Dithieno[3,4-b:3',4'-d]arsole: A Novel Class of Hetero[5]radialenes

Article abstract of DOI:10.1002/ejoc.202000442

[5]Radialene is known as an unstable cyclic hydrocarbon with a cross-conjugated system. Incorporation of heteroatoms into [5]radialene skeleton is an effective strategy for stabilization. Herein we synthesized dithieno[3,4-b:3',4'-d]arsole (1) as a novel class of hetero[5]radialenes since trivalent arsenic atom is much more stable than phosphorus one, which was used for hetero[5]radialene but unstable in air. The characteristic nature of the arsa[5]radialene was experimentally and computationally studied by comparing with the isomer, dithieno[3,2-b:2',3'-d]arsole (2). Structural analysis by X-ray crystallography and computational evaluation of aromaticity revealed the radialene character of 1. Interestingly, 1 showed phosphorescence though only fluorescence was observed for 2. Time-dependent density functional theory (TD-DFT) calculations implied that intersystem crossing could readily occur upon excitation for 1. Furthermore, it was computationally elucidated that dimerization and/or oligomerization via Diels-Alder reaction, which convert [5]radialene, were circumvented to offer stability for 1.

Full text of DOI:10.1002/ejoc.202000442

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Dithieno[3,4-b:3’,4’-d]arsole: a Novel Class of Hetero[5]radialenes
Authors: Aya Urushizaki, Takashi Yumura, Yuichi Kitagawa, Yasuchika
Hasegawa, Hiroaki Imoto, and Kensuke Naka
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000442
Link to VoR: https://doi.org/10.1002/ejoc.202000442
01/2020

10.1002/ejoc.202000442
European Journal of Organic Chemistry
FULL PAPER
Dithieno[3,4‑b:3’,4’‑d]arsole: a Novel Class of Hetero[5]radialenes
Aya Urushizaki,[a] Takashi Yumura,[b] Yuichi Kitagawa,[c,d] Yasuchika Hasegawa,[c,d] Hiroaki Imoto,*[a,e]
and Kensuke Naka*[a,e]
Abstract: [5]Radialene is known as an unstable cyclic hydrocarbon
with a cross-conjugated system. Incorporation of heteroatoms into
[5]radialene skeleton is an effective strategy for stabilization. Herein
we synthesized dithieno[3,4‑b:3’,4’‑d]arsole (1) as a novel class of
hetero[5]radialenes since trivalent arsenic atom is much more stable
than phosphorus one, which was used for hetero[5]radialene but
unstable in air. The characteristic nature of the arsa[5]radialene was
experimentally and computationally studied by comparing with the
isomer, [3,2‑b:2’,3’‑d]arsole (2). Structural analysis by X-ray
crystallography and computational evaluation of aromaticity revealed
positions to which heteroatoms are substituted: 1) around the
sp2 carbons, and 2) on the ring (Figure 1). The first examples of
isolated [5]radialene derivatives are 1,3-dithiol-2-ylidene (DT)-
substituted ones, and the electron-donating DT units give unique
redox behavior.[6]
As examples of the on-the-ring strategy, some kinds of
heterocycle-based radialenes have been reported (Figure 2).
Dithieno[3,4‑b:3’,4’‑d]thiophene (DTT) is one of the most typical
hetero[5]radialene backbones (Figure 2a).[7] Reactivity and
optoelectronic properties of DDT derivatives have been studied,
and polymeric and cyclic architectures of DDT have been
constructed for further modification. Besides sulfur, phosphorus
has in particular contributed the chemistry of radialene;
phospha[n]radialenes (n = 3, 4, and 5) have been synthesized
so far.[8,9] Imahori and Higashino reported the first
phospha[5]radialene derivative, phosphole[2,3-c:4,5-c’]dipyrrole
(PDP), in which 3,3’-bipyrrole is bridged by a phosphorus atom
(Figure 2b).[9a] However, the phosphorus atom is readily oxidized
under ambient condition, and thus the phosphorus atom should
be protected by coordination to gold(I) chloride, resulting in
losing the bare lone pair. Although dithieno[3,4-b:3’,4’-
d]phosphole derivatives were recently reported as well, their
phosphorus atoms were sulfated to be stable pentavalent
state.[9b]
the radialene character of 1. Interestingly,
1
showed
phosphorescence though only fluorescence was observed for 2.
Time-dependent density functional theory (TD-DFT) calculations
implied that intersystem crossing could readily occur upon excitation
for 1. Furthermore, it was computationally elucidated that
dimerization and/or oligomerization via Diels-Alder reaction, which
convert [5]radialene, were circumvented to offer stability for 1.
Introduction
Radialenes are cyclic hydrocarbons containing only sp2-
hybridized carbons.[1] [5]Radialene, C10H10, is much less stable
than [3]-, [4]-, and [6]radialenes. Actually, Sherburn and Paddon-
Row reported the first synthesis of [5]radialene in 2015,[2]
whereas those of [3]-, [4]-, and [6]radialenes were achieved in
1960s-1970s.[3-5] Their computational study revealed that
[5]radialene
is
readily
converted
via
Diels-Alder
dimerization/polymerization. Introduction of heteroatoms into
[5]radialene backbone is an effective strategy to attain high
stability and functional diversity. There are mainly two kinds of
Figure 1. Strategies for heteroatom-substituted [5]radialenes.
[a]
A. Urushizaki, Dr. H. Imoto*, Prof. Dr. K. Naka*
Faculty of Molecular Chemistry and Engineering, Graduate School
of Science and Technology, Kyoto Institute of Technology
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
E-mail: himoto@kit.ac.jp (H.I.); kenaka@kit.ac.jp (K.N.)
Homepage: http://www.cis.kit.ac.jp/~kenaka/index_eng.html
Prof. Dr. T. Yumura
Faculty of Material Science and Technology Graduate School of
Science and Technology, Kyoto Institute of Technology
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
Dr. Y. Kitagawa, Prof. Dr. Y. Hasegawa
Herein dithieno[3,4‑b:3’,4’‑d]arsole (1) was designed (Figure
2c) because trivalent arsenic atom has high oxidative resistance
in air compared with phosphorus one. Actually, we previously
reported that dithieno[3,2‑b:2’,3’‑d]arsole (2), which is an isomer
of 1, is stable enough to be treated under ambient condition
(Figure 2d).[10] The structure, electronic properties, and stability
of 1 were experimentally and computationally studied in this
work.
[b]
[c]
Faculty of Engineering, Hokkaido University,
Kita-13 Jo, Nishi-8 Chome, Kita-ku, Sapporo, Hokkaido 060-8628
(Japan)
[d]
[e]
Institute for Chemical Reaction Design and Discovery (WPI-
ICReDD), Hokkaido University
Sapporo, Hokkaido 001-0021 (Japan)
Materials Innovation Lab, Kyoto Institute of Technology,
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, (Japan)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.202000442
European Journal of Organic Chemistry
FULL PAPER
Figure 2. Chemical structures of (a-c) hetero[5]radialenes (DDT, PDP, and 1),
and (d) 2 (isomer of 1).
Results and Discussion
Synthetic procedure of 1 is summarized in Scheme 1. The key
intermediate for the synthesis is diiodophenylarsine, which can
be generated from hexaphenylcyclohexaarsine.[11] This strategy
allowed us to circumvent usage of volatile and toxic arsenic
intermediates such as phenylarsine and dichlorophenylarsine,
which are required for traditional synthetic routes toward
organoarsenic compounds.[12] The THF solution of the generated
diiodophenylarsine was employed for the reaction with 4,4’-
dilithio-3.3’-bithiophene, which was given from the dibromide
precursor. The product was isolated by alumina column
chromatography and recrystallization from dichloromethane and
methanol. It is notable that solid sample of 1 was highly stable;
no change in the NMR spectrum was observed even after
leaving for more than a month under ambient condition.
Figure 3. ORTEP drawings and selected bond lengths of 1 and 2. The data of
2 were cited from reference 10a. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level.
Aromaticity of 1 and 2 was estimated by nucleus-independent
chemical shift (NICS) value (Figure 4).[13] In addition,
[5]radialene and benzene were also examined as references.
The geometries of these compounds were determined based on
full optimization by DFT calculation at the B3LYP/6-31G+(d,p)
level of theory. The NICS values were calculated at GIAO-
B3LYP/6-311G+(d,p).[14] The NICS(1)zz values of 1 and 2 were
around zero (4.5 and 3.7 ppm) likely to [5]radialene (-2.3 ppm),
whereas that of benzene was -29.1 ppm. This means that the
central five-membered rings of 1, 2, and [5]radialene are non-
aromatic. The shape of their NICS scans showed non-aromatic
character of 1, 2, and [5]radialene as well, considering the result
of benzene.
Scheme 1. Synthesis of dithieno[3,4‑b:3’,4’‑d]arsole 1.
A single crystal of 1 suitable for X-ray diffraction analysis
(Figure 3a) was fabricated by recrystallization from chloroform
and hexane. The C−C bond lengths of the arsole ring were
1.427(3), 1.435(3), and 1.463(3) Å, and those of the As−C ones
were 1.947(2) and 1.960(2) Å, meaning that the five membered
ring is composed of C−C and As−C single bonds. On the other
hand, the other C−C bonds of the two thiophene rings had
double bond character; their bond lengths were 1.379(3),
1.365(3), 1.365(3), and 1.352(5) Å. These results indicated that
1 is likely to adopt [5]radialene structure. In contrast, the results
of 2 that we previously reported showed that the arsole ring is
likely to cyclopentadiene (Figure 3b); two C−C double bonds
(1.375(5) and 1.380(5) Å), two As−C single bonds (1.951(4) and
1.956(4) Å), and one C−C single bond (1.440(5) Å).[10]
Figure 4. NICS scan plots and NICS(1)zz values of 1, 2, [5]radialene, and
benzene.
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10.1002/ejoc.202000442
European Journal of Organic Chemistry
FULL PAPER
Table 1. Optical properties of 1 and 2.[a]
[b]
[d,e]
[d,f]
λabs
ε[c]
λex
λem
Φ[g]
τ[d,h]
[nm]
[L·mol-1·cm-1]
[nm]
[nm]
260
(298)
313
(488)
0.47 ns
(145 ms)
1
2
299
11262
8463
0.04
0.09
331
(335)
405
(394)
334
0.50 ns
[a] Measured in 2-methyltetrahydrofran (1.0
×
10-4 mol/L). [b] Longest
absorption maxima. [c] Molar extinction coefficients for the longest absorption
maxima. [d] Measured at room temperature (bare) and 77 K (in parentheses).
[e] Excitation maxima monitored at the emission maxima. [f] Emission maxima
excited at the excitation maxima. [g] Absolute quantum yield. [h] Emission
lifetimes monitored at the emission maxima.
The optical properties of 1 and 2 are summarized in Table 1.
The UV-vis absorption spectra were measured in 2-
methyltetrahydrofran (2-MeTHF) solution (c = 1.0 × 10-4 mol/L).
DFT calculation (B3LYP/6-31G+(d,p)) was conducted to
understand the relatively short wavelength of the longest
absorption maximum (λabs) of 1 (λabs = 299 nm (1), 334 nm (2))
The highest occupied molecular orbitals (HOMOs) and lowest
unoccupied molecular orbitals (LUMOs) of 1 and 2 were
evaluated based on the full-optimized structures (Figure 5). The
calculated HOMO and LUMO levels well corresponding to those
estimated by cyclic voltammetry (Table S3). The lower HOMO
Figure 5. Energy levels of frontier orbitals of 1 and 2 estimated by DFT
calculations. Electron transition energies from the HOMO to the LUMO (E)
and the oscillator strengths (f) were estimated by TD-DFT calculations.
The photoluminescence (PL) spectra of 1 and 2 were
measured in solutions (c = 1.0 × 10-4 mol/L in 2-MeTHF) under
N2 atmosphere (Figure 6). Both of 1 and 2 emitted fluorescence
at room temperature with short emission lifetimes (τ) and
moderate quantum yields (Φ); τ = 0.47 ns (1), 0.50 ns (2), and Φ
= 0.04 (1), 0.09 (2). On the other hand, emission behaviours of 1
and 2 at 77 K were drastically different. The longest emission
maximum (λem) of 1 at 77 K (λem = 488 nm) was much red-shifted
from that at room temperature (λem = 313 nm), whereas little
difference was observed in the case of 2 (λem = 405 nm (room
temperature), 394 nm (77 K)). Lifetime measurement strongly
supports the idea that the longer wavelength emission of 1 at 77
K was phosphorescence (τ = 145 ms). The photophysical
properties of 1 in the solid state agreed with those in a solution
(Figure S4); 1 exhibited only fluorescence at room temperature
while phosphorescence was observed at 77 K.
and higher LUMO levels of
1 indicated that the cross-
conjugation system of the 3,3’-bithiophene moiety less
expanded the conjugation length than the 2,2’-bithiophene
moiety of 2. Time-dependent-DFT (TD-DFT) calculations
evaluated the transition energies (ΔE) and oscillator strengths
(f) the HOMO-LUMO transitions. The results of the calculations
revealed that the HOMO-LUMO transitions of 1 and 2 were
allowed (f = 0.1388 and 0.2045, respectively), and that the
transition energies (ΔE = 285.83 nm (1) and 339.94 nm (2))
corresponded to the λabs values.
Figure 6. PL spectra (c = 1.0 × 10-4 mol/L in 2-MeTHF) of (a) 1 and (b) 2 at
room temperature (blue) and 77 K (red).
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10.1002/ejoc.202000442
European Journal of Organic Chemistry
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The excited states of 1 and 2 were evaluated by TD-DFT
calculations based on the fully optimized geometries in their
ground states (B3LYP/6-31G(d) for C, H, S and SDD for As).
The energy levels against the ground states of the singlet (Sn)
and triplet (Tn) excited states were listed in Figure 7. In the case
of 1, the energy level of the S1 state (4.51 eV) was higher than
those of T1, T2, T3, T4, T5, and T6 (2.85, 3.46, 3.69, 3.92, 4.39,
4.51 eV, respectively),[15] indicating that the intersystem crossing
was likely to occur. On the other hand, among the triplet excited
states of 2, only the energy level of the T1 state (2.37 eV) was
lower than that of the S1 state (3.78 eV).[15] The energy gap
between S1 and T1 was quite large, and these electron
structures were the same; they were generated from the HOMO-
LUMO transitions (the occupancies were 97% (S1) and 96%
(T1)). It is thus evident that intersystem crossing of 2 is hard to
occur. These results can explain why
1
showed
phosphorescence while 2 showed only fluorescence.
Figure 8. Energetics of Diels-Alder dimerization of 1.
Conclusions
We have synthesized a new hetero[5]radialene derivative,
dithieno[3,4‑b:3’,4’‑d]arsole 1, which is an isomer of
dithieno[3,2‑b:2’,3’‑d]arsole 2. The obtained sample of 1 was
highly stable under ambient conidtion thanks to the air-stable
trivalent arsenic atom. The single crystal X-ray analysis revealed
that the thiophene-fused arsole ring of 1 adopts a characteristic
strucure of [5]radialene unlikely to 2. Optical measurements and
DFT calculations well exhibited the difference between 1 and 2
Figure 7. Energy levels of (a) 1 and (b) 2 in the excited states.
in
electronic
properties.
Interestingly,
1
showed
phosphorescence at 77 K whereas only fluorescence was
observed in the case of 2. The new class of arsenic heterocycle
will contribute to the advancement of heteroatom and π-
conjugated chemistries.
Finally, B3LYP calculations were also employed to investigate
stability of 1, compared with [5]radialene. As mentioned above,
[5]radialene displayed extraordinarily high reactivity toward
Diels-Alder dimerization/polymerization.[2] This analysis focuses
on two types of reaction path on Diels-Alder dimerization of 1.
B3LYP calculations obtained optimized geometries for local
minima and the transition states connecting the two local minima, Experimental Section
which are displayed in Figure 8. DFT calculations obtained the
transition states where a double bond in a thiophene ring of the
below molecule reacts with the diene moiety in a thiophene ring
of the above molecule. Both reaction paths require the activation
energy around 45 kcal/mol, whose values are larger than that in
Diels-Alder dimerization of [5]radialene (5.3 kcal/mol), reported
in the previous paper.[2] These higher activation energies are
due to significant disrupting of the planarity of thiophene-fused
arsole moiety in the transition states. This reaction analyses
indicate that 1 is stable due to its low degree of reactivity toward
Diels-Alder dimerization in contrast to [5]radialene.
Materials
n-Butyllithium (n-BuLi, 1.6ꢀM in hexane) was purchased from Sigma–
Aldrich (Hattiesburg, Mississippi, US). Iodine (I2), tetrahydrofuran (THF,
super dehydrated grade), dichloromethane (CH2Cl2), methanol (MeOH),
hexane, and sodium sulfate (Na2SO4) were purchased from Nacalai
Tesque (Kyoto, Japan). Distilled water was purchased from Wako Pure
Chemical Industry (Osaka, Japan). Hexaphenylcyclohexaarsine,[16] 4,4’-
dibromo-3,3’-bithiophene,[17] and dithieno[3,2‑b:2’,3’‑d]arsole (2)[10a] were
prepared according to literature procedures.
Measurement
1H (400 MHz) and 13C (100 MHz) nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker AVANCE III 400 NMR spectrometer.
High resolution mass spectra (HRMS) were obtained on a JEOL JMS-
SX102A spectrometer. UV/Vis spectra were recorded on a JASCO
spectrophotometer V-670 KNN. Emission and excitation spectra were
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10.1002/ejoc.202000442
European Journal of Organic Chemistry
FULL PAPER
obtained on an FP-8500 (JASCO) spectrometer and the absolute PL
quantum yields (Φ) were determined by using a JASCO ILFC-847S; the
quantum yield of quinine sulfate as reference was 0.52, which is in
agreement with the literature value.[18] Emission lifetimes were measured
by using a Quantaurus-Tau (Hamamatsu Photonics, Shizuoka, Japan)
instrument.
and the mixture was stirred for 2 h. The reaction was quenched by the
addition of distilled water, and the organic layers were extracted with
CH2Cl2. The combined organic layers were dried over Na2SO4, and after
filtration, the volatiles were removed in vacuo. The crude product was
purified by column chromatography on alumina (eluent: hexane). The
solvents were removed in vacuo and the residue was subjected to
recrystallization (slow mixing of a solution of the product in CH2Cl2 with
MeOH at ambient temperature) to obtain 1 as colorless crystals (0.17 g,
0.54 mmol, 30ꢀ%). 1H NMR (acetone-d6, 400 MHz): δ = 7.70 (s, 4ꢀH),
7.39–7.37 (m, 2ꢀH), 7.26–7.24 ppm (m, 3ꢀH); 13C{1H} NMR (acetone-d6,
100 MHz): δ = 148.2, 143.8, 141.4, 131.3, 128.6, 128.3, 126.4, 130.3,
115.6 ppm; HR-FAB-MS: m/z calcd for C14H9AsS2: 315.9362 [M]+; found:
315.9357.
X-ray crystallographic data for single crystalline products
The single crystal was mounted on a nylon loop. Intensity data were
collected at room temperature on a Rigaku XtaLAB mini with graphite-
monochromated MoKα radiation. The readout was performed in the 0.073
mm pixel mode. The data were collected at room temperature to a
maximum 2θ value of 55.0°. Data were processed by Crystal Clear.[19] An
empirical absorption correction[20] was applied. The data were corrected
for Lorentz and polarization effects. The structure was solved by direct
methods[21] and expanded by using Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
refined by using the riding model. The final cycle of the full-matrix least-
squares refinement on F2 was based on observed reflections and
variable parameters. All calculations were performed by using the
CrystalStructure[21] crystallographic software package except for the
refinement, which was performed by using SHELXL2013.[22] Crystal data
and more information on the X-ray data collection are summarized in
Tables S1 and S2. CCDC 1993056 (1) contains the supplementary
crystallographic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
Acknowledgments
The work in KIT was supported by JSPS KAKENHI, grant
Number 19H04577 (Coordination Asymmetry), to HI as well as
by a Grant-in-Aid for Scientific Research (C) (No. 18K04864)
from JSPS for TY. The work in Hokkaido was supported by a
Grant-in-Aid for grant numbers 17K14467, 19H04556,
18H04497, and 18H02041.
Keywords: hetero[5]radialene • arsole • DFT calculation •
aromaticity • phosphorescence
Cyclic voltammetric (CV) analyses
CV analyses were carried out on an ECstat-300 (EC Frontier, Inc.)
Potentio/Galvanostat Analyzer at a scan rate of 100 mV s−1. All the
[1]
[2]
G. Maas in Cross Conjugation: Modern Dendralene, Radialene and
Fulvene Chemistry (Eds.: H. Hopf, M. S. Sherburn), Wiley-VCH,
Weinheim, 2016, pp. 79-116.
measurements were performed in THF containing 0.1
M
tetrabutylammonium hexafluorophosphate at ambient temperature using
a three-electrode system, with each solution being purged with N2 prior to
measurement. The working electrode was a glassy carbon, the counter
electrode was a platinum wire, and the reference electrode was an
Ag0/Ag+ electrode.
E. G. Mackay, C. G. Newton, H. Toombs-Ruane, E. J. Lindeboom, T.
Fallon, A. C. Willis, M. N. Paddon-Row, M. S. Sherburn, J. Am. Chem.
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Computational detail
DFT calculations were carried out to investigate the frontier orbitals and
the NICS values. In addition, the HOMO–LUMO transition energies (ΔE),
their oscillator strengths (f), and the energy levels of the singlet (Sn) and
triplet (Tn) excited states were estimated by TD-DFT calculations. The
calculations of the frontier orbital energies and the electron transition
energies employed B3LYP/6-31G+(d,p) set combinations by using the
Gaussian 16 program package.[14] The NICS values were calculated at
the GIAO-B3LYP/6-311G+(d,p) level of theory. The Sn and Tn states
were calculated at the B3LYP/6-31G(d) for C, H, S and SDD for As. For
analyses on Diels-Alder dimerization of 1, B3LYP/6-31G(d,p) calculations
were employed to obtain local minima and transition states in the
reaction. Harmonic vibrational frequencies were systematically computed
to confirm that each optimized geometry corresponds to a local minimum
that has only real frequencies or a saddle point that has only one
imaginary frequency. Lengths of bonds in B3LYP-optimized structure of 1
are fully consistent with those obtained from X-ray analyses (Figure 3),
indicating this level of theory is accurate enough to study properties of
hetero[5]radialene species.
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Synthesis
[10] (a) T. Kato, H. Imoto, S. Tanaka, M. Ishidoshiro, K. Naka, Dalton Trans.
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4-Phenyldithieno[3,4-b:3’,4’-d]arsole (1): To a solution of 4,4’-dibromo-
3,3’-bithiophene (0.59 g, 1.8 mmol) in THF (14 mL), a solution of n-BuLi
(1.6ꢀM, 1.0 mL, 1.6 mmol) in hexane was added slowly with a syringe at
−100 °C under N2 atmosphere, and the mixture was stirred for 1 h. To the
solution, a mixture of hexaphenylcyclohexaarsine (0.34 g, 0.38 mmol)
and I2 (0.56 g, 2.2 mmol) in THF (26 mL) was added slowly at −100 °C
[11] T, Kato, S. Tanaka, K. Naka, Chem. Lett. 2015, 44, 1476.
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Entry for the Table of Contents
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Arsenic hetero[5]radialene
A. Urushizaki, T. Yumura, Y. Kitagawa,
Y. Hasegawa, H. Imoto,* and K. Naka*
Page No. – Page No.
Dithieno[3,4‑b:3’,4’‑d]arsole: a Novel
Arsenic-substituted [5]radialene has been developed as a stabile [5]radialene
derivative. Its structure, optical and electronic properties, and stability have been
experimentally and computationally studied here.
Class of Hetero[5]radialenes
*one or two words that highlight the emphasis of the paper or the field of the study
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