Breaking the symmetry in the molecular motor family: synthesis of a dissymmetrized pentaphenyl cyclopentadienyl ligand and its ruthenium tris(indazolyl)borate complex

Article abstract of DOI:10.1016/j.tetlet.2006.10.013

In order to demonstrate a movement of rotation in a family of rotary molecular motors, 1-bromo-1-(4-tolyl)-2,3,4,5-tetra(4-bromophenyl) cyclopentadiene has been synthesized. The coordination of this C_2v cyclopentadiene to a ruthenium trisindazo

Full text of DOI:10.1016/j.tetlet.2006.10.013

Tetrahedron Letters 47 (2006) 8741–8744
Breaking the symmetry in the molecular motor family: synthesis
of a dissymmetrized pentaphenyl cyclopentadienyl ligand and
its ruthenium tris(indazolyl)borate complex
Guillaume Vives and Gwenael Rapenne*
¨
NanoSciences Group, CEMES-CNRS, 29 rue Jeanne Marvig, BP 94347, F-31055 Toulouse Cedex 4, France
Received 7 September 2006; revised 29 September 2006; accepted 2 October 2006
Available online 20 October 2006
Abstract—In order to demonstrate a movement of rotation in a family of rotary molecular motors, 1-bromo-1-(4-tolyl)-2,3,4,5-
tetra(4-bromophenyl) cyclopentadiene has been synthesized. The coordination of this C2v cyclopentadiene to a ruthenium tris-
indazolylborate complex and the quadruple coupling reaction with ethynylferrocene led to the dissymmetrized motor (1).
Ó 2006 Elsevier Ltd. All rights reserved.
Me
In the field of nanotechnology, a growing interest has
Fe
Fe
appeared in the design and synthesis of molecular
machines.1 Among theses machines, molecular rotary
motors represent a particular challenge for the control of
a unidirectional movement.2 In our laboratory we have
designed an electrically fuelled molecular rotary motor
with the aim to study it as a single molecule by near field
microscopy techniques.3 This motor is based on a piano
stool ruthenium complex with a tripodal hydro-
tris(indazolyl)borate ligand4 functionalized to be an-
chored on a surface (i.e., the stator) and a substituted
cyclopentadienyl (Cp) ligand with five arms terminated
by electroactive groups (i.e., the rotor). The principle
is to deposit the molecule anywhere between two elec-
trodes of a nano junction and to control the rotation
by the flow of electrons. Physical studies are underway
but a crucial problem is to evidence the rotation. Due
to the high symmetry of the molecule, every 72°, there
is no way to distinguish two images with a C5-symmetric
rotor such as a Cp substituted by five identical
substituents.
Fe
Fe
Ru
N
N
N
N
N
N
B
H
O
O
O
O
O
O
1
Figure 1. Molecular motor (1) with one missing ferrocene group.
symmetry of the molecule should help to prove a move-
ment and maybe to monitor the rotation, the missing
ferrocene acting as a probe for the position of the rotor.
Since the coupling of ethynylferrocene with 1-bromo-
1,2,3,4,5-p-bromophenylcyclopentadiene can only give
access to a statistical mixture of penta-, tetra-, tri-,
di- and mono-ferrocenyl compounds which cannot be
separated, we decided to prepare the tetra-ferrocenyl
derivative through a controlled synthesis.
This major issue can be partly addressed by using com-
pounds of lower symmetry. For that purpose, we
designed the molecular motor 1 (Fig. 1) in which a
ferrocene electroactive group is missing. Lowering the
The key feature of our strategy is to block one position
of the Cp ring during the synthesis, in order to differen-
tiate it from the other four substituents.
Keywords: Cyclopentadienone; Cyclopentadienyl ligand; Disymmet-
rized ligand; Molecular motor; Grignard reagent; Ruthenium.
*
In this letter, we present the synthesis and characteriza-
tion of a dissymmetrized pentaphenyl cyclopentadienyl
Corresponding author. Tel.: +33 562 25 78 41; fax: +33 562 25 79
99; e-mail: rapenne@cemes.fr
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.013

8742
G. Vives, G. Rapenne / Tetrahedron Letters 47 (2006) 8741–8744
CH3
CH3
Br
Br
O
+
O
Br
HO
Br
Br
Br
Br
Br
Br
a
b
c
O
O
Br
Br
Br
Br
Br
Br
4
3
2
Br
Br
Scheme 1. Reagents and conditions: (a) KOH, EtOH, reflux, 40 min, 70%; (b) p-TolMgBr, THF, 25 °C, 2 h, 88%; (c) HBr, AcOH, 95 °C, 2 h, 60%.
Compound 4 is obtained as a mixture of three regioisomers, the two others are shown in Figure 2.
ligand (4) bearing four bromide substituents for the sub-
sequent coupling reaction and a methyl group to block
the fifth position. We also present the synthesis of its
ruthenium tris(indazolyl)borate complex (1).
As shown in Scheme 1, the first step is the condensation
of 1,3-di(4-bromophenyl)propan-2-one5 with 4,40-di-
bromo benzile6 under basic conditions yielding the tetra-
brominated cyclopentadienone7 2 as a purple solid in
70% yield after recrystallization in ethanol. The p-tolyl
substituent is then introduced by a nucleophilic addition
of the Grignard reagent p-tolylmagnesium bromide8 on
a suspension of 2 at room temperature for 2 h in THF.
The functionalized cyclopentadienol 3 was obtained
after recrystallization in hexane in a 88% yield as a single
regioisomer.
In the pentaphenylcyclopentadiene family, it is known9
Figure 3. Top: 1H NMR (CD2Cl2, 250 MHz) spectra of 4 with the
that coordination is not possible via the classical cyclo-
three singlets corresponding to the three regioisomers in the 2–3 ppm
region and the complex signals in the aromatic region. Bottom: 1H
NMR (CD2Cl2, 250 MHz) spectra of 6 with one singlet corresponding
to the methyl group in the 2–3 ppm region.
pentadienide generated under basic conditions, and this
may be due to steric hindrance. Coordination of ruthe-
nium to this type of ligand can only take place if there
is a bromine atom on the Cp ring, following Manner’s
methodology10 which consists in the oxidative addition
of the carbon–bromine bond on the ruthenium carbonyl
cluster. For that purpose, the hydroxy group of 3 was
replaced by a bromine atom.11 After reaction with
HBr in acetic acid, the brominated Cp 4 was purified
by column chromatography (SiO2:cyclohexane/CH2Cl2
10%, Rf = 0.53) and was obtained in 60% yield as a mix-
ture of three regioisomers (Scheme 1 and Fig. 2). The
regioisomers of 4, formed due to the SN1 mechanism
Three singlets were obtained (Fig. 3, top), corresponding
to the three regioisomers with a 1.4:1.7:1.9 ratio which
differs strongly from the statistical mixture 1:2:2, show-
ing the difference of stability of the carbocations
involved. It must be stated that the postulated Cp+
intermediate is antiaromatic but since the mixture ob-
tained is not separable, it has not been possible to check
if the latter was the thermodynamic mixture. Anyway,
the presence of this mixture of regioisomers is not a
problem since in the next step, the aromatization of
the Cp ring through its coordination leads to the same
compound for the three regioisomers.
1
of this reaction, can be quantified by H NMR, using
the methyl group of the tolyl substituent as a probe.
Br
Br
As shown in Scheme 2, coordination of the three regio-
isomers of ligand 4 with Ru3(CO)12 gave complex 5 with
a 43% yield after purification by column chromatogra-
phy (SiO2:cyclohexane/CH2Cl2 10%, Rf = 0.50). The
two carbonyl groups and the bromide ligand can be
substituted by the scorpionate ligand hydrotris[6-(ethoxy-
carbonyl)indazol-1-yl]borate by heating under microwave
irradiation in a sealed tube at 150° for 10 min. The
ruthenium precursor of the molecular motor 6 was
purified by column chromatography (SiO2:CH2Cl2,
Rf = 0.40), and it was subsequently reacted using the
Br
Br
Br
CH3
Br
Br
Br
Br
Br
CH3
Figure 2. The two other regioisomers were formed simultaneously
during the synthesis of the brominated cyclopentadiene 4.

G. Vives, G. Rapenne / Tetrahedron Letters 47 (2006) 8741–8744
8743
Me
2
1
4
3
Me
Br
Br
Me
5
6
Br
Br
Br
Br
Br
Br
Br
Ru
a
b
c
a
d
1
Br
Br
b
Ru
N
N
N
N
N
N
c
B
H
OC
CO
Br
O
Br
Br
O
O
O
O
Mixture of three regioisomers
O
4
5
6
Scheme 2. Reagents and conditions: (a) Ru3(CO)12, toluene, 2 h, reflux, 43%; (b) potassium hydrotris[6-(ethoxycarbonyl)indazol-1-yl]borate,
microwave, DMF/CH3CN, 150 °C, 10 min, 30 %; (c) [(ferrocenyl)ethynyl]zinc chloride, Pd(PPh3)4, THF, reflux, 24 h, 49%.
3. (a) Carella, A.; Rapenne, G.; Launay, J.-P. New J. Chem.
2005, 29, 288–290; (b) Vives, G.; Carella, A.; Sistach, S.;
Launay, J.-P.; Rapenne, G. New J. Chem. 2006, 1429–
1438.
4. (a) Rheingold, A. L.; Haggerty, B. S.; Yap, G. P. A.;
Trofimenko, S. Inorg. Chem. 1997, 36, 5097–5103;
(b) Carella, A.; Jaud, J.; Rapenne, G.; Launay, J.-P.
Chem. Commun. 2003, 2434–2435; (c) Carella, A.; Vives,
G.; Cox, T.; Jaud, J.; Rapenne, G.; Launay, J.-P. Eur. J.
Inorg. Chem. 2006, 980–987.
5. Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.;
Rabe, J. P.; Mullen, K. Chem. Eur. J. 2000, 6, 4327–4342.
6. Purchased from Aldrich and used without further
purification.
7. Coan, S. B.; Trucker, D. E.; Becker, E. I. J. Am. Chem.
Soc. 1955, 77, 60–66.
Negishi cross-coupling conditions. Heating at reflux, a
solution of [(ferrocenyl)ethynyl]zinc chloride in the pres-
ence of 6 and Pd(PPh3)4 in THF yielded, after purifica-
tion by chromatography (SiO2:CH2Cl2, Rf = 0.40), the
dissymmetrized motor 1 in a 49% yield which corre-
sponds to an 84% yield per coupling reaction. The pres-
ence of the ferrocenyl moieties was confirmed by 1H
NMR spectroscopy with an integration of 36 for the
protons of the ferrocenyl groups and 3 for the protons
1
of the tolyl group. Moreover the H NMR also clearly
showed three AA0BB0 patterns for the phenyl protons
of the rotor with a 2:2:1 ratio. MALDI-TOF spectro-
metry also confirmed the presence of the four ferrocene
units.12
8. Broser, W.; Kurreck, H.; Siegle, P. Chem. Ber. 1967, 100,
788–794.
9. Beck, C. U.; Field, L. D.; Hambley, T. W.; Humphrey, P.
A.; Masters, A. F.; Turner, P. J. Organomet. Chem. 1998,
565, 283–296.
In summary, we have succeeded in developing a strategy
to obtain a dissymmetrized cyclopentadienyl ligand.
This ligand has been incorporated in the family of elec-
tron-fuelled molecular rotary motors by coordination to
a ruthenium tris(indazolyl)borate complex. Scanning
probe microscopy studies are underway to prove the
rotation of the molecule.
10. Connelly, N. G.; Manners, I. Dalton Trans. 1989, 283–
288.
11. Field, L. D.; Ho, K. M.; Lindall, C. M.; Masters, A. F.;
Webb, A. G. Aust. J. Chem. 1990, 43, 281–291.
12. All new compounds were fully characterized by 1H NMR,
13C NMR, UV/vis and MS. The numbering scheme is
given for molecule 6 (see Scheme 2).
Acknowledgements
Ligand 4: yellow solid; UV/vis (CH2Cl2) kmax/nm (e/
1
molꢀ1 L cmꢀ1) 260 (33,100), 285 (36,100), 373 (3600); H
G.V. thanks the French Ministry of National Education
NMR: (250 MHz, CD2Cl2) d 7.4–6.8 (m, 20H), 2.32
(s, 0.84H, regioisomer 1CH3), 2.29 (s, 1.14H, regioiso-
mer 2CH3), 2.24 (s, 1.02H, regioisomer 3CH3); 13C NMR
(75 MHz, CD2Cl2) d 149.23; 148.26; 147.63; 147.38;
146.69; 142.57; 141.80; 141.35; 140.72; 140.19; 138.52;
138.02; 137.88; 134.55; 134.52; 133.31; 133.13; 133.03;
133.00; 132.90; 132.78; 132.66; 132.05; 131.98; 131.86;
131.74; 131.70; 131.66; 131.64; 131.41; 131.28; 131.22;
131.05; 130.93; 130.70; 130.19; 129.75; 129.47; 129.36;
129.30; 128.96; 128.59; 127.31; 122.32; 122.22; 122.03;
121.99; 121.92; 121.90; 121.82; 121.66; 21.08; 20.98; 20.85.
MS: (DCI/NH3) 856 [Mꢀ]; HR-FABꢀ-MS (m-NBA/m/z)
849.7654 (M+H+, calcd for C36H23Br5: 849.7717).
´
and the Ecole Normale Superieure of Lyon for a PhD
Fellowship. Christine Viala is also thanked for technical
assistance in the measurement of the NMR spectra. This
work was supported by the CNRS and the University
Paul Sabatier (Toulouse, France). We are also grateful
to Professor Jean-Pierre Launay for fruitful discussions.
References and notes
1. (a) Special issue on molecular machines: Acc. Chem. Res.
2001, 34, 409–522; (b) Balzani, V.; Venturi, M.; Credi, A.
Molecular Devices and Machines—A Journey into the
Nanoworld; Wiley-VCH: Weinheim, 2003; (c) Molecular
Motors; Schillwa, M., Ed.; Wiley-VCH: Weinheim, 2003.
2. Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem.
Rev. 2005, 105, 1281–1376.
1
Complex 6: yellow solid; H NMR: (500 MHz, CD2Cl2) d
8.76 (s, 3H, Hd), 7.96 (s, 3H, Ha), 7.68 (d, 3H, J = 8.5 Hz,
Hc), 7.44 (d, 3H, J = 8.5 Hz, Hb), 7.21 (s broad, 18H, H1
H3–6), 6.89 (d, 2H, J = 8.0 Hz, H2), 4.47 (q, 6H, J = 7.1 Hz,
CH2), 2.23 (s, 3H, PhCH3), 1.48 (t, 9H, J = 7.1 Hz, CH3).
13C NMR: (126 MHz, CD2Cl2) d 166.77; 143.07; 140.73;

8744
G. Vives, G. Rapenne / Tetrahedron Letters 47 (2006) 8741–8744
137.98; 135.15; 135.07; 133.26; 132.26; 131.94; 130.73;
4.20 (s, 10H, Cp), 4.19 (s, 10H, Cp), 2.25 (s, 3H, CH3), 1.48
(t, 9H, J = 7.1 Hz, CH3). 13C NMR: (126 MHz, CD2Cl2) d
166.86; 143.09; 140.87; 137.75; 133.56; 133.49; 133.39;
132.99; 132.82; 130.27; 130.22; 129.48; 128.88; 128.21;
125.36; 123.13; 123.03; 120.97; 119.75; 113.85; 89.82;
89.52; 89.46; 88.58; 88.29; 85.18; 71.38; 70.66; 69.93;
130.67; 129.02; 128.89; 128.31; 125.25; 121.99; 121.87;
121.05; 119.73; 113.85; 90.17; 88.05; 87.28; 61.29; 20.87;
14.24. MS: (DCI/NH3) 1474 [M+NH4]+, 1457 ([M+H]+
calcd for C66H52BBr4N6O6Ru: 1456.6).
Complex 1: orange solid; 1H NMR: (500 MHz, CD2Cl2) d
8.79 (s, 3H, Hd), 8.07 (s, 3H, Ha), 7.67 (d, 3H, J = 8.5 Hz,
Hc), 7.48 (d, 3H, J = 8.5 Hz, Hb), 7.36 (m, 8H, H3 H5),
7.27 (d, 2H, J = 8.2 Hz, H1), 7.18 (m, 8H, H4 H6), 6.91
(d, 2H, J = 8.2 Hz, H2), 4.47 (q, 6H, J = 7.1 Hz, CH2), 4.44
(t, 8H, J = 1.8 Hz, subs Cp), 4.21 (m, 8H, subs Cp),
68.98; 64.87; 61.27; 22.71; 14.25; UV/vis (CH2Cl2) kmax
/
nm (e/molꢀ1 L cmꢀ1) 262 (97,000), 308 (85,500), 365
(42,600). MS: (MALDI/TOF) 1972 [M]+ HR-FAB+-MS
(m-NBA/m/z) 1972.4070 ((M+), calcd for C114H87BFe4-
N6O6Ru: 1972.3221).