Stereochemistry, Stereodynamics, and Redox and Complexation Behaviors of 2,2′-Diaryl-1,1′-Biazulenes

Article abstract of DOI:10.1002/cplu.201900262

2,2′-Diaryl-1,1′-biazulenes were synthesized and electronic communication between the azulene subunits was suggested based on redox measurements. The linkage of azulene at the 1-position also appeared to increase the HOMO levels. In addition, cyclic voltammetry measurements of 2-arylazulenes showed a return peak associated with the oxidation, which was not observed for azulene. The stabilization of the single-electron oxidant may be due to the SOMO-HOMO energy inversion phenomenon. X-ray crystallography of the azulene dimers revealed that this species possessed a syn-type structure in which both aryl groups in the 2-positions formed π-stacks. The twisted structure was indicated to be in the (R)- or (S)-configuration for all molecules in the unit cell. Spontaneous resolution was also shown. Furthermore, from the solid circular dichroism (CD) spectral measurements, the relationship between the absolute configuration of the molecules and the CD spectra was determined. A racemization rotational barrier of ca. 27 kcal mol^?1 was calculated. Moreover, the pyridylazulene dimer cyclized upon reaction with PdCl₂ to form a 3 : 3 complex, in which the biazulene units cyclized to give ratios between the (R)- and (S)-forms of either 2 : 1 or 1 : 2.

Full text of DOI:10.1002/cplu.201900262

Accepted Article
Title: Stereochemistry, Stereodynamics, and Redox and Complexation
Behavior of 2,2ʹ-Diaryl-1,1ʹ-biazulenes
Authors: Takahiro Tsuchiya, Yuka Katsuoka, Kenji Yoza, Hiroyasu
Sato, and Yasuhiro Mazaki
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Stereochemistry, Stereodynamics, and Redox and Complexation
Behaviors of 2,2ʹ-Diaryl-1,1ʹ-biazulenes
Takahiro Tsuchiya,*[a] Yuka Katsuoka,[a] Kenji Yoza,[b] Hiroyasu Sato,[c] and Yasuhiro Mazaki*[a]
Dedication ((optional))
Abstract: 2,2'-Diaryl-1,1'-biazulenes were synthesized and the
presence of an electronic communication between the azulene
subunits was suggested based on redox measurements. The linkage
of azulene at the 1-position also appeared to increase the HOMO
levels. In addition, cyclic voltammetry measurements of 2-
arylazulenes showed a return peak toward the oxidation, which was
not observed for azulene. The stabilization of the first-electron
oxidant may be due to the SOMO-HOMO energy inversion
phenomenon. X-ray crystallography of the azulene dimers revealed
that this species possessed a syn-type structure in which both aryl
groups in the 2-positions formed π-stacks. The twisted structure was
indicated to be in the (R)- or (S)-configuration for all molecules in the
unit cell. Spontaneous resolution was also revealed. Furthermore,
from the solid circular dichroism (CD) spectral measurements, the
relationship between the absolute configuration and the CD spectra
was determined. A racemization rotational barrier of ~27 kcal mol−1
was calculated. Moreover, pyridylazulene dimer and PdCl2 cyclized
through the formation of a 3:3 complex, while the biazulene units
cyclized to give ratios between the (R)- and (S)-forms of 2:1 and 1:2,
respectively.
catalyst ligands BINAP[3] and BINOL.[4] The stereodynamics[5]
and the relationship between conformational change and CD
spectra[6] of 2,2'-substituted-1,1'-binaphthalenes have been well
clarified. Moreover, the applications of these binaphthalenes for
chiral molecular switches,[7] chiral polymers,[8] and chiral dopants
for liquid crystals[9] have also been developed.
In contrast, azulene,[10] which is
a structural isomer of
naphthalene and possesses different photophysical properties
from naphthalene due to its polarized structure, exhibits
attractive redox properties.[10c] Furthermore, unlike naphthalene,
whose HOMO and LUMO are distributed on all carbon atoms
where substituents can be introduced, the HOMO of azulene is
distributed across its odd-positioned carbon atoms, while the
coefficients of the LUMO and HOMO-1 are greater in its even-
positioned carbons atoms[10a,c] (Figure 1). The perturbed orbital
of azulene therefore varies according to the substituent
position.[10a,j] For example, there is almost no effect on the
LUMO or HOMO-1 energy levels when substituents are
introduced onto the odd-positioned carbon atoms and the
HOMO energy levels rise or fall. Hence, azulene-connected
systems behave differently from naphthalene-connected
systems, and there is significant research interest in aspects
such as changes in the electronic properties of the former.
Theoretical calculations predict that azulene oligomers linked
at the 1,3- and 2,6-positions would display conductivity due to
their polaron mechanisms, as well as the relationship between
the conductivity and the dihedral angles between azulenes.[11]
Recently, azulene trimers linked at the 2,6-position, which bears
the high LUMO coefficient, have been found to display either n-
type or ambipolar semiconducting properties.[10f] Meanwhile,
optically active 2,2'-dimethyl-1,1'-biazulene has been isolated.[12]
Furthermore, proton capturing behavior has been observed in
the 1,1ʹ-biazulene bearing a 2-pyridyl group;[13] however, the
dynamic behavior of 1,1ʹ-biazulene itself remains poorly
understood, as do the substituent effects of its geometric
structure and electronic properties.
Introduction
In recent years, organic semiconductor materials, which are
known for their light weight and excellent flexibility, have
attracted attention as replacements for inorganic materials in
electronic components.[1] Among such materials, acenes, which
possess a relatively small HOMO-LUMO gap, have been widely
researched for application in organic electronics and photonic
materials, among others.[2] For example, the use of
a
naphthalene dimer, which has the simplest acene structure, as a
light-emitting component[2b] or charge-transfer layer in organic
electronics has been explored.[2a] Moving beyond applications in
organic electronics, some studies have also explored the use of
naphthalene in catalysis, e.g., the well-known asymmetric
[a]
Prof. Dr. T. Tsuchiya, Yuka Katsuoka, Prof. Dr. Y. Mazaki
Department of Chemistry, Graduate School of Science
Kitasato University, 1–15-1 Kitasato
Minami-ku, Sagamihara, Kanagawa 252–0373 (Japan)
E-mail: ttsuchi@kitasato-u.ac.jp
mazaki@kitasato-u.ac.jp
[b]
[c]
Dr. K. Yoza
Bruker Japan
3-9 Moriya-cho, Kanagawa-ku, Yokohama 221-0022 (Japan)
Dr. H. Sato
Figure 1. a) Numbering of atoms in azulene and its polarized resonance
structure. b) HOMO–1 (right), HOMO (center), and LUMO (left) of azulene.
Rigaku Corporation
3-9-12 Matsubara, Akishima, Tokyo 196-8666 (Japan)
Supporting information for this article is given via a link at the end of
the document.
Thus, we herein report the synthesis of 1-position-linked
dimers of azulene bearing a phenyl or 4-pyridyl group at the 2-
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position (1 and 2). The electronic properties, geometric
structures, and stereodynamics of the resulting compounds are
evaluated on the basis of their ultraviolet-visible (UV-vis)
absorption and CD spectra, redox potentials, X-ray
crystallography data, and effect of the substituent at the 2-
position. The complexation of compounds bearing a pyridyl
group is also examined.
as shown in Figure S8a. The downfield shift of the H2 signal (the
proton signal corresponding to the α-position from the dimerized
position) in 1,1ʹ-biazulene shows a similar trend as that for
biphenyl, 1,1'-binaphthyl, and 2,2'-binaphthyl (Figure S8b).[17]
Because there is no notable change in the chemical shift of the
H8 signal in 1,1'-biazulene, it is suggested that 1,1ʹ-biazulene
exists primarily in this anti state in solution. The 13C NMR
spectra of 1, 2, and 1,1ʹ-biazulene show the downfield shifts of
the signals of the dimerized positions of the C1 carbon atoms
(Figure S9). These phenomena are similar to the relationship
between naphthalene and 1,1ʹ-binaphthalene.[17]
Results and Discussion
Variable temperature (VT)-1H NMR measurements of
indicated that as the temperature was reduced, the signal
corresponding to the H8 proton underwent significant
2
Synthesis and Characterization
2,2ʹ-Diaryl-1,1ʹ-biazulenes
1 and 2 were synthesized from
a
azulene over four steps in total yields of 32 and 20%,
respectively (Scheme 1). 2-Phenylazulene (3)[14] and 2-(4-
pyridyl)azulene (4)[15] were synthesized based on a previously
reported method, prior to bromination using N-bromosuccinimide
(NBS) to yield 1-bromo-2-phenylazulene (5, 84%) and 1-bromo-
2-(4-pyridyl)azulene (6, 88%), respectively. Dimers 1 and 2 were
obtained by performing a homocoupling reaction of 5 and 6,
respectively, using a nickel(0) catalyst.
downfield shift, while the pyridyl H10 proton signal underwent an
upfield shift (Figure S10). These results are believed to signify
an inclination towards a more stable conformation and a greater
exertion of the ring current effect of the pyridyl group in the
paired azulene at low temperatures. However, it is difficult to
assess whether this inclination is towards the syn- or anti-form
by NMR measurements alone.
Scheme 1. Synthetic scheme of 1 and 2.
1H and 13C NMR measurements were performed for 1, 2, and
1
1,1ʹ-biazulene, and changes in the H NMR spectra associated
with dimerization were compared. All signals were assigned by
HSQC, HMBC, and 1H-1H COSY experiments (Figures S1−S7,
Supporting Information). For compounds 1 and 2, significant
upfield shifts were observed for the proton signal corresponding
to the internal 8-position (Figure 2, abbreviated as H8) and the
H10 and H11 atoms of the aryl group. This may be due to the
increased electron density surrounding the coupling position due
to the inductive effect. It is also possible that the H8, H10, and
H11 protons underwent a ring current shielding effect from
dimerized paired azulene or from the aryl group on the opposite
side. In contrast, in the case of 1,1ʹ-biazulene,[16] only a
downfield shift of the H2 proton signal was observed. If
increasing the electron density surrounding the coupling position
is a dominant factor in determining the upfield shifts of the
signals of the H8, H10, and H11 protons in 1 and 2, the H8
signal in 1,1ʹ-biazulene would also be expected to show an
upfield shift. Therefore, the upfield shifts of the H8, H10, and
H11 signals in 1 and 2 are thought to be derived from the ring
current effect of the dimerized paired azulene or the aryl group,
Figure 2. Comparison of 1H NMR chemical shifts of (a) 3 and 1, (b) 4 and 2,
and (c) azulene and 1,1ʹ-biazulene in CDCl3 at 300 K.
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Density functional theory (DFT) calculations for 1, 2, and 1,1ʹ-
biazulene (B3LYP method, 6-31G** basis set)[18] demonstrated
that for 1 and 2, the syn-form is more stable (1: 0.09 kcal/mol, 2:
0.17 kcal/mol) than the anti-form, while for 1,1ʹ-biazulene, the
anti-form is more stable (0.54 kcal/mol). Furthermore, the
estimated relative energy difference was greater when the
calculations[19] were performed using M06-2X[19b-e] and ωB97X-
D[19f] functionals for 1 (M06-2X: 2.15 kcal/mol; ωB97X-D: 1.83
–1
–3
–5
2.03 eV
2.43 eV
510 nm
2.33 eV
532 nm
2.38 eV
521 nm
2.09 eV
593 nm
616 nm
2.33 eV
532 nm
kcal/mol) and
2 (M06-2X: 2.29 kcal/mol; ωB97X-D: 2.25
kcal/mol) (Figure S11), indicating that the syn-form is probably
derived from intramolecular π-π interactions between the aryl
groups at the 2-position of the azulene moieties.
3
1
4
2
azulene 1,1’-biazulene
To clarify the electronic properties of the obtained compounds,
we measured the UV-vis absorption spectra in THF for 1, 2, 1,1ʹ-
biazulene, and their corresponding monomers (Figure 3).
Consequently, the longest absorption wavelength at ~580 nm for
3, 4, and azulene exhibited a red shift (~20–30 nm) due to
conversion to dimers 1, 2, and 1,1ʹ-biazulene. This change was
greatest from azulene to 1,1ʹ-biazulene, i.e., when a substituent
was present at the 2-position.
Figure 4. MO diagrams of 1–4, azulene, and 1,1ʹ-biazulene.
Redox properties
The redox properties of 1–4 and 1,1ʹ-biazulene were then
examined by square wave voltammetric (SWV) measurements
using THF as a solvent (Figure 5). For dimers 1, 2, and 1,1ʹ-
biazulene, stepwise reduction and oxidation peaks were
obtained, likely due to two one-electron steps. This suggests the
presence of an electronic communication between the azulene
subunits. In addition, for each dimerization, the first oxidation
potential exhibited a significant cathodic shift, which was larger
than the change in the first reduction potential, thereby
demonstrating that the linking of azulene at the 1-position
increases the HOMO distributed across its odd-positioned
carbon atoms. Interestingly, for 3 and 4, a return reduction peak
was observed by cyclic voltammetry (CV) measurements,
though the separation between oxidation and return reduction
peaks was rather large (Figure S15). In this context, it should be
noted that azulene, the oxidation peak of which is irreversible, is
reported to form polymers via the radical polymerization of a
single-electron oxidized species.[20] This suggests that 3 and 4
contain more stable single-electron oxidants than azulene.
Time-dependent DFT (TD-DFT) calculations were also
performed to predict the UV-vis absorption spectra for azulene,
2-aryl azulene, and their dimers. Calculations were performed
for both the syn and anti structures, and the obtained spectra
were comparable (Figure S12). In addition, the respective
experimental and predicted spectra were relatively consistent
with one another (Figures S13 and S14). The molecular orbital
diagrams for azulene, 2-aryl azulene, and their dimers as
determined by theoretical calculations are shown in Figure 4.
The energy splitting width of the HOMO associated with
dimerization was larger than that of the LUMO; consequently,
the HOMO level increased due to dimerization. Furthermore, the
energy splitting width of the respective orbitals associated with
dimerization were somewhat greater in azulene than for 3 and 4.
The dihedral angles between the azulene subunits of the
optimized structures of 1, 2, and 1,1ʹ-biazulene were estimated
to be 66.7, 55.3, and 46.6° at the B3LYP/6-31G** level of theory,
and 56.1, 55.4, and 46.8° for the M06-2X/6-31G** level,
respectively, thereby indicating the more planar form between
the two azulene subunits of 1,1ʹ-biazulene. It therefore appears
that the two azulene subunits of 1,1ʹ-biazulene are conjugated
more strongly than those of 1 and 2.
3
–1.99
1
–2.23
–2.04
0.56
–1.83
4
0.18
0.51
2
–1.85
–2.04
0.18
0.47
–2.11
azulene
0.74
1,1’-biazulene
–2.04
–2.31
0.56
0.51 0.18
1.0
0
–1.0
–2.0
–3.0
0/+
Potential / V vs. FeCp2
Figure 5. SWV curves of 1, 2, 3, 4, azulene, and 1,1ʹ-biazulene (1.0 mM) on a
Figure 3. UV-vis absorption spectra of (a) 1, 2, 1,1ʹ-biazulene and (b) 3, 4, and
azulene in THF.
Pt electrode in 1.0 M nBu4NPF6, THF.
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Figure 6. MO diagrams of azulene, 3, and 4 and their radical cationic species.
The stabilities of the radical cationic species of 1 and 2 were
then examined by theoretical calculations as shown in Figure 6.
In the case of 3, the HOMO corresponds to the HOMO of the
azulene moiety, and the HOMO–1 is formed by the antibonding
interaction between the HOMO–1 of azulene and one of the
doubly degenerated HOMOs of benzene. In addition, the
HOMO–2 of 3, which is localized at the phenyl group,
corresponds to the other degenerated HOMO of benzene. In the
case of 4, the HOMO corresponds to the HOMO of the azulene
moiety, and the HOMO–1 is formed by the antibonding
interaction between the HOMO–1 of azulene and the HOMO–2
of pyridine, whose coefficient is large at the linking position to
azulene. Furthermore, the HOMO–2 and HOMO–3 of 4, which
are localized at the pyridyl group, correspond to the HOMO and
HOMO–1 of pyridine, respectively. If an electron is removed
from the HOMO of a neutral azulene species, the HOMO is
transformed into a SOMO in association with the decrease in its
energy level. In the case of azulene, the SOMO plays the role of
a frontier orbital. However, in the case of 3 or 4, the decrease in
the energies of HOMO–1 and HOMO–2 (3) or HOMO–1,
HOMO–2, and HOMO–3 (4) becomes lower than that of the
HOMO. This results in the SOMO of the oxidized species of 3 or
4 lying lower than the orbitals, which are originally the HOMO–1
and HOMO–2 (3) or HOMO–1, HOMO–2, and HOMO–3 (4) of
the neutral species. This suggests that the reactivity of 3 caused
by radical polymerization decreases as the energy level of the
orbital containing the unpaired electron is buried deeper.
Recently, a radical cation system with a SOMO-HOMO energy
inversion has received attention despite its cause not having
been clarified.[21] This system is believed to propose a new
approach that indicates a similar phenomenon.
CHCI3 solutions (Figure 7). Crystallographic data are listed in
Table S7. The crystals of 1 and 2 were merohedral twin crystals
and exhibited similar unit lattices. A tetrad was found to
crystallographically exist independently in these lattices in both
crystals, giving the P21 chiral space group. The tetrads (A, B, C,
and D) contained comparable bond lengths and angles (Figures
S16 and S17).
X-ray crystallographic analysis
X-ray crystallography of compounds 1 and 2 was carried out
on single crystals obtained from the slow evaporation of their
Figure 7. ORTEP drawings of one of the tetrads of (a) 1 and (b) 2. The
thermal ellipsoids were drawn at the 50% probability level.
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Both 1 and 2 adopted the syn-form with both aryl groups in the
2-position, and the nearest face-to-face distances between
intramolecular phenyl or pyridyl groups ranged from 3.36(1) to
3.39(1) Å for 1 and from 3.33(1) to 3.39(1) Å for 2, which
strongly suggests the presence of π–π interactions.[22] The
dihedral angles between the azulene subunits (Az–Az) ranged
from 48(1) to 53(1)° for 1 and from −51(1) to −52(1)° for 2, while
the dihedral angles between azulene and the aryl rings (Az–Ar)
ranged from 33(1) to 46(1)° for 1 and from −32(1) to −45(1)° for
2, which contrasts to the values obtained for the crystal
structures of 2,2ʹ-bis(pyridine-2-yl)1,1ʹ-biazulene [Az–Az: 86(1)–
88(1)°, Az–Ar: 45(1)–52(1)°], in which a proton acted as a bridge
between the two pyridyl moieties.[13]
40
20
(S)-2
(R)-2
0
–20
–40
300
400
Wavelength / nm
Figure 8. Solid state CD spectra of (R)- and (S)-2.
500
Examination of the crystal packing of 1 and 2 showed that the
molecular dipole of each tetrad was aligned parallel along the
crystallographic b axis (Figure S18, computed dipole moments
of 1 and 2 are µ = 1.515 and 6.444 D, respectively, at the M06-
2X/6-31G** level). The tetrads were stacked three dimensionally
through CH/π interactions in the crystal packing. The twisted
structure of 2 was indicated to be in the (R)-configuration for all
tetrads. Interestingly, X-ray structural analysis of another crystal
indicated the presence of a reverse absolute configuration [(S)-
configuration] of the atropisomer (Figure S19). It was also
revealed that all tetrads of 1 in the unit lattice had the same
configuration, which was similar to the crystal packing of 2,
although the absolute configuration was not determined due to
the fact that 1 possesses only carbon and hydrogen atoms (2
also contains the heavier nitrogen atom). These results therefore
Table 1. Thermodynamic and kinetics data for thermal racemization reaction
of 1 and 2 in chlorobenzene.
ΔG‡
ΔH‡
ΔS‡
temp. rate constant, k half life, t1/2
compd
[kcal mol–1] [kcal mol–1] [cal mol–1 K–1
]
[˚C]
[s-1]
[h]
1
27.0 ± 0.1
26.9 ± 0.1
25.2
25.1
–0.10
–0.10
60
70
80
60
70
80
6.81 × 10-6
2.49 × 10-5
6.29 × 10-5
7.72 × 10-6
2.61 × 10-5
6.82 × 10-5
28.3
7.7
3.1
24.9
7.4
2.8
2
indicate that
1 and 2 undergo spontaneous resolution to
generate the optically active (R)- or (S)-enantiomer upon
crystallization.
Complexation of 2 with PdCl2
CD measurements
Moving on to the fixation and/or expansion of the torsion in 2,
complex formation with PdCl2 was examined. The reactions of 2
with PdCl2(PhCN)2 in 1,1,2,2-tetrachloroethane for 4 h at room
temperature (ca. 25 °C) and at 100 °C were then performed.
Solid state cyclic dichroism (CD) spectral measurements were
then conducted on each crystal exhibiting an (R)- or (S)-
configuration upon X-ray structural analysis. As a result, the
spectra showed a positive-negative and/or a negative-positive
cotton effect at both 330 and 294 nm, as shown in Figures 8 and
S20. The correlation between the absolute configuration and the
CD spectra had therefore been determined. In contrast, the CD
spectra of the mother liquor resulted in the formation of single
crystals suitable for X-ray structural analysis, and no cotton
effect was observed in this case.
An arbitrary crystal was thus dissolved in chlorobenzene to
measure the time course CD spectra. At 25 °C, almost no
change was observed after 1 d, thereby indicating particularly
slow racemization. However, at higher temperatures,
racemization was observed for both 1 and 2, with the half-lives
being estimated as 28.3 and 24.9 h at 60 °C, 7.7 and 7.4 h at
70 °C, and 3.1 and 2.8 h at 80 °C (Table 1 and Figures S21 and
S22). The rotational barriers for the racemization of 1 and 2
were then calculated using the racemization reaction rate
constants at each temperature, giving values of 27.0 and 26.9
kcal/mol, respectively.
1
Although the H NMR spectrum of the reaction mixture obtained
from the reaction at room temperature was complex, a simple
spectrum was obtained for the reaction carried out at 100 °C as
shown in Figure 9. This was attributed to convergence of the
products to the thermodynamically more stable structures at
higher temperatures. After the reaction, the complex was
precipitated by the addition of diethyl ether to the reaction
mixture, and a dark green solid corresponding to the desired
complex was obtained by filtration. 13C NMR measurements
produced 39 signals (Figure S23), which suggests that the
complex contains three different pyridyl-azulene moieties (i.e.,
13 signals each).
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Figure S24 shows the UV-vis absorption spectra of 23•(PdCl2)3,
2, and PdCl2(PhCN)2. Although the shape of the absorption
spectrum of 23•(PdCl2)3 is similar to that of free ligand 2,
redshifts of the absorption maxima were observed from 293, 607,
and 639 nm for 2 to 301, 621, and 655 nm for 23•(PdCl2)3. The
redox properties of 23•(PdCl2)3 were then examined by square
wave voltammetry (SWV) measurements in benzonitrile over a
potential range from –2.3 to 1.5 V vs. Cp2Fe0/+ along with 2 and
PdCl2(PhCN)2, using ferrocene as an internal standard (Figure
11). As a result, complex 23•(PdCl2)3 exhibited three reduction
peaks at −1.56, −1.81, and −2.03 V, in addition to overlapped
combined oxidation peaks, which gave a peak minimum and
shoulders at 0.42, 0.59, 0.81, and 1.18 V vs. Cp2Fe0/+. As no
reduction and oxidation waves were observed for the reference
complex PdCl2(PhCN)2, it was suggested that both the reduction
and oxidation peaks were derived from the azulene moieties.
The observed stepwise redox behavior therefore indicates the
presence of an electronic communication between the azulene
moieties within the complex.
Figure 9. 1H NMR spectra in CDCl3 at 300 K of 2, reaction mixtures of 2 and
PdCl2(PhCN)2, and the Pd complex with 2 obtained after recrystallization.
Recrystallization from PhCl/MeOH gave single crystals
suitable for X-ray crystallography. It was found that 2 formed a
3:3 cyclic structure with palladium chloride, namely [23•(PdCl2)3]
(Figure 10). The three biazulene units were found to cyclize
between the (R) and (S)-forms in a ratio of either 2:1 or 1:2
_
(space group: P1), possessing C2 symmetry overall. Therefore,
the 39 signals in the 13C NMR spectra were revealed to be
derived from three spectroscopically non-equivalent pyridyl-
azulene moieties, rather than the mixture of conformers. The
dihedral angles between the azulene subunits (Az–Az) of 2 were
increased ~9° upon complexation with PdCl2. The average
absolute values of the dihedral angles for Az–Az and Az–Ar
were changed from 52(1)° and 37(1)°, respectively, for 2 to
61(1)° and 41(1)°, respectively, for 23•(PdCl2)3.
Figure 11. SWV curves of 2, PdCl2(PhCN)2, and 23•(PdCl2)3 (1.0 mM) on a Pt
electrode in 1.0 M nBu4NPF6, PhCN. All solutions contain a ferrocene internal
standard (1.0 mM).
Conclusion
We herein reported the synthesis of 2,2'-diaryl-1,1'-biazulenes 1
and 2, where these dimers, in addition to 1,1ʹ-biazulene,
exhibited stepwise reduction and oxidation peaks by redox
measurements, and these were considered to be two one-
electron steps. This result suggests the presence of an
electronic communication between the azulene subunits. In
addition, the first oxidation potential of the dimers exhibited a
clear cathodic shift, which was larger than the change observed
Figure 10. ORTEP drawing of 23•(PdCl2)3. The thermal ellipsoids were drawn
at the 50% probability level.
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1462, 1447, 1398, 757, 728, 689, 581 cm-1; MS (GC): m/z = [M+] calcd
for [C16H11Br]+ 282.00, found 282.01; UV-vis (THF) λmax (ε): 246 (14487),
310 (54241), 363 (6019), 383 (5859), 591 (354), 627 (328) nm; Anal.
Calcd for C16H11Br: C, 67.87; H, 3.92. Found: C, 67.79; H, 3.88.
for the first reduction potential, thereby indicating that the linkage
of azulene at the 1-position increases the HOMO distributed
across the odd-positioned carbon atoms. In addition, CV
measurements of 2-arylazulenes 3 and 4 showed a return peak
toward the oxidization, which was not observed for azulene.
Theoretical calculations suggested that this can be attributed to
stabilization of the first-electron oxidant derived from the SOMO-
HOMO energy inversion phenomenon. X-ray crystallography
revealed that this species possessed a syn-type structure in
which both aryl groups in the 2-positions of 1 and 2 formed π-
stacks. Spontaneous resolution was also revealed. Furthermore,
from the solid CD spectral measurements, the relationship
between the absolute configuration and the CD spectra was
determined. More specifically, when racemization was examined
for solutions of 1 and 2, a racemization rotational barrier of
approximately 27 kcal mol–1 was calculated. It was found that
rotation of the bonding at the 1,1ʹ-position of the azulene
skeleton was inhibited in solution at room temperature,
indicating that each optical isomer existed with relative stability.
Moreover, 2 and PdCl2 cyclized through the formation of a 3:3
complex. The biazulene units cyclized to give ratios between the
(R)- and (S)-forms of 2:1 and 1:2, respectively. Electronic
communication between the azulene moieties in the complex
was also observed by redox measurements. Currently, the
construction of a PdCl2 complex with optically active 2 is
underway in our laboratory. We believe that such systems could
provide a novel design concept for functional electronic and
optical devices, or catalysts, and these could be employed in
various applications such as supramolecular architectures,
medicines, biomimetic receptors, molecular pores, and chiral
recognition agents, among others.
Synthesis of 1-bromo-2-(4-pyridyl)azulene (6)
A solution of N-bromosuccinimide (1.7 mmol) in dichloromethane (40 mL)
was added dropwise to
a stirred solution of 4 (1.7 mmol) in
dichloromethane (65 mL) at –78 °C during ca. 1 h. The resulting solution
was gradually warmed to 0 °C and stirring continued at 0 °C for 1 h, and
then a saturated aqueous solution of sodium hydrogen carbonate was
added to the reaction mixture, which was then extracted with
dichloromethane. The solution was dried over anhydrous sodium sulfate,
filtrated, and concentrated, followed by chromatography on silica gel with
chloroform/acetone (10:1) as the eluent to give the product (88%) as a
green solid. m.p. 135–136 °C; 1H NMR (400 MHz, CDCl3) δ 8.74 (d, J = 5.2 Hz,
2H), 8.50 (d, J = 10.0 Hz, 1H), 8.32 (d, J = 9.6 Hz, 1H), 7.79 (d, J = 5.2 Hz, 2H),
7.68 (dd, J = 9.6, 10.0 Hz, 1H), 7.56 (s, 1H), 7.35 (dd, J = 10.0, 10.0 Hz, 1H), 7.27
(dd, J = 9.6, 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 144.6, 143.6, 139.7,
139.2, 138.0, 137.24, 137.22, 124.7, 124.6, 123.9, 116.6, 102.2; IR (KBr)
νmax 3031, 1596, 1573, 1398, 1217, 992, 791, 740 cm-1; MS (GC): m/z =
[M+] calcd for [C15H10BrN]+ 283.00, found 283.06; UV-vis (THF) λmax (ε):
241 (15600), 299 (57200), 356 (6290), 374 (4190), 599 (390), 637 (390)
nm; Anal. Calcd for C15H10BrN: C, 63.40; H, 3.35; N, 4.93. Found: C,
63.53; H, 3.41; N, 5.06.
Synthesis of 2,2'-diphenyl-1,1'-biazulene (1)
A mixture of 5 (0.33 mmol), bis(1,5-cyclooctadiene)nickel(0) (0.33 mmol),
and 2,2'-bipyridine (0.33 mmol) in THF (44 mL) was heated at 60 °C for 2
d. After cooling to 25 °C followed by the addition of brine, the mixture
was extracted with chloroform. The organic layer was dried over
anhydrous sodium sulfate and filtered, followed by chromatography on
silica gel with hexane/dichloromethane (5:1) as the eluent to give the
product (38%) as bluish green solid. m.p. 230–231 °C; 1H NMR (400 MHz,
CDCl3) δ 8.36 (d, J = 9.6 Hz, 2H), 7.83 (d, J = 9.6 Hz, 2H), 7.67 (s, 2H),
7.48 (dd, J = 9.6, 10.0 Hz, 2H), 7.17 (dd, J = 9.6, 10.0 Hz, 2H), 6.96-7.06
(m, 10H), 6.95 (dd, J = 9.6, 10.4 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ
150.9, 141.0, 139.9, 137.3, 136.9, 136.0, 128.8, 128.0, 127.0, 123.6, 123.6,
123.6, 123.1, 116.9; IR (KBr) νmax 2919, 2359, 1568, 1446, 1397, 1261,
Experimental Section
Synthesis
All manipulations were performed under a nitrogen atmosphere. Solvents
were dried and distilled prior to use. 2-phenylazulene (3)[14a] and 2-(4-
pyridyl)azulene (4)[15] were prepared according to the literature. Other
reagents were obtained commercially and used without purification.
1029, 803, 757, 738, 691, 581 cm-1; MS (GC): m/z = [M + H]+ calcd for
+
[C32H23
]
407.18, found 407.18; UV-vis (THF) λmax (ε): 299 (78200), 597
(650), 633 (620) nm; Anal. Calcd for C32H22: C, 94.55; H, 5.45. Found: C,
94.34; H, 4.62.
Synthesis of 1-bromo-2-phenylazulene (5)
Synthesis of 2,2'-di(pyridin-4-yl)-1,1'-biazulene (2)
A solution of N-bromosuccinimide (1.5 mmol) in dichloromethane (40 mL)
A mixture of 6 (0.36 mmol), bis(1,5-cyclooctadiene)nickel(0) (0.35 mmol),
and 2,2'-bipyridine (0.36 mmol) in THF (45 mL) was heated at 60 °C for 2
d. After cooling to 25 °C followed by the addition of brine, the mixture
was extracted with chloroform. The organic layer was dried over
anhydrous sodium sulfate and filtered, followed by chromatography on
silica gel with chloroform/acetone (2:1) as the eluent to give the product
(60%) as bluish green solid. m.p. 275 °C (decomp.); 1H NMR (400 MHz,
CDCl3) δ 8.46 (d, J = 9.6 Hz, 2H), 8.21 (d, J = 5.2 Hz, 4H), 7.95 (d, J = 10.0 Hz,
2H), 7.68 (s, 2H), 7.62 (dd, J = 9.6, 10.0 Hz, 2H), 7.28 (dd, J = 8.0, 9.6 Hz, 2H),
7.07 (dd, J = 9.6, 9.6 Hz, 2H), 6.78 (d, J = 5.6 Hz, 4H); 13C NMR (100 MHz,
CDCl3) δ 149.4, 147.1, 144.6, 141.0, 139.6, 138.6, 137.8, 137.2, 124.4, 124.2,
123.1, 122.3, 116.9; IR (KBr) νmax 3028, 2925, 1568, 1398, 840, 799, 749,
643 cm-1; MS (GC): m/z = [M + H]+ calcd for [C30H21N2]+ 409.17, found
409.17; UV-vis (THF) λmax (ε): 242 (29500), 293 (78200), 607 (800), 639
was added dropwise to
a stirred solution of 3 (1.5 mmol) in
dichloromethane (100 mL) at –78 °C during ca. 1 h. The resulting
solution was gradually warmed to 0 °C and stirring continued at 0 °C for 1
h, and then a saturated aqueous solution of sodium hydrogen carbonate
was added to the reaction mixture, which was then extracted with
dichloromethane. The solution was dried over anhydrous sodium sulfate,
filtrated, and concentrated, followed by chromatography on silica gel with
hexane/ethyl acetate (20:1) as the eluent to give the product (84%) as a
1
blue solid. m.p. 95–96 °C; H NMR (400 MHz, CDCl3) δ 8.44 (d, J = 9.6 Hz,
1H), 8.24 (d, J = 9.6 Hz, 1H), 7.88 (d, J = 8.0 Hz, 2H), 7.59 (dd, J = 9.6, 10.0
Hz, 1H), 7.51 (s, 1H), 7.50 (dd, J = 7.2, 7.6 Hz, 2H), 7.41 (dd, J = 7.2, 7.6 Hz,
1H), 7.29 (dd, J = 9.6, 10.0 Hz, 1H), 7.20 (dd, J = 9.6, 9.6 Hz, 1H); 13C NMR
(100 MHz, CDCl3) δ 148.3, 139.8, 137.9, 137.2, 136.7, 136.1, 136.0,
129.7, 128.4, 128.1, 124.2, 124.2, 116.7, 102.6; IR (KBr) νmax 3052, 1572,
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(810) nm; Anal. Calcd for C30H20N2: C, 88.21; H, 4.93; N, 6.86. Found: C,
88.13; H, 4.85; N, 6.77.
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Synthesis of 23•(PdCl2)3
A mixture of 2 (0.14 mmol) and bis(benzonitrile)palladium(II)chloride
(0.14 mmol) in 1,1,2,2-tetrachloroethane (21 mL) was heated at 100 °C
for 4 h. After cooling to 25 °C followed by the addition of diethyl ether,
the complex 23•(PdCl2)3 was precipitated as green solid. The precipitate
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1
was collected and dried in vacuo (76%). m.p. 250–251 °C; H NMR (600
MHz, CDCl3) δ 8.60 (d, J = 6.0 Hz, 4H), 8.55 (dd, J = 1.2, 5.4 Hz, 4H), 8.50 (m,
8H), 8.45 (d, J = 9.0 Hz, 2H), 8.23 (d, J = 3.6 Hz, 2H), 8.18 (dd, J = 2.4, 9.6 Hz,
4H), 7.73 (m, 6H), 7.58 (s, 2H), 7.52 (s, 2H), 7.49 (s, 2H), 7.34 (m, 6H), 7.20 (m,
6H), 6.59 (dd, J = 1.2, 1.8, 5.1, 4H), 6.55 (dd, J = 1.2, 1.8, 4.2 Hz, 4H), 6.52 (dd,
J = 1.2, 6.0 Hz, 4H).; 13C NMR (100 MHz, CDCl3) δ 152.61, 152.53, 152.43,
147.22, 147.18, 147.14, 145.89, 145.61, 145.59, 141.549, 141.536, 141.43,
139.54, 139.51, 139.46, 138.69, 138.61 (2C), 138.51 (2C), 138.44, 138.14,
138.08, 138.05, 125.22, 125.10, 124.92, 124.91, 124.86, 124.84, 124.59,
124.48, 124.44, 121.70, 121.55, 121.36, 117.93, 117.61, 117.42.; IR (KBr)
νmax 2913, 2368, 1614, 1575, 1558, 1542, 1508, 1419, 1399, 1066, 845,
806, 741, 581 cm-1; MS (GC): m/z = [M – Cl]+ calcd for [C90H60Cl5N6Pd3]+
1721.04, found 1721.05; UV-vis (THF) λmax (ε): 234 (92300), 300
(188000), 621 (2020), 654 (1950) nm; Anal. Calcd for C90H60Cl6N6Pd3: C,
61.51; H, 3.44; N, 4.78. Found: C, 61.03; H, 3.12; N, 4.57.
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Data of 1 and (S)-2 were collected using a Bruker D8 VENTURE
diffractometer with Cu Kα radiation (λ = 1.54178 Å). Data of (R)-2 were
collected using a Rigaku XtaLAB P200 diffractometer with Cu Kα
radiation (λ = 1.54187 Å). Data of 23•(PdCl2)3 were collected using a
Bruker APEX II CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å).
Single crystals were mounted on MiTeGen Dual-Thickness MicroMounts
using a trace of mineral oil. Frames were collected, reflections were
indexed and processed, and the files were scaled and corrected for
absorption using Bruker APEX3 for 1, (S)-2, and 23•(PdCl2)3 and Rigaku
CrysAlisPro for (R)-2 programs The space groups were assigned, and the
structures were solved by direct methods using XPREP within the
SHELXTL[23] suite of programs and refined by full-matrix least-squares
against F2 with all reflections using SHELXL-2014 (1 and 2) or SHELXL-
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1936303 for 1, CCDC 1936304 for (R)-2, CCDC 1936306 for (S)-2, and
CCDC 1936307 for 23•(PdCl2)3 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
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Entry for the Table of Contents (Please choose one layout)
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2,2'-Diaryl-1,1'-biazulenes were
synthesized and characterized. X-
ray crystallography revealed that the
species possessed a syn-type
structure in which both aryl groups
formed π-stacks. The relationship
between the absolute configuration
and the CD spectra was determined.
A racemization rotational barrier of
~27 kcal mol−1 was achieved.
Moreover, 2,2'-di(pyridin-4-yl)-1,1'-
biazulene formed a 3:3 cyclic
complex with PdCl2. Electronic
communication between the azulene
moieties in the complex was
Prof. Dr. Takahiro Tsuchiya,* Yuka
Katsuoka, Dr. Kenji Yoza, Dr. Hiroyasu
Sato, and Prof. Dr. Yasuhiro Mazaki*
Page No. – Page No.
Stereochemistries, dynamics, and
redox and complexation behaviors of
2,2ʹ-diaryl-1,1ʹ-biazulenes
observed.
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