Synthesis and characterisation of 1,2-difluoro-1,2-bis(5-trimethyl-silyl-2-thienyl)ethenes. A new family of conjugated monomers for oxidative polymerisation

Article abstract of DOI:10.1039/b203589h

The synthesis of a series of (E)-1,2-difluoro-1,2-bis(2-thienyl)ethenes (DFDTEs) by low temperature reaction of 2-lithiothiophenes with tetrafluoroethene (TFE) is described. A possible explanation of the mechanism leading to the formation of higher oligomers is also elucidated together with the use of TMS as protecting group to prevent it. All compounds are thoroughly characterised and their E - configuration proved by vibrational spectroscopy. The crystal structure at 120 K of a representative system evidences its essential planarity and suggests significant delocalisation of π-electrons. Fluorine atoms are involved both in short intra- and intermolecular interactions with sulfur atoms and hydrogen atoms, apparently stabilising the planar molecular arrangement.

Full text of DOI:10.1039/b203589h

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Synthesis and characterisation of 1,2-difluoro-1,2-bis(5-trimethyl-
silyl-2-thienyl)ethenes. A new family of conjugated monomers for
oxidative polymerisation
a
a
a
a
2
Luca Albertin, Chiara Bertarelli, Maria C. Gallazzi,* Stefano V. Meille and
b
Silvia C. Capelli
a
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano,
Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Swiss-Norvegian Beam Lines, European Synchrotron Radiation Facility - BP 220, 6 rue Jules
Horowitz, 38043 Grenoble Cedex, France
b
Received (in Cambridge, UK) 12th April 2002, Accepted 1st August 2002
First published as an Advance Article on the web 12th September 2002
The synthesis of a series of (E)-1,2-difluoro-1,2-bis(2-thienyl)ethenes (DFDTEs) by low temperature reaction of
-lithiothiophenes with tetrafluoroethene (TFE) is described. A possible explanation of the mechanism leading to
2
the formation of higher oligomers is also elucidated together with the use of TMS as protecting group to prevent it.
All compounds are thoroughly characterised and their E – configuration proved by vibrational spectroscopy.
The crystal structure at 120 K of a representative system evidences its essential planarity and suggests significant
delocalisation of π-electrons. Fluorine atoms are involved both in short intra- and intermolecular interactions
with sulfur atoms and hydrogen atoms, apparently stabilising the planar molecular arrangement.
Introduction
Poly[(E)-1,2-bis(2-thienyl)ethenes] (PDTEs) have been investi-
gated by various authors because of their reduced energy gap
(
E ) (i.e. the energy difference between the HOMO and LUMO),
g
if compared to the parent polythiophenes, and because of the
possibility of preparing them by facile oxidative polymerisation
of (E)-1,2-bis(2-thienyl)ethene monomers by chemical or
electrochemical means. Because low energy gap polymers are
in demand for their intrinsic conductivity and third order
non-linear optical properties, to further reduce the E value of
g
the PDTEs obtained, up to now three main strategies have been
followed:
1
introduction of electron donating or electron withdrawing
1,2
substituents on the polymer main chain;
2
a regular alternation of electron donating and electron
3
withdrawing substituents on the polymer main chain;
3
stiffening of the monomer unit around the vinylene
4
linkage.
The second approach belongs to the general donor-acceptor
strategy to low band gap polymers and, in the case of PDTEs,
was implemented by Sotzing et al. through the alternation
1
Scheme 1 Repeating units and E of the lowest band gap PDTEs
g
reported in the literature.
of ethylenedioxy-substituted thiophene rings and cyano-
substituted ethene units. Scheme 1 summarises the PDTEs, with
the lowest band gaps reported in the literature: as shown, the
cyano group is the only electron withdrawing substituent used
up to now. In fact, this is one of the few groups that can be
introduced on the ethene double bond of a poly[(arylene-2,5-
diyl)ethene] system without producing a major distortion of its
planar conformation; more sterically demanding substituents
have been shown in some cases to dramatically decrease the
rings. The idea stemmed from the simple consideration that
fluorine is the most electronegative element of the periodic table
and its covalent radius (0.72 Å) is quite small, even compared to
that of carbon atoms (0.77 Å), so that no steric drive to major
distortions of the planar π-system is foreseen. In 1956, Dixon,
a researcher at the Jackson Laboratory of the then E. I. du
Pont de Nemours and Co., reported the preparation of a wide
range of fluorinated olefin derivatives through low temperature
reaction of aromatic and aliphatic lithium derivatives with
5
conjugation degree of the polymer distorting the macro-
molecular chain around the arylene–ethene single bond.
Furthermore, cyanoethene units are readily obtained by the
Knoevenagel condensation of an aryl-aldehyde with aryl-
acetonitrile.
Searching new electron-withdrawing groups for PDTE sys-
tems, we addressed the possibility of introducing fluorine atoms
on the vinylene linkage between two subsequent thiophene
6
fluoroolefins; among others, three 1,2-difluoro-1,2-bis(2-aryl)-
ethenes were described (Scheme 2). Unfortunately, the article
provides scarce experimental details and only outlines a general
synthetic procedure for all the compounds prepared. Moreover
nothing is said about the cis- or trans- configuration of the
1,2-difluoro-1,2-bis(2-aryl)ethenes obtained.
1
752
J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b203589h

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Scheme 3 Synthesis of 1,2-difluoro-1,2-dithienylethenes following the
6
experimental conditions reported by Dixon.
Scheme
2
Synthesis of three 1,2-difluoro-1,2-diarylethenes as
6
II. It is known from the literature that α-protons of
oligothiophenes are more acidic than the α-protons of the
corresponding monomers. For instance, in the synthesis of
reported by Dixon.
In the present work we provide a detailed description of the
synthesis and the characterisation of a series of (E)-1,2-
difluoro-1,2-bis(2-thienyl)ethenes (DFDTEs) by low temper-
ature reactions of 2-lithiothiophenes with tetrafluoroethene
2
,2Ј-bithiophene by oxidative coupling of 2-lithiothiophene
the equilibrium in Scheme 4 takes place and the formation
9
of higher oligomers is observed.
(
TFE). We also report the low temperature crystal structure and
discuss the solid state organisation of a representative system.
Spectroscopic and structural data pertaining to these molecules
provide a unique insight into the effect of fluorination of the
ethene double bond on the chemical, optical and electronic
properties of 1,2-bis(2-aryl)ethenes such as PDTEs. The interest
of these systems is heightened by their potential as monomers
for oxidative polymerisation to give poly[(E)-1,2-difluoro-
1
,2-bis(2-thienyl)ethenes]. The synthetic method outlined
can also be extended, by changing the stoichiometry, to the
one pot synthesis of 1,2-difluoro-2-thienylethene oligomers or
polymers.
Scheme 4 Equilibrium reactions during the oxidative coupling of 2-
lithiothiophene.
III. In the reaction of 2-lithiothiophene derivatives with
TFE, the resulting product bears a fluoroethene group in the
Results and discussion
2
-position with respect to the 5-α-proton of the thiophene
A first attempt was made to prepare alkoxy-substitued
DFDTEs by reacting 2 equivalents of the lithium derivatives of
the alkoxythiophenes with TFE according to the procedure
ring. Under our working hypothesis (i.e. exploiting the electro-
negativity of fluorine atoms), this group should withdraw elec-
trons from thiophene thus increasing the acidity of its α-proton.
This hypothesis is also confirmed by the low field 0.2 ppm
shift of the 5-proton of 2-thienyltrifluoroethene (isolated as an
intermediate of the reaction) with respect to the starting thio-
phene unit. As a result, the unreacted 2-lithiothiophene
derivatives present in the reaction mixture could favourably
exchange lithium with the formed compounds, leading to a
chain lengthening irrespective of the starting stoichiometric
ratios (Scheme 5).
If this explanation is correct, the same experiment carried
out on an α-capped thiophene compound would afford a higher
yield of the (E)-1,2-difluoro-1,2-bis(2-thienyl)ethene derivative
and no higher oligomers, since the product would be unable to
exchange lithium with the unreacted lithiothiophene molecules
at any stage of the reaction. We thus performed a synthesis
analogous to those in Scheme 3 but starting from 2-methyl-
thiophene (Scheme 6): after flash-chromatographic purification
and recrystallisation from petroleum ether pure compound 11
was obtained in a 43% yield. Furthermore, parallel TLC
analysis of 11 and of the gross product showed the former as
being the main product of only two products, the other one
most probably being the cis-isomer.
6
described in the literature. We used two alkoxy derivatives
namely a methoxy and an octyloxythiophene; the lithium
derivatives were prepared using LDA at Ϫ78 ЊC and butyl-
lithium at 0 ЊC to room temperature in THF, in order to obtain
the desired regiochemistry: 2-lithium-4-alkoxy and 2-lithium-3-
alkoxythiophene respectively. In all cases, the gross product was
a red pitch from which we were unable to isolate the expected
compounds even after repeated flash chromatographic purifi-
cation. TLC analysis showed that we had indeed obtained
a complicated mixture of compounds, suggested by UV-vis
absorption spectra to be higher oligomers (λmax(CHCl )/nm
3
4
30–450). Faced with these problems, we decided to repeat the
synthesis starting from thiophene itself, attempting to repro-
duce Dixon’s work with respect to this etherocycle (Scheme 3).
The result was similar to our previous results, but in this case we
were able to isolate both the expected product 8 and one of its
higher analogues 9 as yellow solids. Yields of pure compounds
were quite low (9% and 4% respectively). Under these condi-
tions the reaction clearly does not stop at the stoichiometric
product. On the other hand, the same experiment carried out at
lower temperatures did not proceed at all, in agreement with
7,8
observations reported by Starostina et al.
In accordance with the results reported above, we addressed
the possibility of using a protective group for one of the two α
positions of the thiophene ring in order to prepare alkyl and
alkoxy substituted (E)-1,2-difluoro-1,2-bis(2-thienyl)ethene
in higher yields. The trimethylsilyl (TMS) group was chosen
since it can be easily and selectively introduced and removed
from the thiophene ring and is claimed to favour the oxidative
A thorough analysis of all available information lead us to
formulate the following hypothesis on the reaction mechanism:
I. In a broad perspective, the lithiation process can be
considered an acid-base reaction in which the strongest acid
(
(
i.e. the substrate) displaces the weakest acid from its salt
i.e. the lithiating agent).
J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759
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Table 1 Yields, melting points and UV-vis absorption spectra data of compounds 8, 9, 11 and 14(a–c) in CHCl solution. Commercial (Aldrich)
3
1
,2-bis(2-thienyl)ethene (DTE) reported for comparison. Notation (sh) means shoulder
UV-vis
a
Compound
Yield (%)
Mp/ЊC
λ
ε(λmax
)
DTE
8
—
9
4
43
31
8
132
90
160
79
105
120
72
338
25000
318, 332, 352
374, 396, 420
328, 344, 362
332, 348, 368
353 (sh), 370, 388
327, 352 (sh), 372 (sh)
36000 (332)
62000 (396)
39000 (344)
49500 (348)
31600 (370)
20000 (327)
9
1
1
1
1
1
4a
4b
4c
20
a
Determined as onset by differential scanning calorimetry (DSC).
Scheme 5 Proposed mechanism for the formation of higher oligomers in the reaction between 2-lithiothiophene derivatives and TFE.
Scheme 6 Synthesis of (E)-1,2-difluoro-1,2-bis(5-methyl-2-thienyl)-
ethene.
10-19
polymerisation of the final compounds.
TMS protected thio-
phene derivatives 13(a–c) were thus prepared by the lithiation
of thiophene compounds 12(a–c) followed by quenching with
trimethylsilyl chloride (Scheme 7). After distillation under
reduced pressure, all the products were isolated in high yields.
Further reaction with butyllithium (BuLi) followed by addition
of TFE in a 2 : 1 ratio, purification by flash chromatography
and recrystallisation from petroleum ether afforded pure 14 as
a yellow solid. In the case of unsubstituted thiophene 13a, pro-
tection of one of the α positions with TMS resulted in a three-
fold increase (from 9% to 31%) of the reaction yield. The same
protective group allowed us to isolate a β-methoxy (14b) and a
β-methyl (14c) substituted DFDTE though with lower yields
and in the presence of some by-products, as shown by the TLC
analysis of the gross products. Two of the by-products of 14b
and 14c, even if present in small amounts were isolated and
characterised as the cis isomer and the monosubstituted tri-
fluoroethene. Even taking into consideration the difficulty in
measuring accurately the volume of TFE actually introduced
into the reaction mixture, these data suggest that the presence
of other substituents at the β position of the thiophene ring
decreases the effectiveness of TMS as a protecting group. Since
it is known that trimethylsilyl bonds are easily cleaved by a
fluorine ion because of the high affinity that fluorine has for
silicon and the strength of the Si–F bond, we performed the
reactions in diethyl ether instead of THF in order to reduce
the concentration of fluorine ions in solution derived from the
presence of the by-product LiF.
Scheme 7 Trimethylsilyl protection in the synthesis of (E)-1,2-
difluoro-1,2-dithienylethenes.
(DTE), i.e. the non-fluorinated analogues of 8, are also
1
13
19
reported. H, C and F NMR, IR and Raman characteris-
ation are reported in the Experimental section.
All spectroscopic data are consistent with the expected
structures and vibrational spectra clearly indicate that the
configuration of all the compounds is trans. In fact, in all cases
the C᎐C stretching of the ethene double bond is Raman active
but not IR active, as is the case for symmetrically substituted
bonds. The most intense band in the Raman spectra pertain to
the double bonds collective skeletal motion frequently called
20
Ϫ1
R mode between 1400 and 1450 cm . Furthermore all
fluorinated products have C᎐C stretching frequencies of the
Ϫ1
Table 1 summarises the yields, melting points, and UV-vis
absorption data of compounds 8, 9, 11 and 14. For the sake
of comparison, the same data for (E)-1,2-bis(2-thienyl)ethene
ethene double bond 13–48 cm higher than for DTE. In the IR
spectra, the C–F symmetric stretching band can be seen around
Ϫ1
1100–1130 cm .
1
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J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759

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are shielded by 0.25–0.35 ppm with respect to 8 and a coupling
constant of 1.1 Hz can be observed between the methyl protons
and C(4)H. A strong hyperconjugation between the α-methyl
substituents and the π electrons can thus be inferred. Com-
parison of the UV-vis spectra of (E)-1,2-bis(2-thienyl)ethene
and 8 shows the latter as having an absorption band much
better resolved in its vibrational components. A similar vibronic
structure is also observed in compounds 9, 11, 14a and, to
a lesser degree, in 14b. On the contrary, 14c presents a quite
different spectrum with a single broad absorption with a
maximum at 327 nm and two shoulders at 352 and 372 nm.
UV-vis spectra of 8, 14a and 14b are reported in Fig. 2.
Fig. 2 UV-vis spectra in CHCl of 8, 14a and 14b.
3
To improve our understanding of the effect of 1,2 fluorine
substitution on conjugated double bonds in (E)-1,2-bis(2-
thienyl)ethenes a single crystal X-ray diffraction study of 14b
was carried out. Because crystals were very small and highly
unstable in the beam, data had to be acquired at low tempera-
ture (120 K) using synchrotron radiation at ESRF. A view of
the molecular structure of 14b is shown in Fig. 3 and selected
1
1
19
Fig. 1 NMR spectra (CDCl ) of 8: H-NMR (a), H-NMR, F broad
3
19
1
decoupling (b), F-NMR, H coupled (c).
1
For the correct interpretation of the H-NMR spectra,
19
decoupling from F nuclei was necessary, the later causing a
splitting of the signals with a JH,F of about 1.7 Hz.
As a representative example of the NMR characterisation,
the spectra of compound 8 are reported in Fig. 1. The com-
parison of H spectra (Fig. 1a and 1b) shows that proton 3
1
Fig. 3 A view of molecule 19b at 120 K, with atomic displacement
parameters drawn at 90% probability level with the labelling used in
the text. The molecule has a crystallographic inversion centre at the
midpoint of the ethene double bond while symmetry related atoms are
primed in the text and in Table 2.
does not couple with fluorine while protons 4 and 5 are coupled
with both fluorine atoms with the same coupling constant
19
(
J = 1.7 Hz). The quintet in the F spectrum can be explained
considering the coupling of fluorine with protons 4, 5, 4Ј and 5Ј
with the same coupling constant. A comparison of the chemical
shifts of the aromatic protons of DTE and of 8 show the latter
as being shifted towards lower magnetic fields of 0.12–0.34
ppm, depending on the position. This effect is distinctive of
an electron-withdrawing group and can also be observed for 9
and 14a while in 14b and 14c it is partially counterbalanced by
the presence of the electron-donating β-methoxy and -methyl
substituents. The aromatic protons of compound 11, instead,
molecular dimensions are reported in Table 2. The trans-ethene
system is very close to planarity, as apparent from the torsion
angles in Table 2, and has a crystallographic inversion centre at
its midpoint. The atomic displacement parameters of C(1)
and F(1) are small and closely comparable to those of the thio-
phene ring atoms. This fact implies the absence of significant
torsional disorder along the C(2)–C(1) bond, of the kind found,
especially at room temperature, in some molecules of the (E)-
J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759
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Table 2 Selected molecular dimensions of 14b
electron diffraction data on fluorinated ethenes, the tendency
27,30
of fluorine substitution to shorten double bonds
is hardly
Bond lengths (Å)
relevant in the present case: note that the average length of
alternated double and single bonds in hydrocarbon multiply
S(1)–C(5)
S(1)–C(2)
F(1)–C(1)
O(1)–C(4)
C(2)–C(3)
C(2)–C(1)
C(1)–C(1)Ј
C(5)–C(4)
C(3)–C(4)
1.7189(15)
1.7253(15)
1.3492(17)
1.3614(17)
1.378(2)
1.4402(19)
1.341(3)
31
conjugated systems are 1.345 and 1.443 Å respectively.
Irrespective of the dissymmetric substitution pattern, endo-
cyclic thiophene bond lengths are quite symmetric and closely
correspond with the expected values. The bond angle at the
oxygen substituted C(4) atom is nearly 4 degrees larger than
all the other intraannular bond angles, consistent with the
1.379(2)
1.4208(19)
29,32
expected substituent effects.
A similar but much larger
widening occurs at the fluorine substituted C(1) atom, the C(2)–
C(1)–C(1)Ј bond angle measuring 129.7(2)Њ as compared to
angles C(2)–C(1)–F(1) and F(1)–C(1)–C(1)Ј which are 114.4(1)Њ
and 115.9(2)Њ respectively. Comparable values of 127.7(7)Њ and
Bond angles (Њ)
1
29.2(9)Њ respectively of the H–C–C bond angles at fluorine
C(5)–S(1)–C(2)
C(4)–O(1)–C(6)
C(3)–C(2)–C(1)
C(3)–C(2)–S(1)
C(1)–C(2)–S(1)
C(1)Ј–C(1)–F(1)
C(1)Ј–C(1)–C(2)
F(1)–C(1)–C(2)
C(4)–C(5)–S(1)
C(2)–C(3)–C(4)
C(5)–C(4)–C(3)
O(1)–C(4)–C(5)
O(1)–C(4)–C(3)
C(4)–C(5)–Si(1)
S(1)–C(5)–Si(1)
93.36(7)
substituted carbons have been reported in gas-phase electron
diffraction studies of mono and trans-difluoroethene. At
fluorine substituted C–C–C bond angles, values ranging from
116.23(12)
124.84(13)
111.08(11)
124.05(11)
115.91(16)
129.68(17)
114.41(12)
109.20(11)
111.37(13)
114.99(13)
119.22(13)
125.78(13)
128.80(11)
122.00(8)
27
1
27.2(3) to 132.6(2)Њ result also in α,ω-diarylperfluoro-
25,26
polyenes
while 131.4(1) and 127.7(4)Њ were found in
24
potassium hydrogen difluoromaleate and fumarate. The
widening of this angle in fluorinated conjugated systems needs
further investigation; we suggest that steric factors are likely to
play some role, especially determining the larger values, while
electronic effects may well be dominant in most other cases.
The torsion angles in Table 2 show that, with the obvious
exception of the trimethylsilyl groups, the molecule is closely
planar and even the methoxy group, which shows the largest
deviation, is only 4Њ away from coplanarity with the conjugated
system [C(6)–O(1)–C(4)–C(5) = 176.1(2)Њ]. The angle between
the central ethene and the thiophene planes is roughly 2Њ as
compared to ca. 18Њ found at 118 K in (E)-stilbene.
The planar conformation of 14b appears to be favoured by
weak interactions involving the fluorine atoms. The distances
from F(1) to H(3), the hydrogen atom on C(3), and to S(1)Ј are
2
0
Torsion angles (Њ)
21,22
C(5)–S(1)–C(2)–C(1)
C(3)–C(2)–C(1)–C(1)Ј
S(1)–C(2)–C(1)–C(1)Ј
C(3)–C(2)–C(1)–F(1)
C(6)–O(1)–C(4)–C(3)
Ϫ177.49(13)
Ϫ177.4(2)
0.5(3)
1.4(2)
Ϫ3.6(2)
33
.547 and 2.831(2) Å respectively, amounting to about 0.1 and
.5 Å less than the sum of Van der Waals radii (1.20, 1.47 and
2
1,22
stilbene family.
Accordingly the bond lengths in the crystal
1.80 Å for H, F and S). The F ؒ ؒ ؒ S distance is identical to the
shorter O ؒ ؒ ؒ S interactions (2.835(2) Å) found in planar 3,3Ј-
structure of 14b are expected to be accurate. The C(1)–C(1)Ј
bond measures 1.341(3) Å and is at the upper end of values
found in (E) and (Z)-difluoro substituted, alternated double
bonds: for tetrafluoro-p-benzoquinone (at 113 K) a value of
29
dialkoxy-2,2Ј-bithiophene. The angles C(1)–C(2)–C(3) and
C(1)–C(2)–S(1) are 124.8(1)Њ and 124.1(1)Њ, suggesting the more
favourable nature of the F ؒ ؒ ؒ S interaction as compared to
the H ؒ ؒ ؒ F interaction. An additional short intermolecular
contact involving F(1) and one of the trimethylsilyl group
hydrogens [H(9A)Љ ؒ ؒ ؒ F(1) = 2.477 Å] is likely to play a
stabilising role in the packing.
23
1
.339(2) Å was reported, 1.333(2) and 1.307(9) for potassium
hydrogen difluorofumarate (at 123 K) and maleate (at rt)
24
respectively, while values between 1.329(5) and 1.315(3) Å,
depending essentially on the degree of conjugation and the
deviations from planarity, have been reported at rt for two α,ω-
25,26
27
diarylperfluoropolyenes.
Electron diffraction studies of
Conclusions
(
E)- and (Z)-difluoroethenes give C᎐C double bond values of
᎐
1
1
.331(4) and 1.329(4) Å, to be compared with 1.331(1) and
.311(3) Å for monofluoroethene and tetrafluoroethene. In 14b
In this paper we present effective synthetic routes to 1,2-di-
fluoro-1,2-bis(2-thienyl)ethenes along with their spectroscopic
and structural characterisation. It was demonstrated that
protection of the α position of the thiophene units greatly
enhanced the reaction yield, suppressing H–Li exchange in the
intermediate products. If unprotected thiophene units were
used with a 1 : 1 stoicheometry between the Li derivative and
TFE, longer oligomers or polymers could be obtained. One
short oligomer 9 was isolated and characterised. Fluorine
substitution appears to be compatible with significant π-
delocalisation and favours the planarity of the systems. The
crystal structure of derivative 14b shows intramolecular inter-
actions which may play a role stabilising planar arrangements:
they involve the fluorine atoms and both the hydrogen and the
sulfur atoms on adjacent thiophenes. Dynamic torsional dis-
ordering of the kind observed in (E)-stilbenes is virtually
absent in our systems at 120 K. Comparative bond length exam-
ination suggests that effective conjugation occurs: the C–C
bonds adjacent to the difluorinated double bond are in fact
quite short, while the ethene bond in the fluorinated system
the length of the C(1)–C(2) bond, adjacent to the double bond
is 1.440(2) Å: it is at the lower end of values of corresponding
bond lengths in conjugated 1,2 difluorinated systems. In the
case of tetrafluoro-p-benzoquinone we have 1.475 (2) Å,
1
and 1.502(6) Å for the corresponding non-planar fumarate.
Values between 1.455(3) and 1.431(5) Å, depending upon the
degree of conjugation and the deviations from planarity are
found in the two cited α,ω-diarylperfluoropolyenes.
that in low temperature crystal structures of non-fluorinated
stilbenes, bonds adjacent to the central double bond measure
typically between 1.479(1) and 1.472(1) Å
crystal structure of a vinylene thiophene has a value of 1.445(5)
Å. Inter-ring C–C bond lengths of 1.454(4) and 1.467(5) Å,
respectively, have been reported for coplanar 3,3Ј- and 4,4Ј-
dipentoxy-2,2Ј-bithiophenes. All these data give a subtle but
consistent picture confirming that in 14b the difluorovinylene
system is significantly conjugated. Consistent with gas-phase
23
.499(1) Å for the planar potassium hydrogen difluoromaleate
24
25,26
Note
21,22
while the only
28
28
1
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J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759

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seems marginally shorter than in non-fluorinated conjugated
Materials
systems, consistent with the higher C᎐C stretching frequency
᎐
Butyllithium (hexane solution), diisopropylamine, 2-methyl-
thiophene, 3-methylthiophene, thiophene and trimethylsilyl
choride were purchased from Aldrich. Sodium sulfate,
ammonium chloride and the molecular sieves were purchased
from Fluka. All solvents were Riedel-de Haën. Petroleum ether
refers to bp 40–60 ЊC. Tetrafluoroethene was a kind gift of
Ausimont SpA., 3-methoxythiophene was prepared according
determined in fluorinated systems. In 14b it is on the other
hand significantly longer than in most 1,2-difluorinated ethene
moieties. The observed features relate the modest steric
requirements of the fluorine substituent to the fact that its
strong σ-acceptor properties are complemented by significant
π-donor abilities. Rather small effects on bond lengths result
while large effects can be expected on charge mobility. Syn-
thesis, characterisation and potential applications of polymers
based on the building blocks described herein, have been
38
to the literature.
(
[
E )-1,2-Difluoro-1,2-bis(2-thienyl)ethene (8) and 2,5-bis{(E )-
1,2-difluoro-2-(2-thienyl)]ethenyl}thiophene (9)
34
discussed elsewhere.
Butyllithium (1.6 M, 27.5 ml, 44 mmol) was cautiously added
dropwise over 10 min to a stirred solution of thiophene (3.7 g,
Experimental
4
4 mmol) in THF (100 ml) at 0 ЊC. The resulting mixture was
General methods
allowed to warm up to room temperature over a period of 1 h
before being cooled down to Ϫ40 ЊC and a solution of TFE
All reactions were carried out under an inert atmosphere of
argon and in anhydrous conditions. Reaction solvents were
dried by distillation over a K–Na alloy while reagents were
dried overnight on 4 Å molecular sieves. Trimethylsilyl chloride
and TFE were used as received and the required amount of
gas calculated according to the equation of state for a perfect
gas. TFE solutions were prepared by dissolving the gas in the
reaction solvent at Ϫ80 ЊC (bp of TFE Ϫ75.6 ЊC). Melting
points were determined as onset by Differential Scanning
Calorimetry using a Mettler DSC 30 with Mettler TA3000 pro-
(
1
20.3 mmol) in THF (200 ml) at Ϫ70 ЊC being added. After
5 min at Ϫ40 ЊC the reaction was quenched with saturated
aqueous NH Cl and the solvent evaporated under reduced
4
pressure. The gross product was then extracted with diethyl
ether (2 × 100 ml) and the combined extracts washed with water
to neutrality, dried over sodium sulfate, filtered and evaporated
under reduced pressure to leave a red–orange oil. Purification
by flash chromatography using petroleum ether as eluent and
recrystallisation from the same solvent afforded 8 (0.46 g, 9%)
and 9 (0.20 g, 4%) as yellow solids.
Ϫ1
cessor (heating rate 10 ЊC min ). UV-vis absorption spectra in
CHCl solution were recorded by a Jasco V-570 spectrometer.
Vibrational spectroscopy was performed using a Nicolet FT-IR
Magna 560 (solid in KBr), a Nicolet FT-Raman 910 (solid).
3
(
E )-1,2-Difluoro-1,2-bis(2-thienyl)ethene (8). δH (500 MHz;
19
CDCl3; F decoupling) 7.13 (2H, dd, J4,3 3.8, J4,5 5.0, Th(4)-H),
.40 (2H, dd, J3,4 3.8, J3,5 1.2, Th(3)-H), 7.45 (2H, dd, J5,3 1.2,
J5,4 5.0, Th(5)-H); δH (500 MHz; CDCl ) 7.13 (2H, m, J 3.8,
7
NMR spectra in CDCl were collected using Bruker Instru-
3
3
4,3
ments as specified for single spectra data.
J4,5 5.0, J4,F 1.7, Th(4)-H), 7.40 (2H, dd, J3,4 3.8, J3,5 1.2, Th(3)-
H), 7.45 (2H, m, J5 1.2, J5,4 5.0, J5,F 1.7, Th(5)-H); δF (500
,3
1
MHz; CDCl ; H broad decoupling) Ϫ148.4 (2F, s); δ (250
X-Ray analysis†
3
C
1
MHz; CDCl ; H broad decoupling) 125.8 (2C, t, J 3.7, JCF2
3
CF1
Preliminary data collections of 14b with a laboratory X-ray
source yielded very poor quality data because of small crystal
size and very rapid crystal decay. Intensity data were therefore
collected at the Swiss–Norwegian Beamlines at ESRF, at T =
3
.7), 127.8 (2C, t, JCF1 3.7, JCF2 3.7), 128.0 (2C, s), 131.7 (2C, t,
JCF1 9.25, J 9.25), 144.7 (2C, dd, JCF1 275.5, JCF2 92.6, = CF);
CF2
Ϫ1
νmax/cm (FTIR) 3112s (ring CH), 1439s (ring C᎐C), 1356s
(
(
ring C᎐C), 1256s, 1228s, 1121s (CF), 1046s (ring CH), 853s
᎐
1
0
20(2) K on a minute crystal of 14b (size 0.10 × 0.02 ×
.02 mm) with a MAR345 image plate using synchrotron
ring CH), 828s (ring CH), 714vs (ring CH), 698vs (ring CH);
Ϫ1
νmax/cm (FT-Raman) 3112w (ring CH), 1665vs (vinyl C᎐C),
radiation (λ = 0.7000(2) Å). The 345 mm diameter of the image
plate has been used with a pixel resolution of 0.15 µm. A full
rotation around the spindle axis of the MAR345 has been per-
formed, with a rotation width of 2Њ; the sample to detector
distance was set to 120 mm for a resolution at the edge of the
image plate of 0.70 Å (2θmax = 59.82Њ). 178 images were pro-
cessed and scaled with the program CrysAlisRED. Cell
dimensions and space group were determined from 2894 very
intense reflections selected throughout the data collection. A
total of 22604 reflections have been integrated, 3262 of which
are unique (Rint = 0.0364), for a final completeness of the data
set of 98%. The structure was solved by direct methods using
1
649vs (vinyl C᎐C), 1420vs ( R m ode), 1080w, 853w, 635w.
᎐
2
,5-Bis[(E )-1,2-difluoro-2-(2-thienyl)ethenyl]thiophene
(9).
19
δ (500 MHz; CDCl ; F decoupling) 7.14 (2H, dd, J4,3 3.8, J4,5
H
3
5
.1, Th(4)-H), 7.38 (2H, s, Th(3Ј)-H and Th(4Ј)-H), 7.43 (2H,
dd, J3,4 3.7, J3,5 1.2, Th(3)-H), 7.48 (2H, dd, J5,3 1.2, J5,4 5.1,
Th(5)-H); δH (500 MHz; CDCl ) 7.14 (2H, m, J 3.7, J4,5 5.1,
35
3
4,3
J4,F 1.5, Th(4)-H), 7.38 (2H, br s, Th(3Ј)-H), 7.43 (2H, dd, J3,4
.7, J3,5 1.2, Th(3)-H), 7.48 (2H, m, J 1.2, J5,4 5.1, J5,F 3.2,
3
5,3
1
Th(5)-H); δ (500 MHz; CDCl ; H broad decoupling) Ϫ144.4
(2F, d, JF1,F2
F
3
Ϫ1
114.9), Ϫ147.6 (2F, d, JF2,F1 114.9); νmax/cm
(
(
(
(
FTIR) 3103br (ring CH), 1428s (ring C᎐C), 1240s, 1133vs
᎐
36
the program SIR92 and was refined by full matrix least-
CF), 1052s (ring CH), 851s ( ring CH), 804vs (ring CH), 712
2
37
squares on F using SHELX97. All non-hydrogen atoms have
been refined anisotropically. Clear difference Fourier maps
enabled the location and free refinement of all the hydrogen
atoms in the structure.
Ϫ1
ring CH), 696vs (ring CH), 525s; νmax/cm (FT-Raman) 1646s
vinyl C᎐C), 1630vs (vinyl C᎐C), 1449s, 1418vs ( R m ode).
᎐
᎐
(E )-1,2-Difluoro-1,2-bis[5-methyl-(2-thienyl)]ethene (11)
Crystal data for 14b: C H F O S Si , FW 432.69, Mono-
clinic, space group C2/c, T = 120(2) K, a = 15.537(3) Å, b =
18
26
2
2
2
2
Butyllithium (1.6 M, 20 ml, 32 mmol) was cautiously added
dropwise over 10 min to a stirred solution of 2-methylthiophene
(3.1 ml, 32 mmol) in THF (50 ml) at 0 ЊC. The resulting mixture
was allowed to warm up to room temperature over a period of
3
1
6.739(2) Å, c = 10.481(2) Å, β = 126.46(2)Њ, V = 2192.3(14) Å ,
Ϫ3
Ϫ1
Z = 4, D = 1.311 mg m , µ = 0.379 mm , F(000) = 912, 3262
C
unique reflections, 162 parameters, final agreement factors (all
data): R1 = 0.0389, wR2 = 0.0975, Goof = 1.136.
1
h before being cooled down to Ϫ40 ЊC and a solution of
TFE (16 mmol) in THF (200 ml) at Ϫ70 ЊC being added. The
reaction mixture was then allowed to warm up to room tem-
perature overnight and was quenched with saturated aqueous
†
CCDC reference number 185122. See http://www.rsc.org/suppdata/
NH Cl. Work-up and purification as for 8 afforded 11 (1.75 g,
p2/b2/b203589h/ for crystallographic files in .cif or other electronic
4
19
format.
43%) as a yellow solid; δ
H
(500 MHz; CDCl
;
3
F decoupling)
J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759
1757

View Article Online
2
.52 (6H, d, JMe,4 1.2, Th(5)Me), 6.76 (2H, dd, J4,3 3.7, J4,Me 1.1,
to leave a pale yellow liquid which was mainly the 5-trim-
Th(4)-H), 7.15 (2H, d, J3,4 3.7, Th(3)-H); δ (500 MHz; CDCl )
ethylsilyl derivative, with a minor content of the 2-trimethylsilyl
isomer. Distillation under reduced pressure (70 ЊC, 15 mmHg)
afforded 13c (2.6 g, 61%) as a colourless liquid (98% GC pure).
δH (300 MHz; CDCl ) 0.32 (9H, s, 5-SiMe ), 2.32 (3H, s,
H
3
2
3
.52 (6H, dd, JMe,4 1.2, JMe,F 0.5, Th(5)Me), 6.76 (2H, m, J4,3
.7, J4,Me 1.1, J4,F 1.7, Th(4)-H), 7.15 (2H, d, J 3.7, Th(3)-H);
3,4
1
δF (500 MHz; CDCl ; H broad decoupling) Ϫ148.7 (2F, s);
3
3
3
1
δC (300 MHz; CDCl ; H decoupling) 15.6 (2C, s, Th(5)Me),
3-Me ), 7.06 ( 1H, d, J 0.9, 4-H), 7.17 (1H, d, J2,4 1.0, 2-H).
3
3 4,2
1
9
25.4 (2C, t, JC,F1 5.0, JC,F2 5.0), 126.1 (2C, s), 129.3 (2C, t, JC,F1
.9, JC,F2 9.9), 142.4 (2C, t, JC,F1 4.1, J
4.1), 144.0 (2C, dd,
C,F2
Ϫ1
(E )-1,2-Difluoro-1,2-bis(5-trimethylsilyl-2-thienyl)ethene (14a)
JC,F1 273.5, JC,F2 92.6, = CF); νmax/cm (FTIR) 1472s (ring
C᎐C), 1378s (ring C᎐C), 1225s, 1118vs (CF), 1039s (ring CH),
Butyllithium (2.5 M, 10.8 ml, 27 mmol) was cautiously added
dropwise over 10 min to a stirred solution of 2-trimethyl-
silylthiophene (4.5 ml, 27 mmol) in diethyl ether (100 ml) at
᎐
᎐
Ϫ1
7
97vs (ring CH), 788vs (ring CH), 524s; νmax/cm (FT-Raman)
1
662vs (vinyl C᎐C), 1651vs (vinyl C᎐C), 1447vs ( R m ode).
᎐
᎐
0
ЊC. The resulting mixture was then allowed to warm up to
room temperature over a period of 1 h before being cooled
down to Ϫ30 ЊC and a solution of TFE (13.5 mmol) in diethyl
ether (200 ml) added at Ϫ70 ЊC. The reaction mixture was kept
at Ϫ40 ЊC overnight and was finally quenched with saturated
2
-Trimethylsilylthiophene (13a)
Butyllithium (2.5 M, 54 ml, 134 mmol) was cautiously added
dropwise over 10 min to a stirred solution of thiophene
(
10.7 ml, 128 mmol) in THF (80 ml) at 0 ЊC. The resulting
aqueous NH Cl. Work-up and purification as for 8 afforded 14a
4
mixture was allowed to warm up to room temperature over a
period of 1 h before being cooled down again to 0 ЊC and
trimethylsilyl chloride (17.9 ml, 141 mmol) was cautiously
added dropwise over a period of 20 min. The reaction mixture
was then allowed to warm up to room temperature and was
quenched with 3 ml of water. The solvent was evaporated under
reduced pressure and the gross product extracted with diethyl
ether (2 × 100 ml). The combined extracts were washed with
water to neutrality, dried over sodium sulfate, filtered and evap-
orated under reduced pressure to leave a pale yellow liquid.
Distillation under reduced pressure (60 ЊC, 15 mmHg) afforded
(1.57 g, 31%) as a white–pale yellow solid; δ (500 MHz;
H
19
CDCl3; F decoupling) 0.36 (18H, s, -SiMe ), 7.23 (2H, d, J
3
4,3
3.7, Th(4)-H), 7.44 (2H, d, J3,4 3.7, Th(3)-H); δH (500 MHz;
CDCl ) 0.36 (18H, s, -SiMe ), 7.23 (2H, m, J 3.7, J4,F 1.8,
3
3
4,3
Th(4)-H), 7.44 (2H, d, J3 3.7, Th(3)-H); δ (500 MHz; CDCl ;
,4
F
3
1
H broad decoupling) Ϫ145.1 (2F, s); δC (250 MHz; CDCl3)
0.46 (6C, s, Th(5)SiMe ), 127.0 (2C, t, J 3.7, JC,F2 3.7),
3
C,F1
134.8 (2C, s), 136.6 (2C, t, JC,F1 9.2, JC,F2 9.2), 143.4 (2C, t, JC,F1
3.7, JC,F2 3.7), 144.7 (2C, dd, JC,F1 275.5, JC,F2 92.5, = CF);
Ϫ1
νmax/cm (FTIR) 2953 (CH ), 1515s, 1252vs, 1029s, 1118s (CF),
3
1061s (ring CH), 988vs, 839vs (ring CH), 811vs (ring CH),
Ϫ1
1
3a (18.3 g, 91%) as a colourless liquid (99% GC pure).
758s, 538s; νmax/cm (FT-Raman) 1653vs (vinyl C᎐C), 1422vs
δH (300 MHz; CDCl ) 0.35 (9H, s, -SiMe ), 7.21 (1H, dd,
R m ode).
3
3
J4,3 3.3, J4,5 4.6, 4-H), 7.29 (1H, dd, J3,4 3.3, J3,5 0.8, 3-H), 7.62
(
1H, dd, J5,3 0.8, J5,4 4.6, 5-H).
(E )-1,2-Difluoro-1,2-bis(4-methoxy-5-trimethylsilyl-2-thienyl)-
ethene (14b)
3
-Methoxy-2-trimethylsilylthiophene (13b)
Butyllithium (1.6 M, 12.1 ml, 19.4 mmol) was cautiously added
dropwise over 10 min to a stirred solution of 3-methoxy-2-
trimethylsilylthiophene (3.61 g, 19.4 mmol) in diethyl ether
(100 ml) at 0 ЊC. The resulting mixture was then allowed to
warm up to room temperature over a period of 1 h before being
cooled down to Ϫ30 ЊC and a solution of TFE (9.7 mmol) in
diethyl ether (250 ml) added at Ϫ70 ЊC. The reaction mixture
was kept at Ϫ40 ЊC overnight and was finally quenched with
Butyllithium (1.6 M, 52.5 ml, 84 mmol) was cautiously added
dropwise over 10 min to a stirred solution of 3-methoxythio-
phene (9.13 g, 80 mmol) in THF (50 ml) at 0 ЊC. The resulting
mixture was allowed to warm up to room temperature over a
period of 1 h before being cooled down again to 0 ЊC and
trimethylsilyl chloride (11.2 ml, 88 mmol) cautiously added
dropwise over a period of 20 min. The reaction mixture
was then allowed to warm up to room temperature and was
quenched with 3 ml of water. The solvent was evaporated under
reduced pressure and the gross product extracted with diethyl
ether (2 × 100 ml). The combined extracts were washed with
water to neutrality, dried over sodium sulfate, filtered and
evaporated under reduced pressure to leave a pale yellow
liquid which was mainly the 2-trimethylsilyl derivative, with a
minor content of the 5-trimethylsilyl isomer. Distillation under
saturated aqueous NH Cl. Work-up and purification as for 8
4
afforded 14b (0.33 g, 8%) as a pale yellow solid; δ (500 MHz;
H
19
CDCl3; F decoupling) 0.32 (18H, s, -SiMe ), 3.85 (6H, s,
3
-OMe), 7.16 (2H, s, Th(3)-H); δ (500 MHz; CDCl ) 0.32 (18H,
H
3
s, -SiMe ), 3.85 (6H, s, -OMe), 7.16 (2H, s, Th(3)-H); δ (500
3
F
1
MHz; CDCl ; H broad decoupling) Ϫ147.6 (2F, s); δ (400
MHz; CDCl ; H broad decoupling) 0.57 (6C, s, Th(5)SiMe ),
3
C
1
3
3
71.8 (2C, s, OMe), 115.0 (2C, t, JC,F1 4.8, JC,F2 4.8), 117.8 (2C, s),
134.6 (2C, t, JC,F1 9.6, JC,F2 9.6), 144.8 (2C, dd, J 274.6,
Ϫ1
reduced pressure (40 ЊC, 2 × 10 mbar) afforded 13b (11.2 g,
C,F1
Ϫ1
7
5%) as a colourless liquid (98% pure).
JC,F2 91.4, = CF), 163.9 (2C, s); νmax/cm (FTIR) 2957s (CH ),
3
δH (300 MHz; CDCl ) 0.37 (9H, s, -SiMe ), 3.87 (3H, s,
1352s (ring C᎐C), 1460s (ring C᎐C), 1374vs, 1249s, 1214s,
3
3
-
OMe ), 6.99 ( 1H, d, J 5.0, 4-H), 7.47 (1H, d, J5,4 4.9, 5-H).
1172s, 1092s (CF), 1026vs (ring CH), 842 (ring CH), 823s
3
4,5
Ϫ1
(
ring CH); νmax/cm (FT-Raman) 1651vs (vinyl C᎐C), 1409vs
R m ode).
3
-Methyl-5-trimethylsilylthiophene (13c)
Butyllithium (1.6 M, 17 ml, 27.2 mmol) was cautiously added
dropwise over 10 min to a stirred solution of diisopropylamine
(
E )-1,2-Difluoro-1,2-bis(3-methyl-5-trimethylsilyl-2-thienyl)-
ethene (14c)
(
3.4 ml, 25 mmol) in THF (180 ml) at 0 ЊC. The resulting
mixture was cooled to Ϫ80 ЊC over a period of 1 h after which
time 3-methylthiophene (2.4 ml, 25 mmol) was added. After
Butyllithium (1.6 M, 9.6 ml, 15.3 mmol) was cautiously added
dropwise over 10 min to a stirred solution of 3-methyl-5-
trimethylsilylthiophene (2.61 g, 15.3 mmol) in diethyl ether
(100 ml) at 0 ЊC. The resulting mixture was then allowed to
warm up to room temperature over a period of 1 h being cooled
down to Ϫ30 ЊC and a solution of TFE (7.7 mmol) in diethyl
ether (250 ml) added at Ϫ70 ЊC. The reaction mixture was
kept at Ϫ40 ЊC overnight and finally quenched with saturated
1
h at Ϫ80 ЊC, trimethylsilyl chloride (3.5 ml, 37.5 mmol) was
cautiously added dropwise over a period of 20 min and the
reaction maintained at the same temperature for 30 min before
being quenched with 3 ml of water. The solvent was then
evaporated under reduced pressure and the gross product
extracted with diethyl ether (2 × 100 ml). The combined
extracts were washed with water to neutrality, dried over
sodium sulfate, filtered and evaporated under reduced pressure
aqueous NH Cl. Work-up and purification as for 8 afforded 14c
4
19
(0.60 g, 20%) as a pale-yellow solid; δH (500 MHz; CDCl3;
F
1
758
J. Chem. Soc., Perkin Trans. 2, 2002, 1752–1759

View Article Online
15 P. Hapiot, L. Gaillon, P. Audebert, J. J. E. Moreau, J. P. Lereporte
decoupling) 0.35 (18H, s, -SiMe ), 2.39 (6H, s, Th(3)-Me), 7.02
3
and M. W. C. Man, Synth. Met., 1995, 72, 129.
6 J. L. Sauvajol, C. Chorro, J. P. Lereporte, R. J. P. Corriu,
J. J. E. Moreau, P. Thepot and M. W. C. Man, Synth. Met., 1994,
(
2H, s, Th(4)-H); δH (500 MHz; CDCl ) 0.35 (18H, s, -SiMe ),
3
3
1
2
.39 (6H, t, J
2, Th(3)-Me), 7.02 (2H, s, Th(4)-H); δ (500
Me,F F
1
MHz; CDCl ; H broad decoupling) Ϫ137.0 (2F, s); δ (250
MHz; CDCl ; H broad decoupling) 0.60 (6C, s, Th(5)SiMe ),
3
C
6
2, 233.
1
3
3
17 H. Masuda, Y. Taniki and K. Kaeriyama, J. Polym. Sci., Part A:
Polym. Chem., 1992, 30, 1667.
1
5.9 (2C, t, JC,F1 3.7, JC,F2 3.7, Th(3)Me), 129.9 (2C), 138.2
(
=
(
2C, br s), 140.3 (2C, s), 141.7 (2C, dd, JC,F1 353.4, JC,F2 183.1,
CF), 143.0 (2C, s); νmax/cm (FTIR) 1526s, 1122s (CF), 1021
18 J. Roncali, R. Garreau and A. H. Hoa, J. Electroanal. Chem., 1991,
312, 277.
Ϫ1
1
9 M. Lemaire, A. Guy and J. Roncalli, J. Electroanal. Chem., 1990,
81, 293.
2
ring CH), 1000s (ring CH), 831s (ring CH), 702s (ring CH);
2
Ϫ1
νmax/cm (FT-Raman) 2896w (CH ), 1656s (vinyl C᎐C), 1406s
(
3
0 M. Gussoni, C. Castiglioni and G. Zerbi in, Spectroscopy of
Advanced Materials, eds. R. J. H. Clark and R. E. Hester, Wiley
VCH, New York, 1991, vol. 19, p. 251.
R mode), 626s, 459vs.
2
1 K. Ogawa, T. Sano, S. Yoshimura, Y. Takeuchi and K. Toriumi,
J. Am. Chem. Soc., 1992, 114, 1041.
2 K. Ogawa, J. Harada and S. Tomoda, Acta Crystallogr., Sect. B:
Struct. Sci., 1995, 51, 240.
Acknowledgements
2
This work was performed with the financial support of the
Italian Ministry of Scientific Research MURST 2000. We
gratefully acknowledge Professor Fronza and Walter Panzeri
23 K. Hagen and D. G. Nicholson, Acta Crystallogr., Sect. C: Cryst.
Struct. Commun., 1987, 43, 1959.
2
2
19
for F NMR spectra and helpful discussions.
4 R. Mattes and D. Gohler, J. Mol. Struct., 1980, 68, 59.
5 V. M. Yurchenko, M. Y. Antipin, Y. T. Struchkov and
L. M. Yagupolski, Cryst. Struct. Commun., 1978, 7, 81.
6 V. M. Yurchenko, M. Y. Antipin, Y. T. Struchkov and
L. M. Yagupolski, Cryst. Struct. Commun., 1978, 7, 77.
2
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