Synthesis and Physical Properties of Trioxotriangulene Having Methoxy and Hydroxy Groups at α-Positions: Electronic and Steric Effects of Substituent Groups and Intramolecular Hydrogen Bonds

Article abstract of DOI:10.1021/acs.joc.1c00880

New 4,8,12-trioxotriangulene (TOT) neutral radical derivatives having three methoxy and hydroxy groups at the α-positions were synthesized, and the substituent effects on the electronic spin and redox properties were elucidated in the theoretical and expe

Full text of DOI:10.1021/acs.joc.1c00880

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Synthesis and Physical Properties of Trioxotriangulene Having
Methoxy and Hydroxy Groups at α‑Positions: Electronic and Steric
Effects of Substituent Groups and Intramolecular Hydrogen Bonds
Tsuyoshi Murata,* Yosuke Yamamoto, Akira Ueda, Tomoaki Ise, Daisuke Shiomi, Kazunobu Sato,
Takeji Takui, and Yasushi Morita*
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ABSTRACT: New 4,8,12-trioxotriangulene (TOT) neutral radical
derivatives having three methoxy and hydroxy groups at the α-
positions were synthesized, and the substituent effects on the
electronic spin and redox properties were elucidated in the theoretical
and experimental methods. Due to the small SOMO coefficients at the
α-positions of TOT, the methoxy groups in the TOT neutral radical
had negligible effects on the electronic spin structure and redox ability.
On the other hand, methoxy groups greatly increased the LUMO
energy having large coefficients at α-positions and, thus, caused a
remarkable negative-potential shift of the redox wave of anion species
involving the dianion and trianion species. Converting the methoxy
groups to hydroxy groups caused a dramatic change in the electronic structure of TOT, where the intramolecular hydrogen bonds
between hydroxy groups and oxo groups strongly attracted a minus charge on the TOT skeleton. The HOMO energy of the
monoanion species was significantly reduced, causing a blue shift of the HOMO−LUMO transition and an anodic shift of the redox
potential. In addition, due to the steric repulsion smaller than that of the methoxy group, the hydroxy derivative showed a more
planar molecular structure and a strong π-stacking ability.
protection due to the delocalization of the electronic spin over
the 3-fold symmetric 25π-electronic system.11 The unsub-
stituted TOT neutral radical 1 possesses a strong electron-
accepting ability with a redox potential of ca. − 0.1 V vs Fc/Fc+
and also exhibits a stable four-stage redox ability generating
polyanionic species due to the doubly degenerated the lowest
unoccupied MO (LUMO) (Figure 1b). As previous research
on our study on TOT, Bushby and co-workers synthesized the
monoanion species of 1 and succeeded in the generation of its
radical dianion and diradical trianion species, although the
neutral radical 1 was not investigated.12 These redox features
of TOT derivatives greatly contributed to the development of
electronic functions such as a high-capacity organic recharge-
able battery13 and an electrochemical catalyst for the oxygen
reduction reaction.14 Furthermore, the TOT neutral radical
also exhibits a strong self-assembling ability via “two-electron-
multicenter bonding” based on the strong singly occupied MO
INTRODUCTION
■
Air-stable organic neutral radicals have attracted tremendous
attention in the past several decades because they possess
unique electronic structures and exhibit various functions
originating from unpaired electrons.1 These features of neutral
radicals are recently utilized in the development of various
electronic devices, including organic (semi)conductors,2
rechargeable batteries,3 and quantum information technolo-
gies.4,5 Among them, polycyclic π-radicals, where the electronic
spin delocalizes over the π-electronic system, can exhibit
further intriguing physical properties and functions.6,7 Their
flexible electronic structures are highly sensitive to chemical
modifications such as substituent effects that are useful for the
exploration and control of electronic functions.8 For example,
in the study of phenalenyl-based π-radicals, we have clarified
the effects of chemical modifications inside and around the
molecular skeleton on their stability and electronic properties.9
In particular, when the periphery of the phenalenyl skeleton is
modified with oxygen atoms, the topology of electronic spin
structure and frontier molecular orbital (MO) energy change
drastically, and the chemical and electronic properties are
modified flexibly.10
Received: April 15, 2021
Published: July 20, 2021
4,8,12-Trioxotriangulene (TOT 1, Figure 1a), where the
triangulene skeleton is functionalized with three oxo groups, is
a highly stable neutral radical under air even without steric
https://doi.org/10.1021/acs.joc.1c00880
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substituents on the crystal structure and self-assembling ability
of TOT neutral radicals was clarified.
RESULTS AND DISCUSSION
■
MO Energies of Monoanion Species. In order to discuss
the substituent effects on the frontier MO energies of TOT,
density functional theory (DFT) calculations were performed
for the monoanion species of 2a− and 2b−. The results were
summarized in Table 1, where those of the TOT derivatives
Table 1. Frontier MO Energy Levels and Charge Density at
the OCO Atoms of TOT Monoanion Species Obtained by
a
DFT Calculation
HOMO/ LUMO/ HOMO−LUMO charge density
eV
eV
gap/eV
on OCO
2a−
−1.60
−2.07
−1.55
+1.17
+0.93
+1.12
2.76
3.00
2.67
−0.576
−0.654
−0.580
2b− (H bond)
b
2b− (no H
bond)
3−
−1.53
−1.31
+1.09
+1.07
2.62
2.38
−0.571
−0.571
4−
a
Calculated by the DFT method at the B3LYP/6-31G(d,p) level of
b
theory with the optimized structure. The calculation was performed
using a model where the C−C−O−H bonds of the OH groups were
twisted by 90° from those in the optimized structure (2b− H bond) to
prevent intramolecular H bonds.
Figure 1. (a) Molecular structures of TOT derivatives. R and R′
indicate the substituent groups. (b) Energy levels and coefficient
distributions of frontier MOs of monoanion species 1− calculated at
the B3LYP/6-31(d,p) level of theory. (c) π-Stacking column of TOT
neutral radical in the crystal structure of 3.14 Pale blue dotted lines
represent atomic contacts within the π-stackings, and tert-butyl groups
are omitted.
having tert-butyl groups (t-Bu, 3) and n-butoxy groups (n-
BuO, 4) at the β-positions are also listed for comparison.15a
Comparing the TOT monoanions having substituent groups at
the β-positions, the n-BuO derivative 4− has a significantly
higher HOMO energy by 0.22 eV than that of 3− due to the
stronger electron-donating ability of the n-BuO groups. On the
other hand, the LUMO energy of 4− was close to that of 3−.
The result that substituent groups at the β-positions have a
greater effect on the HOMO energy than that of LUMO is in a
good agreement with the consideration from the coefficient
distributions of HOMO and LUMO in Figure 1b (see also
Figure S1 and S2). In comparing 3− with 2a− having three
MeO groups at the α-positions, the HOMO energy of 2a− was
lower by 0.07 eV than that of 3−. It is considered that 2a−
could not obtain the electron-donating effect of the MeO
groups because the α-positions had very small HOMO
coefficients. On the other hand, the energy of LUMO of
2a−, which has large coefficients at the α-positions, was
increased by 0.08 eV by the introduction of MeO groups. Due
to these substituent effects, the HOMO−LUMO energy gap of
2a− increased by 0.15 eV than that of 3−. These situations
dramatically changed by substituting MeO groups of 2a− to
OH groups 2b−. In the optimized structure of 2b−, the
hydrogen atoms of the OH groups locate on the same plane
with the TOT skeleton and exist between the two oxygen
atoms at the α-positions (Oα) and the oxo groups (OCO) of
the TOT skeleton to form strong intramolecular H bonds
(Figure 2). This is contrastive to the fact that the O−C bonds
of the MeO groups at 2a− are largely inclined to the TOT
plane. Both HOMO and LUMO energies of 2b− were
significantly lower than those in 2a− and 3− in spite of the
strong electron-donating ability of OH groups. In particular,
the decrease in the HOMO energy, 0.5 eV, was remarkable,
and the HOMO−LUMO energy gap of 2b− expanded to 3.00
eV. These results cannot be explained only by the simple
substituent effect and probably caused by the electronic effect
(SOMO)−SOMO interaction to form a π-dimer or 1D π-
stacking column, the latter of which causes the high electrical
conductivity (Figure 1c).15,16 We have designed and
synthesized TOT derivatives having various substituent groups
at the 2,6,10-positions (β-positions), and their electronic or
steric effects highly affected on the electronic spin structure,
redox properties and molecular packing controlling the
physical properties, and functions of TOT derivatives.17
As shown in the frontier MOs of TOT monoanion (1−,
Figure 1b), the distribution of the highest occupied MO
(HOMO) and LUMO of TOT is significantly different from
each other, where the β- and α-positions possess large HOMO
and LUMO coefficients, respectively. This suggests that the
substituent effect on the redox ability, electronic-spin structure,
and photophysical characteristics of TOT greatly depends on
the substitution positions. Thus, the substituent groups at the
β-positions and α-positions strongly affect the LUMO and
HOMO/SOMO of TOT monoanion and neutral radical,
respectively (vide infra). In this study, in order to elucidate the
substituent effect at the α-positions TOT skeleton, we have
newly synthesized 2a having three methoxy (MeO) groups at
the 1,5,9-positions, respectively (Figure 1a). The electronic
effects of these substituents on the redox ability, electronic spin
structure, and photophysical characteristics of TOT mono-
anion and neutral radical species have been clarified
theoretically and experimentally. We have also prepared a
trihydroxy (OH) derivative 2b and elucidated the electronic
effects of intramolecular hydrogen bonds (H bonds) on their
physical properties. Furthermore, the steric effect of these
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material, and the TOT skeleton was formed by a route via
triarylmethane similarly to those of other TOT derivatives.17d
The lithiation of 5 with n-butyllithium and the following
reaction with diethylcarbonate gave the triphenylmethanol
derivative 6. The reduction of 6 using iodine and phosphinic
acid yielded triphenylmethane 7. The methoxycarbonyl groups
were regioselectively introduced by the reaction of iodinated
derivative 8 with carbon monooxide under the presence of Pd
catalyst and methanol. The intramolecular Friedel−Crafts
acylation using trifluoroacetic anhydride of the carboxylic acid
10, which was obtained by the hydrolysis of the methyl ester 9,
yielded the radical precursor 11a. The treatment of 11a with
(n-Bu4N+)(OH−) gave the monoanion salt (n-Bu4N+)(2a−),
and the neutral radical 2a was prepared by the oxidation of 11a
using PbO2 in a CH2Cl2 solution. The neutral radical 2a was
stable under air in the solution state (Figure S3) and was
soluble in toluene, CH2Cl2, THF, etc. The OH derivative 11b
was obtained by heating 11a in the presence of excess LiI.
Similar to the MeO derivative, the reactions of 11b with (n-
Bu4N+)(OH−) gave the monoanion salt (n-Bu4N+)(2b−). The
neutral radical 2b was obtained by the oxidation of 11b using
DDQ. Unlike the MeO derivative 2a, the solubility of neutral
radical 2b was very poor and only slightly soluble in CH2Cl2
and toluene, probably due to the strong intermolecular
interactions via strong π−π stacking, as in the case of the
unsubstituted TOT 1.11 The stability of 2b in a saturated
CH2Cl2 solution under air (half-life of ∼2 days) was lower than
that of 2a, probably due to the presence of phenolic OH
groups (Figure S4). In the solid state, both neutral radicals 2a
and 2b were extremely stable and showed no decomposition
under air at room temperature for several years.
Figure 2. Distribution of HOMO coefficients and optimized
molecular structures of 2a− and 2b− in the DFT calculation at the
B3LYP/6-31(d,p) level of theory.
of intramolecular H bonds between Oα and OCO atoms. When
positively charged hydrogen atoms are close to the H bonded
OCO atoms, the minus charge (π-electron) is attracted to the
OCO atoms. In fact, the charge densities on the OCO atoms in
the optimized structure of 2b− remarkably increased to −0.654
in comparison with those of 2a− (−0.576) and 3− (−0.571).
As a result, the electron density on the TOT skeleton
decreased causing a significant decrease in the HOMO energy.
When the O−H bonds of the OH groups are tilted by 90° to
prevent the H bond formation (2b− (no H bond) in Table 1),
the charge densities on OCO atoms and HOMO and LUMO
energies were almost the same as those of 2a−. Such an
electronic effect of H bonds affecting the charge density
distribution and MO energies have also been reported in our
previous studies such as H bonded charge-transfer complexes18
and donor−acceptor-type open-shell molecules.19
Synthesis. Scheme 1 shows the synthetic method of 2a and
2b. The 3-bromoanisole derivative 520 was used as the starting
a
Scheme 1. Synthesis of 2a and 2b
Crystal Structures of Monoanion Salts. Figure 3a−d
shows the molecular structures of the monoanion species 2a−
and 2b− in the crystal structures of their n-Bu4N+ salts. The
TOT skeleton in the 2a− molecule possessed a 3-fold
symmetry; however, the π-plane was slightly distorted due to
the steric repulsion of MeO groups with neighboring t-Bu and
oxo-(OCO) groups (Figure 3a,b). In the case of the 2b− salt,
non-negligible electron density assignable to the oxygen atoms
of hydroxy groups were located at all six α-positions. This is
probably caused by the random arrangement of two possible
orientations of the 2b− molecules in the crystal. Site occupancy
factors of 0.7 and 0.3 were assigned to these oxygen atoms of
corresponding orientations (Oα and Oα′, respectively, Figure
3c). Since the methyl groups were removed from 2a−, the
TOT skeleton was planar, and the Oα and Oα′ atoms were
located on the π-plane (Figure 3d). C−O bond lengths of the
oxo groups in 2a− and 2b− were 1.239−1.259 Å and 1.288−
1.296 Å, respectively (Table S4), and the latter of which are
close to those of 9-hydroxyphenalenone21 and its derivatives22
forming intramolecular H bonds between two oxygen atoms. It
should be noted that these systems may exhibit interesting
dielectric properties originating from the dynamic properties of
protons between the two oxygen atoms.22a,b In the crystal
packing of 2a− salt, the molecules did not form any π-stacking
structures due to the steric effect of the MeO groups (Figure
S7). On the other hand, the 2b− molecules formed a staggered
type π-dimer similar to those formed in TOT neutral
radicals,11,15a because the steric hindrance of the substituent
groups became smaller (Figure 3e,f). The intermolecular face-
to-face distance within the π-dimer was 3.50 Å. The π-dimer
formation in the 2b− salt may also be associated with the
reduction of charge repulsion due to the delocalization of
a
Reagents and conditions: (i) n-BuLi, THF, −90 to −78 °C; 0.3
equiv of (C2H5O)2CO, −90 to −78 °C, (ii) I2, excess H3PO2, AcOH,
60 °C, (iii) 4.2 equiv I2, 8 equiv HIO4·2H2O, conc. H2SO4, 5:1
AcOH−H2O, 110 °C, (iv) 0.5 MPa CO, 30 mol % dppp, 30 mol %
Pd(OAc)2, 12 equiv of Et3N, 5:1 DMF−MeOH, 100 °C, (v) 40 equiv
of KOH, 5:1 EtOH−H2O, reflux, (vi) 30 equiv of (CF3CO)2O,
CH2Cl2, 0 to 38 °C, (vii) 30 equiv of LiI, HMPA, 130 °C; 2 M HCl
aq, (viii) excess (n-Bu4N+)(OH−), MeOH, reflux, (ix) excess PbO2,
CH2Cl2, rt, (x) DDQ, MeCN, rt.
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a
Table 2. Redox Potentials (E1 and E2) and Absorption
b
Maximum Wavelength (λmax) of TOT Monoanion Salts
E1/V vs Fc/
E2/V vs Fc/
Fc+
Fc+
E1 − E2/V
λmax
(n-Bu4N+)(2a−)
(n-Bu4N+)(2b−)
(n-Bu4N+)(3−)
−0.31
−2.26
−2.10
−2.03
1.95
632 nm
1.96 eV
545 nm
2.28 eV
677 nm
1.83 eV
0.00
2.10
1.67
−0.36
a
Measured by the DPV method in CH2Cl2 solution. Potentials were
b
calibrated with Fc/Fc+ couple = 0 V. Measured in CH2Cl2 solution.
was irreversible probably due to the precipitation of the neutral
radical 2b. As for the reduction wave E2, 2a− showed an
irreversible wave in the CH2Cl2 solvent probably due to the
precipitation of the resulting radical dianion species (Figure
S8), while the E2 wave and also an additional reduction wave
(E3) of 3−, 2a−, and 2b− were observed as reversible ones in
highly polar DMF solvent (Figure S9). The oxidation
potentials E1 of 2b− showed a small anodic shift from that of
3− (−0.31 and −0.36 V vs Fc/Fc+, respectively), indicating the
weak electronic effect of substituent groups at the α-positions
of the TOT skeleton. This result relates to the small
distribution of the HOMO coefficients at the α-positions
and sharply contrastive to the result of the β-substituted TOT
derivatives, where the n-BuO groups at the β-position caused a
large low-potential shift of the oxidation wave E1.11 On the
other hand, in the reduction wave relating to LUMO, the
potential E2 of 2− (−2.26 V) showed a negative shift compared
to 3 (−2.03 V) since the α-positions having large LUMO
coefficients were substituted with electron-rich MeO groups.
As a result, the potential difference between E1 and E2, which
corresponds to the HOMO−LUMO gap of the monoanion
species, was 1.95 V for 2a−. This value was ∼0.3 V larger than
that of 3−, 1.67 V. These results are in good agreement with
the DFT calculation (Table 1). In the case of the OH
derivative 2b−, the oxidation wave E1 largely shifted to a higher
potential (0.00 V). This positive shift is probably caused by the
effect of intermolecular H bonds between OCO and Oα atoms,
as shown by the DFT calculation (Figure 2). On the other
hand, the reduction wave E2 was almost the same as that of 3−,
and the potential difference between E1 and E2 was 2.10 V, and
considerably larger than those of 2a− and 3−. In the DMF
solvent, an additional reduction wave E3 corresponding to the
reduction to diradical trianion species was also observed, and a
negative shift similar to that in E2 was also observed for 2a−
(Figure S9).
Figure 3. Crystal structures of (n-Bu4N+)(2a−) and (n-Bu4N+)(2b−)
salts. Molecular structure of 2a−, (a) top view and (b) side view.
Molecular structure of 2b−, (c) top view and (d) side view. (e)
Overlap pattern in the π-dimer of 2b−. (f) Molecular packing of the
(n-Bu4N+)(2b−) salt viewed along the c-axis. Blue-colored molecules
are n-Bu4N+. The thermal ellipsoids were scaled to a 50% probability
level.
minus charges to oxygen atoms by the intramolecular
hydrogen-bond formation.
Electrochemical Properties of Monoanion Salts. To
investigate the substituent effect on the redox ability of the
TOT unit, the electrochemical behavior in the CH2Cl2
solution was investigated. Figure 4 shows the differential
UV−Vis Spectroscopy of Monoanion Salts. Figure 5
shows the solution-state UV−vis absorption spectra of the n-
Bu4N+ salts of 2a−, 2b−, and 3−. The α-nonsubstituted 3−
shows a strong and broad absorption band attributed to the
HOMO−LUMO transition at 550−750 nm with the
absorption maximum λmax of 677 nm (Blue line in Figure 5).
Similar absorption bands were also observed for 2a− and 2b−,
and their absorption maxima showed significant blue shifts to
632 and 545 nm, respectively (Table 2). The trend is
consistent with the DFT calculations (Table 1) and electro-
chemical measurement (Figure 4), where both the HOMO−
LUMO energy gap and E1−E2 potential gap expanded by the
substituent effects. The color of the solution changed
significantly with the blue shift of the HOMO−LUMO
Figure 4. DPV of 2a−, 2b−, and 3− in CH2Cl2 solution. The results
were calibrated with the ferrocene/ferrocenium couple (Fc/Fc+).
Corresponding salts with n-Bu4N+ were used for the measurements.
pulse voltammetry (DPV) of the monoanion salts (n-
Bu4N+)(2a−) and (n-Bu4N+)(2b−), where two oxidation
waves from monoanions to neutral radical and monocation
(E1 and E0, respectively) and one reduction wave to radical
dianion (E2) were observed (Table 2). In the cyclic
voltammetry measurement (Figure S8), the redox wave
corresponding to E1 was reversible for 3− and 2a−, suggesting
the stability of the neutral radical species; however, that of 2b−
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Figure 5. UV−vis spectra of 2a−, 2b−, and 3− in CH2Cl2 solution (3−
4 × 10−5 M). Corresponding salts with n-Bu4N+ were used for the
measurements. The photographs show the solution of each salt in
CH2Cl2.
transition, and the 2b− solution turned red (Figure 5 inset). In
the UV−vis spectra of the 2b− salt in several solvents (Figure
S10), a significant change was not found in the absorption
wavelength even in a highly polar solvent (DMF) and a protic
solvent (MeOH). This result suggests that the intramolecular
hydrogen bond in 2b− is strong.
ESR Spectroscopy and Solution State Properties of
Neutral Radicals. Figure 6a shows the observed and
simulated ESR spectra of neutral radicals 2a in toluene
solution (2.0 × 10−4 M), showing a well-resolved multiline
spectrum with a g-value of 2.0040. Four large splits originating
from the hyperfine couplings of three hydrogen atoms at the α-
positions (3,7,11-positions) without MeO groups, and multiple
fine splits derived from t-Bu and MeO groups were observed.
The hyperfine coupling constant of the α-hydrogen atom (aH1)
1
Figure 6. (a) Observed and simulated ESR spectra of 2a. (b) H-
ENDOR/TRIPLE spectra of 2a. The measurements were performed
in a degassed toluene solution (2 × 10−4 M) at 270 K. The ESR
simulation was achieved by optimizing the hfcc of the 13C nucleus and
line width of 0.0053 mT (Lorentzian line shape) with the hydrogen
hfccs (aHs) fixed, which are experimentally determined by 1H ENDOR
spectroscopy. The upward and downward arrows in (b) denote an
increase and decrease in the intensity of an ENDOR signal,
respectively, when the pump frequency at 13.218 MHz was applied.
(c) Electronic spin density of 2a calculated with the help of the DFT
calculations at a UBLYP/6-31G(d,p) level of theory. Since aH2 and
aH3 are smaller than the line width, they could not be separated in the
ENDOR spectrum; however, they were determined by the ESR
simulation.
1
of 2a determined by the H ENDOR/TRIPLE spectroscopies
(Figure 6b) was +2.50 MHz, which was very close to that of 3
(2.46 MHz).15a This result highly contrasts with the dramatic
reduction of aH1 in 4 having n-BuO groups at β-positions (aH1
= 2.07 MHz)15a due to the delocalization of electronic spin
into the substituent groups. The experimental and calculated
hyperfine coupling constants are in good agreement. It has
been theoretically and experimentally proven that substituent
groups at the α-positions have a negligible effect on the
electronic spin structure of TOT neutral radicals. The intensity
of the ESR signal of 2a gradually decreased upon cooling due
to the formation of a nonmagnetic π-dimer as observed in
other TOT derivatives11,15a (Figure S12), indicating the strong
multicenter bonding nature of the TOT neutral radical (vide
infra). The enthalpy and entropy changes for the π-
dimerization were determined to be −6.7 kcal mol−1 and
−9.6 eu, respectively, from which the dimerization constant
Kdimer was determined as 6.0 × 102 M−1.16a These parameters
were much smaller than those of the corresponding TOT
derivative 3, where Kdimer is 1.2 × 105 M−1.16a It is considered
that the origin of the weaker π-dimerization ability in the
solution sate is the steric bulkiness and mobility of the MeO
groups.
9.7 × 103 and 1.8 × 104 cm−1 mol−1 L, respectively, and this
similarity suggests that the π-stacking structures within the π-
dimers of 2a and 3 were similar to each other. Notably, the
intensity of the π-dimer band in 2a was much smaller than 3.
The Kdimer of 2a in CH2Cl2 calculated from the temperature-
dependence of the intensity of the π-dimer band was 0.47
mol−1, which was considerably smaller than that of 3 (3600
mol−1). This result is consistent with the result that the Kdimer
of 2a obtained from the temperature-dependence of ESR
intensity was much smaller than that of 3.
The weak π-dimerization of 2a was also found in the
solution-state electronic spectra, where a low energy broad
band with the absorption maximum λmax = 848 nm appeared at
a low temperature (Figure 7a). A similar absorption band was
also observed for 3 (λmax = 834 nm, Figure 7a), and this band
is assignable as an intermolecular transition within the π-dimer
(π-dimer band),14 indicating the π-dimerization of 2a. The
absorption coefficients of the π-dimer band of 2a and 3 were
The highly symmetric π-dimeric structure of 2a in the
solution state was proved by temperature-dependent 1H NMR
spectroscopy in CD2Cl2 solution (8.3 × 10−3 M).11,16b At 273
K, 1H NMR signals of 2a species were not observed due to the
paramagnetic effect of the unpaired electron. Then the
concentration of the nonmagnetic π-dimer increased with
cooling, and three signals appeared at 1.52, 3.56, and 7.90 ppm
at 183 K (Figure 7b). The ratio of these signal intensities was
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1
Figure 8. (a) Observed and simulated ESR spectra of 2b. (b) H-
ENDOR/TRIPLE spectra of 2b. The measurements were performed
in a degassed and saturated toluene solution at 270 K. The ESR
simulation was achieved by a line width of 0.0069 mT (Lorentzian
line shape) with the hydrogen hfccs (aHs) fixed which are
experimentally determined by 1H ENDOR spectroscopy. The upward
and downward arrows in (b) denote an increase and decrease in the
intensity of an ENDOR signal, respectively, when the pump frequency
at 13.274 MHz was applied. (c) Electronic spin density of 2b
calculated with the help of the DFT calculations at a UBLYP/6-
31G(d,p) level of theory.
Figure 7. (a) Temperature-dependent electronic spectra of 2a in
CH2Cl2 (1.0 × 10−3 M) from 300 to 180 K. The dotted line shows
the spectrum of 3 at 180 K (8.3 × 10−5 M in CHCl3). (b)
Temperature-dependent 1H NMR spectra (500 MHz) of 2a in a
CD2Cl2 solution (8.3 × 10−3 M).
27:9:3, suggesting that they are attributable to the hydrogen
atoms of three t-Bu and MeO groups and three C−H groups at
3,7,11-positions (Figure 1a), respectively. This appearance/
disappearance of the signals was reversible upon cooling and
heating. The observed behavior of 2a in the 1H NMR
spectrum well coincides with that observed in 3,11 and
indicates that 2a forms a diamagnetic π-dimer. In addition, the
fact that each t-Bu and MeO group and C−H group at the α-
positions showed only one signal indicates that the π-dimer of
2a has a 3-fold symmetry averaged within a NMR time scale.
The OH derivative 2b in a saturated toluene solution
showed an ESR spectrum similar to that of 2a (Figure 8a),
suggesting that 2a and 2b have almost the same electronic spin
structures. The aH1 value obtained by the 1H ENDOR/
TRIPLE spectroscopies was +2.34 MHz, and an additional
split of aH = −0.25 MHz was observed (Figure 8b). The latter
split is assignable as the hyperfine couplings of the hydrogen
atoms in the OH group (Figure 8c).
Crystal Structure and Solid-State Properties of
Neutral Radicals. The electronic and steric effects of the α-
MeO groups on the physical properties of 2a were further
studied in terms of the X-ray crystal structure analysis and
solid-state magnetic properties. In the crystal structure of 2a,
the TOT skeleton possessed a nearly 3-fold symmetry;
however, the π-plane slightly distorted the steric repulsion of
MeO groups with the neighboring t-Bu and oxo groups of the
TOT skeleton (Figure 9a,b). In the packing structure, 2a
formed a π-dimer in a staggered manner through the strong
multicenter bonding (Figure 9c).11 The averaged face-to-face
distance between the TOT skeleton within the π-dimer was
3.22 Å, and the shortest intermolecular contact (3.16 Å) was
observed between the central carbon atoms, which possess the
largest electronic spin density (Figure 8c). All MeO groups
were oriented in the opposite direction of the π-dimer to
reduce the steric repulsion (Figure 9c). Any effective
intermolecular interactions were not observed between the π-
dimers due to the substituent groups around the TOT skeleton
(Figure 9d). In the temperature dependence of magnetic
susceptibility (χp, Figure S14), almost all electronic spins of 2a
were quenched even at 350 K, suggesting a quite strong π-
dimerization in the crystal structure. From the temperature
dependence of signal intensities of the solid-state ESR spectra,
the antiferromagnetic interaction within the π-dimer was
calculated as 2J/kB ∼ 3300 K based on the singlet−triplet
model (Figure S15),23 which is similar to those in π-dimers of
other TOT derivatives.11
Figure 10 shows the solid-state electronic spectra of neutral
radicals 2a and 2b. In the case of 2a, a low energy absorption
band attributed to the π-dimer band was observed at 800−
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CONCLUSION
■
In conclusion, we have synthesized new TOT derivatives
having three MeO and OH groups at the α-positions, 2a and
2b, respectively, and investigated the substituent effects
theoretically and experimentally. The substituent groups at
the α-positions showed completely different effects from those
at the β-positions. The MeO groups of 2a have a negligible
effect on SOMO (neutral radical) and HOMO (monoanion)
of TOT and highly affected the orbital energy of LUMO,
having large distributions at the α-positions. This was
experimentally demonstrated from the electronic spin structure
of neutral radical and the redox property. In the case of OH
derivative 2b, the orbital energies of SOMO and HOMO were
significantly reduced due to the electronic effect of the
intramolecular H bonds between the OH and oxo groups of
the TOT skeleton, causing a significant effect on the redox and
spectroscopic properties. The interaction between substituent
groups at the α-positions and those at the adjacent positions
also affected the molecular structures. The steric repulsion
between the MeO groups of 2a and t-Bu groups at the β-
positions caused a distortion of the TOT skeleton and a
suppression of the self-assembly ability of neutral radical
species. On the other hand, the intramolecular H bonds in 2b
caused a planar molecular structure and a stronger π-stacking
ability in both monoanion and neutral radical states.
The present results will open an opportunity for a new
strategy to explore electronic functions based on open-shell π-
electronic systems such as electron-transport and energy
storage as well as redox catalysts. We are also interested in
stimuli-response properties of 2b based on the dynamics of
protons within the intramolecular H bonds as observed in 9-
hydroxyphenalenone derivatives.21,21 Furthermore, inspired by
pioneering studies on the catalytic activity of metal
complexes24 and the high electrical conductivity of a single
component of open-shell boron complexes25 derived from 9-
hydroxyphenalenone derivatives, our current research is also
aiming to synthesize metal complexes of 2b utilizing the OH
and oxo groups as chelating ligands. These materials will
extend to multidimensional aggregates such as metal−organic
framework (MOF)16 and covalent organic framework
(COF),26 including TOT, and are expected to provide an
opportunity for exploring intriguing functional materials.
Figure 9. Crystal stricture of 2a neutral radical. Molecular structure of
2a, (a) top view and (b) side view. (c) Overlap pattern in the π-dimer
of 2a. (d) Molecular packing of 2a. The thermal ellipsoid was scaled
to 50% probability level.
EXPERIMENTAL SECTION
■
Materials and Methods. 2-Bromoanisole and other reagents
were used as purchased. Solvents were dried (drying reagent in
parentheses) and distilled under an argon atmosphere prior to use:
DMF and toluene (CaH2); THF (Na−benzophenone ketyl). Rf
values on TLC were acquired on E. Merck precoated (0.25 mm)
silica gel 60 F254 plates. Silica gel 60 (100−200 mesh) was used for
column chromatography. Recycle preparative gel permeation
chromatography (GPC) was performed using tandemly connected
two polystyrene gel columns (JAIGEL 1H, Japan Analytical Industry)
with CHCl3 as an eluant.
Figure 10. Solid-state electronic spectra of neutral radicals 2a (green)
and 2b (red) in KBr.
1000 nm.15a The absorption maximum (λmax = 876 nm) was
almost the same as that of the solution spectrum (834 nm).
This result coincides with the result that the 2a molecule
formed an isolated π-dimer in the crystal structure (Figure 9d).
On the other hand, in the OH derivative 2b, the intermolecular
transition showed a large low-energy shift, and the λmax was
1416 nm, similarly to those of 3 (1134 nm) and 4 (1488
nm).15a This result suggests that 2b molecules form a 1D π-
stacking column by the strong intermolecular interaction
between the π-dimers or by the uniform π-stacking in the solid
state. Although the crystal structure of 2b neutral radical was
not clarified, it is speculated that the smaller steric hindrance of
the substituent groups than that of 2a makes the π-dimers of
2b easier to approach each other.
Characterization. Melting and decomposition points were
measured with a hot-stage apparatus with a Yanako MP-J3 and
were uncorrected. Melting and/or decomposition were detected by
eye observation. Elemental analyses were performed at the Graduate
1
School of Science, Osaka University. H and 13C{1H} NMR spectra
were obtained on a JEOL GSX400 MHz or ECA-500 with CDCl3 or
DMSO-d6 using Me4Si as an internal standard. EI-MS spectra were
taken at 70 eV on a Shimadzu QP-5000 mass spectrometer. The ESI-
MS spectrum was measured in MeOH solution on an Applied
Biosystems QSTARElite. The infrared spectrum was recorded on a
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JASCO FT/IR-660 Plus spectrometer using a KBr plate (resolution 4
cm−1).
into pH 7 phosphate buffer solution (100 mL) and was extracted with
ethyl acetate (250 mL) and water (100 mL). The organic extract was
washed with saturated NaCl aqueous solution and then dried over
Na2SO4. The organic extract was filtered and concentrated under
reduced pressure to give crude 6 (10.3 g, crude 85%, purity ∼75%) as
a white solid. The resulting crude product was used for the following
reaction without further purification. The purification for the
characterization and analysis was performed by the GPC with
CHCl3 as an eluant. Mp: 68−75 °C. TLC Rf: 0.50 (5:1 hexane/ethyl
Magnetic Measurements. Magnetic resonance measurements
were performed on a Bruker ELEXSYS E500 spectrometer for X-band
liquid-phase ESR spectra and on Bruker ESR/ENDOR spectrometers
ESP 300/350 for 1H-ENDOR and 1H-TRIPLE spectra equipped with
a wide-band 500 W radiofrequency amplifier. The solution of the
radical was degassed by the freeze−pump−thaw method before the
measurements were recorded. The magnetic susceptibility was
measured on a Quantum Design SQUID magnetometer MPMS-XL
at 2−350 K. The magnetic responses were corrected with diamagnetic
blank data of the sample holder obtained separately. The diamagnetic
contribution of the sample itself was estimated from Pascal’s
constants.
UV−Vis Absorption Spectroscopy. Electronic spectra were
measured for KBr pellets or solutions on a Shimadzu UV−vis
scanning spectrophotometer, UV-3100PC. Temperature-dependent
measurements were performed using an Oxford ITC502 temperature
controller and an Oxford OptistatDN valuable temperature liquid
nitrogen cryostat.
Electrochemical Measurements. Electrochemical measure-
ments were made with an ALS Electrochemical Analyzer, model
730A. Cyclic voltammograms were recorded with a 5 mm diameter
carbon plate and Pt wire counter electrodes in CH2Cl2 containing 0.1
M n-Bu4NClO4 as a supporting electrolyte at room temperature. The
experiment employed a Ag/AgNO3 reference electrode, and the final
results were calibrated with a Fc/Fc+ couple.
X-ray Crystallography. X-ray crystallographic measurements
were made on a Rigaku RAXIS-RAPID II imaging plate with graphite
monochromated Mo Kα (λ = 0.71073 Å). The structure was
determined by a direct method, and refinement was performed by a
full-matrix least-squares on F2. All non-hydrogen atoms were refined
anisotropically, and all hydrogen atoms were included but not refined.
Empirical absorption correction was applied. Crystal data are listed in
Table S3. All calculations were performed using the OLEX-2 software
program.27,28
1
acetate). H NMR (CDCl3, 500 MHz): δ 7.16 (d, 3H, J = 8.0 Hz),
6.90 (d, 3H, J = 2.3 Hz), 6.66 (dd, 3H, J = 8.0 and 2.3 Hz), 3.72 (s,
9H), 2.76 (s, 1H), 1.35 (s, 27H). 13C{1H} NMR (CDCl3, 500 MHz):
δ 158.1, 145.8, 137.1, 125.7, 120.1, 111.4, 81.8, 55.0, 34.6, 29.8. EI-
MS: m/z, 518 (M+, 47%), 355 (M+−C4H9C6H4OCH3, 64%), 191
(M+−2(C4H9C6H4OCH3)−H, 100%). IR (KBr): 3482, 2999, 2959,
2869, 1607, 1568, 1497, 1464, 1406, 1242 cm−1. Anal. Calcd for
C34H46O4: C, 78.72; H, 8.94; N, 0.00%. Found: C, 78.65; H, 9.15; N,
0.00%.
Tris(4-tert-butyl-3-methoxyphenyl)methane (7). In a 200 mL
round-bottomed flask, crude 6 (3.97 g) was placed and dissolved with
acetic acid (100 mL). To this mixture, iodine (1.95 g, 7.68 mmol) and
50% phosphinic acid (8.5 mL) were added, and the reaction mixture
was stirred for 3 h at 60 °C in an oil bath. After the mixture was
cooled to the room temperature, the mixture was filtered, and the
resulting solid was washed with hexane to give crude 7 (2.65 g, crude
59% in 2 steps, purity ∼81%) as a gray powder. The resulting crude
product was used for the following reaction without further
purification. The purification for the characterization and analysis
was performed by the recrystallization from CHCl3. Mp: 248−250
°C. TLC Rf: 0.40 (20:1 hexane/ethyl acetate). 1H NMR (CDCl3, 500
MHz): δ 7.15 (d, 3H, J = 8.0 Hz), 6.68 (d, 3H, J = 1.8 Hz), 6.63 (dd,
3H, J = 8.0 and 1.7 Hz), 5.36 (s, 1H), 3.72 (s, 9H), 1.34 (s, 27H).
13C{1H} NMR (CDCl3,500 MHz): δ 158.3, 143.0, 136.1, 126.1,
121.2, 113.1, 56.3, 55.1, 34.6, 29.9. EI-MS: m/z, 502 (M+, 79%), 487
(M+−CH3, 100%). IR (KBr): 3008, 2960, 2911, 1608, 1568, 1499,
1462, 1413, 1254, 1233 cm−1. Anal. Calcd for C34H46O3: C, 81.23; H,
9.22; N, 0.00%. Found: C, 79.73; H, 9.11; N, 0.14%.
Computational Methods. Density functional theory (DFT)
calculations were performed using the Gaussian 09 program package.
The calculation of electronic-spin structures of the neutral radicals
were performed at the UBLYP/6-31G(d,p) level of theory with
optimization of the geometries, and their MO energy levels were
calculated at the ROB3LYP/6-31G(d,p) level of theory with the
geometries obtained from UB3LYP/6-31G(d,p). The MO energy
levels of the monoanion species were calculated at the B3LYP/6-
31G(d,p) level of theory with optimization of the geometries.
Synthesis. 2-tert-Butyl-5-bromoanisole (5).20 To a mixture of 3-
bromoanisole (31.0 g, 166 mmol), 1-chloro-2-methylpropane (27 mL,
248 mmol), and CH2Cl2 (40 mL) in a 100 mL round-bottomed flask
was added AlCl3 (309 mg, 2.32 mmol). The mixture was stirred at
room temperature for 2.5 h. After being poured into a mixture of 2 M
HCl aqueous solution and ice, the resulting mixture was extracted
with CH2Cl2 (50 mL × 2). The combined organic extract was washed
with saturated NaHCO3 aqueous solution (100 mL) and then dried
over Na2SO4. The solution was filtered and then concentrated under
reduced pressure. The residual oil was dried in vacuo at 75 °C to
remove the unreacted substrate, and the mixture was subjected to
silica gel column chromatography with hexane as an eluant to give 5
Tris(4-tert-butyl-2-iodo-5-methoxyphenyl)methane (8). In a 30
mL round-bottomed flask, crude 7 (400 mg) was placed and dissolved
with a mixture of acetic acid (5 mL) and water (1 mL), and periodic
acid dihydrate (1.45 g, 6.37 mmol), iodine (808 mg, 3.18 mmol), and
H2SO4 (0.1 mL) were added. The mixture was refluxed for 16 h in an
oil bath. After the mixture was cooled to room temperature, a
saturated NaHSO3 aqueous solution was added to this mixture and
then extracted with ethyl acetate (50 mL). The organic extract was
washed with saturated NaHSO3 aqueous solution (30 mL) and
saturated NaHCO3 aqueous solution (30 mL × 5), and the combined
organic extract was dried over Na2SO4. The mixture was filtered, and
the filtrate was concentrated under reduced pressure. The residue was
washed with a small amount of hexane to give crude 8 (630 mg, 53%
in three steps) as a white powder. Mp: 222−225 °C. TLC Rf: 0.35
(40:1 hexane/ethyl acetate). 1H NMR (CDCl3, 500 MHz): δ 7.67 (s,
3H), 6.22 (s, 3H), 5.63 (s, 1H), 3.52 (s, 9H), 1.33 (s, 27H). 13C{1H}
NMR (CDCl3, 500 MHz): δ 158.6, 143.0, 139.2, 137.6, 114.6, 93.0,
68.8, 55.0, 34.5, 29.5. EI-MS: m/z, 790 (M+, 30%), 479 (M+ − 2I−
C4H9, 100%). IR (KBr): 2997, 2954, 2866, 1593, 1539, 1485, 1362,
1225, 1090 cm−1. Anal. Calcd for C34H43I3O3: C, 46.38; H, 4.92; N,
0.00%. Found: C, 46.30; H, 4.86; N, 0.00%.
1
(17.1 g, 42%) as a colorless oil. TLC Rf: 0.50 (hexane). H NMR
(CDCl3, 400 MHz): δ 7.10 (d, 1H, J = 8.3 Hz), 7.00 (dd, 1H, J = 2.2
and 8.3 Hz), 6.97 (d, 1H, J = 2.0 Hz), 3.81 (s, 3H), 1.33 (s, 9H). EI-
MS: m/z 242 (M+, Br = 79, 22%), 244 (M+, Br = 81, 20%), 227 (M+
− CH3, Br = 79, 100%), 229 (M+ − CH3, Br = 81, 96%).
Tris(4-tert-butyl-3-methoxyphenyl)methanol (6). To a mixture of
the mixture of 5 (16.84 g, 71.9 mmol) and THF (250 mL) in a 1 L
Schlenk flask was added n-BuLi (1.6 M hexane solution, 48.0 mL, 76.8
mmol) at −90 °C dropwise for 5 min. The mixture was stirred for 1 h
at −78 °C in an ethanol−dry ice bath. Diethylcarbonate (2.85 mL,
23.5 mmol) was added to this mixture at −90 °C in an ethanol−liquid
nitrogen bath, and the reaction mixture was gradually warmed up to
room temperature. After being stirred for 1 h, the mixture was poured
Tris[(4-tert-butyl-2-methoxycarbonyl-5-methoxy)phenyl]-
methane (9). In a 500 mL autoclave, 8 (2.00 g, 2.27 mmol), 1,3-
bis(diphenylphosphino)propane (dppf, 281 mg, 0.681 mmol),
palladium(II) acetate (153 mg, 0.681 mmol), and triethylamine
(3.8 mL, 27.3 mmol) were dissolved in a mixture N,N-
dimethylformamide (60 mL) and methanol (12 mL). The mixture
was stirred under a 0.5 MPa carbon oxide atmosphere for 40 h at 100
°C in an oil bath. After the mixture was cooled to room temperature, a
saturated NaCl aqueous solution (70 mL) was added to this mixture
and then extracted with ethyl acetate (100 mL). The organic extract
was washed with a saturated NaHCO3 aqueous solution (100 mL ×
3), and the combined organic extract was dried over Na2SO4. The
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mixture was filtered, and the filtrate was concentrated under reduced
pressure. The residue was subjected to silica gel column
chromatography with 10:1 to 6:1 hexane/ethyl acetate as an eluant
to give 9 (1.29 g, 83%) as a white powder. Mp: 187−190 °C. TLC Rf:
0.65 (1:1 hexane/ethyl acetate). 1H NMR (CDCl3, 500 MHz): δ 8.04
(s, 1H), 7.90 (s, 3H), 6.36 (s, 3H), 3.68 (s, 9H), 3.54 (s, 9H), 1.34 (s,
27H). 13C{1H} NMR (CDCl3, 500 MHz): δ 168.0, 160.6, 145.2,
135.7, 130.2, 122.2, 113.8, 54.7, 51.5, 47.3, 34.5, 29.6. EI-MS: m/z,
676 (M+, 3%), 644 (M+ − H−OMe, 23%), 585 (M+ − H−OMe−
CO2Me, 100%). IR (KBr): 3007, 2961, 2911, 1609, 1568, 1499, 1462,
1414, 1233 cm−1. Anal. Calcd for C40H52O9: C, 70.98; H, 7.74; N,
0.00%. Found: C, 70.83; H, 7.73; N, 0.00%.
50 mL round-bottomed flask, 11a (64.0 mg, 0.110 mmol) and
tetra(n-butyl)ammonium hydroxide (10 wt % solution in methanol,
1.0 mL, 0.32 mmol) were dissolved in methanol (6 mL), and the
mixture was stirred for 0.5 h at 60 °C in an oil bath under an argon
atmosphere. The mixture was evaporated under reduced pressure.
Water (30 mL) was added to the residue, and the mixture was stirred
for 1 h at 60 °C in an oil bath. The resulting precipitate was collected
by filtration and then washed with water and a small amount of
hexane to give (n-Bu4N+)(2a−) (76.1 mg, 74%) as a deep blue
powder. Single crystals suitable for X-ray analysis were obtained by
the recrystallization from DMSO as deep blue block crystals. Mp:
1
135−137 °C. H NMR (DMSO-d6, 500 MHz): δ 8.75 (s, 3H), 3.97
(s, 9H), 3.12−3.15 (m, 8H), 1.50−1.58 (m, 35H), 1.30 (sextet, 8H, J
= 7.3 Hz), 0.92 (t, 12H, J = 7.3 Hz). 13C{1H} NMR (DMSO-d6, 500
MHz): δ 179.0, 166.1, 133.9, 133.0, 130.6, 123.0, 117.1, 90.8, 61.4,
57.5, 34.7, 30.9, 22.9, 19.0, 13.2. IR (KBr): 2960, 2874, 1593, 1569,
1545, 1484, 1378, 1296, 1273, 1218 cm−1. ESI-MS (negative mode):
Tris[(4-tert-butyl-2-carboxyl-5-methoxy)phenyl]methane (10). In
a 300 mL round-bottomed flask, 9 (1.83 g, 2.70 mmol) was dissolved
with ethanol (140 mL), and then a solution of KOH (6.07 g, 108
mmol) in water (28 mL) was added. The mixture was refluxed for 5.5
h in an oil bath. After the mixture was cooled to room temperature, 2
M HCl aqueous solution was added to this mixture (pH < 1) and
then extracted with ethyl acetate (100 mL). The organic extract was
washed with 2 M HCl aqueous solution (50 mL) and dried over
Na2SO4. The mixture was filtered, and the filtrate was concentrated
under reduced pressure. The residue was washed with hexane to give
10 (1.74 g, quant) as a white powder. Mp: 291−295 °C. TLC Rf:
+
m/z 579 (M−NBu4 ). Anal. Calcd for (C53H75N1O6)(H2O)2: C,
74.18; H, 9.28; N, 1.63%. Found: C, 73.90; H, 9.13; N, 1.67%.
1,5,9-Trihydroxy-2,6,10-tri-tert-butyl-4,8-dioxo-4H,8H-dibenzo-
[cd,mn]pyren-12-ol (11b). In a 100 mL round-bottomed flask, 11a
(117 mg, 0.202 mmol) was dissolved in hexamethylphosphoric
triamide (HMPA, 20 mL), and lithium iodide (810 mg, 6.05 mmol)
was added. The mixture was stirred for 15 min at 130 °C in an oil
bath under an argon atmosphere. After the mixture was cooled to
room temperature, a small amount of 2 M HCl aqueous solution and
saturated NaCl aqueous solution was added, and the mixture was
extracted with ethyl acetate (50 mL). The organic extract was washed
with 2 M NaOH aqueous solution (10 mL), 2 M HCl aqueous
solution (10 mL), saturated NaHCO3 aqueous solution (10 mL), and
saturated NaCl aqueous solution (10 mL). The organic extract was
dried over Na2SO4 and then filtered and concentrated under reduced
pressure. The residue was washed with chloroform and hexane to give
11b (95.1 mg, 87%) as a deep blue powder. Mp: ∼265 °C (gradual
decomposition, in air). TLC Rf: 0.25 (10:1 ethyl acetate/methanol).
1H NMR (DMSO-d6, 500 MHz): δ 8.70 (s, 3H), 1.55 (s, 27H). ESI-
1
∼0.5 (ethyl acetate). H NMR (CDCl3, 500 MHz): δ 8.39 (s, 1H),
7.87 (s, 3H), 6.90 (s, 3H), 3.73 (s, 9H), 1.34 (s, 27H). 13C{1H} NMR
(CDCl3, 500 MHz): δ 174.5, 160.8, 143.4, 136.2, 130.4, 122.3, 113.5,
81.8, 54.8, 44.1, 34.6, 29.5. ESI-MS (negative mode): m/z 633 (M −
H+). IR (KBr): 3600−2500, 2997, 2959, 1686, 1607, 1558, 1506,
1458, 1231 cm−1. Anal. Calcd for C37H46O9: C, 69.81; H, 7.32; N,
0.00%. Found: C, 70.01; H, 7.30; N, 0.00%.
1,5,9-Trimethoxy-2,6,10-tri-tert-butyl-4,8-dioxo-4H,8H-dibenzo-
[cd,mn]pyren-12-ol (11a). In a 250 mL Schlenk flask, 10 (300 mg,
0.47 mmol) was dissolved in dry CH2Cl2 (45 mL), and the mixture
was cooled to 0 °C in an ice bath. To this mixture, trifluoroacetic
anhydride (1.3 mL, 9.50 mmol) was added, and the mixture was
stirred at 0−38 °C for 9 h under an argon atmosphere. After the
mixture was cooled to room temperature, the mixture was extracted
with CH2Cl2 (50 mL × 2) and ice−water (100 mL), the organic
extract was washed with a saturated NaCl aqueous solution (50 mL)
and water (50 mL). The organic extract was dried over Na2SO4 and
then filtered and concentrated under reduced pressure. The residue
was subjected to silica gel column chromatography with 1:1 to 0:1
hexane/CH2Cl2 as an eluant to give 11a (74.5 mg, 27%) as a deep
MS (negative mode): m/z 537 (M− − H). IR (KBr): 3434, 2957,
1607, 1443, 1341, 1272, 1232 cm−1.
Tetra(n-butyl)ammonium-trihydroxy-2,6,10-tri-tert-butyl-4,8-
dioxo-4H,8H-dibenzo[cd,mn]pyren-12-olate (n-Bu4N+)(2b−). In a
20 mL round-bottomed flask, 11b (20.0 mg, 37.1 μmol) and tetra(n-
butyl)ammonium hydroxide (10 wt % solution in methanol, 1.0 mL,
0.32 mmol) were dissolved in methanol (6 mL), and the mixture was
stirred for 0.5 h at 60 °C in an oil bath under an argon atmosphere.
The mixture was evaporated under reduced pressure. Water (10 mL)
was added to the residue, and the mixture was stirred for 1 h at 60 °C
in an oil bath. The resulting precipitate was collected by filtration, and
then washed with water and small amount of hexane, to give (n-
Bu4N+)(2b−) (25.9 mg, 89%) as a reddish purple powder. Single
crystals suitable for X-ray analysis were obtained by the vapor
diffusion method using dimethoxyethane−hexane as deep red block
1
blue powder. Mp: 272−274 °C (dec.). TLC Rf: 0.25 (CH2Cl2). H
NMR (CDCl3, 270 MHz): δ 10.75 (s, 1H), 9.04 (s, 1H), 8.89 (s,
1H), 8.88 (s, 1H), 4.21 (s, 3H), 4.09 (s, 3H), 4.05 (s, 3H), 1.66 (s,
9H), 1.62 (s, 9H), 1.58 (s, 9H). IR (KBr): 3433, 2959, 2870, 1650,
1576, 1555, 1457, 1376, 1296, 1252, 1221 cm−1. EI-MS: m/z 581 (M
+ H+, trace), ESI-MS (negative mode): m/z 579 ([M − H]−). Anal.
Calcd for (C37H40O6)(H2O)0.1: C, 76.29; H, 6.96; N, 0.00%. Found:
C, 76.16; H, 6.92; N, 0.00%.
1
crystals. Mp: 217−227 °C. H NMR (DMSO-d6, 500 MHz): δ 8.71
1,5,9-Trimethoxy-2,6,10-tri-tert-butyl-4,8-dioxo-4H,8H-dibenzo-
[cd,mn]pyren-12-oxyl (2a). In a 30 mL round-bottomed flask, 11a
(49.7 mg, 85.6 μmol) was dissolved in CH2Cl2 (12 mL). PbO2 (123
mg, 0.51 mmol) was added to the mixture, and the mixture was stirred
at room temperature for 1 h under an argon atmosphere. The reaction
mixture was filtered through a Celite column, and the filtrate was
concentrated under reduced pressure. The residue was washed with
hexane to give 2a (35.5 mg, 72%) as a dark green powder. The single
crystals of 2a were obtained by the slow evaporation at −30 °C as
dark green platelet crystals. The solid-state ESR and magnetic
susceptibility measurements were performed using the polycrystalline
sample prepared by the same method. Mp: 257 °C (gradually
decomposition, in air). TLC Rf: 0.25 (CH2Cl2). IR (KBr): 2950,
1625, 1567, 1464, 1342, 1222 cm−1. ESI-MS (positive mode): m/z
581 (M + H+), (negative mode), m/z 579 (M−). Anal. Calcd for
(C37H39O6)(CH2Cl2)1.1: C, 67.98; H, 6.17; N, 0.00%. Found: C,
67.75; H, 6.03; N, 0.00%.
(s, 3H), 3.12−3.15 (m, 8H), 1.50−1.58 (m, 35H), 1.30 (sextet, 8H, J
= 7.3 Hz), 0.92 (t, 12H, J = 7.3 Hz). 13C{1H} NMR (DMSO-d6, 500
MHz): δ 178.3, 174.9, 133.2, 130.3, 129.6, 112.0, 109.8, 97.5, 57.5,
34. 6, 29.2, 22.9, 19.0, 13.2. IR (KBr): 3434, 2960, 2875, 1606, 1466,
1388, 1343, 1275, 1231 cm−1. ESI-MS (negative mode): m/z 537
+
(M−NBu4 ). Anal. Calcd for (C50H69N1O6)(H2O)0.2: C, 76.63; H,
8.93; N, 1.79%. Found: C, 76.33; H, 8.82; N, 1.90%.
1,5,9-Trihydroxy-2,6,10-tri-tert-butyl-4,8-dioxo-4H,8H-dibenzo-
[cd,mn]pyren-12-oxyl (2b). In a 20 mL Schlenk flask, 11b (20.0 mg,
37.1 μmol) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ, 10
mg, 44 μmol) were dissolved in acetonitrile (8 mL), and the mixture
was stirred at room temperature for 0.5 h under an argon atmosphere.
The resulting mixture was concentrated under a vacuum to ∼4 mL of
the volume, and the resulting precipitate was collected by
centrifugation and washed with acetonitrile and dichloromethane to
give 2b (7.9 mg, 40%) as a dark green powder. Mp: ∼230 °C (gradual
decomposition, in air). IR (KBr): 3446, 2954, 1599, 1545, 1480,
1444, 1395, 1229 cm−1. ESI-MS (negative mode): m/z 537 (M−).
Tetra(n-butyl)ammonium-trimethoxy-2,6,10-tri-tert-butyl-4,8-
dioxo-4H,8H-dibenzo[cd,mn]pyren-12-olate (n-Bu4N+)(2a−). In a
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J. Org. Chem. 2021, 86, 10154−10165

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Article
Anal. Calcd for (C34H33O6)(C2H3N)0.1(H2O)0.4: C, 73.65; H, 6.00;
N, 0.49%. Found: C, 73.66; H, 3.93; N, 0.35%.
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00880.
■
ACKNOWLEDGMENTS
■
sı
This work was supported by the Grants-in-Aid for Scientific
Research B (25288022 and 16H04114) and C (20K0548),
Elements Science and Technology Project from the Ministry of
Education, Culture, Sports, Science and Technology, Japan,
and Core Research for Evolutional Science and Technology
(CREST) Basic Research Program “Creation of Innovative
Functions of Intelligent Materials on the Basis of Element
Strategy” of Japan Science and Technology Agency (JST).
Crystallographic data, selected bond lengths, charge and
spin densities, and FMO energy diagrams in DFT
calculations, temporal observation of UV−vis spectra of
neutral radicals, UV−vis spectra of 11a and 11b, and
2b− salt, cyclic voltammograms, molecular structures in
the crystal structures, temperature dependence of
solution-state and solid-state ESR spectra of 2a,
temperature dependence of magnetic susceptibility of
2a, and spectroscopic data for characterization (1H
NMR, 13C{1H} NMR, ESI-MS), Cartesian coordinates,
and total energy of optimized structures in the DFT
calculation (PDF)
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CCDC 2077864−2077866 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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AUTHOR INFORMATION
Corresponding Authors
■
Tsuyoshi Murata − Department of Applied Chemistry, Faculty
of Engineering, Aichi Institute of Technology, Toyota, Aichi
470-0392, Japan; orcid.org/0000-0001-6861-5456;
Email: tmurata@aitech.ac.jp
Yasushi Morita − Department of Applied Chemistry, Faculty
of Engineering, Aichi Institute of Technology, Toyota, Aichi
470-0392, Japan; orcid.org/0000-0002-2124-0201;
Email: moritay@aitech.ac.jp
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Yosuke Yamamoto − Department of Chemistry, Graduate
School of Science, Osaka University, Toyonaka, Osaka 560-
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Akira Ueda − Department of Chemistry, Kumamoto
University, Kumamoto 860-8555, Japan; orcid.org/0000-
0002-9600-5409
Tomoaki Ise − Department of Chemistry and Molecular
Materials Science, Graduate School of Science, Osaka City
University, Osaka 558-8585, Japan
Daisuke Shiomi − Department of Chemistry and Molecular
Materials Science, Graduate School of Science, Osaka City
University, Osaka 558-8585, Japan; orcid.org/0000-
0002-7135-547X
Kazunobu Sato − Department of Chemistry and Molecular
Materials Science, Graduate School of Science, Osaka City
University, Osaka 558-8585, Japan; orcid.org/0000-
0003-1274-7470
Takeji Takui − Department of Chemistry and Molecular
Materials Science, Graduate School of Science, Osaka City
University, Osaka 558-8585, Japan; orcid.org/0000-
0001-6238-5215
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