Synthesis of triester-functionalized molecular motors incorporating bis-acetylide trans-platinum insulating fragments

Article abstract of DOI:10.1039/b605509e

Bis-ferrocene compounds linked either by two triple bonds (1,4-di(ferrocenyl)butadiyne 1), or by the triple bond-platinum-triple bond sequence (trans-bis(ferrocenylethynyl)bis(triethylphosphine)platinum(ii), 2) have been synthesized. The electronic coupli

Full text of DOI:10.1039/b605509e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis of triester-functionalized molecular motors incorporating
bis-acetylide trans-platinum insulating fragmentswz
´
Guillaume Vives, Alexandre Carella, Stephanie Sistach, Jean-Pierre Launay and
´ ¨
Gwenael Rapenne*
Received (in Montpellier, France) 18th April 2006, Accepted 21st June 2006
First published as an Advance Article on the web 18th July 2006
DOI: 10.1039/b605509e
Bis-ferrocene compounds linked either by two triple bonds (1,4-di(ferrocenyl)butadiyne 1), or by
the triple bond–platinum–triple bond sequence (trans-bis(ferrocenylethynyl)
bis(triethylphosphine)platinum(II), 2) have been synthesized. The electronic coupling between the
ferrocene groups has been estimated from the intensity of the intervalence transition in the
electrochemically generated mixed valence complexes. Upon insertion of a platinum fragment a
weak attenuation was observed, with the Vab parameter decreasing from 0.036 eV for 1 to 0.025
eV for 2. A theoretical study has also been performed, using a combination of DFT for geometry
optimization and Extended Huckel Theory for the estimation of the electronic coupling. It was
¨
found that the electronic coupling decreases from 0.090 eV for 1 to 0.022 eV for a model of 2. In
a second part of this work, we describe the synthesis of two molecular motors incorporating the
ligand hydrotris[6-(ethoxycarbonyl)indazol-1-yl]borate which exhibits three pendant ester groups
dedicated to be anchored onto an oxide surface. This stator is connected through a ruthenium
centre to a pentasubstituted cyclopentadienyl rotor bearing ferrocene terminal electroactive
groups, linked either by a phenylethynyl spacer (complex 4) or a spacer containing bis-acetylide
trans-platinum insulating fragments (complex 8).
tadienyl trisindazolylborate complex with five ferrocenyl units
Introduction
located at the periphery. The general principle is that the
trisindazolylborate part would be grafted on a solid surface,
and thus should stay still (stator), while the pentasubstituted
cyclopentadienyl part would be able to rotate under the
influence of a suitably oriented electrical field (rotor). The
two parts are rigid, and the molecule has only one significant
possible degree of internal rotation (Fig. 1, left).
In the field of nanosciences, the design and chemical synthesis
of artificial molecular machines1 is becoming an important
challenge. A molecular-level machine can be defined as an
assembly of molecular components designed to perform a
controlled movement in response to a stimulus. Among these
machines, a motor is a machine that converts energy into work
via a unidirectional and controlled movement. Molecular
rotary motors present a very particular challenge, such as
the control of the directionality of a repetitive 3601 rotary
motion. But among the many examples of molecular rotors
which have been described,2 only a few of them present a
control of the directionality and can be considered as potential
molecular motors. Indeed, in view of recovering the mechan-
ical work produced by the rotation of a motor, their structure
must be relatively rigid and the number of degrees of freedom
as low as possible.
In such systems, the major difficulty lies in the capacity to
control the parameters which favor the desired process over
the unwanted ones.4 In our case the rotation has to compete
with the undesired intramolecular electron transfer without
rotation (Fig. 1, right). The rotation, a fifth of a turn in the
case of a C5-symmetric rotor, has to be significantly faster than
the intramolecular electron transfer between two electroactive
units. If not, the electrons will travel through the carbon
skeleton without rotation of the whole rotor. Since a con-
jugated skeleton can clearly not separate electronically the
electroactive groups, our goal is to use platinum acetylide
fragments to insulate the ferrocene groups by disrupting the
electronic communication between them, possibly due to the s
character of the Pt–C bonds which breaks the conjugation of
the different submodules. Therefore the charge localization on
one single oxidized ferrocenyl unit, which should be the seat of
the oxidoreduction cycles in the desired process, should be
favoured.
Recently, we have presented the concept of an electrically
fuelled single molecular rotary motor.3 The motor is based on
an organometallic centre built around a ruthenium cyclopen-
NanoSciences Group, CEMES-CNRS, 29 rue Jeanne Marvig,
F-31055 Toulouse Cedex 4, France. E-mail: rapenne@cemes.fr;
Fax: (+33) 562 25 79 99; Tel: (+33) 562 25 78 41
w Electronic supplementary information (ESI) available: molecular
modelling coordinates of 1, 1b and 2. Parameters used in the calcula-
tions for the atoms C, H, Fe and Pt, energy of the orbitals and Vab of
1, 1b and 2b as a function of the energy of the iron 3d orbitals. See
DOI: 10.1039/b605509e
In the first part of this paper we will present our contribu-
tion to the debate about the insulating role of trans-platinum
centres.5 For that purpose, we used a combination of theore-
tical calculations and experiments to compare the electronic
z The HTML version of this article has been enhanced with colour
images.
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Fig. 2 Complexes used to study experimentally the electronic influ-
ence of a trans-platinum complex.
For both approaches, simple model compounds incorporat-
ing (or not) a platinum linker have been chosen. Although not
resembling the target complexes 4 and 8, the study of such
models will give some information about the electronic char-
acter of a trans-platinum bridge.
Experimental approach
The two symmetrical complexes used in this study (Fig. 2)
incorporate ferrocene units as electroactive groups,6 linked by
an alkynyl bridge. One complex contains a trans-platinum
complex intercalated between two C–C triple bonds. The
preparation of platinum acetylide complexes with rigid alkynyl
backbones has been described by Stang and Takahashi for
their use in the formation of organometallic dendrimers7
which are thermally robust and stable, even when exposed
to air and moisture, and can be obtained in high yields by a
well-established synthetic methodology. 1,4-Di(ferrocenyl)
butadiyne (1) was obtained by the Glaser homocoupling
Fig. 1 Desired behaviour (top): electron transport with the rotation
of the rotor. Undesired behaviour (bottom) electron transport via
intramolecular electron transfers without rotation.
communication in two model compounds with two ferrocene
electroactive groups bridged by a conjugated organic linker or
a platinum fragment. In a second part, we will report the
synthesis of two star-shaped molecules which can potentially
act as molecular motors since they incorporate ester groups to
anchor the molecules onto oxide surfaces. These motors bear
ferrocene terminal electroactive groups linked either by a
phenylethynyl spacer in complex 4 or a spacer containing bis-
acetylide trans-platinum insulating fragments in complex 8.
of
ethynylferrocene8
and
trans-bis(ferrocenylethynyl)
bis(triethylphosphine)platinum(II) (2) was obtained by reac-
tion of two equivalents of ethynylferrocene with trans-
dichlorobis(triethylphosphine)platinum(II).9 A partial oxida-
tion of the complexes yields the mixed valence species, which
grants access to the electronic coupling parameter, Vab. This
parameter is used as a probe to estimate the electronic com-
munication between the two ferrocene units. Despite the
slightly longer Fe–Fe distance in 2, the comparison of Vab in
compounds 1 and 2 can give an estimation of the insulating
role of the trans-platinum complex.
Results and discussion
Investigation of the insulating role of a trans-platinum complex
The question concerning the electronic role of trans-platinum
centres has still not fully been addressed.5 On the one hand,
the conductivity of a single molecule placed between two gold
electrodes has been investigated by Mayor et al.5a The resis-
tance of the platinum-containing molecule was much higher
than with a conjugated organic spacer, which was justified by
the pure s character of the Pt–C(sp) bonds. On the other
hand, Marder and co-workers have compared a trans-plati-
num complex with a p-phenylene group as spacers between
The redox properties of compounds 1 and 2 have been
studied by cyclic voltammetry. The conjugated bisferrocene
compound 1 shows two reversible waves sufficiently resolved
to measure the potentials (0.58 and 0.68 V/SCE) correspond-
ing to the successive oxidation of both ferrocenes, first yielding
the mixed valence complex [Fc-(CC)2-Fc]+ and then the
dication [Fc-(CC)2-Fc]2+ 10
The potential of the first oxida-
.
two electroactive organic groups (dianisylphenylamine).5b
A
tion wave is very close to the oxidation potential obtained in
the same conditions for ethynylferrocene (0.59 V/SCE). Simi-
larly compound 2 shows two waves at 0.32 and 0.40 V
corresponding to the mixed valence complex and to the
dication, however strongly shifted towards lower potentials.
The marked stabilization of the oxidized forms can be ex-
plained by the strong s-donating character of the trans-
platinum complex electronically enriched by the two triethyl-
phosphine ligands. It must be noted that no oxidation or
reduction of the platinum complex has been observed in
dichloromethane from ꢁ1.8 V to +1.5 V, which shows the
spectroscopic study of the mixed-valence species and calcula-
tions have shown that the electronic coupling between the two
redox centres is only slightly decreased with a trans-platinum
spacer. Diederich also observed an almost complete lack of
p-electron conjugation in platinum-tetraethynylethene scaffol-
ding.5c In view of confirming the electronic influence of a
trans-platinum complex incorporated between two ferrocene
electroactive units, we have combined two complementary
approaches: an experimental approach based on spectroelec-
trochemistry and a theoretical approach combining geometry
optimization by DFT and extended Huckel calculations.
¨
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redox stability of this centre. The reduction of similar plati-
num centres is indeed known to occur around ꢁ2.4 V.9
A spectroelectrochemical study allowed the evaluation of
the electronic communication between the two ferrocenes in
both complexes. Partial oxidation of the complex at different
potentials and examination of the visible–near infrared region
of the absorption spectrum grants access to the intervalence
band which successively appears and disappears during the
process. The intervalence transition can be considered as a
special case of charge transfer transition, the donor group
being the reduced site and the acceptor group the oxidized site.
The mixing of the two electronic states is due to an electronic
term, Vab, which is also responsible for the intensity of the
intervalence transition. This transition gives a measure of the
electronic coupling parameter (Vab) following the Hush
formula.11
thus decreases the energy of the LMCT transition by destabi-
lizing the highest occupied ligand-centred orbital. In this case,
the intervalence transition is strongly mixed with the other
transition and only appears as a residual absorption in the
1200–1600 nm region, which increases and decreases during
the oxidation process. A deconvolution of the two bands
enabled the parameters of the intervalence transition to be
obtained, in particular Vab = 0.025 eV, a weakly coupled class
II mixed valence system, corresponding to an attenuation of
the electronic communication by a factor of only 1.5.
Experimentally, the complex incorporating a trans-platinum
fragment displays a weaker ferrocene–ferrocene coupling i.e.
an attenuated electronic communication, which goes in the
desired direction. However the attenuation factor is moderate
and in order to confirm this trend, we have performed
theoretical calculations on three model complexes 1, 1b and
2b (Fig. 4).
1=2
2:05 ꢂ 10ꢁ2ðemaxnꢀmaxDnꢀ1=2Þ
Vab
¼
Theoretical study of the insulating role of the trans-platinum
RMM
complex
with emax the maximum molar extinction coefficient, nꢀmax the
The influence of a trans-platinum complex on the electronic
communication between two metallic centres has been studied
by examining the molecular orbitals involved by DFT and by
position of the transition (cmꢁ1), Dn1/2 the bandwidth at half-
ꢀ
height (cmꢁ1) and RMM the metal–metal distance (A).
The spectrum of the mixed valence species obtained by
partial oxidation of 1 displays an absorption band at
760 nm (e = 670 molꢁ1 L cmꢁ1) corresponding to the LMCT
transition of the ferrocenium group. The broad band at
1180 nm (e = 400 molꢁ1 L cmꢁ1) corresponds to the inter-
valence transition. This is clearly shown by the fact that during
electrolysis, the intensity of this band increases before decreas-
ing, whereas the band at 760 nm always increases. The Hush
formula allows the evaluation of the electronic coupling para-
meter. Assuming a trans geometry for the two ferrocenes
corresponding to a metal–metal distance of 9.58 A, this gives
Vab = 0.036 eV, i.e. a moderately coupled class II system.
The visible–near ir spectrum measured during gradual
oxidation of complexes 1 and 2 are given in Fig. 3. The
monocation of compound 2 displays an absorption band at
1000 nm (e = 1900 molꢁ1 L cmꢁ1) corresponding to the
LMCT transition of the ferrocenium group. The bathochro-
mic shift observed by comparison to compound 1 is due to the
platinum complex, which makes the ligand a better donor and
realizing an extended Huckel calculation. We compared 1 and
¨
bis(ferrocenylethynyl)bis(trimethylphosphine)platinum(II) 2b
instead of 2, replacing the triethylphosphines by trimethyl-
phosphines. The comparison between 1 and 2b will give some
information about the effect of the incorporation of the
platinum moiety, bearing in mind the different Fe–Fe distance
in these two complexes (9.78 A in 1, 12.39 A in 2b). To free
ourselves from the influence of the Fe–Fe distance, we also
considered complex 1b incorporating three triple bonds be-
tween the two ferrocenyl groups. Complexes 1b and 2b have
almost identical Fe–Fe distances (12.22 and 12.39 A, respec-
tively), which enables us to focus on the pure influence of the
platinum complex on Vab
.
Contrary to molecular mechanics calculations, where the
nature of the interatomic bonds is fixed, DFT calculations
yield a geometry which takes into account a possible cumu-
lenic form due to electronic delocalization through the bridge.
It is known indeed that pure DFT tends to overestimate the
electronic delocalization.12 On the contrary, Hartree–Fock
Fig. 3 Visible and near infrared spectra of the partially oxidized complexes obtained during the gradual oxidation of complexes 1 and 2 (CH2Cl2,
n
0.1 M Bu4NBF4). The concentration of mixed-valence complexes is maximum for 0.5 electron per ferrocene centre.
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Fig. 6 Overlap between the iron 3d orbitals and the orbitals of the
bridge.
et al.15 In the optimized structure the Cp cycles are almost
coplanar (torsion angle 21) and the Pt–P distance is relatively
close to the crystallographic data (2.329 A instead of 2.285 A).
The electronic coupling parameter can be determined from
the calculation of molecular orbitals by the dimer splitting
method.16 In the case of complexes bearing a symmetry
element, such as here an inversion centre, the system is reduced
to two levels: only the two significant metallic molecular
orbitals (symmetrical and antisymmetrical) with the largest
energy gap are taken into consideration. The energy difference
Fig. 4 Complexes used to study theoretically the electronic influence
of a trans-platinum complex.
calculations tend to underestimate the electronic delocaliza-
tion13 and for this reason we have chosen DFT with an hybrid
functional (B3PW91) to obtain some more reliable results. The
starting structure for DFT optimization has been constructed
from the X-ray structure of fragments in order to decrease the
CPU time. The structures of two ferrocenyl units separated by
two and four carbon–carbon triple bonds have been de-
scribed14 and therefore we have taken the average C–C
distances to describe a bridge with three C–C triple bonds.
The calculation of vibration frequencies has shown the
absence of imaginary frequencies, which confirms that the
optimized structure is indeed a minimum of energy. On the
optimized structures (Fig. 5) the two Cp cycles covalently
linked to the bridge are in the same plane (torsion angle 0.61),
as found in the starting X-ray structures. Besides, the Fe–C
distances within the ferrocenes are very similar in the X-ray
(2.045 A) and the calculated (2.041 A) structures.
between these two MOs corresponds to 2 Vab
.
A significant overlap can take place between the iron 3d
orbitals and the orbitals of the bridge. In the ferrocenyl unit,
the 3d orbitals of iron split into three groups: (dxz, dyz), dz2
,
(dxy, dx2ꢁy2). Among the filled orbitals, the dz2 orbitals have
almost no overlap with the bridge orbitals and therefore do
not take part in the electronic communication. However the
dxy and dx2ꢁy2 orbitals strongly overlap (Fig. 6) with the bridge
orbitals and are involved in the electronic delocalization
through the two orthogonal p systems of the conjugated
alkyne bridge.
Since the DFT method tends to underestimate the energy
Similarly the structure of complex 2b was optimized by DFT
starting from a crystallographic structure published by Mori
gap between the different orbitals, in particular the
HOMO–LUMO gap,17 we have estimated Vab from Huckel
¨
orbitals. An extended Huckel calculation was carried out with
¨
the HyperChem Professional 6 program package, using para-
meters optimized for iron and platinum,18 and performed on
the basis of the DFT-optimized geometry.
Vab was determined from the energy difference between the
significant u- and g-symmetry orbitals on the 3dx2-y2 orbitals of
the iron atoms represented on Fig. 6 for 1, 1b and 2b. The
contribution of the 3dxy orbitals in the electronic communica-
tion was neglected because the splitting between the u- and
g-symmetry MOs was significantly smaller than that of the
3dx2ꢁy2 orbitals.
Considering the Huckel parameter for the iron 3d orbital to
¨
be ꢁ11 eV, the electronic coupling parameter Vab for complex
1 (with two triple bonds between the ferrocenyl groups) is
0.090 eV, which is higher than the value obtained for 1b (0.077
eV with three triple bonds) and corresponds to a moderately
coupled system. As expected this difference illustrates the
attenuation of the electronic coupling with an increased dis-
tance. The same calculations for complex 2b gave a Vab value
equal to 0.022 eV, i.e. it is a weakly coupled system. In the case
of 1 and 2b the highest energy orbital has g-symmetry,
contrary to 1b. For a given Fe–Fe distance, the insertion of
platinum in a conjugated acetylenic system results in a 3-fold
decrease in the electronic coupling, which demonstrates the
slight insulating role of the trans-platinum complex. This can
be interpreted in terms of the pure s character of the Pt–C
Fig. 5 DFT-optimized structures of complexes 1, 1b and 2b.
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Scheme 1 Palladium-catalysed coupling strategy.
bond, which should insulate the p systems located on either
side of the platinum. Platinum thus hinders the electronic
delocalization through the molecule and decreases the Vab
coupling parameter.
Cp ring, the quadruplet and triplet system of the ethyl groups
of the esters and an integration of 45 protons for the ferrocene
moieties. The presence of the five ferrocene units was also
confirmed by MALDI-TOF spectrometry.
The Vab value depends on the parameters used for the
Synthesis of the platinum precursors
extended Huckel calculation, in particular the energy of the
¨
iron 3d orbitals. To validate our calculations, the energy of the
3d orbitals of free iron was varied from ꢁ10.5 to ꢁ11.5 eV,
which greatly influences the absolute Vab value but does not
alter the observed trend: the value for the platinum-containing
complex is always significantly lower than for the fully-con-
jugated complex. (see Supplementary Information for detailsw).
Despite the discrepancy observed between the results ob-
tained from the experimental and theoretical approaches, both
converge towards the conclusion that the insertion of a trans-
platinum complex in the bridging unit increases its insulating
character by decreasing the electronic communication within
mixed valence complexes.
trans-Bis(alkynyl)bis(trialkylphosphine)platinum(II)
com-
plexes have been chosen to maintain the rigidity and linearity
of the assembly and to keep the electroneutrality of the
molecule which is essential for its deposition on a surface.
Moreover the diameter of the molecule is significantly in-
creased (from 2 to 3 nm) which is interesting for the future
near-field microscopy studies. As shown in Schemes 2 and 3, a
modular strategy was followed in order to build separately the
arms containing the electroactive groups and the ruthenium
core, and to couple them in a last step. The synthesis of
complex 7, bearing a free alkyne function for the last coupling
step with the ruthenium precursor, was envisaged starting
from trans-dichlorobis(triethylphosphine)platinum(II), by the
successive displacement of the two chloride ligands.21 The first
chloride ligand was substituted by a ferrocenylethynyl frag-
ment, by heating trans-Pt(PEt3)2Cl2 with ethynylferrocene in
the presence of a base under microwave irradiation (300 W) in
a sealed tube for 15 min. Complex 5 was obtained in 86% after
chromatography. Its trans geometry was confirmed by 31P
NMR with the 1J(31P–195Pt) coupling constant. For complex 5
the value of the coupling constant was found to be 2405 Hz,
which corresponds to a trans geometry, a cis coupling constant
being much larger (about 3500 Hz).22 The second alkyne
function was introduced as tri(isopropylsilsyl) (TIPS) pro-
tected group under Sonogashira conditions in 72% isolated
yield. The deprotection of complex 6 was attempted under the
classical conditions, using tetrabutylammonium fluoride
(TBAF) in THF containing 5% water, at room temperature.
Even by heating, using microwaves or KF in combination with
a crown ether, no deprotection occurred. This could be
Synthesis of the molecular motor without platinum fragments
Our strategy is based on a quintuple palladium catalysed
coupling reaction between the pentabrominated ruthenium
complex (3) and ethynyl ferrocene. The synthetic scheme is
presented in Scheme 1.
The ruthenium centre 3 was prepared by heating under
microwave irradiation in a sealed tube bromo Z5-1,2,3,4,5-
penta-(p-bromophenyl)cyclopentadienyl dicarbonyl ruthe-
nium(II) with potassium hydrotris[6-(ethoxycarbonyl)inda-
zol-1-yl]borate in a mixture of DMF and acetonitrile. After
purification by column chromatography 3 was obtained in a
19% yield. The coupling between an alkyne and an aryl
bromide is in general achieved under Sonogashira condi-
tions,19 i.e. using catalytic CuI and Pd(PPh3)4. These condi-
tions were unsuccessful, maybe due to the high electron
density of the bromide derivative which is covalently linked
to a formal anionic entity (Cp). The electron-rich character of
the carbon–bromide bond precludes the oxidative addition on
the palladium catalyst. However the synthesis of the motor 4
was achieved by a quintuple Negishi coupling20 using a
solution of [(ferrocenyl)ethynyl]]zinc chloride and Pd(PPh3)4
in refluxing THF ꢃ 4 was isolated after purification by column
chromatography in 20% yield, which correspond to 72% by
1
coupling reaction. H-NMR spectroscopy clearly showed an
AA0BB0 pattern for the phenyl groups attached to the central
Scheme 2 Synthesis of the platinum-containing electroactive arms.
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Scheme 3 Synthesis of the motor with platinum insulating fragments.
explained by the electronic effect of the platinum complex on
the acetylenic TIPS group. Complex 7 could nevertheless be
obtained in 87% yield by reaction of gaseous acetylene with
complex 5, in the presence of catalytic CuI and diethylamine.
the TIPS protecting groups were removed by overnight reac-
tion with TBAF to yield after chromatography the desired
complex 14 in 54% isolated yield.
As mentioned previously, the target molecule 8 could not be
obtained when the halide groups were carried on the ruthe-
nium complex 3. On the contrary, the second approach was
successful and the quintuple coupling of chloroplatinum com-
plex 5 with complex 14 gave the desired product in 35%
isolated yield, which corresponds to a yield of 81% per
coupling. The complex was fully characterised by multinuclear
NMR. The AA0BB0 pattern of the protons of the phenyl
groups attached to the central Cp ring and an integration of
45 protons for the signals of the ferrocene units were observed
Connection to the ferrocenyl-terminated platinum fragments
Following a modular strategy, the five ferrocene-platinum
arms remained to be coupled to the core through a classical
palladium-catalyzed coupling reaction between ruthenium
complex 3 and the platinum moiety 7. The Negishi conditions
which were successful for the synthesis of the motor 4 failed to
prepare the molecular motor incorporating platinum frag-
ments (8). This can be due to the instability of the platinum
complex in the presence of a strong base such as BuLi.
Therefore a second route was followed (Scheme 3), in which
the alkyne functions carried by the ruthenium core (14) are
coupled to a chloroplatinum complex (5).
1
in H NMR. Both 31P and 195Pt NMR22,30 are in agreement
with the formation of only the trans isomer of the platinum
1
diphosphino complex with a coupling constant J195
of
PtꢁP
2393 Hz. The presence of the five platinum units was also
confirmed by MALDI-TOF mass spectrometry.
The ruthenium complex 14 was prepared in five steps
starting from 1-bromo-1,2,3,4,5-penta(p-bromophenyl)cyclo-
pentadiene 9.23 The pentayne compound 10 was obtained from
9 in a remarkable 81% isolated yield by a quintuple Sonoga-
shira coupling with mono-TIPS-protected acetylene.24 Many
We then investigated the electronic communication between
ferrocene units in complexes 4 and 8 by spectroelectrochem-
istry. We performed a partial oxidation of the ferrocene sites,
in search for intervalence transitions but no such transitions
were observed, showing that the electronic communication
between these sites is not measurable.
25
attempts to coordinate directly 10 with RuCl3 or [Ru
26
(p-cymene)Cl2]2 failed, probably due to the sterical hin-
drance of the phenyl groups on the Cp ligand. The only
alternative to introduce ruthenium was Manners methodology
which consists in an oxidative addition of the brominated
Cp 11 on the ruthenium carbonyl cluster Ru3(CO)12.27 The
bromine atom was introduced on the cyclopentadiene ring
using nBuLi and NBS at low temperature. The ruthenium was
subsequently coordinated, yielding complex 12 in a 72% yield
after chromatography. The complex 13 incorporating a tripo-
dal ligand with ester anchoring groups28 derived from the
scorpionate family29 was obtained using microwave irradia-
tion in 17% yield after chromatographic purification. Finally
Conclusion
A theoretical approach combining geometry optimization by
DFT and extended Huckel calculations on bis-ferrocenyl
¨
model compounds allowed the electronic communication
parameter Vab between ferrocene units to be estimated, show-
ing a four-fold attenuation in the presence of a trans-platinum
spacer between the two iron centres. The experimental ap-
proach based on spectroelectrochemistry led to the same
conclusion, although by a factor of only 1.5. Thus the bis
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acetylide trans-platinum spacer has some insulating character,
as shown in the decrease in the electronic coupling parameter.
On the basis of these results, we have synthesized two star-
shaped molecules incorporating ester anchoring groups on the
trisindazolylborate part (stator). One of these ruthenium
complexes bears platinum spacers on the rotor, and thus fulfils
all the requirements for an electrically driven molecular motor.
Work is now underway to anchor these molecules on an oxide
surface to prepare their addressing with two metallic electro-
des. The demonstration of a controlled rotary movement will
then need further experimental developments by physical
methods such as scanning probe microscopies or the analysis
of the time dependence of the current in a two-electrode
configuration.
for quadruplet; m: for multiplet. The numbering scheme is
given in Schemes 1 and 2 (vide supra).
UV–visible–near infra-red spectra were recorded on a Shi-
madzu UV-3100 spectrometer. FAB and DCI mass spectro-
metry was performed using
a Nermag R10-10. Cyclic
voltammetry was performed with an AUTOLAB PGSTAT
100 potentiostat using a Pt disc (1 mm diameter) as working
electrode and a Pt counter electrode. The reference electrode
used was the saturated calomel electrode (SCE).
trans-Bis(ferrocenylethynyl)-bis(triethylphosphine) platinum(II)
(2). CuI (10 mg, 0.05 mmol) was added to a solution of trans-
dichlorobis(triethylphosphine) platinum(II) (250 mg, 0.5
mmol, 1 eq.) and ethynylferrocene (210 mg, 1 mmol, 2 eq.)
in degassed diethylamine (10 mL). The solution was stirred at
room temperature during 2 h. Solvents were removed under
reduced pressure and the crude material was column chroma-
tographed (SiO2: cyclohexane–Et2O, (0–5%)). The product
was isolated as an orange solid (300 mg, 70%). MS: (DCI/
NH3) 850 [M + H]+; High Resolution LSI Calculated [M]+:
849.1578 (C36H48Fe2P2Pt) Found: 849.1582 (100% [M]+); Mp
= 193 1C. 1H NMR: (250 MHz, CDCl3) d 4.20 (4H, t, J = 1.6
Hz, subst. Cp), 4.13 (10H, s, Cp), 4.04 (4H, t, J = 1.6 Hz,
subst. Cp), 2.17 (12H, m, CH2CH3), 1.24 (18H, m, CH2CH3);
13C NMR: (100 MHz, CD2Cl2) d 104.44; 102.72; 73.13; 70.07;
69.27; 66.93; 16.32; 8.21; 31P NMR (160 MHz, CDCl3) d: 10.98
Experimental
Computational details
Geometries of the ferrocene derivatives were fully optimized
at the BP86/6-31G** and B3PW91/6-31G** level or at the
BP86/6-31G**/LANL2DZ(Pt)
and
B3PW91/6-31G**/
LANL2DZ(Pt) level for the platinum derivative using Gaus-
sian98.31 The second generation functional BP86 was chosen
because it seemed particularly appropriate for modelling
ferrocenyl groups.32 In the case of the platinum complex, the
same BP86 functional was used but the platinum and phos-
phorus atoms have been modelled by using the effective core
potential and the corresponding valence orbitals LanLDZ in
order to decrease the number of basis functions. In all cases,
the atoms were described by the zeta 6-31G** double base
which takes into account the polarization orbitals of all atoms,
including hydrogens. Vibrational analysis was performed at
the same level in order to check the obtention of a minimum
on the potential energy surface.
(s, J195
= 2391 Hz).
PtꢁP
g5-1,2,3,4,5-Penta-(4-bromophenyl) cyclopentadienyl hydro-
tris[6-(ethoxycarbonyl) indazol-1-yl]borate ruthenium(II) (3).
Bromo Z5-1,2,3,4,5-penta-(p-bromophenyl)cyclopentadienyl
dicarbonyl ruthenium(II) (55 mg, 0.05 mmol, 1 eq.) and
potassium
hydrotris[6-(ethoxycarbonyl)indazol-1-yl]borate
(62 mg, 0.1 mmol, 2 eq.) were heated in a sealed tube at
150 1C under microwave irradiation during 10 minutes in a
mixture of 2 ml of acetonitrile and 1 ml of DMF. The crude
reaction mixture was evaporated under vacuum. The product
was adsorbed on silica and purified by column chromatogra-
phy (SiO2, dichloromethane) to give a yellow solid (15 mg,
19%). MS: (APCI) 1522 [M]+; 1H NMR: (250 MHz, CD2Cl2)
d 8.76 (s, 3H, Hd), 7.95 (s, 3H, Ha), 7.68 (d, 3H, J = 8.5 Hz,
Hc), 7.45 (d, 3H, J = 8.5 Hz, Hb), 7.21 (s broad, 20H, Ho-m),
4.47 (q, 6H, J = 7.1 Hz, CH2), 1.48 (t, 9H, J = 7.1 Hz, CH3);
13C NMR: (63 MHz, CD2Cl2) d 166.81; 143.22; 140.77; 135.10;
131.82; 130.92; 129.30; 125.35; 122.24; 121.26; 119.86; 113.97;
88.05; 61.38; 14.31.
Syntheses
All commercially available chemicals were of reagent grade
and were used without further purification. Ethynylferrocene
and trans-dichlorobis(triethylphosphine)platinum(II) were
purchased from Aldrich. Ruthenium carbonyl and 1,2,3,4,5-
pentaphenylcyclopentadiene were purchased from Strem. 1,4-
di(ferrocenyl)butadiyne (1),12 1-bromo-1,2,3,4,5-penta(p-bro-
mophenyl)cyclopentadiene (9),23 bromo Z5-1,2,3,4,5-penta-
(p-bromophenyl)cyclopentadienyldicarbonyl ruthenium (II),23
and potassium hydrotris[6-(ethoxycarbonyl)indazol-1-yl]
borate27 were prepared according to literature procedures.
Toluene was dried over CaH2, THF over sodium with
benzophenone and diethylamine over KOH. All reactions
were carried out using standard Schlenk techniques under an
argon atmosphere. Flash column chromatography was carried
out on silica gel 230–400 mesh from SDS. NMR Spectra were
recorded on Bruker AM 250 or Avance 500 spectrometers and
full assignments were made using COSY, ROESY, HMBC
and HMQC methods. Chemical shifts are defined with respect
to TMS = 0 ppm for 1H and 13C NMR spectra and to H3PO4
for 31P NMR spectra and were measured relative to residual
solvent peaks. The following abbreviations have been used to
describe the signals: s for singlet; d for doublet; t for triplet; q
g5-1,2,3,4,5-Penta-(4-(ferrocenylethynyl)phenyl) cyclopenta-
dienyl [6-(ethoxycarbonyl) indazol-1-yl]borate ruthenium(II)
(4). In a two-necked flask, a solution of Z5-1,2,3,4,5-penta-
(4-bromophenyl) cyclopentadienyl hydrotris[6-(ethoxycarbo-
nyl)indazol-1-yl] borate ruthenium(II) (3, 40 mg; 0.026 mmol)
and Pd(PPh3)4 (16 mg; 0.5 eq.) in 5 mL of freshly distilled THF
was degassed. A solution of (ferrocenylethynyl)zinc chloride
0.2M (0.52 mmol) was then added. The mixture was heated
under reflux for 24 h. Additional reactants were added (16 mg
Pd(PPh3)4 and 0.5 mmol of (ferrocenylethynyl)zinc chloride)
and heating at reflux was maintained for another 24 h. The
crude reaction mixture was evaporated under vacuum. The
product was adsorbed on silica and purified by flash column
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chromatography (SiO2: cyclohexane–CH2Cl2 0–100%, to give
an orange solid (11 mg, 20%). MS: (MALDI-TOF) 2166
[M]+; 1H NMR: (500 MHz, CD2Cl2) d 8.79 (s, 3H, Hd),
8.09 (s, 3H, Ha), 7.70 (d, 3H, J = 8.5 Hz, Hc), 7.49 (d, 3H, J =
8.5 Hz, Hb), 7.36 (d, 10H, J = 8.4 Hz, Ho), 7.19 (d, 10H, J =
8.4 Hz, Hm) 4.47 (q, 6H, J = 7.1 Hz, CH2), 4.44 (t, 10H, J =
1.8 Hz, subs Cp), 4.22 (t, 10H, J = 1.8 Hz, subs Cp), 4.20 (s,
25H, Cp), 1.48 (t, 9H, J = 7.1 Hz, CH3); 13C NMR: (126
MHz, CD2Cl2) d 166.84; 143.10; 140.83; 133.46; 132.64;
130.32; 128.95; 125.36; 123.22; 121.01; 119.78; 113.86; 89.58;
88.62; 85.15; 71.38; 69.93; 68.98; 64.85; 61.27; 14.24.
102.28; 100.61; 94.03; 72.94; 70.08; 69.31; 66.96; 16.05; 8.11;
31P NMR: (160 MHz, CDCl3) d 10.70 (s, J195
= 2380 Hz).
PtꢁP
g5-1,2,3,4,5-Penta[4-{trans-ethynyl-(ethynylferrocenyl)-bis
(triethylphosphine)platinum(II)} phenyl] cyclopentadienyl hydro-
tris[6-(ethoxycarbonyl) indazol-1-yl]borate ruthenium(II) (8). In
a Schlenk tube, Z5-1,2,3,4,5-penta(4-(ethynyl)phenyl)cyclo-
pentadienylhydrotris[6-(ethoxycarbonyl) indazol-1-yl]borate
ruthenium(II) (14) (16 mg, 0.013 mmol, 1 eq.), THF (2 mL)
and diethylamine (1 mL) were degassed under argon during 20
min. trans-Chloro-(ferrocenylethynyl)-bis(triethylphosphine)-
platinum(II) (5) (50 mg, 0.073 mmol, 5.6 eq.) and CuI (5 mg,
0.010 mmol, 2 eq.) were added and the mixture was stirred at
room temperature for 8 h. The same amounts of additional
CuI and platinum complex were added and the solution was
stirred at room temperature overnight. Solvents were removed
under reduced pressure and the crude material was column
chromatographed (SiO2, cyclohexane–Et2O (0–20%)) to give
an orange solid (20 mg, 35%). MS: (MALDI-TOF) 4442.4
trans-Chloro-(ferrocenylethynyl)-bis(triethylphosphine) plati-
num(II) (5). trans-Dichlorobis(triethylphosphine)platinum(II)
(250 mg, 0.5 mmol, 1 eq.) and ethynylferrocene (105 mg, 0.5
mmol, 1 eq.) were dissolved in a mixture of chloroform (4 mL)
and diethylamine (1 mL). The mixture was heated in a sealed
tube at 113 1C under microwave irradiation over a 15 min
period. Solvents were removed under reduced pressure and the
crude material was column chromatographed (SiO2: cyclohex-
ane–Et2O (0–5%)). The product was isolated as an orange
solid (290 mg, 86%). MS: (DCI/NH3) 677 [M + H]+; High
Resolution LSI Calculated [M]+: 675.1213 (C24H39ClFeP2Pt)
1
[M]+ (Calc. 4442.19); H NMR: (500 MHz, C6D6) d 9.24 (s,
3H, Hd), 8.70 (s, 3H, Ha), 8.02 (d, 3H, J = 7.2 Hz, Hc), 7.78 (d,
10H, J = 7.6 Hz, Ho), 7.36 (d, 10H, J = 7.6 Hz, Hm), 6.99 (d,
3H, J = 7.2 Hz, Hb), 4.53 (s broad, 10H, subst. Cp), 4.33 (s,
25H, Cp), 4.1 (q, 6H, J = 6.4 Hz, OCH2CH3), 4.12 (s broad,
10 H, subst. Cp), 2.09 (m, 60H, CH2), 1.43 (t, 9H, J = 6.4 Hz,
OCH2CH3), 1.22 (m, 90H, CH3); 13C NMR: (125 MHz, C6D6)
d 166.40; 143.55; 140.99; 133.73; 130.32; 125.90; 121.30;
120.30; 113.94; 70.51; 69.61; 67.29; 60.61; 16.57; 8.33; 31P
1
Found: 675.1215 (100% [M]+); Mp = 94 1C. H NMR: (250
MHz, CDCl3) d 4.20 (2H, t, J = 1.8 Hz, subst. Cp), 4.12 (5H,
s, Cp), 4.05 (2H, t, J = 1.8 Hz, subst. Cp), 2.06 (12H, m,
CH2CH3), 1.20 (18H, m, CH2CH3); 13C NMR: (100 MHz,
CD2Cl2) d 96.70; 76.47; 72.72; 70.82; 69.35; 67.07; 14.49; 7.86;
31P NMR: (160 MHz, CDCl3) d 14.77 (s, J195
= 2405 Hz).
NMR: (200 MHz, C6D6) d 11.30 (s, J195
195Pt NMR: (500 MHz, C6D6) dꢁ4745.
= 2393 Hz);
PtꢁP
PtꢁP
trans-(Triisopropysilyl)ethynyl-(ferrocenylethynyl)-bis(tri-
ethylphosphine) platinum(II) (6). CuI (20 mg, 0.1 mmol, 0.3 eq.)
and TIPSA (0.1 mL, 0.44 mmol, 1.3 eq.) were added to a
solution of trans-chloro-(ferrocenylethynyl)-bis(triethylpho-
sphine)platinum(II) (5) (230 mg, 0.34 mmol, 1 eq.) in degassed
diethylamine (5 mL). The solution was stirred at room tem-
perature during 40 min. Solvents were removed under reduced
pressure and the crude material was column chromatographed
(SiO2: cyclohexane–Et2O (0–5%)). The product was isolated
as an orange solid (200 mg, 72%). MS: (DCI/NH3) 822 [M +
H]+. 1H NMR: (250 MHz, CDCl3) d 4.22 (2H, broad s, subst.
Cp), 4.13 (5H, s, Cp), 4.05 (2H, broad s, subst. Cp), 2.15 (12H,
m, CH2CH3), 1.19 (18H, m, CH2CH3), 1.04 (21H, m, TIPS);
1,2,3,4,5-Penta(4-(triisopropylsilylacetylene)phenyl)cyclo-
pentadiene (10). In a Schlenk tube, 1-bromo-1,2,3,4,5-penta(4-
bromophenyl)cyclopentadiene (9) (300 mg, 0.32 mmol, 1 eq.),
TIPSA (1.4 mL, 6.4 mmol, 20 eq.), diisopropylamine (2 mL)
and freshly distilled THF (4 mL) were degassed under argon
during 20 min. CuI (12 mg, 0.06 mmol, 20 mol%) and
Pd(PPh3)4 (40 mg, 0.03 mmol, 10 mol%) were added and
the mixture was heated at 80 1C overnight. The same amounts
of additional CuI, TIPSA and Pd0 were added and the mixture
was heated at 80 1C for another 24 h. Solvents were removed
under reduced pressure and the crude material was column
chromatographed (SiO2, cyclohexane–CH2Cl2 (0–10%)) to
give a fluorescent yellow glassy solid (350 mg, 81%). MS:
(DCI/NH3) 1349 [MH]+, 1381 [M + 2NH3]+; High Resolu-
tion LSI Calculated [M]+: 1346.8706 (C90H126Si5) Found:
1346.8687 (100% [M]+). 1H NMR: (250 MHz, CD2Cl2) d
7.34 (d, 6H, J = 8 Hz, Hbꢁf), 7.21 (d, 4H, J = 8 Hz, Hd), 7.17
(d, 2H, J = 8 Hz, Ha), 7.02 (d, 4H, J = 8 Hz, He), 6.96 (d, 4H,
J = 8 Hz, Hc), 5.16 (s, 1H, Hg), 1.14 (m, 105 H, TIPS); 13C
NMR: (100 MHz, CD2Cl2) d 146.80; 144.11; 137.92; 135.71;
135.23; 132.37; 131.74; 131.49; 129.95; 128.76; 128.30; 122.24;
122.02; 121.74; 106.83; 106.78; 106.72; 91.34; 91.23; 90.57;
62.11; 18.43; 18.39; 11.30; 11.26.
31P NMR: (160 MHz, CDCl3) d 11.30 (s, J195
= 2409 Hz).
PtꢁP
trans-Ethynyl(ferrocenylethynyl)-bis(triethylphosphine) plati-
num(II) (7). CuI (10 mg, 0.05 mmol) was added to a solution of
trans-chloro-(ferrocenylethynyl)-bis(triethylphosphine)plati-
num(II) (5) (290 mg, 0.43 mmol) in degassed diethylamine
(5 mL). Acetylene was bubbled in the solution with stirring at
room temperature for 20 min. Solvents were removed under
reduced pressure and the crude material was column chroma-
tographed (SiO2: cyclohexane–Et2O (0–5%)). The product
was isolated as an orange solid (250 mg, 87%). MS: (FAB+,
MNBA) 665 [M]+; High Resolution LSI Calculated [M]+:
665.1603 (C26H40FeP2Pt) Found: 665.1604 (100% [M]+); Mp
= 71 1C. 1H NMR: (250 MHz, CDCl3) d 4.19 (2H, t, J = 2.0
Hz, subst. Cp), 4.10 (5H, s, Cp), 4.02 (2H, t, J = 2.0 Hz, subst.
Cp), 2.16 (1H, s, H alkyne, 2.13 (12H, m, CH2CH3), 1.18
(18H, m, CH2CH3); 13C NMR: (100 MHz, CD2Cl2) d 104.24;
1-Bromo-1,2,3,4,5-penta(4-(triisopropylsilylacetylene)phenyl)
cyclopentadiene (11). In a Schlenk tube, 1,2,3,4,5-penta(4-
(triisopropylsilylacetylene)phenyl)cyclopentadiene (10) (380
mg, 0.28 mmol, 1 eq.) was dissolved in freshly distilled THF
n
(10 mL). BuLi (0.28 mmol, 1 eq.) was added dropwise under
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argon at ꢁ78 1C and the mixture was stirred at this tempera-
ture for 30 min. NBS (55 mg, 0.30 mmol, 1.1 eq.) was
subsequently added and the mixture was stirred at ꢁ78 1C
for 30 min. The temperature was left to raise slowly to room
temperature. Solvents were removed under reduced pressure
and the crude material was column chromatographed (SiO2,
cyclohexane) to give an orange glassy solid (220 mg, 55%).
MS: (DCI/NH3) 1427 [MH]+, 1444 [M + NH4]+; High
Resolution LSI Calculated [M]+: 1424.7811 (C90H125BrSi5)
Found: 1424.7863 (100% [M]+); 1H NMR: (500 MHz,
CDCl3) d 7.36 (broad s, 4H, Haꢁb), 7.28 (d, 4H, J = 8 Hz,
Hf), 7.21 (d, 4H, J = 8 Hz, Hd), 6.89 (d, 4H, J = 8 Hz, He),
6.88 (d, 4H, J = 8 Hz, Hc), 1.15 (s, 21H, TIPS), 1.14 (s, 42H,
TIPS), 1.11 (s, 42H, TIPS); 13C NMR: (125 MHz, CDCl3) d
148.34; 141.55; 135.27; 134.03; 133.52; 132.32; 131.86; 131.56;
131.4; 130.08; 129.80; 127.36; 123.32; 122.69; 122.51; 106.90;
106.81; 106.37; 91.81; 91.64; 91.51; 75.35; 18.67; 11.32.
133.04; 131.04; 129.03; 125.31; 122.78; 121.03; 119.81; 113.86;
106.53; 91.85; 88.53; 61.25; 18.39; 14.25; 11.28.
g5-1,2,3,4,5-Penta[4-(ethynyl)phenyl]cyclopentadienyl hydro-
tris [6-(ethoxycarbonyl)indazol-1-yl] borate ruthenium(II) (14).
Z5-1,2,3,4,5-Penta-(4-(triisopropylsilylacetylene)phenyl) cyclo-
pentadienyl hydrotris [6-(ethoxycarbonyl)indazol-1-yl] borate
ruthenium(II) (13) (30 mg, 0.015 mmol, 1 eq.) was dissolved in
2 ml of TBAF 1M in THF (2 mmol, 135 eq.) with 5% of water.
The solution was stirred at room temperature overnight and
the solvent was evaporated. The crude product was purified by
column chromatography (SiO2, CH2Cl2) to give a yellow solid
(10 mg, 54%). MS: (APCI) 1246 [M]+; 1H NMR: (250 MHz,
CD2Cl2) d 8.76 (s, 3H, Ha), 7.98 (s, 3H, Hd), 7.68 (d, 3H, J =
8.5 Hz, Hc), 7.44 (d, 3H, J = 8.5 Hz, Hb), 7.31 (d, 10H, J =
8.4 Hz, Ho), 7.17 (d, 10H, J = 8.4 Hz, Hm), 4.47 (q, 6H, J =
7.1 Hz, CH2), 3.11 (s, 5H, CH), 1.48 (t, 9H, J = 7.1 Hz, CH3);
13C NMR: (63 MHz, CD2Cl2) d 166.85; 143.19; 140.83; 133.82;
133.43; 131.32; 129.21; 125.38; 121.53; 121.18; 119.85; 113.95;
88.48; 83.04; 78.16; 61.36; 14.32.
Bromo-g5-1,2,3,4,5-penta[4-(triisopropylsilylacetylene)phenyl]
cyclopentadienyl dicarbonyl ruthenium(II) (12). Ruthenium car-
bonyl (37 mg, 0.058 mmol, 1 eq.) and 1-bromo-1,2,3,4,5-penta
(4-(triisopropylsilylacetylene)phenyl)cyclopentadiene (11) (250
mg, 0.17 mmol, 3 eq.) were heated under argon at reflux for 2 h
in 4 mL of freshly distilled toluene. The solution, initially
yellow, rapidly turned to dark green and then to cherry red.
Solvents were removed under reduced pressure and the crude
material was column chromatographed (SiO2, cyclohexa-
ne–CH2Cl2 (0–10%)) to give a yellow solid (200 mg, 72%).
MS: (DCI/NH3) 1618 [M + 2NH3]+, 1601 [M + NH3]+, 1563
[M + 2NH3 ꢁ 2CO]+, 1545 [M + NH3 ꢁ 2CO]+, 1528 [M ꢁ
2CO]+; High Resolution LSI Calculated [M]+: 1582.6753
(C92H125BrO2RuSi5) Found: 1582.6791 (100% [M]+); Mp =
180 1C (with decomposition). 1H NMR: (250 MHz, CD2Cl2) d
7.28 (d, 10H, J = 8.6 Hz, Ho), 7.00 (d, 10H, J = 8.6 Hz, Hm),
1.13 (s, 105H, TIPS); 13C NMR: (100 MHz, CD2Cl2) d 196.17;
132.28; 131.70; 129.36; 123.99; 106.31; 105.99; 92.76; 18.53;
Acknowledgements
G. V. thanks the French Ministry of National Education and
the Ecole Normale Superieure of Lyon for a PhD Fellowship.
´
A. C. thanks the French Ministry of National Education and
the Institut Universitaire de France for a PhD Fellowship.
Dr Christine Lepetit (LCC-CNRS) is gratefully acknowledged
for her assistance in the DFT calculations and we also would
like to thank CALMIP (Calculation centre, Toulouse, France)
for computing facilities. Dr Isabelle M. Dixon is warmly
acknowledged for her corrections and comments on this
manuscript. Christine Viala is also thanked for technical
assistance in the measurement of the NMR spectra.
References
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11.43; IR: nCQO 2004 (s) and 2048 (s) cmꢁ1
.
g5-1,2,3,4,5-Penta-(4-(triisopropylsilylacetylene)phenyl)cyclo-
pentadienyl hydrotris [6-(ethoxycarbonyl)indazol-1-yl] borate
ruthenium(II) (13). Bromo-Z5-1,2,3,4,5-penta(4-(triisopropylsi-
lylacetylene)phenyl)cyclopentadienyl dicarbonyl ruthenium(II)
(12) (160 mg, 0.1 mmol, 1 eq.) and potassium hydrotris
[6-(ethoxycarbonyl)indazol-1-yl]borate (125 mg, 0.2 mmol, 2 eq.)
were heated in a sealed tube at 150 1C under microwave irradia-
tion during 10 minutes in a mixture of 2 ml of acetonitrile and
1 ml of DMF. The crude reaction mixture was evaporated under
vacuum. The product was adsorbed on silica and purified by
column chromatography (SiO2, cyclohexane–CH2Cl2 80%) to a
yellow solid (35 mg, 17%). MS: (APCI) 2029 [M + H]+; High
Resolution LSI Calculated [M]+: 2026.9835 (C120H153BBrO2-
N6O6RuSi5) Found: 2026.9872 (100% [M]+); 1H NMR: (250
MHz, CD2Cl2) d 8.76 (s, 3H, Ha), 7.99 (s, 3H, Hd), 7.68 (d, 3H, J
= 8.6 Hz, Hc), 7.46 (d, 3H, J = 8.6 Hz, Hb), 7.31 (d, 10H, J =
8.2 Hz, Ho), 7.17 (d, 10H, J = 8.2 Hz, Hm), 4.47 (q, 6H, J = 7.1
Hz, CH2), 1.48 (t, 9H, J = 7.1 Hz,CH3), 1.08 (m, 105H, TIPS);
13C NMR: (63 MHz, CD2Cl2) d 166.80; 143.11; 140.84; 133.31;
6 D. Astruc, Acc. Chem. Res., 1997, 30, 383.
ꢀc
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