Synthesis of tetrahydroxy-π-extended tetrathiafulvalenes as new supramolecular redox building blocks

Article abstract of DOI:10.1016/S0040-4039(02)02779-X

A novel p-quinonoid π-extended tetrathiafulvalene (exTTF) endowed with four hydroxy groups with different reactivity (phenol and alcohol) has been synthesized as a supramolecular redox building block. The redox properties, studied by cyclic voltammetry, reveal a strong donor ability and, despite the different substitution pattern on the 1,3-dithiole rings, only one oxidation wave involving two electrons to form the dication species.

Full text of DOI:10.1016/S0040-4039(02)02779-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 945–948
Synthesis of tetrahydroxy-p-extended tetrathiafulvalenes as new
supramolecular redox building blocks
Marta C. D´ıaz, Beatriz Illescas and Nazario Mart´ın*
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad Complutense, E-28040 Madrid, Spain
Received 22 October 2002; accepted 4 December 2002
Abstract—A novel p-quinonoid p-extended tetrathiafulvalene (exTTF) endowed with four hydroxy groups with different reactivity
(phenol and alcohol) has been synthesized as a supramolecular redox building block. The redox properties, studied by cyclic
voltammetry, reveal a strong donor ability and, despite the different substitution pattern on the 1,3-dithiole rings, only one
oxidation wave involving two electrons to form the dication species. © 2003 Elsevier Science Ltd. All rights reserved.
One of the most appealing challenges in materials sci-
ence is the creation of supramolecular materials in
which the constituent units are highly regular molecular
nanostructures. In this regard, learning how to create
large supramolecular units, and the rules that mediate
their macroscopic organization into functional materi-
als, represents a fascinating prospect for technology.1
central hydrocarbon skeleton (1)6 or on the 1,3-dithiole
ring (27, 38) have been reported (Chart 1).
Previously, hydroxy containing exTTF molecules were
reported bearing one or two hydroxy groups on either
the 1,3-dithiole ring or the hydrocarbon skeleton (1–3).
In this communication we describe the multistep syn-
thesis of a novel tetrahydroxy-9,10-bis(1,3-dithiol-2-yli-
Among the different known strategies to construct
supramolecular assemblies by using intermolecular
interactions, hydrogen bonding is at the forefront due
to its relative strength, high directionality and
selectivity.2
dene)-9,10-dihydroanthracene (11) as
a
suitable
building block endowed with four hydroxy groups,
located simultaneously on the carbo and heterocyclic
rings, for further applications in the construction of
exTTF-based supramolecular ensembles.
Tetrathiafulvalene (TTF) derivatives endowed with sub-
stituents (hydroxy, amido, etc.) able to form inter-
molecular hydrogen bonding have been used for the
construction of redox-active supramolecular systems
with application in different areas.3,4
In order to gain a better understanding of electroactive
compound 11, the electrochemical properties as well as
the molecular geometry determined by semiempirical
calculations at the semiempirical PM3 level were car-
ried out.
In contrast, suitably functionalized p-quinodimethane
analogues of TTF (exTTF) have been much less studied
for the construction of redox-active supramolecular
ensembles despite the interesting electron donor proper-
ties they display.5 This fact can be accounted for by the
lack of available procedures for the synthesis of exTTFs
bearing groups which can participate in intermolecular
hydrogen bonding interactions.
To the best of our knowledge, only recently the chemi-
cal functionalization of extTTFs has been undertaken
and some derivatives bearing a functional group on the
* Corresponding author. Tel.: 34-91-3944227; fax: 34-91-3944103;
e-mail: nazmar@quim.ucm.es
Chart 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02779-X

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M. C. D´ıaz et al. / Tetrahedron Letters 44 (2003) 945–948
The synthesis of the exTTF-tetraol (11) was carried out
as depicted in Scheme 1. Thus, dihydroxy substituted
anthrone (5) was prepared from commercially available
2,6-dihydroxyanthraquinone (4) (anthraflavic acid) by
refluxing in a solution of tin dichloride in hydrochloric
acid as reducing system.9 Compound 5 precipitates
quantitatively in the reaction medium with a high
purity.
expected aromatic signals, the protons of the dithiole
ring as a singlet at 6.33 ppm and the methyleneprotons
as two singlets at 4.23 and 4.24 ppm. The hydroxy
protons were not observed in the spectrum registered in
CD3OD. However, these protons were observed when
the spectrum was recorded in deuterated acetone, the
phenolic hydroxy groups appearing as a broad signal at
8.7 ppm and the hydroxymethyl OH protons at 4.58
ppm. The OH groups were also detected in the precur-
sor 7 registered in DMSO-d6 at 10.65 and 9.98 ppm for
the phenolic protons and as a broad signal at around
5.5 ppm for the remaining OH groups. The 13C NMR
spectrum of 11 showed as significant signals the methyl-
ene groups at 56.0 ppm.
Olefination reaction of the anion of the dihydroxyan-
throne (5) with bis-hydroxymethyldithiolium salt (6),
which was in turn prepared in a three-step synthetic
procedure,10 led to the tetrahydroxy derivative 7 in 54%
yield. Introduction of the second dithiole ring in 7
requires protection of the hydroxy groups which was
achieved by reaction with tert-butyldiphenylsilyl chlo-
ride. Further Wittig–Horner reaction of the tetrakis-
protected tetraol 8 with phosphonate ester 911 under
basic conditions (LDA/THF) afforded the protected
quinonoid-exTTF (10) as a stable yellow solid in 65%
yield. Deprotection of 10 was carried out by treatment
with tetrabutylammonium fluoride to give the 2,6-dihy-
droxy-9-(4,5-dihydroxymethyl-1,3-dithiol-2-ylidene)-10-
(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (11) in
65% yield, which was purified by flash chromatography
(silica gel) using hexane:ethyl acetate (1:9) as eluent.
The UV–vis spectrum of 11 shows the umax at 422 nm
(CH2Cl2) similarly to the parent unsubstituted exTTF
(1: R=X=X%=H, umax=415 nm, CH2Cl2). However,
compound 7 showed a strong bathochromic shift
(umax=469 nm) which corresponds to a charge-transfer
transition from the donor 1,3-dithiole moiety to the
acceptor carbonyl group, similarly to that observed for
other related systems.13
Due to the interest of compound 11 as an electron
donor building block for the preparation of electrically
conducting materials with high dimensionality, as well
as for the construction of supramolecular ensembles
based on hydrogen bonding and p–p interactions, we
determined the redox properties of 11 by cyclic voltam-
metry at room temperature. Table 1 shows the redox
properties of 11 together with 7 and the parent exTTF
(1: R=X=X%=H) for comparison purposes.
Compound 11 was poorly soluble in non-polar com-
mon organic solvents. All attempts to obtain a single
crystal of 11 were unsuccessful and revealed the rela-
tively low stability of this compound on air.
Tetrahydroxy TTF 11 and its precursors were fully
characterized by using analytic and spectroscopic meth-
ods.12 Thus, compounds 5, 7 and 11 bearing hydroxy
groups showed a broad band at around 3420–3215
cm−1 in the FTIR spectra which disappears in the
In contrast to the parent TTF, p-quinonoid p-extended
TTFs showed the presence of a quasireversible oxida-
tion wave involving a two-electron process to form the
dication species.14
1
hydroxy protected compounds 8 and 10. The H NMR
spectrum of tetraol 11 showed, in addition to the
Scheme 1.

M. C. D´ıaz et al. / Tetrahedron Letters 44 (2003) 945–948
947
observed experimentally for the X-ray data determined
for the related tetramethylthio substituted exTTF (h=
143.8° and k=33.3°).14
Compound 11 bearing the hydroxymethyl and hydroxy
groups shows a quasireversible oxidation wave with the
oxidation potential at 0.55 V. This value is anodically
shifted 80 mV in comparison with the unsubstituted
exTTF (Table 1). Thus, although a different substitu-
tion pattern is present in both 1,3-dithiole rings in 11,
only one oxidation wave is observed, which indicates
that the presence of two hydroxymethyl groups do not
alter significatively the electrochemical behaviour of the
1,3-dithiole ring.
In summary, we have synthesized a novel exTTF sys-
tem bearing four hydroxy groups with different nature
and reactivity (alcohol and phenol) in a multistep syn-
thetic procedure, as an appealing redox active building
block for further applications in supramolecular and
materials science. The redox properties determined by
cyclic voltammetry show a good electron donor ability
which can be used for the preparation of novel salts
and charge-transfer complexes exhibiting a strong
hydrogen bonding network in the solid state. Work is
currently in progress in these directions.
Compound 7 showed an amphoteric redox behaviour
exhibiting a quasireversible oxidation wave at remark-
able more positive potential values, since only one
1,3-dithiole ring is present in the molecule, and a
quasireversible reduction wave at −0.99 V, correspond-
ing to the reduction of the carbonyl group.
Acknowledgements
Theoretical calculations carried out at semiempirical
PM3 level on donor 11 predict a molecular geometry
quite similar to other previously reported exTTF
molecules with a butterfly-shaped structure with the
central ring adopting a boat conformation.15
Financial support by the DGI of the Spanish Ministry
of Science and Technology (project BQU2002-00855) is
gratefully acknowledged. We are indebted to Professor
M. R. Bryce for his help in attempting to obtain single
crystals of compound 11.
Distorsion from planarity (Fig. 1) can be described in
terms of the angles h and k (h corresponds to the angle
formed by the outer benzene rings, and k to the tilting
of the dithiole units). These calculated angles (h=
139.6° and k=33.8°) are in good agreement with that
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