An extremely redox-active air-stable neutral π radical: Dicyanomethylene-substituted triangulene with a threefold symmetry

Article abstract of DOI:10.1002/chem.201203755

Radically active: A redox-active air-stable neutral π radical (1 ^.) with three dicyanomethylene groups introduced with threefold symmetry into a triangulene ?? skeleton instead of the oxygen atoms of TOT^. was designed, synthesized, and characterized (see figure). The enhanced electron-accepting ability and extended π-electronic system of this chemical modification gave an extremely small SOMO-LUMO gap and significantly lowered the frontier orbital energies, leading to the remarkable increase in the redox stages.

Full text of DOI:10.1002/chem.201203755

DOI: 10.1002/chem.201203755
An Extremely Redox-Active Air-Stable Neutral p Radical:
Dicyanomethylene-Substituted Triangulene with a Threefold Symmetry
Akira Ueda,[a] Hideki Wasa,[a] Shinsuke Nishida,[b] Yuki Kanzaki,[b] Kazunobu Sato,[b]
Daisuke Shiomi,[b] Takeji Takui,*[b] and Yasushi Morita*[a]
Organic molecules possessing multistage redox ability
play important roles in a wide range of scientific fields from
chemistry and biology to materials science and physics.[1–4]
p-Benzoquinone derivatives are typical examples of such
multistage redox systems.[1] They usually exhibit two-stage
one-electron reduction behavior and form the radical anion
and dianion species (Figure 1a). This multistage redox fea-
However, some kinds of stable open-shell neutral organic
molecules (neutral radicals), such as phenalenyl and verda-
zyl derivatives, also show the multistage redox behavior.[5,6]
For example, we reported that tri-tert-butylated 6-oxophena-
lenoxyl (6OPO in Figure 2a), a phenalenyl-based air-stable
Figure 1. a) Two-stage redox behavior of the p-benzoquinone system. b)
General transformation scheme from p-benzoquinone derivatives to their
tetracyano-p-quinodimethane analogues.
ture is attributable to the electron-accepting ability of the
two carbonyl groups as well as an aromatic stabilization
effect in the six-membered ring in the anionic states. Fur-
thermore, transformation to the corresponding tetracyano-p-
quinodimethane (TCNQ)-type analogues by the replace-
ment of the oxygen atoms of the p-benzoquinone derivatives
with dicyanomethylene groups, which have stronger elec-
tron-accepting ability (Figure 1b), has frequently been per-
formed to modulate their redox abilities, electronic struc-
tures, and salient physical properties.[2]
Figure 2. a) Two-stage redox behavior of the 6OPO system. Chemical
À
À
C
C
structures of b) TOT and TOT and c) 1 and 1 .
neutral radical with two carbonylic oxygen atoms, shows re-
versible two-stage redox behavior and gives the anion and
radical dianion species.[5,7] Importantly, in this redox process,
charge and spin states of the generated chemical species are
in sharp contrast to the quinoid-type system that forms the
radical anion and dianion species (Figure 1a). This unique
redox feature in the oxophenalenoxyl systems enabled us to
explore various exotic electronic-spin properties and func-
tionalities.[5,7,8] Notably, we recently revealed that tri-tert-bu-
Most of the well-studied multistage redox systems are
based on intrinsically closed-shell neutral organic molecules.
[a] Dr. A. Ueda, H. Wasa, Prof. Dr. Y. Morita
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
C
tylated trioxotriangulene (TOT in Figure 2b), an extremely
air-stable neutral p radical with a C3-symmetric extended p-
conjugated system and is derived from 6OPO, exhibits a
four-stage reduction process due to the small SOMO–
LUMO gap as well as the doubly degenerate LUMOs.[7e]
Taking advantage of this four-stage redox ability, we suc-
ceeded in developing a new secondary battery that shows a
higher discharge capacity than Li-ion batteries.[7e]
[b] Dr. S. Nishida, Dr. Y. Kanzaki, Prof. Dr. K. Sato, Prof. Dr. D. Shiomi,
Prof. Dr. T. Takui
Department of Chemistry and Molecular Materials Science
Graduate School of Science
Osaka City University
Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax : (+81)6-6605-2522
E-mail: takui@sci.osaka-cu.ac.jp
In order to further investigate this kind of the fused poly-
cyclic neutral p radical with the multistage redox ability, in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203755.
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COMMUNICATION
this study we have designed, synthesized, and isolated a
[9]
C
novel C3-symmetric triangulene -type neutral p radical 1
(Figure 2c), in which three dicyanomethylene groups with
stronger electron-accepting ability are introduced instead of
C
C
the oxygen atoms of TOT . High air-stability of both 1 and
the anionic species 1À has allowed us to experimentally dis-
close electronic effects of the dicyanomethylene groups on
the redox properties and solid-state properties of fused poly-
cyclic giant p-electronic molecular systems with the help of
theoretical calculations. In particular, we emphasize that this
C
chemical modification enables the 1 system to exhibit eight-
C
stage redox ability exceeding the TOT system, due to the
extremely small SOMO–LUMO gap and the significantly
C
lowered frontier orbital energies of 1 together with the de-
generate LUMOs.
C
As depicted in Scheme 1, the neutral radical 1 was syn-
thesized in four steps from diketoalcohol 2[7e] in a relatively
high total yield (71%). The two carbonyl oxygen atoms in 2
C
Figure 3. a) Observed ESR spectrum of 1 in a degassed toluene (2ꢁ
10À4 m) at 293 K. The microwave frequency used is 9.40005 GHz and the
observed g-value is 2.0028. The field modulation frequency of 12.5 kHz
with low modulation depth was used to observe a sideband-free and
C
Scheme 1. Synthesis of neutral radical 1 : a) CH2(CN)2, TiCl4, pyridine,
highly resolved spectrum. b) Simulated ESR spectrum. c) 1H-ENDOR
CHCl3, 708C, 90%; b) TsCl, pyridine, CHCl3, room temperature, 86%;
c) i) NaH, CH2(CN)2, [PdCl2ACHTNUTRGNE(NUG PPh3)2], THF, room temperature; ii)
Bu4NCl, MeOH, room temperature, 94%; d) DDQ, CH2Cl2, room tem-
perature, 97%. Ts=p-toluenesulfonyl, DDQ=2,3-dichloro-5,6-dicyano-
1,4-benzoquinone.
1
C
and d) H-TRIPLE (pump frequency, 15.691 MHz) spectra of 1 in de-
gassed toluene (2ꢁ10À4 m) at 273 K.
C
trast to TOT with an aroxyl-radical character. This is due to
C
the replacement of the oxygen atoms of TOT with the di-
1
were converted to dicyanomethylene groups by a condensa-
tion reaction with malononitrile in the presence of TiCl4 and
pyridine, to give 3. After the hydroxyl group of 3 was tos-
cyanomethylene groups. Furthermore, H-ENDOR/TRIPLE
spectroscopy allowed us to unequivocally determine the hy-
perfine coupling constants (hfccs) and relative signs of the
protons in the triangulene skeleton (H(Ar)) and tert-butyl
ACHTUNGTRENNUNGylated, the coupling reaction of the tosylate 4 with malono-
nitrile and subsequent treatment with Bu4NCl were per-
formed to yield the anion salt Bu4N+·1À as air-stable green
crystals.[10] Oxidation of Bu4N+·1À by 2,3-dichloro-5,6-dicya-
no-1,4-benzoquinone (DDQ) quantitatively gave neutral
groups (HACTHNGUTERNNUG
(tBu); Figure 3c, d).[14] These hfccs were success-
fully assigned with the help of the DFT calculations
(Table 1).[15] Also, the hfcc of the nitrogen nucleus in the di-
cyanomethylene groups was determined by ESR spectral
simulation (Figure 3b, Table 1, and see the Supporting Infor-
C
C
radical 1 as brown crystals. Similar to TOT , this neutral p
C
radical 1 is stable in air at room temperature, not only in
the crystalline state but also in solution.[11]
C
Table 1. Observed and calculated hfccs [in mT] of 1 . H(Ar), HACHTUNGTRENNUNG(tBu), and
Firstly, we investigated the electronic-spin structure of the
N represent hfccs of the protons in the triangulene skeleton, tert-butyl
protons, and nitrogen nucleus in the dicyanomethylene groups, respec-
tively. The g-value for 2.0028 is used for the unit conversion of the hfccs
from MHz to mT.
obtained neutral radical 1 by liquid-phase ESR and 1H-
C
ENDOR/TRIPLE spectroscopy (Figure 3). An ESR spec-
trum of 1 in a degassed toluene (2ꢁ10À4 m; Figure 3a)
C
H(Ar)
H
G
N
showed a well-resolved hyperfine structure. The observed g-
obsd[a]
calcd[c]
+0.090
+0.146
+0.0082
+0.008
Æ0.0535[b]
C
value of 1 was 2.0028, which is similar to that of tri-tert-bu-
+0.073
tylated phenalenyl (g=2.0028),[12] but significantly smaller
[a] Values and relative signs were determined by the 1H-ENDOR/
TRIPLE spectroscopy. [b] Value was determined by the ESR spectral
simulation. [c] Hfccs of the optimized structure calculated at the
[13]
C
than that of TOT (g=2.0042).
This result indicates that
C
heteroatomic contribution to the g-value of 1 is less, that is,
C
the unpaired electron of 1 has a substantial hydrocarbon-
UB3LYP/6-31GACTHNUGRTNEUNG(d,p) level of theory. Considering the molecular symme-
radical character (no significant influence by CN), in con-
try, hfccs for the equivalent nuclei were averaged.
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16273

T. Takui, Y. Morita et al.
mation for the detailed analysis).[16] The experimental and
calculated hfccs are in good agreement. Thus, as shown in
C
Figure 4a, the unpaired electron of 1 in solution extensively
delocalizes over the whole 37p-conjugated molecular skele-
C
Figure 4. Spin density distribution of a) 1 (left: top view, right: side view)
C
and b) TOT calculated at the UB3LYP/6-31GACTHNUTRGNE(NUG d,p) level of theory. The
red and blue colors denote positive and negative spin densities, respec-
tively. Hydrogen atoms are omitted for clarity.
ton with C3 symmetry. In addition, the spin density on the
C
triangulene p skeleton of 1 (68% of the total spin densities,
C
Figure 4a) is much less than that of TOT (88% of the total
Figure 6. Frontier molecular orbital energy levels and distributions of the
neutral radicals TOT· (left)[7e] and 1 (right) calculated at the ROB3LYP/
C
spin densities, Figure 4b), indicating that significant portion
of spin density delocalizes onto the dicyanomethylene
groups from the triangulene skeleton. This change in the
spin density distribution can also be interpreted in terms of
the electronic resonance effect of the dicyanomethylene
group (see the Supporting Information).
The electronic effect on the multistage redox ability by
the introduced dicyanomethylene groups was evaluated by
means of cyclic voltammetry (CV) of the salts Bu4N+·1À and
Bu4N+·TOTÀ in DMF (Figure 5) with the help of theoretical
6-31GACTHNUGTRENNUG(d,p)//UB3LYP/6-31GACHUTNGTREN(NNGU d,p) level of theory. Each SOMO–LUMO
energy gap is denoted by the number in blue.
found that all of the peak potentials in 1À (Epred1 =À1.17,
Erped2 =À1.28, Epred3 =À2.09, and Eopx1 =+0.09 V, Figure 5b)
are significantly positively shifted from those of TOTÀ
(Erped1 =À2.14, Erped2 =À2.53, Epred3 =À3.18, and Epox1
=
À0.26 V, Figure 5a). According to the molecular orbital cal-
culations of 1 and TOT (Figure 6),[17] these four redox
events should be related to the SOMO and doubly degener-
ate LUMOs: the three reduction processes provide the radi-
cal dianion, diradical trianion, and radical tetraanion, and
the one oxidation process generates the neutral radical.[18]
Thus, the experiments suggest that the SOMO and LUMO
C
C
C
energy levels of 1 remarkably decrease in comparison with
C
those of TOT . Furthermore the potential difference be-
tween Epred1 and Epox1, corresponding to the SOMO–LUMO
energy gap, significantly decreases from 1.88 V of TOTÀ to
1.26 V of 1À.[19] The calculated results (Figure 6) show the
similar trends with the experimental observation: the theo-
C
retical energy levels of the SOMO and LUMOs of 1 (À3.53
C
and À3.46 eV) are much lower than those of TOT (À3.11
and À2.29 eV) and thus the estimated SOMO–LUMO
Figure 5. Cyclic voltammograms (vs. Fc/Fc+) of a) Bu4N+·TOTÀ (3 mm)
and b) Bu4N+·1À (1 mm) measured in DMF with 0.1m Bu4NClO4 as a
supporting electrolyte at room temperature.
C
energy gap of 1 is much smaller (0.07 eV) than that of TOT
(0.82 eV). Also, the potential difference between Erped2 and
Erped1 of 1À is also found to be much smaller (0.09 V) than
that of TOTÀ (0.39 V; Figure 5). This means that the tri
ACHTUNGTRENNUNG
calculations (Figure 6). As illustrated in Figure 5b, the di-
cyanomethylene derivative Bu4N+·1À exhibited six reduction
processes and two oxidation ones. A series of the redox
waves were observed reversibly and reproducibly. These re-
sults mean that the number of redox stages increased to
ACHTUNGTRENNUNG
Namely, the intramolecular Coulomb repulsion in 13À is
smaller than that in TOT3À due to the extended p-electronic
system. At more negative potentials, we observed three
other reduction processes (À2.49, À2.80, À2.98 V) in the
system 1. One can imagine that the further three electrons
are sequentially added to the LUMO and LUMO+1 of
C [18b]
C
eight from four of the TOT system by the replacement of
C
the three oxygen atoms of TOT with the dicyanomethylene
groups. Focusing on the first, second, and third reduction
processes and the first oxidation one of both the systems, we
1 ,
because the LUMO+1 is located at still a lower
C
energy level (À2.16 eV) similar to the LUMO of TOT
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Dicyanomethylene-Substituted Triangulene
COMMUNICATION
(À2.29 eV) as shown in Figure 6. Furthermore, the system 1
showed the second oxidation wave at +0.56 V, suggesting
that a cationic species might be formed.[18b] Taking account
al chain structure along the a axis (Figure 7e).[21] This is in
sharp contrast to the p-dimer-based columnar structure of
[7e]
C
the TOT crystal. This structural feature is also probably
C
C
C
of the lower SOMO energy level of 1 than TOT , this obser-
vation is probably not due to the electronic effect of the di-
cyanomethylene groups, but rather to the higher solubility
and/or stability of the 1+ ion compared to TOT+.[20] All
due to the modulation of the spin density distribution of 1 :
the dicyanomethylene groups have a certain proportion of
the spin density (Figure 4a and Table 1). In fact, the mag-
netic susceptibility measurements and DFT calculations of 1
C
C
these intriguing multistage redox and MO features of 1 are
illustrate that there are significant intermolecular magnetic
exchange interactions attributable to the intra- and inter-
dimer short contacts through the dicyanomethylene
groups.[22]
In conclusion, we have designed, synthesized and charac-
terized an extremely redox-active, air-stable, neutral p radi-
attributable to both the enhanced electron-accepting ability
and the extended p-electronic system due to the replace-
ment of the oxygen atoms of TOT with the dicyanometh
ene groups.
The electronic and steric effects of the introduced dicya-
C
ACHTUNGERTNyNUNG l-
U
C
nomethylene groups were further studied in terms of the
crystal structure and solid-state magnetic properties. Single-
crystal X-ray structure analysis revealed that the triangulene
cal 1 with three dicyanomethylene groups introduced with
threefold symmetry into a triangulene p skeleton. Most im-
portantly, besides the degenerate LUMOs due to the inher-
ent C3-symmetric p-electronic molecular skeleton, the en-
hanced electron-accepting ability and the extended p-elec-
tronic system by this simple chemical modification gave an
extremely small SOMO–LUMO gap and significantly low-
ered the frontier orbital energies, leading to the remarkable
increase in the redox stages. The spectroscopic identification
C
skeleton of 1 is significantly distorted, to form a nonplanar
p-conjugated network (Figure 7a, b), which is ascribable to
of the redox species[18] as well as the investigation of the
À
C
properties of 1 and 1 as an electrode-active material for
our secondary “molecular spin battery”[7e] are underway.
Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific Re-
search on Innovative Areas (No. 20110006 and Quantum Cybernetics)
and Elements Science and Technology Project from Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and Toyota Motor
Corporation. The support for the present work by Japan Science and
Technology Agency through CREST project, “Implementation of Molec-
ular Spin Quantum Computers” and by the FIRST project on “Quantum
Information Processing”, JSPS, Japan is also acknowledged.
C
C
Figure 7. X-ray crystal structures of 1 : a) top and b) side views of 1 mol-
ecule (hydrogen atoms on tert-butyl groups are omitted), c) top and d)
side views of a dimeric pair (hydrogen atoms are omitted), and e) one-di-
mensional chain structure based on the dimeric pair (hydrogen atoms
and tert-butyl groups are omitted).
Keywords: cyclic voltammetry
·
density functional
calculations · EPR spectroscopy · radicals · X-ray diffraction
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and the hydrogen atoms at the peri-positions. All the dicya-
nomethylene groups locate at the same (upper) side to the
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C
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C
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groups or the triangulene skeleton of another molecule of
C [21]
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[16] The simulated ESR spectrum well reproduces the splitting pattern
of the experimental spectrum, indicating that the experimental hfccs
were correctly determined. The side-band free hyperfine spectrum
with the ENDOR data allowed us not only to accurately determine
the hfccs, but also to identify a line broadened component with the
same g-value (g=2.0028), which is probably attributable to a dimer-
C
ic species of 1 . The detailed ESR analysis is given in the Supporting
Information.
[17] Molecular orbital calculations for the anion species 1À and TOTÀ
C
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show the similar trend with those for the neutral radicals 1 and
C
TOT (Figure 6). For details, see the Supporting Information.
[18] a) As a preliminary result, we have spectroscopically identified the
radical dianion and diradical trianion species of the TOT system;
b) Spectroscopic identification and detailed characterization of all
the redox species of the system 1 except for the neutral radical and
anion species seen in the CV studies are underway.
C
C
[9] J. Inoue, K. Fukui, T. Kubo, S. Nakazawa, K. Sato, D. Shiomi, Y.
Morita, K. Yamamoto, T. Takui, K. Nakasuji, J. Am. Chem. Soc.
2001, 123, 12702–12703.
[19] Solution-phase UV/Vis spectra of the neutral radicals 1 , TOT and
the salts Bu4N+·1À, Bu4N+·TOTÀ also suggest the remarkable de-
C
crease of the SOMO–LUMO and HOMO–LUMO energy gaps in 1
and 1À, respectively. For details, see the Supporting Information.
[20] In fact, 1 and Bu4N ·1 are more soluble in organic solvents than
[10] In the crystal, the 1À ion forms a p-dimeric pair in a staggered ar-
rangement. For details, see the Supporting Information.
+
À
C
+
À
C
C
[11] The decomposition point of 1 is 1958C under aerobic conditions.
TOT and Bu4N ·TOT , respectively, whereas all of them are stable
[12] K. Goto, T. Kubo, K. Yamamoto, K. Nakasuji, K. Sato, D. Shiomi,
T. Takui, M. Kubota, T. Kobayashi, K. Yakushi, J. Ouyang, J. Am.
Chem. Soc. 1999, 121, 1619–1620.
in air.
C
[21] The more detailed crystal structure of 1 is illustrated in the Support-
ing Information.
[13] Y. Morita, A. Ueda, M. Moriguchi, K. Fukui, K. Sato, T. Takui, un-
[22] The experimental and theoretical magnetic exchange interactions
2J/kB are obtained as À81 and À60 K for the intradimer contacts
and À12 and À13 K for the interdimer ones, respectively. For details,
see the Supporting Information.
published result.
14
C
[14] N-ENDOR/TRIPLE measurements of 1 were also carried out;
however, the signal attributable to the nitrogen nucleus of the dicya-
nomethylene groups was not observed.
[15] All DFT calculations were performed with Gaussian03: Gaussi-
an 03, Revision E.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
Received: October 20, 2012
Published online: November 23, 2012
16276
www.chemeurj.org
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Chem. Eur. J. 2012, 18, 16272 – 16276