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Dication charge transfer salts of TTF vinylogues
ELSEVIER
Synthetic Metals86 (1997) 1831-1832
Dication Charge Transfer Salts of TTF Vinylogues D. Lorcya, P. Le Magueresb,
C. Rimbaudb,
L. Ouahabb. P. Delhaesc
R. CarlieP,
A. Talleca, and A. Roberta
a - Laboratoire
de synthhse et &ectrosynth&e organiques. awocitf au CNRS, Universite’ de Rennes, Campus de Beaulieu, 35042 Rennes, France. b - LCSIM, assock! au CNRS, Universitt de Rennes, Campus de Beaulieu, 35042 Rennes. France. c - CRPP CNRS et UniversitP de Bordeaux 1. Avenue Albert Schweitzer, 33600 Pessac. France.
Abstract TTF vinylogues were electrochemically synthesized and dication charge transfer salts were isolated. Important conformational changes between the neutral and oxidized states were observed by X-ray structure analysis. Keywords : Electrochemical
methods, Coupling reactions, Single-crystal growth, X-ray diffraction.
1. INTRODUCTION Among the recent trends relative to n-donors, extended TTF (tetrathiafulvalene) including a cyclic or an aliphatic conjugated spacer group between the two dithiole rings have focused attention in attempt to prepare novel organic materials [l]. Within this frame, TTF vinylogues involvi g an re an ethanediylidene unit in the central conjugation interesting type of ethylene analogues of TTF. The syn esis of such extended ‘ITFs reported in the literature is main1 1 based on the Wittig reaction of a (1. 3-dithiole) phosphoni m salt with an appropriate aldehyde [2]. We have recently r %ported that the electrochemical synthesis is a convenient way for preparing this kind of extended TTF [3]. We ha& now prepared dication charge transfer salts and in the present paper, we report the important conformational changes which occur between the neutral state and the oxidized one of these TTF vinylogues. Me H
SMe
Me )_(
SMe -
vs SCE (depending on R) and, without any treatment, reduced at -0.2 V vs SCE. Mechanism of the oxidative dimerization of DTF to form ‘ITF vinylogues has been investigated by cyclic voltammetry. It involves first the formation of the cation radical DTF+. which couples to form the protonated dication. This dication slowly deprotonates to give the TTF vinylogue [4].
3. RESULTS
This family of TTF vinylogues displays a peculiar voltammetric behavior compared to their non substituted analogues. These compounds (R # H) present one two-electron reversible oxidation wave in CH3CN in contrast with their analogue (R = H) which presents two reversible monoelectronic waves as it can be seen in table 1. Important conformational changes between the neutral state and the oxidized one of these compounds could explain this unusual feature for TTF vinylogues.
Table 1 Oxidation potentials of ‘ITF vinylogues. in CH$N, electrode with 0.1 M n-BtqNPF6. lOOrnV/s
ox I red -
R
DTF
2.
MeS
J-c
Me
SYNTHESIS
The ‘ITF vinylogues were obtained in one pot by a two-step electrochemical synthesis : first an oxidation and then a reduction starting from substituted 1,4-dithiafulvenes (DTF). The macroscale electrolyses were performed in a divided cell under controlled potential. The solution is oxidized at 0.6-0.8 V 03794779/97/$17.00 0 1997ElseviesScienceS.A All rig@ reserved
PII 303794779(96)04610-3
AND DISCUSSION
E,,‘in
V vs SCE
Pt working
(&a-&c
a pMcOC6H4
0.32
30mV
b C6l-Q
0.34
30mV
H
Eox’ (0.22)/Eox2
(0.32)
)
30/30mV
The single-crystal X-ray analysis of a neutral TTF vinylogue (R = pMeOC6H4) shows that in contrast with planar TTF vinylogues (R = H) [S], this compound is nonplanar [3]. The non planarity is associated with steric interactions caused by the two R groups. We chemically oxidized a with a solution
1832
D. Lorcy et al. /Synthetic
of Cu(ClO4)2.6H2O in THF, and b in an iodine atmosphere. The stoichiometry of both charge transfer salts was determined by X-ray structure analysis : one donor dication for two anions. Fig. 1 shows the crystal packing viewed of a-(C104)2 onto the 11cplane [6].
Metals 86 (1997) 1831-1832
The ESR spectrum of a polycrystalline sample does not exhibit any intrinsic resonance line. The formed dicationic state is quite stable as it is necessary to heat up the powder to 8O“C during a few hours for detecting a sizeable resonance line associated with the occurrence of a radical-cation. R.T. absorption spectrum recorded between 15000 cm-l and 500 cm-l diluted KBr pellet_ presents a strong electronic absorption
on a band
around 15000 cm-l with a shoulder at 13340 cm-l. This observation is reminiscent of the near IR transitions recently observed on a BEDT-TTF++ salt [7] and attributed to localized intramolecular excitations. This salt has a room temperature conductivity
values in the range of 106-10-7S.cm-*.
REFERENCES
[ll F. Ogura, T. Otsubo and Y.Aso. Sulfur. Rep., II (1992) 439.and ref.cited. 14 (1993) 245.
G. Schukat. E. Fangh&nel, Sulfur. Rep. ,
VI See for example T. Sugimoto,
H. Awaji, I. Sugimoto. Y. Misaki, T. Kawase, S. Yoneda and ZYoshida, Chem. Mater., 1 (1989) 535.
[31 D. Lorcy, R. Carlier, A. Robert, A. Tallec, P. Le Magueres and L. Ouahab, .I. Org. Chem., 60 (1995) 2443. [41 P. Hapiot. D. Lorcy, A. Tallec, R. Carlier and A. RoberL J. Phys. Chem.. in press. Fig. 1 Crystal structure of a-(ClO4)2. Projection onto the UC plane. In the dication form the donor adopts a different conformation compared to the neutral species : the extended TTF core becomes planar while the phenyl groups are located in a plane perpendicular to the one formed by the dithiole rings. The interplanar separation between successive extended TTF cores seems too important to involve any orbital overlap. Similar conformational modifications were observed in the case of b. Spectroscopic experiments were carried out on the dicationic salt (b2+)(1332.
[5] A. J. Moore, M. R. Bryce, D. J. Ando, M. B. Hursthouse. J. Chem. Sot., Chem. Commun., (1991) 320.
[61Crystal data for a-(ClO4)2:
C26H26C120lOS6. monoclinic, space group P21/n, a= 11.087(5) A, b= 10.433( 1) A.
c= 14.325(g) A, p= 104.30(2)“. V= 1605(l) A3, Z= 2 and d talc= 1.583 g.cmm3. [71 L. K. Chou, M. A. Quijada. M. B. Clevenger. G. F. de Oliveira K. A. Abboud, D. B. Tanner and D. R. Talham. Chem. Maler.. 7 (1995) 530.
Synthetic Metals86 (1997) 1831-1832
Dication Charge Transfer Salts of TTF Vinylogues D. Lorcya, P. Le Magueresb,
C. Rimbaudb,
L. Ouahabb. P. Delhaesc
R. CarlieP,
A. Talleca, and A. Roberta
a - Laboratoire
de synthhse et &ectrosynth&e organiques. awocitf au CNRS, Universite’ de Rennes, Campus de Beaulieu, 35042 Rennes, France. b - LCSIM, assock! au CNRS, Universitt de Rennes, Campus de Beaulieu, 35042 Rennes. France. c - CRPP CNRS et UniversitP de Bordeaux 1. Avenue Albert Schweitzer, 33600 Pessac. France.
Abstract TTF vinylogues were electrochemically synthesized and dication charge transfer salts were isolated. Important conformational changes between the neutral and oxidized states were observed by X-ray structure analysis. Keywords : Electrochemical
methods, Coupling reactions, Single-crystal growth, X-ray diffraction.
1. INTRODUCTION Among the recent trends relative to n-donors, extended TTF (tetrathiafulvalene) including a cyclic or an aliphatic conjugated spacer group between the two dithiole rings have focused attention in attempt to prepare novel organic materials [l]. Within this frame, TTF vinylogues involvi g an re an ethanediylidene unit in the central conjugation interesting type of ethylene analogues of TTF. The syn esis of such extended ‘ITFs reported in the literature is main1 1 based on the Wittig reaction of a (1. 3-dithiole) phosphoni m salt with an appropriate aldehyde [2]. We have recently r %ported that the electrochemical synthesis is a convenient way for preparing this kind of extended TTF [3]. We ha& now prepared dication charge transfer salts and in the present paper, we report the important conformational changes which occur between the neutral state and the oxidized one of these TTF vinylogues. Me H
SMe
Me )_(
SMe -
vs SCE (depending on R) and, without any treatment, reduced at -0.2 V vs SCE. Mechanism of the oxidative dimerization of DTF to form ‘ITF vinylogues has been investigated by cyclic voltammetry. It involves first the formation of the cation radical DTF+. which couples to form the protonated dication. This dication slowly deprotonates to give the TTF vinylogue [4].
3. RESULTS
This family of TTF vinylogues displays a peculiar voltammetric behavior compared to their non substituted analogues. These compounds (R # H) present one two-electron reversible oxidation wave in CH3CN in contrast with their analogue (R = H) which presents two reversible monoelectronic waves as it can be seen in table 1. Important conformational changes between the neutral state and the oxidized one of these compounds could explain this unusual feature for TTF vinylogues.
Table 1 Oxidation potentials of ‘ITF vinylogues. in CH$N, electrode with 0.1 M n-BtqNPF6. lOOrnV/s
ox I red -
R
DTF
2.
MeS
J-c
Me
SYNTHESIS
The ‘ITF vinylogues were obtained in one pot by a two-step electrochemical synthesis : first an oxidation and then a reduction starting from substituted 1,4-dithiafulvenes (DTF). The macroscale electrolyses were performed in a divided cell under controlled potential. The solution is oxidized at 0.6-0.8 V 03794779/97/$17.00 0 1997ElseviesScienceS.A All rig@ reserved
PII 303794779(96)04610-3
AND DISCUSSION
E,,‘in
V vs SCE
Pt working
(&a-&c
a pMcOC6H4
0.32
30mV
b C6l-Q
0.34
30mV
H
Eox’ (0.22)/Eox2
(0.32)
)
30/30mV
The single-crystal X-ray analysis of a neutral TTF vinylogue (R = pMeOC6H4) shows that in contrast with planar TTF vinylogues (R = H) [S], this compound is nonplanar [3]. The non planarity is associated with steric interactions caused by the two R groups. We chemically oxidized a with a solution
1832
D. Lorcy et al. /Synthetic
of Cu(ClO4)2.6H2O in THF, and b in an iodine atmosphere. The stoichiometry of both charge transfer salts was determined by X-ray structure analysis : one donor dication for two anions. Fig. 1 shows the crystal packing viewed of a-(C104)2 onto the 11cplane [6].
Metals 86 (1997) 1831-1832
The ESR spectrum of a polycrystalline sample does not exhibit any intrinsic resonance line. The formed dicationic state is quite stable as it is necessary to heat up the powder to 8O“C during a few hours for detecting a sizeable resonance line associated with the occurrence of a radical-cation. R.T. absorption spectrum recorded between 15000 cm-l and 500 cm-l diluted KBr pellet_ presents a strong electronic absorption
on a band
around 15000 cm-l with a shoulder at 13340 cm-l. This observation is reminiscent of the near IR transitions recently observed on a BEDT-TTF++ salt [7] and attributed to localized intramolecular excitations. This salt has a room temperature conductivity
values in the range of 106-10-7S.cm-*.
REFERENCES
[ll F. Ogura, T. Otsubo and Y.Aso. Sulfur. Rep., II (1992) 439.and ref.cited. 14 (1993) 245.
G. Schukat. E. Fangh&nel, Sulfur. Rep. ,
VI See for example T. Sugimoto,
H. Awaji, I. Sugimoto. Y. Misaki, T. Kawase, S. Yoneda and ZYoshida, Chem. Mater., 1 (1989) 535.
[31 D. Lorcy, R. Carlier, A. Robert, A. Tallec, P. Le Magueres and L. Ouahab, .I. Org. Chem., 60 (1995) 2443. [41 P. Hapiot. D. Lorcy, A. Tallec, R. Carlier and A. RoberL J. Phys. Chem.. in press. Fig. 1 Crystal structure of a-(ClO4)2. Projection onto the UC plane. In the dication form the donor adopts a different conformation compared to the neutral species : the extended TTF core becomes planar while the phenyl groups are located in a plane perpendicular to the one formed by the dithiole rings. The interplanar separation between successive extended TTF cores seems too important to involve any orbital overlap. Similar conformational modifications were observed in the case of b. Spectroscopic experiments were carried out on the dicationic salt (b2+)(1332.
[5] A. J. Moore, M. R. Bryce, D. J. Ando, M. B. Hursthouse. J. Chem. Sot., Chem. Commun., (1991) 320.
[61Crystal data for a-(ClO4)2:
C26H26C120lOS6. monoclinic, space group P21/n, a= 11.087(5) A, b= 10.433( 1) A.
c= 14.325(g) A, p= 104.30(2)“. V= 1605(l) A3, Z= 2 and d talc= 1.583 g.cmm3. [71 L. K. Chou, M. A. Quijada. M. B. Clevenger. G. F. de Oliveira K. A. Abboud, D. B. Tanner and D. R. Talham. Chem. Maler.. 7 (1995) 530.