Ruthenium complexes of sterically-hindered pentaarylcyclopentadienyl ligands

Article abstract of DOI:10.1039/d1ra03875c

The synthesis of ruthenium complexes incorporating an overcrowded pentaarylcyclopentadienyl ligand has been investigated, and higher efficiency has been reached using chlorine-functionalised precursors when compared with their brominated counterparts. A new methodology for the preparation of chlorocyclopentadienes has been developed which is well adapted for highly sterically hindered compounds and works with either electron rich or poor systems.

Full text of DOI:10.1039/d1ra03875c

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Ruthenium complexes of sterically-hindered
pentaarylcyclopentadienyl ligands†
Cite this: RSC Adv., 2021, 11, 20207
ab
b
c
c
Ryosuke Asato,
Tsuyoshi Kawai,
Colin J. Martin,
Claire Kammerer
Yohan Gisbert,
Seifallah Abid,
ab
c
abc
*
and Gwenael Rapenne
´ ¨
The synthesis of ruthenium complexes incorporating an overcrowded pentaarylcyclopentadienyl ligand has
Received 18th May 2021
Accepted 1st June 2021
been investigated, and higher efficiency has been reached using chlorine-functionalised precursors when
compared with their brominated counterparts.
A new methodology for the preparation of
DOI: 10.1039/d1ra03875c
chlorocyclopentadienes has been developed which is well adapted for highly sterically hindered
compounds and works with either electron rich or poor systems.
rsc.li/rsc-advances
and their large volume is reported to confer enhanced kinetic
stability towards organometallic derivatives.6 Interestingly, the
Introduction
reactivity pathways of pentaarylcyclopenta-dienyl ligands (Cp5Ar
)
A molecular-level machine can be dened as a molecule or an
assembly of molecular components designed to perform precise
functions in response to controlled stimulation. Driven by the
seminal works of Sauvage, Stoddart and Feringa, the eld of
articial molecular machines1 has developed signicantly over
the last few decades with the many types of machines prepared
revolutionising the way chemists think about motional molec-
ular systems. Remarkable synthetic masterpieces have opened
the door to new dimensions of chemistry with the control of
motion at the molecular level moving from a static to a dynamic
view where animated objects take the place of immobile
structures.
Coordination chemistry is a very versatile and efficient way to
assemble mechanical subunits, allowing for the production of
a large and diverse range of molecular machines. Many ligands
are available and a vast number of metal centres can be chosen to
vary and complexify molecular architectures.2 In this eld the
pentaphenylcyclopentadienyl anion is a common and useful
ligand, with functionalised analogues already applied as rotors in
various molecular machines3 including molecular motors4 and
gears.5 Interest in this ligand started as a result of the availability
of pentaphenylcyclopentadiene precursors which can be readily
synthesised in large quantities and are air stable. Such hindered
ligands are also more electron-withdrawing than their cyclo-
pentadienyl and pentamethylcyclopentadienyl anionic analogues
seems to vary signicantly when compared to cyclopentadiene
(Cp) and pentamethylcyclopentadiene, due to both differences in
the steric hindrance at the Cp ring and the electronic contribu-
tions from the metal-Cp coordination. These propeller shaped
ligands are capable of conferring novel steric and electronic
properties to metal centres7 and can also be deposited on metal
surfaces chirally, with both le- and right-handed propeller
chirality being represented and recognised.4c In such case,
a
pentaphenylcyclopentadienyl ligand functionalised with
bromine atoms in the ve para positions is exploited both as an
anchoring subunit and as a chiral surface, contributing to the
unidirectionality in the movement of the upper rotating units.
Unique properties also arise from a combination of electronic
effects and the steric hindrance provided by the ve phenyl
substituents, including protection of the metallic centre and
inuence on the electron releasing ability of the complex. It has
also been shown that coordination of the peripheral phenyl rings
can occur in place of the central cyclopentadienyl one,8 giving rise
to highly dissymmetric compounds.
Among the available metals, ruthenium offers a very inter-
esting target for preparing stable and inert heteroleptic
complexes, however the coordination of ruthenium to highly
sterically constrained ligands is not an easy task. Many previous
attempts to directly coordinate pentaarylcyclopentadienyl
(Cp5Ar) ligands from RuCl3 (ref. 9) or [Ru(p-cymene)Cl2]2 (ref. 10)
failed, mainly due to the steric hindrance of the phenyl groups
on the Cp ligand. As an alternative, the triruthenium dodeca-
carbonyl cluster Ru3(CO)12 is recognised as a reliable source of
ruthenium(0) for the preparation of halogenodicarbonyl(h5-
1,2,3,4,5-pentaaryl)cyclopentadienyl ruthenium(II) complexes
Cp5ArRu(CO)2X. Indeed, in 1989 Manners reported the synthesis
of Cp5PhRu(CO)2Br starting from Ru3(CO)12 and 5-bromo-
1,2,3,4,5-pentaphenylcyclopenta-1,3-diene in reuxing toluene
aDivision of Materials Science, Nara Institute of Science and Technology, NAIST,
8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan. E-mail: gwenael-rapenne@ms.
naist.jp
bInternational Collaborative Laboratory for Supraphotoactive Systems, NAIST-CEMES,
CNRS UPR 8011, 29 rue Marvig, F-31055 Toulouse Cedex 4, France
c
´
CEMES, Universite de Toulouse, CNRS, 29 rue Marvig, F-31055 Toulouse Cedex 4,
France
† Electronic supplementary information (ESI) available: HR-MS, IR, 1H and
13C-NMR spectra. See DOI: 10.1039/d1ra03875c
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substituents located on the cyclopentadiene ring on this
complexation reaction, we decided to investigate the reactivity
of a series of brominated tetraphenylcyclopentadienes bearing
one aryl group with particular electronic and/or steric proper-
ties. As an alternative, the chlorinated analogues were also
prepared and their reactivity studied.
Here we report the preparation of a series of ruthenium
pentaarylcyclopentadienyl complexes via halogen containing
intermediates, in good to excellent yields even with very crow-
ded Cp ligands such as mesityl or terarylenyl substituents.
Scheme
1 Coordination of ruthenium to the pentaphenylcyclo-
pentadiene ligand by previously published procedures.11,14
(Scheme 1, le).11 This reaction proceeds via the formal oxida-
tive addition of the brominated cyclopentadiene precursor onto
ruthenium(0) and involves a transient cyclopentadienyl radical
intermediate. In the last decades, the scope of this reaction has
been expanded to include a variety of para-substituted penta-
phenylcyclopentadienes, as precursors of ruthenium-based
molecular machines.12 In addition, the analogous chloror-
uthenium complex Cp5PhRu(CO)2Cl has been successfully
prepared from the corresponding chlorocyclopentadiene and
exploited as a catalyst for the dynamic kinetic resolution of
secondary alcohols.13 To avoid preparation of potentially
unstable halogenocyclopentadienes, new conditions for the
synthesis of this family of complexes were developed by Martin-
Matute et al., involving the direct oxidative addition of bare
Results and discussion
For this study, we selected four aryl-functionalised tetraphenyl
Cp ligands (Scheme 3). The rst two to serve as representative
model compounds, without signicant steric hindrance, but
instead having opposite electronic effects to each other. Here,
the electron donating 4-tert-butylphenyl (Ar1) and the electron
accepting 5-pyrimidyl (Ar2) aryl moieties were chosen as they
have similar steric volume to the phenyl groups already present
on the Cp. Two further aryl fragments were selected to study
more sterically overcrowded substituents; a mesityl model (Ar3)
offering a similar electronic effect to the 4-tert-butylphenyl
group, and the sterically demanding terarylene already dis-
cussed (Ar4), which acts as a strong electron acceptor due to the
thiazole ring at the point of connection to the Cp ring. This
group has previously been used by our group as a photoactive
brake subunit to hinder the rotation of a molecular motor with
the rotational braking observed using both NMR and UV/Visible
analysis.15
pentaphenylcyclopentadiene Cp5Ph
H
onto Ru3(CO)12.14a
However this reaction only proceeds under very harsh condi-
tions (160 ꢀC for several days in a decane/toluene mixture)
giving the corresponding ruthenium hydride intermediate,
which nally yields Cp5PhRu(CO)2X (X ¼ I, Br, Cl) aer treat-
ment with the appropriate haloform (Scheme 1, right). Even
though some variations in aryl groups have been achieved,14 the
relatively high temperatures required has limited the use of this
process when cyclopentadienyl ligands bearing sensitive
substituents are involved.
Synthesis of the aryl-functionalised cyclopentadienol
precursors
In our efforts towards the development of photo-controlled
molecular machines, we aimed to introduce a terarylene pho-
tochrome on the cyclopentadienyl rotating subunit of a ruthe-
nium complex.15 Given the synthetic cost and sensitivity of the
terarylene moiety, the direct oxidative addition of the cyclo-
pentadiene precursor Cp5ArH was ruled out and we instead
turned our attention to Manners' method, starting from the
bromocyclopentadiene carrying four phenyl groups and a ter-
arylene fragment (Scheme 2). Strikingly, its reaction with
Ru3(CO)12 was inoperative due to decomposition of the bromine
precursor via radical side-reactions, preventing formation of the
desired ruthenium complex. To understand the inuence of the
Reaction of 2,3,4,5-tetraphenylcyclopentadienone with organo-
lithium or organomagnesium aryl derivatives, is known to be
a very efficient route to pentaarylcyclopentadienols.7c,7d,16 In
a typical reaction, the 2,3,4,5-tetraphenylcyclopentadienone was
treated with the appropriate aryllithium reagent in tetrahydro-
furan at ꢁ78 ꢀC (Scheme 3). The corresponding pentaar-
ylcyclopentadienols CpOHAr1,16b CpOHAr2, CpOHAr3,7d and
CpOHAr4 (ref. 15) were isolated in moderate to good yields,
from 20% in the case of the mesityl derivative (CpOHAr3) to
81% for the tert-butyl precursor (CpOHAr1). As expected, this
step is strongly affected by the steric hindrance close to the
lithium centre. For reactants of similar steric effect (CpOHAr1
Scheme 2 Photochrome-functionalised tetraphenylcyclopentadiene
brominated precursor unable to coordinate to ruthenium via reaction Scheme
3 Synthetic pathway to form the functionalised cyclo-
with Ru3(CO)12
.
pentadienol precursors.
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or CpOHAr2), the more electron-rich derivative gave better yield and the sterically-hindered bromocyclopentadiene
addition yield.
carrying a mesityl substituent CpBrAr3 was prepared in 44%
yield. Interestingly for the attempted reaction of CpOHAr4 with
thionyl bromide, the desired CpBrAr4 product was obtained as
an inseparable mixture with its chlorinated counterpart
CpClAr4 and the hydrogenated analogue CpHAr4. This side-
reactivity likely results from the washing of the reaction
mixture during workup with dilute aqueous HCl,7d and high-
lights the increased stability of the chlorinated cyclopentadiene
CpClAr4 as compared to its brominated analogue.
Activation of the Cp ring by halogenation
For pentaphenycyclopentadienols, it is well known11,12 that
conversion to the halogen derivative is required prior to ruthe-
nium coordination.17 The cyclopentadienol is generally con-
verted to its brominated analogue by reaction with HBr in acetic
acid.18 In the case of our electron rich model compound func-
tionalised with a 4-tert-butylphenyl fragment, the brominated
Cp ring (CpBrAr1) was obtained using HBr in 78% yield
(Scheme 4).16b This reaction is highly sensitive to electronic
effects with the electron poor 5-pyrimidyl precursor giving no
brominated product. This can be explained by protonation of
the nitrogen centres of the heterocycle under acidic conditions,
making this fragment highly electron poor and dramatically
impacting on the reactivity of the cyclopentadienol.
The same reaction conditions applied to the two sterically-
hindered Cp ring containing molecules CpOHAr3 and CpO-
HAr4 were also found to be ineffective. We noticed that using
HBr in acetic acid, led to the decomposition of the compounds
with no starting material detected. This may be due to the
instability of the brominated Cp's generated and/or because an
electron accepting group destabilises the cation, not allowing
for the elimination of the oxonium ion generated under acidic
conditions.
In the case of SOX2 with or without pyridine, the envisioned
mechanisms will be a competitive mixture of SN1, SNi, SN2 and
SN20 processes depending on the substrate, the solvent and the
presence of pyridine (Scheme 5). As with the HBr reaction, the
SN1 mechanism gives a mixture of three regioisomers (in red)
while the SNi pathway (in blue) gives only one and the SN2 gives
one direct compound (in black) along with two others via a SN20
variation (in green). In consequence a 66 : 30 : 4 ratio of
regioisomers was obtained for CpBrAr2, as determined by
integrating the three sets of pyrimidyl protons signals on the 1H
NMR spectrum (Fig. S3†). In the case of the mesityl-containing
CpBrAr3, only one regioisomer has been obtained with or
without pyridine, in agreement with the results obtained by
Thepot and Lapinte.7d In this case, the bromine atom is located
´
at the 3-position of the Cp ring related to the mesityl moiety, i.e.
on the less sterically hindered position. This indicates that the
SN1 mechanism might not be followed for this substrate.
Given the higher stability of chloro- vs. bromocyclopenta-
diene observed with CpBrAr4, we next explored the possibility of
selectively forming the chlorinated derivatives of the sterically-
To overcome this lack of reactivity towards acidic bromina-
´
tion, J.-Y. Thepot and Lapinte reported the conversion of
cyclopentadienol derivatives to their brominated analogues
using a SOBr2–pyridine mixture instead of HBr in acetic acid.7d
Since the mechanism of halogenation is different in the case of
SOX2 compared to HX (X ¼ Cl or Br), different results could be
expected, especially as the sulfur-containing by-products
further decompose to volatile SO2 gas rendering the process
irreversible (Scheme 5). As shown on Scheme 4, the SOBr2–
pyridine reagents allowed the conversion of the electron de-
cient cyclopentadienol CpOHAr2 to its bromo derivative in 82%
hindered cyclopentadienol rings using
a SOCl2–pyridine
system in diethyl ether.19 The reaction proceeded smoothly with
the chloride derivative of the terarylene (CpClAr4) obtained with
a yield of 74%. The pyridine is generally used with thionyl
Scheme 4 Synthetic pathway to form the brominated and chlorinated Scheme 5 Mechanisms involved in the halogenation of secondary
derivatives. aThe product is obtained as
a
mixture of three alcohols by SOX2 (X ¼ Cl or Br) with and without pyridine. SN1
regioisomers, with the halogen (X) in Cp position 1, 2 or 3 relative to the mechanism gives access to three regioisomers (in red) while SNi gives
Ar moiety. bThe product is obtained as a single regioisomer, with the only one (in blue). When pyridine is used, a SN2 pathway is followed,
halogen (X) in Cp position 3 relative to the Ar moiety. cThe yield could giving access to one regioisomer (in black) plus two others via SN20
not be determined as the product was obtained as an inseparable variations (in green). A monosubstituted cyclopentadienol instead of
mixture of chlorinated and hydrogenated analogues.
a pentasubstituted one has been considered for clarity.
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chloride or bromide for the halogenation of secondary alcohols
via a SN2-type mechanism, whereas if pyridine is not used, an
intramolecular SNi mechanism can also take place. In the case
of the sterically constrained precursors, the SN2 mechanism is
kinetically difficult because of the low access available to the
carbon atom. This is the reason why we next tested the reaction
without pyridine. Reaction of the cyclopentadienol derivatives
with SOCl2 in benzene proceeded well in all cases, presumably
via a SN1 or SNi nucleophilic substitution mechanisms,20 with
yields from 61% for the highly sterically constrained mesityl
derivative (CpClAr3) to 87, 91 and 96% for compounds
CpClAr4,15 CpClAr1 and CpClAr2 respectively. This method is
very well adapted for highly sterically hindered compounds and
is working very well for both electron rich and poor systems.
CpClAr1 was obtained as a mixture of three regioisomers
with the chlorine atom at three possible positions of the Cp ring
due to the SN1 mechanism (Scheme 5, red products). Their
Scheme 6 Formation of the ruthenium complexes from bromo or
chloro precursors (Ar2 is not suitable for ruthenium complexation due
to multiple coordination sites available which might give many poly-
nuclear complexes as well as coordination polymers of various size).
signicantly improved yields over the brominated ones, with
59% obtained in the case of the mesityl-functionalised tetra-
phenylcyclopentadiene CpClAr3 compared to 14% for CpBrAr3.
In the case of the electron poor terarylene-functionalised
cyclopentadienyl ligand, the ruthenium complex has been ob-
tained with a yield of 56% (ref. 15) from the chlorinated
precursor CpClAr4, while it is inaccessible using its brominated
analogue. Despite both the changes in halogen atom present
and the varying steric effects of the Ar groups the electronic
character of these complexes remains largely the same as re-
ected in the similar CO stretches in their IR spectra. It appears
that the use of a chloride instead of a bromide offers a new
1
respective proportion can be quantied by H-NMR, using the
equivalent methyl groups of the tert-butyl substituent as
a probe. Three singlets were obtained (Fig. S5†), corresponding
to the three regioisomers, with a 46 : 34 : 20 ratio which slightly
differs from the statistical mixture of 40 : 40 : 20 expected from
a SN1 mechanism, due to the varying stabilities of the carbo-
cations involved as a result of the presence of the tert-butyl-
phenyl donor substituent. A 57 : 37 : 6 ratio of regioisomers has
been obtained for CpClAr2 (Fig. S7†). In the case of CpClAr4, it
has not been possible to determine a ratio as the methyl signals
are usually very broad in such terarylene fragments and no
other region of the spectrum could be exploited for this
purpose. However, the presence of these mixtures of
regioisomers is not an issue as in the next step the aromatisa-
tion of the Cp ring through h5-coordination to the ruthenium
centre leads to the same single cyclopentadienide complex in all
cases.
pathway
for
the
preparation
of
novel
cyclo-
pentadienylruthenium complexes in cases where the steric
hindrance around the Cp ligand is large. The use of chlorine-
functionalised pentaarylcyclopentadienyl precursors, similar
to those developed here opens up a new synthetic route for the
preparation of molecular motors containing sterically hindered
pentaarylcyclopentadienyl ligands.
As a representative example of this pathway, exploiting this
new synthetic methodology we recently reported the use of
these chlorinated derivatives in the preparation of a photo-
chromic molecular motor containing the terarylene unit Ar4.
This was achieved using CpClAr4 and Ru3(CO)12 in conjunction
with our previously reported tris[(ethylsulfanyl)methyl]indazo-
lylborate surface anchor to give a new ruthenium based
molecular motor.15
In the case of the sterically crowded, electron donating,
mesityl substituent CpClAr3 (Fig. S9†) one single regioisomer is
obtained as for the brominated analogue. The H NMR spec-
trum also shows that rotation of the mesityl moiety is blocked,
as evidenced by the non-equivalence of the aromatic and methyl
protons located on this bulky group.
1
Coordination to the ruthenium
Next, coordination to ruthenium using the triruthenium
dodecacarbonyl cluster in toluene was attempted for each of the
Conclusions
halogenated derivatives. For CpBrAr2 and CpClAr2 this led In this work investigations into the preparation of halogen
apparently to the formation of polynuclear metal complexes pentaarylcyclopentadienyl ligands as intermediates for ruthe-
through the nitrogens on the 5-pyrimidyl ring, so they were nium containing molecular motors have been carried out. The
discounted. For the other six systems this was found to yield, effect of changes in sterics and electronics on one of the ve
aer purication by column chromatography, the correspond- rings on the cyclopentadiene ligand have been investigated for
ing cyclopentadienylruthenium(II) complexes [RuCpArBr(CO)2] a series of four model compounds. A new methodology for the
or [RuCpArCl(CO)2] (Scheme 6, complexes are named as preparation of chlorine functionalised intermediates has been
RuCpXAr).
developed which is well adapted for highly sterically hindered
In the case of precursors CpXAr1, the reaction yields are compounds and works with either electron rich or poor
similar whichever halogen derivative is present (63 and 74% for systems. It has been used for the preparation of new function-
Br and Cl respectively) but surprisingly for the sterically over- alised tetraphenyl cyclopentadiene complexes in yields well
crowded Cp systems the chlorinated precursors gave beyond those previously reported for similar highly sterically
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hindered molecular motors. This methodology has already been was added dropwise while carefully maintaining the tempera-
used successfully for the synthesis of a photo-controlled ture. The suspension was stirred for 30 minutes at this
molecular motor functionalized with a terarylene photo- temperature and
a
degassed solution of 2,3,4,5-
chrome on the cyclopentadienyl rotating subunit.15
tetraphenylcyclopenta-2,4-dienone (1 g, 2.6 mmol, 1 eq.) in
30 mL of anhydrous tetrahydrofuran was added dropwise via
a cannula. The reaction was stirred for two hours at ꢁ78 ꢀC
before being neutralised by pouring it slowly into 20 mL of
a saturated aqueous ammonium chloride solution. The crude
product was then extracted with diethyl ether, washed three
times with water and once with brine. The organic layer was
dried over magnesium sulfate and the solvents were removed
under vacuum. The crude product was adsorbed onto SiO2 and
puried by a quick column chromatography on SiO2, eluting
impurities with dichloromethane followed by pure ethyl acetate
to collect the product. Solvents were evaporated to afford
a yellow solid, still contaminated with impurities. This solid was
then recrystallised from a minimal amount of boiling chloro-
form, to give CpOHAr2 as a clear white solid in a 65% yield
(780 mg, 1.7 mmol).
Experimental
Materials and methods
All commercially available chemicals were of reagent grade and
were used without further purication. Anhydrous tetrahydro-
furan, anhydrous toluene, anhydrous diethyl ether, magnesium
sulfate, HCl, ammonium chloride, n-butyllithium (2.5 M in
hexanes),
5-bromopyrimidine
and
2,3,4,5-
tetraphenylcyclopenta-2,4-dienone were purchased from
Aldrich. Triruthenium dodecacarbonyl was purchased from
Acros or Fluorochem. Benzene was purchased from Fluka.
Thionyl chloride and thionyl bromide were purchased from
Wako or Aldrich. CpOHAr1,16b CpBrAr1,16b CpOHAr3,7d
CpBrAr3,7d CpOHAr4,15 CpClAr4,15 RuCpClAr4 (ref. 15) were
prepared according to the corresponding published procedures.
Reactions were carried out using standard Schlenk techniques
under an argon atmosphere. Thin layer chromatography (TLC)
was performed on pre-coated aluminium-backed silica gel 60
UV254 plates (Macherey-Nagel) with visualisation effected with
ultraviolet irradiation (l ¼ 254, 366 nm). Flash column chro-
matography was carried out on 230–400 mesh silica gel
(Aldrich) unless otherwise stated. NMR spectra were recorded
with a Bruker Avance 300, a Bruker Avance 500 or a JEOL JNM-
ECA600 spectrometer and assignments were made with the
assistance of COSY, HMBC and HSQC spectra when necessary.
1H and 13C NMR chemical shis (d) are reported in ppm relative
to the signal of tetramethylsilane (TMS). Residual solvent
signals were used as an internal reference. Coupling constants
(J) are given in Hz and the following abbreviations have been
used to describe the signals: singlet (s); doublet (d); triplet (t);
multiplet (m). The numbering system used for the assignment
of signals in some compounds is provided in the ESI,† along
with the spectra of new compounds. IR spectra were recorded
with a Jasco 4200 FTIR-ATR. Only selected characteristic peaks
are listed. High-resolution mass spectra (HRMS) were per-
formed with a Waters GCT Premier spectrometer (DCI), with
a Waters Xevo G2 QTof spectrometer (ESI) and a JEOL JMS-
Q1000TD spectrometer with JMS-700 Mstation (MALDI).
Elemental analyses have been measured on a Perkin Elmer 2400
Series II CHNS/O system. Melting points were measured using
Rf ¼ 0.4 (SiO2, ethyl acetate/hexane 3 : 7); mp 233 ꢀC; 1H
NMR (300 MHz, (CD3)2SO, 25 ꢀC): d 8.98 (s, 1H, Ha), 8.81 (s, 2H,
Hb), 7.22–7.14 (m, 6H, HPh), 7.12–6.95 (m, 14H, HPh), 6.87 (s,
13
1
ꢀ
1H, OH); C{ H} NMR (75 MHz, CD2Cl2, 25 C): d 168.4 (Ca),
168.4 (Cb), 161.3 (Cquat), 157.0 (Cquat), 149.3 (Cquat), 149.2 (Cquat),
148.0 (Cquat), 143.8 (CPh–H), 143.3 (CPh–H), 142.4 (CPh–H), 142.1
(CPh–H), 141.7 (CPh–H), 141.4 (CPh–H), 101.5 (Cquat–OH); HR-MS
(DCI-CH4): calcd for C34H24N2O [MH]+: 465.1940, found
465.1960; and calcd for C33H23N2O [M]+: 464.1889, found
464.1881.
5-Bromo-1,2,3,4-tetraphenyl-5-(pyrimidin-50-yl)cyclopenta-1,3-
diene (CpBrAr2)
Obtained as a 66 : 30 : 4 mixture of 3 regioisomers.
2,3,4,5-Tetraphenyl-1-(pyrimidin-50-yl)cyclopenta-2,4-dien-1-
ol CpOHAr2 (200 mg, 0.43 mmol, 1 eq.) was placed in a Schlenk
tube containing a magnetic stir bar and anhydrous diethyl ether
(10 mL) and freshly distilled pyridine (44 mL, 0.54 mmol, 1.25
ꢀ
eq.) were added. The mixture was cooled down to ꢁ10 C and
thionyl bromide (42 mL, 0.54 mmol, 1.25 eq.) was added. The
medium was then allowed to warm up to room temperature over
one hour, under stirring. It was then neutralised by adding it
slowly to 20 mL of a 1 M aqueous hydrochloric acid solution.
The product was extracted with ethyl acetate (150 mL) and
washed three times with water (3 ꢂ 150 mL). The organic layer
was dried over magnesium sulfate and the solvents were
removed by rotary evaporation. The crude product was puried
by column chromatography (SiO2, ethyl acetate/cyclohexane
1 : 9) to afford the desired brominated product. Nevertheless,
traces of impurities were still observed in some batches, so the
product was further recrystallised from boiling heptane and
rinsed with ice-cold pentane to give CpBrAr2 in 82% yield
¨
a Kruss M5000 melting point meter or a MEL-TEMP capillary
melting point apparatus. Melting points were not reported for
compounds obtained as a mixture of regioisomers.
2,3,4,5-Tetraphenyl-1-(pyrimidin-50-yl)-cyclopenta-2,4-dien-1-
ol (CpOHAr2)
5-Bromopyrimidine (620 mg, 3.9 mmol, 1.5 eq.) was placed in (187 mg, 0.36 mmol) as a yellow solid composed of a 66 : 30 : 4
a Schlenk ask with a stir bar, and anhydrous tetrahydrofuran mixture of regioisomers.
(10 mL) was added. The solution was quickly degassed by
Rf ¼ 0.41 (SiO2, ethyl acetate/hexane 3 : 7); elemental anal-
bubbling argon and cooled down to ꢁ78 ꢀC. Then, a 2.5 M ysis: found: C, 75.0; H, 4.18; N, 5.26. Calc. for C33H23BrN2: C,
solution of n-butyllithium in hexanes (2 mL, 5.2 mmol, 2 eq.) 75.14; H, 4.40; N, 5.31; 1H NMR (300 MHz, CD2Cl2, 25 ꢀC,
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66 : 30 : 4 mixture of regioisomers): d 9.05 (s, 0.04H, Haregio3), chromatography (SiO2, dichloromethane/hexane 1 : 1) to give
8.91 (s, 0.30H, Haregio2), 8.86 (s, 0.66H, Haregio1), 8.78 (s, 0.08H, pure product CpClAr1 in 91% yield (94 mg, 0.18 mmol) as
Hbregio3), 8.27 (s, 1.32H, Hbregio1), 8.24 (s, 0.60H, Hbregio2), 7.55– a pale-orange solid composed of a 46 : 34 : 20 mixture of
7.46 (m, 2H, HPh), 7.36–6.84 (m, 18H, HPh); 13C{1H} NMR (75 regioisomers.
MHz, CD2Cl2, 25 ꢀC, 66 : 30 : 4 mixture of regioisomers): d 157.6
Rf ¼ 0.8 (SiO2, dichloromethane/hexane 1 : 1); 1H NMR (300
(Cpyr–H), 157.3 (Cpyr–H), 157.2 (Cpyr–H), 156.4 (Cpyr–H), 150.2 MHz, CD2Cl2, 25 ꢀC, 46 : 34 : 20 mixture of regioisomers) d 7.48–
(Cquat), 145.5 (Cquat), 142.4 (Cquat), 142.2 (Cquat), 134.9 (Cquat), 7.34 (m, 2H, HAr), 7.25–6.78 (m, 22H, HAr), 1.22 (s, 1.84H,
134.8 (Cquat), 134.5 (Cquat), 134.4 (Cquat), 134.0 (Cquat), 133.9
(Cquat), 133.5 (Cquat), 131.1 (CPh–H), 130.9 (CPh–H), 130.7 (CPh
H
tBu,regio3), 1.15 (s, 2.97H, HtBu,regio2), 1.11 (s, 4.04H, HtBu,regio1);
C{ H} NMR (126 MHz, CD2Cl2, 25 C, 46 : 34 : 20 mixture of
13
1
ꢀ
–
H), 130.6 (CPh–H), 130.3 (CPh–H), 130.1 (CPh–H), 129.3 (CPh–H), regioisomers) d 151.5 (Cquat–tBu), 150.8 (Cquat–tBu), 150.5
129.0 (CPh–H), 128.9 (CPh–H), 128.7 (CPh–H), 128.5 (CPh–H), (Cquat–tBu), 148.4 (Cquat), 148.3 (Cquat), 147.9 (Cquat), 147.6
128.4 (CPh–H), 128.3 (CPh–H), 128.1 (CPh–H), 128.0 (CPh–H), (Cquat), 143.4 (Cquat), 143.2 (Cquat), 143.1 (Cquat), 143.0 (Cquat),
127.9 (CPh–H), 127.7 (CPh–H), 75.9 (Cquat–Br); HR-MS (ESI+): 136.8 (Cquat), 136.5 (Cquat), 135.5 (Cquat), 135.1 (Cquat), 135.0
calcd for C33H24BrN2 [MH]+: 529.1108, found 529.1114.
(Cquat), 134.5 (Cquat), 134.3 (Cquat), 133.1 (Cquat), 131.8 (Cquat),
130.9 (Cquat), 130.6 (CPh–H), 130.5 (CPh–H), 130.3 (CPh–H), 130.2
(CPh–H), 129.9 (CPh–H), 129.8 (CPh–H), 128.9 (CPh–H), 128.3
(CPh–H), 128.1 (CPh–H), 128.0 (CPh–H), 127.8 (CPh–H), 127.6
(CPh–H), 127.5 (CPh–H), 126.7 (CPh–H), 126.6 (CPh–H), 126.4
(CPh–H), 125.8 (CPh–H), 125.0 (CPh–H), 124.8 (CPh–H), 82.4
(Cquat–Cl), 82.3 (Cquat–Cl), 82.2 (Cquat–Cl), 34.8 (CtBu), 34.7 (CtBu),
31.4 (CtBu), 31.3 (CtBu), 31.2 (CtBu).; HR-MS (spiral-TOF) signal:
calcd for C39H33Cl [M]+: 536.2265, found. 536.2266.
2-(1-Bromo-2,3,4,5-tetraphenylcyclopenta-2,4-dien-1-yl)-4,5-
bis(2-methylbenzo[b]thiophen-3-yl)thiazole (CpBrAr4)
1-(4,5-Bis(2-methylbenzo[b]thiophen-3-yl)thiazol-2-yl)-2,3,4,5-
tetraphenylcyclopenta-2,4-dien-1-ol
CpOHAr4
(100
mg,
0.13 mmol, 1 eq.) was placed in a Schlenk tube containing
a magnetic stir bar and anhydrous diethyl ether (10 mL) and
freshly distilled pyridine (61 mL, 0.79 mmol, 6.0 eq.) were added.
ꢀ
The mixture was cooled down to ꢁ10 C and thionyl bromide
5-Chloro-1,2,3,4-tetraphenyl-5-(pyrimidin-50-yl)cyclopenta-1,3-
diene (CpClAr2)
(64 mL, 0.79 mmol, 6.0 eq.) was added. The medium was then
allowed to warm up to room temperature over two hours, under
stirring. It was then neutralised by adding it slowly to 1 mL of
a 1 M aqueous hydrochloric acid solution. The product was
extracted with diethyl ether (100 mL) and washed with three
times water (3 ꢂ 100 mL). The organic layer was dried over
magnesium sulfate and the solvents were removed by rotary
evaporation. The crude mixture was puried by column chro-
matography (SiO2, dichloromethane/hexane 1 : 4) to give the
brominated product CpBrAr4 along with its chlorinated coun-
terpart CpClAr4 (ref. 15) and the hydrogenated product CpHAr4
as an inseparable mixture (40 mg). Each of these products is
present as a mixture three regioisomers and the structures of all
products are very similar, therefore it was not possible to
provide a proper NMR characterization of CpBrAr4.
Obtained as a 57 : 37 : 6 mixture of 3 regioisomers.
2,3,4,5-Tetraphenyl-1-(pyrimidin-50-yl)cyclopenta-2,4-dien-1-
ol CpOHAr2 (100 mg, 0.22 mmol, 1 eq.) was placed in a round-
bottom ask equipped with a magnetic stirring bar. Benzene
(2.5 mL) was added and the suspension was heated to reux
using a preheated oil bath. Thionyl chloride (94 mL, 1.29 mmol,
6 eq.) was then added, and the mixture was reuxed for 30
minutes. The reaction medium was then cooled down, diluted
with ethyl acetate (20 mL) and washed with a saturated aqueous
solution of sodium hydrogen carbonate (20 mL) followed by
distilled water (2 ꢂ 20 mL). The organic layer was then dried
with magnesium sulfate and the solvents were evaporated to
dryness. TLC of the crude (SiO2, ethyl acetate/hexane 3 : 7)
showed only one spot resulting from a quantitative conversion
of the starting material. The crude product was dissolved in
ethyl acetate and ltered through a silica plug, using the same
solvent for elution, to remove eventual impurities or salts. The
solvent was then removed using rotary evaporation to give pure
product CpClAr2 in 96% yield (99.3 mg, 0.21 mmol) as a pale-
yellow solid composed of a 57 : 37 : 6 mixture of regioisomers.
Rf ¼ 0.5 (SiO2, ethyl acetate:hexane 3 : 7); 1H NMR (500 MHz,
Rf ¼ 0.8 (SiO2, dichloromethane/hexane 1 : 4); MS (DCI-NH3)
of the mixture of products: m/z 826 (CpBrAr4, [M + H]+, 9%), 780
(CpClAr4, [M + H]+, 100), 746 (CpHAr4, [M + H]+, 52).
5-[4-(tert-Butyl)phenyl]-5-chloro-1,2,3,4-tetraphenylcyclo-
penta-1,3-diene (CpClAr1)
Obtained as a 46 : 34 : 20 mixture of 3 regioisomers.
1-(4-(tert-Butyl)phenyl)-2,3,4,5-tetraphenylcyclopenta-2,4-
dien-1-ol CpOHAr1 (100 mg, 0.19 mmol, 1 eq.) was placed in CD2Cl2, 25 ꢀC, 57 : 37 : 6 mixture of regioisomers): d 9.06 (s,
a Schlenk tube containing a magnetic stir bar. Benzene (2 mL) 0.06H, Haregio3), 8.91 (s, 0.57H, Haregio2), 8.85 (s, 0.37H,
and thionyl chloride (84 mL, 1.16 mmol, 6 eq.) were added and Haregio1), 8.80 (s, 0.12H, Hbregio3), 8.25 (s, 0.75H, Hbregio1), 8.23
the suspension was reuxed using a preheated oil bath for 30 (s, 1.15H, Hbregio2), 7.59–7.46 (m, 2H, HPh), 7.41–6.83 (m, 18H,
minutes. The reaction medium was then cooled down, diluted
with diethyl ether (20 mL) and washed with a saturated aqueous of regioisomers): d 157.5 (Cpyr–H), 157.4 (Cpyr–H), 157.3 (Cpyr
H
Ph); 13C{1H} NMR (126 MHz, CD2Cl2, 25 ꢀC, 57 : 37 : 6 mixture
–
solution of sodium hydrogen carbonate (20 mL) followed by H), 157.2 (Cpyr–H), 155.5 (Cquat), 152.3 (Cquat), 149.7 (Cquat),
distilled water (2 ꢂ 20 mL). The organic layer was dried with 148.7 (Cquat), 146.4 (Cquat), 143.0 (Cquat), 141.9 (Cquat), 136.3
magnesium sulfate and the solvent was then removed using (Cquat), 135.4 (Cquat), 135.3 (Cquat), 134.4 (Cquat), 134.3 (Cquat),
rotary evaporation. The crude product was puried by column 133.7 (Cquat), 133.6 (Cquat), 133.2 (Cquat), 130.7 (CPh–H), 130.6
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(CPh–H), 130.4 (CPh–H), 130.3 (CPh–H), 130.1 (CPh–H), 129.4 (CAr), 128.7 (CAr), 128.1 (CAr), 125.1 (CAr), 107.6 (CCp), 107.3
(CPh–H), 129.1 (CPh–H), 129.0 (CPh–H), 128.9 (CPh–H), 128.8 (CCp), 106.6 (CCp), 34.9 (CtBu), 31.3 (CtBu); HR-MS (ESI+): calcd for
(Cquat), 128.7 (CPh–H), 128.6 (CPh–H and Cquat), 128.5 (CPh–H),
128.4 (CPh–H), 128.3 (CPh–H), 126.7 (CPh–H), 126.5 (CPh–H), 82.1
(Cquat–Cl), 81.9 (Cquat–Cl); HRMS (DCI-CH4): calcd for
C
41H33BrNaO2Ru [M + Na]+: 763.0607, found. 763.0619.
C
33H24N2Cl [MH]+: 483.1628, found 483.1642.
Bromodicarbonyl-h5-5-mesityl-1,2,3,4-tetraphenylcyclopenta-
dienylruthenium(II) (RuCpBrAr3)
5-Chloro-2-mesityl-1,3,4,5-tetraphenylcyclopenta-1,3-diene
(CpClAr3)
5-Bromo-2-mesityl-1,3,4,5-tetraphenylcyclopenta-1,3-diene
CpBrAr3 (48 mg, 0.085 mmol, 1 eq.) and triruthenium dodeca-
1-Mesityl-2,3,4,5-tetraphenylcyclopenta-2,4-dien-1-ol CpOHAr3 carbonyl Ru3(CO)12 (33 mg, 0.053 mmol, 0.6 eq.) were placed in
(50 mg, 0.10 mmol, 1 eq.) was placed in a Schlenk tube con- a Schlenk tube containing a magnetic stir bar under argon.
taining a magnetic stir bar. Benzene (2 mL) and thionyl chloride Anhydrous and degassed toluene (2 mL) was then added and
(43 mL, 0.60 mmol, 6 eq.) were added and the suspension was the mixture was reuxed for 2 hours. The colour of suspension
reuxed using a preheated oil bath for 30 minutes. The reaction changed from orange to brown. The reaction mixture was
medium was then cooled down, diluted with diethyl ether (20 allowed to reach rt and the solvent was then removed using
mL) and washed with a saturated aqueous solution of sodium rotary evaporation. The crude product was puried by column
hydrogen carbonate (20 mL) followed by distilled water (2 ꢂ 20 chromatography (SiO2, dichloromethane/hexane 1 : 1 to 2 : 1) to
mL). The organic layer was dried with magnesium sulfate and give RuCpBrAr3 as a yellow solid in 14% yield (8.6 mg, 0.012
the solvent was then removed using rotary evaporation. The mmol).
crude product was puried by column chromatography (SiO2,
Rf ¼ 0.2 (SiO2, dichloromethane/hexane 1 : 1); mp 203–
1
dichloromethane/hexane 1 : 1) to give CpClAr3 as a yellowish- 205 ꢀC (dec.); IR: nmax/cmꢀꢁ1 2040 (CO) and 1989 (CO); H NMR
orange solid in 61% yield (32 mg, 0.061 mmol).
(500 MHz, (CD3)2CO, 25 C): d 7.32–7.23 (m, 10H, HPh), 7.16–
Rf ¼ 0.8 (SiO2, dichloromethane/hexane 1 : 1); mp 93–95 ꢀC; 7.13 (m, 2H, HPh), 7.02–6.99 (m, 4H, HPh), 6.94–6.93 (m, 5H, HPh
1H NMR (600 MHz, (CD3)2CO, 25 ꢀC): d 7.59–7.58 (m, 2H, HPh), and HMes), 6.77 (s, 1H, HMes), 2.57 (s, 3H, HMe), 2.22 (s, 3H, HMe),
7.36–7.25 (m, 3H, HPh), 7.11–7.01 (m, 11H, HPh), 6.97–6.93 (m, 1.99 (s, 3H, HMe); 13C{1H} NMR (126 MHz, (CD3)2CO, 25 ꢀC):
4H, HPh), 6.81 (s, 1H, HMes), 6.79 (s, 1H, HMes), 2.20 (s, 6H, HMe), d 198.0 (CO), 138.9 (Cquat–Me), 138.0 (Cquat–Me), 137.2 (Cquat
–
2.15 (s, 3H, HMe); 13C{1H} NMR (151 MHz, (CD3)2CO, 25 ꢀC) Me), 132.6 (CAr), 131.2 (CAr), 130.5 (CAr), 129.6 (CAr), 129.0 (CAr),
d 148.7 (Cquat), 146.7 (Cquat), 143.1 (Cquat), 142.5 (Cquat), 137.2 128.6 (CAr), 128.5 (CAr), 128.2 (CAr), 127.7 (CAr), 127.2 (CAr), 113.9
(Cquat), 136.7 (Cquat), 136.1 (Cquat), 135.8 (Cquat), 134.6 (Cquat), (CCp), 103.2 (CCp), 22.9 (CMe), 21.1 (CMe), 20.1 (CMe); HR-MS
134.0 (Cquat), 133.7 (Cquat), 131.8 (Cquat), 130.3 (CPh–H), 129.3 (ESI+): calcd for C40H31BrNaO2Ru [M + Na]+: 747.0450, found
(CPh–H), 128.7 (CPh–H), 128.4 (CMes–H), 128.3 (CMes–H), 128.3 747.0451.
(CPh–H), 128.0 (CPh–H), 127.7 (CPh–H), 127.6 (CPh–H), 127.5
(CPh–H), 127.4 (CPh–H), 127.3 (CPh–H), 126.4 (CPh–H), 81.8
Chlorodicarbonyl-h5-5-[4-(tert-butyl)phenyl]-1,2,3,4-
tetraphenylcyclopentadienylruthenium(II) (RuCpClAr1)
(Cquat–Cl), 20.3 (CMe), 20.3 (CMe), 19.5 (CMe); HR-MS (MALDI):
calcd for C38H31Cl [M]+: 522.2109, found 522.2106.
5-(4-(tert-Butyl)phenyl)-5-chloro-1,2,3,4-tetraphenylcyclopenta-
1,3-diene CpClAr1 (50 mg, 0.093 mmol, 1 eq.) and triruthenium
dodecacarbonyl Ru3(CO)12 (36 mg, 0.056 mmol, 0.6 eq.) were
Bromodicarbonyl-h5-5-[4-(tert-butyl)phenyl]-1,2,3,4-
tetraphenylcyclopentadienylruthenium(II) (RuCpBrAr1)
5-(4-(tert-Butyl)phenyl)-5-bromo-1,2,3,4-tetraphenylcyclopenta-
placed in a Schlenk tube containing a magnetic stir bar under
1,3-diene CpBrAr1 (50 mg, 0.086 mmol, 1 eq.) and triruthenium argon. Anhydrous and degassed toluene (3 mL) was then added
dodecacarbonyl Ru3(CO)12 (32 mg, 0.052 mmol, 0.6 eq.) were and the mixture was reuxed for 2 hours. The colour of
placed in a Schlenk tube containing a magnetic stir bar under suspension changed from orange to brown. The reaction
argon. Anhydrous and degassed toluene (3 mL) was then added, mixture was allowed to reach rt and the solvent was then
and the mixture was reuxed for 2 hours. The colour of removed using rotary evaporation. The crude product was
suspension changed from orange to brown. The reaction puried by column chromatography (SiO2, dichloromethane/
mixture was allowed to reach rt and the solvent was then hexane 1 : 1 to 2 : 1) to give RuCpClAr1 as a yellow solid in
removed using rotary evaporation. The crude product was 74% yield (48 mg, 0.069 mmol).
puried by column chromatography (SiO2, dichloromethane/
Rf ¼ 0.2 (SiO2, dichloromethane/hexane 1 : 1); mp 197–
1
hexane 1 : 2 to 2 : 1) to give RuCpBrAr1 as a yellow solid in 199 ꢀC (dec.); IR: nmax/cmꢁ1 2038 (CO) and 1985 (CO); H NMR
63% yield (40 mg, 0.0.54 mmol).
(600 MHz, CD2Cl2, 25 ꢀC):d 7.24–7.20 (m, 4H, HAr), 7.13–7.04 (m,
Rf ¼ 0.2 (SiO2, dichloromethane/hexane 1 : 1); mp 206– 18H, HAr), 6.96–6.95 (m, 2H, HAr), 1.24 (s, 9H, HtBu); 13C{1H}
1
208 ꢀC (dec.); IR: nmax/cmꢁ1 2037 (CO) and 1988 (CO); H NMR NMR (126 MHz, CD2Cl2, 25 C): d ¼ 197.8 (CO), 152.1 (Cquat
–
ꢀ
ꢀ
3
(600 MHz, CD2Cl2, 25 C): d 7.24–7.21 (m, 4H, HAr), 7.13–7.04 tBu), 132.7 (CAr), 132.6 (CAr), 132.2 (CAr), 130.3 (CAr), 130.2 (CAr),
(m, 18H, HAr), 6.95 (d, J ¼ 8.9 Hz, 2H, HAr), 1.24 (s, 9H, HtBu); 128.7 (CAr), 128.2 (CAr), 128.1 (CAr), 125.1 (CAr), 107.8 (CCp), 107.4
13C{1H} NMR (151 MHz, CD2Cl2, 25 ꢀC): d 197.4 (CO), 152.2 (CCp), 106.6 (CCp), 34.9 (CtBu), 31.3 (CtBu); HR-MS (ESI+): calcd for
(Cquat–tBu), 132.8 (CAr), 132.8 (CAr), 132.4 (CAr), 130.3 (CAr), 130.2
C
41H33ClNaO2Ru [MNa]+: 717.1110, found 717.1097.
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Chlorodicarbonyl-h5-5-mesityl-1,2,3,4-tetraphenylcyclopenta-
dienylruthenium(II) (RuCpClAr3)
G. Rapenne and J.-P. Sauvage, Chirality, 1998, 10, 125; (d)
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CpClAr3 (17 mg, 0.032 mmol, 1 eq.) and triruthenium dodeca-
carbonyl Ru3(CO)12 (12 mg, 0.019 mmol, 0.6 eq.) were placed in
a Schlenk tube containing a magnetic stir bar under argon.
Anhydrous and degassed toluene (2 mL) was then added and
the mixture was reuxed for 2 hours. The colour of suspension
changed from orange to brown. The reaction mixture was
allowed to reach rt and the solvent was then removed using
rotary evaporation. The crude product was puried by column
chromatography (SiO2, dichloromethane/hexane 1 : 1 to 2 : 1) to
give the ruthenium complex RuCpClAr3 as a yellow solid in 59%
yield (13 mg, 0.019 mmol).
´
´
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S.-W. Hla, Nat. Nanotechnol., 2013, 8, 46; (c) Y. Zhang,
J. P. Calupitan, T. Rojas, R. Tumbleson, G. Erbland,
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38, 7805; (b) S. Brydges, L. E. Harrington and
M. J. McGlinchey, Coord. Chem. Rev., 2002, 233–234, 75; (c)
G. Erbland, S. Abid, Y. Gisbert, N. Saffon-Merceron,
Rf ¼ 0.2 (SiO2, dichloromethane/hexane 1 : 1); mp 189–
1
191 ꢀC (dec.); IR: nmax/cmꢁ1 2045 (CO) and 1996 (CO); H NMR
(500 MHz, (CD3)2CO, 25 ꢀC): d 7.32–7.23 (m, 10H, HPh), 7.15 (t, 3J
¼ 7.4 Hz, 2H, HPh), 7.03–6.99 (m, 4H, HPh), 6.94–6.92 (m, 5H,
H
H
Ph and HMes), 6.77 (s, 1H, HMes), 2.57 (s, 3H, HMe), 2.21 (s, 3H,
Me), 1.99 (s, 3H, HMe); 13C{1H} NMR (126 MHz, (CD3)2CO, 25
ꢀC): d 198.0 (CO), 138.9 (Cquat–Me), 138.0 (Cquat–Me), 137.2
(Cquat–Me), 132.6 (CAr), 131.2 (CAr), 130.5 (CAr), 129.6 (CAr), 129.6
(CAr), 129.0 (CAr), 128.6 (CAr), 128.5 (CAr), 128.2 (CAr), 127.2 (CAr),
113.9 (CCp), 103.2 (CCp), 22.9 (CMe), 21.1 (CMe), 20.1 (CMe); HR-
MS (ESI+): calcd for C40H31ClNaO2Ru [MNa]+: 703.0954, found
703.0976.
´
Y. Hashimoto, L. Andreoni, T. Guerin, C. Kammerer and
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and G. Rapenne, Chem. Commun., 2019, 55, 14689; (e)
¨
¨
K. H. Au Yeung, T. Kuhne, F. Eisenhut, M. Kleinwachter,
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N. Saffon-Merceron, J. Cuny, C. Kammerer and
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Conflicts of interest
There are no conicts to declare.
6 K. Broadley, G. A. Lane, N. G. Connelly and W. E. Geiger, J.
Am. Chem. Soc., 1983, 105, 2486.
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M. D. Rausch, Organometallics, 1986, 5, 1635; (b) C. U. Beck,
L. D. Field, T. W. Hambley, P. A. Humphrey, A. F. Masters
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This work was supported by the MEXT Program for Promoting
the Enhancement of Research Universities in NAIST, the CNRS
and the University Paul Sabatier (Toulouse). It has also received
funding from the European Union's Horizon 2020 research and
innovation program under the project MEMO, grant agreement
no. 766864 and from the JSPS KAKENHI grant in aid for
Scientic Research on Innovative Areas “Molecular Engine (No.
8006)” 18H05419. R. A. thanks the NAIST foundation for
nancial support. Y. G. thanks the French Ministry of National
Education for a PhD Fellowship. C. J. M. thanks the JSPS
KAKENHI Grant-in-Aid for Early-Career Scientists (19K15312)
and G. R. the JSPS KAKENHI Grant-in-Aid for Challenging
Research (20K21131).
´
1996, 35, 1986; (d) J.-Y. Thepot and C. Lapinte, J.
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