Exceptional electron donating ability of an extended tetrathiafulvalene derivative

Article abstract of DOI:10.1016/j.tetlet.2004.02.003

We report the synthesis, crystal structures and redox chemistry of two new TTF derivatives bearing fused triaryl units. Both compounds show significant planarisation within conjugated regions, assisted by close intramolecular S?O contacts. The extended-TTF analogue displays multi-redox activity and can be oxidised sequentially to the tetracation species.

Full text of DOI:10.1016/j.tetlet.2004.02.003

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2535–2539
Exceptional electron donating ability of an extended
tetrathiafulvalene derivative
b
b
c
Tahir Khan,a Peter J. Skabara,a, Pierre Frere, Magali Allain, Simon J. Coles and
*
ꢀ
Michael B. Hursthousec
aDepartment of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK
bGroupe Systꢀemes Linꢁeaires Conjuguꢁes, CIMMA CNRS UMR 6200, 2 Boulevard Lavoisier, 49045 Angers, France
cDepartment of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
Received 29 January 2004; accepted 2 February 2004
Abstract—We report the synthesis, crystal structures and redox chemistry of two new TTF derivatives bearing fused triaryl units.
Both compounds show significant planarisation within conjugated regions, assisted by close intramolecular Sꢀ ꢀ ꢀO contacts. The
extended-TTF analogue displays multi-redox activity and can be oxidised sequentially to the tetracation species.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The p-electron donor tetrathiafulvalene (TTF) and its
derivatives have been studied extensively for their use in
a variety of applications such as organic metals and
superconductors,1 Langmuir–Blodgett films,2 molecular
sensors,3 non-linear optics,4 field effect transistors5 and
photovoltaics.4 In many cases, the bulk electronic
properties of TTF derivatives stem from their potential
to self-assemble into well-defined structures featuring
close intermolecular contacts. Such interactions are
particularly beneficial where charge transport is of
concern. From this viewpoint, derivatisation of the TTF
framework affords the possibility of integrating addi-
tional valuable features into the molecule, namely: (i)
enhanced redox behaviour, (ii) stronger and more
widespread intermolecular interactions and (iii) control
of the shape of the molecule itself. In recent work,6 we
have shown that 1-3-dithiole-2-thione species, appended
with fused 2,5-diphenylthiophene and 2,5-diphenylfuran
derivatives, are efficient electron donors capable of
undergoing two reversible oxidation processes to afford
the corresponding radical cation and dication species.
ortho-substitution of the phenyl rings with methoxy
groups induces planarisation of the triaryl units through
intramolecular Sꢀ ꢀ ꢀO interactions and this situation is
responsible for enhanced stabilisation of the products of
oxidation. In this paper, we present the first soluble TTF
derivatives bearing fused 4,6-bis(2,4-dimethoxyphe-
nyl)thieno[3,4-d][1,3]dithiole and 4,6-bis(2,4-dimethoxy-
phenyl)furano[3,4-d][1,3]dithiole moieties and discuss
the novel geometrical and electrochemical features that
are attributed to the incorporation of these triaryl units.
Due to the insolubility of symmetrical TTF derivatives
derived from compounds such as 1,6 we attempted to
synthesise the unsymmetrical derivatives 4 and 10,
bearing ethylenedithiodithiole units. Compound 4 was
prepared according to Scheme 1. Thus, 1 was stirred at
80 ꢁC with oxone 27 in the presence of trimethyl phos-
phite to afford the cross-coupled product 3 in 26% yield.
The cyanoethylene protecting groups of 3 were removed
by CsOH in THF and the resulting dithiolate interme-
diate was reacted immediately with 1,2-dibromoethane
to give the product in 57% yield. The reason for pro-
ceeding in two steps from the oxone 2, rather than using
the ethylenedithio analogue, resides in the easy separa-
tion of 3 and tetracyanoethylsulfanyl-TTF.
Scheme 2 depicts the route towards the p-extended TTF
analogue 10. Methylation of compounds 5 and 1 was
achieved using methyl triflate, to give the dithiolium
salts 6a,b in 73% and 81% yield, respectively. Com-
pounds 7a,b were isolated from the reactions of 6a,b
with sodium borohydride (79% and 88% yields). Treat-
ment of 7a,b with triflic acid gave salts 8a,b in 67% and
74% yield, but these materials proved to be unstable and
slowly decomposed upon isolation. The phosphonium
Keywords: Tetrathiafulvalene; Synthesis; X-ray crystallography; Elec-
trochemistry.
* Corresponding author. Tel.: +44-161-275-4781; fax: +44-161-275-
4598; e-mail: peter.skabara@man.ac.uk
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.003

2536
T. Khan et al. / Tetrahedron Letters 45 (2004) 2535–2539
CN NC
The second benzene ring is twisted significantly away
from the plane of the thiophene unit, with a torsion
angle of 44.4ꢁ. Compound 4 forms dimers in the solid
S
S
S
ꢂ
S
S
O
O
state through intermolecular Sꢀ ꢀ ꢀS contacts (3.296(8) A)
+
between the S(5) atoms of the ethylenedithio fragment.
There are no further significant intermolecular short
contacts. Compound 1012 crystallises as a 1:1 toluene
solvate and shows a high degree of planarity (Fig.1(b)).
S
S
S
O
O
O
1
2
ꢂ
Two Sꢀ ꢀ ꢀO interactions (2.777(3) and 2.754(2) A) are
observed exclusively between the o-methoxy substituents
and the corresponding sulfurs of the adjacent dithiole
ring. Hydrogen bonding is also observed between the
marked hydrogens of the two benzene rings (Fig.1(b))
and the oxygen atom of the furan unit (2.313(3) and
(i)
O
ꢂ
2.309(2) A). This arrangement of short contacts is also
evident in the oxone analogue of compound 56 and is
thought to be responsible for the high degree of pla-
narity within the triaryl fragment, which comprises the
two benzene rings and the thienofurano unit. The tor-
sion angle between benzene A and the furan is 0.03ꢁ,
whilst that between ring B and the furan heterocycle is
4.2ꢁ. There are no close intermolecular interactions in
the crystal structure of compound 10, apart from weak
H-bonds involving aliphatic C–H groups.
O
O
S
S
S
S
CN
CN
3
S
S
S
O
The redox properties of compounds 4 and 10 have been
studied by cyclic voltammetry.13 Compound 4 under-
goes two reversible oxidation processes at +0.56 and
+0.97 V, respectively. A third irreversible peak is also
observed at +1.47 V. Compound 1 also undergoes two
reversible oxidation processes at +0.86 and +1.16 V.
From these values it can be concluded that the first
oxidation of 4 is attributed to the TTF unit of the
molecule, whilst the second oxidation originates from
the triaryl component. The third redox process is also
assumed to arise from the triaryl fragment and its irre-
versible nature is probably due to the high potential at
which this oxidation takes place.
(ii)
O
O
O
S
S
S
S
S
S
S
4
The cyclic voltammogram of compound 10 is depicted in
Figure 2. In contrast to the electrochemical behaviour of
4, the extended TTF analogue is oxidised to the tetra-
cation through four sequential, reversible oxidation
O
Scheme 1. Reagents and conditions: (i) P(OMe)3, toluene, 80 ꢁC, 12 h;
(ii) THF, CsOH, then 1,2-dibromoethane.
processes (E11=2 ¼ þ0:35 V, E21=2 ¼ þ0:76 V, E31=2
¼
þ1:09 V and E41=2 ¼ þ1:25 V). The first two oxidation
processes are assigned to the extended TTF unit. Com-
paring the electrochemistries of compounds 4 and 10,
the key role of the additional vinylene spacer group
between the two dithiole rings is to minimise couloum-
bic repulsion within the dication of the TTF species. In
comparison with the values reported for the symmetri-
cal bis(ethylenedithio) analogue 11 (E11=2 ¼ þ0:48 and
E21=2 ¼ þ0:71 vs Ag/AgCl in dichloromethane),14 E11=2 for
10 is 130 mV lower, whilst E21=2 is higher by 50 mV. E1 for
compound 10 is therefore assigned to the triaryl fused
dithiole, leaving the 4,5-ethylenedithio1,3-dithiole ring
as the site for E2. The difference between the first two
redox processes for each compound (E2 ꢁ E1) is 0.23 V
for 11 and 0.41 V for 10. This discrepancy is due to the
unsymmetrical nature of compound 10 and the stronger
electron donating effect of the triaryl moiety, compared
to the relatively unchanged donor ability of the 4,5-
ethylenedithio1,3-dithiole unit. It is surprising to see
salt of 8a was prepared by reaction of the latter with
triphenyl phosphine in THF. After treatment in situ
with n-BuLi at )78 ꢁC, compound 98 was added to give
the product 10 in 71% yield.9 Unfortunately, we were
not able to repeat this procedure for salt 8b to give the
thiophene analogue of 10.
Recrystallisation of compounds 4 (CHCl3–EtOH 10/2
v/v) and 10 (toluene) gave single crystals suitable for
X-ray crystallographic analysis. Compound 410 displays
two intramolecular short Sꢀ ꢀ ꢀO contacts11 (Fig.1(a)), the
first involving one o-methoxy group and the adjacent
dithiole ring (2.948(7) A) and the other between the
second o-methoxy substituent and the sulfur of the thio-
ꢂ
phene ring (2.626(7) A). The latter interaction is associ-
ated with a reasonable degree of planarity between
the corresponding benzene ring and the thiophene unit;
the torsion angle between these two cyclic units is 11.8ꢁ.
ꢂ

T. Khan et al. / Tetrahedron Letters 45 (2004) 2535–2539
2537
S
X
SCH3
S
CF3SO3
S
S
S
(i)
O
O
O
O
X
O
O
O
O
5 X = O
1 X = S
6a X = O
6b X = S
(ii)
H
H
S
SCH3
CF3SO3
S
S
S
(iii)
O
O
O
O
X
X
O
O
O
O
8a X = O
8b X = S
7a X = O
7b X = S
O
O
O
S
S
(iv), (v)
S
S
O
8a
S
S
S
S
S
S
O
9
O
10
Scheme 2. Reagents and conditions: (i) CF3SO3CH3, CH2Cl2; (ii) NaBH4, CH3CN/IPA; (iii) CF3SO3H, CH2Cl2; (iv) PPh3, THF, rt; (v) n-BuLi,
)78 ꢁC, then 9.
that molecule 10 undergoes two further reversible oxi-
dation processes, despite the inevitability of strong
coulombic repulsion within the triaryl unit. The
extended TTF 12 with two dithiafulvenyl arms is
also known to lead to a tetracation state, but with an
irreversible fourth oxidation step.15 Compared to com-
pound 1, E31=2 and E41=2 for 10 are 230 and 90 mV higher
than the corresponding redox processes and are slightly
inferior to those observed for 12 (E31=2 ¼ 1:15 V and
E41=2 ¼ 1:40 V). Undoubtedly, this increase in redox
potentials is due to coulombic factors, but the overall
reversibility of the oxidation processes, together with the
narrow electrochemical window for attaining a tetraca-
tion state, illustrate the new extended TTF as an out-
standing and promising p-electron donor.
Initial attempts at forming charge-transfer (CT) mate-
rials with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-
dimethane (TCNQF4) has resulted in the isolation of an
amorphous powder with a donor–acceptor stoichiome-
try of 1:1.16 The conductivity of this material was found
to be 2.4 · 10ꢁ4 Scmꢁ1 by the two-probe compressed
pellet method17 (average of five readings).
S
S
S
S
p-Tolyl
S
S
S
S
S
S
S
S
S
S
S
In conclusion, we have presented some new TTF
materials featuring fused, electroactive and highly pla-
nar triaryl fragments. Compound 10 is the highlight in
this series, due to its exceptional p-electron donor
properties and its ability to be reversibly oxidised to the
tetracation species.
S
S
S
11
12

2538
T. Khan et al. / Tetrahedron Letters 45 (2004) 2535–2539
Figure 1. X-ray crystal structures of (a) compound 4 and (b) derivative 10 with the solvent of crystallisation (toluene) omitted.
Acknowledgements
We thank the EPSRC for a grant to TK (GR/M36342)
and the Royal Society of Chemistry (PJS) and the
British Council (PF) for travel grants.
References and notes
1. Day, P.; Kurmoo, M. J. Mater. Chem. 1997, 8, 1291;
Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K.
D.; Geiser, U.; Wang, H. H.; Kini, A. M.; Whangbo,
M.-H. Organic Superconductors (including fullerenes);
Prentice Hall: New Jersey, 1992.
2. Bryce, M. R. Chem. Soc. Rev. 1991, 20, 355.
3. Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Petty, M.
C.; Monkman, A. P.; Morenco, C.; Yarwood, J.; Joyce,
M. J.; Port, S. N. Adv. Mater. 1998, 10, 395; Le Derf, F.;
Mazari, M.; Mercier, N.; Levillain, E.; Richomme, P.;
Figure 2. Cyclic voltammogram of compound 10 vs Ag/AgCl reference
electrode, using a Au working electrode, Pt counter electrode and
Bu4NPF6 as the supporting electrolyte.
ꢁ
Becher, J.; Garın, J.; Orduna, J.; Gorgues, A.; Salle, M.
Chem. Commun. 1999, 1417.
ꢁ

T. Khan et al. / Tetrahedron Letters 45 (2004) 2535–2539
2539
ꢁ
4. Segura, J. L.; Martın, N. Angew. Chem., Int. Ed. 2001, 40,
1372.
5. Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.;
Rovira, C. J. Am. Chem. Soc. 2004, 126, 984.
6. Khan, T.; McDouall, J. J. W.; McInnes, E. J. L.; Skabara,
tary data in the form of a CIF has been deposited with the
Cambridge Crystallographic Data Centre (CCDC 231107).
11. Close contacts refer to interatomic distances shorter than
the sum of the van der Waals radii for the two
corresponding atoms.
ꢀ
P. J.; Frere, P.; Coles, S. J.; Hursthouse, M. B. J. Mater.
Chem. 2003, 13, 2490.
12. Crystallographic
data
for
compound
10:
C28H24O5S6 Æ C7H8, triclinic, P-1, a ¼ 7:7619ð1Þ, b ¼
ꢂ
7. Svenstrup, N.; Rasmussen, K. M.; Karen, M.; Hansen, T.
K.; Becher, J. Synthesis 1994, 809.
14:1309ð4Þ, c ¼ 15:3911ð4Þ A, a ¼ 100:642ð2Þꢁ, b ¼
3
ꢂ
91:195ð2Þꢁ, c ¼ 100:785ð2Þꢁ, volume ¼ 1627.11(7) A ,
8. Hansen, T. K.; Lakshmikantham, M. V.; Cava, M. P.;
Metzger, R. M.; Becher, J. J. Org. Chem. 1991, 56, 2720;
Moore, A. J.; Bryce, M. R. Synthesis 1991, 26.
9. Selected data for compound 10: Mp, 154–156 ꢁC; MS
(APCI) ¼ 632.0; (Found: C, 53.01; H, 3.82; S, 29.87. Calcd
T ¼ 120 K, Z ¼ 2, Dc ¼ 1:48 Mg/m3, l ¼ 0:464 mmꢁ1
,
hmax ¼ 27:48ꢁ, 36270 measured and 7418 unique
(Rint ¼ 0:0653) reflections, R1 (obs) ¼ 0.0647 and wR2 (all
data) ¼ 0.1721, qmax=qmin ¼ 2:614= ꢁ 0:716 eAꢁ3. Supple-
ꢂ
mentary data in the form of a CIF has been deposited with
the Cambridge Crystallographic Data Centre (CCDC
228893).
1
for C28H24O5S6: C, 53.14; H, 3.82; S, 30.40%); H NMR
(CDCl3, 300 MHz, d) 7.65 (2H, d, J 8.65), 6.58–6.45 (4H,
m), 6.08 (1H, d, J 11.12), 5.83 (1H, d, J 11.13), 3.92 (6H, d,
J 7.14), 3.84 (6H, s) 3.30 (4H, s); 13C NMR (CDCl3,
75 MHz, d) 161.0, 160.9, 156.2, 156.0, 141.2, 139.3, 139.2,
127.6, 127.4, 126.5, 123.1, 122.3, 113.1. 112.2, 112.0, 111.5,
105.7, 105.6, 98.8, 98.7, 55.8, 55.5, 55.3 and 30.2; Vmax
(KBR)/cmꢁ1 1611, 1504, 1287, 1226, 1160, 1030 and 819.
10. Crystallographic data for compound 4: C26H22O4S7,
monoclinic, space group P121/n1, a ¼ 16:286ð2Þ, b ¼
13. Experiments were run in CH2Cl2–CH3CN (1:1) with
substrate concentration ca. 10ꢁ4 M, using a Ag/AgCl
reference electrode, Au working electrode, Pt counter
electrode and Bu4NPF6 (0.1 M) as the supporting electro-
lyte.
14. Moore, A. J.; Bryce, M. R.; Ando, D. J.; Hursthouse, M.
B. J. Chem. Soc., Chem. Commun. 1991, 320.
ꢀ
15. Frere, P.; Boubekeur, K.; Jubault, M.; Batail, P.; Gorgues,
A. Eur. J. Org. Chem. 2001, 3741.
ꢂ
10:6501ð7Þ, c ¼ 16:636ð1Þ A, a ¼ 90ꢁ, b ¼ 111:379ð9Þꢁ,
3
ꢂ
c ¼ 90ꢁ, volume ¼ 2686.9(8) A , T ¼ 293 K, Z ¼ 4, l ¼
0:620 mmꢁ1, 3211 reflections measured and 1808 symme-
try-independent with I > 3r(I), full matrix least-squares
refinement on F2, Rall ¼ 0:053, wRall ¼ 0:075. Supplemen-
16. Found C, 51.82; H, 2.42; N, 6.01. Calcd for
C40H24F4N4O5S6: C, 52.85; H, 2.66 N, 6.16%); Vmax (KBr)/
cmꢁ1 2190, 1609, 1505, 1438, 1497, 1336, 1191 and 1021.
17. Wudl, F.; Bryce, M. R. J. Chem. Educ. 1990, 67, 717.