A star-shaped ruthenium complex with five ferrocenyl-terminated arms bridged by trans-platinum fragments

Article abstract of DOI:10.1039/b603508f

We present the synthesis of the new heteropolytopic penta(4-ethynylphenyl) cyclopentadiene ligand, its complexation through the Cp ring to ruthenium tris(indazolyl)borate and through the terminal alkyne groups to five ferrocenyl ethynyl platinum units, yi

Full text of DOI:10.1039/b603508f

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A star-shaped ruthenium complex with five ferrocenyl-terminated arms
bridged by trans-platinum fragments{
Guillaume Vives, Alexandre Carella, Jean-Pierre Launay and Gwe´nae¨l Rapenne*
Received (in Cambridge, UK) 9th March 2006, Accepted 6th April 2006
First published as an Advance Article on the web 28th April 2006
DOI: 10.1039/b603508f
purpose, trans-bis(alkynyl) bis(trialkylphosphine)platinum(II) com-
plexes, although moderately insulating,11 have been selected for a
combination of reasons: (i) the rigidity and linearity of the
assembly are maintained, (ii) the electroneutrality of the molecule
is preserved, essential for its deposition on a surface, and (iii) the
diameter of the molecule is significantly increased (from 2 to 3 nm),
favourable for future near-field microscopy studies. Moreover, the
preparation of platinum acetylide complexes with rigid alkynyl
backbones has been described by Stang et al. and Takahashi et al.
for use in the formation of organometallic dendrimers that 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.12 A modular strategy was followed in
order to prepare separately the different metallic building blocks
and to assemble them step-by-step around the penta(4-ethynyl-
phenyl)cyclopentadiene ligand.
We present the synthesis of the new heteropolytopic penta(4-
ethynylphenyl)cyclopentadiene ligand, its complexation
through the Cp ring to ruthenium tris(indazolyl)borate and
through the terminal alkyne groups to five ferrocenyl ethynyl
platinum units, yielding an undecanuclear heterotrimetallic
complex.
With increasing interest in the unique chemical and physical
properties possessed by metal complexes, the development of novel
functionalized nanosized materials incorporating transition metal
complexes is a promising area of research. In the last few years,
there has been a growing interest in the design of star-shaped
molecules incorporating different metal centres.1 Their magnetic2
or multi-redox properties have yielded molecules which have found
applications in the fields of catalysis,3 artificial photosynthesis4 and
sensors.5 However, they have not been extensively studied due to
synthetic difficulties.
Starting from the previously described 1-bromo-1,2,3,4,5-
penta(para-bromophenyl)cyclopentadiene (3),13 compound 4 was
prepared in a remarkable 81% isolated yield by a quintuple
coupling with mono-TIPS-protected acetylene (Scheme 1).{ At this
stage, it is preferable to keep the alkyne functions protected in
order to coordinate selectively the cyclopentadienyl (Cp) ring. This
also allows us to vary the nature of the metals coordinated to the
two different types of binding sites offered by this polytopic ligand.
By following Connelly and Manners’ methodology, which uses a
brominated Cp with Ru3(CO)12,14 a bromine atom was introduced
back at the 1-position of the Cp ring by means of nBuLi and
N-bromosuccinimide (NBS) at a low temperature to yield
compound 5. Ruthenium was subsequently coordinated, giving
Polyalkyne compounds are particularly well-suited as rigid
building blocks for such supramolecular assemblies.6 Hexaethynyl-
benzene and its derivatives have received extensive attention over
the last two decades, starting with the pioneering work performed
by Vollhardt et al.7 The analogous five-membered (pentaethynyl)-
cyclopentadiene has been prepared and coordinated to
a
manganese tricarbonyl fragment by Bunz et al.,8 and it has been
shown recently by Rubin et al. to have the remarkable property,
for a free ligand, to exist as a radical that can be easily oxidized to
the corresponding antiaromatic cation.9 However, nothing is
known about the synthesis, reactivity and stability of penta(para-
ethynylphenyl)cyclopentadiene, in which the para-phenylene units
are intercalated between the Cp ring and the alkyne functions.
We present here the synthesis of the new TIPS-protected
penta(4-ethynylphenyl)cyclopentadiene ligand (4) (TIPS = tri(iso-
propyl)silyl). Following a stepwise approach, its polytopicity
allowed us to obtain star-shaped molecule 1 (Fig. 1) by complexa-
tion through the Cp ring to a ruthenium tris(indazolyl)borate
fragment and to five ferrocenyl ethynyl platinum units through the
terminal alkyne groups. This star-shaped molecule is the heart of
our electrically-triggered rotary motor project,10 in which the
ferrocenes will act as electroactive units for reversible redox cycles.
To favour charge localization on a ferrocene unit, it is preferable to
break the conjugation between the electroactive groups. For this
NanoSciences Group, Centre d’Elaboration de Mate´riaux et d’Etudes
Structurales–CNRS, 29 rue Jeanne Marvig, BP 94347, F-31055,
Toulouse Cedex 4, France. E-mail: rapenne@cemes.fr;
Fax: +33 562 25 79 99; Tel: +33 562 25 78 41
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation (analytical and spectral) of compounds 1 and 4. See DOI:
10.1039/b603508f
Fig. 1 Chemical structure of 1, an undecanuclear heterotrimetallic
complex.
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Scheme 1 Synthetic route followed to yield the heterotrimetallic undecanuclear complex 1.
complex 6 in a 72% yield after chromatography. By reacting 6 with
potassium tris(indazolyl)borate (KTIB), complex 7, incorporating
a tripodal ligand derived from the scorpionate family,15 was
formed, together with another unidentified complex of similar
polarity that was hardly separable by chromatography. An
analytically pure sample of 7 could be obtained in 30% yield after
a second chromatographic purification, although the yield of the
reaction was clearly higher. Finally the TIPS protecting groups
were removed by an overnight reaction with tetrabutylammonium
fluoride (TBAF) to yield the desired complex 8, bearing five free
acetylene groups, now available for coordination. The problem of
purifying complex 7 could be avoided by performing the TIB
complexation and TIPS deprotection in a one-pot fashion, with a
37% overall yield.
constant was found to be 2374 Hz, corresponding to a trans
geometry; a cis coupling constant is usually much larger (about
3500 Hz).17 The presence of the five ethynylferrocene units was
also confirmed by mass spectrometry. Oxidation of the five iron
centres occurs simultaneously at a potential of 0.31 V/SCE,
followed by oxidation of the ruthenium centre at 0.60 V (Fig. 2).
These values are lower than the potentials measured for ethynyl
ferrocene, which presents an oxidation wave at 0.59 V/SCE,
showing the electron-rich character of the diphosphinoplatinum
fragments that are able to stabilize the Fe(III) oxidation state. It
must be noted that the platinum centres were not oxidized within
the potential window used (21.8 to +1.5 V/SCE in dichloro-
methane). In addition, preparative electrolysis of 1 was performed
on a platinum grid, first to oxidize the ferrocene sites, then to
oxidize the ruthenium site. After back reduction, the starting
material was recovered unchanged, showing the robustness of the
molecule towards oxidation. Finally, after performing a partial
oxidation of the ferrocene sites, no intervalence transition between
Fe(II) and Fe(III) was observed, thus confirming the absence of
Ethynylferrocenyl platinum fragment 2 was prepared starting
from trans-dichlorobis(triethylphosphine)platinum(II) by the sub-
stitution of one chloride ligand for a ferrocenylethynyl group.16 It
was achieved by heating trans-Pt(PEt3)2Cl2 with ethynylferrocene
for 36 h in the presence of diethylamine. Complex 2 was obtained
in 74% yield after chromatography by using conventional heating
to reflux. The yield of this step could be improved to 86% by using
microwave irradiation (300 W) at 113 uC in a sealed tube for
15 min. The five ferrocene–platinum units were subsequently
introduced by a quintuple coupling of chloro-platinum complex 2
with complex 8, yielding the desired heterotrimetallic complex 1 in
41% isolated yield after column chromatography (SiO2, cyclohex-
ane/Et2O (0–20%)), corresponding to a yield of 84% per coupling.§
1H NMR spectroscopy clearly showed an AA9BB9 pattern for
the phenyl groups attached to the central Cp ring and an
integration of 45 protons for the signals of the ferrocene units. Free
rotation of the Cp ligand was observed down to 290 uC. The trans
geometry around the platinum-diphosphino centres was confirmed
Fig. 2 Cyclic voltammogram of 1 (CH2Cl2, nBu4NPF6, Pt working
1
by 31P NMR17 and 195Pt NMR18 spectroscopy from the JP–Pt
and counter electrode). All waves were reversible. The sweep rate was
100 mV s21
.
coupling constant. For complex 1, the value of the coupling
2284 | Chem. Commun., 2006, 2283–2285
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measurable electronic communication between the electroactive
ferrocene units.
We have reported the synthesis of the new TIPS-protected
penta(4-ethynylphenyl)cyclopentadiene (4). This versatile ligand is
well suited to giving access to a large variety of heteropolynuclear
complexes due to its polytopicity. Its coordination takes place step-
by-step, in our case binding the Cp ring first and then the five
alkyne functions. However, the reverse approach could be more
appropriate for other types of metals. We prepared a hetero-
trimetallic undecanuclear star-shaped complex built around a
ruthenium centre by following a building block strategy. The final
step was a quintuple complexation of the terminal alkyne groups
by platinum fragments, occurring with a 41% isolated yield.
Electrochemical measurements showed the oxidation processes
were independent for each metal. We are now exploring the
possibility of using this star-shaped heterotrimetallic complex as
the prototype for an electron-fuelled rotary motor.
We thank the CNRS, the European Union and the University
of Toulouse for financial support, the French Ministry of National
Education for a fellowship to A. C. and the Ecole Normale
Supe´rieure Lyon for a fellowship to G. V.
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Notes and references
{ 4: MS: (DCI/NH3) 1349 [MH]+, 1381 [M + 2NH3]+. High resolution LSI:
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1
calc. [M]+ = 1346.8706 (C90H126Si5), found = 1346.8787 (100% [M]+). H
NMR (250 MHz, CD2Cl2): d 7.34 (d, 6 H, J 5 8 Hz, H2–H6), 7.21 (d, 4 H,
J 5 8 Hz, H4), 7.17 (d, 2 H, J 5 8 Hz, H1), 7.02 (d, 4 H, J 5 8 Hz, H5),
6.96 (d, 4 H, J 5 8 Hz, H3), 5.16 (s, 1 H, H7) and 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 and
11.26.
§ 1: MS: (ES+) 2113 [M]2+ and 1409 [M]3+. MALDI-TOF: 4226.2 [M]+
(calc. 4226.98). 1H NMR (500 MHz, CD2Cl2): d 8.04 (d, 3 H, J 5 6.8 Hz,
He), 8.02 (s, 3 H, Ha), 7.45 (d, 3 H, J 5 8.7 Hz, Hb), 7.38 (m, 3 H, Hc), 7.25
(d, 10 H, J 5 8.5 Hz, H1), 7.03 (t, 3 H, J 5 7.6 Hz, Hd), 6.97 (d, 10 H,
J 5 8.5 Hz, H2), 4.22 (t, 10 H, J 5 1.8 Hz, subst. Cp), 4.15 (s, 25 H, Cp),
4.07 (t, 10 H, J 5 1.8 Hz, subst. Cp), 2.17 (m, 60 H, CH2) and 1.22 (m,
90 H, CH3). 13C NMR (125 MHz, CD2Cl2): d 143.37, 140.31, 133.24,
130.92, 129.61, 127.69, 126.01, 123.06, 120.03, 119.98, 111.36, 109.21,
108.78, 104.86, 102.62, 87.52, 72.95, 70.09, 69.31, 66.97, 16.23 and 8.15. 31
P
NMR (200 MHz, CD2Cl2): d 11.30 (s, 1J31P–195Pt 5 2374 Hz). 195Pt NMR
(500 MHz, CD2Cl2): d 24742. UV-vis (CH2Cl2; lmax/nm (e/mol21
L cm21)): 264 (225000), 306 (169200), 359 (58000) and 436 (10400). CV
n
(CH2Cl2, Bu4NPF6) EFe( )–Fe( (V/SCE) = +0.31 rev. (5 e); ERu( )–Ru(
18 A. L. Rieger, G. B. Carpenter and P. H. Rieger, Organometallics, 1993,
12, 842.
II
III
)
II
III
)
(V/SCE) = +0.60 rev. (1 e) (sweep rate = 100 mV s21).
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Chem. Commun., 2006, 2283–2285 | 2285