Synthesis and properties of novel tetrathiafulvalene vinylogues

Article abstract of DOI:10.1039/a803531h

The cation radical states of novel TTF vinylogues with o-substituted phenyl groups at the vinyl positions are thermodynamically stable, and X-ray analyses of the cation radical salts have revealed the planar structures of the TTF vinylogue parts and unique crystal structures.

Full text of DOI:10.1039/a803531h

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Synthesis and properties of novel tetrathiafulvalene vinylogues
Yoshiro Yamashita,*† Masaaki Tomura, M. Badruz Zaman and Kenichi Imaeda
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
The cation radical states of novel TTF vinylogues with
o-substituted phenyl groups at the vinyl positions are
thermodynamically stable, and X-ray analyses of the cation
radical salts have revealed the planar structures of the TTF
vinylogue parts and unique crystal structures.
1a,m and the phenyl ones 1b,n show reversible one-stage two-
electron redox waves. The p-substituted derivatives 1g,h,q and
the m-substituted one 1i also undergo the similar two-electron
oxidation. This fact suggests that the cation radical states of
these compounds are thermodynamically unstable due to non-
planar structures, similar to the reported examples. On the other
hand, the o-substituted derivatives 1c–f,j,o,p,r showed two
reversible one-electron oxidation waves similar to the non-
substituted one 1t. This result suggests that the cation radicals of
the o-substituted phenyl derivatives are thermodynamically
stable. This can be explained by considering that the TTF
vinylogue skeletons are planar when there are o-substituents,
which increase the steric interactions and make the phenyl parts
twist away from the p-conjugated framework. In the case of
1-naphthyl derivatives 1k,s and 1-pyrenyl one 1l, the cation
radicals are also stable since they are also o-substituted phenyl
derivatives. The values of the oxidation potentials are depend-
ent on the substituents. BEDT-TTF analogues 1o,s show lower
first oxidation potentials than the non-substituted one 1t and
even TTF, indicating that they are stronger electron donors. The
differences between the first and second oxidation potentials are
smaller than those of TTF and BEDT-TTF, indicating the
reduced on-site Coulombic repulsion in 1 due to the extended
p-conjugation.
Tetrathiafulvalene (TTF) vinylogues
1
with extended
p-conjugation are promising electron donors for organic
R
R
R
R1
R1
H
S
S
S
S
R
S
S
R
R
R1
R
R
1
2
R1
S
S
+
S
*
S
R
R1
R
3
4
= +
= •
*
*
conductors due to their highly electron-donating properties and
reduced on-site Coulombic repulsion.1 The parent compound2
and the ethylenedithio derivative3 were synthesized several
years ago. Recently the derivatives of 1 containing substituents
such as aryl groups at the vinyl positions have been prepared by
several groups.4 They have been considered to be non-planar
molecules due to the steric interactions between the substituents
and the 1,3-dithiole parts, and X-ray analyses revealed the
severely deformed structures.4c,d,e Therefore, those TTF mole-
cules have not been used as components for organic conductors.
However, the TTF vinylogue skeletons can be planar when the
aryl substituents are twisted away from the p-conjugated
framework. These compounds are expected to afford con-
ductors with unusual structures since good molecular over-
lapping is disturbed by the bulky substituents. We have now
prepared various derivatives 1 by a convenient method and have
found that the cation radicals of 1 containing o-substituted
phenyl groups are thermodynamically stable and can be isolated
as single crystals.
The derivatives of 1 were previously prepared by electro-
chemical oxidative coupling of the corresponding 1,4-dithia-
fulvenes 2.4b,c,e,5 We have recently reported oxidative intra-
molecular cyclization to give bis(1,3-dithiole) electron donors
by using tris(4-bromophenyl)aminium hexachloroantimonate
as the oxidation reagent.6 We have now used this chemical
oxidation for the synthesis of 1. First, 1,4-dithiafulvene
derivatives 2 were prepared by either Wittig reaction of a
phosphonium salt (for the dibenzo derivatives) or Wittig–
Horner reaction of a phosphonium ester (for the ethylenedithio
derivatives) with the corresponding aldehydes. Reaction of 2
with the aminium salt in CH2Cl2 afforded dication salts 3 which
are formed by oxidative dimerization followed by oxidation.
The salts 3 were reduced with zinc in MeCN to give 1. The
yields of 1 from 2 were 20–85%. It is noteworthy that the
naphthyl and pyrenyl derivatives as well as phenyl derivatives
were prepared.
Electrochemical oxidation of 1j and 1p gave the cation
radical salts 1j·PF6 and 1p·Au(CN)2 as stable single crystals
with stoichiometries of 1:1. These are the first examples of
cation radical salts of TTF vinylogues containing substituents at
the vinyl positions. In order to investigate the geometries of the
donors, X-ray structure analyses of the neutral compounds 1a,j
Table 1 Oxidation potentials of donors 1a
Compound
R,R
R1
E/V
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
(CHNCH)2
SCH2CH2S
SCH2CH2S
SCH2CH2S
SCH2CH2S
SCH2CH2S
SCH2CH2S
SCH2CH2S
SCH2CH2S
H
Me
Ph
0.71 (2e)
0.61 (2e)
0.44, 0.62
0.53, 0.75
0.42, 0.63
0.59, 0.76
0.67 (2e)
0.58 (2e)
0.73 (2e)
0.66, 0.85
0.47, 0.71
0.50, 0.70
0.51 (2e)
0.49 (2e)
0.30, 0.54
0.38, 0.60
0.52 (2e)
0.51, 0.70
0.33, 0.58
0.37, 0.56
0.42, 0.65
0.35, 0.79
0.50, 0.85
o-MeC6H4
o-ClC6H4
o-OMeC6H4
o-NO2C6H4
p-ClC6H4
p-OMeC6H4
m-NO2C6H4
2,6-F2C6H3
1-naphthyl
1-pyrenyl
Me
Ph
o-MeC6H4
o-ClC6H4
p-ClC6H4
2,6-F2C6H3
1-naphthyl
H
1q
1r
1s
1t
1u
TTF
BEDT-TTF
2,6-F2C6H3
a 0.1 mol dm23 Bu4NPF6 in PhCN, Pt electrode, scan rate 100 mV s21, E/V
vs. SCE.
The oxidation potentials of the donors 1 (Table 1) were
measured by cyclic voltammetry (CV). The methyl derivatives
Chem. Commun., 1998
1657

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S(2*)
C(1*)
S(1*)
F(2*)
S(4)
S(1)
S(1)
C(1)
F(1*)
C(8)
C(8*)
F(1)
S(3)
S(2)
S(2)
F(2)
1a
F(2*)
S(2*)
F(2)
F(1*)
C(8)
F(1)
C(8*)
S(2)
C(1*)
C(1)
S(1)
S(1*)
1j
Fig. 1 Molecular structures of 1a and 1j
and the cation radical salts 1j·PF6 and 1p·Au(CN)2 were carried
out.‡ As expected from the CV data, the methyl derivative 1a
has a non-planar structure (Fig. 1) as found in the substituted
TTF vinylogues.4c,d,e The o-substituted phenyl derivative 1j
which undergoes step-wise oxidation was expected to have a
planar TTF vinylogue framework. However, the structure was
revealed to be similarly non-planar, as shown in Fig 1. In
contrast, the molecular structure is drastically changed upon
oxidation. The cation radical salt 1j·PF6 has a planar TTF
vinylogue skeleton, as shown in Fig. 2. The aryl groups are
twisted and almost orthogonal to the TTF vinylogue framework.
The dihedral angle between the aryl group and the 1,3-dithiole
ring is 83.9°. The C(1)–C(8) bond length [1.400(8) Å] in the
cation radical salt is longer than that of the neutral 1j [1.354(6)
Å], while the C(8)–C(8*) bond length [1.41(1) Å] is shorter than
the corresponding one in the neutral species [1.481(8) Å]. This
fact suggests the contribution of a delocalized structure 4. The
donor molecules form an interesting two-dimensional columnar
structure, as shown in Fig. 2. The bulky aryl groups disturb good
overlapping between molecules. Instead, one molecule bridges
two others; the 1,3-benzodithiole moieties form a stacking with
an equal intermolecular distance of 3.6 Å. The side view of the
crystal structure (Fig. 2, bottom) clearly shows a stair-like
network where two-dimensional interactions can be expected.
The donor molecule in 1p·Au(CN)2 also has a planar geometry
of its TTF vinylogue framework. The unique crystal structure
will be reported elsewhere. Unfortunately, these cation radical
salts have a 1:1 stoichiometry and so they showed semiconduct-
ing behavior (1j·PF6: s = 3 3 1026 S cm21, Ea = 0.18 eV;
1p·Au(CN)2: s = 1022 S cm21, Ea = 0.11 eV). Studies on the
preparation of other cation radical salts of 1 are now in
progress.
Fig. 2 Crystal structure of 1j•PF6. Top: structure of the donor molecule.
Middle: projection along the b-axis. Bottom: view perpendicular to the
stacking axis b.
reflections 3640, R
C28H14F4S4, M
=
0.055 for 2812 data with I
>
=
3s(I). For 1j:
20.124(9), b =
=
554.65, orthorhombic, Pbcn, a
8.724(7), c = 14.258(5) Å, V = 2503(3) Å3, Z = 4, T = 296 K, µ(Mo-Ka)
= 4.26 cm21, independent reflections 3283, R = 0.037 for 1166 data with
I > 3s(I). For 1j·PF6: C28H14F10PS4, M = 699.62, monoclinic, C2/c, a =
19.814(2), b = 7.1181(6), c = 20.069(1) Å, b = 104.588(6)°, V =
2739.3(4) Å3, Z = 4, T = 296 K, m(Cu-Ka) = 45.65 cm21, independent
reflections 3036, R = 0.058 for 1303 data with I > 3s(I). For 1p·Au(CN)2:
C28H16AuCl2N2S8, M = 880.78, monoclinic, P21/c, a = 10.527(1), b =
11.025(1), c = 13.001(3) Å, b = 100.26(2)°, V = 1484.8(4) Å3, Z = 2, T
= 296 K, m(Cu-Ka) = 164.1 cm21, independent reflections 3021, R =
0.056 for 2173 data with I > 2s(I). CCDC 182/905
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This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture, Japan.
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Notes and References
† E-mail: yoshiro@ims.ac.jp
‡ Crystal data for 1a: C18H14S4, M = 358.55, monoclinic, P21/c, a =
12.1233(4), b = 7.4016(3), c = 18.5788(6) Å, b = 98.196(3)°, V =
1650.08(9) Å3, Z = 4, T = 296 K, m(Cu-Ka) = 52.11 cm21, independent
Received in Cambridge, UK, 12th May 1998; 8/03531H
1658
Chem. Commun., 1998