Ruthenium nanoparticles ligated by cholesterol-derived NHCs and their application in the hydrogenation of arenes

Article abstract of DOI:10.1039/c8cc02833h

Herein we present ruthenium nanoparticles (Ru-NPs) stabilized with two rigid NHC ligands derived from cholesterol. The obtained nanoparticles were fully characterized and applied in the hydrogenation of various aromatic compounds under mild conditions. Interestingly, the more bulky ligand gives a slightly lower ligand coverage and a faster catalyst.

Full text of DOI:10.1039/c8cc02833h

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Chem. Commun., 2018, DOI: 10.1039/C8CC02833H.
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DOI: 10.1039/C8CC02833H
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Ruthenium nanoparticles ligated by cholesterol-derived NHCs and
their application in the hydrogenation of arenes †
Lena Rakersa,‡, Luis M. Martínez-Prieto b,‡, Angela M. López-Vinascob, Karine Philippotc, Piet W. N.
Received 00th January 20xx,
Accepted 00th January 20xx
M. van Leeuwenb, Bruno Chaudret b,* and Frank Glorius a,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein we present ruthenium nanoparticles (Ru-NPs) stabilized in the backbone of the imidazolium core. For this study, we
with two rigid NHC ligands derived from cholesterol. The obtained wanted to generate the NHCs of the steroidal-based
nanoparticles were fully characterized and applied in the imidazolium salts for the first time and test if they could act as
hydrogenation of various aromatic compounds under mild stabilizing ligands for NPs and if the rigid moiety influences the
conditions. Interestingly, the more bulky ligand gives a slightly properties of the obtained NPs.
lower ligand coverage and a faster catalyst.
N-Heterocyclic carbenes (NHCs) are well established stabilizers
in transition metal catalysis exhibiting attractive characteristics
such as their electron-donating nature, strong bond to metals
and broad structural variation.1 The benefits of these ligands are
also of high interest for the stabilization and functionalization of
Figure 1: Cholesterol (left) and cholesterol-based imidazolium
salts 1·HOMs and 2·HI (right).
metal nanoparticles (MNPs).2 Indeed, as for molecular
complexes, it has been shown that the characteristics of the
MNPs can be tuned by the NHCs. In this regard, structural
variability of the ligands can range from simple NHCs such as
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or (N,N’-
di(tert-butyl)imidazol-2-ylidene) (ItBu),3 to the introduction of
long alkyl chains,4 additional binding motives,5 hydrophilic
groups6 or chiral motives.7 In addition to other potentially
interesting parameters, the integration of a natural product into
a ligand is attractive due to the diversity of the compounds, the
readily accessible functional complexity and the possibility to
act as a recognition unit. Very recently, we synthesized two
The hydrogenation of aromatic substrates is of great interest
from a synthetic organic and an industrial point of view9 since
the generated cyclohexanes and cyclohexenes can serve as
precursors for further functionalization10 or as motifs in
functional materials like pharmaceuticals.11 The high stability of
arenes due to their aromaticity makes their reduction difficult
and catalysts in forcing conditions are usually required.12 As
ruthenium nanoparticles (Ru-NPs) proved to be efficient
materials to probe NP surface properties13 and feature
interesting catalytic activities14 including the hydrogenation of
imidazolium salts derived from cholesterol (
(Figure 1) and examined them in biophysical and cellular
studies, identifying compound ·HI as a functional analogue of
natural cholesterol in
cellular environment.8 Unlike
1·HOMs and 2·HI)
arenes3c we investigated the rigid ligands
synthesis of Ru-NPs. The characteristics and catalytic
performances of the obtained NPs (Ru@ and Ru@2,
1 and 2 in the
2
1
a
respectively) were examined to establish the influence of the
ligand.
imidazolium salts bearing long alkyl chains these molecules
contain a rigid, lipophilic moiety either as the N-substituent or
a.Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster, Germany. E-mail: glorius@uni-muenster.de
b.LPCNO; Laboratoire de Physique et Chimie des Nano-Objets, UMR5215 INSA-CNRS-
UPS, Institut des Sciences Appliquées, 135, Avenue de Rangueil, F-31077 Toulouse,
France. E-mail: chaudret@insa-toulouse.fr
c.
LCC-CNRS, Université de Toulouse, CNRS, 205, Route de Narbonne, F-31077
Toulouse, France.
Scheme 1: Synthesis of Ru@1 and Ru@2.
†Electronic
supplementary
DOI: 10.1039/x0xx00000x
‡ These authors contributed equally to this work.
information
(ESI)
available.
See
DOI:
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According to a previous procedure for the synthesis of Ru-NPs NMR spectroscopy (ESI, Figures S5 and S7). In both cases, a
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with non-isolable NHCs,15 the preparation of the NPs was broad peak at δ ≈ 220 ppm and a sharp one at δ ≈ 190 ppm were
DOI: 10.1039/C8CC02833H
achieved by controlled decomposition of Ru(cyclooctadiene) observed which could be attributed to COb and COt,
(cyclooctatriene) [Ru(COD)(COT)] at 3 bar of H2 [room respectively. The FT-IR spectra of Ru@1 and Ru@2 (ESI, Figures
temperature (r.t.) in THF] in the presence of 0.2 equivalent S12 and S13) indicated the presence of a CO absorption band
(equiv.) of each NHC (Scheme 1). In situ deprotonation of the (ca. 1920 cm-1 for both NPs) already before the reaction with
imidazolium salts studied by NMR demonstrated the possibility CO, which indicates the decarbonylation of THF during the NP
to generate the free NHCs
conditions (ESI, Figure S14 and S15).
Both samples, Ru@ and Ru@2, contain spherical and well- into a THF solution during 5 min), the absorption band showed
1
and
2
by the above mentioned synthesis demonstrating the high reactivity of the NPs, as
previously observed.17 After the reaction with CO (bubbling CO
1
distributed NPs with a mean diameter of ca. 1.5 (0.3) nm and a higher intensity as well as a shift to ca. 1970 cm-1. An
ca. 1.4 (0.3) nm, respectively. High-resolution TEM (HRTEM) additional band appeared at ca. 2030 cm-1, that may correspond
analysis revealed for both systems crystalline NPs with a to CO coordinated in a multi-terminal mode Ru(CO)2 as was
hexagonal close-packed (hcp) structure, as for bulk Ru (ESI, previously observed for rhodium NPs.18
Figures S1 and S2). Analysis via Wide-Angle X-Ray Scattering Thanks to the high solubility of the NPs, liquid NMR studies
(WAXS) confirmed the HRTEM observations by showing a could be performed and confirmed the existence of the NHC on
pattern consistent with small NPs of metallic Ru in the hcp the NP surface (Figure 3 and ESI, Figures S8 and S9). From
structure and a coherence length of ca. 1.6 nm with no sign of previous studies on NPs, it is known that groups closely located
oxidation (ESI, Figure S3). Evidence for the presence of the NHCs to the particles suffer from a peak broadening or even a peak
on the NPs could be found by 13C magic angle spinning solid- disappearance in 1H and 13C NMR.19 In 1H NMR spectra of Ru@
state NMR (MAS-NMR) (ESI, Figures S4 and S6). MAS 13C{1H} and Ru@
the methyl groups of the N-substituents (ca. 4.1 ppm
NMR spectra of Ru@ and Ru@ ·HOMs and ca. 4.0 ppm for ·HI), the NHC-backbone
performed with 1H-13C cross- for
polarization (CP) presented signal at ca. 55 ppm protons (ca. 7.5 ppm for ppm for ·HOMs) and the proton of the
corresponding to the CH3-group attached to the nitrogen atoms cholesterol ring in direct neighbourhood to the NHC core
1
2
1
2
1
2
a
1
and a group of resonances between 40 and 10 ppm which (ca. 4.5 ppm for
belongs to the steroidal body of the NHC. The amount of
ruthenium in the NPs was determined by Atomic Absorption
Spectroscopy (AAS) displaying a metal content of ca. 50% (51%
1·HOMs) vanished.
Ru@
4.0:1 for Ru@
with the bulky N-substituent (
coordination to the Ru surface, and as a result, Ru@
higher metal/ligand ratio than Ru@ . Consequently, given the
similar mean sizes of the NPs Ru@ is expected to have slightly
more free available Ru sites than Ru@
1
, 47% Ru@
(ESI, Table S1). As one might expect the ligand
) needs more room for
has a
2) and a Ru/ligand ratio of 4.8:1 for Ru@1 and
2
1
1
2
1
2
.
Figure 2: TEM micrographs and the corresponding size
histograms of Ru@ (A) and Ru@ (B).
Figure 3: 1H liquid-NMR spectra (in d8-THF) of Ru@
·HOMs ( ) and Ru@ and ·HI ( ). Peaks that correspond to
1
and
1
2
1
A
2
2
B
To learn whether the free surface sites are accessible for
substrates, the coordination of CO onto Ru@ and Ru@ was
protons at or close to the imidazolium core are labelled. Peaks
marked with an asterisk (*) correspond to CH2Cl2 (δ 5.5 ppm),
which comes from the purification step of the ligands, and
1
2
examined by MAS-NMR and infrared spectroscopy (FT-IR). CO
can coordinate to the metal surface in two different modes: a
bridging mode (COb), the sites of which are located on the faces
of the NPs, and a terminal binding mode (COt), the sites of which
are located on apexes and edges.16 For this purpose, the Ru-NPs
were reacted with 13CO (1 bar, r.t., 20 h) and analysed by 13C{1H}
MeOH (δ 3.2 ppm), which arises from the purification of Ru@
and Ru@
1
2
.
Thus, as expected, line broadening and peak disappearance was
observed for most of the ligand signals, indicating not only the
2 | J. Name., 2012, 00, 1-3
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coordination of the NHCs on the NP surface, but also a full (No
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6
) could be performed highlighting the potential of our Ru-
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rigidity of the ligands and absence of dynamical exchange NPs for the transformation of ligDnOoI:c1e0ll.1u0lo39s/iCc8CbCi0o2m83a3sHs
between the surface of the NP and the solution. The lack of derivatives 20
. TEM analysis performed after the hydrogenation
mobility of the ligands and their strong coordination to the Ru of styrene (ESI, Figures S16 and S17) revealed similar sizes for
surface makes these NPs perfect candidates to become the NPs as before catalysis thus demonstrating their stability
chemioselective and stable catalysts.
Furthermore, diffusion-ordered spectroscopy (DOSY) NMR Ru@
yielded the diffusion coefficients of the Ru-NPs in comparison naphthalene (
under catalytic conditions. Differences in activity of Ru@
could be observed for more complex substrates like
), biphenyl (10) and acetophenone (17). Ru@
1 and
2
7
1
to those of the free ligands (ESI, Figures S10 and S11). As enabled not only their complete conversion but also presented
expected, diffusion of the surface coordinated ligand is slower a higher reactivity for the fully hydrogenated compounds. A
than that of the free ligand in solution: 2.4 x 10-10 m2s-1 for Ru@
against 5.0 x 10-10 m2s-1 for ·HOMs; 2.5 x 10-10 m2s-1 for Ru@
against 6.0 x 10-10 m2s-1 for
1
2
lower activity was observed for Ru@
2 with these three
1
substrates, since the hydrogenation of the compounds was
2
·HI. These results confirm once either not quantitative (No 4a) or the amount of fully
again the coordination of the ligands to the NP surface (further hydrogenated products was significantly lower (No 3, 4a, 7
details are given in the ESI). leading to moderate selectivities. Lowering the hydrogen
pressure (No 4e) in the hydrogenation of biphenyl (10) slightly
Table 1: Application of Ru-NPs in the mild hydrogenation of improves selectivity in the case of Ru@ but additionally lowers
)
1
arenes.a
the reactivity for both systems. Interestingly, when the catalytic
loading was considerably reduced (from 5% to 0.5%; No 1d),
both Ru-NPs presented totally different selectivities for the
No
1
cat
starting materials
productsb
conv./product ratiosc
100%/4:5 = 100:0a
hydrogenation of styrene (
hydrogenated product (ethylcyclohexane), Ru@
selectivity switch by completing selective partial
3
). While Ru@
1
produced the fully
Ru@1
Ru@2
100%/4:5 = 100:0d
100%/4:5 = 100:0a
100%/4:5 = 0:100d
2
performed a
a
Ru@1
Ru@2
100%
hydrogenation process, forming only ethylbenzene. Similar
results were observed in a multiple addition test were a new
batch of styrene was added to the reaction mixture four times
(ESI, S12). During the first reaction run a complete
hydrogenation of styrene occurred for both systems resulting in
the formation of ethylcyclohexane. Interestingly, the following
additions of styrene resulted in its full conversion for both NPs
2
3
100%
Ru@1
Ru@2
100%/8:9 = 60:40
100%/8:9 = 87:13
100%/11:12 = 60:40a
Ru@1
Ru@2
Ru@1
91%/11:12 = 74:26e
4
70%/11:12 = 75:25a
56%/11:12 = 75:25e
100%
5
6
but in the case of Ru@
ethylbenzene was observed, while Ru@
ethylcyclohexane for all additions. These results illustrate once
more the superior activity of Ru@ compared to Ru@2. This
2
only the partial hydrogenation to
Ru@2
Ru@1
Ru@2
100%
100%
100%
1
exclusively formed
Ru@1
Ru@2
100%/16:17 = 0:100
100%/18:19 = 20:80
1
7
difference may result from the ligand structures. Due to the
more flexible connection of the steroid to the NHC core in ligand
, it possesses a higher steric demand than
lower NHC amount on the Ru@ surface (ESI, table S1), and
therefore in a higher quantity of free faces. As faces are
necessary to hydrogenate aromatic rings, Ru@ presents a
a: Reaction conditions: substrate (0.2 mmol), Ru-NPs (2 mg, 0.01 mmol
Ru assuming ≈ 50% Ru from AAS), THF (1 mL), hydrogen (5 bar), r.t., 20 h;
b: products were identified by GC-MS analysis; c: conversions and ratios
were determined by GC-FID analysis with mesitylene (0.2 mmol) as
internal standard; d: substrate (2 mmol); e: hydrogen (1 bar).
1
2 which results in a
1
1
higher activity in hydrogenation of arenes as was already
observed for other stabilized Ru-NPs.3c,17 In molecular complex
catalysts similar phenomena have been reported, i.e. bulky
ligands giving more active catalysts.21 Unfortunately, the
selectivity switch by lowering the catalyst loading or changing
the conditions (temperature, solvent, pressure, time) could not
Knowing that NHCs bearing rigid moieties are capable of
stabilizing Ru-NPs, the effect of the steroidal body on the
catalytic activity should be tested. In former reports, Ru-NPs
were recognized as active catalysts for the hydrogenation of
various arenes,3c,14,17 e.g. the selective hydrogenation of the
arene moiety in benzoic acid.14g Additionally, dependencies of
the ligand bulkiness on the selectivities were observed in the
hydrogenation of arenes that bear other functional groups.3c,17
Inspired by the activities of known catalysts we wanted to
be applied for other substrates (7, 10, 17).
Herein, for the first time we report biomimetic NHCs derived
from cholesterol and the successful application of these rigid
NHCs as stabilizers of Ru-NPs. The particles obtained were fully
characterized, including analysis by liquid NMR techniques
given their high solubility induced by the NHCs ligands. They
were applied as catalysts in the hydrogenation of arenes at r.t.
investigate the catalytic differences of Ru@1 and Ru@2 in the
mild hydrogenation of arenes to learn about the impact of the
rigid moiety (2 mg NPs, 5 bar H2, r.t., 20 h – table 1). Various
aromatic compounds were successfully hydrogenated resulting
in a complete hydrogenation of the aromatic moiety (No 1a, 2,
Ru@1 showed a higher activity than Ru@2, especially at lower
catalyst loading and in the multi-addition experiments. This
difference in activity is attributed to steric differences between
the two NHCs which in fact appears as a way to tune the
5
) and the functional group (No 1a, 2) illustrating the high
activity of the catalysts. Moreover, the hydrogenation of phenol
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nanocatalyst surface
hydrogenation of phenol was successfully performed showing
the interest of these catalysts for the transformation of
lignocellulosic biomass derivatives. The application of the rigid
ligands without ligand loss not only opens the way to catalysis
in the solid state but also offers the possibility to utilize it as a
recognition unit for biological systems as well as for substrate
interactions in catalysis which we will focus on in the future.
Journal Name
T. Zamora, L. N. Saunders, A. Rousina-Webb, M. Nambo and
C. M. Crudden, J. Am. Chem. Soc., 201D8,O1I:4100,.110539V7i/6eCw.8ACrtCic0le2O8n3l3inHe
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properties.
Furthermore,
the
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We thank the Deutsche Forschungsgemeinschaft (SFB 858) and
CNRS, UPS-Toulouse, INSA, IDEX/Chaires d’attractivité
l’Université Fédèrale Toulouse Midi-Pyrénées for generous
financial support. We also thank L. Datas for TEM facilities (UMS
Castaing), S. Cayez for HRTEM micrographs, Y. Coppel (LCC,
CNRS) for NMR measurements and P. Lecante (CEMES, CNRS)
for WAXS analysis.
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There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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