Efficient synthesis, structure, and complexation studies of electron-donating thiacalix[n]dithienothiophene

Article abstract of DOI:10.1002/anie.201410970

A series of thiacalix[n]dithiothiophenes (n = 4-10) was prepared by a facile method and X-ray analysis was used to determine the molecular structures of square-(4-mer) and pentagonal-shaped macrocycles (5-mer). In the cyclic voltammograms, reversible multielectron redox processes, which are due to electronic delocalization, were observed at low oxidation potentials. The cyclic 4-mer acted as a "Janus-head"cavitand for two C₆₀ molecules, whereas the 5- and 6-mer formed stable 1:1 complexes with C₆₀.

Full text of DOI:10.1002/anie.201410970

Angewandte
Chemie
DOI: 10.1002/anie.201410970
3
D Macrocycles
Efficient Synthesis, Structure, and Complexation Studies of Electron-
Donating Thiacalix[n]dithienothiophene**
Ryota Inoue, Masashi Hasegawa,* Tohru Nishinaga, Kenji Yoza, and Yasuhiro Mazaki
Abstract: A series of thiacalix[n]dithiothiophenes (n = 4–10)
was prepared by a facile method and X-ray analysis was used
to determine the molecular structures of square- (4-mer) and
pentagonal-shaped macrocycles (5-mer). In the cyclic voltam-
mograms, reversible multielectron redox processes, which are
due to electronic delocalization, were observed at low oxida-
tion potentials. The cyclic 4-mer acted as a “Janus-head”
cavitand for two C60 molecules, whereas the 5- and 6-mer
formed stable 1:1 complexes with C .
6
0
C
yclization reactions to prepare well-defined macrocycles
composed of oligoarylene units are usually the most difficult
[
1]
step in many reaction systems. Recently, a variety of
synthetic methods have been used to synthesize three-dimen-
sional (3D) hoop-shaped conjugated molecules. Their syn-
thesis usually involves the cyclization of preorganized pre-
[1d]
cursors as the key reaction step. However, preparation of
the particular precursor for cyclization is quite costly, and the
need for preorganization hampers comprehensive studies of
homologue series with different sizes. Alternatively, simple
cyclization of several components affords thermally robust or
kinetically reproducible 3D cyclic oligomers, such as cucurbi-
Scheme 1. Synthesis and structures of 1, 2, and 6.
[2,3]
turil, calixarene, and pillerarene.
Although they can be
Thiacalix[n]thiophenes (1; Scheme 1), which are cavitand
usually obtained by a one-pot reaction, facile cyclization
methods for conjugated macrocycles are still limited with the
exception of Rothemund-type synthesis.
molecules consisting of thienylene and sulfide linkages, were
first reported in the late 1990s by two groups independently.
[5]
[
4]
In 1997, Kçnig and co-workers reported a one-pot synthesis of
thiacalix[n]thiophenes from the reaction of a 2,5-thiophene
dianion with SCl2 in a yield of < 1%. Soon after that,
Nakayama and co-workers developed a more practical multi-
step synthesis from acyclic precursors (ca. 24% yield).
Although these sulfur-bridged oligothiophenes can be used
not only as host molecules but also as novel redox-active
conjugated molecules, there are few detailed studies of their
properties. Bearing in mind the need for an accessible
synthesis, we decided to prepare thienylene-containing sul-
fide-linked 3D macrocycles as a new class of electron-
donating cavitand compounds. Here we report the facile
and selective synthesis of a series of thiacalix[n]dithieno[3,2-
b:2’,3’-d]thiophenes (thiacalix[n]DTTs), 2a–g (n = 4–10),
through a simple Pd-catalyzed coupling reaction of stannyl
[
*] R. Inoue, Dr. M. Hasegawa, Prof. Y. Mazaki
Department of Chemistry, School of Science
Kitasato University
Sagamihara, Kanagawa 252-0373 (Japan)
E-mail: masasi.h@kitasato-u.ac.jp
Homepage: http://www.kitasato-u.ac.jp/sci/resea/kagaku/
HP_kinou/Index.html
Prof. T. Nishinaga
Department of Chemistry
Graduate School of Science and Engineering
Tokyo Metropolitan University
Hachioji, Tokyo 192-0397 (Japan)
Dr. K. Yoza
Bruker AXS
Yokohama, Kanagawa 221-0022 (Japan)
[
6]
sulfide. The resulting cyclic oligomers are conjugated
through the sulfide linkers, meaning that the electronic
properties should depend on the size of the homologue.
An efficient cyclization is shown in Scheme 1. When
[
**] We acknowledge financial support from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan through a Grant-
in-Aid for Scientific Research (grant number 20438120). We thank
Prof. Dr. Takahiro Tsuchiya (Kitasato University) for helpful dis-
cussions, and Dr. Yasutoshi Kasahara (Kitasato University) for
Raman spectroscopy. All calculations were performed at the
Research Center for Computational Science, Okazaki (Japan).
[7]
compound 4 was used as the starting material, the reaction
proceeded cleanly to afford various sizes of macrocycles.
After purification by gel permeation chromatography (GPC),
4–10 mers 2a–g together with acyclic polymers were
[
8]
obtained. The combined yield of the obtained cyclic
homologue series was as high as 75% when toluene was
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410970.
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used as the solvent (entry 1; Table 1). When dimethylform-
amide (DMF) was employed, 2a precipitated from the
reaction mixture. Thus, the formation of the larger macro-
cycles is suppressed in the catalytic cycle. In fact, the reaction
intermolecular contact between the macrocycles in the
packing diagram, 2a aligns in a channel-like columnar
structure along the c-axis (Figure 1b). Two PhCl molecules
crystallized in the crystal lattice with 2a.
On the other hand, when there is an odd number of DTT
units in the macrocycle, the molecules do not adopt the
alternating geometry as a stable conformation due to the
steric repulsion among the TMS groups. Compound 2b has an
envelope-shaped pentagonal geometry in which four of the
five S atoms at the corners are nearly coplanar (Figure 1c),
and the other one is out of the plane. Although the three
DTT rings that are confined to the plane of the four S atoms
alternate so that their TMS groups are oriented in opposite
directions, the other two DTT rings, which are adjacent to
each other, align nearly perpendicular to each other. As
a result, the macrocycle forms a roughly U-shaped cavity, and
one of the DTT rings of the neighboring molecules mutually
fills the cavity of the other (Figure 1d). The diagonal S···S
lengths of the pentagonal structure were determined to be
within the range of 13.2–15.6 ꢀ.
[
a]
Table 1: Synthesis of thiacalix[n]DTT 2a–c under various conditions.
[
b]
Entry
Solvent
Temp.
Yield [%]
[
c]
2
a
2b
2c
Others
1
2
3
toluene
DMF
DMF/toluene
reflux
1208C
1208C
43
56
77
19
–
5
8
–
2
<5
–
<1
[
9]
[
d]
[
a] 0.5 mmol of 4 and (Bu Sn) S were used. [b] Yield of isolated product
after GPC separation. [c] Sum of other cyclic compounds. [d] v/v=1:1.
3 2
performed in a 1:1 mixture of DMF/toluene showed the
highest conversion. In contrast, when the identical cyclization
conditions were applied to compound 5, no cyclization
product was obtained, possibly due to low solubility and/or
a conformational flexibility of intermediates caused by the
absence of bulky TMS groups. Monomeric compound 6 was
synthesized for comparison.
The molecular structures of 2a–c were calculated by using
a DFT method with the B3LYP/6-31G(d) basis set (Fig-
[
10]
ure 2a–c).
For 2a, a molecular geometry, in which four
Figure 1a shows an X-ray crystal structure of 2a with
[
9]
chlorobenzene (PhCl). Compound 2a crystallizes in the
tetragonal space group I4 /a (#88). The four DTT rings are
1
Figure 2. a) HOMOs of 2a, b) 2b, and c) 2c. d) UV/Vis absorption
spectra for 2a, 2b, 2c, 2d, and 6 in CH Cl .
2
2
Figure 1. a) ORTEP diagram of 2a. b) Packing diagram of 2a and PhCl
molecules. c) ORTEP diagram of 2b. d) Packing diagram of 2b.
DTT moieties adopt a 1,3-alternating conformation with D2d
symmetry, was found to be a minimum energy structure. On
the contrary, from DFT calculations on 2b, the structure was
connected together by nonplanar sulfur atoms at the corners.
The macrocycle forms a puckered quadrilateral conforma-
tion. Adjacent DTT rings are connected in a nearly anti-form
arrangement through the CÀSÀC bond to reduce the steric
shown to have a folded arrangement with C symmetry, which
1
is consistent with the X-ray structure. In addition, from DFT
calculations, 2c was determined to have a hexagonal geom-
etry with D3d symmetry (Figure 2c). The lower symmetry in
2b is attributed to the steric repulsion between the TMS
groups at the corners of S(1) (Figure 2b); the parity of the “n”
results in the structural differences. All of the S atoms of 2a
and 2c at the corner are almost coplanar, and there is an
orbital interaction between the adjacent DTT rings through
repulsion of the TMS groups so that the macrocycle adopts
a nearly 1,3-alternating conformation in comparison to
calix[4]arene. Compound 2a was found to have a large
square cavity with a diagonal distance of 12.8 ꢀ (S1a···S1c),
and the shortest distance between facing walls was found to
be 8.6 ꢀ (S2a···S4d). Although there is no remarkable
2
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+
the 3p orbitals on the bridged S atoms. For this reason, the
that 2aC , whose electrons are delocalized due to the
conjugation with the corner linkage atoms, forms easily.
Subsequently, the second oxidation process leads to the
HOMO of 2a and 2c basically spreads over the cyclic core
(
Figure 2a and 2c). On the other hand, the coefficients of the
2
+
HOMO in 2b are clearly localized within part of the DTT
rings due to its envelope-like geometry.
formation of 2a , and the third oxidation process involves
+
2
4+
simultaneous two-electron oxidation of 2a to 2a . Elec-
n+
Figure 2d shows UV/Vis absorption spectra for 2a–d and
tronic spectra of cationic species 2a (1 < n < 4), generated
6
in CH Cl . In each spectrum, a similar absorption band with
by spectroelectrochemistry, clearly support the redox pro-
cesses, described above (Figure S21). Upon oxidation, a broad
shoulder absorption band appeared at about 1500 nm due to
the delocalized electron among the DTT units, and it
2
2
a different molar extinction coefficient was observed. The
absorption maxima of 2a (360 nm), 2b (355 nm), 2c (358 nm),
and 2d (349 nm) are red-shifted in comparison to that of 6
2
+
[11]
(
348 nm). These bathochromic shifts are attributed to the
disappeared when 2a formed. Since the electronic spectra
+
conjugation among the DTT units of each macrocycle.
The redox properties of 2a–c and 6 were investigated by
cyclic voltammetry (Figure S18) and differential pulse vol-
tammetry (DPV; Figure 3 and Table 2). In the cyclic voltam-
mograms (CVs) for 2a, three reversible redox waves were
observed at 0.46, 0.54, and 1.00 V. Rotating disk electrode
of 6C exhibited an absorption maximum at 827 nm as the
lowest energy band, the observed red-shifts in the spectrum
+
for 2aC depend on the p-conjugation through the sulfide
linkages.
À
In the CVs of 2b and 2c, five and six 1e reversible redox
waves, respectively, were observed. The E value for 2b is
1
ox
(
RDE) analysis on the CVs for 2a indicated that these redox
positively shifted in comparison with those of 2a and 2c and is
more negative than that of 6. We think that this is due to the
different conjugation of 2b with a pentagonal U-shaped
geometry. In the spectroelectrochemistry, the monocationic
species (2bC and 2cC ) exhibited broad shoulder absorption
with tailing to 2000 nm due to the delocalized electron.
Next, we examined the incorporation of C60 by 2a–c.
When a solution of 2a was mixed with C60 (> 4 equiv) in
À
À
À
processes corresponded to 1e , 1e , and 2e redox processes,
respectively. In the CVs of 6, two 1e redox processes were
observed at 0.52 and 0.86 V. These results indicate that the
potentials of the redox processes correspond to the number of
sulfide linkages. The first oxidation wave of 2a is at a lower
À
+
+
1
potential than that of 6. The lower first potential (E ox) implies
CDCl , the peak corresponding to the TMS protons in
3
1
H NMR spectra clearly shifted downfield. However, a dark
precipitate, which was insoluble in common organic solvents,
formed within several minutes. On the contrary, 2b and 2c
exhibited clear complexation behavior with excess C in
6
0
1
solution. The H NMR signals for both 2b and 2c with C
6
0
shifted downfield by 0.03 and 0.05 ppm, respectively
1
3
(
Figure 4). Moreover, the C NMR signals for C60 clearly
Figure 3. DPV charts for a) 2a, b) 2b, c) 2c, and d) 6.
Table 2: Redox potentials of 2a–c and 6 and absorption maxima of their
[
a,b]
cationic species.
1
Compd. Redox potentials
Monocation Dication
Figure 4. H NMR spectra at room temperature in CDCl for a) 2b and
3
+
[
V vs. Fc/Fc ]
[nm]
[nm]
a mixture of 2b with C , and b) 2c and as mixture of 2b with C .
6
0
60
À
À
2
2
2
6
a
b
c
0.46 (1e ), 0.54 (1e ),
1
507, 797
ca. 1500
493, 785
À
[c]
.00 (2e )
[12]
shifted upfield (Figure S34). UV/Vis titrations of C60 with
2b or 2c in PhCl exhibited an increase in the absorption
intensity over the range of 400–500 nm upon the addition of
each macrocycle solution. On the basis of Job plots, which
showed maxima at a molar fraction of 0.5, 1:1 complexes
formed in each case. The calculated binding constants (Ka)
from the titration experiments, assuming 1:1 complexes, were
À
À
À
0.50 (1e ), 0.61 (1e ), 0.76 (1e ), 508, 806
503, 798
494, 790
550
À
À
[c]
0
.98 (1e ), 1.21 (1e )
ca. 1300
À
À
À
0.43 (1e ), 0.65 (1e ), 0.83 (1e ), 507, 805
À
À
À
[c]
1
.10 (1e ), 1.25 (1e ), 0.96 (1e )
ca. 1600
À
À
0.52 (1e ), 0.86 (1e )
506, 827
3
À1
4
À1
(
1.6 Æ 0.08) ꢁ 10 m for 2b and (5.3 Æ 0.44) ꢁ 10 m for 2c.
[
a] All potentials were recorded with DPV in CH Cl containing 0.1m
2 2
+
n
The larger value of K for 2c indicates facile incorporation of
a
Bu PF at 258C. Potentials were measured against the Ag/Ag electrode
4
6
+
C . In the optimized D geometry of 2c from the DFT
60 3d
calculations, the distance between opposite DTTrings was ca.
15.5 ꢀ, and thus, the cavity of 2c is large enough to
and adjusted to the Fc/Fc potential. [b] Absorption maxima were
obtained by using spectroelectrochemistry with a Pt mesh electrode at
each potential. [c] Observed as a shoulder.
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incorporate C . On the other hand, the U-shape geometry of
the redox processes depends on the number of sulfide
linkages. In the electronic spectra of electrically generated
2a–cC , broad absorption bands due to electronic delocaliza-
6
0
2
b would somewhat restrain the complexation. Difference
+
spectrum obtained by subtraction of 2b/2c and C60 from the
absorption of the mixture displays a broad absorption band
associated with a weak CT band in the range of 400–650 nm
tion were observed. Macrocycle 2a formed a Janus-head
complex with two molecules of C , whereas 2b and 2c
6
0
[
13]
(
Figures S39 and S40).
Mixing of 2a and C60 by slow diffusion gave black crystals
formed stable 1:1 complexes with C . The highly symmetrical
60
structure of the Janus-head complex resulted in a condensed
1D array of C60 molecules in the crystal.
[9]
suitable for X-ray analysis. The crystals contain 2a and two
disordered C60 molecules together with o-xylene from the
solvent in a tetragonal crystal system. The two C60 molecules
are incorporated in the cavities on each side of 2a with 1,3-
alternating geometry (Figure 5a). Opposite pairs of DTT
Received: November 12, 2014
Published online: && &&, &&&&
Keywords: cross-coupling · electrochemistry · macrocycles ·
.
sulfur heterocycles · supramolecular chemistry
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2
6
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of 2a help hold the C60 molecules in the cavities. The Raman
and FTIR spectra do not exhibit obvious differences between
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[
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the crystal and free C . Since the crystal has an S axis in
6
0
4
[8] For each compound, simple signals corresponding to their highly
symmetrical structures were found. The shape of the signals did
not change even at À808C except for their chemical shifts.
the center of the macrocycle, the cavities are crystallograph-
ically identical, which gives rise to the “Janus-head” in 2a.
Although there are several examples of 1:2 the macrocycle–
C60 complex, the number of 1:2-complex crystal structures
[
9] CCDC 1030028 (2a), 1030029 (2b), and 1030030 (2a-C ),
60
contain 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.
[15]
with highly symmetrical orientations is small. In the crystal
packing, C60 molecules form a 1D-array along the c-axis
(
Figure 5b and c). The high symmetry of the 1:2 complexes
[
[
10] All calculations were performed using Gaussian09, Revision
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allows for the formation of a condensed C array in the
6
0
[
16]
crystal.
In conclusion, we developed an efficient synthesis of
a homologue series of thiacalix[n]DTT derivatives. From X-
ray analysis, 2a and 2b were shown to have square and
pentagonal shape, respectively. DFT calculations and UV/Vis
absorption spectra clearly indicated p-conjugation through
the corners. On the bases of CVs and DPVs, the potential of
[
12] We assume that this upfield shift is due to the shielding effects of
the DTT walls and/or CT interactions.
4
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Angewandte
Chemie
[
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Angewandte
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Communications
3
D Macrocycles
Binding two fullerenes: A series of thia-
calix[n]dithiothiophenes (n=4–10) was
synthesized. The molecular geometry
depends on the parity of “n” of the neutral
state, and electronic delocalization was
found in the cationic species. The 4-mer
acted as a “Janus-head” cavitand for two
C60 molecules, whereas the 5- and 6-mer
formed stable 1:1 complexes with C .
R. Inoue, M. Hasegawa,* T. Nishinaga,
K. Yoza, Y. Mazaki
&&& — &&&
Efficient Synthesis, Structure, and
Complexation Studies of Electron-
Donating Thiacalix[n]dithienothiophene
6
0
6
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These are not the final page numbers!