An extended tetrathiafulvalene redox-ligand incorporating a thiophene spacer

Article abstract of DOI:10.1016/j.tetlet.2008.07.005

An extended tetrathiafulvalene derivative incorporating a thiophene spacer and a fused crown-ether unit has been synthesized. This highly delocalized system exhibits remarkable electrochemical recognition properties for Na⁺ and Ba²⁺ as shown by cyclic voltammetry in methylene chloride. This result is attributed to the proximity between the guest metal cation and the positive charge of the oxidized ligand, which is located on the central conjugated thiophenic part.

Full text of DOI:10.1016/j.tetlet.2008.07.005

Tetrahedron Letters 49 (2008) 5452–5454
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An extended tetrathiafulvalene redox-ligand incorporating a thiophene spacer
*
*
Gaëlle Trippé, David Canevet, Franck Le Derf, Pierre Frère , Marc Sallé
Université d’Angers, CIMA UMR CNRS 6200, UFR Sciences, 2 boulevard Lavoisier, 49045 Angers, France
a r t i c l e i n f o
a b s t r a c t
Article history:
An extended tetrathiafulvalene derivative incorporating a thiophene spacer and a fused crown-ether unit
has been synthesized. This highly delocalized system exhibits remarkable electrochemical recognition
properties for Na and Ba as shown by cyclic voltammetry in methylene chloride. This result is attri-
buted to the proximity between the guest metal cation and the positive charge of the oxidized ligand,
which is located on the central conjugated thiophenic part.
Received 3 May 2008
Revised 28 June 2008
Accepted 1 July 2008
Available online 4 July 2008
+
2+
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Thiophene
Tetrathiafulvalene
Redox-responsive ligand
Electrochemical
Crown-ether
Redox-responsive ligands1–5 are built from the association of
a binding unit and an electroactive moiety (ferrocene, quinone,
tetrathiafulvalene (TTF), etc.). In this context, a broad range of
TTF-based receptors presenting various binding sites have been
redox-responsive ligands is strongly dependent on electrostatic
interactions occurring between the guest cation and the electroac-
tive unit, it is of strong interest to study the case in which the redox
moiety consists in an extended-TTF derivative. One case is A,31 and
related polyether derivatives which have been described re-
6
–9
10–13
described,
with a coordinating acyclic domain.
such as crown-ether derivatives
or receptors
14–16
32
The recognition process
cently. Also, a crown-ether vinylogous TTF receptor B has been
3
3
between the receptor and a cationic guest is accompanied by a
modification of the redox potentials of the TTF unit, resulting in
an electrochemical sensing of the binding event. Anion sensing
very recently used for tuning molecular movements, and a new
extended TTF-based receptor C presenting two pyridyl functional
groups for guest coordination has been built by inserting a fur-
ano-quinonoid spacer between both 1,3-dithiol-2-ylidene rings.34
It is well established that the nature of the conjugated spacer in
has also been recently described from analogous TTF-based sys-
tems incorporating suitable binding units.1
7–24
One important
interest of TTF-based systems lies in their two fully reversible
extended-TTF derivatives has a strong influence on the electro-
1
2
27
one electron oxidation steps ðEp ; Ep Þ, which are associated to
chemical properties of the
p-donors. In particular, insertion of a
a
a
three stable oxidation states. Therefore, the binding ability of these
host assemblies can be tuned as a function of the oxidation state of
the electroactive TTF part.15 Another interest of the TTF unit resides
on chemical structural flexibility, which allows to tune the redox
potentials of the electroactive signalling unit by a suitable substi-
S
S
S
tution on the periphery, or by insertion of a
p
-conjugated spacer
O
O
S
O
N
between the 1,3-dithiol-2-ylidene rings.2
5,26
In that case, a strong
S
S
O
O
S
C
N
modification of the charge distribution is observed in the donating
molecule, which explains why -extended TTF structures have
been extensively explored in particular as precursors for the prep-
p
S
S
O
O
O
O
O
2
7–29
S
O
aration of solid-state electroconducting cation radical salts,
or
O
O
as strong donating (D) counterparts in donor–acceptor (D–A)
O
3
0
dyads. Extended-TTF skeletons are much rarely found in redox-
active receptors. Considering that recognition process with
O
O
1 (R=CO2Me)
A
S
S
B
S
S
S
S
S
S
S
*
Corresponding authors. Tel.: +33 241 73 54 39; fax: +33 241 73 54 05 (M.S.).
E-mail addresses: pierre.frere@univ-angers.fr (P. Frère), marc.salle@univ-
Me
Me
S
S
Me
Me
Me
R
R
R
R
Me
angers.fr (M. Sallé).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.005

G. Trippé et al. / Tetrahedron Letters 49 (2008) 5452–5454
5453
O
O
O
O
HO
OH
CO Et
TsOCH (CH OCH ) CH OTs
O
2
2
2 3
2
EtO C
2
S
2
CsF, CH CN
3
EtO2C
CO2Et
S
3
2
MeO2C
S
BF4-
H
O
O
O
O
O
O
O
S
PBu3
MeO C
2
6
LiAlH4
O
PDC / CH2Cl2
O
O
1
Et N, CH CN
3
3
OHC
CHO
S
4
S
5
HO
OH
Scheme 1.
Table 1
Peak—potentials versus Fc/Fc+, C = 10 mol L
ꢀ3
ꢀ1
in CH
2
Cl
2
4 6
(Bu NPF , 0.1 M),
v = 100 mV sꢀ1, Pt, rt
1
D45
E46
1
E
E
0.18
0.39
0.50
0.90
0.22
0.41
ox
2
ox
S
MeO
2
C
CO
2
Me
S
S
S
S
S
S
S
S
MeO C
CO Me
2
2
MeO
2
C
CO
2
Me
CO Me CO Me
2
2
D
E
Comparison of 1 with the reference TTF analogue D bearing four
tetracarbomethoxy substituents is meaningful (Table 1). The sig-
nificantly lower oxidation potentials observed for 1 relative to D
are due to the combined effect of the occurrence of a more
extended conjugation in the former case and the presence of
donating alkoxy substituents at the 3,4-positions of the central
thiophene ring. The difference between the two oxidation poten-
ꢀ
3
ꢀ1
Figure 1. Cyclic voltammograms of 1 (10 mol L in 0.1 M Bu NPF
4
6 2 2
in CH Cl ) in
+
the presence of increasing amounts of NaClO (a) and Ba(ClO ) (b); vs Fc/Fc .
4
4 2
thiophene ring as a spacer has been achieved,35,36 and the revers-
ible electrochemical properties of these
-donors28 make this
tials in 1 ð
D
E
2ꢀ1 ¼ E ꢀ E ¼ 210 mVÞ is lower than the value
2
1
ox
ox
45
p
observed for D (DE2ꢀ1 = 400 mV). Indeed, extension of the conju-
extended skeleton a suitable candidate as an electroactive moiety
in redox sensors. Finally, it has been demonstrated that crown-
ethers fused to thiophene units present a good ability to recognize
alkali ions in corresponding polythiophene and oligothiophene
gated spacer between both dithiole units favors the access to the
dication state by decreasing the coulombic repulsion between both
positive charges. Conversely, the value of
D
E
2ꢀ1for 1 is higher than
for the parent system E devoid of any substituent on the central
thiophene ring ( 2ꢀ1= 190 mV). This observation indicates that in
4
6
3
7–41
systems.
DE
In this context, recent reports on ligands B and C prompt us to
report the synthesis and the recognition properties of a molecular
receptor 1, presenting a thiophene-based extended-TTF skeleton as
the redox-active unit.
the case of 1, a larger coulombic repulsion exists in the dication
state. A similar effect was also observed for extended-TTFs built
with a 3,4-dialkoxythiophene unit as the spacer, and reveals a
tendency for the oxygen atoms directly linked to the thiophene
moiety, to localize positive charge in the central position.47
Such an electrochemical characteristic is of course of impor-
tance when considering the context of redox-responsive ligands,
for which electrostatic effects between a bound cation and the
electroactive unit play a crucial role.
Diester derivative 3 was synthesized according to the literature
4
2
procedure, by [1+1] cyclocondensation of 3,4-dihydroxythioph-
ene-2,5-dicarboxylate 2 with tetraethylene glycol ditosylate in
the presence of CsF (Scheme 1). Reduction using lithium alumin-
ium hydride afforded diol 4 in 45% yield. Conversion to the 2,5-
diformyl thiophene derivative 5 was carried out using pyridinium
dichromate (40%). Note that attempts to directly reduce diester 3–
Binding ability of receptor 1 for various cations was studied by
ꢀ
1
CV(CH
2
Cl
2
, Bu
4
NPF
6
0.1 mol L ). Important modifications could be
or
5
with Dibal-H failed. The final step involves a double Wittig-type
observed upon addition of increasing amounts of NaClO
4
4
3
olefination of 5 with an excess of the phosphonium-ylid 6 bear-
ing a 1,3-dithiol moiety, in the presence of triethylamine. The tar-
get compound 1 was obtained as a red powder, in good yields
Ba(ClO
4
)
2
. Addition of controlled amounts of sodium perchlorate
1
caused a strong positive shift ð
DE
¼ þ90 mVÞ of the first oxida-
ox
1
2
tion potential ðE Þ, whereas E remained approximately constant
ox
ox
(
70%) considering the double olefination step.44
(Fig. 1a). The reason why the first oxidation wave is shifted to more
anodic potentials is explained by the electrostatic inductive effect
promoted by the bound metal cation, which decreases the
Redox properties of 1 have been evaluated by cyclic voltamme-
try (CV) in methylene chloride (Fig. 1). As expected, ligand 1 pre-
sents two fully reversible monoelectronic oxidation processes
p-donating ability of the adjacent redox unit. Such a positive shift
1
(Table 1), corresponding to the successive formation of cation rad-
of
DE
is remarkably high and comparable to the highest values
ox
ical and dication species.
reported for TTF-based redox-responsive ligands. This result is

5
454
G. Trippé et al. / Tetrahedron Letters 49 (2008) 5452–5454
.
+
attributed to the localization of the positive charge in 1 on the
median thiophene ring47 (see above), that is near the bound
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2
cationic guest. On the other hand, the constant value of E upon
ox
addition of sodium cation indicates that for the dicationic stage
2
+
(
1 ), expulsion of the metal cation outside from the polyether-
cavity takes place, because of the repulsive electrostatic interac-
tion. A very similar electrochemical behaviour was observed for 1
1
1
in presence of LiClO
4
(see Supplementary data) ð
DE
¼ þ55 mV,
ox
2
DE
ꢁ 0:0 mV. Also, the addition of a group II cation (Ba(ClO
4 2
) )
ox
1
is accompanied by an electrochemical signature
ð
DE
¼
ox
2
ox
þ90 mV;
DE
ꢁ 0:0 mVÞ (Fig. 1b). In this case, no additional
1
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tor 1 in the presence of alkali and alkali earth cations (see Supple-
mentary data). In methylene chloride, the addition of controlled
amounts of sodium perchlorate up to 1 equiv provokes a strong
variation of the shape of the absorption band with in particular
the decreasing of the vibronic fine structure and the formation of
isosbestic points. For more than 1 equiv, no significant additional
modification of the absorption spectrum is observed. In presence
1
2
2
1164–1167.
of Ba(ClO
case, an evolution of the absorption band until the addition of
.5 equiv, as expected for the 2:1 stoichiometry observed by the
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4
)
2
, the change in the spectrum is similar with, in this
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by addition of sodium or barium cation with CH CN as solvent.
1
Attempts for reproducing these results by H NMR failed
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3
9. Roncali, J. J. Mater. Chem. 1997, 7, 2307–2321.
in the NMR tube, when using a chlorinated solvent (CDCl
Conversely, use of a more solvating mixture (CDCl –CD
did not allow observation of any chemical shift upon introduction
of a metallic salt (NaClO , Ba(ClO ).
3
, CD
2 2
Cl ).
0. For representative recent C60-exTTF conjugates, see: (a) Giacalone, F.; Martín,
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3
3
CN; 1/1)
2005, 11, 7440–7447; (c) Sanchez, L.; Sierra, M.; Martín, N.; Guldi, D. M.;
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4 2
)
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Barbero, J.; Sarova, G.; Guldi, D. M. Tetrahedron 2006, 62, 1998–2002.
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In summary, we have presented a synthetic access to a redox-
responsive ligand built around an extended tetrathiafulvalene unit
incorporating a thiophene ring as spacer. The electrochemical rec-
3
3
3
3
3
3
+
2+
ognition properties for Na and Ba are remarkable in methylene
chloride. This ability to electrochemically detect a guest cation is
assigned to the conjunction of a suitable location of the binding
site related to the redox unit, associated to a contribution of the
electrodonating O-atoms which are directly connected to the
central conjugated thiophene spacer.
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Acknowledgements
3
3
7. Bauerle, P.; Scheib, S. Acta Polym. 1995, 46, 124–129.
8. Bauerle, P.; Mitschke, U.; Gruner, G.; Rimmel, G. Pure Appl. Chem. 1999, 71,
The authors thank the PIAM of the University of Angers for ana-
lytical characterizations; MS is indebted to the Institut Universi-
taire de France.
2153–2160.
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4
4
1. Demeter, D.; Blanchard, P.; Grosu, I.; Roncali, J. J. Inclusion Phenom. 2008, 1–13.
2. Sone, T.; Ohba, Y. Bull. Chem. Soc. Jpn. 1989, 62, 838–844.
Supplementary data
43. Lakshmikantham, M. V.; Cava, M. P.; Caroll, P. J. J. Org. Chem. 1984, 49, 726–728.
4
4. All new compounds exhibited spectral properties consistent with the assigned
structures: Compound 4: white viscous oil, 1H NMR (500 MHz, CDCl
): 2.41 (br
s, 2H, OH), 3.68 (br s, 8H, OCH ), 3.85 (t, 4H, OCH ), 4.21 (t, 4H, OCH ), 4.63 (s,
4H, CH OH); C NMR (125.7 MHz, CDCl ) 56.8, 70.5, 70.6, 71.0, 73.3, 123.5,
3
Uv-vis and CV titration studies of receptor 1 in presence of alkali
and alkali earth cations. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2008.07.005.
2
2
2
13
2
3
1
1
3
46.1. Compound 5: pale yellow solid, mp 88 °C; H NMR (500 MHz, CDCl
.68 (br s, 4H, OCH ), 3.72 (br s, 4H, OCH ), 3.92 (t, 4H, OCH ), 4.52 (t, 4H,
), 10.06 (s, 2H, CHO); C (125.7 MHz, CDCl ): 70.0, 70.6, 70.8,75.1, 130.4,
3
):
2
2
2
13
OCH
54.4, 181.9; MS (EI) 330 [M ]. Compound 1: red–orange solid, mp 159 °C;
NMR (500 MHz, CDCl ): 3.71 (s, 8H, OCH ), 3.85 (s, 6H, OCH ), 3.88 (s, 6H,
OCH ), 3.89 (t, 4H, OCH ), 4.16 (t, 4H, OCH ), 6.64 (s, 2H, @CH); C NMR
125.7 MHz, CDCl ): 53.3, 70.3, 70.7, 71.1, 73.6, 105.9, 122,3, 127.1, 130.9,
31.2, 144.8, 159.9, 160.0; HRMS (ESI+) Calcd for C28 : 734.0290;
2
3
+
Å
1
1
H
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3
2
3
13
3
2
2
(
3
1
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H O S
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