New planar and soluble tris-tetrathiafulvalene derivatives with threefold-symmetry

Article abstract of DOI:10.1016/S0040-4039(00)00344-0

Novel trimeric tetrathiafulvalene (TTF) derivatives with threefold- symmetry are built on the 1,3,5-benzene core. Long alkyl chains have been incorporated on the peripheral positions of the TTF moieties in order to increase the solubility of the final products. Electrochemical and spectroscopic studies of the target compounds suggest that there are no significant interactions between the TTF units because of their meta orientation. Semiempirical theoretical calculations indicate that the new systems exhibit a high structural anisotropy resembling that exhibited by discotic liquid crystals. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00344-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3083–3086
New planar and soluble tris-tetrathiafulvalene derivatives with
threefold-symmetry
∗
Alfonso González, José L. Segura and Nazario Martín
Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, E-28040 Madrid, Spain
Received 17 January 2000; accepted 25 February 2000
Abstract
Novel trimeric tetrathiafulvalene (TTF) derivatives with threefold-symmetry are built on the 1,3,5-benzene core.
Long alkyl chains have been incorporated on the peripheral positions of the TTF moieties in order to increase
the solubility of the final products. Electrochemical and spectroscopic studies of the target compounds suggest
that there are no significant interactions between the TTF units because of their meta orientation. Semiempirical
theoretical calculations indicate that the new systems exhibit a high structural anisotropy resembling that exhibited
by discotic liquid crystals. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: tetrathiafulvalenes; electrochemistry; trimers.
Many efforts have been devoted in recent years to the development of new systems containing
1
–3
two or more tetrathiafulvalene (TTF) units
as well as to the synthesis of star-shaped thiophene or
4
TTF oligomers. Because of the intramolecular through-bond or through-space interactions between
the TTF units, these kinds of systems may display multi-stage redox behaviour which might provide
the possibility to control the stoichiometry, band filling and molecular assembly in desired conductive
complexes. Depending upon the nature of the bridging groups between the TTF units and the geometry of
the molecule, the intramolecular interaction between the redox moieties can be modified in a controlled
way (Fig. 1).
Thus, although systems containing three TTF units should be oxidizable up to the hexacation, because
each TTF unit can release two electrons, the number of electrochemical oxidation processes observed
by using cyclic voltammetry measurements is strongly variable. Only two electrochemical processes
corresponding to the oxidation to the trication and hexacation are observed for those systems in which the
5
TTF units are not interacting while up to six electrochemical processes are observed for other systems
with strong interactions between the TTF units.6
TTF dimers and trimers in which the TTF units are introduced at 1,3- or 1,3,5-positions of aromatic
rings are especially interesting, given that a ferromagnetic interaction between TTF units might occur in
the CT complex or radical-cation salts which are usually known to show conductivity. Thus, Iyoda and
∗
Corresponding author. Tel: +34-91-3944227; fax: +34-91-3944103; e-mail: nazmar@eucmax.sim.ucm.es (N. Martín)
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00344-0
tetl 16624

3
084
Fig. 1.
co-workers proposed that the triradical–trication salts derived from 1 should exhibit a quartet ground state
7
if the three TTF radical cations show a ferromagnetic interaction. In 1993, Johannsen and co-workers
8
attempted the synthesis of 2 which is the vinylogue of trimer 1. However, a dark red solid, insoluble in all
usual solvents, was obtained but could not be investigated. In this communication we report the synthesis
and the electrochemical and structural study of alkyl substituted analogues of 2 in which the presence
of long alkyl chains efficiently improves their solubility in organic solvents, thus allowing appropriate
spectroscopic and electrochemical characterization.
The synthesis of the hexakis(hexylthio)-, hexakis(dodecylthio)-derivatives 3a,b as well as the hexa-
kis(dodecyl)substituted vinylogue 3c were carried out following the synthetic route depicted in Scheme
1
. The tris-(triphenylphosphonium) salt 7 was synthesized following the synthetic route previously
9
reported by Storck and Manecke, starting from 1,3,5-tris-(methoxycarbonyl)benzene 4. Reduction of
with lithium aluminium hydride followed by treatment with phosphorus tribromide yielded the 1,3,5-
4
tris-(bromomethyl)derivative 6. Reaction of 6 with triphenylphosphine in refluxing dimethylformamide
afforded 1,3,5-tris-(triphenylphosphonium)benzene bromide 7 as a white solid that was used for the next
synthetic step without further purification.
On the other hand, treatment of the dialkylthiosubstituted TTFs 8a,b with LDA at −78°C in
1
0
11
THF, followed by addition of either N-methylformanilide 10 or N,N-dimethylaminoacrolein 11,
0
0
0
0
respectively, gave 4-formyl-4 ,5 -bis(alkylthio)tetrathiafulvalene 12a,b and 4-(2-formylvinyl)-4 ,5 -
bis(dodecylthio)tetrathiafulvalene 12c.
Lithium ethoxide-catalyzed Wittig reaction of the tris-(triphenylphosphoniummethyl)benzene bromide
7
with formyl TTFs 12a–c was carried out in chloroform:ethanol (3:5) at 50°C, leading to a mixture
of various cis/trans isomers. Attempts at isomerization to the all-trans stereoisomer by using the usual
iodine-catalyzed conditions was not succesful due to the rapid formation of a charge-transfer salt between
the TTF donor fragments and iodine. However, the major isomer (TLC) formed in the synthesis of
†
compounds 3a–c could be finally isolated by careful chromatography in moderate yields (30–50%).
†
1
Compounds 3a–c gave satisfactory analytical and spectroscopic data. Selected spectroscopic data for 3a: H NMR (C
00 MHz): δ (ppm) 6.83 (s, 3H), 6.49 (d, 3H, J=15.3 Hz), 6.31 (d, 3H, J=15.3 Hz), 5.83 (s, 3H), 2.47 (t, 6H), 2.46 (t, 6H), 1.19
, 50 MHz): δ (ppm) 137.2, 135.9, 128.2, 119.3, 114.4, 109.3, 36.5, 31.6, 30.2, 28.5,
90
2.9, 14.2. Anal. calcd for C66H S18 (%): C: 54.32; H: 6.17; S: 39.51. Found: C: 54.20; H: 6.14; S: 39.72.
6 6
D ,
2
(
m, 48H), 1.16 (m, 18H). 13C NMR (C
D
6
6
2

3
085
Scheme 1.
Spectroscopic analyses confirmed that the isolated isomers correspond to the more stable all-trans
1
configurations, the coupling constant measured in the H NMR spectra for the vinyl protons being around
16 Hz in all cases.
Cyclic voltammetry measurements (in dichloromethane, reference electrode: SCE; working electrode:
+
−
platinum, supporting electrolyte: Bu N ClO , at 200 mV/s) performed for 3a–c clearly confirm the lack
4
4
of electronic interaction between the three TTF units in these systems. Thus, only two redox processes
were observed, corresponding to the simultaneous oxidation of the three TTF units to the tris-(radical
cation) (0.48 V) and hexacation (0.85 V) species.
Semiempirical theoretical calculations performed for systems 3 (AM1 level) show molecular geome-
tries close to planarity. Fig. 2 depicts the minimum energy conformation obtained for 3a. Long alkyl
chains have been replaced by methyl groups in order to facilitate calculations. The minima obtained for
these systems show a high structural anisotropy which resembles the disc form exhibited by discotic
liquid crystals.
In conclusion, we have synthesized a new family of planar trimeric TTF systems with threefold
symmetry, in which the TTF units are electronically independent. Because of the high structural
anisotropy of these systems and the presence of the peripheral long alkyl chains, they are appealing
candidates for the obtention of TTF-based discotic liquid crystals (LC). On the other hand, the meta

3
086
Fig. 2. Geometry of the most favourable conformation for 3a determined by AM1 calculations
linking of the tetrathiafulvalene systems can produce interesting magnetic interactions in the charge-
transfer salts of the trimers. Work is in progress in order to explore their potential behaviour as organic
ferromagnets as well as LC in the search of novel multiproperty materials.
Acknowledgements
We are indebted to DGES of Spain (Project PB98-0818) for financial support.
References
1
2
. For a review on TTF dimers, see: Otsubo, T.; Aso, Y.; Takimiya, K. Adv. Mater. 1996, 8, 203.
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