Synthesis of a photoswitchable azobenzene-functionalized tris(indazol-1-yl) borate ligand and its ruthenium(II) cyclopentadienide complex

Article abstract of DOI:10.1016/j.tet.2010.01.024

In this paper, the design and synthesis of a new indazole bearing a photoisomerizable fragment at its 4-position are presented as well as the photoisomerization studies on both the indazole precursor and the final ruthenium model complex. It was obtained after five steps, the last one being the cleavage of the indazole protecting group. Reaction of 1 equiv of this functionalized indazole with 2 equiv of plain indazole and dibromophenylborane gave access to a mixture of four tripodal ligands of the tris(indazolyl)borate family. In a last step, complexation of this mixture with [RuCp(CH₃CN)₃]PF₆ yielded the corresponding ruthenium complexes from which the target ruthenium complex coordinated to a dissymmetric azobenzene-functionalized tripodal ligand was successfully isolated. Photoisomerization occured reversibly upon irradiation with UV light at 365 nm.

Full text of DOI:10.1016/j.tet.2010.01.024

Tetrahedron 66 (2010) 1885–1891
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Tetrahedron
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Synthesis of a photoswitchable azobenzene-functionalized tris(indazol-1-yl)
borate ligand and its ruthenium(II) cyclopentadienide complexq
a
a
a,b,
*
´
¨
Henri-Pierre Jacquot de Rouville , Damien Villenave , Gwenael Rapenne
a CNRS, CEMES (Centre d’Elaboration des Mate´riaux et d’Etudes Structurales), BP 94347, 29 rue J. Marvig, F-31055 Toulouse, France
b Universite´ de Toulouse, UPS, 29 rue J. Marvig, F-31055 Toulouse, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, the design and synthesis of a new indazole bearing a photoisomerizable fragment at its
4-position are presented as well as the photoisomerization studies on both the indazole precursor and
the final ruthenium model complex. It was obtained after five steps, the last one being the cleavage of the
indazole protecting group. Reaction of 1 equiv of this functionalized indazole with 2 equiv of plain
indazole and dibromophenylborane gave access to a mixture of four tripodal ligands of the tris(inda-
zolyl)borate family. In a last step, complexation of this mixture with [RuCp(CH3CN)3]PF6 yielded the
corresponding ruthenium complexes from which the target ruthenium complex coordinated to a dis-
symmetric azobenzene-functionalized tripodal ligand was successfully isolated. Photoisomerization
occured reversibly upon irradiation with UV light at 365 nm.
Received 6 October 2009
Received in revised form 4 January 2010
Accepted 5 January 2010
Available online 11 January 2010
Keywords:
Diazobenzene
Scorpionate ligand
Cyclopentadienide ligand
Dissymmetric ligand
Ruthenium
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
a penta(ethynyl ferrocenyl)phenylcyclopentadienyl rotor and
a hydrotris (indazol-1-yl) borate stator (Fig.1). Our current goal is to
integrate an additional functionality in the molecule by introducing
an isomerizable fragment in the backbone of the tripodal ligand.
With this aim in view, a dissymmetric tripodal ligand was designed
and synthesized, one indazole being functionalized by an azo-
benzene fragment.
In the past 10 years, chemists have been able to build more and
more complex molecular architectures following a bottom-up
strategy.1 This approach consists to synthesize molecules and to use
them as elementary building blocks to construct multifunctional
nanomachines.2 Among others, biomimetic or technomimetic3
nanomachines like molecular gears,4 molecular muscles,5 nano-
wheels,6 nanovehicles,7 wheelbarrows,8 molecular rotors,9 mo-
tors10 and turnstiles11 are the archetypes of such as molecular
machinery. They provide a basis for the future design of bottom-up
nanoscale systems and materials to perform tasks as varied as
nanoscale manipulation, information storage, molecular electron-
ics and mechanics. Study of the features of such molecules could be
achieved with the help of sophisticated tools such as close field
microscopies (STM and NC-AFM), which are able to image and
manipulate single molecular objects on atomically-controlled sur-
faces.12 In this perspective, a family of molecular motors was
designed and developed in the group to study the possibility to
control and exploit a controlled rotation on a surface.13 These
molecules belong to a family of half-sandwich ruthenium com-
plexes, in which the metallic atom acts as a joint between
Since the discovery of the tris(pyrazolyl)borate ligand in the late
60s by Trofimenko,14 several structural modifications have been
developed to such tripodal ligands by adjustment of the pyrazole or
the indazole rings.15 Electronic properties and steric hindrance
could be easily modified through the modulation of the azole
building blocks. In our family of molecular motors, a series of
tris(indazolyl)borate ligands bearing functional groups at the
6-position of each indazole was recently described.16 Ester func-
tions were introduced to anchor the molecule by chemisorption to
an oxide surface, while functionalization by thioethers is expected
to allow covalent attachment of the ruthenium complexes on me-
tallic surfaces. Moreover, such molecular motors were designed in
order to be placed between two electrodes of a nanojunction. These
molecules have ferrocenyl electroactive groups connected to
a central cyclopentadiene (Cp) ring. Each of them will be the siege
of successive oxidoreduction processes inducing the rotation of the
upper part (the Cp ligand) compared to the lower part (the tripodal
ligand). In this paper we present the integration of a photo-
switchable azobenzene fragment aimed at acting as a light-driven
brake to control the rotor’s speed of rotation.
q
In memory of Christiane O. Dietrich-Buchecker (1942–2008).
* Corresponding author. Tel.: þ33 5 62 25 78 41; fax: þ33 5 62 25 79 99.
E-mail address: rapenne@cemes.fr (G. Rapenne).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.01.024

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H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
Figure 1. Simplified isomerization mechanism of the target cyclopentadienyl ruthenium(II) complex. In the Z conformation, rotation of the Cp ring would be blocked due to the
insertion of the azobenzene between two arms of the rotor. Irradiation of the diazobenzene fragment and isomerization to the E conformation releases the brake and allows
rotation.
Azobenzenes are well-known to photoisomerize reversibly from
the Z to the E conformation, both isomerization processes occurring
under irradiation at different wavelengths. Isomerization from the
E to the Z form occurs under UV irradiation, whereas the reverse
transformation follows irradiation in the visible range. Besides, the
Z/E conversion is also thermally triggered. The most likely
mechanism for this isomerization is presented in Figure 1.17 This
diagram illustrates the bistability of the azobenzene fragment.
Nevertheless, both isomers always coexist due to the photosta-
tionary equilibrium, the minor one being present in a non-negli-
gible ratio. Isomerization from one conformation to the other by
establish if the stator is properly (i.e., covalently) fixed on the
surface.19
2. Results and discussion
2.1. Molecular modelling
Once the functional brake fragment had been chosen, the
question of the best position to incorporate it in the design was
addressed by molecular modelling. Theoretical calculations on the
Z and E isomers substituted at the 4- and 5-position of the indazole
showed that in only the E isomer substituted at the 4-position, the
rotation is effectively blocked by an efficient steric hindrance (Fig. 2
top left) since the diazobenzene fragment fits between two phe-
nylethynylferrocenyl substituents of the pentaphenylcyclopenta-
dienyl ligand. On the contrary, the rotor appears to be free to rotate
in the Z conformation, this geometry being reached after UV irra-
diation (Fig. 2, top). The calculations also showed (Fig. 2 bottom
left) that the functionalization at the 5-position is inefficient with
respect to locking the rotation, the steric hindrance between the
azobenzene fragments and the pentaphenylcyclopentadiene frag-
ments being totally supressed.
light excitation occurs via a pN]N
/
pN]N transition through an
*
excited state SN. Relaxation follows a non-radiative pathway due to
several intersystem crossings18 and isomerization occurs via rota-
tion around the N–N bond.
According to the geometry of the N]N bond and the steric
hindrance it induces, control of the rotation in the target molecular
motor is expected to occur as shown in Figure 1. For that purpose an
azobenzene fragment was introduced in the design. In this paper,
the synthesis and characterization of a dissymmetrized tris(inda-
zolyl) borate ligand bearing an azobenzene substituent and its
cyclopentadienyl ruthenium(II) complex is described, as well as the
photoisomerization studies on the azobenzene-functionalized
indazole and on the model ruthenium complex.
2.2. Retrosynthetic analysis
Moreover, a tripodal ligand with one functionalized indazole is
also of special interest for subsequent studies on surfaces since it
presents the advantage to integrate a tag. One indazole ring being
different from the two unfunctionalized indazoles, it enables to
Two retrosynthetic approaches were considered for the syn-
thesis of an indazole bearing an azobenzene substituent (Scheme 1).
The first one envisages to form the diazo bridge first and the

H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
1887
Figure 2. Top: azobenzene fragment connected to the 4-position of the indazole ring (left the E isomer, right the Z isomer). Bottom: azobenzene fragment connected to the
5-position of the indazole ring (left the E isomer, right the Z isomer). Clearly, the E azobenzene fragment connected at the 4-position (top left) is the most suitable isomer to lock the
rotation. Whereas in the 5-position, the photoisomerizable group does not have sufficient interaction with the rotor (upper ligand in red).
indazole ring subsequently. The second one proceeds in the reverse
order. Purification is expected to be facilitated in the first case be-
cause pyrazole and indazole derivatives show strong H-bonding
interactions with silica gel, which usually renders their chromato-
graphic separation tedious.
steps of the synthesis, such as O-coordination to the boron atom
during the tripode formation step but also the tautomeric equi-
librium between the phenol and the quinone–hydrazone form.21
An overall yield of 31% was obtained for these two steps. Reduction
of the nitro functional group to amino failed using various condi-
tions such as classical hydrogenation catalyzed with Pd/C, tin
chloride in stoichiometric amount or tin(0) in acidic conditions.22
Many side reactions involving the diazo bridge could explain this
result since reduction of the diazo group to form hydrazine de-
rivatives has been observed, but one could also imagine a reduction
to an oxime or Mills’ reaction.23
2.3. Synthesis of the azobenzene-functionalized indazole
The second synthetic route shown in Scheme 3 yielded the
desired indazole 7. The synthesis of the indazole functionalized
with an azobenzene fragment was achieved in five steps. The
indazole ring was formed in a first step according to Jacobson’s
procedure.24 1-Methyl-2-nitro-aniline was reacted with acetic
anhydride and potassium acetate followed by isoamylnitrite,
yielding the desired protected indazole (3). It is interesting to
note that cleavage of the N-acetyl protecting group by hydro-
chloric acid was not performed as it is usually done. This presents
the advantages to mask the nucleophilicity of the indazole
fragment for further steps and to ease the purification by column
chromatography by avoiding the presence of a free NH group,
which would strongly interact with the stationary phase.
N-Acetyl-4-nitro-indazole (3) was obtained in 60% yield. A cat-
alytic hydrogenation was then performed under a hydrogen at-
mosphere using Pd/C as catalyst, which gave access to 4 with
a 82% yield. The next step involved the formation of the azo-
benzene bridge. Compound 4 was first reacted with sodium ni-
trite and hydrochloric acid to give the corresponding diazonium
salt. Subsequently, an aqueous solution of phenolate was added
to yield the diazobenzene-functionalized indazole (5) with
a satisfying 78% yield. As previously described (vide supra), the
phenol function was methylated to avoid any possible side-re-
action of the phenolic proton. The methylated derivative 6 was
thus obtained in 74% yield. The last step consisted in the selective
cleavage of the N-acetyl protective group of the indazole ring
Scheme 1. Retrosynthetic analysis for the formation of the diazobenzene-functional-
ized indazole. The first synthetic route introduces the diazo bridge at the beginning of
the strategy whereas the second one creates the indazole ring first.
As shown in Scheme 2, compound 2 was obtained in two steps.
Among all the possible ways to create the diazo bridge, a phenolic
coupling under basic conditions between 1-methyl-2-nitroaniline
and phenol was selected.20
Scheme 2. First synthetic route for the formation of the functionalized indazole. a)
NaNO2, HCl, H2O/acetone (1/1), 15 min, 0 ꢀC; b) PhOH, NaOH, Na2CO3, 2 h, 0 ꢀC, 59%; c)
MeI, K2CO3, toluene, 3 h, 65 ꢀC, 53%.
First, conversion of the amino functional group to the corre-
sponding diazonium salt was achieved, followed by an electrophilic
substitution with phenolate. After acidic work-up, the phenol was
methylated via a nucleophilic substitution using methyliodide un-
der basic conditions. This protection was performed to avoid un-
wanted reactions of the phenolic protons during the subsequent
performed quantitatively by action of
a 0.01 M of sodium

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H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
Scheme 3. Synthesis of 4-(diazenyl(4-methoxyphenyl))indazole (7). a) AcOK, Ac2O, toluene, 15 min, 25 ꢀC; b) isoamylnitrite, toluene, reflux, overnight, 60%; c) H2, Pd 10%/C, THF,
12 h, 25 ꢀC, 82%; d) NaNO2, HCl, H2O/acetone (1/1), 15 min, 0 ꢀC; e) PhOH, NaOH, Na2CO3, 2 h, 0 ꢀC, 78%; f) MeI, K2CO3, THF, overnight, 65 ꢀC, 74%; g) MeONa, MeOH, 20 min, 25 ꢀC,
98%.
methoxide solution.25 In summary, the synthesis of the indazole
integrating an azobenzene function (7) was realized in five steps
with a global yield of 28%.
2.4. Synthesis of the tripodal ligand: first attempt
The synthesis of the target tripodal ligand was first attempted fol-
lowing the method developed by Trofimenko for tris(pyr-
azolyl)borates. This strategy is based on the thermal control of the
number of coordinated pyrazoles on the boron centre: coordination of
two pyrazoles requires heating with KBH4 at 120 ꢀC without any sol-
vent. A temperature of 180 ꢀC allows the coordination of a third pyr-
azole and finally a fourth pyrazole at a temperature above 220 ꢀC.
However following the procedure described in the literature for the
synthesis of dihydrobis(pyrazolyl) borate, formation of dihydrobis
(indazolyl)borate (8)couldnotbeachieved.26 In the indazole series, the
optimized preparation of potassium dihydrobis(indazolyl)borate in-
volved heating 4 equivof indazolewith KBH4 at170 ꢀC(Scheme4)until
the expected volume of dihydrogen had been evolved.
Scheme 5. One pot synthesis of potassium phenyl-(4-(p-methoxyphenyl) (dia-
zenyl)indazolylbis(indazolyl)borate and formation of its cyclopentadienyl ruth-
enium(II) complex. a) 7 and indazole (1/2), benzene, NEt3, 12 h, 25 ꢀC; b) tBuOK, THF/
benzene (1/1), 12 h, Ar, 60 ꢀC; c) [RuCp(NCCH3)3]PF6, MeCN, 12 h, Ar, reflux, 3%.
1 equiv of the azobenzene-functionalized indazole (7) in the presence of
2 equiv of triethylamine. It is important to note that a higher proportion
of base led to an unidentified product. After filtration under argon,
a solution of 1 equiv of potassium tert-butoxide was added to the me-
dium. After evaporation of the solvents, the mixture of the corre-
sponding tripodes was extracted with diethylether. Their presence was
confirmed by mass spectrometry, which presented a clear spectrum
with only four signals corresponding to the statistical mixture of the four
possible tripodal ligands: phenyltris(indazolyl) borate, the desired phe-
nyl-(4-(p-methoxyphenyl)(diazenyl)indazolyl)bis(indazolyl)borate, ph-
enylbis((4-(p-methoxyphenyl)diazenyl) indazolyl)indazolylborate and
phenyltris(4-(p-methoxy phenyl)diazenyl)indazolyl borate as potas-
sium salts. At this stage, the relative ratio of the four tripodes could not be
established by 1H NMR due to the superimposition of broad multiplets,
the molecules being highly dissymmetric. Moreover, no separation was
possible through classical chromatographic techniques due to the an-
ionic nature of the molecules.
Coordination to the ruthenium centre allowed to separate the
desired coordinated tripodal ligand. After coordination, the
complexes are neutral, which facilitates the chromatographic
separation. Cyclopentadienyltris(acetonitrile)ruthenium(II) hexa-
fluorophosphate was chosen as ruthenium precursor since it gave
excellent results in this family of sterically hindered Cp ring in past
studies.28 After heating under reflux in acetonitrile, the medium
turned dark green and after purification, the desired green complex
was characterized by 1H NMR, 13C NMR, mass spectrometry and UV–
vis spectroscopy. The absorption spectrum showed two major bands
characteristic of the azobenzene fragment at 295 and 384 nm. In
addition, the ruthenium-based transition is expected to be found
around 390 nm as described in analogous complexes,16 and is
therefore most probably hidden beneath the 384 nm transition. This
could be explained because the azobenzene fragment is a stronger
chromophore than the metal-centred chromophore.
Scheme 4. Synthesis of potassium dihydrobis(indazolyl)borate and attempt of for-
mation of the corresponding potassium hydro-4-((p-methoxyphenyl)diazenyl)
indazolylbis(indazolyl)borate. a) 4 equiv Indazole, KBH4, 170 ꢀC, argon, 3 h; b) 7, 180 ꢀC,
Ar, 3.5 h.
It is obvious to note that the temperature should be precisely
monitored to avoid the formation of hydrotris(indazolyl)borate,
which could be initiated at such temperature.
Since column chromatography is totally inefficient for such
compounds due to the strong and similar interactions of the dif-
ferent borate ligands with the stationary phase (silica or alumina),
the desired compound was purified by sublimation and then trit-
uration in hot toluene. This work-up protocol allowed isolation of
potassium dihydrobis(indazolyl) borate 8, as confirmed by 1H NMR
analysis. The synthesis of the dissymmetric tripodal ligand was
attempted by mixing 7 and 8 (1/1) in a mortar and heating this
homogeneous powder at 180 ꢀC during 3.5 h under an argon at-
mosphere. The medium turned black and an evolution of gas was
observed but the desired compound was not present in the
mixture.
2.6. Photoisomerization studies
2.5. Alternative synthesis of the tripodal ligand and its
cyclopentadienyl ruthenium(II) complex
The isomerization reactions were achieved and monitored by
UV–vis spectroscopy on the azobenzene-functionalized indazole
and on the model ruthenium complex. The azobenzene moieties in
both molecules undergo reversible photoisomerization upon irra-
diation with UV light at 365 nm (E/Z). The photostationary state is
obtained after 10 min for the free indazole 7 and after 15 min for
the ruthenium complex 9. Figure 3 shows the UV–vis spectral
changes.
An alternative strategy presented in Scheme 5 was finally employed
to synthesize the tripodal ligand. It was based on the statistical reaction
of two different types of indazoles with phenyldibromoborane as boron
precursor.27 This compound was obtained by reaction of boron tri-
bromide on trimethylsilylbenzene via an electrophilic substitution at the
ipso-position. Then, 2 equiv of indazole were statistically reacted with

H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
1889
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 Avance 300 or Avance
500 spectrometers and full assignments were made using COSY,
ROESY, HMBC and HMQC methods. Chemical shifts are defined
with respect to TMS¼0 ppm for 1H and 13C NMR spectra and were
measured relative to residual solvent peaks. The following abbre-
viations have been used to describe the signals: s for singlet; d for
doublet; t for triplet; q for quadruplet; dd for doublet of doublet, dt
for doublet of triplet, td for triplet of doublet, m: for multiplet. The
numbering scheme used in this article is given in Scheme 2 (mol-
ecule 2), Scheme 3 (molecule 6) and Scheme 5 (molecule 9). FAB
and DCI mass spectrometry were performed using a Nermag R10-
10. UV spectrum was performed with a Varian 5000 UV–vis–NIR
spectrometer. All geometries were minimized with Cerius2 soft-
ware29 (version 4.2) using the Universal Force Field 1.02.
4.2. Photoisomerization studies
Photoirradiation experiments were performed on solutions of
the sample dissolved in dichloromethane of 2ꢁ10ꢂ4 mol Lꢂ1 (for
ligand 7) and 4ꢁ10ꢂ4 mol Lꢂ1 (for complex 9). The solutions were
irradiated in a quartz cuvette with a 6 W 365 nm lamp (Vilber
Lourmat VL-6.LC).
4.3. Synthesis
4.3.1. 4-(2-(2-Methyl-3-nitrophenyl)diazenyl)phenol (1). To a solu-
tion of 1-methyl-2-nitroaniline (1 g, 6.56 mmol, 1 equiv) in a water/
acetone mixture (20 mL, 1/1), was added concentrated HCl (2 mL)
Figure 3. UV–vis spectral changes (upon irradiation with 365 nm light) of dichloro-
methane solutions in of 7 (top, 2ꢁ10ꢂ4 mol Lꢂ1 after 0, 15 s, 30 s, 1 min, 5 min and
10 min) and 9 (bottom, 4ꢁ10ꢂ4 mol Lꢂ1 after 0, 15 s, 30 s, 1 min, 5 min, 10 min and
15 min).
at 0 ꢀC. After 15 min.,
a solution of sodium nitrite (0.86 g,
12.5 mmol, 1.9 equiv) in water (20 mL) was added slowly. After
stirring for 15 min, phenol (0.62 g, 6.56 mmol, 1 equiv), sodium
hydroxide (0.40 g, 10 mmol, 1.5 equiv) and sodium carbonate (1 g,
9.43 mmol, 1.4 equiv) dissolved in water (20 mL) were added
dropwise and the mixture was stirred at 0 ꢀC for 2 h. After con-
centration of the filtrate, the crude product was purified on column
chromatography (SiO2, CH2Cl2). 1 was obtained as a yellow solid in
59% yield. MS (DCI/CH4): 258.09 ([MþH]þ, calcd 258.08); 1H NMR:
(300 MHz, CD2Cl2) 7.92 (2H, m, Ho); 7.82–7.85 (2H, m, Hb–d); 7.42
(1H, t, J¼9 Hz, Hc); 7.01 (2H, m, Hm); 5.50 (1H, s, OH); 2.85 (3H, s,
3. Conclusion
Synthesis of a new half-sandwich ruthenium complex equipped
with an azobenzene-functionalized tripodal ligand to act as a brake
for the full size Cp rotor (represented in Fig. 1) has been presented.
The key intermediate was the azobenzene-functionalized indazole,
which was prepared in five steps with easy and efficient chro-
matographic purification thanks to the introduction of a protecting
group on the indazole ring. The tripodal ligand was obtained by
a statistical reaction of phenyldibromoborane with 2 equiv of
indazole and 1 equiv of 4-((p-methoxyphenyl)diazenyl)indazole.
The model ruthenium complex bearing an azobenzene moiety in
the 4-position of its indazole ligand undergoes UV-induced E/Z
photoisomerization. Work is now underway to combine this brake-
integrated tripodal ligand and the pentaferrocenyl substituted Cp
ruthenium complex to obtain a fully equipped molecular motor
able to switch from a stationary state to a rotary motion with light.
Ha); 13C NMR (CDCl3)
d: 159.0; 151.8; 151.2; 147.2; 131.9; 126.5;
125.5; 125.3; 120.0; 115.95; 13.0.
4.3.2. 1-(p-Methoxyphenyl)-2-(2-methyl-3-nitrophenyl)diazene
(2). Dried potassium carbonate (8 g, 57.9 mmol, 30 equiv) and 4-(2-
(2-methyl-3-nitrophenyl)diazenyl)phenol (1) (500 mg, 1.94 mmol,
1 equiv) were heated at 65 ꢀC in DMF (25 mL). A solution of iodo-
methane (180 mL, 2.91 mmol, 1.5 equiv) in DMF (50 mL) was added
dropwise. After 3 h at 65 ꢀC, the red mixture was poured in ice and the
salts were filtered. After evaporation of the solvent from the filtrate,
the crude product was purified by column chromatography (SiO2,
CH2Cl2/cyclohexane 3/7). Compound 2 was obtained as an orange
solid in 53% yield. MS (DCI/NH3): 272.4 ([MþH]þ, calcd 272.09); 1H
4. Experimental section
4.1. General
NMR: (300 MHz, CD2Cl2)
d 7.94 (2H, m, Ho); 7.87 (1H, m, Hb–d); 7.81
(1H, m, Hb–d); 7.38 (1H, dd, Hc); 7.01 (2H, m, Hm); 3.89 (3H, s, OCH3);
2.84 (3H, s, Ha); 13C NMR: (75 MHz, CD2Cl2)
d: 162.9; 151.8; 151.1;
All commercially available chemicals were of reagent grade and
were used without further purification. 1-Methyl-2-nitroaniline,
isoamylnitrite, phenol and methyliodide were purchased from
Aldrich. Cyclopentadienyl tris(acetonitrile)ruthenium(II) hexa-
fluorophosphate was purchased from STREM chemicals. Indazole,
boron tribromide (1 M in dichloromethane) and trimethylsi-
lylbenzene were purchased from Acros. Diethylether and THF were
dried over sodium with benzophenone and dichloromethane was
dried over CaH2. All reactions were carried out using standard
147.1; 131.9; 126.6; 125.3; 119.9; 114.4; 55.7; 12.8.
4.3.3. N-Acetyl-4-nitroindazole (3). Acetic anhydride (6.9 mL,
70.8 mmol, 3.6 equiv) was added slowly to a solution of 1-methyl-
2-nitroaniline (3 g, 19.7 mmol, 1 equiv) and potassium acetate
(2.01 g, 21.7 mmol, 1.1 equiv) in toluene (60 mL) at room tempera-
ture. After 15 min of stirring, isoamylnitrite (5.52 mL, 41.4 mmol,
2.1 equiv) was added dropwise and the mixture was refluxed
overnight. After evaporation of the solvent, the crude product was

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H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
purified by column chromatography (SiO2, CH2Cl2/hexane 1/1).
(1H, m, Hd); 8.03 (2H, m, Ho); 7.85 (1H, m, Hb); 7.60 (1H, m, Hc);
7.06 (2H, m, Hm); 3.92 (3H, s, OCH3); 13C NMR: (75 MHz, CDCl3)
Compound 3 was obtained as a yellow solid in 60% yield. MS (DCI/
NH3): 205.1 ([M]þ, calcd 205.1); 1H NMR: (300 MHz, CDCl3)
d
8.86
d
162.3; 147.5; 146.1; 127.3; 124.8; 123.4; 114.4; 112.5; 55.7; lmax
(1H, m, Hb); 8.80 (1H, s, Ha); 8.29 (1H, m, Hd); 7.71 (1H, t, J¼9 Hz,
(
3
) (CH2Cl2).
Hc); 2.85 (3H, s, He); 13C NMR: (75 MHz, CDCl3)
138.2; 129.1; 122.2; 121.0; 23.2.
d: 220.5; 171.3;
4.3.8. Dihydrobis(indazolyl)borate (8). Indazole (2 g, 16.93 mmol,
4 equiv) and potassium borohydride (228 mg, 4.22 mmol, 1 equiv)
were grinded in a mortar. The solid mixture was heated at 170 ꢀC
under an argon atmosphere until the expected volume of dihy-
drogen had been evolved, and the mixture was allowed to room
temperature. After sublimation and trituration in hot toluene, the
purified compound was stored in a dessicator. Compound 8 was
obtained as a white solid in 47% yield. MS (DCI/CH4): 247 ([MꢂK]ꢂ,
0
4.3.4. N-Acetyl-4-aminoindazole (4). Palladium on carbon (10%,
1.248 g, 1.17 mmol, 0.1 equiv) was added to a solution of N-acetyl-
4-nitroindazole (3) (2.4 g, 11.7 mmol, 1 equiv) in distilled THF
(250 mL) at room temperature. After stirring 12 h under a dihy-
drogen atmosphere, the mixture was filtered over Celite, evapo-
rated and purified by column chromatography (SiO2, CH2Cl2).
Compound 4 was obtained as an orange solid in 82% yield. MS (DCI
NH3): 176.09 ([MþH]þ, 100%, calcd 176.08); 1H NMR: (300 MHz,
100%, calcd 247); 1H NMR: (300 MHz, CDCl3)
d
7.33 (2H, dt, He ,
0
0
J¼8,5 Hz, J¼0.9 Hz); 7.32 (2H, d, Ha , J¼0.9 Hz); 7.08 (2H, dt, Hd ,
0
CDCl3)
d
8.07 (1H, s, Ha); 7.81 (1H, m, Hd); 7.33 (1H, m, Hc); 6.56 (1H,
J¼8 Hz, J¼1.1 Hz); 6.61 (2H, td, Hb , J¼8.5 Hz, J¼1.1 Hz); 6.38 (2H, td,
m, Hb); 4.18 (2H, s, NH2); 2.77 (3H, s, He); 13C NMR (CDCl3)
140.3; 139.9; 136.6; 131.0; 115.6 108.5; 105.6; 23.1.
d: 171.0;
Hc , J¼8.0 Hz, J¼0.9 Hz); 13C NMR: (75 MHz, CDCl3)
d: 143.3; 132.8;
0
131.5; 121.5; 120.5; 120.0; 112.1.
Microanalysis calculated for C14H12BN4K: C 58.76; H 4.23; N
19.58. Found: C 58.31; H 4.06; N 19.17.
4.3.5. N-Acetyl-4-((p-hydroxyphenyl)diazenyl)indazole
(5). Concentrated HCl (2.2 mL) at 0 ꢀC was added to a solution of
N-acetyl-4-amino-indazole (4) (1.24 g, 7.08 mmol, 1 equiv) in
a water/acetone mixture (60 mL, 1/1). A solution of sodium nitrite
(0.79 g, 13.3 mmol, 1.6 equiv) in water (20 mL) was added slowly.
After 15 min of stirring, phenol (0.67 g, 7.08 mmol, 1 equiv), sodium
hydroxide (0.43 g, 10.75 mmol, 1.5 equiv) and sodium carbonate
(1.07 g, 9.91 mmol, 1.4 equiv) dissolved in water (20 mL) were
added dropwise. The mixture was then stirred at 0 ꢀC for 2 h. After
evaporation of the solvent, the product was extracted with
dichloromethane and washed with a saturated NaCl solution. After
concentration in vacuum, the crude product was purified by col-
umn chromatography (SiO2, CH2Cl2/AcOEt 99/1). Compound 5 was
obtained as a yellow solid in 78% yield. MS (DCI/CH4): 281.09
4.3.9. Phenyl-4-((p-methoxyphenyl)diazenyl)indazolylbis(indazolyl)
borate cyclopentadienylruthenium (II) (9). In
equipped with a Teflon stopper, a 1 M solution of boron tri-
bromide in dichloromethane (39 L, 0.39 mmol, 1 equiv) was
added to trimethylsilylbenzene (68.5 L, 0.39 mmol, 1 equiv). Af-
a schlenk flask
m
m
ter 3 days at room temperature, solvent and bromotrimethylsilane
were evaporated under vacuum. The freshly synthesized dibro-
mophenylborane was dissolved in 3 mL of distilled benzene and
a solution of triethylamine (111.1 mL, 0.79 mmol, 2 equiv), indazole
(93.5 mg, 0.79 mmol, 1.5 equiv) and 4-(diazenyl(4-methox-
yphenyl))indazole (7) (100 mg, 0.36 mmol, 1 equiv) in 3 mL of
benzene was transferred via canula. The medium was stirred for
12 h at room temperature. The mixture was filtered under vacuum
to remove the colourless ammonium salt. A solution of potassium
tert-butoxide (44.4 mg, 0.36 mmol, 1 equiv) in 3 mL of THF/ben-
zene (1/1) was added and the medium was stirred for 12 addi-
tional hours under argon at 60 ꢀC. After evaporation of the
solvents, the mixture of tripodal ligands was extracted with ether.
The compounds were used without further purification. In
a schlenk flask, cyclopentadienyl tris(acetonitrile) ruthenium(II)
hexafluorophosphate (46 mg, 0.10 mmol, 1 equiv) was added un-
der argon to the mixture of tripodal ligands (65 mg, 0.10 mmol,
1 equiv) in 3 mL of anhydrous acetonitrile. The medium was stir-
red under reflux for 12 h. After evaporation of the solvent, the
crude material was purified by column chromatography
(SiO2$CH2Cl2/AcOEt 8/2). Compound 9 was obtained in 3% yield as
a green solid. MS (ESI): 740.10 ([MþH]þ, 100%, calcd 740.2); 1H
0
([MþH]þ, 100%, calcd 281.28); 1H NMR: (300 MHz, CDCl3)
d 8.80
(1H, s, Ha); 8.56 (1H, m, Hd); 7.98 (2H, m, Ho); 7.94 (1H, m, Hb); 7.71
(1H, dd, Hc); 7.02 (2H, m, Hm); 5.23 (1H, s, OH); 2.84 (3H, s, He); 13
NMR (CDCl3) : 171.4; 158.9; 149.9; 147.4; 139.7; 129.8; 125.2;
123.3; 118.2; 117.4; 115.9; 26.9; 23.2.
C
d
4.3.6. 1-Acetyl-4-((p-methoxyphenyl)diazenyl)indazole (6). Dried
potassium carbonate (23 g, 0.166 mol, 30 equiv) and N-acetyl-4-
(diazenyl(4-hydroxyhenyl))indazole (5) (1.54 g, 5.50 mmol,
1 equiv) were heated at 65 ꢀC in distilled THF (120 mL). A solu-
tion of iodomethane (1.0 mL, 16.5 mmol, 3 equiv) in distilled THF
(80 mL) was added dropwise. After stirring overnight at 65 ꢀC,
the solvent was evaporated and the crude product purified by
column chromatography (SiO2, CH2Cl2/cyclohexane 8/2). Com-
pound 6 was obtained as an orange solid in 74% yield. MS (DCI
/CH4): 295.12 ([MþH]þ, 100%, calcd 295.31); 1H NMR: (300 MHz,
NMR (CDCl3) d: 9.76 (1H, m, Ha); 7.72 (2H, m, Ha ); 7.61 (2H, m);
CDCl3)
(1H, m, Hb); 7.70 (1H, dd, Hc); 7.05 (2H, m, Hm); 3.82 (3H, s,
OCH3); 2.83 (3H, s, He); 13C NMR: (75 MHz, CDCl3)
:171.4; 162.6;
d
8.79 (1H, s, Ha); 8.54 (1H, m, Hd); 8.01 (2H, m, Ho); 7.94
7.55 (3H, m); 7.52 (3H, m); 7.35 (5H, m, BPh); 7.13 (4H, m); 6.91
(3H, m); 4.07 (5H, m, Cp); 3.91 (3H, m, OCH3) 162.8; 143.2; 139.7;
138.9; 138.7; 137.4; 137.1; 136.9; 136.2; 135.6; 134; 2122.7; 122.3;
d
147.2; 145.4; 140.2; 139.8; 129.8; 125.0; 123.2; 118.2; 117.4;
114.4; 55.6; 23.2.
119.3; 112.4; 111.0; 70.4; 36.4; 25.2; 54.7; UV lmax (3) (CH2Cl2)/nm:
261 (703), 272 (666), 295 (500), 384 (518).
4.3.7. 4-((p-Methoxyphenyl)diazenyl)indazole (7). To a solution of
sodium (780 mg, 3.41 mmol, 1 equiv) dissolved in dried methanol
(340 mL), 1-acetyl-4-(diazenyl(4-methoxyphenyl))indazole (6)
was added (1 g, 3.41 mmol, 1 equiv). After 20 min of stirring at
room temperature, the mixture was acidified to pH¼1 by addition
of aqueous HCl (5 M). After evaporation of the methanol, the
aqueous layer was extracted with dichloromethane. The organic
layers were washed with a saturated NaCl solution, dried over
sodium sulfate and then evaporated. Compound 7 was obtained as
an orange solid in 98% yield. MS (DCI /CH4): 253.10 ([MþH]þ, 100%,
Acknowledgements
This work was supported by the CNRS, the University Paul
Sabatier (Toulouse) and the European Community. H.-P.J.d.R.
thanks the French Ministry of National Education for a Ph.D. Fel-
´
lowship. Prof. J.-P. Launay and Dr. Andre Gourdon are thanked for
fruitful discussions. We also would like to thank Christine Viala for
her assistance and Garreth Thompson for preparation of starting
materials. Dr. Isabelle M. Dixon is warmly acknowledged for her
corrections and comments on this manuscript.
calcd 253.10); 1H NMR: (300 MHz, CDCl3)
d: 8.83 (1H, s, Ha); 8.54

H.-P. Jacquot de Rouville et al. / Tetrahedron 66 (2010) 1885–1891
1891
12. (a) Grill, L. J. Phys. Condens. Matter 2008, 20, 053001; (b) Carella, A.; Coudret, C.;
Guirado, G.; Rapenne, G.; Vives, G.; Launay, J.-P. Dalton Trans. 2007, 177–186 and
references therein.
13. (a) Vives, G.; Carella, A.; Sistach, S.; Launay, J.-P.; Rapenne, G. New J. Chem. 2006,
30, 1429–1438; (b) Vives, G.; Gonzalez, A.; Jaud, J.; Launay, J.-P.; Rapenne, G.
Chem.dEur. J. 2007, 13, 5622–5631; (c) Vives, G.; Jacquot de Rouville, H.-P.;
Carella, A.; Launay, J.-P.; Rapenne, G. Chem. Soc. Rev. 2009, 38, 1551–1561.
14. Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 6288–6294.
References and notes
1. Feynman, R. P. In The Pleasure of Finding Things Out; Perseus Books: Cambridge,
MA, 1999.
2. (a) Balzani, V.; Venturi, M.; Credi, A. Molecular devices and machines. In Con-
cepts and Perspectives for the Nanoworld, 2nd ed.; Wiley-VCH: Weinheim, 2008;
(b) Browne, W. R.; Feringa, B. L. Nat. Nanotech. 2006, 1, 25–35.
3. Rapenne, G. Org. Biomol. Chem. 2005, 3, 1165–1169.
15. (a) Trofimenko, S. In Scorpionates – Polypyrazolylborate Ligands and Their Co-
ordination Chemistry; Imperial College: London, 1999; (b) Trofimenko, S. Chem.
Rev. 1993, 93, 943–980; (c) Pettinari, C.; Santini, C. In Comprehensive Co-
ordination Chemistry II; Lever, A. B. P., Ed.; Elsevier: Amsterdam, 2003; Vol. 1,
pp 159–210; (d) Reger, D. L. Coord. Chem. Rev. 1996, 147, 571–595; (e) Etienne,
M. Coord. Chem. Rev. 1996, 156, 201–236; (f) Jacquot de Rouville, H.-P.; Vives, G.;
Tur, E.; Crassous, J.; Rapenne, G. New J. Chem. 2009, 2, 293–299.
16. Carella, A.; Vives, G.; Cox, T.; Jaud, J.; Rapenne, G.; Launay, J.-P. Eur. J. Inorg.
Chem. 2006, 980–987.
17. Zimmermann, G.; Chow, L.-Y.; Paik, U.-J. J. Am. Chem. Soc. 1958, 80, 3528–3531.
18. Conti, I.; Garavelli, M.; Orlandi, G. J. Am. Chem. Soc. 2008, 130, 5216–5230.
19. Vives, G.; Rapenne, G. Tetrahedron 2008, 64, 11462–11468.
20. Yagai, S.; Iwashima, T.; Kishikawa, K.; Nakahara, S.; Karatsu, T.; Kitamura, A.
Chem.dEur. J. 2006, 12, 3984–3994.
21. Luboch, E.; Wagner-Wysiecka, E.; Rzymowski, T. Tetrahedron 2009, 63,
10671–10678.
22. Schartz, M. K.; Tumelty, D.; Gallop, M. A. J. Org. Chem. 1999, 64, 2219–2231.
23. March, J. In Advanced Organic Chemistry, 4th ed.; Wiley: New York, NY, 1992;
p 818.
24. Ruechardt, C.; Hassmann, V. Synthesis 1972, 7, 375–376.
25. Castro-Palomino, J. C.; Schmidt, R. R. Tetrahedron Lett. 1995, 36, 6871–6874.
26. (a) Rheingold, A. L.; Brian, B. S.; Yap, G. P. A.; Trofimenko, S. Inorg. Chem. 1997,
36, 5097–5103; (b) Craven, E.; Mutlu, E.; Lundberg, D.; Temizdemir, S.; Dechert,
S.; Brombacher, H.; Janiak, C. Polyhedron 2002, 21, 553–562.
27. Reger, D. L.; Gardinier, J. R.; Smith, M. D.; Shahin, A. M.; Long, G. J.; Rebbouh, L.;
Grandjean, F. Inorg. Chem. 2005, 44, 1852–1866.
4. (a) Chance, J. M.; Geiger, J. H.; Mislow, K. J. Am. Chem. Soc. 1989, 111, 2326–2327;
(b) Clayden, J.; Pink, J. H. Angew. Chem., Int. Ed. 1998, 37, 1937–1939; (c) Carella,
A.; Launay, J.-P.; Poteau, R.; Rapenne, G. Chem.dEur. J. 2008, 14, 8147–8156; (d)
Chiaravalloti, F.; Gross, L.; Rieder, K. H.; Stojkovic, S.; Gourdon, A.; Joachim, C.;
Moresco, F. Nat. Mater 2007, 6, 30–33.
5. (a) Jimenez-Molero, M. C.; Dietrich-Buchecker, C. O.; Sauvage, J.-P. Angew. Chem.,
Int. Ed. 2000, 39, 3284–3287; (b) Barboiu, M.; Vaughan, G.; Kyritsakas, N.; Lehn,
J.-M. Chem.dEur. J. 2003, 9, 763–769.
6. (a) Grill, L.; Rieder, K. H.; Moresco, F.; Rapenne, G.; Stojkovic, S.; Bouju, X.;
Joachim, C. Nat. Nanotech. 2007, 2, 95–98; (b) Schmittel, M.; Mal, P. Chem.
Commun. 2008, 960–962.
7. Vives, G.; Tour, J. M. Acc. Chem. Res. 2009, 42, 473–487 and references therein.
8. Rapenne, G.; Jimenez-Bueno, G. Tetrahedron 2007, 63, 7018–7026.
9. (a) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev. 2005, 105,
1281–1376; (b) Leigh, D. A.; Zerbetto, F.; Kay, E. R. Angew. Chem., Int. Ed. 2007,
46, 72–191.
10. (a) Koumura, N.; Zijistra, R. W. J.; Van Delden, R. A.; Harada, N.; Feringa, B. L.
Nature 1999, 401, 152–155; (b) Kelly, T. R.; De Silva, H.; Silva, R. A. Nature 1999,
401, 150–152; (c) Tashiro, K.; Konishi, K.; Aida, T. J. Am. Chem. Soc. 2000, 122,
7921–7926; (d) Ikeda, M.; Takeuchi, M.; Shinkai, S.; Tani, F.; Naruta, Y.; Saka-
moto, S.; Yamaguchi, K. Chem.dEur. J. 2002, 8, 5541–5550; (e) Ter Wiel, M. K. J.;
Van Delden, R. A.; Meetsma, A.; Feringa, B. L. J. Am. Chem. Soc. 2003, 125, 15076–
15086; (f) Horinek, D.; Michl, J. J. Am. Chem. Soc. 2003, 125, 11900–11910; (g)
Jian, H.; Tour, J. M. J. Org. Chem. 2003, 68, 5091–5103; (h) Leigh, D. A.; Wong,
J. K. Y.; Dehez, F.; Zerbetto, F. Nature 2003, 424, 174–179.
11. (a) Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662–10671; (b) Ste-
vens, A. M.; Richards, C. J. Tetrahedron Lett. 1997, 38, 7805–7808; (c) Tashiro, K.;
Fujiwara, T.; Konishi, K.; Aida, T. Chem. Commun. 1998, 1121–1122; (d) Brydges,
S.; Harrington, L. E.; McGlinchey, M. J. Coord. Chem. Rev. 2002, 75, 233–234.
28. Vives, G.; Carella, A.; Launay, J.-P.; Rapenne, G. Coord. Chem. Rev. 2008, 252,
1451–1459.
29. Accelrys, Cerius2 4.6; Accelrys: San Diego, 2001.