Resonance balance shift in stacks of delocalized singlet biradicals

Full text of DOI:10.1002/anie.200901382

Communications
DOI: 10.1002/anie.200901382
Singlet Biradicals
Resonance Balance Shift in Stacks of Delocalized Singlet Biradicals**
Akihiro Shimizu, Mikio Uruichi, Kyuya Yakushi, Hiroyuki Matsuzaki, Hiroshi Okamoto,
Masayoshi Nakano, Yasukazu Hirao, Kouzou Matsumoto, Hiroyuki Kurata, and Takashi Kubo*
Dedicated to Professor Ichiro Murata on the occasion of his 80th birthday
Recently we succeeded in the isolation of delocalized singlet
biradicals[1,2] utilizing the spin-delocalizing character of the
phenalenyl radical.[3] We demonstrated that the singlet
biradical 1 has strong spin–spin interactions between mole-
cules through the overlap of phenalenyl rings in the one-
dimensional (1D) chain even though the closed-shell Kekulꢀ
structure 1 can be drawn as a resonance contributor
(Scheme 1).[1b] Huang and Kertesz gave further insight into
the spin–spin interactions from a theroretical point of view
and showed that the spin–spin interaction between the
molecules was predicted to be stronger than that within the
molecule.[4] These experimental and theoretical findings are
associated with very fundamental issues: Do delocalized
singlet biradicals actually have open-shell character? Are the
electrons coupled within a molecule involved in covalent
Scheme 1. Resonance structures of 1 and 2. The arrows in the biradical
structure represent antiparallel spins.
bonding between molecules? In this study we will demon-
strate that intra- and intermolecular spin–spin interactions
strongly correlate and can be altered in magnitude by an
applied external field. Our proposal is based on the exper-
imentally determined molecular structure of 2, a temper-
ature-dependent reflection spectrum of 2, and a pressure-
dependent reflection spectrum of 1. Methyl groups at the
2- and 10-positions in 2, where the frontier molecular orbital
has very small coefficients, are expected to alter the distance
between the overlapping phenalenyl rings with respect to the
analogous separation in 1, and as a result, the magnitude of
the intermolecular spin–spin interaction should be affected.
The synthesis of 2 is outlined in Scheme 2. The 3,10- and
[*] A. Shimizu, Dr. Y. Hirao, Dr. K. Matsumoto, Dr. H. Kurata,
Prof. Dr. T. Kubo
3,11-bis(bromomethyl) compounds
3 were synthesized
Department of Chemistry Graduate School of Science, Osaka
University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043 (Japan)
Fax: (+81)6-6850-5387
according to the previously reported procedures.[1b] The
individual isomers were not isolated because both were
expected to lead to the single compound 2. Bis(2-methylpro-
pionic acid) derivatives 5 were obtained in three steps by
standard methods. Friedel–Crafts cyclization of the acyl
chloride derivatives of 5 with AlCl3 afforded diketones 6.
These were reduced with NaBH4 and subsequently dehy-
drated with a catalytic amount of p-toluenesulfonic acid to
afford the dihydro compounds 8. Dehydrogenation of 8 with
p-chloranil afforded the hydrocarbon 2 as green prisms.
Compound 2 was found to be stable in the solid state at room
temperature.
E-mail: kubo@chem.sci.osaka-u.ac.jp
Dr. M. Uruichi, Prof. Dr. K. Yakushi
Department of Applied Molecular Science, Institute for Molecular
Science, Okazaki, Aichi 444-8585 (Japan)
Dr. H. Matsuzaki, Prof. Dr. H. Okamoto
Department of Advanced Material Science, Graduate School of
Frontier Science, The University of Tokyo, Kashiwa, Chiba 277-8561
Prof. Dr. M. Nakano
Department of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
[**] This work was supported by Yamada Science Foundation (T.K.),
Grants-in-Aid for Scientific Research (nos. 19550039 (T.K.),
18350007 (M.N.), and 20340072 (H.O.)) from the Japan Society for
the Promotion of Science (JSPS) and from the Ministry of
Education, Culture, Sports, Science and Technology-Japan (MEXT)
(no. 20110002 (H.O.)), and the Global Center of Excellence program
“Global Education and Research Center for Bio-Environmental
Chemistry” of Osaka University (A.S.). A.S. acknowledges the Japan
Society for the Promotion of Science (JSPS) Fellowship for Young
Scientists.
The small HOMO–LUMO gap of 2, which is an essential
factor for a singlet biradical electronic structure, was con-
firmed by electrochemical and optical methods. The cyclic
voltammogram of 2 shows four reversible redox waves: E2ox
=
+ 0.51, Eo1x = + 0.11, E1red = ꢀ1.09, and Er2ed = ꢀ1.62 (V vs.
ferrocene/ferrocenium couple (Fc/Fc+), see Figure S1 in the
Supporting Information), which led to an electrochemical
HOMO–LUMO gap of 1.20 eV. The electronic absorption
spectrum of 2 in CH2Cl2 shows an intense low-energy band at
756 nm (13200 cmꢀ1 = 1.64 eV, e = 115000, f = 0.605, see
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901382.
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Figure 1. Top view (a) and side view (b) of the 1D chain of 2/PhCl at
200 K. Hydrogen atoms and chlorobenzene solvent molecules are
omitted for clarity. Ellipsoids are shown at the 50% probability level.
overlap at the phenalenyl rings.[5] This stacking pattern is
quite similar to that of 1 except for larger intermolecular p–p
separation (3.225 ꢁ for 2 vs. 3.137 ꢁ for 1 at 200 K), which is
nevertheless shorter than sum of the van der Waals contacts of
carbon atoms (3.4 ꢁ). The average intermolecular p–p
distance decreased upon cooling from room temperature:
3.279 ꢁ (300 K), 3.225 ꢁ (200 K), and 3.208 ꢁ (100 K).
Interestingly, there is a strong correlation between the p–p
separation distance and the length of bond a (Figure 2a).[6] As
shown in Table 1, the shorter the p–p separation, the greater
Table 1: The p–p separation distances (R) between stacked phenalenyl
rings and the lengths (L) of bond a in crystals of 2 with various solvent
molecules and at various temperatures.
Scheme 2. Synthesis of 2. a) CH3CH(CO2Et)2, EtONa, EtOH/toluene
(1:2), room temperature, 51%; b) 1. aq KOH, EtOH, reflux, 2. aq HCl,
reflux, 87%; c) 1. (COCl)2, reflux; 2. AlCl3, CH2Cl2, ꢀ78!208C, 25%;
d) NaBH4, EtOH/CH2Cl2 (5:2), room temperature, 99%; e) cat.
p-toluenesulfonic acid, toluene, 908C, 90%; f) p-chloranil, toluene,
908C, 60%.
Benzene
PhCl
PhCl
PhCl
Toluene
T [K]
200
100
200
300
200
R [ꢀ]
3.160
1.476(2)
3.208
1.472(2)
3.225
1.469(2)
3.279
1.465(3)
–
L [ꢀ][a]
1.457(2)
[a] Mean values.
Figure 3). These values are almost the same as those of 1,
indicating that the methyl substituents in 2 have a minimal
effect on the electronic structure. Thus 2 possesses biradical
character identical to that of 1 (30% at the CASSCF(2,2)/6-
31G level of calculation).[1b] 1H NMR spectrum of 2 at ꢀ308C
displays sharp signals. This finding indicates that 2 behaves
intrinsically as a closed-shell molecule in the single-molecule
state. Upon heating, the signals of the ring protons, but not the
phenyl groups, undergo progressive line broadening (see
Figure S2 in the Supporting Information). Similar results have
been obtained for 1, and the broadening at elevated temper-
atures is ascribed to triplet species that are thermally
accessible as a result of the small HOMO–LUMO energy gap.
Fortunately, we could obtain many kinds of polymorphic
crystals of 2, which included solvent molecules and were
suitable for X-ray crystallographic structure analysis, by
recrystallization from chlorobenzene (PhCl), benzene, or
toluene solutions (see Figures S3–5 and Table S1 in the
Supporting Information). In the crystal of 2/PhCl, the
methyl groups are effective in controlling the p–p separation
distance of the overlapping phenalenyl rings (Figure 1).
Compound 2 forms a 1D p–p chain with superimposed p–p
the length of bond a. This result indicates that the intra- and
intermolecular spin–spin interactions strongly correlate in the
1D chain. A decrease in the p–p separation distance enhances
the intermolecular orbital overlap and strengthens the
intermolecular bonding interaction. The enhanced intermo-
lecular interaction makes the unpaired electrons more
localized on the phenalenyl rings and consequently weakens
the intramolecular bonding interaction.
The strongest intermolecular spin–spin interaction for 2
was found in the crystal of 2/benzene. Here, 2 also forms a 1D
p–p chain with the superimposed phenalenyl rings and
considerably short average p–p distances (3.160 ꢁ at
200 K).[7] The strongest intermolecular interaction results in
the longest bond a, fully consistent with the findings in 2/PhCl.
It is noted that in the crystals of 2/PhCl and 2/benzene the
spin–spin interaction is restricted to within the 1D chains as a
result of the lack of effective p–p overlap between the chains.
Fortunately, a zero-limit of intermolecular spin–spin inter-
action could be found in the crystal of 2/toluene. Individual
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molecules of 2 were isolated because the toluene solvent
molecules prevent effective p–p overlap between phenalenyl
rings. Therefore the bonding interaction of the unpaired
electrons on phenalenyl radicals is restricted to within the
molecule. Bond a in 2/toluene is the shortest among the
polymorphic crystals of 2.
Based on these results, the electronic structure of the 1D
chain is best rationalized in terms of the Resonating Valence-
Bond (RVB) model proposed by Pauling[8] and Anderson.[9]
This model describes a Li–Li bond as a resonating singlet pair
of two electrons. By replacing the lithium atom by the
phenalenyl radical, we can apply the model to represent the
intra- (A) and intermolecular (B) bonding in the 1D chain of
2. The electronic structure of the 1D chain can be described as
the superposition of formulas A and B. (Figure 2b).[10] The
shorter distance between overlapping phenalenyl rings gives
greater weight to formula B in the electronic structure of the
1D chain. This “resonance balance shift” to formula B at
lower temperature results in weaker bonding interaction
within the molecule, that is, the greater length of bond a.
Figure 3. Optical spectra of 2. Absorption spectrum of a solution of 2
in CH2Cl2 (dashed line) and reflection spectrum (solid line) of a single
crystal of 2/PhCl at 300 K obtained with light polarized along the
c axis.
Figure 2. a) Schematic drawing of the resonance structures of a single
molecule of 2. b) The RVB model for the electronic structure in the 1D
chain of 2.
Figure 4. Temperature dependency of the energy of the lowest energy
reflection bands on a single crystal of 2/PhCl (black circles) at ambient
pressure, and pressure dependency of the energy of the lowest energy
reflection bands on a single crystal of 1/PhCl (open circles) at room
temperature.
The resonance balance shift in the 1D chain is related to
the temperature- and pressure-dependent electronic spectra
observed with single crystals of 2/PhCl and 1/PhCl. In the
reflection spectrum of a single crystal of 2/PhCl (Figure 3) the
absorption band is shifted to substantially lower energy with
respect to the band recorded for the solution of 2 in CH2Cl2, as
observed in 1/PhCl.[1b] This finding indicates that the elec-
tronic structure of the molecule is affected by the strong
intermolecular spin–spin interaction in the 1D chain. Upon
cooling, the lowest energy band of 2/PhCl shifts to the higher
energy region, contrary to our expectation. Figure 4 shows the
temperature dependency of the lowest-energy reflection
bands recorded on a single crystal of 2/PhCl. In addition,
the pressure-dependent reflection spectra of a single crystal of
1/PhCl showed a similar higher energy shift of the lowest
energy band (see also Figure S6 in the Supporting Informa-
tion).[11]
The coexistence of intra- and intermolecular spin–spin
interactions in 2/PhCl and 1/PhCl indicates that their 1D
chains mimic the electronic structure of an infinite polyene,
although the magnitude of the bonding interactions of
unpaired electrons are quite different (0.2–0.3 eV for 1[4] vs.
2.0–2.4 eV for an infinite polyene[12]). As stated in refer-
ence [12], individual double bonds in an infinite polyene are
corresponding to “molecules” and single bonds to weak
intermolecular orbital overlap in a 1D molecular crystal
composed of a closed-shell compound. Following this simple
model, we initially expected a lower energy shift of the lowest
energy band with more effective intermolecular orbital
overlap caused by decrease of the p–p separation distance,
because 2 and 1 seem to be intrinsically closed-shell
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Angew. Chem. 2004, 116, 595 – 597; Angew. Chem. Int. Ed. 2004,
compounds, at least in the single-molecule state according to
the H NMR results. Consequently their 1D chains should
1
43, 585 – 587; b) M. Bendikov, H. M. Duong, K. Starkey, K. N.
Houk, E. A. Carter, F. Wudl, J. Am. Chem. Soc. 2004, 126, 7416 –
7417; c) A. Kikuchi, F. Iwahori, J. Abe, J. Am. Chem. Soc. 2004,
126, 6526 – 6527; d) M. Abe, M. Hattori, A. Takegami, A.
Masuyama, T. Hayashi, S. Seki, S. Tagawa, J. Am. Chem. Soc.
2006, 128, 8008 – 8014; e) S. Hiroto, K. Furukawa, H. Shinokubo,
A. Osuka, J. Am. Chem. Soc. 2006, 128, 12380 – 12381; f) R. P.
Ortiz, J. Casado, V. Hernꢂndez, J. T. L. Navarrete, P. M. Viruela,
E. Ortꢃ, K. Takimiya, T. Otsubo, Angew. Chem. 2007, 119, 9215 –
9219; Angew. Chem. Int. Ed. 2007, 46, 9057 – 9061; g) J. Casado,
S. Patchkovskii, M. Z. Zgierski, L. Hermosilla, C. Sieiro, M. M.
Oliva, J. T. L. Navarrete, Angew. Chem. 2008, 120, 1465 – 1468;
Angew. Chem. Int. Ed. 2008, 47, 1443 – 1446; h) S. Ito, J. Miura,
N. Morita, M. Yoshifuji, A. J. Arduengo III, Angew. Chem. 2008,
120, 6518 – 6521; Angew. Chem. Int. Ed. 2008, 47, 6418 – 6421.
[3] Recent phenalenyl chemistry; a) K. Goto, T. Kubo, K. Yama-
moto, K. Nakasuji, K. Sato, D. Shiomi, T. Takui, M. Kubota, T.
Kobayashi, K. Yakushi, J. Ouyang, J. Am. Chem. Soc. 1999, 121,
1619 – 1620; b) P. A. Koutentis, Y. Chen, Y. Cao, T. P. Best, M. E.
Itkis, L. Beer, R. T. Oakley, A. W. Cordes, C. P. Brock, R. C.
Haddon, J. Am. Chem. Soc. 2001, 123, 3864 – 3871; c) Y. Morita,
T. Aoki, K. Fukui, S. Nakazawa, K. Tamaki, S. Suzuki, A.
Fuyuhiro, K. Yamamoto, K. Sato, D. Shiomi, A. Naito, T. Takui,
K. Nakasuji, Angew. Chem. 2002, 114, 1871 – 1874; Angew.
Chem. Int. Ed. 2002, 41, 1793 – 1796; d) S. Zheng, J. Lan, S. I.
Khan, Y. Rubin, J. Am. Chem. Soc. 2003, 125, 5786 – 5791; e) S.
Nishida, Y. Morita, K. Fukui, K. Sato, D. Shiomi, T. Takui, K.
Nakasuji, Angew. Chem. 2005, 117, 7443 – 7446; Angew. Chem.
Int. Ed. 2005, 44, 7277 – 7280; f) S. Zheng, J. D. Thompson, A.
Tontcheva, S. I. Khan, Y. Rubin, Org. Lett. 2005, 7, 1861 – 1863;
g) S. Suzuki, Y. Morita, K. Fukui, K. Sato, D. Shiomi, T. Takui, K.
Nakasuji, J. Am. Chem. Soc. 2006, 128, 2530 – 2531; h) L. Beer,
S. K. Mandal, R. W. Reed, R. T. Oakley, F. S. Tham, B. Donna-
dieu, R. C. Haddon, Cryst. Growth Des. 2007, 7, 802 – 809; i) Y.
Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H.
Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro,
K. Sasaki, D. Shiomi, K. Sato, T. Takui, K. Nakasuji, Nat. Mater.
2008, 7, 48 – 51; j) L. Beer, R. W. Reed, C. M. Robertson, R. T.
Oakley, F. S. Tham, R. C. Haddon, Org. Lett. 2008, 10, 3121 –
3123.
[4] J. Huang, M. Kertesz, J. Am. Chem. Soc. 2007, 129, 1634 – 1643.
[5] For similar overlap of phenalenyl rings, see references [1d,3a,h]
and a) X. Chi, M. E. Itkis, K. Kirschbaum, A. A. Pinkerton, R. T.
Oakley, A. W. Cordes, R. C. Haddon, J. Am. Chem. Soc. 2001,
123, 4041 – 4048; b) M. E. Itkis, X. Chi, A. W. Cordes, R. C.
Haddon, Science 2002, 296, 1443 – 1445; c) S. K. Pal, M. E. Itkis,
F. S. Tham, R. W. Reed, R. T. Oakley, R. C. Haddon, Science
2005, 309, 281 – 284; d) S. K. Mandal, S. Samanta, M. E. Itkis,
D. W. Jensen, R. W. Reed, R. T. Oakley, F. S. Tham, B. Donna-
dieu, R. C. Haddon, J. Am. Chem. Soc. 2006, 128, 1982 – 1994;
e) S. K. Pal, M. E. Itkis, F. S. Tham, R. W. Reed, R. T. Oakley, B.
Donnadieu, R. C. Haddon, J. Am. Chem. Soc. 2007, 129, 7163 –
7174.
[6] The length of bond a would be the most sensitive to the
contribution weight of the each canonical structure in Figure 2a,
because bond a has both single- and double-bond character in
the Kekulꢀ structures but only single-bond character in the
biradical structure.
[7] No changes in the average intermolecular p–p distances were
observed in crystals of 1/PhCl upon cooling (3.137 ꢁ at 200 K
and 290 K).
have stronger intramoleculer spin–spin interactions pertur-
bed by weaker intermolecular interactions. The experimental
results strongly contradict our expectation, which suggests
that the 1D chain of 2 and 1 has stronger interactions between
the molecules than within individual molecules. In other
words, the “double bond” can be drawn in the phenalenyl
overlap and the “single bond” within the molecule. In the 1D
chains, the contribution weight of formula B is unexpectedly
large even at atmospheric pressure, and the resonance
balance shift to formula B induced by lowering temperature
or applying pressure to the crystal results in a larger “bond
alternation” of the electronic structure, that is; more “double-
bond” character in the phenalenyl overlap. The larger “bond
alternation” causes a greater separation in energy between
the HOMO and LUMO of the 1D chains and thus the higher
energy shift of the lowest energy band is observed.[13]
In conclusion, we have synthesized the delocalized singlet
biradical 2 consisting of two phenalenyl rings. We determined
that the intra- and intermolecular spin–spin interactions
correlate in the p–p 1D chains and that the electronic
structure of the 1D chain is best represented by the RVB
model. Understanding the interaction of two electrons is
fundamental to understanding the covalent bond itself. In this
view our singlet biradical molecules provide the fundamental
insight that an intermediate bonding interaction of two
electrons can lead to bifunctionality in covalent bonds in
molecular systems, that is, the coexistence of intra- and
intermolecular chemical bonding.
Experimental Section
Detailed synthetic procedure and crystallographic data of 2 are
described in the Supporting Information. CCDC 731071 (2/benzene),
731072 (2/PhCl at 100 K), 731073 (2/PhCl at 200 K), 731074 (2/PhCl at
300 K), and 731075 (2/toluene) contain the supplementary crystallo-
graphic 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.
Received: March 12, 2009
Published online: June 22, 2009
Keywords: covalent bonding · phenalenyl ·
.
resonating valence bond · radicals · spin–spin interaction
[1] a) T. Kubo, M. Sakamoto, M. Akabane, Y. Fujiwara, K.
Yamamoto, M, Akita, K. Inoue, T. Takui, K. Nakasuji,
Angew. Chem. 2004, 116, 6636 – 6641; Angew. Chem. Int. Ed.
2004, 43, 6474 – 6479; b) T. Kubo, A. Shimizu, M. Sakamoto, M.
Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y.
Morita, K. Nakasuji, Angew. Chem. 2005, 117, 6722 – 6726;
Angew. Chem. Int. Ed. 2005, 44, 6564 – 6568; c) T. Kubo, A.
Shimizu, M. Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K.
Sato, T. Takui, Y. Morita, K. Nakasuji, Org. Lett. 2007, 9, 81 – 84;
d) T. Kubo, Y. Goto, M. Uruichi, K. Yakushi, M. Nakano, A.
Fuyuhiro, Y. Morita, K. Nakasuji, Chem. Asian J. 2007, 2, 1370 –
1379.
[8] L. Pauling, Nature 1948, 161, 1019 – 1020.
[9] P. W. Anderson, Mater. Res. Bull. 1973, 8, 153 – 160.
[10] A resonating valence-bond ground state in a phenalenyl-based
neutral radical conductor was proposed in reference [5c].
[2] Recent studies on singlet biradicals; a) D. Scheschkewitz, H.
Amii, H. Gornitzka, W. W. Schoeller, D. Bourissou, G. Bertrand,
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Communications
[11] We used a single crystal of 1 for the pressure-dependent
would depend on the balance in the strengths of J1 and J2 and
might approach a zero-limit in the case of J1 = J2 (no bond
alternation). The absorption energy increases with the unbal-
ance of J1 and J2 (J1 > J2 or J1 < J2). If J1 and J2 are taken as
intra- and intermolecular spin–spin interactions, respectively, the
former case is found in crystals of most closed-shell compounds
and J1 is usually much greater than J2, but surprisingly our
singlet biradical compounds were found to correspond to the
latter case.
experiment, because the methyl groups of 2 might sterically
hinder the closer approach of the phenalenyl rings under high
pressure.
[12] J. A. Pople, S. H. Walmsley, Trans. Faraday Soc. 1962, 58, 441 –
448.
[13] Alternatively, the electronic structure of the 1D chains can be
described by the antiferromagnetic 1D Heisenberg chain model
for the system with two spin–spin interactions, J1 and J2, as
mentioned in reference [4]. Absorption energy in the solid state
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