Long-chain NHC-stabilized RuNPs as versatile catalysts for one-pot oxidation/hydrogenation reactions

Article abstract of DOI:10.1039/c6cc01130f

The synthesis and catalytic activity of long-chain NHC-stabilized RuNPs are presented. Full characterization of these novel nanostructures including surface state studies show that the ligand influences the number and the location of Ru active sites which impacts the NP catalytic activity, especially in hydrogenation reactions. The high stability and versatility of these nanosystems make them successful catalysts for both oxidation and hydrogenation reactions that can even be performed successively in a one pot-fashion.

Full text of DOI:10.1039/c6cc01130f

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. M. Martinez-
Prieto, A. Ferry, L. Rakers, C. Richter, P. Lecante, K. Philippot, B. Chaudret and F. Glorius, Chem.
Commun., 2016, DOI: 10.1039/C6CC01130F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C6CC01130F
Journal Name
COMMUNICATION
Long-Chain NHC-stabilized RuNPs as Versatile Catalysts for One-
pot Oxidation/Hydrogenation Reactions
L. M. Martínez-Prieto,a,b,† A. Ferry,c,† L. Rakers,c C. Richter,c P. Lecante,d K. Philippot,b,* B.
Chaudret,a,* and F. Glorius c,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis and catalytic activity of long-chain NHC-stabilized reported the use of NHC ligands to stabilize well-defined
RuNPs are presented. Full characterization of these novel ruthenium nanoparticles (RuNPs) thanks to their electron-rich
nanostructures including surface state studies show that the property and their capability to form strong metal ligand
ligand influences the number and the location of Ru active sites interactions.5c Moreover, some of us demonstrated that NHCs
which impacts the NP catalytic activity, especially in possessing long aliphatic chains in positions 4 and 5 of the
hydrogenation reactions. The high stability and versatility of these imidazole ring increase the stability of palladium nanoparticles,
nanosystems make them successful catalysts for both oxidation forming a protective monolayer around the metallic surface.5h
and hydrogenation reactions that can be even performed Based on these previous works, we opted to investigate the
successively in a one pot-fashion.
synthesis of two types of RuNPs stabilized with long aliphatic
chain NHCs (LC-NHCs), containing either small or bulky
substituents in positions 1 and 3, namely LC-IMe (1,3-dimethyl-
The use of metal nanoparticles (MNPs) as catalysts has gained
growing interest in the scientific community, with applications
4,5-diundecyl
imidazoline)
and
LC-IPr
(1,3-bis(2,6-
in
a wide range of organic transformations such as
diisopropylphenyl)-4,5-diundecyl imidazoline) (Figure 1). With
these two different LC-NHCs in hand, we evaluated the
influence of the N-substituents on the size and reactivity of the
RuNPs. The small methyl groups will minimize steric hindrance
at the metallic surface, whereas diisopropylphenyl
substituents are very bulky and might coordinate to the metal
surface through van der Waals or π-interactions. We expected
these novel RuNPs to possess high stability compared to those
previously described. Their stability was investigated by TEM,
HRTEM and WAXS analysis and their surface chemistry was
probed by MAS-NMR and FT-IR after reaction with CO. The
catalytic performance of these nanosystems was evaluated in
hydrogenation and oxidation reactions including one-pot
oxidation-hydrogenation processes. The route recently
developed for RuNPs capped with non-isolable NHCs,5g was
applied to prepare the long-chain NHC-stabilized RuNPs,
namely Ru@LC-IPr and Ru@LC-IMe. Stable ruthenium colloidal
solutions were hence obtained (Scheme 1) by the
hydrogenation, polymerization, oxidation and C-C coupling
reactions.1 Their importance in catalysis comes from their large
metallic surface area possessing numerous isolated active sites
thus combining the benefits of homogeneous and
heterogeneous catalysts. However, the high reactivity of
unsupported NPs makes them, most of the time, very sensitive
to air and moisture. The catalytic activity of MNPs is intensely
affected by their size and the nature of the ligands used as
stabilizers.2 Indeed, the stabilizing agents are capable of tuning
the surface reactivity by modification of electronic and steric
properties of the metallic surface.
N-Heterocyclic carbenes (NHCs) are among the most versatile
ligands for transition metals in molecular coordination
chemistry, broadly used in many catalytic reactions.3 Their use
as ligands/stabilizers/modifiers of metal nanoparticles4-6 and
flat metal surfaces7 has been explored recently. Some of us
Figure 1. LC-IMe and LC-IPr long chain carbene ligands.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C6CC01130F
COMMUNICATION
Journal Name
decomposition of the complex (1,5-cyclooctadiene)(1,3,5- after (red) exposure to CO (1 bar, 20 h, r.t.). Remarkably, both
cyclooctatriene)ruthenium [(Ru(COD)(COT)] (THF; room FT-IR spectra exhibit the CO absorption band (between 1900
temperature (r.t.); 3 bar H2) in the presence of 0.3 or 0.1 and 2000 cm-1) before the exposure to CO, which probably
equivalent (equiv.) of LC-NHCs (LC-IMe or LC-IPr), previously results from a decarbonylation process of THF during the NP
generated in solution from their corresponding imidazolium synthesis, as recently observed.2d This evidences the high
salts. After purification by washing with MeOH, the RuNPs, reactivity of 1a and 2a. In both cases, the coordination of CO
obtained as a black powder, were re-dispersed in THF for was confirmed through the increase in intensity and shift to
Transmission Electronic Microscopy (TEM) analysis (Figure 2). higher energy of the aforementioned band after CO exposure,
Ru@LC-NHCs prepared with 0.3 equiv., Ru@LC-IMe0.3
Ru@LC-IPr0.3 2a) reveal spherical, crystalline and well- The coordination of the LC-NHCs on the Ru surface was
( .
1a) and specifically at 2011 cm-1 for 1a and 2012 cm -1 for 2a
(
dispersed NPs with a mean diameter of 1.3 (0.2) and 1.2 (0.3) attested by 13C{1H} MAS-NMR. The spectra of 1a and 2a
nm, respectively and a narrow size dispersity. With 0.1 equiv. samples (bottom of Figures S7 and S8, ESI) present most of
of LC-IPr (Ru@LC-IPr0.1
;
2b) larger NPs were obtained (1.5 (0.4) their characteristic peaks. The broad and low intensity signal
1b near 120-130 ppm corresponds to the aromatic rings of the LC-
nm) with a higher size dispersity while Ru@LC-IMe0.1
(
)
presented a similar size to 1a (Figure 2). High resolution TEM IPr ligand and/or the imidazole backbone. The peaks at 29 ppm
(HRTEM) micrographs of 1a (Figure S1, ESI) and 2a (Figure S2, and 22 ppm belong to CH2 of the long alkyl chains as well as
ESI) indicate the presence of crystalline NPs displaying the some alkyl groups from the nitrogen-bonded substituents (iPr
hexagonal close packed (hcp) structure of bulk Ru. Wide-Angle or Me). The methyl groups at the extremity of the long alkyl
X-ray Scattering (WAXS) analysis of 1a and 2a evidences the chains are visible at 13 ppm for 1a and at 14 ppm for 2a. These
presence of unoxidized crystalline hcp RuNPs having
a
signals are not visible on CP-MAS 13C{1H} NMR spectra (top of
coherence length of ca. 1.3 nm, which is in accordance with Figures S7 and S8, ESI) probably due to the high mobility of CH3
HRTEM observations (Figures S3 and S4, ESI). 1a and 2a were groups.
characterized by infrared spectroscopy (FT-IR) and magic angle Coordination of CO was previously used to probe the NP
1
spinning solid-state NMR (MAS-NMR) with and without H-13C metallic surface and it was proposed that COs coordinated in a
cross-polarization (CP) using the coordination of CO as a tool bridging mode (COb) are located on the faces of the NPs and
to locate the actives sites on the Ru surface. Figures S5 and S6 terminal COs (COt) on their apexes and edges.8 When 1a and
(see ESI) show FT-IR spectra of 1a and 2a before (blue) and 2a were reacted with 13CO (1 bar, r.t., 20 h), MAS 13C{1H} NMR
spectra (Figure 3) exhibited two new signals. The broad peak
at δ ~ 230 ppm can be assigned to COb and the sharper
resonance at δ = 198-196 ppm to COt. In the 2a spectrum, the
COb peak has a higher intensity than in the 1a one, evidencing
as expected from the bulkiness of the ligand that 2a has more
available face sites than 1a. In CP MAS, 13C{1H} spectra of 1a
and 2a (Figure S9, ESI) the broad peak of COb is not visible and
the intensity of COt peak decreases, potentially indicating that
COb are not located close to LC-NHCs (hydrogen carriers) while
COt are only partially located nearby the ligands. Interestingly,
after reaction with CO, the peaks at 130-125 ppm (aromatic
rings and imidazolium backbones) are not broad any more,
probably because of their displacement from the surface by
CO which allows them to tumble in the vicinity of the particle.
Figure 2. TEM micrographs and the corresponding size histograms of
Ru@LC-IMe0.3 1a (left top), Ru@LC-IMe0.1 1b (right top), Ru@LC-IPr0.3 2a
(left bottom) and Ru@LC-IPr0.1 2b (right bottom).
Figure 3. 13C MAS NMR spectra of Ru@LC-IMe0.3 1a (top) and Ru@LC-
IPr0.3 2a (bottom) after exposure to 13CO (1 bar, 20h, at r.t.). The signal
with asterisk corresponds to grease.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C6CC01130F
Journal Name
COMMUNICATION
All these data confirm that the RuNPs have different surface converted into the corresponding amino-acid 12 only in the
states depending on the ligand. In particular, 2a has more case of 2a (Table 1, entries 7 and 8). These experiments clearly
available face sites than 1a. We can thus expect different show a high influence of the nitrogen-bonded substituents on
catalytic behavior (vide infra).
the reactivity of the RuNPs in agreement with the number of
NHC-stabilized RuNPs are efficient catalysts for the free active sites on metallic surface (2a having more available
hydrogenation of arenes, ketones and olefins.5c,d,g, Moreover, it face sites than 1a), whereas change of the loading of the ligand
was recently reported that LC-NHC-stabilized PdNPs present has only little influence.
interesting chemoselectivities in the hydrogenation of To test the stability of these NPs against oxygen, 1a and 2a
olefins.5h Influence of this micelle-type NHC-backbone on the were exposed to air for 3 days at r.t. In these conditions, only a
reactivity and chemoselectivity in hydrogenation (Table 1) as partial oxidation was observed by WAXS (Figures S3 and S4,
well as in oxidation reactions9 (Table 2) of the RuNPs was thus ESI). The oxide formed is mostly amorphous while the
investigated. Significant differences of reactivity between the ruthenium core displays a small residual coherence length.
two nanosystems were observed in hydrogenation reactions. Consequently, oxidation reactions using oxygen as oxidant
Indeed, neither ketone function nor the aromatic part were were tested (Table 2). Oxidation of primary alcohols into
reduced with 1a under tested conditions (Table 1, entries 1, 3 aldehydes (Table 2, entries 1-4), and benzyl amine into
and 5) whereas 2a mainly led to the over-reduced compound benzonitrile (Table 2, entries 5 and 6) were performed with
(Table 1, entries 2, 4 and 6). The change of LC-NHC loading both catalysts. The order of activity again is 2a
from 0.3 to 0.1 equiv. makes Ru@LC-IMe catalyst slightly more are less active (around 25% yield in the oxidation of 13 for
active (19% versus 0% in the hydrogenation of leading mainly both 1b and 2b and around 55% in the oxidation of 15). The
to ). 1a and 1b having a similar size (1.3 nm), the difference of lower reactivity of 1b and 2b can be explained by the lower
>1a. 1b and 2b
3
4
reactivity can be explained by the higher accessibility of the quantity of LC-NHCs in 1b and 2b which makes them less
metallic surface of 1b, allowing a better diffusion of the stable under oxidative conditions. Successive oxidation
substrate on the surface. In contrast to Ru@LC-IMe, the use of reactions were performed complementary with 1a and 2a
2b led to slight decrease of the reactivity in the After a first oxidation run (16 h) with 13, a second substrate
hydrogenation of (89% versus 100%), probably due to the was added (15) for another 24 h run under fresh O2. For both
larger size and the polydispersity of 2b compared to 2a 1a and 2a a higher conversion for 13 was then observed as
.
a
3
.
Following the same reactivity trend, acetamide 11 was well as a good conversion for 15 (Scheme S1, stability
experiments, ESI), showing the stability of the system for at
Table 1. Hydrogenation reactions catalyzed by 1a and 2a.a
least 40 h. “Post-mortem” TEM and HRTEM analysis showed
that the size and dispersion of both NPs did not change
Entry/ Ru-
Conv. (%)c
Substrate
Products b
Cond.
significantly after the first run despite
a loss of their
NP
crystallinity (Figures S10 and S11, ESI), in accordance with
previous WAXS analyses. The possibility to run successively
oxidation and hydrogenation reactions in a one-pot process
was investigated (Table 3).
1/1a
0
H2 (5 bar)
hexane 16
h, 25 °C
2/2a
98
Oxidation conditions were first optimized to obtain
quantitative conversions. Once the oxidation was complete
(determined by GC/MS analysis), O2 atmosphere was replaced
3/1a
65
(only 6)
Table 2. Oxidation reactions catalyzed by 1a and 2a.a
H2 (5 bar)
toluene
16 h,
Entry/ Ru-
NP
Productsb
Conv. (%)c
4/2a
quant
(only 7 & 8 :
8>>7)
Substrate
25 °C
48
79
1/1a
2/2a
5/1a
6/2a
0
H2 (35 bar)
hexane
16 h,
65
90
3/1a
4/2a
quant
30 °C
7/1a
8/2a
H2 (5 bar)
toluene
16 h,
Trace
quant
15
30
5/1a
6/2a
25 °C
a. Reaction conditions: Substrate (0.2 mmol), Ru-NPs (1mg) under inert
atmosphere. The pressure of H2, solvent, temperature and time are indicated
in the Table.
b. Determined by GC/MS
c. Conversions were determined by GC/FID
a. Reaction conditions: Substrate (0.2 mmol), Ru-NPs (1 mg), O2 (1 bar),
trifluorotoluene (1 mL), 25 °C, 16 h under inert atmosphere.
b. Determined by GC/MS
c. Conversions were determined by GC/FID
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C6CC01130F
COMMUNICATION
Journal Name
Datas for TEM facilities (UMS-Castaing) and Y. Coppel (LCC) for
NMR.
Table 3. One-pot oxidation-hydrogenation reactions.a
Entry/ Ru-
NP
Subs
t.
Conv.c,d (%)
Select.d
1
(a) A. Roucoux, J. Schulz and H. Patin, Chem. Rev. 2002, 102,
Cond.
Prod.b
3757; (b) Nanoparticles and Catalysis, Ed. D. Astruc, Wiley-
VCH, Weinheim, 2008; (c) Nanomaterials in Catalysis, Eds. P.
Serp, K. Philippot, Wiley-VCH, Weinheim 2013.
1/1a
(1.0 mg)
65
19/20/21:
83/16/1
1) O2 (1 bar)
PhCF3,
2
(a) K. An, G. A. Somorjai, Chem. Cat. Chem. 2012,
D. González-Gálvez, P. Nolis, K. Philippot, B. Chaudret, P. W.
N. M. van Leeuwen, ACS Catal. 2012, , 317; (c) L. M.
4, 1512; (b)
16 h, r.t.
15
2
quant
19/20/21:
0/8/92
2/2a
(1.0 mg)
Martínez-Prieto, S. Carenco, C. H. Wu, E. Bonnefille, S.
Axnanda, Z. Liu, P. F. Fazzini, K. Philippot, M. Salmeron, B.
Chaudret, ACS Catal. 2014, 4, 3160; (d) L. M. Martínez-Prieto,
2) H2 (5 bar),
r.t., 4 h
C. Urbaneja, P. Palma, J. Campora, K. Philippot, B. Chaudret,
Chem. Comm. 2015, 51, 4647.
(a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius,
1) O2 (1 bar)
PhCF3,
16 h, r.t.
3/1a
(1.5 mg)
54
22/23: 99/1
3
Nature 2014, 510, 485; (b) N-Heterocyclic Carbenes in
Synthesis, Ed. S. P. Nolan, Wiley-VCH, New York, 2006; (c) N-
Heterocyclic Carbenes in Transition Metal Catalysis Ed. F.
Glorius, Springer, Berlin, 2007; (d) E. A. B. Kantchev, C. J.
O’Brien, M. G. Organ, Angew. Chem. Int. Ed. 2007, 46, 2768;
(e) S. Würtz, F. Glorius, Acc. Chem. Res. 2008, 41, 1523; (f) F.
E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122;
(g) S. Diez-Gonzalez, N. Marion, S. P. Nolan, Chem. Rev. 2009,
109, 3612; (h) T. Dröge, F. Glorius, Angew. Chem. Int. Ed.
2010, 49, 6940; (i) N-Heterocyclic Carbenes in Transition
Metal Catalysis, Ed. C. S. J. Cazin, Springer, Berlin, 2011.
Review on the use of NHCs for nanoparticles and surfaces: A.
V. Zhukhovitskiy, M. J. MacLeod, J. A. Johnson, Chem. Rev.
2015, 115, 11503.
For NHCs as ligands for unsupported NPs see for example: (a)
J. Vignolle and T. D. Tilley, Chem. Commun. 2009, 7230; (b) E.
C. Hurst, K.Wilson, I. J. S. Fairlamb and V. Chechik, New J.
Chem. 2009, 33, 1837; (c) P. Lara, O. Rivada-Wheelaghan, S.
Conejero, R. Poteau, K. Philippot and B. Chaudret, Angew.
Chem. Int Ed. 2011, 50, 12080; (d) D. Gonzalez-Galvez, P.
Lara, O. Rivada-Wheelaghan, S. Conejero, B. Chaudret, K.
Philippot and P. W. N. M. van Leeuwen, Catal. Sci. Technol.
2) isopropyl-
amine (1.5
equiv.),
99
22/23:
78/22
13
4/2a
(1.5 mg)
2 h, r.t.
3) H2
(5 bar),
45 °C, 16 h
a. Reaction conditions: Substrate (0.1 mmol) under inert atmosphere. The
quantity of Ru-NPs, the pressure of H2 and O2, solvent, temperature and time are
indicated in the Table.
b. Determined by comparison of GC/MS data with commercial or synthesized
references
4
5
c. Conversions correspond to the hydrogenation step.
d. Conversions and selectivities were determined by GC-FID.
by H2 for further hydrogenation to take place. Thus aldehyde
19 was obtained in a one-pot process using 1a, via quantitative
oxidation of 15 into 16 followed by hydrogenation
predominantly leading to aldehyde 19 (Table 3, entry 1).
2013,
Jesús and B. Chaudret, Angew. Chem. Int. Ed. 2014, 53
13220; (f) A. Ferry, K. Schaepe, P. Tegeder, C. Richter, K. M.
Chepiga, B. J. Ravoo, F. Glorius, ACS Catal. 2015, , 5414; (g)
L. M. Martínez-Prieto, A. Ferry, P. Lara, C. Richter, K.
Philippot, F. Glorius and B. Chaudret, Chem. Eur. J, 2015, 21
3, 99; (e) E. A. Baquero, S. Tricard, J. C. Flores, E. de
Contrary to 1a, 2a mainly led to the over-reduced compound
,
21 (Table 3, entry 2). Likewise, amine 22 was obtained in one
pot from 13. Indeed, intermediate 14, formed by oxidation of
13, reacted with isopropylamine to form the corresponding
imine which was selectively reduced with 54% to quant
conversion (Table 3, entries 3 and 4).
5
,
17495; h) C. Richter, K. Schaepe, F. Glorius, B. J. Ravoo,
Chem. Commun. 2014, 50, 3204. For the application of these
long chain NHCs in micellar catalysis, see: i) A. Rühling, H.-J.
Galla, F. Glorius, Chem. Eur. J. 2015, 21, 12291.
For NHCs on supported metal NPs, see: (a) K. V. S.
Ranganath, J. Kloesges, A. H. Schäfer, F. Glorius, Angew.
Chem. Int. Ed. 2010, 49, 7786; (b) K. V. S. Ranganath, A.
Conclusions
6
7
In summary, two long-chain NHC-stabilized RuNP samples
were successfully synthesized and fully characterized. When
tested in hydrogenation and oxidation reactions, these new
Ru-NPs were shown active, thus attesting the presence of
accessible active sites at their surface despite the long alkyl
chains of the ligands. Interestingly, a clear influence of the
ligand on selectivity and activity of the NPs was observed.
Successful application of these RuNPs in one-pot oxidation-
hydrogenation processes evidenced the value of these new
systems of NPs as versatile air-stable catalysts for several
organic transformations.
Schäfer, F. Glorius, Chem. Cat. Chem. 2011, 3, 1889; (c) D. Yu,
M. X. Tan and Y. Zhang, Adv. Synth. Catal. 2012, 354, 969.
For NHCs on metal surfaces, see: (a) T. Weidner, N. Ballav, U.
Siemeling, D. Troegel, T. Walter, R. Tacke, D. G. Castner, M.
Zharnikov, J. Phys. Chem. C. Nanomater. Interfaces 2009,
113, 19609; (b) T. Weidner, J: E. Baio, A. Mundstock, C.
Große, S. Karthäuser, C. Bruhn, U. Siemeling, Aust. J. Chem.
2011, 64, 1177; (c) A. V. Zhukhovitskiy, M. G. Mavros, T. V.
Voorhis, J. A. Johnson, J. Am. Chem. Soc. 2013, 135, 7418; (d)
C. M. Crudden, J. H. Horton, I. I. Ebralidze, O. V. Zenkina, A.
B. McLean, B. Drevniok, Z. She, H.-B. Kraatz, N. J. Mosey, T.
Seki, E. C. Keske, J. D. Leake, A. Rousina-Webb, G. Wu,
Nature Chem. 2014, 6, 409.
Notes and references
8
9
F. Novio, K. Philippot, B. Chaudret, Catal. Lett. 2010, 140, 1.
(a) M. Tristany, K. Philippot, Y. Guari, V. Collière, P. Lecante,
B. Chaudret, J. Mater. Chem. 2010, 20, 9523; (b) E. J. García-
Suárez, M. Tristany, A. B. García, V. Collière, K. Philippot,
Micropor. Mesopor. Mat. 2012,153, 155.
‡ We thank CNRS, UPS-Toulouse, INSA, EU (ERC Advanced Grant,
NANOSONWINGS 2009-246763)
Forschungsgemeinschaft (SFB 858
and
the
Deutsche
Synergetic effects in
–
chemistry) for financial support. We also thank V. Collière and L.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins