N-Heterocyclic Carbene (NHC)-Stabilized Ru0 Nanoparticles: In Situ Generation of an Efficient Transfer Hydrogenation Catalyst

Article abstract of DOI:10.1002/chem.202000142

Tethered and untethered ruthenium half-sandwich complexes were synthesized and characterized spectroscopically. X-ray crystallographic analysis of three untethered and two tethered Ru N-heterocyclic carbene (NHC) complexes were also carried out. These RuNHC complexes catalyze transfer hydrogenation of aromatic ketones in 2-propanol under reflux, optimally in the presence of (25 mol %) KOH. Under these conditions, the formation of 2–3 nm-sized Ru⁰ nanoparticles was detected by TEM measurements. A solid-state NMR investigation of the nanoparticles suggested that the NHC ligands were bound to the surface of the Ru nanoparticles (NPs). This base-promoted route to NHC-stabilized ruthenium nanoparticles directly from arene-tethered ruthenium–NHC complexes and from untethered ruthenium–NHC complexes is more convenient than previously known routes to NHC-stabilized Ru nanocatalysts. Similar catalytically active RuNPs were also generated from the reaction of a mixture of [RuCl₂(p-cymene)]₂ and the NHC precursor with KOH in isopropanol under reflux. The transfer hydrogenation catalyzed by these NHC-stabilized RuNPs possess a high turnover number. The catalytic efficiency was significantly reduced if nanoparticles were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures during the reaction.

Full text of DOI:10.1002/chem.202000142

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Accepted Article
Title: NHC stabilized Ru(0) nanoparticles: in-situ generation of an
efficient transfer hydrogenation catalyst
Authors: Lakshay Kathuria, NoorUDin Reshi, and Ashoka Samuelson
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NHC stabilized Ru(0) nanoparticles: in-situ generation of an
efficient transfer hydrogenation catalyst
Lakshay Kathuria, Noor U Din Reshi, and Ashoka G. Samuelson*[a]
Abstract: Tethered and untethered ruthenium half-sandwich
complexes were synthesized and characterized spectroscopically. X-
ray crystallographic analysis of three untethered and two tethered
RuNHC complexes were also carried out. These RuNHC complexes
catalyze transfer hydrogenation of aromatic ketones in refluxing 2-
propanol, optimally in the presence of (25 mol%) KOH. Under these
conditions, the formation of 2-3 nm-sized Ru(0) nanoparticles was
detected by TEM measurements. A solid-state NMR investigation of
the nanoparticles suggested that the NHC ligands were bound to the
surface of RuNPs. This base promoted route to NHC stabilized
ruthenium nanoparticles directly from arene-tethered ruthenium-NHC
complexes and from untethered ruthenium-NHC complexes is more
convenient than previously known routes to NHC stabilized Ru nano-
catalysts. Similar catalytically active RuNPs were also generated from
the reaction of a mixture of [RuCl2(p-cymene)]2 and the NHC
precursor with KOH in refluxing isopropanol. The transfer
hydrogenation catalyzed by these NHC stabilized RuNPs possess a
high turn-over number. The catalytic efficiency significantly reduced if
nanoparticles were exposed to air or allowed to aggregate and
precipitate by cooling the reaction mixtures during the reaction.
Most nanoparticulate catalysts are best stabilized by surfactants,
solvents or ligands which provide stability by binding to the metal
surface.[10,17–20] Since the catalytic properties of MNPs are
strongly influenced by their surface state,[21,22] many N, S, P, C
based ligands have been used to control the size of NPs and tune
the stereoelectronic properties of the surface. It is not surprising
then, that N-heterocyclic carbenes (NHCs), which have emerged
as ligands par excellence in organometallics,[23] have found utility
in stabilizing MNPs. Their good binding capacity to metals[24–26]
and their modular nature make them attractive.[27,28] Glorius et al.
have reported the stabilization effect of NHC on various metal
nanoparticle systems.[29–31] Chaudret and co-workers utilized
[Ru(cod)(cot)] as the metal precursor to prepare RuNPs[25] in the
presence of reducing dihydrogen and subsequently added
externally generated NHC ligands to stabilize the nanoparticles
and study their physical and catalytic properties.[25,32,33] These
NHC stabilized RuNPs carry out a variety of reactions including
hydrogenation of various unsaturated compounds such as olefins,
ketones, and arenes.[34] Chaudret and co-workers introduced a
betaine adduct of NHC and a carbodiimide ligand to generate
ultra-small ruthenium nanoparticles.[35] Although catalytically
active MNPs can be generated using hydrogen gas and an
organometallic complex,[14,36,37] an easier approach to prepare
NHC stabilized MNPs would be welcome. Catalytically active
ruthenium nanoparticles inadvertently generated from metal
complexes have been found decorated with the ligands used in
the precursor complexes.[15,16] We report here our serendipitous
discovery of a route to catalytically active NHC-stabilized-RuNPs
generated in-situ from tethered and untethered RuNHC half-
sandwich complexes. These NHC stabilized RuNPs catalyze
transfer hydrogenation of ketones far more effectively than the
precursor RuNHC complexes which remain intact in the absence
of a base and also more stable than RuNPs generated in the
absence of the NHC ligands.
Introduction
Catalysis by metal nanoparticles (MNPs) has drawn significant
attention in recent years.[1–3] The large surface area and the
presence of numerous active sites make metal nanoparticles
(MNPs) superior to bulk metals for catalysis. At the same time,
they retain the advantages of heterogeneous catalysts in terms of
high TON, TOF, and the easy separation of catalysts from
products.[4,5] Hence, nanoparticles are promising and high activity
catalysts.[6] Ru and Rh metal-based NPs are the most common
nano-catalysts used for hydrogenation reactions.[7–9] For example,
Liu and co-workers reported hydrogenation of benzene by Ru
nanoparticles immobilized on montmorillonite in ionic liquids.[10]
Song et al. have demonstrated the use of RuNPs for catalyzing
the hydrogenation of nitroaromatic compounds and azo-dyes and
showed that the catalytic activity was dependent on the size of
NPs.[11] Similarly, Wang et al. reported highly efficient and
recyclable PEG stabilized RhNPs for hydrogenation of quinoline
and its derivatives.[12] More recently, flat and pristine fcc RuNP
based catalysts have been reported to perform reductive
amination of various carbonyl compounds for the synthesis of
primary amines.[13] Examples of transfer hydrogenation by Ru
nanoparticles are also known where the catalysts have been
generated from metal complexes.[14–16]
Results and Discussion
A series of benzimidazole derived NHC precursors (L1-L10) were
synthesized by varying one of the wing-tip groups on the nitrogen
and keeping a methyl on the other nitrogen (Scheme S1, and S2).
The 13C resonance of the carbon atom between the nitrogen
atoms of these NHC precursors occurs around 150 ppm. Further
details of the synthesis and characterization of these NHC
precursors are described in the supplementary section.
The untethered ruthenium NHC complexes were synthesized by
transferring the NHC from AgNHC generated in situ[38,39] from the
NHC precursor (L) and silver oxide[38] in the dark to Ru in the
dimer of dichloro(p-cymene)ruthenium(II)[40,41] (Scheme 1).
[a]
L. Kathuria, Dr. N.U Din Reshi and Prof. Dr. A. G. Samuelson
Inorganic and Physical Chemistry Department
Indian Institute of Science
Bangalore, Karnataka, 560012 (India)
E-mail: ashoka@iisc.ac.in
FAX: 91- 80-23601552
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Based on these studies, 25 mol% of the base was used in all other
reactions unless indicated otherwise. Under the optimized
conditions used ([S]/[C]/[KOH] = 100/1/25, in refluxing 2-propanol,
acetophenone underwent complete conversion to 1-phenyl
ethanol in 4 h (Entry 1, Table 1) using RuNPs as catalyst
generated from the complex N-DM. Almost all the other
complexes exhibited similar activity.
When the reaction was performed without maintaining a dry
nitrogen atmosphere the yield dropped precipitously (Entry 18,
Table 1). Although the reaction was found to be air-sensitive,
the reduction of acetophenone could be carried out in the
presence of 10-mole equivalents of deionized distilled water
w.r.t. heterogeneous complex (Entry 19, Table 1). Since the
reaction was found to be moisture tolerant (Entry 19, Table 1),
a reaction was attempted in water using sodium formate as a
base. Unfortunately, under these conditions, the reduction
could not be effected (Entry 20, Table 1).
Scheme 1. Synthesis of untethered RuNHC complexes by transmetallation
from silver.
Tethered complexes (Scheme 2) were prepared for generating
catalytically active RuNPs in-situ to probe the influence of the
arene tethering on the catalytic transfer hydrogenation of ketones.
We synthesized the tethered complexes by refluxing their
untethered analogs in degassed toluene or xylene following a
literature report by Cetinkaya et al.[42] Successful tethering
depended on the electron density on the benzyl substituent on the
N of NHC compared to the p-cymene ring on the ruthenium
(Figure S1). Only, the untethered RuNHC complexes N-mOMe,
N-piPr, N-ptBu, N-DM, and N-TM were converted to arene
tethered complexes (Figure S2).
From Table 1, it can be observed that the NPs generated from
the RuNHC complexes show better activity than NPs
generated from the precursor complex namely [RuCl2(p-
cymene)]2 (Entry 21, Table 1). But the in-situ generated RuNPs
from tethered, as well as from untethered complexes reduced
acetophenone with almost equal efficiency.
Scheme 2. Synthesis of arene-tethered RuNHC complexes.
Table 1. Transfer hydrogenation of acetophenone [S] by in-situ generated
RuNPs [C] using untethered and tethered RuNHC complexes.
A series of untethered (Figure S1) and tethered RuNHC half
sandwich complexes (Figure S2) was prepared. All the RuNHC
complexes were isolated in good to excellent yields as air-stable,
orange to red-colored crystalline solids. In some cases, filtering
the crude material through a short silica column was necessary to
obtain the pure complex. The RuNHC complexes were
characterized by NMR spectroscopy, HRMS, and elemental
analysis. The NMR spectroscopic analysis of these RuNHC
complexes reveals a Ru-Ccarbene signal around 190 ppm. The
complexes were crystallized by diffusing pentane into the
chloroform solution of the complex at RT. The complexes N-
mOMe, N-piPr, N-pNO2, T-TM, and T-mOMe were characterized
by single-crystal X-Ray diffraction studies (Figure S3). In the solid-
state, the complexes have the expected piano-stool geometry.
[b]Conversion [%]
(Isolated yield)
[a]Entry
Catalyst
Time (h)
1
2
N-DM
N-TM
100 (93)
4
6
4
6
6
4
4
4
4
4
6
6
4
8
97 (95)
98 (95)
97 (94)
98 (94)
100 (96)
97 (91)
96 (89)
98 (92)
98 (93)
98 (92)
96 (94)
97 (91)
97 (96)
3
N-mOMe
N-piPr
4
5
N-ptBu
N-pNO2
N-mMe
N-pOMe
N-Bn
6
7
TH of the aromatic ketones using untethered complexes and the
tethered complexes as catalysts were investigated in refluxing 2-
propanol using potassium hydroxide as a base. For optimization
of the amount of base required, transfer hydrogenation of
acetophenone using 1 mol% of complex N-DM as a catalyst, was
tested with different amounts of KOH. A plot of the product yield
as a function of time is given in the supplementary section (Figure
S4). The rate of formation of the product improved with the
amount of KOH used and 25 mol% appeared to be the optimum
amount required.
8
9
10
11
12
13
14
N-EB
T-mOMe
T-ptBu
T-piPr
T-TM
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15
16c
17d
18e
19f
20g
21h
T-DM
100 (96)
98 (94)
99 (96)
15 (-)
4
12
12
24
4
6
185
4
180
2
N-DM
N-DM
7
8
180
163
178
0
4
24
6
182
157
157
174
4
24
4
N-DM
N-DM
98 (91)
5 (-)
N-DM
4
[RuCl2(p-cymene)]2
98 (90)
8
9
[a] Ketone 0.423 mmol, [S]/[C]/[KOH] = 100/1/25, reflux, IPA 10 mL. [b]
Conversion (%) based on 1H NMR analysis. The values in parenthesis represent
the isolated yield. [c] [S]/[C]/[KOH] = 100/1/5, [d] [S]/[C]/[KOH] = 100/1/15, reflux
conditions. [e] Reaction was performed without dinitrogen atmosphere in AR
grade 2-propanol. [f] Added 10 mole equivalents of deionized water w.r.t the
catalyst (as a solution in IPA). [g] Reaction was carried out in deionized water
under following conditions [S]/[C]/[HCOONa] = 100/1/excess. [h] NoorUDin et
al. J. Organomet. Chem. 2018, 866,189-199.
10
24
24
11
185
4
182
4
To further confirm this similarity, the transfer hydrogenation of
other aromatic ketones having different steric and electronic
properties was tested using five test complexes.
[a] Ketone 0.423 mmols, [S]/[C]/[KOH] = 100/1/25, reflux, IPA 10 mL. [b]
Average particle size of RuNPs generated from complex NDM was found to be
2.12 nm and from TDM is 2.19 nm. [c] TON was calculated (after complete
conversion and isolating the product) based on the available surface atoms;
[43,44] the real TON based on total concentration of NDM and TDM is 0.524 times
the calculated TON. [d] Time required for complete conversion.
Almost all the substrates were reduced efficiently by the NPs
generated from these RuNHC complexes. A comparison of the
activity of the untethered RuNHC complexes and their tethered
analogs (Table 2) showed that the catalytic activity of the RuNPs
generated from both types of complexes was similar not only in
the case of acetophenone but also in the reduction of other
substrates (Also see Table S3 and S4).
Next, we tested the reduction of a variety of substrates including
heteroatom-containing ketones and aromatic ketones substituted
with other functional groups on the ring by using RuNPs
generated from complex NDM under optimized conditions. The
catalyst could efficiently reduce 2-acetylthiophene (Entry 2, Table
3) and 2-acetylpyridine (Entry 5, Table 3) and therefore do not
poison the catalyst. Similarly, 4-acetyl benzoic acid (Entry 3,
Table 3), and methyl 4-acetylbenzoate (Entry 4, Table 3) could
also be reduced without affecting the amino, acid, and ester group
respectively on the ring. Amino substituted aromatic ketone (Entry
1, Table 3), could also be reduced efficiently by the catalyst.
Table 2. Transfer hydrogenations of ketones [S] by in-situ generated RuNPs
[C] using untethered RuNHC complexes.
[b]NDM
[d]Time(h)
[b]TDM
[d]Time(h)
Table 3. Transfer hydrogenations of ketones [S] by in-situ generated RuNPs [C]
using NDM.
[a]Entry
Substrate
[c]TON
178
[c]TON
184
4
4
2
[a]Entry
Substrate
Product
Time (h)
12
[b]yield (%)
91
1
2
1
184
187
176
2
185
184
184
2
3
6
2
96
96
3
4
4
8
4
4
4
2
97
5
184
12
176
12
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heterogeneous catalyst is likely to be involved. Therefore,
poisoning experiments for one of the complexes (N-DM) were
carried out with sub-stoichiometric amounts of known poisons
such as CS2 and 1, 10-phenanthroline using standard catalytic
conditions (N-DM: KOH: acetophenone = 1: 25:100).
5
2
92
[a] Ketone 0.423 mmols, [S]/[C]/[KOH] = 100/1/25, reflux, IPA 10 mL.
[b] Yield (%) was calculated from 1H NMR using ferrocene as an internal
standard after complete conversion.
Standard conditions
15 min delayed addition of substrate
Tethering was expected to provide the RuNHC complexes
additional stability and/or a different reactivity pattern, but no
major difference was observed in the catalytic activity of the
untethered and tethered complexes. This made us check if the
untethered complexes were converted to the tethered systems
under the reaction conditions.
100
80
60
50
60
40
30
40
20
10
It could be shown that an untethered complex when heated in
refluxing 2-propanol in the absence of the base and substrate,
remained intact even after 24 h of heating. This observation, and
the fact that complexes that could not be converted to tethered
counterparts reacted in a similar fashion, led us to conclude that
there must be other reasons why all complexes show similar
reactivity. The transformation of these complexes to a common
more catalytically active species such as nanoparticles seemed
to be a possibility.[45]
20
0
5
10
15
20
25
30
Time (min)
0
0
50
100
150
200
250
Time (min)
Figure 1. Transfer hydrogenation profile of acetophenone using RuNPs
generated from the complex NDM under two different conditions. Black: A
standard catalytic run. Red: Acetophenone was added after 15 min. reflux of a
mixture of the complex NDM and KOH (inset 0 – 30 min).
To a standard reaction mixture containing 1 mg of the complex N-
DM, KOH, and acetophenone in refluxing 2-propanol (N-DM:
KOH: acetophenone = 1:25:100) was added a sub-stoichiometric
amount of 1, 10-phenanthroline (0.5 equiv w.r.t. N-DM) after 30
minutes and an aliquot was withdrawn from the reaction mixture
at the same time. The aliquot showed 57% of the product
formation by 1H NMR. The reaction was continued after the
addition of 1, 10-phenanthroline and product formation monitored
after 120 and 240 min of the initial start time of the reaction, but
the reaction had been completely inhibited and only 57% of the
product was present even after 4 h of reaction in the presence of
the poison.
Nature of the active catalyst: Homogeneous vs.
Heterogeneous
Testing a catalytic reaction for the nature of a catalyst has been a
contentious issue for several decades. However, various methods
are known in the literature to evaluate the nature of active species
if it is a molecule or a colloidal nanoparticle or a heterogeneous
solid.[46]
Induction time
The complex NDM was used to check for induction time in the
acetophenone reduction (Figure 1). Two independent reactions
were carried out. In the first case, the complex and KOH were
mixed at RT along with acetophenone. The reaction flask was
placed in the oil bath and quickly brought to reflux temperature.
The progress of the reaction was followed by NMR at specific time
intervals. In the second case, the complex and KOH were refluxed
for 15 min. before addition of acetophenone. The progress of both
reactions after addition of acetophenone suggests that; the
induction period if any, is very short or the formation of
nanoparticles is much faster than the reduction reaction.
A
similar experiment was performed by adding a sub-
stoichiometric amount of CS2 (0.5 equiv w.r.t. N-DM) to a standard
reaction mixture (1 mg of the complex N-DM, KOH, and
acetophenone in refluxing 2-propanol; N-DM: KOH:
acetophenone = 1:25:100) after 30 minutes when 56% of the
product formation was confirmed by 1H NMR. The addition of CS2
caused complete inhibition of catalysis as shown by analysis of
the reaction mixture after 120 min and 240 min. The addition of
sub-stoichiometric amounts of poisons used in these experiments
clearly suggests that NPs are the active catalysts.
A hot filtration test of the catalyst solution, carried out as
suggested by Finke[47] resulted in complete deactivation of the
filtrate suggesting it’s heterogeneous nature. The residual celite
used as filteraid was colored, visibly indicating adsorption of the
nanoparticles. However, the celite exhibited no catalytic activity
from the MNPs adhering to the surface probably due to the
deactivation of NPs by cooling or by the adsorption on celite.
Poisoning experiments
Ruthenium nanoparticles including many other nanoparticles are
unaffected by the presence of a mercury drop because of the
inability of mercury to form an amalgam with those metals. It has
been shown that if the catalysis is stopped by the addition of less
than one equivalent of a ligand (poison) w.r.t. the metal, a
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Nitro-arene reduction test
e
f
The hydrogenation of the nitro group on aromatic rings has been
used in the past as a test to distinguish between homogeneous
and heterogeneous catalysts.[47] Reduction of the nitroaromatic
ketone to an amino aromatic ketone suggests catalysis by
heterogeneous species. When complex N-DM (1 mol%) was used
to reduce p-nitro acetophenone, only 17% of p-nitrophenyl
ethanol was observed after 24 h of reaction. Increasing the
amount of catalyst to 5 mol%, led to complete conversion within 4
h, giving p-nitrophenyl ethanol and p-amino acetophenone in the
ratio 2:1 with other unidentified reduction products. The major
product p-nitrophenyl ethanol was isolated in 49% yield. When 5
mol% of complex T-DM was used, complete disappearance of the
starting material occurred within 4 h and from 1H NMR the ratio of
p-nitrophenyl ethanol and p-amino acetophenone was 2:1. The
major product p-nitrophenyl ethanol was isolated in 64% yield
(See SI). The reduction of the nitro group to the amino group
provides further confirmation that the catalytically active species
is Ru(0) nanoparticles.
Figure 2. (a) HRTEM images of the reaction mixture after 30 min of reflux and
(b) corresponding size histogram. (c) The area used for STEM imaging. (d) Ru
mapping in the area marked in the panel (e) EDS (f) SAED pattern of the
reaction mixture.
It was observed that the reaction stopped on cooling the reaction
mixture to room temperature. It could not be resumed by refluxing
the solution again. Although TEM analysis of the sample after
cooling the reaction mixture to RT showed nanoparticles of similar
sizes could still be identified, DLS data of the cooled reaction
mixture suggested a significant amount of aggregated NPs which
could be inactive (Figure S6f).
Characterization of nanoparticles by transmission electron
microscopy (TEM)
After the formation of NPs from the RuNHC complexes was
confirmed, we proceeded to check the oxidation state of Ru by
XPS measurements. Figure S5a shows the XPS spectra of
RuNPs generated from NDM. Two bands were observed, one at
484.1151 eV corresponding to Ru 3P1/2 and other at 461.8419 eV
corresponding to Ru 3P3/2. This confirmed the zero-oxidation state
of Ru.[49] N atom from NHC was found to be present on the RuNPs
surface. When a sub-stoichiometric amount of CS2 was used as
a poison, a band at 483.8350 eV for Ru 3P1/2 was observed
(Figure S5e). This decrease in the binding energy could be due
to the coordination of CS2 on the free sites of the Ru surface.
These NPs were also found to be air sensitive. The catalytic
action was inhibited if the reaction mixture was exposed to air.
The reaction mixture containing NPs generated using NDM was
exposed to air and XPS was recorded to check if the particles are
oxidized. Two bands were observed, one at 484.7195
corresponding to Ru 3P1/2 and other at 462.3618 eV
corresponding to Ru 3P3/2 (Figure S5c). Similarly, a reaction was
performed completely under air and the XPS of the reaction
mixture was recorded. Two bands were observed at 484.3766 eV
and 462.1835 eV corresponding to Ru 3P1/2 and Ru 3P3/2
respectively (Figure S5g). The increase in the binding energy of
Ru and the observed binding energy of O(1s) at 531.6933 eV and
532.2275 eV in these two cases, could be attributed to the
chemisorption of oxygen on the Ru surface as suggested by
Winograd et al.[50]
Aliquots were withdrawn from the reaction mixture obtained by
refluxing complex N-DM and KOH in 2-propanol ([C]/[KOH] =
1/25) after 30 min and deposited on a TEM grid. The HR-TEM
images showed the presence of nearly spherical, 2-3 nm-sized
NPs. The size histogram is represented in figure 2b. One can
conclude that the formation of NPs is very rapid and the size of
these RuNPs is in a similar range as the other reported NHC
stabilized MNPs.[33,48] Energy-dispersive X-ray spectra (EDS)
showed the presence of Ru, K, and Cl (Figure 2e). Figure 2f is the
selected area electron diffraction (SAED) spectrum of the
nanoparticles.
a
25
b
20
15
10
5
0
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Size (nm)
c
d
Since [RuCl2(p-cymene)]2 was also capable of performing TH of
acetophenone under the optimized conditions used, XPS and
TEM measurements were done (Figure S5f, S6d). TEM showed
similar particle size of the NPs generated from [RuCl2(p-
cymene)]2. However, the XPS data suggested that these
nanoparticles were readily oxidized to ruthenium dioxide.
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Three-phase test
The 3-phase test is a useful method to distinguish a heterogenous
catalyst from a homogenous one. Therefore, we synthesized a
cross-linked copolymer of an aromatic ketone containing 1%
divinylbenzene/monomer. The monomer (Figure 3) was
synthesized by using the standard DCC coupling method from p-
OH benzophenone and acrylic acid.
Figure 3. The monomer used for synthesizing the crosslinked polymer.
The cross-linked polymer having a ketonic substrate on the side
chain was synthesized using AIBN as an initiator and
divinylbenzene as a crosslinking agent. The polymer was then
subjected to TH under the optimized conditions using 1 mol% of
NDM for a period of 24 h. The reaction was followed by FTIR.
Even after 24 h of reaction, no significant reduction of the
benzophenone moiety was observed in the FTIR when compared
with the initial polymer (Figure S8). This suggests that the catalyst
generated under these conditions is heterogeneous in nature and
the catalysis was not carried out by leached metal fragments.
Figure 4. 13C CP-MAS NMR spectrum of the solid obtained after generating
RuNPs from complex NDM.
The solid obtained was probed by 13C CP-MAS NMR technique.
The NMR showed peaks in the aromatic and aliphatic regions
(Figure 4). This suggests that apart from 2-propanol[53] and NHC,
a
significant amount of p-cymene is coordinated to the
ruthenium(0) surface. In the aromatic region, three major
resonances were observed, one at 223 ppm and the other at 209,
and 196 ppm. The presence of these different resonances
suggests different chemical environments for the carbene ligand,
which could result from coordination to different sites on the
surface of ruthenium nanoparticles.[32]
NMR characterization
Complex N-DM was reacted with KOH in the ratio [N-DM]: [KOH]
= 1:25 in refluxing 2-propanol. After 30 min of reaction, the solvent
was removed under vacuum without exposing the reaction
mixture to air. The solid obtained was dissolved in dried and
degassed CDCl3 and NMR was recorded. No peaks
corresponding to the intact complex or free NHC were observed
in 1H NMR. Even in the case of the tethered complex, very similar
results were obtained. Neither the intact complex nor free NHC
1
was detected in H NMR (See SI for NMR). Our observation is
b
a
1
similar to that made by Chaudret and coworkers, where no H
NMR peaks corresponding to the NHC coordinated to the
Ru(0)NPs could be seen in solution. The NHC ligand could
coordinate to the ruthenium surface via the carbene carbon and
1
via the π-electron cloud of the benzyl group. The absence of H
NMR peaks in the solution indicates that these NHC ligands are
not free but possibly strongly bound to the surface of
Ru(0)NPs.[9,33]
Surface study of Ru(0)NPs
Figure 5. 13C CP-MAS NMR spectrum of the solid obtained after generating
RuNPs from (a) complex NDM, and (b) [RuCl2(p-cymene)]2.
The presence of carbenes as transient species in metal NPs in
imidazolium-based ionic liquids was observed and proposed
earlier by Finke[51] and Dupont[52]. In order to probe the nature of
the stabilizing agents on the nanoparticles by CP-MAS NMR, a
large-scale reaction was carried out. Complex N-DM and 25 mol%
The peaks observed in the aromatic region can be assigned to
carbene carbon and the aromatic carbons of NHC precursor
bound to the Ru(0) NPs surface. The peaks in the range 80-10
ppm are probably due p-cymene and 2-propanol coordinated to
KOH were refluxed together in 2-propanol in absence of substrate. the nanoparticle surface. The IR spectrum of the solid showed
After 2 h of reflux, 2-propanol was removed by distillation under
vacuum without exposing it to air.
peaks corresponding to (C=N stretching) at 1563 cm-1 and at
1369 cm-1 (C=C stretching).[54]
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To evaluate the strength of the interaction between the carbene
ligands and the surface of the nanoparticles, ligand substitution
was attempted by adding good σ-donor ligands. We chose chiral
donors so that their interaction with the ruthenium surface would
result in a chiral surface leading to chirality induction. Firstly, in an
independent experiment, NPs were generated using [RuCl2(p-
cymene)]2 under standard reaction conditions and the surface
was decorated with chiral cinchonidine. Acetophenone was
reduced in 8 h to 1-phenyl ethanol in 96% yield with 20% ee. On
the other hand, when poorly coordinating (1R, 2R)-(+)-1,2-
Diaminocyclohexane L-tartrate was used as a capping ligand, a
racemic mixture of 1-phenyl ethanol was obtained in 8 h after
complete reduction of acetophenone.
potassium hydroxide), suggests that the one-pot method of
making NHC stabilized catalytically active Ru(0)NPs is very
similar and as good as the nanoparticle generated from the
preformed complex (see Table S5 in the supplementary section).
Conclusions
The untethered and the arene-tethered RuNHC complexes are
excellent catalysts for the TH of aromatic ketones as a result of
the formation of nanoparticles which were confirmed by TEM
measurements to have an average size of 2-3 nm. The turnover
frequency of transfer hydrogenation carried out by these
nanoparticles are greater than TOF of nanoparticles without NHC.
Sub-stoichiometric amounts of Lewis base poisoning provide
support for catalysis by NHC decorated nanoparticles. Hot
filtration test and the 3-phase test also point to the heterogeneous
nature of the catalyst. Chemical proof for nanoparticle catalysis
also came from the reduction of the nitro groups in substrates to
amino groups. These catalytically active RuNPs from RuNHC
complexes could also be formed in-situ by a one-pot generation
of NHC and RuNPs from a mixture of NHC precursors and
[RuCl2(p-cymene)]2.
In another experiment, the achiral catalyst N-DM was used to form
the NHC coated Ru(0)NPs as described earlier using KOH, in
refluxing 2-propanol (N-DM: KOH: acetophenone = 1:25:100).
When 1 mol% of cinchonidine (w.r.t. catalyst) was added to these
NHC coated nanoparticles and reduction of acetophenone
attempted, complete conversion of the reactant to a racemic
mixture of 1-phenyl ethanol was observed in 4 h. These
observations showed that these carbenes are possibly firmly
coordinated to the ruthenium surface and are not easily displaced
by other ligands. This observation is in good agreement with those
made by Chaudret et al.[32,33,55]
Experimental Section
The catalytic activity of the nanoparticles reduced tremendously
on cooling the reaction mixture to room temperature. However, if
the nanoparticles are not allowed to cool down to room
temperature, the reaction mixture retains its catalytic activity and
could be used to reduce a fresh batch of the substrate. In order to
calculate TON of the catalyst, a reaction was performed with N-
DM (1eq, 0.0018 mmol), KOH (25eq, 0.045 mmol) and
acetophenone (1.8 mmol) as a substrate in 2-propanol. After 24 h
of reaction, the complete reduction of acetophenone was
observed. This experiment was performed with the tethered
complex T-DM as well, and a similar result was obtained. This
experiment showed that a high TON of 1912 and TOF of 120 h-1
(calculated at 50% yield) can be achieved with these ruthenium
nanoparticles.
Materials and Methods:
All reactions and manipulations were carried out under an atmosphere of
dinitrogen using standard Schlenk techniques. [RuCl2(p-cymene)]2 was
synthesized using a literature procedure.[56] Unless otherwise stated, all
the reagents were obtained from commercial sources and used without
further purification. Solvents were purified and dried by standard methods
using appropriate drying agents under dry nitrogen before use. Deionized
water was degassed before use. The reactions were monitored by thin-
layer chromatography (TLC) with precoated aluminum plates supplied by
Merck. NMR spectra were recorded at room temperature on a Bruker AMX
400 spectrometer operating at 400 MHz for 1H and 100 MHz for 13C.
Chemical shifts for 1H and 13C{1H} NMR spectra are reported as chemical
shift, δ, in ppm with respect to the residual protio solvent peak. Silica gel
GF was used for preparative scale TLC. Electrospray ionization mass
spectrometry (ESI-MS) was carried out on an Agilent 6538 Ultra High
Definition (UHD) Accurate-Mass Q-TOF using mass spectroscopic grade
solvents. Elemental analysis was carried out on a vario MICRO CUBE
CHNS instrument supplied by Elementar GmbH. Bright-field (BF) imaging,
high resolution (HR) TEM imaging, and selected area diffraction patterns
were obtained using 2100 FETEM supplied by JEOL. Energy-dispersive
X-ray spectroscopy (EDS) was performed using an OXFORD EDS
spectrometer attached to the TEM. The operating voltage of the field
emission gun is 200 kV. FT-IR spectra were recorded using Bruker ALPHA
FT-IR spectrometer. The DLS and Zeta potential measurements were
carried out using Zeta Phase Analysis Light Scattering (Zeta PALS).
An alternative route to NHC stabilized RuNPs
Since the Ru(0)NPs were formed by the in-situ transformation of
RuNHC complexes under basic conditions which are also suitable
for the generation of the NHC ligands, we wondered if refluxing
[RuCl2(p-cymene)]2 and the NHC precursor in the presence of
KOH can also be used to generate NHC stabilized Ru-
nanoparticles. The dimeric ruthenium precursor [RuCl2(p-
cymene)]2 was refluxed in 2-propanol in the presence of 25 mol%
KOH for half an hour and then the imidazolium cation (the NHC
precursor) was added to the reaction mixture. After 15 min,
acetophenone was added as a substrate. A complete reduction of
the substrate was observed after 4 h. A comparison of the
reactivity profile of the two catalysts, one formed from the RuNHC
complex, and the other formed by stabilization of the RuNPs
(generated from [RuCl2(p-cymene)]2, the NHC precursor and
General procedure for the catalytic transfer hydrogenation reaction
The complex (4.23 x 10-3 mmol) was dissolved in 10 mL of 2-propanol in a
25 mL two-necked round bottom flask and ketone (0.423 mmol) was added
followed by KOH (0.105 mmol) as a solution in 2-propanol. The resulting
solution was refluxed using an oil bath controlled by a thermostat. Small
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10.1002/chem.202000142
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portions of the reaction mixture were withdrawn from the reaction vessel
using a glass syringe through a septum at specified intervals. The solvent
was removed from the aliquots under vacuum and 1H NMR spectra were
recorded in CDCl3. The conversion was calculated from the intensity ratios
of the peaks arising from the reactant and product. After the reaction was
completed, the product was separated by preparative TLC and the isolated
yield was calculated.
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Lakshay Kathuria, Noor U Din Reshi, and
Ashoka G. Samuelson*
Page No. – Page No.
NHC stabilized Ru(0) nanoparticles:
in-situ generation of an efficient
transfer hydrogenation catalyst
NHC stabilized Ru(0) nanoparticles were generated in-situ from the tethered and
untethered ruthenium half-sandwich complexes. These catalytically active Ru(0)
nanoparticles were explored for transfer hydrogenation of aromatic ketones in
refluxing 2-propanol. The transfer hydrogenation catalyzed by these NHC
stabilized Ru(0)NPs possess a high turn-over number. The catalytic efficiency
significantly reduced if these nanoparticles were exposed to air or allowed to
aggregate and precipitate by cooling the reaction mixtures.
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