Singlet biradical character of phenalenyl-based Kekule hydrocarbon with naphthoquinoid structure

Article abstract of DOI:10.1021/ol062604z

(Chemical Equation Presented) A new Kekule polycyclic hydrocarbon with a singlet biradical index of 50% was synthesized. The singlet biradical character was assessed with UV and ¹H-NMR spectroscopy, cyclic voltammetry, SQUID magnetic susceptibi

Full text of DOI:10.1021/ol062604z

ORGANIC
LETTERS
2007
Vol. 9, No. 1
81-84
Singlet Biradical Character of
Phenalenyl-Based Kekule´ Hydrocarbon
with Naphthoquinoid Structure
Takashi Kubo,*,† Akihiro Shimizu,† Mikio Uruichi,‡ Kyuya Yakushi,‡
Masayoshi Nakano,§ Daisuke Shiomi, Kazunobu Sato, Takeji Takui,
|
Yasushi Morita,*,†, and Kazuhiro Nakasuji*,†
Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka,
Osaka 560-0043, Japan, Department of Applied Molecular Science, Institute for
Molecular Science, Okazaki 444-8585, Japan, Department of Materials Engineering
Science, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka
560-8531, Japan, Departments of Chemistry and Materials Science, Graduate School
of Science, Osaka City UniVersity, Sumiyoshi-ku, Osaka 558-8585, Japan, and
PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
kubo@chem.sci.osaka-u.ac.jp; morita@chem.sci.osaka-u.ac.jp
Received October 23, 2006
ABSTRACT
A new Kekule´ polycyclic hydrocarbon with a singlet biradical index of 50% was synthesized. The singlet biradical character was assessed
with UV and 1H-NMR spectroscopy, cyclic voltammetry, SQUID magnetic susceptibility measurement, and quantum chemical calculations.
Continuing attention and considerable effort have been
devoted to the understanding of the electronic structure of
singlet biradicals.1-8 The chemical and physical properties
are characterized by the weak bonding interactions between
two unpaired electrons which occupy a different part of space
with a small sharing region. Many works have focused on
the spin-spin interaction of the singlet biradicals in which
methyl,1 triphenylmethyl,2 nitronyl nitroxides,3 nitroxides,4
verdazyls,5 phenoxyls,6 semiquinones,7 and imidazolyl moi-
eties8 are connected by σ- or π-fragments. These molecules
† Graduate School of Science, Osaka University.
‡ Institute for Molecular Science.
§ Graduate School of Engineering Science, Osaka University.
Osaka City University.
(1) (a) Berson, J. A. Acc. Chem. Res. 1997, 30, 238-244. (b) Matsuda,
K.; Iwamura, H. J. Am. Chem. Soc. 1997, 119, 7412-7413. (c) McMasters,
D. R.; Wirz, J. J. Am. Chem. Soc. 2001, 123, 238-246. (d) Abe, M.;
Kawakami, T.; Ohata, S.; Nozaki, K.; Nojima, M. J. Am. Chem. Soc. 2004,
126, 2838-2846.
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10.1021/ol062604z CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/08/2006

are categorized into conjugated and nonconjugated biradicals,
and the former is further divided into Kekule´ and non-Kekule´
systems.9 In contrast to a large number of investigations for
non-Kekule´ and nonconjugated biradicals, the electronic
structure of singlet biradicals with Kekule´ structures has been
less thoroughly examined. Although the most extensive
studies in the Kekule´ singlet biradical have been performed
on Chichibabin’s hydrocarbon,10 the potential high reactivity
and flexible molecular framework make a definitive elec-
tronic structure still open for discussion. Recently, we have
prepared and characterized a stable phenalenyl-based11
Kekule´ singlet biradical 1b with a rigid structure, and have
clarified a prominent feature of strong intermolecular spin-
spin interactions.12
Herein, we will report the synthesis and single-molecular
electronic structure of a novel Kekule´ polycyclic hydrocarbon
with substantial biradical character. Our newly designed
molecule 2 has a naphthoquinoid structure along with two
phenalenyl rings. The singlet biradical index y of 2 is
expected to be larger than that of 1 (y of 1a ) 30%), because
two benzene rings appear in the biradical resonance structure
2′ in contrast to only one in 1. The CASSCF(2,2)/6-31G//
RB3LYP/6-31G** calculation of 2a gave the LUMO oc-
cupation number of 0.504, from which the y value was
determined to be 50%, according to the NOON (natural
orbital occupation number) analysis.13 Spin density distribu-
tion was estimated by using a broken-symmetry (BS)
Figure 1. Phenalenyl-based singlet biradicals.
UB3LYP/6-31G** method, which indicated large spin
density on two phenalenyl rings with antiparallel spins
retaining the characteristic spin distribution pattern of phen-
alenyl radical (Figure 2). Thus the electronic structure of 2
in a ground state is best represented by the resonance of
Kekule´ and biradical forms as shown in Figure 1.
(2) (a) Montgomery, L. K.; Huffman, J. C.; Jurczak, E. A.; Grendze, M.
P. J. Am. Chem. Soc. 1986, 108, 6004-6011. (b) Rajca, A.; Rajca, S. J.
Am. Chem. Soc. 1996, 118, 8121-8126.
(3) (a) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.; Darcy, R. J.
Am. Chem. Soc. 1972, 94, 7049-7059. (b) Alies, F.; Luneau, D.; Laugier,
J.; Rey, P. J. Phys. Chem. 1993, 97, 2922-2925. (c) Frank, N. L.; Cle´rac,
R.; Sutter, J.-P.; Daro, N.; Kahn, O.; Coulon, C.; Green, M. T.; Golhen, S.;
Ouahab, L. J. Am. Chem. Soc. 2000, 122, 2053-2061. (d) Ziessel, R.; Stroh,
C.; Heise, H.; Ko¨hler, F. H.; Turek, P.; Claiser, N.; Souhassou, M.; Lecomte,
C. J. Am. Chem. Soc. 2004, 126, 12604-12613.
(4) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J. Am. Chem. Soc. 1993,
115, 847-850.
(5) (a) Brook, D. J. R.; Fox, H. H.; Lynch, V.; Fox, M. A. J. Phys. Chem.
1996, 100, 2066-2071. (b) Brook, D. J. R.; Yee, G. T. J. Org. Chem. 2006,
71, 4889-4895.
(6) (a) Sugimoto, T.; Sakaguchi, M.; Ando, H.; Tanaka, T.; Yoshida,
Z.; Yamauchi, J.; Kai, Y.; Kanehisa, N.; Kasai, N. J. Am. Chem. Soc. 1992,
114, 1893-1895. (b) Shultz, D. A.; Boal, A. K.; Farmer, G. T. J. Am. Chem.
Soc. 1997, 119, 3846-3847. (c) Rebmann, A.; Zhou, J.; Schuler, P.; Rieker
A.; Stegmann, H. B. J. Chem. Soc., Perkin Trans. 2 1997, 1615-1617.
(7) Shultz, D. A.; Kumar, R. K. J. Am. Chem. Soc. 2001, 123, 6431-
6432.
(8) Kikuchi, A.; Ito, H.; Abe, J. J. Phys. Chem. B 2005, 109, 19448-
19453.
(9) Borden, W. T. In Diradicals; Borden, W. T., Ed.; Wiley: New York,
1982; in preface, which indicates biradicals include carbenes, non-
conjugated, conjugated non-Kekule´, and anti-aromatic. We used the term
“conjugated Kekule´” instead of “anti-aromatic” for wider application of
the categorization.
(10) Platz, M. S. In Diradicals; Borden, W. T., Ed.; Wiley: New York,
1982; pp 201-209. Reference 2a also summarizes the results of investiga-
tions.
Figure 2. HOMO, LUMO, and spin density of 2a. Red and blue
surfaces represent positive and negative signs of molecular orbitals
calculated with a RB3LYP/6-31G** method, respectively. Light
blue and light red surfaces represent R and â spin densities,
respectively.
(11) Our recent works of the phenalenyl system: (a) Morita, Y.; Aoki,
T.; Fukui, K.; Nakazawa, S.; Tamaki, K.; Suzuki, S.; Fuyuhiro, A.;
Yamamoto, K.; Sato, K.; Shiomi, D.; Naito, A.; Takui, T.; Nakasuji, K.
Angew. Chem., Int. Ed. 2002, 41, 1793-1796. (b) Nishida, S.; Morita, Y.;
Fukui, K.; Sato, K.; Shiomi, D.; Takui, T.; Nakasuji, K. Angew. Chem.,
Int. Ed. 2005, 44, 7277-7280. (c) Suzuki, S.; Morita, Y.; Fukui, K.; Sato,
K.; Shiomi, D.; Takui, T.; Nakasuji, K. J. Am. Chem. Soc. 2006, 128, 2530-
2531.
Because most polycyclic aromatic compounds suffer from
low solubility, we have decided to introduce phenyl and tert-
butyl substituents on the rings of 2a. The synthetic procedure
for the derivative 2b is shown in Scheme 1. The starting
(12) Kubo, T.; Shimizu, A.; Sakamoto, M.; Uruichi, M.; Yakushi, K.;
Nakano, M.; Shiomi, D.; Sato, K.; Takui, T.; Morita, Y.; Nakasuji, K.
Angew. Chem., Int. Ed. 2005, 44, 6564-6568.
(13) (a) Do¨hnert, D.; Koutecky´, J. J. Am. Chem. Soc. 1980, 102, 1789-
1796. (b) Jung, Y.; Head-Gordon, M. ChemPhysChem 2003, 4, 522-525.
For the calculation details of 2a, see the Supporting Information.
82
Org. Lett., Vol. 9, No. 1, 2007

Scheme 1. Synthetic Procedure for 2b
Figure 3. ORTEP drawings of 2b at the 50% probability level.
Hydrogen atoms and toluene molecules were omitted for clarity.
character, we performed time-dependent (TD) B3LYP cal-
culations, which were summarized in Table 1. The broken-
compound 3, which was obtained according to the reported
procedure,14 was converted to diformyl derivatives as a
mixture of 3,11- and 3,12-isomers.15 Carboxylic acid deriva-
tives 8 were prepared in five steps. Intramolecular Friedel-
Crafts cyclization of the acyl chloride of 8 with AlCl3 gave
diketones 9, which were reduced and subsequently dehy-
drated to afford dihydro compound 11. The dehydrogenation
of 11 with DDQ provided the target compound 2b as dark
violet plates. The solid 2b was found to be stable at room
temperature in air for a couple of days. The molecular
structure was determined by the X-ray crystallography of a
single crystal of 2b (Figure 3).16 The crystal of 2b included
toluene solvent molecules. The main core rings have planar
geometries, and phenyl groups are perpendicular to the plane.
There are no appreciable π-π contacts between molecules
(Figure S1).
Figure 4. Solution absorption spectra of 1b (blue) and 2b (red).
symmetry UB3LYP calculations reproduce quite well the
transition energies (∆E) and oscillator strengths (f) observed
in 1b and 2b, whereas the RB3LYP calculations show
smaller ∆E and much larger f values than the observed ones.
In general, a TD-RDFT (Restricted DFT) calculation
Table 1. Observed and Calculated Transition Energies (∆E)
and Oscillator Strengths (f) of 1 and 2
1a, 1b
∆E (eV)
2a, 2b
∆E (eV)
The electronic absorption spectrum of 2b in a hexane/
dichloromethane (49:1, v/v) solution gave an intense low-
energy band at 865 nm (ꢀ ) 78 433) as shown in Figure 4
(red line). To assess the band with respect to biradical
f
f
observed
RB3LYP a
UB3LYPb
1.66
1.59
1.61
0.65c
0.86
0.60
1.43
1.38
1.46
0.63c
1.03
0.55
(14) Neudorff, W. D.; Schulte, N.; Lentz, D.; Schlu¨ter, A. D. Org. Lett.
2001, 3, 3115-3118.
a Time-dependent RB3LYP/6-31G** calculation at the RB3LYP/6-
31G** optimized geometry of 1a and 2a. b Time-dependent UB3LYP(BS)/
6-31G** calculation at the UB3LYP(BS)/6-31G** optimized geometry of
1a and 2a. c The oscillator strengths were obtained by the integration of
the absorbance in the range of 560-830 nm for 1b and 630-1000 nm for
2b.
(15) Individual isomers were not isolated for further transformations
because both isomers could lead to a single compound 2b.
(16) Crystal data for 2b: monoclinic, P21/n (no. 14), a ) 16.109(7) Å,
b ) 9.036(3) Å, c ) 19.383(8) Å, â ) 94.544(7)°, V ) 2812.5(1) Å3, Z )
2, T ) 150 K, R1(wR2) ) 0.070 (0.184) for 370 parameters and 3823
independent reflections, GOF ) 1.14.
Org. Lett., Vol. 9, No. 1, 2007
83

provides an optical gap in good agreement with the experi-
ment for most closed-shell compounds.17 A single-determi-
nant restricted calculation is, however, insufficient for the
description of the ground state of singlet biradicals.18a In
contrast, a broken-symmetry UDFT (Unrestricted DFT)
method is suitable for the approximate description of the
singlet biradical electronic structure.18b Better agreement of
the UB3LYP results with the observed transition energies
implies the biradical contribution to the ground state in 1b
and 2b.19 The oscillator strengths f are more informative for
biradical character. The RB3LYP, which have no biradical
contribution, predicts the larger f for the more expanded
conjugation system 2a than for 1a,20 in clear contrast to the
experimental result. The RB3LYP calculations of 1a and 2a
afford fully delocalized frontier orbitals (see ref 12 and Figure
2), whereas the UB3LYP gives the MO pictures localizing
on one side of the molecule (Figure S2). In the UB3LYP
method, the wave function in the HOMO occupies regions
in space different from that in the LUMO as a result of the
biradical contribution, and the intensity-determining integrals
for the transition would be small. The theoretical and
experimental f values of 2 are smaller than those of 1. These
UV results along with the TD-UB3LYP calculations imply
2b is a Kekule´ system with larger biradical character, that
is a more spin-localized system,21 compared to 1b.
susceptibility above 250 K in a SQUID measurement (Figure
S4). The singlet-triplet energy gap (∆ES-T) was estimated
to be ∼1900 K, which was consistent with the theoretically
calculated value of 2250 K using the B3LYP/6-31G**
method, and was smaller than that of 1b. The ∆ES-T is,
however, much larger than those in most non-Kekule´ and
nonconjugated singlet biradicals, indicaing an adequate
antiparallel spin-spin interaction. We could not discern ESR
signals of the triplet species in the powdered 2b, since a
∆Ms ) 1 peak of a monoradical impurity (peak-to-peak
width, ∆Hpp ) 1.4 mT) would obscure the triplet species
signal which should have a small D value (∼1.6 mT)
according to the distance between the centers of phenalenyl
rings. The monoradical impurity would originate from partial
oxidation or hydrogenation of 2b. In a toluene solution of
2b at room temperature, the ESR spectrum consisted of 15
sharp peaks centered at g ) 2.0029. Double integral of the
signal indicated the monoradical concentration of ∼2%.23,24
1
The CD2Cl2 solution of 2b showed no H-NMR signals
of the main core rings at +20 °C. Upon cooling, progressive
line sharpening of the signals on the ring protons was
observed. Even at -100 °C, however, the signals were
slightly broad (Figure S6). Thermally excited triplet species
are responsible for this signal broadening. Although a similar
behavior has also been observed in 1b, sharp signals are
obtained at -30 °C.12 The signal broadening of 2b at lower
temperature supports the smaller ∆ES-T compared to 1b.
In conclusion, the experimental assessment of 2b strongly
supported the larger biradical character as compared to 1b.
The UV spectra combined with the TD-UB3LYP calculations
suggested the more localized spin structure in 2b than 1b,
while the SQUID and NMR measurements indicated 2b still
has sufficient spin-spin coupling in antiparallel manner. This
bifunctional character, that is spin localization and pairing,
is consistent with the intermediate biradical character (y )
50%) calculated with the CASSCF method. Currently, we
are investigating an intermolecular interaction of a Kekule´
singlet biradical with the intermediate biradical character.
The cyclic voltammogram of 2b showed four reversible
ox
ox
red
redox waves: E2 ) +0.26, E1 ) -0.13, E1 ) -1.17,
red
and E2 ) -1.55 (V vs Fc/Fc+, see Figure S3). From the
difference between the first oxidation and reduction poten-
tials, the electrochemical HOMO-LUMO gap is estimated
to be 1.04 eV, which is 0.11 eV smaller than that of the
tetra-tert-butyl derivative of 1a.22 The smaller energy gaps
are consistent with the larger biradical character of 2b,
because a HOMO-LUMO gap is closely related to the
promotion of electrons from HOMO to LUMO. Admixing
a doubly excited configuration 1Φ H,HfL,L into a ground state
leads to a strong static correlation between two unpaired
electrons, and consequently the system has pronounced
singlet biradical character.
Acknowledgment. This work is partly supported by
Mitsubishi Chemical Corporation Fund, PRESTO-JST, and
a Grant-in-Aid for Scientific Research (17550034) and in
priority areas “Application of Molecular Spins” (Area No.
769) from the Ministry of Education, Science, Sports, and
Culture, Japan.
The weakening spin-spin coupling originating from the
small HOMO-LUMO gap leads to thermal excitation to a
triplet state. A powdered sample of 2b showed an increasing
(17) (a) Heinze, H. H.; Go¨rling, A.; Ro¨sch, N. J. Chem. Phys. 2000,
113, 2088-2099. (b) Dierksen, M.; Grimme, S. J. Phys. Chem. A 2004,
108, 10225-10237.
(18) (a) The biradical contribution like 2′ to the ground state requires an
appropriate electron-electron repulsion effect (static electron-electron
correlation), which can be calculated by using a multiconfigurational wave
function such as a CASSCF method. (b) With the spin-symmetry broken
method, the static correlation is simulated, and the ground state bears
biradical character.
(19) For a successful application of the UB3LYP(BS) calculation for
UV transition of singlet biradical states, see: Gao, Y.; Liu, C.-G.; Jiang,
Y.-S. J. Phys. Chem. A 2002, 106, 5380-5384.
Supporting Information Available: The detailed syn-
thetic procedure of 2b, and crystallographic data in CIF
format. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL062604Z
(20) Oscillator strength f is known to be related to the square of a
transition dipole moment, and the dipole moment is represented by ∑er,
where r is the dipole length. Thus the RB3LYP would produce the larger
f for 2a with a longer molecular skeleton.
(21) We used the term “localized” for describing the spin structure of
the conjugated Kekule´ biradical. The term originates from the distribution
pattern of wave functions where the highest occupied R and â molecular
orbitals are localized in different regions of the molecule.
(23) The hyperfine coupling constants determined were 0.510 (4H) and
0.157 mT (2H), which could be assigned to R and â protons on the
phenalenyl ring, respectively (Figure S5). The parent phenalenyl radical
has hyperfine coupling constants of 0.63 and 0.18 mT for R and â protons,
respectively. See: Gerson, F. HelV. Chim. Acta 1966, 49, 1463-1467.
(24) A reveiwer pointed out a possibility that the monoradical impurity
might be responsible for the increasing magnetic susceptibility above 250
K. However, the closest π-π overlap on the main core ring of 2b between
the molecules is over 5 Å, which would not lead to a sizable J value.
(22) Ohashi, K.; Kubo, T.; Masui, T.; Yamamoto, K.; Nakasuji, K.; Takui,
T.; Kai, Y.; Murata, I. J. Am. Chem. Soc. 1998, 120, 2018-2027.
84
Org. Lett., Vol. 9, No. 1, 2007