A Pyrrolo-Tetrathiafulvalene Belt and Its TCNQ Complex: Syntheses and X-ray Crystal Structures

Article abstract of DOI:10.1021/ol025622z

(Matrix Presented) A general and efficient four-step synthesis of a tetrathiafulvalene-belt 6, starting from the monopyrrolo-tetrathiafulvalene building block 1, is described, together with its 7,7,8,8-tetracyano-p-quinodimethane charge transfer complex. The complexation of the electron acceptor 7,7,8,8-tetracyano-p-quinodimethane by the tetrathiafulvalene-belt 6 was investigated both in solution and in the solid state.

Full text of DOI:10.1021/ol025622z

ORGANIC
LETTERS
2002
Vol. 4, No. 8
1327-1330
A Pyrrolo-Tetrathiafulvalene Belt and Its
TCNQ Complex: Syntheses and X-ray
Crystal Structures
,†
Kent Nielsen,† Jan O. Jeppesen,† Niels Thorup,‡ and Jan Becher*
Department of Chemistry, UniVersity of Southern Denmark (Odense UniVersity),
CampusVej 55, DK-5230, Odense M, Denmark, and Department of Chemistry,
The Technical UniVersity of Denmark, DK-2800, Lyngby, Denmark
jbe@chem.sdu.dk
Received January 25, 2002
ABSTRACT
A general and efficient four-step synthesis of a tetrathiafulvalene-belt 6, starting from the monopyrrolo-tetrathiafulvalene building block 1, is
described, together with its 7,7,8,8-tetracyano-p-quinodimethane charge transfer complex. The complexation of the electron acceptor 7,7,8,8-
tetracyano-p-quinodimethane by the tetrathiafulvalene-belt 6 was investigated both in solution and in the solid state.
Since the discovery of the first metallic charge transfer (CT)
complex1 of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-
p-quinodimethane (TCNQ), the chemistry1b of TTF and its
derivatives has been intensively studied with the ultimate
goal of preparing superconducting CT complexes and radical
cation salts.2 Furthermore, there has recently been an
increasing interest in incorporating TTF into molecular,
macrocyclic, and supramolecular systems capable of acting
as, for example, sensors, catalysts, or switches at the
molecular level.3 Dimeric TTF molecules4 and TTF-belts5
in which two TTF units are linked by one or more spacer
groups have received particular attention, due to the pos-
sibility of affecting the formation, structure, and physical
properties of their CT complexes and ion radical salts.4
Versatile TTF building blocks have been developed in recent
years, and many problems concerning the selective func-
tionalization of TTF have been solved.3a,c,6 However, the four
attachment sites of TTF usually result in a mixture of two
inseparable isomers, a cis and a trans isomer. This problem
can be circumvented using the newly described bis-cyano-
ethyl thiolate protected monopyrrolo-TTF building block6 1
(Scheme 1), which only possesses three attachment sites and
thereby is devoid of cis/trans problems.
In this letter, we describe a general and efficient method
for the preparation of TTF-belts employing the TTF building
block 1, together with a crystal structure analysis of the
neutral TTF-belt 6. Furthermore, we describe our preliminary
complexation studies between the TTF-belt 6 and TCNQ,
(3) (a) Bryce, M. R. J. Mater. Chem. 2000, 10, 589-598. (b) Collier,
C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.;
Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 298, 1172-1175.
(c) Nielsen, M. B.; Lomholt, C.; Becher, J. Chem. Soc. ReV. 2000, 29, 153-
164. (d) Jeppesen, J. O.; Perkins, J.; Becher, J.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2001, 40, 1216-1221. (e) Segura, J. L.; Mart´ın, N. Angew. Chem.,
Int. Ed. 2001, 40, 1372-1409. (f) Collier, C. P.; Jeppesen, J. O.; Luo, Y.;
Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc.
2001, 123, 12632-12641.
(4) Otsubo, T.; Aso, Y.; Takimiya, K. AdV. Mater. 1996, 8, 203-211.
(5) Spanggaard, H.; Prehn, J.; Nielsen, M. B.; Levillain, E.; Allain, M.;
Becher, J. J. Am. Chem. Soc. 2000, 39, 9486-9494 and references therein.
(6) Jeppesen, J. O.; Takimiya, K.; Jensen, F.; Brimert, T.; Nielsen, K.;
Thorup, N.; Becher, J. J. Org. Chem. 2000, 65, 5794-5805.
† University of Southern Denmark (Odense University).
‡ The Technical University of Denmark.
(1) (a) Ferraris, J.; Cowan, D. O.; Walatka, V. V.; Perlstein, J. H. J. Am.
Chem. Soc. 1973, 95, 948-949. (b) Schukat, G.; Fangha¨nel, E. Sulfur Rep.
1996, 18, 1-294.
(2) 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). Synthesis, Structure, Properties, and Theory; Prentice
Hall: Englewood Cliffs, NJ, 1992.
10.1021/ol025622z CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/19/2002

The TTF-belt 6 was synthesized as illustrated in Scheme
1. A solution of the monopyrrolo-TTF building block6 1 was
treated with 1 equiv of CsOH‚H2O. This procedure generated
the TTF-monothiolate, which was alkylated with 0.5 equiv
of 1,2-bis(2-iodoethoxy)ethane (2) to afford the bis-TTF 3
in 85% yield. Subsequently, the two remaining cyanoethyl
protecting groups in 3 were deprotected using 2 equiv of
CsOH‚H2O followed by addition of 1 equiv of 1,2-bis(2-
iodoethoxy)ethane (2), which effected the second deprotec-
tion/alkylation sequence, affording 4 in 64% yield. Depro-
tection of the tosyl groups were carried out in near
quantitative yield by refluxing 4 in a 1:1 mixture of THF
and MeOH in the presence of an excess of NaOMe.
Employing high-dilution conditions allowed a 63% yield of
the TTF-belt10 6 to be obtained following N-alkylation of
the pyrrole units in 5 with 1,2-bis(2-iodoethoxy)ethane (2).
Scheme 1. Synthesis of the TTF-Belt 6, Starting from the
Monopyrrolo-TTF Building Block 1
The redox behavior of the TTF-belt 6 was investigated in
solution by cyclic voltammetry (CV). The CV of 6 exhibited
two well-defined two-electron reversible redox waves at
1
2
E1/2 ) 0.37 V and E1/2 ) 0.91 V corresponding to the
simultaneous formation of two radical cations followed by
formation of two dications at a higher potential.11
Single crystals of 6 were obtained by crystallization from
CH2Cl2-MeOH, and its structure was solved by X-ray
crystallography12 (Figure 1). The TTF-belt is rather elongated
which have been carried out in solution using 1H NMR, UV-
vis, and ESR spectroscopies and in the solid state using IR
spectroscopy and X-ray crystallography.7
Figure 1. Crystal structure of the TTF-belt 6. Hydrogen atoms
are omitted for clarity.
The TTF-belt 6 was designed to participate in host-guest
chemistry,8 where the belt incorporating two identical TTF
electron donors could be expected to act as a host molecule
for electron-deficient guests such as TCNQ. The optimal
distance between two TTF moieties for CT and/or π-π
interaction with the purpose of stabilizing inclusion of a guest
is approximately 7 Å.9 A Cory-Pauling-Koltun (CPK)
model inspection revealed that 1,2-bis(2-iodoethoxy)ethane
spacers between two TTF moieties would allow the distance
between the two TTF moieties to be longer/or equal to 7 Å.
with the two TTF parts having there longer axes parallel.
The interplanar distance is not well-defined but on the order
of 5.5 Å, and one TTF part is rotated approximately 15°
1
(10) Data for TTF-belt 6: H NMR (300 MHz, CDCl3, 25 °C) δ 3.00
(t, J ) 6.1 Hz, 8H), 3.50 (s, 4H), 3.53 (t, J ) 5.2 Hz, 4H), 3.62 (s, 8H),
3.72 (t, J ) 6.1 Hz, 8H), 3.89 (t, J ) 5.2 Hz, 4H), 6.47 (s, 4H); 13C NMR
(75 MHz, CDCl3, 25 °C) δ 36.0, 50.8, 70.5, 70.6, 70.7, 70.9, 110.8, 113.0,
118.6, 121.0, 128.2; MS(EI) m/z 956 (M+); mp 162-163 °C. Anal. Calcd
for C34H40N2O6S12 (957.5): C, 42.51; H, 4.29; N, 2.90; S, 40.03. Found:
C, 42.65; H, 4.21; N, 2.93; S, 40.18.
(7) For examples of TTF‚TCNQ complexes, see: (a) Batsanov, A. S.;
Bryce, M. R.; Chesney, A.; Howard, J. A. K.; John, D. E.; Moore, A. J.;
Wood, C. L.; Gershtenman, H.; Becker, J. Y.; Khodorkovsky, V. Y.; Ellern,
A.; Bernstein, J.; Perepichka, I. F.; Rotello, V.; Gray, M.; Cuello, A. O. J.
Mater. Chem. 2001, 11, 2181-2191. (b) Legros, J.-P.; Dahan, F.; Binet,
L.; Carcel, C.; Fabre, J.-M. J. Mater. Chem. 2000, 10, 2685-2691.
(8) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany,
1995.
(11) CV was performed using a 0.84 mM solution of 6 in anhydrous
CH2Cl2 with n-Bu4NPF6 (0.10 M) as the supporting electrolyte and Ag/
AgCl as the reference electrode at a scan rate of 100 mV/s.
(12) Selected interatomic distances (Å) for 6: S(1)-C(8) 1.746(2),
S(1)-C(1) 1.762(2), S(2)-C(2) 1.757(2), S(2)-C(3) 1.766(2), S(3)-C(3)
1.746(2), S(4)-C(4) 1.745(3), S(5)-C(4) 1.756(2), S(5)-C(2) 1.758(3),
S(6)-C(5) 1.738(2), S(6)-C(1) 1.770(2), N(1)-C(6) 1.361(3), N(1)-C(7)
1.364(2), C(1)-C(2) 1.342(3), C(3)-C(4) 1.344(4), C(5)-C(6) 1.371(3),
C(5)-C(8) 1.411(3), C(7)-C(8) 1.362(3).
(9) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
1328
Org. Lett., Vol. 4, No. 8, 2002

with respect to the other part. In addition to the rotation,
there is a rather long translation in the longitudinal direction
(4.4 Å), thus leading to a ring-over-bond arrangement. The
central C2S4 systems and the pyrrole part of the TTF moieties
are approximately planar. Derivations from planarity of the
moieties can be described by the interplanar angles A-B
4.7°, A-C 17.1°, D-E 16.5°, and D-F 6.4°, where the
planes are defined by the atoms: A(S1, S6, S5, S2, C1, C2),
B(S1, S6, C5, C8), C(S2, S5, C3, C4), D(S7, S12, S8, S11,
C19, C20), E(S7, S12, C16, C17), and F(S8, S11, C21, C22).
Finally, a few shorter intermolecular S‚‚‚S distances indicate
some weak sidewise interactions between neighboring TTF-
belts.
Figure 3. Crystal structure of the CT complex 6‚TCNQ. Hydrogen
atoms are omitted for clarity.
The complexation behavior between the TTF-belt 6 (host)
and TCNQ (guest) was investigated in solution using UV-
1
vis, H NMR, and electron paramagnetic resonance (EPR)
spectroscopies.
Single crystals of 6‚TCNQ (Figure 3) suitable for X-ray
diffraction analysis were grown by slow evaporation of a
CH2Cl2 solution containing 6 and TCNQ in a 1:1 ratio.15
The crystal structure determination16 (Figure 3) surprisingly
revealed that the TCNQ molecule is located between two
different TTF-belts and not as expected within a molecular
cavity. In this structure, the TTF-belt is more collapsed,
leaving no cavity between the two TTF parts. The crystal
structure of 6‚TCNQ comprises mixed stacks of an ‚‚‚
ADDADD‚‚‚ type (Figure 4) having almost parallel molec-
Addition of 1 equiv of TCNQ to a CH2Cl2 solution of 6
(2.5 × 10-4 M) resulted in an immediate color charge from
orange to green and the appearance of two CT absorption
bands, centered on λmax 749 and 849 nm in the UV-vis
spectrum (Figure 2). This might indicate two different forms
Figure 2. Absorption spectra (CH2Cl2, 25 °C) recorded on a 1:1
mixture of 6 and TCNQ.
of complexation in solution, an inside and an outside
(alongside) form. The 1H NMR spectrum (250 MHz)
recorded at 25 °C in CDCl3 revealed a weak upfield shift
(∆δ ) 0.02 ppm) of the resonances associated with the
pyrrole protons. Furthermore, EPR spectroscopy of 6‚TCNQ
recorded in CH2Cl2 revealed a radical signal13 at g ) 2.009.
The line width and g-value are consistent with the presence
of a TTF radical cation.14 These results indicate that a CT
interaction between the host and guest is taking place in
solution. However, these results do not provide any informa-
tion regarding the stoichiometry of the complexation or
whether TCNQ is complexed inside the cavity of 6 or
associated at the outside of 6.
Figure 4. Crystal packing of 6‚TCNQ. CH2Cl2 groups are omitted
for clarity.
ular planes,17 where the interplanar distance between the two
TTF parts have been reduced to ca. 3.5 Å. The structure
(15) Selected interatomic distances (Å) for 6‚TCNQ: S(7)-C(16)
1.735(3), S(7)-C(9) 1.754(3), S(8)-C(10) 1.745(3), S(8)-C(11) 1.761-
(3), S(9)-C(11) 1.748(3), S(10)-C(12) 1.754(3), S(11)-C(10) 1.741(3),
S(11)-C(12) 1.747(3), S(12)-C(13) 1.729(3), S(12)-C(9) 1.755(3), N(2)-
C(14) 1.366(4), N(2)-C(15) 1.370(4), C(9)-C(10) 1.358(4), C(11)-C(12)
1.345(4), C(13)-C(14) 1.371(4), C(13)-C(16) 1.414(4), C(15)-C(16)
1.380(4), C(35)-C(41) 1.383(4), C(35)-C(40) 1.430(4), C(35)-C(36)
1.437(4), C(36)-C(37) 1.345(5), C(37)-C(38) 1.431(5), C(38)-C(44)
1.394(5), C(38)-C(39) 1.434(4), C(39)-C(40) 1.352(4), C(41)-C(42)
1.421(4), C(41)-C(43) 1.427(4), C(44)-C(45) 1.428(5), C(44)-C(46)
1.429(5).
(13) The EPR spectrum also revealed a radical signal at g ) 2.004, which
is consistent with the presence of TCNQ•-; see: Bryce, M. R.; Moore, A.
J.; Tanner, B. K.; Whitehead, R.; Clegg, W.; Gerson, F.; Lamprecht, A.;
Pfenninger, S. Chem. Mater. 1996, 8, 1182-1188.
(14) Wudl, F.; Smith, G. M.; Hufnagel, E. J. J. Chem. Soc., Chem.
Commun. 1970, 1453-1454.
Org. Lett., Vol. 4, No. 8, 2002
1329

also reveals that the degrees of overlap between the TCNQ‚
‚‚TTF (containing C1 and C2) and TCNQ‚‚‚TTF (containing
C9 and C10) are not identical,18 indicating that the charge
transfer can be different for the two TTF parts in the same
molecule. In a TCNQ moiety, bond lengths tend to change
with an increase in the negative charge located on the
acceptor.
and substituted monopyrrolo-TTF25 (1.761(2) Å). 6‚TCNQ
exhibits a lower degree of intermolecular charge transfer than
TTF‚TCNQ,26 which is not surprising, since the oxidation
1
potential of 6 (E1/2 ) 0.37 V, vs Ag/AgCl in CH2Cl2) is
1
slightly higher than that of TTF (E1/2 ) 0.34 V, vs Ag/
AgCl in MeCN). The nitrile absorption frequency in the
solid-state IR spectrum of 6‚TCNQ is shifted to a lower
wavenumber27 (2195 cm-1) with respect to that of TCNQ
(2222 cm-1), which also suggests that a charge transfer
occurs between the TTF part and TCNQ in the solid state.28
In summary, a general and efficient method for the
preparation of TTF-belts has been developed. The TTF-belt
6 was designed to allow complexation of TCNQ inside its
cavity. However, a solid-state X-ray crystal structure analysis
of the CT complex 6‚TCNQ revealed that the TCNQ is
associated outside (alongside) one of the electron donors,
reflecting that the complicated and subtle balance between
all the individual noncovalent forces acting in cooperation
are difficult to predict. The efficient synthetic methodology
described in this letter allows us, in a few steps, to make
related TTF-belts in which the spacers can be varied in order
to optimize the complexation properties of the macrocyclic
host. Work is aimed in this direction, together with further
functionalization generating also TTF-cages incorporating
three or more monopyrrolo-TTF moieties.
It has been found that differences in the bond length of
TCNQ (Figure 5) change practically linearly19 from 0.069
Figure 5. Structure of TCNQ.
Å (b-c) and 0.062 Å (d-c) in neutral TCNQ20 to zero in
the TCNQ radical anion.21 In the case of 6‚TCNQ, the
average bond lengths are a ) 1.349(5) Å, b ) 1.433(4) Å,
c ) 1.389(4) Å, and d ) 1.426(5) Å, suggesting that the
charge transfer is ca. 0.4 e. A similar model of linearly
dependency22 can be applied for the central C-S bonds of
the TTF moiety, suggesting positive charges23 of 0.2 and
0.0 e, respectively, for the two TTFs in the same belt. This
inconsistency in balance between negative and positive
charges can presumably be ascribed to a change in central
C-S bond length between unsubstituted TTF24 (1.757(2) Å)
Acknowledgment. We thank University of Southern
Denmark for a Ph.D. Scholarship to K.N., Carlsbergfondet
for financial support to J.O.J., and Lars Duelund for recording
the EPR spectrum.
Supporting Information Available: Characterization
data for 3-6, together with crystallographic data for 6 and
6‚TCNQ. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) The crystal structure contains one solvent molecule per 6‚TCNQ,
and one of the spacer chains is somewhat disordered, which was modeled
by two slightly different conformations (only one of which is shown in
Figure 3).
(17) Deviations from planarity can be described by the interplanar angles
A-B 1.8°, A-C 3.5°, D-E 2.2°, and D-F 10.6° of 6‚TCNQ; translation
of the two TTF parts in the longitudinal direction was 4.5 Å.
(18) The overlap between TCNQ‚‚‚TTF (containing C9 and C10) is
largest.
OL025622Z
(25) The average bond length for the central C-S bonds in 6. If the
value 1.757 is changed to 1.761 Å in the equation described in ref 23, the
positive charges on the two TTF units are ca. 0.3 and ca. 0.1 e, respectively.
(26) Kistenmacher, T. J.; Phillips, T. E.; Cowan, D. O. Acta Cryst. 1974,
B30, 763-768.
(27) The change in absorption frequencies measured by IR spectroscopy
(2195 cm-1) suggests a charge transfer of ca. 0.7 e according to the model
proposed by Chappell and co-workers.28 However, these results are
inconsistent with the degree of charge transfer suggested by the crystal
structure analysis (ca. 0.4 e). A similar inconsistency has been observed in
CT complexes of BEDT-TTF‚TCNQ; see: Wu, W.; Zhang, D.; Li, H.;
Zhu, D. J. Mater. Chem. 1999, 9, 1245-1249.
(28) (a) Chappell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M. O.;
Poehler, T.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442-2443. (b)
Khatkale, M. S.; Devlin, J. P. J. Chem. Phys. 1979, 70, 1851-1859.
(19) Flandrois, S.; Chasseau, D. Acta Crystallogr., Sect. B 1977, 33,
2744-2750.
(20) Long, R. E.; Sparks, R. A.; Trueblood, K. N. Acta Crystallogr. 1965,
18, 932-939.
(21) Hoekstra, A.; Spoelder, T.; Vos, A. Acta Crystallogr., Sect. B 1972,
28, 14-25.
(22) Clemente, D. A.; Marzotto, A. J. Mater. Chem. 1996, 6, 941-946.
(23) The values 0.2 and 0.0 were determined from the equation f ) 1.757
Å - 0.0385(F+), where f is the average bond length of the central C-S
bonds in the CT complex, 1.757 Å is the average bond length of the central
C-S bonds in neutral unsubstituted TTF, and F+ is the degree of positive
charge on the TTF unit in the CT complex.
(24) Cooper, W. F. N.; Kenny, C.; Edmonds, J. W.; Nagel, A.; Wudl,
F.; Coppers, P. Chem. Commun. 1971, 889-890.
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