Synthesis and characterization of teranthene: A singlet biradical polycyclic aromatic hydrocarbon having kekule structures

Article abstract of DOI:10.1021/ja1049737

A teranthene derivative has been successfully isolated in a crystalline form for the first time. Geometrical considerations and physical property investigations indicate that the molecule possesses prominent biradical character in the ground state.

Full text of DOI:10.1021/ja1049737

Published on Web 07/26/2010
Synthesis and Characterization of Teranthene: A Singlet Biradical Polycyclic
Aromatic Hydrocarbon Having Kekule´ Structures
Akihito Konishi,† Yasukazu Hirao,† Masayoshi Nakano,‡ Akihiro Shimizu,† Edith Botek,§
Benoˆıt Champagne,*,§ Daisuke Shiomi,| Kazunobu Sato,| Takeji Takui,| Kouzou Matsumoto,†
Hiroyuki Kurata,† and Takashi Kubo*,†
Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan,
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka UniVersity,
Toyonaka, Osaka 560-8531, Japan, Laboratoire de Chimie The´orique, Faculte´s UniVersitaires Notre-Dame de la
Paix (FUNDP), rue de Bruxelles, 61, B-5000 Namur, Belgium, and Department of Chemistry, Graduate School of
Science, Osaka City UniVersity, Sumiyoshi-ku, Osaka 558-8585, Japan
Received June 8, 2010; E-mail: benoit.champagne@fundp.ac.be; kubo@chem.sci.osaka-u.ac.jp
Scheme 1. Resonance Structures of Anthenes and the Synthetic
Target Molecule 2aa
Abstract: A teranthene derivative has been successfully isolated
in a crystalline form for the first time. Geometrical considerations
and physical property investigations indicate that the molecule
possesses prominent biradical character in the ground state.
Singlet open-shell polycyclic aromatic hydrocarbons (PAHs) have
been intensively studied1 because they are closely related to the
nanographenes, which display fascinating properties resulting from
the spin-polarized state at the zigzag edges.2 Commonly asked
questions about their ground-state electronic structure encompass
the relative importance of the biradical (or polyradical) structures
and the location of the large spin density. Recent theoretical
studies1e have presented evidence that the biradical (or polyradical)
structures in PAHs with Kekule´ structures originate from the
aromatic stabilization due to the formation of Clar sextets3 in the
quinoidal rings and that the unpaired electrons appear predominantly
at the zigzag edges rather than the armchair edges. Questions of
particular interest also concern the experimental investigation of
the crossover from a closed-shell to an open-shell structure in a
homologous series of singlet-ground-state Kekule´ PAHs. Because
of their simplicity, we paid attention to peri-condensed anthracenes,
the so-called anthenes,4 because all anthenes are subject to the loss
of only one double bond when the structure is written with a
maximum number of Clar sextets (Scheme 1). In these biradical
forms, the Clar sextets, the isolated double bonds, and the unpaired
electrons have fixed positions, i.e. the influence of migration or
delocalization is limited. Therefore, the discussion of the biradical
character can concentrate on the energy balance between the formal
loss of the double bond and the aromatic stabilization. Herein we
report the first synthesis of a teranthene derivative, 2a, and its
characterization in terms of singlet biradical character in comparison
with bisanthene 1a.
a The amplitudes of the biradical character (y) were calculated at the
CASSCF(2,2)/6-31G level. The six-membered rings depicted by bold lines
represent the Clar sextets. Mes ) mesityl group.
carbon 6. Cyclization of 6 with DDQ/Sc(OTf)3 followed by a
N2H4 ·H2O quench8 gave 2a as a dark-green solid. Single crystals
Scheme 2. Synthesis of 2aa
Scheme 2 displays the synthetic procedure for 2a. Lithiation of
3 followed by coupling with 1,5-dichloroanthraquinone and a
subsequent treatment of reductive aromatization with NaI/NaH2PO2
gave the teranthrylene derivative 4.5 Cyclization along with
demethylation of 4 was carried out with KOH/quinoline to give
the partially ring-closed quinone 5.6 Treatment of 5 with mesityl-
7
a Reaction conditions: (a) n-BuLi, ether, 0 °C, then 1,5-dichloroan-
thraquinone, 0 °C to rt; (b) NaI, NaH2PO2 ·H2O, AcOH, 150 °C, 90% (two
steps); (c) KOH, quinoline, 190 °C, 78%; (d) mesitylmagnesium bromide,
CeCl3, 4:1 THF/ether, -30 °C; (e) NaI, NaH2PO2 ·H2O, AcOH, 150 °C,
75% (two steps); (f) DDQ, Sc(OTf)3, toluene, reflux, then N2H4 ·H2O,
toluene, rt, 72%. DDQ ) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
magnesium bromide in the presence of CeCl3 and subsequent
reductive aromatization afforded the partially ring-closed hydro-
† Department of Chemistry, Osaka University.
‡ Department of Materials Engineering Science, Osaka University.
§ Faculte´s Universitaires Notre-Dame de la Paix (FUNDP).
| Osaka City University.
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J. AM. CHEM. SOC. 2010, 132, 11021–11023 11021

C O M M U N I C A T I O N S
suitable for X-ray crystallographic analysis were obtained by
recrystallization from a CH2Cl2/hexane solution under an argon
flow. A toluene solution of 2a showed gradual decomposition with
a half-life of 3 days upon exposure to air under room light at room
temperature.9
which predicts a large LUMO occupation number of 0.42 for 2 in
contrast to only 0.07 for 1.13
Another aspect to consider is the biradical character of these
molecules in terms of the energy balance between the formal loss
of the double bond and the aromatic stabilization. The aromatic
stabilization energy of benzene based on the homodesmic stabiliza-
tion energy is ∼90 kJ/mol,14 three times which is comparable to
the C-C π-bond energy of ∼270 kJ/mol.15 Upon transformation
to the biradical form, 1 contains only two additional sextets;
therefore, the destabilization energy due to the π-bond cleavage
cannot be fully compensated, so electron pairing is favored in 1.
In contrast, 2 includes three additional sextets in its biradical form,
so both the Kekule´ and biradical forms contribute similarly to the
ground state.
The X-ray crystallographic analysis of 2a revealed an effective
D2h symmetry for the teranthene core, with the two mesityl groups
nearly perpendicular (dihedral angle ) 81°) (Figure 1). The
geometry of teranthene reflects the prominent biradical character
of 2a. As shown in Scheme 1, the biradical resonance contribution
enforces a shortening of the a bonds due to their double-bond
character. The length of the a bond in 2a (C8-C18 in Figure 1) is
1.424(4) Å, which is considerably shorter than length of a
C(sp2)-C(sp2) single bond (1.467 Å),10 whereas 1a, the 4-(tri-
fluoromethyl)phenyl derivative11 of bisanthene (1), presents a
relatively longer a bond having a length of 1.447(4) Å. Additionally,
the four corner six-membered rings in 2a (denoted by A in Figure
1) show a considerable decrease in the bond-length alternation, i.e.,
they bear more benzene character than those in 1a (Figure 2). The
geometry-based aromaticity index HOMA (harmonic oscillator
model of aromaticity),12 which has a larger value for the A rings
in 2a (0.854) than for those in 1a (0.704), fully supports the idea.
Moreover, these geometrical findings are in line with the amplitude
of the biradical character (y) estimated at the CASSCF(2,2) level,
Physical property investigations for 2a thoroughly support its
intermediate biradical character. A CD2Cl2 solution of 2a showed
no 1H NMR signals from the teranthene core at room temperature,
while progressive line sharpening was observed upon cooling
(Figure 3). This behavior was also seen in the phenalenyl-based
singlet biradicals we investigated previously16 and is caused by a
thermally excited triplet species. The small singlet-triplet energy
gap (∆ES-T) of 2a was estimated by SQUID measurements on a
powdered sample (Figure S1 in the Supporting Information). The
measurement showed an increasing susceptibility above 220 K.
From careful fitting of the observed increase, the value of ∆ES-T
was determined to be 1920 K (16.0 kJ/mol).17 Thus, the two
unpaired electrons in 2a are coupled very weakly in comparison
with most Kekule´ compounds (∆ES-T > 104 K). It is further noted
1
that the sharp H NMR signals observed for 1a indicate that the
influence of the triplet species is negligible as a result of a larger
18
∆ES-T.
Figure 1. ORTEP drawing of 2a measured at 200 K: (left) top view; (right)
side view.
1
Figure 3. Variable-temperature H NMR spectra of 2a in CD2Cl2 in the
aromatic region.
Thermal excitation to the triplet state requires a small HOMO-
LUMO energy gap. The cyclic voltammogram of 2a shows four
reversible redox waves: E2ox ) +0.09, E1ox ) -0.35, E1red ) -1.40,
and E2 ) -1.76 (all values in V vs Fc/Fc+; Figure 4). On the
red
basis of the difference between the first oxidation and reduction
potentials, the electrochemical HOMO-LUMO gap amounts to
1.05 eV, which is 0.58 eV smaller than that of 1a.11 The electronic
absorption spectrum of 2a in CH2Cl2 (Figure 5) exhibited a low-
energy band at 1054 nm (ε ) 8000 M-1 cm-1), from which the
optical HOMO-LUMO gap was determined to be 1.18 eV. This
value is 0.64 eV smaller than that of 1a. The smaller HOMO-LUMO
gap of 2a is consistent with its larger biradical character, because
Figure 2. Bond lengths (mean values) in the main core rings of (left) 1a11
and (right) 2a.
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Scientific Research on Innovative Areas (2105, T.K.) and a Grant-
in-Aid for Scientific Research (18350007, M.N.) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan;
PRESTO-JST (D.S.); CREST-JST (T.T.); the Global COE Program
“Global Education and Research Center for Bio-Environmental
Chemistry” of Osaka University (A.K. and A.S.); and the Belgian
Government (IUAP N° P06-27 “Functional Supramolecular Sys-
tems”, E.B. and B.C.)
Supporting Information Available: Experimental procedures for
all new compounds, crystal data, a magnetic susceptibility plot, a crystal
packing diagram, details of the theoretical calculations, and a CIF file
for 2a. This material is available free of charge via the Internet at http://
pubs.acs.org.
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the smaller the gap, the easier it is to promote electrons from the
HOMO to the LUMO. Admixing the doubly excited configuration
1ΦH,HfL,L into the ground state leads to a strong static correlation
between two unpaired electrons and consequently to a pronounced
singlet biradical character.
In conclusion, we have demonstrated the uncoupling of the
electron pair in a Kekule´ PAH, the extent of which is governed by
the energy balance between the formal loss of the double bond
and the formation of aromatic sextets. Our study further shows that
the singlet biradical character in PAHs can be investigated
experimentally in an unexpectedly small molecule. This finding
would encourage a bottom-up approach for establishing spintronics
and nonlinear optics, both based on molecular-sized nanographene
compounds.19
Acknowledgment. This work was supported in part by
Mitsubishi Chemical Corporation Fund (Y.H.); a Grant-in-Aid for
JA1049737
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