have Kekule
formula. In this communication, we report the synthesis of
bisphenalenyl Kekule molecules (3a, b) with an anthracene
´
canonical forms in the resonance
´
linker that possesses higher aromatic stabilization energy than
benzene and naphthalene linkers. The higher aromaticity
endows 3a and 3b with more singlet biradical character,
leading to a spectroscopic observation of a covalent bonding
interaction between molecules in a molecular aggregate of 3b.
The synthetic procedure for 3a and 3b is outlined in Scheme 1.
The Diels–Alder reaction of 4a4 with benzyne generated by
1,2,4,5-tetrabromobenzene and butyllithium, and subsequent
a Department of Chemistry, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: kubo@chem.sci.osaka-u.ac.jp
b Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
w This article is part of the ChemComm ‘Aromaticity’ web themed
issue.
z Dedicated to Professor Kazuhiro Nakasuji on the occasion of his
70th birthday.
y Electronic supplementary information (ESI) available: Experimental
procedures for 3a, 3b, 4a and 4b. A spin density map for the
unsubstituted compound of 3 (Fig. S1). Cyclic voltammogram of 3a
(Fig. S2). Optical conductivity of 3b with light polarized perpendicular
to the c-axis (Fig. S3). NICS(1) values of 1, 2 and 3 (Fig. S4). CCDC
871896 and 871897. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc31955a
Chart 1 Molecular structures of 1, 2 and 3.
dehydrogenation with p-chloranil gave a hexahydro compound 5.
Three benzyl carbons were acetylated with Pb3O4 and the acetyl
groups were reduced with LiAlH4 to give triol 6. Dehydration of
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5629–5631 5629

6 gave 3a as purple powder. Recrystallization of 3a from a
chlorobenzene solution in a sealed degassed tube gave black
crystals. The solid 3a was found to be stable at room temperature
in air for a couple of days. 3b was synthesized with a similar
procedure to that for 3a and was obtained as dark purple plates
by recrystallization from a toluene solution. The solid 3b was
also stable at room temperature in air for a couple of days.
The structures of 3a and 3b were confirmed by single crystal
X-ray crystallographic analysis (Fig. 1).5
HOMO–LUMO energy gap, which was confirmed by using
UV–Vis–NIR spectroscopy and cyclic voltammetry as listed in
Table 1. Furthermore, 3a had the longest length in the bond a
(see Chart 1 and Table 1) among the series of the bisphenalenyl
compounds. The length of the bond a is the most sensitive to the
contribution weight of the each canonical form, because the
bond a has both single and double bond character in the Kekule
´
form, whereas only single bond character in the biradical form.
These observations thoroughly support very significant singlet
biradical character of 3a. A singlet–triplet energy gap (DES–T) is
also associated with singlet biradical character. The DES–T
values for the unsubstituted compounds of 1, 2 and 3 were
estimated to be 30, 19 and 12 kJ molꢁ1, respectively, on the
basis of UB3LYP/6-31G** calculations.10,11
The amount of singlet biradical character (y) for unsubstituted
compounds of 1, 2 and 3 was estimated from the natural orbital
occupation number (NOON)6 of the LUMO on the basis of
CASSCF(2,2)/6-31G calculations (see Table 1). The y values
increase with increasing the homodesmotic stabilization energy
(HSE) of the linkers, which is known to be a good measure for
aromaticity.7 This finding can be explained by the energy balance
between destabilization due to the formal loss of one double
bond8 and stabilization due to the formation of aromatic rings in
The non-tert-butyl compound 3b formed a one-dimensional
(1D) chain in the crystal in a slipped stack arrangement
running along the [1 0 1] direction (Fig. 2). 3b reproduces
the superimposed phenalenyl overlap that is identical to that
of 1b and 2b. The average C–C distance between the over-
lapping carbons is 3.122 A, which is substantially shorter than
the van der Waals contact of carbon atoms. This close contact is
ascribed to a covalent bonding interaction between molecules,
as previously discussed for 1b and 2b.3a,d
the linkers, upon transformation from the Kekule to the biradical
´
forms.9 The y value of the unsubstituted compound of 3 amounts
to 68%, and unpaired electrons reside mostly on the phenalenyl
rings in an anti-parallel manner on the basis of a broken-
symmetry UB3LYP/6-31G** calculation (Fig. S1, ESIy).10
Single molecule character of the anthracene-linked bisphenalenyl
was estimated with the tert-butyl substituted compound 3a.
Larger y, that is, weaker coupling of two unpaired electrons
that appear in the biradical form, results in a smaller
In the 1D stack of 3b, a long-range p-conjugation should be
formed through the intra- and inter-molecular covalent bonding
interactions, as in the case of the 1D stacks of 1b and 2b. The
long-range p-conjugation in the 1D stacks of 1b and 2b causes
a drastic redshift for the lowest-energy bands relative to the
solution bands. However, the solid state optical spectrum of 3b,
which was obtained by the Kramers–Kronig transformation
of the reflection spectrum measured with light on the (0 1 0)
surface of a single crystal, gave no significant redshift for the
lowest-energy band centered at 9750 cmꢁ1 relative to the solution
band (Fig. 3). We previously demonstrated that the extent of the
redshift depends on the balance in magnitude between intra- and
inter-molecular covalent bonding interactions3c and that the well-
balanced interactions in the 1D stack of 1b lead to the very large
redshift compared to the less-balanced stack of 2b.3d The
no-redshift in 3b might reflect a lopsided interaction in the
1D stack. In order to determine the direction of the transition
Fig. 1 ORTEP drawings of (a) 3a at 150 K and (b) 3b at 200 K.
Displacement ellipsoids are drawn at the 50% probability level.
Table 1 Amount of singlet biradical character (y), HSE of the linkers,
HOMO–LUMO gaps and the length of the bond a for 1a, 2a and 3a
HSE of linkers/ H–L gap H–L gap Length of
(UV)/eV (CV)/eV bond a/A
Compounds y/% kJ molꢁ1
1a
2a
3a
30
50
68
89.9
137.2
173.4
1.64a
1.43b
1.26c
1.20a
1.04b
0.98d
1.457(2)a
1.465(7)b
1.467(3)e
a
b
Ref. 3c. Ref. 3b. This work, determined from the longest-wavelength
c
d
absorption band. This work, determined from the difference between the
first reduction and the first oxidation potentials (Fig. S2, ESI). e This work,
determined from X-ray crystallographic analysis.
Fig. 2 (a) 1D stack of 3b along with the unit cell. (b) Distances (in A)
of the short C–C contacts in the overlapping phenalenyl rings. The
overlapping carbons are highlighted in gray.
c
5630 Chem. Commun., 2012, 48, 5629–5631
This journal is The Royal Society of Chemistry 2012

Kekule singlet biradical molecules always encounter the
´
difficulty in how their spin structures can be elucidated,
because all p-electrons are covalently paired to form the singlet
ground state. Intrinsically, they are inactive for an ESR
measurement that is the most powerful method for elucidating
the spin structure. Our study presented herein concerns the
investigation of unpaired electrons in a Kekule molecule by
´
examining a covalent bonding interaction between molecules
in a molecular aggregate. A covalent bonding interaction
between molecules would be a good criterion to identify singlet
biradical character for Kekule molecules.
´
This work was supported in part by the Grants-in-Aid for
Scientific Research on Innovative Areas ‘‘Reaction Integration.’’
(No. 2105) and Grants-in-Aid for Scientific Research
(No. 19550039) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
Fig. 3 Optical conductivity of 3b with the light polarized along the c-axis
(solid line), and the absorption spectrum of 3a in CH2Cl2 (dotted line).
Notes and references
1 (a) E. D. Bergmann and B. Pullman, Aromaticity, Pseudo-aromaticity,
and Anti-aromaticity, Academic Press, New York, 1971; (b) P. J.
Garratt, Aromaticity, Wiley, New York, 1986; (c) V. I. Minkin, M. N.
Glukhovtsev and B. Y. Simkin, Aromaticity and Antiaromaticity:
Electronic and Structural Aspects, Wiley, New York, 1994.
2 (a) P. v. R. Schleyer, Chem. Rev., 2001, 101, 1115 and articles therein;
(b) P. Lazzeretti, Phys. Chem. Chem. Phys., 2004, 6, 217–223.
3 (a) T. Kubo, A. Shimizu, M. Sakamoto, M. Uruichi, K. Yakushi,
M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Morita and
K. Nakasuji, Angew. Chem., Int. Ed., 2005, 44, 6564–6568;
(b) T. Kubo, A. Shimizu, M. Uruichi, K. Yakushi, M. Nakano,
D. Shiomi, K. Sato, T. Takui, Y. Morita and K. Nakasuji, Org.
Lett., 2007, 9, 81–84; (c) A. Shimizu, M. Uruichi, K. Yakushi,
H. Matsuzaki, H. Okamoto, M. Nakano, Y. Hirao, K. Matsumoto,
H. Kurata and T. Kubo, Angew. Chem., Int. Ed., 2009, 48,
5482–5486; (d) A. Shimizu, T. Kubo, M. Uruichi, K. Yakushi,
M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Hirao,
K. Matsumoto, H. Kurata, Y. Morita and K. Nakasuji, J. Am.
Chem. Soc., 2010, 132, 14421–14428.
Fig. 4 Schematic drawing of the 1D stack of 3b. Phenyl groups are
omitted for clarity. Red half-arrows represent magnetic spins. Blue
and orange dotted lines represent the intra- and inter-molecular
covalent bonding interactions, respectively.
moment for the lowest-energy band of 3b, we carefully investi-
gated the intensities of the band by rotating polarized light in
steps of 301 on the (0 1 0) surface. The peak had a maximum
intensity at the polarization direction parallel to the c-axis, and
was gradually depressed by rotating the polarization direction,
and finally, disappeared when the polarization direction was
perpendicular to the c-axis (Fig. S3, ESIy).
If a covalent bonding interaction operates only within a
molecule or between molecules, then the direction of transition
moment should be parallel (along the blue dotted line in
Fig. 4) or perpendicular (along the orange dotted line in
Fig. 4) to the molecule, respectively. In the case of the well-
balanced 1D stack of 1b, the transition moment for the lowest-
energy band is directed along the line connecting between the
centers of gravity of the molecule. This indirectly proves that
intra- and inter-molecular covalent bonding interactions are
comparable, being supported with theoretical calculations by
Huang and Kertesz.12 On the other hand, the transition
moment in the 1D stack of 3b tilts to the perpendicular
direction (the orange dotted line direction) with respect to the
line connecting between the centers of gravity of the molecule.
This finding experimentally demonstrates that a covalent bonding
interaction in the 1D stack of 3b is substantially stronger between
molecules compared to within a molecule.
4 The synthetic procedures for 4a and 4b are described in ESIy.
5 Crystal data for compound 3a: T = 150(2) K, monoclinic, space
group P21/n (No. 14), a = 16.1189(8), b = 9.4946(5), c =
21.3361(12) A, b = 100.876(2)1, V = 3206.7(3) A3, Z = 2,
R1 (wR2) = 0.0698 (0.2289) for 397 parameters and 3758 indepen-
dent reflections, GOF = 0.988. CCDC 871896. Crystal data for
%
compound 3b: T = 200(2) K, triclinic, space group P1 (No. 2),
a = 10.152(2), b = 11.5617(18), c = 11.940(2) A, a = 90.925(5),
b = 96.852(7), g = 108.851(6)1, V = 1314.6(4) A3, Z = 1,
R1 (wR2) = 0.0673 (0.2018) for 380 parameters and 4281 indepen-
dent reflections, GOF = 1.185. CCDC 871897.
6 D. Dohnert and J. Koutecky´ , J. Am. Chem. Soc., 1980, 102, 1789–1796.
¨
7 (a) M. N. Glukhovtsev, R. D. Bach and S. Laiter, J. Mol. Struct.:
THEOCHEM, 1997, 417, 123–129; (b) S. W. Slayden and
J. F. Liebman, Chem. Rev., 2001, 101, 1541–1566.
8 The p-bonding energy is 270 kJ molꢁ1, which is estimated from the
rotational barrier in ethylene. See, J. E. Douglas, B. S. Rabinovitch
and F. D. Looney, J. Chem. Phys., 1955, 23, 315–323.
9 Nucleus-independent chemical shifts (NICS(1)) of the six-
membered ring(s) in the linkers negatively increase in order of
the size of the linkers; +0.314 for 1, ꢁ5.609 for 2, and ꢁ6.881
(central) and ꢁ9.686 (both ends) for 3, as shown in Fig. S4, ESIy. For
the NICS, see, Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta
and P. v. R. Schleyer, Chem. Rev., 2005, 105, 3842–3888.
10 A broken symmetry method was applied to evaluation of spin
densities and total energy for the singlet biradical states. See,
(a) L. Noodleman, J. Chem. Phys., 1981, 74, 5737–5743;
(b) K. Yamaguchi, Chem. Phys. Lett., 1975, 33, 330–335.
In conclusion, anthracene-linked bisphenalenyl Kekule
´
molecules (3a and 3b) are endowed with very significant singlet
biradical character by virtue of high aromatic stabilization
energy of anthracene. 3b forms a 1D stack with a superimposed
phenalenyl overlap in the solid state, and in the 1D stack,
unpaired electrons, which appear in a biradical contributor in
the resonance forms, covalently interact within a molecule and
between molecules. Polarized reflection measurements revealed
that the covalent bonding interaction largely operates between
molecules rather than within a molecule.
11 The DES–T values were deduced from the exchange coupling
parameter in the DFT framework by using the equation defined
by Yamaguchi. See, K. Yamaguchi, T. Tsunekawa, Y. Toyoda and
T. Fueno, Chem. Phys. Lett., 1988, 143, 371–376.
12 J. Huang and M. Kertesz, J. Am. Chem. Soc., 2007, 129, 1634–1643.
c
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Chem. Commun., 2012, 48, 5629–5631 5631