Photon-Quantitative 6π-Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium

Article abstract of DOI:10.1002/asia.201500328

The high reactivity of 6π-electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3-diarylbenzo[b]thiophenes with nonsymmetric side-aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed quantum yield of 6π-electrocyclization of 54% in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon-quantitative 6π-cyclization reaction. The nonsymmetrical modification into the side-aryl units was considered to enhance the CH/π interactions between side-aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.

Full text of DOI:10.1002/asia.201500328

DOI: 10.1002/asia.201500328
Full Paper
Electrocyclic Reactions
Photon-Quantitative 6p-Electrocyclization of
a Diarylbenzo[b]thiophene in Polar Medium
Ruiji Li,[a] Takuya Nakashima,*[a] Olivier Galangau,[a, b] Shunsuke Iijima,[a] Rui Kanazawa,[a] and
Tsuyoshi Kawai*[a, b]
Abstract: The high reactivity of 6p-electrocyclization in polar
solvents has remained one of the important challenges for
diarylethenes because of the emergence of a twisted intra-
molecular charge transfer (TICT) state at the excited state in
such polar media, which usually quenches the photocycliza-
tion reaction. Herein we report on the preparation and
highly efficient photocyclization of 2,3-diarylbenzo[b]thio-
phenes with nonsymmetric side-aryl units in a polar solvent.
While the dithiazolylbenzo[b]thiophene showed a suppressed
quantum yield of 6p-electrocyclization of 54% in methanol,
the replacement of a thiazole unit with a thiophene ring led
to a photon-quantitative 6p-cyclization reaction. The non-
symmetrical modification into the side-aryl units was consid-
ered to enhance the CH/p interactions between side-aryl
units to support a photoreactive conformation in methanol.
The stabilization of the photochromic reactive conformation
is expected to suppress the formation of the TICT state at
the excited state, leading to highly efficient photoreactivity.
Introduction
cifically stabilize the photochromic reactive conformation in
less polar solvents. A related strategy has also been employed
by Nakatani, Tian, and Zhu.[16] These methodologies to regulate
the rotational configuration with intra- and intermolecular in-
teractions are in parallel with the design strategy for oligoary-
lene molecules forming specific foldamers.[17,18] Therefore, the
appropriate placement of interacting moieties including heter-
oatoms in a molecule is of major importance to control the
conformational behavior. However, the high reactivity in polar
solvents such as methanol has remained one of the important
challenges for this class of molecules because of the lack of an
effective design to control the molecular geometry of photo-
chromes in polar solvents, wherein the hydrogen bonding in-
teraction is less effective. Moreover, the introduction of polar
units into diarylethene-based photochromes to enhance the
solubility in polar solvents often results in the formation of
a twisted intramolecular charge transfer (TICT) state in the ex-
cited state, which adversely affects the efficiency of the ring-
cyclization reaction.[19]
Photoresponsive molecular materials have attracted much in-
terest because their physicochemical properties are modulated
with precise spatiotemporal control in a remote and noninva-
sive manner.[1–7] In particular, photoswitching systems based on
diarylethenes[8,9] and their aromatic analogs[10] have been ex-
tensively studied because they undergo thermally irreversible
photoisomerization based on 6p-electrocyclization even in the
solid state. Recent studies on these classes of molecules have
highlighted their high photosensitivity in the electrocyclization
reaction.[11–16] These contributions made an extensive effort to
control the molecular folding geometry suitable for the photo-
cyclization reaction proceeding in a conrotatory manner.[8a] For
example, we have recently reported a dithiazolylbenzo[b]thio-
phene derivative, which showed a photon-quantitative cycliza-
tion reaction in hexane.[11a] This extremely high photochromic
reactivity of the molecule was explained in terms of noncova-
lent interactions such as S/N and CH/N interactions, which spe-
Herein we report the design of photochromic molecules
based on diarylbenzo[b]thiophenes (Figure 1) which exhibit
a highly efficient 6p-electrocyclization reaction in methanol. Al-
though the dithiazolylbenzothiophene 1a[11a] with symmetrical-
ly modified side aryl units showed an ordinary photocyclization
quantum yield of 54% in methanol, the nonsymmetrical intro-
duction of side-aryl units improves the reactivity up to unity at
most. The effect of chemical modifications on the conforma-
tional preference is discussed in terms of the contribution of
noncovalent interactions including CH/p interactions between
side-aryl units to stabilize the photoreactive conformation in
particular.
[a] R. Li, Dr. T. Nakashima, Dr. O. Galangau, S. Iijima, R. Kanazawa,
Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology, NAIST
8916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
E-mail: ntaku@ms.naist.jp
tkawai@ms.naist.jp
[b] Dr. O. Galangau, Prof. T. Kawai
NAIST-CEMES International Collaborative Laboratory for Supraphotoactive
Systems
Centre d’Élaboration de MatØriaux et d’Etudes Structurales, CEMES
29, rue Jeanne Marvig, BP 94347, Toulouse 31055 (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500328.
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The spectroscopic properties of 1a–3a were evaluated in
hexane and methanol (Table 1). Upon UV irradiation, both solu-
tions of 2a and 3a in methanol underwent a spectral change
with a decrease in absorbance at around 300 nm and an in-
crease of the band at 610 nm, in a manner similar to that of
the parent molecule 1a[11a] (Figure 2). Each spectral change
was accompanied by an isosbestic point at 315 and 320 nm
for 2 and 3, respectively. These spectral shifts are a clear indica-
tion of the formation of closed-ring isomers 2b and 3b, which
was confirmed by 1H NMR spectroscopy in CDCl3 (Figures S5
and S8 in the Supporting Information) after isolation by re-
verse phase HPLC using methanol as an eluent.
Next, the optimized geometries and the frontier molecular
orbitals (FMOs) for 1a were compared with those of 2a and
3a by DFT calculations using the Gaussian 09 suite.[20] For the
calculations, the wB97XD DFT functional[21] with the 6-31G(d)
Figure 1. Photochromic reaction and chemical structures of diarylbenzo[b]-
thiophenes.
Table 1. Absorption maxima and coefficients of the open- and closed-
ring isomers of 1-3, together with the quantum yields in solutions.
Results and Discussion
[d]
[e]
Isomer
lmax [nm]
(e [104 MÀ1 cmÀ1])
FoÀc
FcÀo
One of the side-aryl thiazole units of 1a was replaced with
a phenylthiophene unit in 2a and 3a (Figure 1) to investigate
specific intramolecular interactions affecting the conformation-
al behavior. 2,3-Diarylbenzo[b]thiophenes 2a and 3a were syn-
thesized as shown in Scheme 1. Newly synthesized molecules
1a[a]
1b[a]
2a
2b
3a
307 (2.9)[b]
597 (0.95)[b]
290 (3.4)[c]
612 (1.1)[c]
300 (3.2)[c]
615 (0.92)[c]
0.98,[b] 0.54[c]
–
–
0.008[b]
–
0.50,[b] 0.91[c]
–
0.007[b]
–
0.036[b]
0.71,[b] 0.99[c]
–
3b
[a] Ref. [11a]. [b] In hexane. [c] In methanol. [d] lirrad. =313 nm.[e] lirrad.
=
480 nm.
Scheme 1. Synthesis of 2,3-diarylbenzo[b]thiophenes: a) 2mK3PO4 aq.,
[Pd(PPh3)4], 1,4-dioxane.
1
were characterized by H and 13C NMR spectroscopy, high-reso-
lution mass spectrometry, and elemental analysis (see the Sup-
porting Information for details). The 2,3-diarylbenzo[b]thio-
phenes were synthesized via simple Suzuki–Miyaura cross-cou-
pling reactions between the corresponding aryl units under
identical conditions. The sequence of connection in the tri-ary-
lene structures was controlled by exploiting the different reac-
tivities of 2- and 3-positions in 2,3-dibromobenzo[b]thiophene.
A thienyl pinacol borate (Ar1) was first reacted with dibromo-
benzothiophene followed by the coupling with a thiazolyl
borate (Ar2) to form 2a and vice versa for the synthesis of 3a.
The sequence of aryl units was confirmed by X-ray single-crys-
tal analysis for all molecules.
Figure 2. UV/Vis absorption spectral change of 2 and 3 in methanol (concen-
tration: 2.0”10À5 m) upon UV irradiation (l=365 nm). Dotted line: open
form, bold line: photostationary state.
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basis set was employed. The solvent polarity was taken into
account by using the polarizable continuum model (PCM).[22]
All molecules gave similar optimized structures with the pho-
toreactive conformation as the most stable geometries (Fig-
ure S12 in the Supporting Information). A negligible difference
between 1a, 2a, and 3a was also found in the shape of FMOs
(HOMO and LUMO) (Figures S14–S16 in the Supporting Infor-
mation). The HOMOs were delocalized in the entire molecule,
whereas the LUMOs were rather localized in a thiazolyl side-
aryl group for all molecules. The energy differences between
the open-ring isomers a-form and the closed-ring isomers b-
form were also similar for all molecules (Figure S12).
tance in the range of 2.8–3.1 Š.[27] On the other hand, signifi-
cantly tight contacts between the sulfur atom in the central
benzothiophene and the thiazolyl nitrogen atom in a side-aryl
unit (S1–N1, S1’–N1’<3.0 Š in Figure 3c) was observed for 3a,
suggesting that the S/N interaction harnesses the rotation
about the benzothiophene-thiazole bond. Although no specific
interaction was suggested between the central and the Ar2
units for 3a, the CH/p interactions with the CH–aryl distance
[27]
in the range of 2.7–3.2 Š between the side-aryl units seem
to support the photoreactive conformations.
Overall, ordinary DFT calculations and X-ray crystallography
data failed to highlight the effect of the substitution of a phe-
nylthiazole with a phenylthiophene on the conformational be-
havior of 2,3-diarylbenzo[b]thiophenes. However, the ring-cycli-
zation quantum yield (FOÀC) in hexane dropped from 0.98 for
the parent compound to 0.50 and 0.71 for 2a and 3a, respec-
tively. Although CH/S and CH/N interactions were expected be-
tween the central and side-aryls in 2a as suggested in the X-
ray crystallographic data, the contribution of an unreactive
conformation with a flipped Ar1-unit stabilized by the Me/S in-
teraction may decrease the FOÀC in hexane.[10a,c] The absence
of CH/N interaction in 3a also resulted in a decreased FOÀC in
hexane. Interestingly, the FOÀC jumped in methanol from 0.54
of 1a to 0.91 and >0.99 for 2a and 3a, respectively. The
reason for the low reactivity of 1a in methanol was attributed,
at least partly, to a weakening of the H-bond-like CH/N interac-
tion between the 4-proton on the central benzothiophene and
the nitrogen in the Ar2-thiazolyl ring (Figure 1) in a protic sol-
vent,[11a] leading to a larger conformational fluctuation regard-
less of the photoreactive conformation in the single crystal.
The NMR chemical shift of the 4-proton (H4) (7.97 ppm) was
almost identical for both 1a and 2a in CD3OD (Figure S18 in
the Supporting Information). Therefore, the CH/N interaction
was expected to have a small effect on the conformational
preference of both molecules in methanol.
These three molecules successfully gave single crystals[23–25]
from methanol solution and exhibited photoreactive confor-
mations with distances between the photoreactive carbon
[26]
atoms below 3.7 Š (Figure 3, see also Figure S21, Tables S1
To gain further insight into the reason for the higher photo-
cyclization efficiency of 2a and 3a in comparison to 1a, the
conformational preferences for these diarylbenzothiophenes
1
were studied by H NMR measurements in CD3OD (Figure S18
in the Supporting Information). Here we focus on the values of
chemical shift of two methyl groups and the splitting between
them (Dd), which efficiently probe the substantial ring-current
effect operating on these methyl groups (Table 2) and relative
stability of the highly symmetric C2-conformation around the
photoreactive 6p system.[11a] Each methyl peak was assigned to
either Ar1- or Ar2-methyl groups based on differential nuclear
Overhauser effect (1D NOE) measurements (Figure S20 in the
Figure 3. Crystal structures of (a) 1a, (b) 2a and (c) 3a with the indication of
close contacts of noncovalent interactions (dotted lines). The dots indicate
the centroids of five-membered aryl rings.
and S2 in the Supporting Information). Since the packing
modes of molecules in the crystals were different from each
other with different space groups and intermolecular packing
manners (Table S1 in the Supporting Information), the atomic
distances of intramolecular interactions could not be directly
compared. Compound 1a gave photoreactive conformers very
similar to that found in the single crystal prepared from
hexane solution.[11a] The CH/S and CH/N interactions between
those atoms on the central benzothiophene and the side-aryl
units were suggested from the close atomic contacts (H5-S1:
3.058; H4-N1: 2.665 Š in Figure 3b) for 2a that appear to stabi-
lize the specific conformation with the aid of CH/p interactions
between methyl groups and aryl rings with the CH–aryl dis-
1
Supporting Information). For H NMR spectral simulations, the
GIAO model at the PBEPBE/6-311 + +G(2d,p) level was used.[28]
Compound 2a gave a set of peaks at 2.06 (Ar2-thiazole) and
2.28 ppm (Ar1-thiophene) with Dd of 0.22 ppm. The former
peak appeared more upfield than that of 1a (2.08 ppm). On
the other hand, 3a, which exhibited a photon-quantitative re-
activity in methanol, provided the peaks at 2.11 (Ar1-thiazole)
and 2.10 ppm (Ar2-thiophene) with a surprisingly small peak
splitting of only Dd=0.01 ppm. The thiazolyl-methyl peak at
2.11 ppm was a more upfield signal compared to the Ar1-thia-
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Table 2. Summary of the chemical shifts of methyl groups of 1a–3a in
CD3OD together with simulated values[a,b] (in parentheses) based on the
GIAO model at the PBEPBE/6-311+ +G(2d,p) level.
Compd.
Ar1-Me [ppm]
Ar2-Me [ppM]
Dd [ppm]
1a
2.17
2.08
0.09
(2.183)[c], (2.166)[d]
2.28
(2.243)[c], (2.976)[d]
2.06
(0.06)[c], (0.81)[d]
0.22
2a
3a
(2.112)[c], (3.002)[d]
2.11
(2.144)[c], (2.172)[d]
(2.236)[c], (2.051)[d]
2.10
(2.166)[c], (2.885)[d]
(0.124)[c], (0.951)[d]
0.01
(0.022)[c], (0.713)[d]
Figure 4. Modes of CH/p interactions working in diarylbenzo[b]thiophenes
(parts of structures were given for clarity) with symmetrically and nonsym-
metrically modified side-aryl units.
[a] Solvent polarity was taken into account by using the polarizable con-
tinuum model. [b] Average values of three protons to take the rotation of
methyl groups into consideration. [c] Based on the most stable photo-
reactive conformation (see Figure S12 in the Supporting Information).
[d] Based on the non-photoreactive conformation with a flipped aryl unit
(see Figure S19 in the Supporting Information).
1a, while the stronger CH/p interaction operating between
the electron-deficient thiazolyl-methyl group and the electron-
rich thiophene ring in the nonsymmetrically modified one po-
sitions these moieties close together (Figure 4). As a result, an-
other methyl-aryl pair is expected to come close to each other.
As a related matter, we have recently demonstrated that a sig-
nificant mono-directional CH/p interaction was still effective to
fix a photoreactive conformation in a diarylethene and a terary-
lene derivative of nonsymmetrical structures.[31]
zolyl-methyl one of 1a (2.17 ppm). These upfield shifts of thia-
zolyl-methyl in comparison with 1a could be attributed to the
larger ring current effect of the oppositely facing thienyl ring
on the thiazolyl-methyl groups for 2a and 3a, which also indi-
cated a stronger CH/p interaction between the electron-defi-
cient methyl and the electron-rich thiophene. The thienyl-
methyl peaks of 2.28 for 2a and 2.11 ppm for 3a were also
found upfield in comparison with the synthetic intermediates
of phenylthienyl parts (2.4–2.7 ppm), also supporting the oper-
To further support the above hypothesis, we additionally
synthesized 2,3-diarylbenzo[b]thiophenes 4a with an electron-
donating methoxy (OMe) group to make one of the side-aryl
units electron-richer relative to the other one (Figure 5). This
compound showed a reversible photochromic reaction in
methanol in a similar manner to the parent molecule 1a (Fig-
ure S23 in the Supporting Information). Meanwhile, a photo-
reactive conformer was given in a single crystal of 4a prepared
from methanol solution (Figure 6, also see Table S1 in the Sup-
porting Information). The noncovalent interactions including
CH/N and S/N interactions with the atomic distances equiva-
1
ation of CH/p interaction. The DFT simulation of the H NMR
spectrum of 3a based on an optimized photoreactive confor-
mation well reproduced the experimental result including
a very small peak splitting Dd (Table 1). Thus, the conformation
of 3a is expected to be well controlled into the photoreactive
one to achieve the photon-quantitative reactivity in methanol.
It is suggested that the attractive interactions to harness the
rotations around the single bond between the central and
side-aryl units are not always necessary at both sides for con-
trolling the molecular geometry.
The results of 2a and 3a with enhanced photosensitivity in
methanol let us to hypothesize that the nonsymmetrical modi-
fication into side-aryl units in terms of electron-donating or de-
ficient properties may direct the side-aryl pair to reinforce the
CH/p interactions. The importance of CH/p interaction on sta-
bilizing the photoreactive conformation was partly verified by
a decrease in FOÀC of a derivative of 1a in which methyl
groups on the reactive carbon atoms were substituted by
a proton and a methoxy group (Figure S22 in the Supporting
Information). The CH/p interaction is supposed to be com-
posed of electrostatic and dispersion interactions as attractive
forces.[29] While the contribution of the electrostatic force to
this interaction has been thought to be relatively small, the en-
hanced electron-donating property in an aryl ring owing to
the substitution of thiazole with thiophene should practically
increase the strength of the CH/p interaction.[26] The nature of
the CH/p interaction characterized by the dispersion force is
most likely to work more efficiently in polar solvents than in
less polar ones with the aid of the solvophobic effect.[30] The
idea is that the opposed CH/p interactions between Ar1 and
Ar2 equally operate for a symmetrically modified molecule like
Figure 5. Introduction of an electron-donating (OMe) group into 1a to form
4a.
Figure 6. Crystal structures of 4a with the indication of close contacts of
noncovalent interactions (dotted lines). The dots indicate the centroids of
five-membered aryl rings.
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placed using AFIX instructions. DFT calculations were performed
with Gaussian 09 at the wB97XD/6–31G(d) level. Photoirradiation
with UV and visible light under microscope observation was con-
ducted using a mercury lamp for epi-illumination through BP330–
385 and BP520–550 filters, respectively. Absolute ring photoreac-
tion yield (FOÀC and FCÀO) values were obtained on a Shimadzu
QYM-01 setup.[33]
lent to or shorter than the sum of van der Waals radii of each
atom (H: 1.2, N: 1.55, S: 1.8 Š) are expected to support the
photoreactive conformations in a manner similar to that of the
parent molecule 1a (Figure 3). It should be noted that the dis-
tances between the methyl groups and the centroids of op-
posing thiazolyl rings were shorter than 3.0 Š, corresponding
to the effective distance of CH/p interaction.[27]
1
The H NMR chemical shifts of two methyl groups on side-
aryl units appeared at 2.04 and 2.15 ppm for 4a in CD3OD (Fig-
ures S18 and S20 in the Supporting Information), showing up-
field shifts in comparison with those of 1a (2.06 and 2.17 ppm,
respectively). These slight upfield shifts from 1a to 4a suggest-
ed that the stronger ring current effect operates on these
methyl protons through the reinforced CH/p interactions in 4a
owing to the asymmetrization of aide aryl units in terms of the
electron-rich or -deficient property. As a consequence, the
ring-cyclization quantum yield in methanol substantially in-
creased from 0.54 for 1a to 0.70 for 4a in methanol. This in-
crease in FOÀC further supports the reinforcement of CH/p in-
teractions between the pair of electron-deficient and -richer
side aryl units (Figure 4), stabilizing the photoreactive confor-
mation in methanol.
Acknowledgements
The authors thank Mr. F. Asanoma, Mr. S. Katao and Ms. Y. Nish-
ikawa at NAIST for their assistance with VT-NMR, X-ray crystal-
lography and mass spectra, respectively. This work was partly
supported by Ministry of Education, Culture, Sports, Science
and Technology (MEXT) of Japan under Green Photonics Proj-
ect at NAIST and the Grand-in-Aid for on Innovative Areas of
“PhotoSynergetics”, NAIST Green-Photonics Project and the
Mazda Foundation.
Keywords: crystals
·
noncovalent
interactions
·
photochromism · quantum yield
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Experimental Section
2,3-Diarylbenzo[b]thiophenes were synthesized through the se-
quential step-by-step Pd-catalyzed cross-coupling reactions be-
tween 2,3-dibromobenzo[b]thiophene and the corresponding aryl-
pinacol borates (see the Supporting Information for details).
1H NMR spectra were recorded on a JEOL AL-300 spectrometer
(300 MHz). Temperature-dependent 1H NMR spectra were mea-
sured using a JEOL ECP 400 spectrometer (400 MHz). Separative
HPLC was performed on a JASCO LC-2000 Plus Series. Mass spectra
were measured with a JEOL JMS-700 mass spectrometer. Absorp-
tion spectra in solution were studied with a JASCO V-670 spectro-
photometer. UV irradiation was carried out using a Panasonic
Aicure UV curing system (LED, l=365 nm) as the exciting light
source. X-ray crystallographic analyses were carried out with
a Rigaku R-AXIS RAPID/s Imaging Plate diffractometer with MoKa ra-
diation. The structures were solved by direct method (SHELXL-97)
and refined by the full-matrix least-squares on F2. All non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms were
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;
33647 reflections measured, 8601 unique reflections (Rint =0.0367);
structure solved by direct methods and refined with a full matrix
against all F2 data; hydrogen atoms were calculated in riding positions;
wR=0.0977, R=0.0414. CCDC-1051199 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[23] Crystallographic data for 1a: C28H20N2S3, a=11.8375(6), b=12.8279(7),
c=15.7808(8) Š, a=95.9310(13), b=101.9037(14), g=92.0425(13)8, tri-
clinic, space group PÀ1(#2), Z=4, V=2328.2(2) Š3, 1calcd =1.371 gcmÀ3
,
39714 reflections measured, 10666 unique reflections (Rint =0.0738);
structure solved by direct methods and refined with a full matrix
against all F2 data; hydrogen atoms were calculated in riding positions;
wR=0.1729, R=0.0625. CCDC-1051196 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[32] Crystallographic data for 4a: C29H22N2OS3, a=7.4606(2), b=28.2772(8),
c=11.7003(3) Š, a=90, b=96.0039(8), g=908, monoclinic, space
group P121/n1(#14), Z=4, V=2454.81(11) Š3, 1calcd =1.382 gcmÀ3
;
41939 reflections measured, 5631 unique reflections (Rint =0.0501);
structure solved by direct methods and refined with a full matrix
against all F2 data; hydrogen atoms were calculated in riding positions;
wR=0.09927, R=0.0403. CCDC-1051197 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[24] Crystallographic data for 2a: C29H21NS3, a=23.2685(5), b=10.1171(2),
c=10.1904(2) Š,
a=b=g=908,
orthorhomibic,
space
group
Pna21(#33), Z=4, V=2398.90(10) Š3, 1calcd =1.328 gcmÀ3; 22949 reflec-
tions measured, 5495 unique reflections (Rint =0.0339); structure solved
by direct methods and refined with a full matrix against all F2 data; hy-
drogen atoms were calculated in riding positions; wR=0.0892, R=
0.0372. CCDC-1051198 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Manuscript received: April 1, 2015
Final article published: June 12, 2015
Chem. Asian J. 2015, 10, 1725 – 1730
www.chemasianj.org
1730
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim