An approach to long and unsubstituted molecular wires: Synthesis of redox-active, cationic phenylethynyl oligomers designed for self-assembled monolayers

Article abstract of DOI:10.1021/jo026735z

Various oligo(phenyleneethynylene)s (OPEs) have been synthesized in the past, as they are considered as prototypes of molecular wires. When the oligomers are capped by a redox site at one end and a thiol at the other end, the resulting molecules can be grafted as a self-assembled monolayer on a gold electrode and fully studied by electrochemical techniques. Unfortunately, such molecules are usually poorly soluble and require the incorporation of solubilizing pendant groups. In this paper, we show that the replacement of the classically used redox group ferrocene by a cationic organometallic ruthenium complex, namely, [Ru(bipy)₂(ppH)]⁺ (bipy, 2,2′-bipyridine; ppH, 2-(2′-yl-phenyl)pyridine), allows a concise synthesis of an unsubstituted thioacetate-capped OPE up to four repetitive units long. The positive charge does not interfere with the conventional organic chemistry used to elongate, purify, or characterize the hexafluorophosphate salts of the molecules. To our knowledge, this represents the first family of long, poorly substituted OPEs designed for self-assembly.

Full text of DOI:10.1021/jo026735z

An Ap p r oa ch to Lon g a n d Un su bstitu ted Molecu la r Wir es:
Syn th esis of Red ox-Active, Ca tion ic P h en yleth yn yl Oligom er s
Design ed for Self-Assem bled Mon ola yer s
Ce´dric Hortholary and Christophe Coudret*
CEMES-CNRS, 29 rue J eanne Marvig BP 4347, 31055 Toulouse Cedex 4, France
coudret@cemes.fr
Received November 19, 2002
Various oligo(phenyleneethynylene)s (OPEs) have been synthesized in the past, as they are
considered as prototypes of molecular wires. When the oligomers are capped by a redox site at one
end and a thiol at the other end, the resulting molecules can be grafted as a self-assembled
monolayer on a gold electrode and fully studied by electrochemical techniques. Unfortunately, such
molecules are usually poorly soluble and require the incorporation of solubilizing pendant groups.
In this paper, we show that the replacement of the classically used redox group ferrocene by a
cationic organometallic ruthenium complex, namely, [Ru(bipy)2(ppH)]+ (bipy, 2,2′-bipyridine; ppH,
2-(2′-yl-phenyl)pyridine), allows a concise synthesis of an unsubstituted thioacetate-capped OPE
up to four repetitive units long. The positive charge does not interfere with the conventional organic
chemistry used to elongate, purify, or characterize the hexafluorophosphate salts of the molecules.
To our knowledge, this represents the first family of long, poorly substituted OPEs designed for
self-assembly.
In tr od u ction
processes (either a light-induced, intramolecular one or
a chemically driven one), chemical approaches such as
spectroscopy and kinetics have also being extensively
used.2,12 Among the last, ultrafast electrochemical tech-
niques have been reported, allowing one to study the
behavior of a small number of active molecules.6,13-18
Experimentally, the molecules probed are embedded in
a self-assembled monolayer (SAM), either pure or made
of a dilute solution of the redox-active molecule in an
inert diluent. By recording the rate constant of the
interfacial electron transfer for various molecule lengths,
one can characterize the distance dependence of the
tunneling parameter. This electrochemical approach has
the advantage of being rapid, and the technique itself
remains simple.
Molecular wires are of critical interest in the develop-
ing field of molecular electronics.1-3 Both the conductivity
and its decay with length are crucial characteristics of
such molecules. For a tunneling process, this decay is
thought to follow an exponential law, but if several
theoretical calculations are available, few experimental
data exist.4-6 It remains a challenge to address a single
molecule with two metallic electrodes;5,7-9 thus, most of
the experiments are based on the average response of a
group of molecules, possibly arranged in a self-assembled
“nanoarchitecture”.10,11 On the other hand, since electron
tunneling is also used to describe electron transfer
(1) J oachim, C.; Gimzewski, J . K.; Aviram, A. Nature 2000, 408,
541-548.
Thus, suitable molecules for such an experiment must
be rigid, rod-shaped, and asymmetrical with a redox site
at one end and a connecting atom at the other end, the
“molecular wire” being the portion comprised between the
redox site and the connecting atom anchoring the elec-
trode. Thiol groups are commonly used along with
(2) Launay, J .-P. Chem. Soc. Rev. 2001, 30, 386-397.
(3) Tour, J . M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.; J agessar, R. C.;
Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C.-W.; Chen, J .; Wang,
W.; Campbell, I. Chem. Eur. J . 2001, 7, 5118-5134.
(4) Sikes, H. D.; Smalley, J . F.; Dudek, S. P.; Cook, A. R.; Newton,
M. D.; Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1519-
1523.
(5) Langlais, V. J .; Schlittler, R. R.; Tang, H.; Gourdon, A.; J oachim,
C.; Gimzewski, J . K. Phys. Rev. Lett. 1999, 83, 2809-2812.
(6) Creager, S.; Yu, C. J .; Bamdad, C.; O’Connor, S.; MacLean, T.;
Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J .; Gozin, M.; Kayyem, J . F. J .
Am. Chem. Soc. 1999, 121, 1059-1064.
(11) Holmlin, R. E.; Ismagilov, R. F.; Haag, R.; Mujica, V.; Ratner,
M. A.; Rampi, M. A.; Whitesides, G. M. Angew. Chem., Int. Ed. 2001,
40, 2316-2320.
(12) Launay, J .-P.; Coudret, C. In Electron Transfer in Chemistry;
Wiley: New York, 2001; Vol. 5, pp 3-47.
(7) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J .; Sankey, O. F.;
Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science
2001, 294, 571-574.
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10.1021/jo026735z CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/27/2003
J . Org. Chem. 2003, 68, 2167-2174
2167

Hortholary and Coudret
SCHEME 2. [Ru (bip y)2(p p H)]+
SCHEME 1. Im p r ovem en t of th e Solu bility of
Asym m etr ica l Oligom er s: (a ) F lexible Ch a in s
Ad d ed on th e Con ju ga ted Sp a cer , (b) Bu lk
P r otectin g Gr ou p of th e Th iol, a n d (c) Ch a r ge
Effect
of arenethiol-terminated molecular wires, one can think
of combining the solubilizing group with the protective
one necessary to avoid oxidation into disulfide (Scheme
1b), as proposed in a general route to symmetrical double
thiol-capped OPV. However, the solubility is lost at the
thiol deprotection step just before the formation of the
SAM. A third strategy, albeit limited to the “electro-
chemical approach”, is to improve the solubility via the
redox site, by charge effect for instance (Scheme 1c),
which to our knowledge has been applied only to prepare
water-soluble but nonredox active OPE.24 In this case,
the solubility is maintained even after the thiol depro-
tection step and, since the redox site will not be buried
within the SAM, no dramatic structural consequences
should be expected.
Ferrocene has been extensively used and proved to be
more suitable than the other possible redox centers
described in the literature.4,6,13,25 However, it does not
improve the global solubility and its persubstitution
requires specific routes of synthesis.26 In our group, the
chemical potentialities of the remarkably robust orga-
nometallic complex [Ru(bipy)2(ppH)]+ (Scheme 2), easily
accessible in two steps from commercially available
products, have been investigated.27-30 Compared to its
N6 analogue [Ru(bipy)3]2+, the N5C donor set around the
ruthenium(II) lowers dramatically the first metal-
centered oxidation redox potential, down to ca. 0.5 V vs
SCE, a value comparable to that of the ferrocene deriva-
tive. Moreover, it allows the regiospecific introduction of
various functionalities such as bromine on the metalated
phenyl ring in an electrophilic way. Thus, the readily
accessible brominated complex [Ru(bipy)2(ppBr)]+ can be
used as a building block in palladium-catalyzed cross-
coupling reactions, the main reactions to build OPE and
OPV. The cationic character of those complexes makes
them soluble in various polar organic solvents, and the
use of hexafluorophosphate as counteranions allows
purification via conventional chromatographic tech-
niques.
conjugated spacers such as oligo(phenylenevinylene)
(OPV) and oligo(phenylethynyl) (OPE).19,20 Whatever the
repetitive unit chosen, the bottleneck that the synthetic
chemist has to go through is the necessary solubility
required throughout all the synthetic steps and the
geometrical constraints due to the desired rodlike shape
of the molecular wire.
In a seminal paper, Sita and co-workers reported the
preparation of unsubstituted rigid OPE capped at one
end by a ferrocene and at the other end by a thiophenol
masked either as a thioacetate or as a methyl thioether
(or its sulfoxide and sulfone derivative).21 Unfortunately,
the solubility was found to depend strongly on the
substituent carried by the sulfur atom. Indeed, the
longest soluble molecule described was a methyl thioet-
her-capped wire of 31 Å, the cleavage of which was not
attempted. Since gold-thioethers interactions are weaker
than thiol-gold ones,22 such capping groups are hardly
interesting for the formation of stable mixed SAM.
Classically, the solubility is improved by grafting
saturated chains (alkyl, alkoxy...) on the repetitive units
of the conjugated skeleton (Scheme 1a).19,20 Despite its
efficiency, this approach could present the following
disadvantages: (i) it requires additional steps of synthe-
sis to prepare the suitable monomeric units, and (ii) the
solubilizing groups may disturb the packing of the SAM
and influence the electron transfer. Grafting the solubi-
lizing groups at one or both ends of the OPE is a way to
bypass the first impediment: a single triisopropylsilyl
group seems to be enough to solubilize an OPE made of
five repetitive phenylene-ethynylene units.23 In the case
In a recent report, we have shown that the electro-
chemical properties of this redox site (standard potential,
(24) Anderson, S.; Aplin, R. T.; Claridge, T. D.; Goodson, T., III;
Maciel, A. C.; Rumbles, G.; Ryan, J . F.; Anderson, H. L. J . Chem. Soc.,
Perkin Trans. 1998, 1, 2383-2397.
(25) Creager, S. E.; Wooster, T. T. Anal. Chem. 1998, 70, 4257-
4263.
(26) Hradsky, A.; Bildstein, B.; Schuler, N.; Schottenberger, H.;
J aitner, P.; Ongania, K.-H.; Wurst, K.; Launay, J .-P. Organometallics
1997, 16, 392-402.
(19) Dudek, S. P.; Sikes, H. D.; Chidsey, C. E. D. J . Am. Chem. Soc.
2001, 123, 8033-8038.
(20) Yu, C. J .; Chong, Y.; Kayyem, J . F.; Gozin, M. J . Org. Chem.
1999, 64, 2070-2079.
(27) Coudret, C.; Fraysse, S. Chem. Commun. 1998, 663-664.
(28) Fraysse, S.; Coudret, C.; Launay, J .-P. Tetrahedron Lett. 1998,
39, 7873-7876.
(21) Hsung, R. P.; Chidsey, C. E. D.; Sita, L. R. Organometallics
1995, 14, 4808-15.
(22) Li, X.-M.; de J ong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J .;
Reinhoudt, D. N. J . Mater. Chem. 2001, 11, 1919-1923.
(23) Anderson, S. Chem. Eur. J . 2001, 7, 4706-4714.
(29) Fraysse, S.; Coudret, C.; Launay, J .-P. Eur. J . Inorg. Chem.
2000, 1581-1590.
(30) Monchaud, D.; Lacour, J .; Coudret, C.; Fraysse, S. J . Orga-
nomet. Chem. 2001, 624, 388-391.
2168 J . Org. Chem., Vol. 68, No. 6, 2003

An Approach to Molecular Wires
SCHEME 3
kinetics) were compatible with the general requirements
of an electrochemical screening of molecular wires em-
bedded in SAMs.31 We wish to present in this paper that
the remarkable solubility of these complexes, combined
with efficient palladium-catalyzed ethynylation reactions,
allows a concise synthesis of conjugated redox active
thiols containing unsubstituted OPE from simple starting
materials, easily purified by column chromatography.
We have shown recently that the ethynylated complex
6 was a readily accessible synthon.29 Thus, the elongation
of the growing wire is rather straightforward. All the
syntheses start with a coupling between complex 6
and compound 2 or 4 (Scheme 4). The molecules 7 and 8
are obtained, with respectively 87% and 92% yield. The
best results have been obtained, at room temperature,
using Pd(PPh3)4, CuI, and diisopropylamine (DIPA).
Since the ruthenium moiety is not soluble in DIPA,
DMF is added as a cosolvent. The three-benzene-ring
complex 9, ended with a trimethylsilyl group, is also
obtained following an analogous procedure. Starting from
8, the TMS group is removed under standard conditions
by use of a carbonate salt (namely potassium carbonate)
in a 1:1 mixture of methanol/dichloromethane. This
provides a terminal alkyne, which is not isolated and
used under the same conditions of cross coupling, as
described above, with the derivative 2 to give 9 with 93%
yield.
Resu lts a n d Discu ssion
Syn th esis. Most of the different routes developped to
prepare unsymmetrical phenylethynyl oligomers rely on
palladium-catalyzed ethynylation reaction and involve
either the dissymmetrization of an existing OPE, a
stepwise growth of the oligomeric chain, or a combination
of the two, especially for the smaller terms. Since no
pendant group has to be included on the aromatic
moieties, the strategy is greatly simplified. The ruthe-
nium organometallic building block is used as a stopper
and the elongation of the chain is achieved by the
repetition of a Sonogashira coupling followed by a
terminal alkyne unmasking. As noticed by Sita and co-
workers, this synthesis suffers from its linearity, due to
the iterative process, and the convergence can be im-
proved by preparing two types of organic units for the
wire’s elongation, containing one (2) or two phenylene
(4) nuclei (Scheme 3) (the synthesis of analogous mol-
ecules with more units would have led to a loss of
solubility).32-35 Iodine has been preferred to bromine,
since it is known to give better yields in Sonogashira
cross-coupling and it allows one to work at room tem-
perature, which can be beneficial since it has been shown
recently that cuprous chloride catalyzed desilylation was
readily observed for trimethylsilyl protected alkyne upon
heating with DMF as solvent.36
Synthesis is terminated by capping the chain with a
derivative of thiophenol, the 1-iodo-4-thioacetylbenzene
5, prepared in one step following the protocol described
by Tour and co-workers (Scheme 5).37
Despite their known lability, thioesters such as thio-
acetate are protecting groups known to be compatible
with palladium-catalyzed cross-couplings, but the use of
a 1:1 mixture of anhydrous THF and diisopropylethy-
lamine (Hunig’s base) is critical for obtaining high
yields.32 Unfortunately, the derivatives 7-9 are not
soluble in such a mixture. Our first attempt was to
replace DIPA by Hunig’s base under the same conditions
as those used to obtain 7-9. The desired molecules 10-
12 have been obtained, albeit in moderate yields (40%).
Addition of THF as cosolvent (2:1:1 DMF:THF:diisopro-
pylethylamine) increases the yield only up to 50%.
Despite this low yield in the last stage of this strategy,
unsubstituted asymmetric molecular wires of various
lengths are easily obtained by the right combination of
only four synthons, without any problem of solubility for
the synthesis, thanks to the cyclometalated ruthenium
redox site. The introduction of the sulfur atom at the very
end of the strategy also enables the use of the simple and
rather labile thioacetate as protective group. However,
a clear decrease in solubility is noticeable by comparison
to the TMS-capped molecules of similar length, and we
(31) Hortholary, C.; Minc, F.; Coudret, C.; Bonvoisin, J .; Launay,
J .-P. Chem. Commun. 2002, 1932-1933.
(32) Hsung, R. P.; Babcock, J . R.; Chidsey, C. E. D.; Sita, L. R.
Tetrahedron Lett. 1995, 36, 4525-8.
(33) Aujard, I.; Baltaze, J .-P.; Baudin, J .-B.; Cogne´, E.; Ferrage, F.;
J ullien, L.; Perez, E.; Pre´vost, V.; Qian, L. M.; Ruel, O. J . Am. Chem.
Soc. 2001, 123, 8177-8188.
(34) Lavastre, O.; Ollivier, L.; Dixneuf, P. H.; Sibandhit, S. Tetra-
hedron 1996, 52, 5495-5504.
(35) Mongin, O.; Papamicae¨l, C.; Hoyler, N.; Gossauer, A. J . Org.
Chem. 1998, 63, 5568-5580.
(36) Nishihara, Y.; Ikegashira, K.; Hirabayashi, K.; Ando, J .-i.; Mori,
A.; Hiyama, T. J . Org. Chem. 2000, 65, 1780-1787.
(37) Pearson, D. L.; Tour, J . M. J . Org. Chem. 1997, 62, 1376-1387.
J . Org. Chem, Vol. 68, No. 6, 2003 2169

Hortholary and Coudret
SCHEME 4 a
a
Reagents and conditions: (a) Pd(PPh3)4, CuI, DIPA, DMF, rt, 10 h; (b) K2CO3, MeOH, rt, 2 h; (c) Pd(PPh3)4, CuI, Hunig’s base, DMF,
THF, rt, 12 h; (d) K2CO3, MeOH-CH2Cl2, rt, 2 h.
SCHEME 5
This could be a drawback of our strategy, if we had to
separate a mixture of two same-charged complexes, but
this never happened. We can explain this as follows. The
desilylation step is quantitative (7-9 are not found in
the analysis of 10-12). The nonprotected alkyne can
react following two paths: the cross-coupling with the
iodo derivative or the oxidative dimerization called Glaser
coupling. This second competitive way of reaction pro-
vides a dinuclear complex with two positive charges. It
is thus easily separated from the required mononuclear
complex, since the elution is controlled by charge effect.
The formation of this dinuclear species can be limited
by the use of well degassed solutions.
have been compelled to change the NMR solvent (vide
infra). Nevertheless, the solubilizing properties of the
ruthenium complex are sufficient to allow the handling
of these molecules with conventional techniques. To our
knowledge, this represent the first family of such poorly
substituted OPEs designed for self-assembly.
Ch a r a cter iza tion a n d P r op er ties. All the complexes
are characterized by 1H NMR, 13C NMR, and mass
spectroscopy. We have reported in Scheme 6 the 1H NMR
spectra (recorded in acetonitrile, concentration 0.02 mol/
L) for molecules 7-9, focusing on the aromatic region.
The peaks remain narrow and well-resolved, whatever
the length of the wire, proving the very good solubilities
of these complexes in this solvent.
P u r ifica tion of Oligom er s. The chromatographic
behavior of cyclometalated derivatives is strongly de-
pendent on the associated counteranion. We have shown
in the past that easy purifications, comparable to classical
organic compounds, were achieved with common solvents
on silica with the readily available hexafluorophosphate
ion.29 The elution of such species is mainly controlled by
the global charge of the molecule. In the present case,
all the oligomers are purified by silica gel using the same
eluent (98:2 dichloromethane:acetonitrile). Indeed, the
mass increase after one coupling is not sufficient enough
to counterbalance the charge effect, and the complexes
exhibit almost the same behavior during the purification.
The behavior of the molecules 10-12 (terminated by
a thioacetate group) is quite different. Indeed, at a
concentration of 0.02 mol/L in acetonitrile the peaks
broaden when the length of the wire is increased (from
10 to 12), showing the decrease of solubility. Well-
2170 J . Org. Chem., Vol. 68, No. 6, 2003

An Approach to Molecular Wires
SCHEME 6. Ar om a tic Region of th e 1H NMR
Sp ectr a betw een 6.2 a n d 8.8 p p m , Recor d ed in
Aceton itr ile, for Molecu les 7, 8, a n d 9a
SCHEME 7. UV-Visible Sp ectr a , Recor d ed in
Eth a n ol, of Molecu les 10 (5 × 10-5 m ol/L), 11 (5 ×
10-5 m ol/L), a n d 12 (3 × 10-5 m ol/L)
site and the oligomeric rod. It has been stressed previ-
ously that the very shielded proton Ha on the cyclometa-
lated ring was an efficient probe of the electronic influ-
ence of the substituent of the complex.38 In the present
case, no significant drift could be observed, indicating a
moderate influence of the length of the spacer on the
redox site. Cyclic voltammogramms, recorded in aceto-
nitrile, led to the same conclusion, since the length of
the spacer affects neither the half-wave potential E1/2 nor
the potential peak difference between the anodic and the
cathodic waves.
The UV-visible electronic absorption spectra of mol-
ecules 10-12, recorded in ethanol, are shown in Scheme
7 and display typically two features: a complex group of
transitions in the region between 200 and 400 nm,
corresponding to the absorption of the conjugated ligands,
and a group of less intense transitions in the visible
region centered around 450-600 nm, where the ruthe-
nium complex is supposed to absorb. As the first group
depends, as expected, on the number of monomeric units,
since it contains intraligands transitions, the second one
is remarkably insensitive to the extension of the pendant
conjugated phenylpyridine substituent. To explain these
rather surprising results, since charge transfer transition
are commonly invoked to explain this part of the elec-
tronic absorption spectrum, we have performed calcula-
tions at the ZINDO/S level of approximation. The results
fit the experimental data well between 450 and 600 nm,
and calculations confirm that this region of the spectrum
is due to the MLCT transitions but only to those of the
ancillary bipyridine ligands, as shown by a closer analysis
of the involved orbitals. However, calculated spectra do
not reproduce the experimental ones for the 200-400-
nm region. This mismatch can be rationalized by keeping
in mind that the geometry of the oligomeric chain has
been arbitrary chosen as coplanar for our calculations.
It was shown recently that the absorption spectrum of
such type of molecule strongly depends on the torsion
angle between adjacent phenylene moieties.39 Because of
the very weak rotational barrier (estimated to ca. 0.5
kcal/mol), a considerable disorder is thus expected. Since
all the conformations are available in solution, a calcula-
tion taking into account all these conformations should
be performed to improve the result in this region.
a
The inset shows the attribution to the most shielded proton
Ha; the arrow indicates the quasidegenerated signal for all the
protons of the OPE part.
resolved spectra are achieved by the use of more polar
solvents such as DMF or DMSO. Nevertheless, these
molecules remain soluble in ethanol in the range of
concentration classically used for the formation of SAMs
(from 10-6 to 10-4 mol/L).6,14
A comparison of the 1H NMR spectra reported in
Scheme 6 shows small differences between them. It
appears that the main modification occurs between 7.4
and 7.6 ppm. For this family of silylated complexes, we
have recorded 2D 1H-1H and H-13C correlation spectra.
1
Starting from the attribution of the signals already
reported by Constable for the [Ru(bipy)2ppH]+ moiety, we
have assigned the signals to the conjugated wire.38 This
shows that the chemical shifts of the OPE part are
quasidegenerate in the 7.4-7.6 ppm region. Thus, 1H
NMR analysis is not the best way of analyzing the
elongation of the wire. A similar feature is observed in
the 13C NMR spectra: the shielding influence of the
complex vanishes quickly and chemical shifts of the triple
bond carbons tend to be degenerate. Unambiguous
characterization of the complexes has been achieved by
mass spectroscopy. All of the complexes behave well
under electrospray mass spectrometry, giving a typical
peak attributed to a monocharged, PF6--free molecular
ion. As previously observed, no starting material could
be detected.
From the 1H NMR and UV-visible spectra, one can
also estimate the degree of interaction between the redox
(38) Constable, E. C.; Holmes, J . M. J . Organomet. Chem. 1986, 301,
203-208.
(39) Levitus, M.; Schmieder, F.; Ricks, H.; Shimizu, K. D.; Bunz, U.
H. F.; Garcia-Garibay, M. A. J . Am. Chem. Soc. 2001, 123, 4259-4265.
J . Org. Chem, Vol. 68, No. 6, 2003 2171

Hortholary and Coudret
TABLE 1
calcium hydride). All experiments were carried out under an
argon atmosphere using standard Schlenck tube techniques,
except otherwise mentioned.
7
8
9
δ (ppm) Ha
λmax (nm)
E1/2 (V)
6.55
485, 539
0.51
6.55
481, 540
0.51
6.55
480, 540
0.51
P h ysicoch em ica l Mea su r em en ts a n d Ca lcu la tion s.
The working electrode for cyclic voltammetry esperiments was
a 1 mm platinium button, and the reference electrode an
aqueous saturated calomel electrode with a double frit system.
The solvent was DNA-synthesis-grade acetonitrile containing
0.1 M tetrabutylammonium hexafluorophosphate.
The same behavior is also observed for the molecules
7-9 ended with a trimethylsilyl group (namely, no
change in the 450-600-nm region), but no calculation has
been performed, since the ZINDO parameters are not
available for silicon. However, we can reasonably suggest
that this behavior could be explained in a similar
manner.
All these results (summarized in Table 1) indicate that
the oligomer chain and the complex are not strongly
coupled. Hence, further electrochemical measurements
should characterize the oligomer itself and allow the
determination of the decay parameter along it.
Molecular geometry was optimized using the Cerius2 pack-
age.45 The force field was Universal with Ru-N and Ru-C
bonds defined by Kb ) 350 kcal mol-1 Å-2 and Ro ) 2.05 Å
(N) or 2.02 Å (C). The ZINDO/S calculations were performed
in the Hyperchem environment,45 using the ruthenium bases
of Krogh-J espersen but with Ruâ(4d) ) -26.29 eV.46,47
4-(Tr im eth ylsilyleth yn yl)a n ilin e (1) was prepared by
following the procedure described in the litterature. Yield:
1
69%. H NMR (250 MHz, CDCl3 ) δ: 7.26 (d, J ) 8.6 Hz, 2H),
6.56 (d, J ) 8.6 Hz, 2H), 3.76 (s, 1H), 0.21 (s, 9H). MS (CI,
NH3) m/z: 190.0 (M + H+).
4-iod o-(tr im eth ylsilyleth yn yl)ben zen e (2): prepared by
following the procedure described in the litterature. Yield:
1
Con clu sion
57%. H NMR (250 MHz, CDCl3 ) δ: 7.63 (d, J ) 8.6 Hz, 2H),
7.17 (d, J ) 8.6 Hz, 2H), 0.23 (s, 9H). MS (CI, NH3) m/z: 301.0
In this paper we have presented the advantages of
capping oligo(phenylethynyl) by a cationic cyclometalated
ruthenium complex, for the synthesis of rodlike, conju-
gated molecules. The presence of the charged center on
the molecule is sufficient to allow the molecule elongation
by conventional palladium-catalyzed cross-couplings up
to 35 Å without any appreciable loss of solubility. And,
since no additional solubilizing group is necessary, their
syntheses require less organic synthons and fewer steps.
Furthermore, the resulting charged species are easily
purified and characterized by standard techniques. Since
several charged organometallic or coordination building
blocks are now available,40-42 we believe that this strat-
egy should provide a convenient access to long, conju-
gated, rigid molecules designed for various purposes such
as light-emitting devices.43
(M + H+), 318.0 (M + NH4+).
4-{[4-(Tr im eth ylsilyleth yn yl)ph en yl]eth yn yl}an ilin e (3).
To a solution of 4-(trimethylsilylethynyl)aniline (1) (378 mg,
2 mmol) in 10 mL of methanol is added 1.3 g of potassium
carbonate in one portion (9.4 mmol). After stirring at room
temperature for 2 h, 10 mL of dichloromethane and 20 mL of
water are added. The aqueous layer is extracted twice with
dichloromethane, and the combined organic layers are dried
over magnesium sulfate. After evaporation, the crude product
is not purified but solubilized in 6 mL of THF and added, under
argon, to 2 mL of a piperidine solution containing the 4-iodo-
(trimethylsilylethynyl)benzene (2) (630 mg, 2.1 mmol), Pd-
(Ph3)4 (36 mg, 0.03 mmol), and CuI (18 mg, 0.09 mmol). The
resulting mixture is stirred overnight under argon. After
hydrolysis with a saturated solution of NH4Cl, the aqueous
layer is extracted twice with dichloromethane. The combined
organic layers are washed with a diluted solution of hydro-
chloric acid (0.5 M) and dried over magnesium sulfate. After
evaporation, the crude product is purified by silica gel flash
chromatography (eluent, cyclohexane:diethyl ether 1:1) to
provide 460 mg of a pale yellow solid. Yield: 80%. 1H NMR
(250 MHz, CDCl3 ) δ: 7.40 (s, 4H), 7.31 (d, J ) 8.6 Hz, 2H),
6.62 (d, J ) 8.6 Hz, 2H), 3.84 (s, 2H), 0.24 (s, 9H). MS (CI,
NH3) m/z: 290.0 (M + H+), 307.0 (M + NH4+).
Since they carry no substituent, we anticipate that
these molecules should have a limited impact on their
environment: in our case fewer disruptions of self-
assembled monolayers are expected. This should facili-
tate their coadsorption as a SAM on a gold electrode for
further electrochemical studies.
1-(Tr im et h ylsilylet h yn yl)-4-[(4-iod op h en yl)et h yn yl]-
ben zen e (4). To a solution of 3 (820 mg, 2.8 mmol) in
hydrochloric acid (6 M, 40 mL), cooled at 0 °C, is added sodium
nitrite (246 mg, 3.6 mmol) in one portion. After 1 h at 0 °C,
this solution is added dropwise to an ice-cold solution of sodium
iodide (750 mg, 5 mmol), and 40 mL of dichloromethane is
poured in this solution. The resulting mixture is allowed to
reach room temperature and is stirred for 4 h. The aqueous
phase is extracted twice with dichloromethane. The combined
organic layers are washed with a saturated solution of sodium
thiosulfate and dried over magnesium sulfate. The solvents
are removed, and the crude product is purified by silica gel
flash chromatography with hexane as eluent, to provide 526
Exp er im en ta l Section
Gen er a l In str u m en ta tion a n d Ma ter ia ls. FID were
analyzed using the MestRE-C 2.3a program.44 Two-dimen-
sional COSY NMR experiments were performed by the Service
de RMN of the Laboratoire de Chimie de Coordination in
Toulouse. ES-MS spectra were recorded by the Service de
Spectrome´trie de Masse of the Universite´ Paul Sabatier and
FAB-HRMS by the CESAMO in Bordeaux. Reagents and
anhydrous DMF were all commercially available and were
used as received; other solvents were distilled from the
appropriate drying agent (THF from sodium/benzophenone,
amines from potassium hydroxide, and dichloromethane from
1
mg of a white solid. Yield: 47%. H NMR (250 MHz, CDCl3 )
δ: 7.68 (d, J ) 8.7 Hz, 2H), 7.43 (s, 4H), 7.22 (d, J ) 8.7 Hz,
2H), 0.24 (s, 9H). MS (CI, NH3) m/z: 401.0 (M + H+), 418.0
(M + NH4+).
(40) Chodorowski-Kimmes, S.; Beley, M.; Collin, J .-P.; Sauvage, J .-
P. Tetrahedron Lett. 1996, 37, 2963-2966.
(41) Tor, Y. Synlett 2002, 1043-1054.
(42) Xu, B.; Yongge, W.; Barnes, C. L.; Peng, Z. Angew. Chem., Int.
Ed. 2001, 40, 2290-2292.
(45) Cerius2; Molecular Simulations Inc: San Diego, CA.
(46) Krogh-J espersen, K.; Westbrook, J . D.; Potenza, J . A.; Schugar,
H. J . J . Am. Chem. Soc. 1987, 109, 7025-7031.
(47) Anderson, W. P.; Cundari, T. R.; Zerner, M. C. Int. J . Quantum
Chem. 1991, 39, 31.
(43) Glussac, K. D.; J iang, S.; S., S. K. Chem. Commun. 2002, 2504-
2505.
(44) Cobas, J . C.; Cruces, J .; Sardina, F. J . MestRe C 2.3, a freeware
program for NMR data processing.
2172 J . Org. Chem., Vol. 68, No. 6, 2003

An Approach to Molecular Wires
1-Iod o-4-(a cetylth io)ben zen e (5). To a solution of 1,4-
diiodobenzene (3 g, 9.1 mmol) in dry ether (100 mL), cooled at
-78 °C, is added dropwise tert-butyllithium (1.7 M in hexane,
10 mL, 17 mmol). The solution is vigorously stirred at this
temperature for 10 min. Then the solution is allowed to warm,
and sulfur is added (290 mg, 1.1 mmol), as soon as the
temperature reaches -10 °C. The reaction is kept at 0 °C for
30 min and then cooled again at -78 °C before the addition of
acetyl chloride in one portion (0.9 g, 11.4 mmol). The solution
is allowed to warm to room temperature overnight. After
hydrolysis, the mixture is extracted with dichloromethane. The
combined organic layers are washed with a dilute solution of
sodium hydrogenocarbonate and dried over magnesium sul-
fate. The crude product is purified by silica gel flash chroma-
tography (eluent, hexane:ether 25:1) and then recrystallized
166.75, 157.98, 157.24, 157.06, 155.38, 154.49, 150.87, 150.70,
150.54, 149.48, 146.78, 136.97, 136.31, 136.17, 135.65, 134.59,
134.50, 132.28, 132.07, 131.96, 131.61, 130.63, 127.46, 126.87,
126.79, 126.62, 126.60, 124.58, 123.89, 123.63, 123.49, 123.46,
123.41, 123.26, 122.30, 119.59, 114.44, 104.65, 96.83, 93,80,
91.26, 90.62, 87.44, -0.76. ES-MS: 864.4 (M - PF6-). HRMS
calcd for RuC52N5SiH40 (M - PF6-): 864.20965. Found:
864.21120.
Com p lex 9: 93% yield from 8. 1H NMR (400 MHz, CD3CN)
δ: 8.48 (d, J ) 8.0 Hz, 1H), 8.40 (d, J ) 8.0 Hz, 1H), 8.33-
8.37 (m, 2H), 8.11 (d, J ) 8 Hz, 1H), 8.07 (dd, J 1 ) 6 Hz, J 2
1.2 Hz, 1H), 8.03 (d, J ) 1.6 Hz, 1H), 8.01 (td, J 1 ) 8 Hz, J 2
)
)
1.6 Hz, 1H), 7.82-7.89 (m, 3H), 7.80 (d, J ) 6 Hz, 1H), 7.73-
7.78 (m, 3H), 7.63 (d, J ) 6 Hz, 1H), 7.43-7.57 (m, 13H), 7.22-
7.28 (m, 3H), 7.00 (pseudo td, J 1 ) 5.6 Hz, J 2 ) 1.6 Hz, 1H),
6.98 (dd, J 1 ) 8 Hz, J 2 ) 1.6 Hz, 1H), 6.56 (d, J ) 8 Hz, 1H),
0.26 (s, 9H). 13C NMR (400 MHz, CD3CN) δ: 199.49, 166.81,
158.02, 157.29, 157.10, 155.42, 154.49, 150.85, 150.70, 150.53,
149.47, 146.79, 136.96, 136.30, 136.17, 135.64, 134.58, 134.49,
132.27, 132.09, 132.06, 132.03, 131.99, 131.62, 130.65, 127.44,
126.84, 126.76, 126.60, 124.62, 123.87, 123.62, 123.55, 123.52,
123.47, 123.44, 123.30, 123.22, 122.36, 119.59, 114.51, 104.67,
96.89, 93.81, 91.33, 91.07, 90.97, 90.76, 87.47, - 0.77. ES-
MS: 964.2 (M - PF6-). HRMS calcd for RuC60N5SiH44 (M -
PF6-): 964.24095. Found: 964.24143.
Com p lex 10: 56% yield from complex 7. 1H NMR (250 MHz,
CD3CN) δ: 8.46 (d, J ) 8.20 Hz, 1H), 8.38 (d, J ) 8.20 Hz),
8.32 (dd, J ) 3.4 Hz, 7.7 Hz, 2H), 8.03 (m, 4H), 7.79 (m, 7H),
7.49 (m, 10 H), 7.23 (m, 4H), 6.96 (m, 2H), 6.53 (d, J ) 7.7 Hz,
1H), 2.41 (s, 3H).13C NMR (250 MHz, CD3CN) δ: 200.17,
194.27, 167.17, 158.43, 157.69, 157.50, 155.82, 154.91, 151.38,
151.16, 151.01, 149.97, 147.24, 137.46, 136.79, 136.64, 136.12,
135.52, 135.07, 135.00, 132.87, 132.61, 132.12, 131.11, 129.81,
127.94, 127.38, 127.27, 127.12, 125.07, 124.73, 124.35, 123.94,
123.76, 122.68, 120.07, 114.84, 94.31, 91.26, 90.84, 87.90,
30.49. FAB-MS: 842(M - PF6-). HRMS calcd for RuC49N5-
SOH34 (M - PF6-): 842.15276. Found: 842.15384.
1
in pentane to provide 1.58 g of a white solid. Yield: 62%. H
NMR (250 MHz, CDCl3 ) δ: 7.73 (d, J ) 8.6 Hz, 2H), 7.12 (d,
J ) 8.6 Hz, 2H), 2.42 (s, 3H). MS (CI, NH3) m/z: 279.0 (M +
H+), 296.0 (M + NH4+), 313.0 (M + N2H7+).
Gen er a l P r oced u r e for th e Dep r otection of th e Tr im -
eth ylsilyl Gr ou p (7-9). To a solution of 0.2 mmol of the
silylated complex either in methanol (12 mL) (7) or in a 1:1
dichloromethane:methanol mixture (6 mL/6 mL) (8 and 9) is
added 270 mg (2.0 mmol, 10 equiv) of potassium carbonate.
The solution is stirred at room temperature for 2 h under
argon. After evaporation of the solvents, the excess potassium
carbonate is eliminated by filtration in dichloromethane. The
crude product is not purified.
Gen er a l P r oced u r e for Son oga sh ir a Cr oss-Cou p lin g
(7-9). To 2 mL of a DMF solution containing the deprotected
alkyne (typically 0.2 mmol) are added the required p-iodo
derivative 2 or 4 (2 equiv), Pd(PPh3)4 (0.05 equiv), CuI (0.1
equiv), and 1 mL of distilled diisopropylamine. The solution
is stirred at room temperature under argon for 10 h. After
evaporation of the solvents, the crude product is purified by
silica gel chromatography (eluent, dichloromethane:acetonitrile
98:2).
Com p lex 11: 50% yield from complex 8. 1H NMR (250 MHz,
DMF-d7) δ: 8.88 (d, J ) 8.2 Hz, 1H), 8.80 (d, J ) 8.2 Hz, 1H),
8.74 (m, 2H), 8.39 (d, J ) 8.2 Hz, 1H), 8.17 (m, 3H), 7.89 (m,
4H), 7.65 (m, 16H), 7.46 (m, 4H), 7.16 (m, 1H), 7.02 (dd, J )
1.6 Hz, J ) 7.8 Hz, 1H), 6.60 (d, J ) 7.6 Hz, 1H), 2.49 (s, 3H).
1H NMR (250 MHz, DMSO-d6) δ: 8.77 (d, J ) 8.2 Hz, 1H),
8.69 (d, J ) 8.2 Hz, 1H), 8.63 (dd, J ) 3.3 Hz, J ) 8.2 Hz, 2H),
8.29 (d, J ) 8.7 Hz, 1H), 8.09 (m, 2H), 7.77 (m, 3H), 7.58 (m,
18 H), 7.39 (m, 4H), 7.10 (m, 1H), 6.94 (dd, J ) 1.2 Hz, J )
7.6 Hz, 1H), 6.46 (d, J ) 7.6 Hz, 1H) 2.45 (s, 3H). 13C NMR
(250 MHz, DMF-d7) δ: 200.36, 193.46, 167.07, 158.38, 157.67,
157.42, 155.74, 154.70, 151.13, 150.96, 150,88, 149.78, 147.16,
137.48, 136.77, 136.26, 135.25, 135.10, 134.98, 132.82, 132.53,
132.44, 132.01, 130.90, 129.88, 129.47, 128.09, 127.46, 127.28,
127.15, 127.09, 124.93, 124.53, 124.21, 124.05, 123.80, 123.43,
122.46, 120.11, 114.51, 84.53, 91.88, 91.25, 91.06, 87.84. ES-
MS: 942.3 (M - PF6-). HRMS calcd for RuC57N5SOH38 (M -
PF6-): 942.18406. Found: 942.18340.
Com p lex 12: 52% yield from complex 9. 1H NMR (250 MHz,
DMF-d7) δ: 8.88 (d, J ) 8.4 Hz, 1H), 8.80 (d, J ) 8.4 Hz, 1H),
8.74 (m, 2H), 8.39 (d, J ) 8.1 Hz, 1H), 8.17 (m, 3H), 7.89 (m,
4H), 7.65 (m, 20H), 7.45 (m, 4H), 7.16 (m, 1H), 7.02 (dd, J )
1.4 Hz, J ) 7.7 Hz, 1H), 6.59 (d, J ) 7.7 Hz, 1H), 2.49 (s, 3H).
1H NMR (250 MHz, DMSO-d6) δ: 8.76 (d, J ) 8.5 Hz, 1H),
8.68 (d, J ) 8.5 Hz, 1H), 8.63 (dd, J ) 3.0 Hz, J ) 8.5 Hz, 2H),
8.29 (d, J ) 8.5 Hz, 1H), 8.10 (m, 2H), 7.77 (m, 3H), 7.59 (m,
22 H), 7.38 (m, 4H), 7.09 (m, 1H), 6.93 (d, J ) 8.5 Hz, 1H),
6.46 (d, J ) 7.6 Hz, 1H).13C NMR (250 MHz, DMF-d7) δ:
200.36, 193.45, 167.07, 158.38, 157.67, 157.41, 155.73, 154.70,
151.12, 150.95, 150.88, 149.81, 147.16, 137.48, 136.77, 136.26,
136.17, 135.25, 135.11, 134.98, 133.85, 132.82, 132.50, 132.01,
130.90, 129.88, 128.09, 127.47, 127.28, 127.14, 127.09, 124.93,
124.52, 124.21, 124.05, 123.80, 123.71, 123.61, 123.56, 123.49,
122.46, 120.11, 114.51, 94.53, 94.44, 91.87, 91.61, 91.24, 91.07,
Gen er a l P r oced u r e for Son oga sh ir a Cr oss-Cou p lin g
(10-12). To 3 mL of a 2:1 mixture of DMF:THF containing
the deprotected alkyne (typically 0.2 mmol) are added the
S-protected derivative 5 (2 equiv), Pd(PPh3)2Cl2 (0.05 equiv),
CuI (0.1 equiv), and 1 mL of distilled diisopropylethylamine
(Hunig’s base). The solution is stirred at room temperature
under argon for 12 h. After evaporation of the solvents, the
crude product is purified by silica gel chromatography (eluent,
dichloromethane:acetonitrile 98:2).
Com p lex 7: 87% yield from 7. 1H NMR (400 MHz, CD3-
CN) δ: 8.48 (d, J ) 8.0 Hz, 1H), 8.40 (d, J ) 8.0 Hz, 1H), 8.33-
8.37 (m, 2H), 8.10 (d, J ) 8.4 Hz, 1H), 8.06 (dd, J 1 ) 4.8 Hz,
J 2 ) 1.2 Hz, 1H), 8.02 (d, J ) 1.6 Hz, 1H), 8.01 (td, J 1 ) 6 Hz,
J 2 ) 1.6 Hz, 1H), 7.81-7.89 (m, 3H), 7.79 (d, J ) 6 Hz, 1H),
7.73-7.77 (m, 3H), 7.63 (d, J ) 5.6 Hz, 1H), 7.43-7.47 (m,
5H), 7.22-7.28 (m, 3H), 7.00 (pseudo td, J 1 ) 5.6 Hz, J 2 ) 1.6
Hz, 1H), 6.98 (dd, J 1 ) 8 Hz, J 2 ) 1.6 Hz, 1H), 6.56 (d, J ) 8
Hz, 1H), 0.26 (s, 9H). 13C NMR (400 MHz, CD3CN) δ: 199.43,
166.80, 158.02, 157.29, 157.10, 155.42, 154.49, 150.85, 150.70,
150.53, 149.46, 146.78, 136.95, 136.29, 136.15, 135.63, 134.57,
134.48, 132.22, 131.51, 130.63, 127.44, 126.84, 126.75, 126.59,
124.60, 123.87, 123.61, 123.47, 123.44, 123.21, 122.52, 119.57,
114.49, 104.89, 96.37, 93.69, 87.31, 62.71, -0.75. ES-MS: 764.1
(M - PF6-). HRMS calcd for RuC44N5SiH36 (M - PF6-):
764.17835. Found: 764.17881.
Com p lex 8: 92% yield from 6. 1H NMR (400 MHz, CD3CN)
δ: 8.48 (d, J ) 8.0 Hz, 1H), 8.40 (d, J ) 8.0 Hz, 1H), 8.33-
8.37 (m, 2H), 8.10 (d, J ) 8 Hz, 1H), 8.06 (dd, J 1 ) 5.6 Hz, J 2
) 1.6 Hz, 1H), 8.03 (d, J ) 1.6 Hz, 1H), 8.01 (td, J 1 ) 8 Hz, J 2
) 1.6 Hz, 1H), 7.81-7.89 (m, 3H), 7.79 (d, J ) 5.6 Hz, 1H),
7.73-7.78 (m, 3H), 7.63 (d, J ) 5.6 Hz, 1H), 7.43-7.57 (m,
9H), 7.22-7.28 (m, 3H), 7.00 (pseudo td, J 1 ) 5.6 Hz, J 2 ) 1.6
Hz, 1H), 6.98 (dd, J 1 ) 8 Hz, J 2 ) 1.6 Hz, 1H), 6.56 (d, J ) 8
Hz, 1H), 0.26 (s, 9H). 13C NMR (400 MHz, CD3CN) δ: 199.59,
J . Org. Chem, Vol. 68, No. 6, 2003 2173

Hortholary and Coudret
87.84. ES-MS: 1042.3 (M - PF6-). HRMS calcd for RuC65N5-
SOH42 (M - PF6-): 1042.21536. Found: 1042.21741.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra,
13C NMR spectra, and UV-visible absorption spectra for all
new molecules, 2D-NMR spectra (COSY 45) and attributions
of the protons in the aromatic region for complexes 7-9, and
representation of the molecular orbitals involved in MLCT
transitions for complexes 10-12. This material is available
free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. This work has been supported
by the IST-FET “Nanomol project” EC program and the
CNRS. The authors thank Prof. J ean-Pierre Launay for
fruitful discussions and Christine Viala for technical
assistance.
J O026735Z
2174 J . Org. Chem., Vol. 68, No. 6, 2003