Negatively Charged N-Heterocyclic Carbene-Stabilized Pd and Au Nanoparticles and Efficient Catalysis in Water

Article abstract of DOI:10.1021/acscatal.5b01160

Herein we describe the synthesis of negatively charged N-heterocyclic carbene (NHC)-functionalized palladium and gold nanoparticles (NPs), which are stable in water for over 3 months. The formation of these NHC-NPs proceeds via an efficient ligand exchange procedure. This method was successfully applied to different negatively charged NHCs bearing sulfonate and carboxylate groups. The obtained PdNPs were investigated as catalysts in hydrogenation reactions and showed high catalytic activity (TON up to 2500 and TOF up to 2000 h^-1).

Full text of DOI:10.1021/acscatal.5b01160

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Negatively Charged NꢀHeterocyclic Carbeneꢀ
Stabilized Pd and Au Nanoparticles and Efficient
Catalysis in Water
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Angélique Ferry‡, Kira Schaepe‡, Patricia Tegeder, Christian Richter, Kathryn M. Chepiga, Bart
Jan Ravoo* and Frank Glorius*
OrganischꢀChemisches Institut, Westfälische WilhelmsꢀUniversität Münster, Corrensstrasse 40,
48149 Münster, Germany
ABSTRACT. Herein we describe the synthesis of negatively charged Nꢀheterocyclic carbene
(NHC)ꢀfunctionalized palladium and gold nanoparticles (NPs), which are stable in water for over
three months. The formation of these NHC−NPs proceeds via an efficient ligand exchange
procedure. This method was successfully applied to different negatively charged NHCs bearing
sulfonate and carboxylate groups. The obtained PdNPs were investigated as catalysts in
hydrogenation reactions and showed high catalytic activity (TON up to 2500 and TOF up to
2000 hꢀ1).
KEYWORDS. metallic nanoparticles, Nꢀheterocyclic carbene, ligand exchange, catalysis,
anionic nanoparticles
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INTRODUCTION
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In the last decades, interest in nanoparticles (NPs) has significantly increased due to their
numerous applications in material science, medicine and catalysis.1 The large surface area of
metal NPs (MNPs) compared to bulk material allows for efficient metalꢀsubstrate interactions to
take place, rendering MNPs attractive candidates for catalysis. Furthermore, the novel physical
and chemical properties of the nanoscale dimension can lead to unprecedented activities and
reaction pathways.2 However, due to their high surface energy, MNPs tend to aggregate which
leads to a decrease in their surface area and thus a loss in activity. Stabilization of MNPs is
generally achieved with the use of ligands such as thiols, amines, disulfides, thioethers,
phosphines3 or, more recently, Nꢀheterocyclic carbenes (NHCs).4,5 The neutral, electronꢀrich
NHCs form a strong covalent bond with the metallic surface which is key to their stabilization of
NPs and allows the NPs to maintain their sizeꢀdependent properties. An advantage of using
NHCs as ligands is the feasible modification of the Nꢀheterocycle with different functional
groups which can give rise to a tunable surface. To date, only a few examples of NHCꢀ
functionalized NPs are described in the literature with limited applications.4,5 Among them, two
different approaches have been used to form the NHCꢀstabilized NP systems: (1) the metal
complex decomposition route, largely exploited for the formation of Ru,4a,d Pt,4b,f Au4c,e,g and
PdNPs4g and (2) the ligand exchange strategy for the synthesis of Pd and AuNPs.5 The ligand
exchange approach is desirable because it uses preformed NPs with defined sizes and shapes.
Several protocols have been established for the synthesis of these NPs.5,6 Very recently, we
presented a new NHC bearing two long alkyl chains in its backbone which is capable of
stabilizing Pd and AuNPs.5b These stericallyꢀstabilized NPs showed good solubility in apolar
solvents with interesting chemoselectivity in hydrogenation reactions. In the continuation of this
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work, we were interested in synthesizing NPs suitable for catalysis in aqueous media. We
considered employing negatively charged NHC ligands in electrostatic stabilization of MNPs.
The electrostatic stabilization of NPs has been previously demonstrated by addition of ionic
liquids7 or salts,8 such as an ammonium halide, citrate, polyacrylate or polyoxometallate during
NP formation. It is known that the stabilization of the NPs relies on repulsive electrostatic
interactions generated from a diffuse double layer at the surface of NPs coated with charged
ligands.8f,g,h However, it has been shown that NHCs bind very strongly to the metal surface,4l
thus giving an interesting platform to tune the reactivity of the metal NP. To date, only one
example of employing charged NHCs for the stabilization of PtNPs has been reported. This work
by Chaudret et al. featured NHCs bearing a negatively charged sulfonate group and the
corresponding NHCꢀMNPs were made by the decomposition protocol.4j However, the catalytic
activity of these nanostructures was not investigated in depth. Herein we present the first
example of Pd and AuNPs stabilized by charged NHCs. We installed various negatively charged
NHCs possessing different functional groups (sulfonates or carboxylates) following the ligand
exchange method. All of these NPs showed longꢀterm stability in water. These PdNPs showed
high activity in hydrogenation reactions conducted in aqueous media (TON up to 2500 and TOF
up to 2000 hꢀ1).
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SYNTHESIS
In order to obtain stable MNPs in water, we synthesized a library of NHCs bearing negatively
charged sulfonate (L1 & L2) and carboxylate groups (L3 & L4, see Scheme 1 A). The use of
ligands L1ꢀL4 for Pd or for MNPs has not been previously reported and there is only one
example of an L4ꢀAu complex.9
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Scheme 1: A. Negatively charged NHCs (L1ꢀL4) for the stabilization of Pd and AuNPs in
aqueous media. B. Schematic representation of the ligand exchange procedure using
DDS@MNPs as precursor.
The subsequent synthesis of NHC functionalized Pd and AuNPs was realized by use of the
ligand exchange method. As was previously demonstrated,5b displacement of weakly binding
thioethers from the NP surface by stronger binding NHCs is a general procedure to attach a
variety of NHC ligands to MNPs. The ligand exchange was performed by mixing
didodecylsulfide (DDS) stabilized Pdꢀ and AuNPs with the desired charged NHCs (cNHCs) in a
biphasic hexane/DMF system (see Scheme 1B). The NHCs were generated in situ by
deprotonation of the corresponding imidazolium salts with KOtBu. After the addition of
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DDS@MNPs, a phase transfer of the MNPs from hexane into the more polar DMF phase could
be observed, indicating the successful ligand exchange on the NP surface. The NPs were then
purified by repeated washing and dialysis against water. Using this approach, we were able to
synthesize the first waterꢀsoluble Pd and AuNPs using cNHCs ligands. These NPs were stable
for more than three months.
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RESULTS AND DISCUSSION
Characterization of the cNHC@MNPs by NMR spectroscopy showed that the thioether DDS
was completely displaced from the NP surface. The 1H NMR spectra, measured in D2O, showed
broad peaks corresponding to the protons of the NHCs (Scheme 2A). This peak broadening is
caused by the coordination of the NHCs to the NP surface and has also been observed for
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comparable PtNP systems.5j MAS C NMR spectroscopy performed for L2@PdNPs confirmed
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the successful coordination of the NHC to the NP surface (see Scheme 2 B). In addition, a C
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labeled derivate of L3 was synthesized and attached to PdNPs. The C NMR spectra obtained
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from CꢀL3@PdNP, measured in D2O, showed a broad peak of high intensity in the typical
region for a metal coordinated carbene (166.7 ppm). The signals corresponding to the other
carbon atoms had lower intensities (∼0.06%).11 This data indicates that a carbene species is
stabilizing the MNPs.
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Scheme 2: A. H NMR spectra, measured in D2O, of imidazolium salt L2 (top) and L2@PdNP
(bottom) showing the successful coordination of the NHC to the NPs as indicated by a
broadening of the proton signals. B. C MASꢀNMR spectra of L2@PdNPs showing a peak at
~170 ppm corresponding to the carbene carbon atom.
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The composition of the purified MNPs was further investigated by thermogravimetric
analysis (TGA) and elemental analysis giving a metal content in the range of 61 – 67% for the
PdNPs and 82ꢀ93% for the AuNPs. Based on the average metal content, detected by TGA and
elemental analysis, a metal:ligand (M:Lig) ratio could be calculated. The results are given in
Tables 1 and 2 together with a summary of the main characteristics for the cNHC@Pdꢀ and
AuNPs. Due to the steric demand of the substituents, L4@PdNPs have a higher M:Lig ratio
compared to the other cNHC@PdNPs. The cNHC@AuNPs show a higher M:Lig ratio because
their larger size distribution decreases the relative amount of surface atoms.
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Table 1. Main Analytics for the cNHC@PdNPs.
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Solubility
at pH 4
∅ Metal contenta
Size / nm 13C NMR shift
M:Lig ratio
L1@PdNP 4.1 (± 0.6)
n. d.
170 ppmb
166.7 ppmc
n. d.
63%
4.5 : 1
dissolved
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L2@PdNP 4.1 (± 0.6)
L3@PdNP 4.1 (± 0.6)
L4@PdNP 4.0 (± 0.6)
63%
65%
70%
4.0 : 1
3.5 : 1
dissolved
precipitated
precipitated
7.0 : 1
a Measured by TGA and elemental analysis. b 13C MASꢀNMR. Measured in D2O.
c
Table 2. Main analytics for the cNHC@AuNPs.
Solubility
at pH 4
dissolved
∅ Metal contenta
Size / nm
L1@AuNP 4.1 (± 1.6)
L2@AuNP 4.7 (± 1.5)
L3@AuNP 4.9 (± 1.7)
M:Lig ratio
10 : 1
92%
90%
14 : 1
dissolved
88%
12 : 1
precipitated
a Measured by TGA and elemental analysis.
The size of the cNHC@MNPs was then investigated by TEM. The PdNPs showed a narrow
size distribution with a mean size of 4.1 (±0.6) nm for L1-L3@PdNP and 4.0 (±0.6) nm for
L4@PdNP (see Figure 1 for TEM images of L1@PdNP). Thus, no changes in size or size
distribution during the ligand exchange were detected (DDS@PdNP (4.0 (±0.6) nm)). On the
other hand, if DDS@AuNPs were applied in the ligand exchange procedure, a significant
decrease in the mean size from 8.5 (±1.7) nm (DDS@AuNP) to 4.1 (±1.5) nm (L1@AuNP),
4.7 (±1.6) nm (L2@AuNP) and 4.9 (±1.7) nm (L3@AuNP) was observed. Due to the
restructuring of the AuNPs, the size distributions increased from 20% to 34ꢀ36%, yielding
polydisperse cNHC@AuNPs. The decrease in size of AuNPs during a ligand exchange with
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NHCs5c seems to be attributed to dissociation of NHCꢀAuI complexes from the NP surface,
leading to a shrinking of the AuNPs. NMR spectroscopic analysis of the cNHC@AuNPs before
and after dialysis provided further insight into this phenomenon. Before dialysis, the NMR
spectra of cNHC@AuNPs showed sharp signals that could be attributed to a molecular NHCꢀAuI
species in solution. This was validated by ESIꢀMS analysis confirming the presence of NHCꢀAuI
complexes. After dialysis of the cNHC@AuNPs, no NHCꢀAuI complexes were detected by
NMR or ESIꢀMS. As mentioned above, the NMR spectra of the purified cNHC@AuNP only
show the expected broad signals of NHC ligands coordinated to the NP surface.10 Also, AuNPs
showed no further size changes or aggregation after purification. The longꢀterm stability of the
cNHC@MNPs was investigated by DLS and UV/vis measurements and no change in the mean
size or the maximum of the SPR band was observed after three months.10 An aqueous solution of
the cNHC@MNPs could even be exposed to liquid nitrogen without aggregation occurring (e.g.
for lyophilisation).10
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Figure 1: TEM images and size histogram of L1@PdNP showing a narrow size distribution of
4.1 (±0.6) nm.
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Concerning the electrostatic stabilization of the NPs, zeta potential measurements were
performed giving values between –30 to –50 mV. As NPs with a zeta potential under –30 mV or
over +30 mV are considered to be stable in aqueous media, these results confirmed the
stabilization of the MNPs by the cNHCs.4b,10 The stability against acidic conditions was tested by
exposing the samples of cNHC@MNPs to pH 4 and pH 1. Pd and AuNPs stabilized by the
sulfonated ligands L1 and L2 were stable in aqueous solution at pH 4 whereas MNPs
functionalized with the carboxylate ligands L3 and L4 aggregated under these conditions.10 This
observation is attributed to the partial protonation of the carboxylate function and the loss in
electrostatic repulsion between the NPs, thus leading to a clustering. When L3@AuNP was used,
this behavior could be monitored with UV/vis spectroscopy by observing changes in the
characteristic SPR band of AuNPs. L3@AuNPs show an absorption maximum at 518 nm, which
is shifted to 560 nm at pH 4 (when the solution was acidified by the addition of 1M acetic acid).
This shift to higher wavelengths is characteristic for aggregated AuNPs. The aggregates of
L3@AuNPs could be redispersed by adjusting the pH to 10 (by addition of 1M sodium
hydroxide). This behavior is reversible for at least three cycles (see Scheme 3). If the NPs were
exposed to pH 4 for ~30 minutes, the NPs completely precipitated out of the solution (Scheme 3,
black curve). The decrease in signal intensity is attributed to the dilution of the sample upon the
addition of acid and base.
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Scheme 3: UV/vis measurements of L3@AuNPs in aqueous solution showing the reversible
aggregation and redispersion of the NPs depending on the pH.
L3@PdNPs showed the same reversible aggregation/redispersion behavior over 3 cycles as
confirmed by DLS measurements. However, L4@PdNP showed the formation of larger
aggregates after the second cycle. This can be explained by the sterically demanding substituents
leading to a weaker metalꢀligand bond and lowering the stabilization of the L4@PdNP in the
aggregated state.10
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When kept under basic conditions, the NPs could repeatedly be dried and redispersed in aqueous
media without changes to their size (monitored by DLS measurements).
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These waterꢀstable NPs are particularly interesting as catalysts, especially the PdNPs, since
they can catalyze the chemical transformations in aqueous media. The catalytic activity and
selectivity of these new PdNPs was investigated in the hydrogenation of olefins with different
degrees of substitution (namely styrene (1), 1ꢀdecene (2), 3ꢀmethylꢀ2ꢀcyclohexenone (3) and
citronellol (4)).5b Competition experiments with these substrates were conducted. We were
mainly interested in evaluating the influence of the nature of the ligand on the catalytic activity.
The first trials were conducted using 0.25 mg of L1@PdNP (~0.16 mg of “Pd”/ 0.2 mmol of
substrate) in different waterꢀmiscible solvents under 10 bar of H2 for 16 h. In various solvents
(toluene/MeOH, CH3CN, THF and DMF), styrene (1) and 3ꢀmethylꢀ2ꢀcyclohexenone (3) were
successfully hydrogenated whereas the hydrogenation of 1ꢀdecene (2) and citronellol (4) resulted
in moderate or poor yields (Table 3, entries 1ꢀ4). The nonꢀquantitative conversion of 1ꢀdecene
(2) is explained by the competitive isomerization of the double bond to internal positions as we
have previously observed with NHCꢀMNP mediated hydrogenations.5b In acetonitrile (Table 3,
entry 2), an interesting chemoselectivity for the α,βꢀunsaturated carbonyl compound 3 compared
to the trisubstituted alkene 4 was observed. This selectivity in acetonitrile was further exploited
in the hydrogenation of citral (6) bearing both an α,βꢀunsaturated carbonyl and a trisubstituted
alkene function, leading to citronellal (6a) as the sole product (Table 4, entry 1). Interestingly,
the use of water as solvent led to reduction of all the four compounds (1ꢀ4) with very good yields
(Table 3, entry 5) even at low pressure of H2 (2 bar) (Table 3, entry 6), after a shorter reaction
time (1 h) (Table 3, entry 10) and with a small amount of L1@PdNP (Table 3, entry 9), leading
to TON values up to 2500.10 The use of dimethyl sulfoxide poisoned the catalyst and resulted in
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a dramatic decrease in the catalytic activity with a total chemoselectivity for styrene (1) (Table 3,
entry 11). Using ligand L4@PdNP gave similar results to L1@PdNP (Table 3, entry 7), whereas
the use of L3@PdNP led to a slight decrease in the catalytic activity (Table 3, entry 8). In order
to investigate the potential difference in activity induced by the different ligands, the reaction
was performed with a very small amount of PdNP (0.063 mg (~ 0.041 mg of “Pd”)/0.2 mmol of
substrate) and short reaction time (1 h). Under these conditions, significant differences were
observed. While L4@PdNP hydrogenated three of the four compounds (1, 2 and 3) in a
quantitative manner (Table 3, entry 13), L1@PdNP and L3@PdNP gave good yields for only
two substrates (1 and 2) (Table 3, entries 12 and 15), and L2@PdNP only reduced styrene (2)
(Table 3, entry 14). The latter finding was further exploited in the chemoselective reduction of
sorbinaldehyde (7) which resulted in the completely selective reduction of the •,•-double bond
(Table 4, entry 2). Among the four ligands tested, L4@PdNPs showed the best catalytic activity
giving TOFs of ~2000 hꢀ1.10 The catalytic activity of these waterꢀsoluble NPs was compared with
a commercial Pd/C 5% catalyst (Evonik). At high catalyst loadings (0.16 mg of Pd/ 0.2 mmol of
substrate), Pd/C gave slightly higher yields compared to L4@PdNP (Table 3, entry 16).
However, at low catalyst loadings (0.041 mg of Pd/ 0.2 mmol of substrate), L4@PdNP remained
active whereas the reactivity of Pd/C was greatly diminished, hydrogenating only styrene (1)
with poor conversion (Table 3, entry 17). The possibility of recycling these PdNPs was also
investigated with L3@PdNPs. After a first catalytic cycle of 16h (Table 3, entry 8), the NPs were
analyzed by TEM measurements and showed nearly the same size distribution (4.0 (±0.7) nm) as
they did prior to catalysis (4.1 (±0.6) nm). The NPs were separated from the reaction mixture by
precipitation at pH 4. After washing, the NPs were redispersed in water by adjusting the pH to
slightly basic conditions and then subjected to a second catalytic cycle. The recycled NPs
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remained active in the hydrogenation of all substrates (1ꢀ3) except substrate 4 (Table 3, entry
18). In order to eliminate the possibility that these hydrogenations were catalyzed by a
homogeneous NHCꢀPd complex (formed by leaching of the PdꢀNPs) rather than by the cNHCꢀ
MNP itself,12 a PEPPSIꢀtype Pd complex bearing L1 as ligand (Figure 2) was synthesized and
tested in hydrogenation. Complex 5 as was found to be inactive in this reaction (Table 3, entry
19), showing that the active catalytic species is most likely heterogeneous in nature.13
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Table 3. Competition experiments.
Entry
Solvent
P (bar)
Time
(h)
Amount of
NP (mg/0.2
mmol)
L
Yield (%)a
1a
2a
3a
4a
1
Toluene/MeOH
(70 : 30)
10
16
0.25
(~ 0.16 mg
“Pd”)
100
97
90
46
L1
2
3
CH3CN
THF
DMF
H2O
10
10
10
10
2
16
16
16
16
16
16
16
16
1
0.25
100
97
63
16
91
79
6
L1
L1
L1
L1
L1
L4
L3
L1
L1
L1
L1
0.25
0.25
0.25
0.25
0.25
0.25
0.05
0.25
0.25
4
4
100
100
100
88
67
100
100
90
60
88
75
71
54
66
65
0
5
100
100
94
6
H2O
7
H2O
2
100
82
8
H2O
2
97
100
100
74
9
H2O
2
98
95
10
11
12
H2O
2
94
92
DMSO
H2O
2
16
1
100
100
6
Trace
45
2
0.063
(~ 0.041 mg
“Pd”)
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H2O
H2O
H2O
H2O
2
2
2
2
1
1
1
1
0.063
0.063
0.063
98
58
96
Trace
88
92
0
18
0
L4
L2
L3
-
99
37
100
0
3.2 mg Pd/C
100
100
98
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(5%)
(= 0.16 mg
“Pd”)
17
18
H2O
2
1
0.8 mg Pd/C
(5%)
(= 0.041 mg
“Pd”)
16
0
0
0
-
H2O
H2O
2
2
16
1
0.25
(recycled
NPs)
98
91
0
65
0
0
0
L3
L1
19
0.89 mg 5
(= 0.16 mg
“Pd”)
trace
a GC yield.
Figure 2: Structure of the NHCꢀPdIIꢀcomplex 5.
After establishing the high catalytic activity of these cNHCꢀPdNPs in water, more complex
substrates were tested. The tetrasubstituted alkene pulegone (8) was completely reduced after 16
hours under 2 bar of H2, albeit as a 1:1 mixture of two diastereoisomers (Table 4, entry 3). We
were also interested in testing the influence of the Nꢀalkyl substituents of L4@PdNP on the
stereochemical outcome of the reaction. We found that L4@PdNP only slightly improved the
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diastereoselectivity in the hydrogenation of (+)ꢀcarvone (9) (55:45 d.r. with L1@PdNP (Table 4,
entry 4) vs. 65:35 d.r. with L4@PdNP (Table 4, entry 5)). Next, the reducible functional group
was varied. While cyano groups were inert under the reaction conditions (Table 4, entries 6 and
7), nitroarene 11 was fully converted into the corresponding amine (11a) (Table 4, entry 7).
Table 4. Investigation of the Reactivity of PdNPs.
Amount of NP
(mg/0.2 mmol)
Entry
Substrate
L
Conditions
Product
Conv (%)
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Selectivitya
H2 (5 bar)
CH3CN, 24
h, 50°C
1
0.25
100
>99:1
L4
H2 (2 bar)
H2O, 1 h,
25°C
2
3
0.063
0.25
100
>99:1
L2
L1
H2 (2 bar)
H2O, 16 h,
25°C
100
d.r. 50:50
H2 (2 bar)
H2O, 1 h,
25°C
4
5
0.25
0.25
63
d.r. 55:45
L1
L4
H2 (2 bar)
H2O, 1 h,
25°C
60
d.r. 65:35
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H2 (2 bar)
6
7
0.25
0.25
100
>99:1
L1
L1
H2O/DMF
(1:1), 16 h,
25°C
NC
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H2 (2 bar)
H2O, 2 h,
25°C
100
>95:5
a Conversions and selectivities determined by GCꢀFID.
CONCLUSION
In summary, novel negativelyꢀcharged NHCꢀstabilized Pdꢀ and AuNPs were synthesized by the
ligand exchange method. Four structurally distinct NHCs bearing either sulfonate or carboxylate
moieties were applied. The resulting cNHCꢀMNPs showed longꢀterm stability in water. Upon
variation of the pH, L3@PdNPs showed a reversibility between the aggregated and soluble form
in water, demonstrating the underlying concept of electrostatic stabilization. Finally, cNHCꢀ
PdNPs were tested in catalytic hydrogenation reactions and were found to be highly active and
chemoselective in hydrogenation reactions in aqueous media. Our Pdꢀnanosystems outperformed
commercial Pd/C at low catalyst loadings. A recycling study showed that recycled cNHCꢀPdNPs
are catalytically active. The lack of catalytic activity of a homogeneous Pdꢀcomplex bearing L1
as ligand indicated that the catalytic species is most likely heterogeneous in nature.
ASSOCIATED CONTENT
Supporting Information.
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Experimental procedures, characterization data, TEM images and 1 H and 13C NMR spectra are
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Bart Jan Ravoo: b.j.ravoo@uniꢀmuenster.de
*Frank Glorius: glorius@uniꢀmuenster.de
Author Contributions
‡These authors contributed equally.
ACKNOWLEDGMENT
Generous financial support by the Deutsche Forschungsgemeinschaft (SFB 858 “Synergetic
Effects in Chemistry”) is gratefully acknowledged. We thank Dr. Martin Peterlechner of the
Institute of Materials Physics (WWU) for TEM measurements and Olga Fromm and Jessica
Hüsker of MEET (WWU) for TGA analysis. Dörthe Dietrich from the Institute of Inorganic and
Analytic Chemistry is acknowledged for ICPꢀMS measurements.
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Insert Table of Contents Graphic and Synopsis Here.
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Stable NP
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Tunable reactivity
Robust synthesis
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