Technomimetic molecules: Synthesis of ruthenium(II) 1,2,3,4,5-penta(p- bromophenyl)cyclopentadienyl hydrotris(indazolyl)borate, an organometallic molecular turnstile

Article abstract of DOI:10.1039/b307577j

A short route to prepare a ruthenium complex with a pentaphenyl substituted cyclopentadienyl and a hydrotris(indazolyl)borate ligand is described: this complex can be seen as an organometallic molecular turnstile.

Full text of DOI:10.1039/b307577j

Technomimetic molecules: synthesis of ruthenium(II
)
1,2,3,4,5-penta(p-bromophenyl)cyclopentadienyl
hydrotris(indazolyl)borate, an organometallic molecular turnstile
Alexandre Carella, Joël Jaud, Gwénaël Rapenne and Jean-Pierre Launay*
NanoSciences Group, CEMES-CNRS, 29 rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex 4, France.
E-mail: rapenne@cemes.fr; Fax: (+33) 562 25 79 99; Tel: (+33) 562 25 78 41
Received (in Cambridge, UK) 3rd July 2003, Accepted 21st August 2003
First published as an Advance Article on the web 4th September 2003
A short route to prepare a ruthenium complex with a
pentaphenyl substituted cyclopentadienyl and a hydro-
tris(indazolyl)borate ligand is described: this complex can be
seen as an organometallic molecular turnstile.
heating 2 with 2 equiv. of potassium hydrotris(indazolyl)borate6
for 24 hours in freshly distilled THF. After purification by
column chromatography (SiO
and crystallization, 3 was obtained as yellow crystals with a
0% yield. Reaction time, temperature and solvent have been
2 2 2
: cyclohexane–CH Cl 0–20%)
3
Technomimetic molecules are molecules designed to transpose
macroscopic objects at the molecular level, including the
motions that these objects are able to undergo. In this paper, we
varied, but none of these changes resulted in an improved yield.
Nevertheless, it must be noted that the coordination of the
tripodal ligand is the result of two steps and the presence of
various ligands (Tp4 , cp, CO, Br) in the reaction mixture may
allow numerous side reactions.
1
Bo
present the synthesis of a ruthenium complex containing two
different ligands, which behaves as an organometallic molec-
2
7
ular turnstile. Interestingly, the rotation of one ligand with
The X-ray structure of 3 confirmed the coordination of both
4
Bo
respect to the other induces a secondary rotation.
ligands. As shown by Trofimenko, Tp
binds in a facial
4Bo
3
Hydrotris(indazolyl)borate ligand (Tp ) is a versatile
tripodal mode (i.e. k –N,NA,NB). The complex has a piano stool
structure with the cp substituents fitting in the vacant spaces of
the tripodal ligand.
3
ligand of the family of scorpionates developed by Trofimenko.
It has been shown that Tp4 binds in a facial tripodal mode and
its geometry is particularly well suited to mimic macroscopic
gears. Traditional synthetic routes to half-sandwich cyclopenta-
dienyl ruthenium(II) complexes have relied extensively on the
Bo
6
photochemical labilisation of an h -arene ring or on the use of
5
4
5 5 3 2
a precursor such as (h –C H )Ru(PPh ) Cl. We present here a
short route to prepare a ruthenium complex with a pentaphenyl
substituted cyclopentadienyl (cp) and a hydrotris(indazolyl)bor-
ate ligand.
The synthesis is presented in Scheme 1. Our strategy starts
with the bromination of 1,2,3,4,5-pentaphenylcyclopentadiene
with neat bromine (20 equiv.) at 20 °C for 2 hours, which
affords 1-bromo-1,2,3,4,5-penta(p-bromophenyl)cyclopenta-
diene selectively. After trituration of the crude material with
cold ethanol (240 °C), several washings with a potassium
Fig. 1 Crystal structure of 3: space filling (side view) and ORTEP (bottom
view) representations. The cp ligand is distant of the ruthenium atom by
1.79 Å and the average distance between the three coordinated nitrogens and
the ruthenium centre is 2.16 Å.
thiosulfate solution and column chromatography (SiO
2
: cyclo-
hexane –CH Cl 0–10%), 1 was obtained as a pale yellow solid
2
2
with a yield of 98%. The presence of the five bromine atoms in
the para position of each phenyl ring should be helpful to
introduce various groups on the cyclopentadienyl ring. Upon
For symmetry considerations, the superimposition of a
tripodal ligand bearing a C
3
axis and a penta-substituted cp of
C
5
symmetry causes differentiation of the five p-bromophenyl
reacting 3 equiv. of 1 with a solution of Ru
reflux, the reaction mixture rapidly turned from yellow to deep
green and finally to cherry red, characteristic of a conjugated cp
3
(CO)12 in toluene at
rings connected to the cp. It is not possible to have a
symmetrical structure, and as shown on the X-ray structure (Fig.
1, ORTEP representation) there are three types of phenyl rings
considering the dihedral angle between the plane of the phenyl
ring and the plane of the cp ring. One is exactly perpendicular
(90°). Two are almost perpendicular (81°), those are the groups
which are not located above indazole. Due to steric hindrance
with the tripodal ligand, the remaining two phenyl rings have to
be tilted at 35° and 36° to minimize these interactions. This
dissymmetry has been exploited to study the dynamics of the
molecule.
2 2 2
radical. Column chromatography (SiO : cyclohexane–CH Cl
40%) gave 2 as a yellow solid in 78% isolated yield. Ligand 1
1
and complex 2 were fully characterized.† H-NMR spectros-
copy clearly shows an AAABBA pattern and in the IR spectrum of
2
1
2
the carbonyl bands at 2003 and 2048 cm are similar in
5
5 5
(h –C H
)].5
frequency to those of [RuBr(CO)
Coordination of the scorpionate ligand was achieved one pot
by substitution of the bromide ligand and CO displacement by
2
2 3
Scheme 1 (a) Br , 20 °C, 2h, 98%; (b) Ru (CO)12, 2 h, Ar, reflux, 78%; (c) Potassium trisindazolylborate, THF, 24 h, Ar, reflux, 30%.
2
434
CHEM. COMMUN., 2003, 2434–2435
This journal is © The Royal Society of Chemistry 2003

This organometallic system is particularly well suited for the
preparation of a single molecule nanomotor. Free rotation
around the ruthenium(II) centre allows movement of the
cyclopentadienyl upper platform (rotor) with respect to the
tripodal ligand (stator) which is intended to be deposited on a
surface after proper functionalisation. Work is underway to
control the direction of the rotation.
We thank the CNRS, the European Union and the University
of Toulouse for financial support and the French Ministry of
National Education for a fellowship to AC.
Notes and references
+
+
1
†
(
1: MS (DCI /NH
250 MHz, CD Cl
NMR (66 MHz, CDCl
3
): m/z 1037 (100%, [M+NH
): d (ppm) = 7.2–7.4 (m, 12H), 6.7–6.9 (m, 8H); 13C-
) d (ppm) = 147.74; 140.86; 134.00; 132.48; 132.06;
31.83; 131.74; 131.47; 131.41; 131.14; 129.09; 127.95; 122.45; 122.24;
4
] , calc 1037) ; H-NMR
2
2
3
1
1
(
+
+
22.15. 2: MS (DCI /NH
KBr/cm ) 2003, 2048 ; H-NMR (250 MHz, CD
3
1
): m/z 1096 (100%, [M+NH
4
] , calc 1096); IR
): d (ppm) = 7.2–7.3
21
2
Cl
2
Fig. 2 1H-NMR (CD
Cl , 250 MHz) spectra of 3 (bottom) compared to
2 2
3
3
13
(
AAABBA, 10H, J = 8.7 Hz), 6.8–6.9 (AAABBA, 10H, J = 8.7 Hz); C-
) d (ppm) = 196.40; 134.45; 132.22; 128.61; 124.03;
4Bo
Ru(cp)(Tp ) (top) at 25 °C.
NMR (66 MHz, CDCl
3
+
+
1
1
06.02. 3: MS (DCI /NH
NMR (250 MHz, CD Cl ): d (ppm) = 8.01 (dd, 3H, J = 8.6 Hz, J = 0.8
Hz), 7.86 (d, 3H, J = 0.8 Hz), 7.34–7.44 (m, 6H); 7.17–7.27 (m, 20H) ,
3
): m/z 1305 (100%, [M+H] , calc 1305) ; H-
1
The H-NMR spectrum of 3 (Fig. 2) shows that, just like in
3
4
2
2
the solid state, the two ligands also fit into each other in
solution. Compared to the spectrum with the non substituted
4
3
3
4
13
7.02 (ddd, 3H, J = 7.1 Hz, J = 6.9 Hz, J = 0.8 Hz); C-NMR (66 MHz,
CDCl ) d (ppm) = 143.60; 140.42; 135.27; 132.27; 130.78; 126.77; 123.07;
cyclopentadienyl ligand : Ru(cp)(Tp4Bo) which we have also
3
1
21.99; 120.70; 120.06; 111.58; 87.20. CV (V/SCE) ERuII/RuIII +0.78 rev.
synthesised, the spectrum of 3 shows striking differences.
Protons 1 and 2 (see numbering scheme on Fig. 3) are
significantly shielded from 8.62 to 7.86 ppm for protons 1 and
from 7.60 to 7.39 ppm for protons 2. This shielding is explained
by the location of these protons in the shielding cones of the p-
bromophenyl rings, evidencing the two ligands fitting into each
other. The rotational freedom of the cp ring has been evidenced
UV/Vis (CH Cl ): lmax (e) 295 (300 300), 311 (268 700), 396
2
2
=
(34 800).
1
2
Technomimetic Molecules part 1 see : G. Jimenez-Bueno and G.
Rapenne, Tetrahedron Lett., 2003, 44, 6261.
(a) T. C. Bedard and J. S. Moore, J. Am. Chem. Soc., 1995, 117, 10662;
(b) K. Tashiro, T. Fujiwara, K. Konishi and T. Aida, Chem. Commun.,
1998, 1121; (c) M. Ikeda, M. Takeuchi, S. Shinkai, F. Tani, Y. Naruta, S.
Sakamoto and K. Yamaguchi, Chem. Eur. J., 2002, 8, 5542.
S. Trofimenko, Scorpionates : The Coordination Chemistry of Poly-
pyrazolylborate Ligands, Imperial College Press, London, 1999.
R. S. Koefod and K. R. Mann, Inorg. Chem., 1991, 30, 541.
N. G. Connelly and I. Manners, J. Chem. Soc., Dalton Trans., 1989, 30,
1
by H-NMR since the three indazolyl groups are equivalent as
shown on Fig. 2. The p-bromophenyl rings are also free to
rotate, as evidenced by the chemical equivalence of the two
ortho protons (H ) and of the two meta protons (H ). These
o m
3
4
5
protons are referred to as an AAABBA spin system. Variable
1
temperature H-NMR from 290 °C to 120 °C did not show any
2
83–288.
significant differences in terms of equivalence of protons. The
rotation barrier could therefore not be measured and must be
6
(a) A. L. Rheingold, B. S. Haggerty, G. P. A. Yap and S. Trofimenko,
Inorg. Chem., 1997, 36, 5097; (b) C. Janiak, S. Temizdemir and S.
Dechert, Inorg. Chem. Commun., 2000, 3, 271.
2
1
low ( < 10 kcal.mol ).
If we confront these NMR data with the X-ray structure
which showed that the two ligands were interpenetrated, the
rotation of the cp ring should only be possible if the p-
bromophenyl groups tipped over to settle in the vacant spaces of
the tripodal ligand. In Fig. 3. is represented the secondary
rotation of the p-bromophenyl rings (action 2) induced by the
rotation of the upper cp ligand (action 1).
7 Crystal structure analysis of 3: orange prismatic crystals suitable for X-
ray analysis (0.344 3 0.197 3 0.087 mm) were obtained by dissolution
of the compound in benzene and slow liquid diffusion of methanol.
5 6 6 6
Crystal data for C56H36BBr N Ru·2C H : M, = 1460.51, orthorhombic,
space group Pbca, a = 14.271 (2), b = 21.144 (7), c = 40.875 (5), V =
3
23
21
1
2333 (5) Å , Z = 8, rcalc = 1.573 g cm , m(MoK
a
) = 3.48 mm
.
Data were collected on a Nonius KappaCCD diffractometer using MoK
a
graphite monochromated radiation (l = 0.71073 Å) at 298 K. A total of
3381 data was collected, 0° < q < 28.00°. The diffraction power of the
8
crystals was poor, only 2938 reflections having I > 3s(I), were used for
structure determination and refinement, 16% of the unique data was
observed. Only Ru and Br atoms were refined anisotropically. H atoms
were introduced as fixed contributors. The structure was solved using
direct methods and refined against ¡F¡. Absorption corrections were
applied. Large residual electron density peaks have been obtained in the
refinement which are located on the Ru atom. For all computations the
Brücker maXus software package8 was used. Final results : R(F) =
0
.088, Rw(F) = 0.139, GoF = 4.39. CCDC 207452. See http://
www.rsc.org/suppdata/cc/b3/b307577j/ for crystallographic data in .cif
or other electronic format.
8
S. Mackay, C. J. Gilmore, C. Edwards, N. Stewart and K. Shankland,
maXus, Computer program for the solution and refinement of crystal
structures; Nonius, Delft, The Netherlands; MacScience, Japan, the
University of Glasgow, 1999.
Fig. 3 Representation of the secondary rotation: the rotation of the upper cp
ligand (action 1) results in the paddles tipping over (action 2). The
1
numbering scheme for the H-NMR is also given.
CHEM. COMMUN., 2003, 2434–2435
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