Practical Synthesis and Properties of 2,5-Diarylarsoles

Article abstract of DOI:10.1021/acs.orglett.5b02416

2,5-Diarylarsoles were easily synthesized from nonvolatile arsenic precursors. Diiodoarsine was generated in situ and reacted with titanacyclopentadienes to give 2,5-diarylarsoles. The structures and optical properties were studied in comparison with those of 2,5-diarylphosphole. It was found that the arsoles were much more stable in the air than the phosphole. Single crystal X-ray diffraction revealed the arsenic atoms adopted a trigonal pyramidal structure, reflecting on the s-character of the lone pair. The obtained 2,5-diarylarsoles and 2,5-diarylphosphole showed intense emission in solutions and solid state. In addition, the optical properties were controlled by transition-metal coordination.

Full text of DOI:10.1021/acs.orglett.5b02416

Letter
pubs.acs.org/OrgLett
Practical Synthesis and Properties of 2,5-Diarylarsoles
Makoto Ishidoshiro,† Yoshimasa Matsumura,‡ Hiroaki Imoto,† Yasuyuki Irie,† Takuji Kato,† Seiji Watase,§
Kimihiro Matsukawa,§ Shinsuke Inagi,‡ Ikuyoshi Tomita,‡ and Kensuke 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
‡Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta-cho 4259-G1-9, Midori-ku,
Yokohama 226-8502, Japan
§Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
S
* Supporting Information
ABSTRACT: 2,5-Diarylarsoles were easily synthesized from
nonvolatile arsenic precursors. Diiodoarsine was generated in
situ and reacted with titanacyclopentadienes to give 2,5-
diarylarsoles. The structures and optical properties were
studied in comparison with those of 2,5-diarylphosphole. It
was found that the arsoles were much more stable in the air
than the phosphole. Single crystal X-ray diffraction revealed
the arsenic atoms adopted a trigonal pyramidal structure,
reflecting on the s-character of the lone pair. The obtained 2,5-
diarylarsoles and 2,5-diarylphosphole showed intense emission
in solutions and solid state. In addition, the optical properties
were controlled by transition-metal coordination.
Scheme 1. As−C Bond Formation via in Situ Iodination of
ive-membered heterocyclopentadienes “heteroles” have
Fbeen extensively studied to date. They have been utilized
in widespread fields from catalyst to optical and electrical
materials. Various kinds of heteroles have been reported:
borole,1 pyrrole,2 furan,3 and analogues having heavier main
group elements.4,5 Among them, arsole is an attractive analogue
because the unique electronic properties are predicted by
theoretical studies.6 Especially, arsole has low aromaticity likely
to phosphole,5a,6b which has been widely studied for nonlinear
optical materials, light emitting materials, and conductive
materials.7−10 In addition, arsole is suitable for practical
application under oxygen environments because it is highly
tolerant to oxygen in comparison with the air-sensitive
phosphole.11
Despite the advantage, however, the chemistry of arsole has
been highly regulated by the fact that practical synthesis of
arsole derivatives remains to be achieved. There is serious
concern for the toxicity of volatile arsenic precursors. For
example, volatile organoarsines such as phenylarsine and
dichlorophenylarsine were employed to obtain arsole deriva-
tives in previous papers.12
Organoarsenic Homocycles13
is a strong tool for As−C bond formation toward the
construction of various arsenic compounds.
Herein, we investigated the synthesis of arsole derivatives by
using the in situ iodination of an organoarsenic homocycle. The
generated diiodoarsine was reacted with titanacyclopentadienes.
Low-valent titanium complexes can regioselectively form
titanacycles with terminal alkynes,14 and thus, the titana-
cyclopentadienes are effective for construction of heterole
skeletons such as thiophene, selenophene, and phosphole. For
example, heterole containing polymers were synthesized by
utilizing reactive polymers having titanacyclopentadiene moiety
in the main chain.15 In this work, diiodophenylarsine and 2,5-
diaryltitanacyclopentadienes were employed to synthesize 2,5-
diarylarsines. Structures, optical properties, and coordination
behavior were examined utilizing 2,5-diarylphosphole for
comparison to explore the intrinsic properties of 2,5-diary-
larsoles. This is the first study on the practical synthesis and
Recently, we have reported the methodology by which
diiodoarsines are generated in situ from nonvolatile organo-
arsenic homocycles (Scheme 1).13 Safely prepared diiodoar-
sines were converted to 9-arsafluorenes. The synthetic
procedure of the in situ iodination of organoarsenic homocycles
includes only mixing an organoarsenic homocycle and iodine,
and diiodoarsine is generated quantitatively. The resultant
solution can be used for the next reaction. The synthetic route
Received: August 21, 2015
© XXXX American Chemical Society
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optical properties of arsole derivatives, to the best of our
knowledge.
Synthesis of 2,5-diaryl-1-phenyl-arsoles 5 and 1,2,5-triphe-
nylphosphole 6a is shown in Scheme 2. Reaction of
phenyl groups was 6.3° (Table S5), and the total angles around
the phosphorus atom was 301.7°. This is because As-lone pair
exhibits a high s-character in comparison to those of N and P.5a
In the case of 5b, the molecular planarity was distorted by the
o-methoxy groups, and the average interplanar angle of the
heterole ring and the phenyl groups was 43.5° (Table S4). The
oxygen atoms of 5b should be interacted to the arsenic atom
because the distance of As−O was 3.19 Å, which was within the
total van der Waals radii of arsenic (1.85 Å) and oxygen (1.52
Å).17
Scheme 2. Synthesis of Arsoles 5 and Phosphole 6a
The highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) were
estimated by density functional theory (DFT) calculation
with the Gaussian 09 suite program at the B3LYP/6-31G+(d,p)
level of theory (Figure S26).18 In all cases, HOMOs were
located at diphenyl butadiene moieties, and LUMOs were
delocalized across the entirety of the molecules. The energy
levels of HOMOs and LUMOs were estimated by cyclic
voltammetry (CV) in tetrahydrofuran solutions (Figures S23−
S25 and Table S9). The results of 5a were similar to those of
6a. On the other hand, those of 5b were raised by the electron-
donating methoxy groups.
phenylacetylene derivatives 3 with a low-valent titanium(II)
complex gave titanacyclopentadiene derivatives 4. A diethyl
ether (Et2O) solution of iodine was added to an Et2O
dispersion of hexaphenylcyclohexaarsine 116 for in situ
preparation of diiodophenylarsine 2, and subsequently the
solution of 2 was added to the solutions of 4 to obtain arsoles 5.
Instead of 2, reaction of 4a with dichlorophenylphosphine gave
1,2,5-triphenylphosphole 6a. The chemical structures were
confirmed by 1H and 13C NMR, high resolution mass
spectroscopy, and single crystal X-ray diffraction.
Absorption spectra and photoluminescence (PL) spectra
were measured in chloroform solutions under air conditions
(Table 1). The absorption and emission wavelengths of 5b
Table 1. Optical Properties of the Products
a
in solution
solid state
b
c
d
c
d
product
λabs [nm]
λem [nm]
Φ
λem [nm]
Φ
5a
5b
6a
374
395
375
458
476
465
0.59
0.49
0.56
482
485
0.21
0.24
0.39
The oxidation resistance tests were carried out to evaluate
the air stability of the products. Chloroform solutions of 5a and
6a were aerated to drying air by bubbling at 25 °C for 3 h. The
extent of the oxidation was estimated by the signal intensity of
504
a
b
Measured in chloroform solutions under air condition. Absorption
c
maxima. Emission maxima (excited at absorption maxima (in
solution) or 350 nm (solid state)). Absolute quantum yields.
1
d
the H NMR spectra (Figure S13). Despite such mild test
conditions, 12% of air-sensitive 6a was oxidized. In the case of
5a, less than 2% of it was oxidized, confirming that arsole is
more stable in an oxidative atmosphere than phosphole.
Structural analyses for the resulting heteroles were conducted
by single crystal X-ray diffraction. The representative
perspective drawing of 5a is shown in Figure 1. The average
were red-shifted from those of 5a, reflecting on the narrower
HOMO−LUMO gap estimated by CV analysis (Table S9).
The absorption and PL properties of 5a were quite similar to
those of 6a, well corresponding to the CV analysis result and
the previous literatures.6 The lifetimes of the emission were
measured; τ1/2 = 2.92 ns (5a), 1.76 ns (5b), and 2.06 ns (6a).
This result implies that the emission is fluorescence (Figures
S20−S22, Table S8).
Notably, all the obtained compounds showed intense
greenish blue emission in the solid states (Figure 2a). For an
understanding of the solid state emission, we investigated the
packing structures in the crystalline states. As a representative
example, the packing structure of 5a is shown in Figure 2b and
2c. The planar 2,5-diphenylheterole moieties avoided cofacial
π−π interactions, which could cause concentration self-
quenching, by the bulky phenyl group substituted on the
arsenic atom. In the cases of 5b and 6a, cofacial π−π
interactions were also not observed in the crystalline states.
This is why 5a, 5b, and 6a exhibited intense emission even in
the solid states.
We demonstrated complex formation with Au(I)Cl for
emission color tuning. Solutions of Au(I)Cl species and each
heterole were stirred at room temperature, and after
evaporation, recrystallization from dichloromethane and
methanol gave gold complexes (Scheme 3). Gold(I) complexes
5−AuCl and 6−AuCl were synthesized from Au(I)Cl and
Figure 1. Results of single crystal X-ray diffraction analysis of 5a. (a)
Front view and (b) top view with ellipsoids at 50% probability.
Hydrogen atoms are omitted for clarity.
interplanar angle of the heterole ring and the phenyl groups was
8.3° (Table S3), suggesting that the 2,5-diphenylheterole
moiety was highly planar. The bond angles of C(1)−As(1)−
C(4), C(1)−As(1)−C(17), and C(4)−As(1)−C(17) of 5a
were 87.0°, 101.7°, and 101.8°, respectively, and the total
angles around the arsenic atom was 290.5°. This result means
that the trivalent arsenic atom has a trigonal pyramidal
structure, differing from trigonal planar pyrrole. In the case of
6a, the average interplanar angle of the heterole ring and the
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Table 2. Optical Properties of AuCl Complexes
a
in solution
solid state
b
c
d
c
d
ligand
λabs [nm]
λem [nm]
Φ
λem [nm]
Φ
5a
5b
6a
383
411
391
476
498
498
0.86
0.77
0.81
498
522
0.17
0.13
0.25
508
a
b
Measured in chloroform solutions under air conditions. Absorption
c
maxima. Emission maxima (excited at absorption maxima (in
solution) or 350 nm (solid state)). Absolute quantum yields.
d
conjugation than the As-lone pair. Actually, a previous paper
reported that the aromatic stabilization energy (ASE) and
nucleus independent chemical shift (NICS) of phosphole were
3.20 kcal/mol and −5.43, respectively, while the ASE and NICS
of arsole were 1.71 kcal/mol and −3.93, respectively.6b Even
though there is a minor difference, the emission color of arsole
can be controlled by metal coordination as several papers
reported about phosphole.10b,c,20 The quantum yields of the
gold complexes were relatively high in solutions (Φ = 0.77−
0.86), and those in solid states were moderate (Φ = 0.13−
0.25). The quantum yields in the solid states were slightly
lowered after coordination because partial overlap of the π-
conjugated systems was observed in the packing structure
(Figure S14). Arsoles can serve as ligands for intensely emissive
transition metal complexes.
Figure 2. (a) Photographs of solid state emissions under UV
irradiation at 365 nm. Packing structures of 5a: (b) top view and (c)
side view. Hydrogen atoms are omitted for clarity.
Scheme 3. Syntheses of Gold Complexes
In conclusion, 2,5-diarylarsoles were readily synthesized from
nonvolatile arsenic precursors by employing the in situ
iodination of an organoarsenic homocycle. Titanacyclopenta-
dienes worked as excellent nucleophiles to construct the arsole
skeleton. The arsenic atoms in 5 adopted a trigonal pyramidal
structure because of the relatively high s-character of the lone
pair. Compounds 5 showed intense emission in solutions and
solid states. The optical properties of 5a were similar to those
of 6, as theoretically studied. The emission color of 5 was
controlled by coordination to Au(I)Cl, and after coordination,
the complexes had high quantum yields (up to 0.86 in
solutions). These results imply that arsole is a potential
candidate for air-stable and intensely emissive materials. Arsole
containing π-conjugated polymer materials based on the
synthetic strategy proposed in this work will be reported soon.
[Au(I)Cl(tetrahydrothiophene)], respectively.19 Their chemical
structures were confirmed by NMR spectroscopy and single
crystal X-ray diffraction. The 1H NMR spectra of these
complexes were shifted downfield in comparison to bare
ligands because electron density of the ligands was lowered by
the coordination. This result means that the gold complexes
were stable in solutions.
Single crystals suitable for X-ray diffraction analysis were
obtained in the case of 5a−AuCl and 6a−AuCl. As shown in
Figure 3a, 5a formed monodentate coordination to the gold
ASSOCIATED CONTENT
* Supporting Information
■
S
and X-ray crystallographic data. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.orglett.5b02416.
Detailed experimental procedures, spectral data of
products (PDF)
Figure 3. (a) Results of single crystal X-ray diffraction analysis of 5a−
AuCl with ellipsoids at 50% probability. Hydrogen atoms are omitted
for clarity. (b) PL spectra measured in chloroform under air condition
before (dash lines) and after (solid lines) the coordination.
X-ray crystallographic data for 5a (CIF)
X-ray crystallographic data for 5a−AuCl (CIF)
X-ray crystallographic data for 5b (CIF)
X-ray crystallographic data for 6a (CIF)
X-ray crystallographic data for 6a−AuCl (CIF)
center. The UV−vis and PL spectra of 5−AuCl and 6−AuCl
were measured (Figure 3b and Table 2). The solutions of 5−
AuCl and 6−AuCl showed green emission, while greenish blue
emission was observed before the coordination. The emission
maxima of 5a−AuCl, 5b−AuCl, and 6a−AuCl were 476, 498,
and 498 nm, respectively, and the red shift caused by the
coordination were 49, 53, and 65 nm, respectively. The effect of
coordination of 6a was a little more profound than those of 5
because the P-lone pair has a larger contribution to π-
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: kenaka@kit.ac.jp.
Notes
The authors declare no competing financial interest.
C
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Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, M. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
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Revision C.01; Gaussian, Inc.: Wallingford, CT, 2009.
(19) The isolated yield of 6a−AuCl was significantly lowered (32%)
by using Au(I)Cl as a precursor.
ACKNOWLEDGMENTS
■
This study is a part of a Grant-in-Aid for Scientific Research on
Innovative Areas “New Polymeric Materials Based on Element-
Blocks (No. 2401)” (No. 24102003) of The Ministry of
Education, Culture, Sports, Science and Technology, Japan.
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