Synthesis of Water-Soluble Palladium Nanoparticles Stabilized by Sulfonated N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/chem.201702204

A strategy involving the decomposition of palladium(II) organometallic complexes with sulfonated N-heterocyclic carbene ligands leads to the formation of stable and water-soluble Pd nanoparticles. Three different methodologies (thermal decomposition, redu

Full text of DOI:10.1002/chem.201702204

A Journal of
Accepted Article
Title: Synthesis of Water-Soluble Palladium Nanoparticles Stabilized by
Sulfonated N-Heterocyclic Carbenes
Authors: Juan Manuel Asensio, Simon Tricard, Yannick Coppel,
Román Andrés, Bruno Chaudret, and Ernesto de Jesus
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Synthesis of Water-Soluble Palladium Nanoparticles Stabilized by
Sulfonated N-Heterocyclic Carbenes
Juan M. Asensio,[a] Simon Tricard,[b] Yannick Coppel,[c] Román Andrés,[a] Bruno Chaudret,*[b] and
Ernesto de Jesús*[a]
Dedication ((optional))
Abstract: A strategy involving the decomposition of palladium(II)
organometallic complexes with sulfonated N-heterocyclic carbene
ligands (NHC) leads to the formation of stable and water-soluble Pd
spectroscopy,[8] and have explored their application as catalysts
in the hydrogenation of arenes.[9] Additionally, NHCs have been
used to synthesize Au,[6,7,10] Pd,[7,11] and Ag[12] NPs as model
nanoparticles. Three different methodologies (thermal decomposition, systems for the formation of conglomerates or 3D networks.
reduction under 13CO atmosphere and reduction with H2) gave
particles with different shapes and sizes, ranging from 1.5 to 7 nm.
The structures of the organometallic intermediates and organic
decomposition products were elucidated by NMR spectroscopy. To
check the accessibility of the surface, the nanoparticles were tested
as catalysts for the chemoselective hydrogenation of styrene in
water. An effect of the particle size on the catalyst activity was
observed. The aqueous phase was recycled up to ten times without
any precipitation of metallic palladium.
Planellas and co-workers,[13] and Richter et. al.[14] have
synthesized and characterized PdNPs stabilized with NHCs and
have studied their application as catalysts in different processes
such as Suzuki couplings or catalytic hydrogenations. However,
these MNPs were insoluble in water and required the use of
organic solvents. We reported the first example of water-soluble
MNPs stabilized with sulfonated NHCs coordinated to the
surface of platinum particles.[15] These MNPs were synthesized
by thermal decomposition of water-soluble dimethyl Pt(II) NHC
complexes.[16,17] The PtNPs thus obtained were highly stable in
aqueous solution. Concerning PdNPs, Glorius and coworkers
have reported the preparation of water-stable Pd nanoparticles
stabilized by charged NHC ligands.[18] In a recent work, we have
also communicated the synthesis and characterization of water-
soluble NHC-stabilized PdNPs prepared by decomposition of a
dimethyl NHC metal complex.[19] The coordination of the NHC
ligand to the metal surface of these nanoparticles was confirmed
by the observation of a Knight-shifted signal for the carbenic
carbon in solid state 13C NMR spectra.
In this work, we report full details regarding the synthesis
and characterization of different sulfonated dimethyl
palladium(II) NHC complexes and nanoparticles, including liquid
and solid-state NMR studies. We also compare different
methods for the generation of the PdNPs involving the
decomposition of Pd(II) complexes. Particular attention has been
paid to the characterization by NMR spectroscopy of the
decomposition products and the organometallic intermediates, in
order to elucidate the NPs formation mechanism.
Introduction
Metallic nanoparticles (MNPs) dispersed in aqueous media are
interesting for their different applications in catalysis, including
green catalysis.[1] In the case of palladium, for instance, MNPs
have been used as catalysts in Suzuki–Miyaura cross-coupling
reactions in water.[2] Surfactants, polymers or dendrimers have
been employed to stabilize palladium nanoparticles (PdNPs)
avoiding their agglomeration in aqueous solution.[3] Ligands
strongly bond to the surface can also induce interesting
reactivity. N-Heterocyclic carbenes (NHC) have shown to form
robust transition-metal complexes that have been applied in
many homogeneous catalytic processes, some of them in
aqueous media.[4] In recent years, some examples of MNPs
stabilized with NHCs directly coordinated to the surface have
been reported.[5,6,7] Some of us have characterized the
coordination of such ligands to RuNPs by solid-state NMR
Results and Discussion
[a]
[b]
[c]
Dr. J. M. Asensio, Dr. R. Andrés, Prof. Dr. E. de Jesús
Departamento de Química Orgánica y Química Inorgánica
Universidad de Alcalá
Edificio de Farmacia, Campus Universitario
E28871 Alcalá de Henares, Madrid (Spain)
E-mail: ernesto.dejesus@uah.es
Dr. S. Tricard, Prof. Dr. B. Chaudret
Laboratoire de Physique et Chimie des Nano-objets
INSA, CNRS, Université de Toulouse
135 avenue de Rangueil
31077 Toulouse (France)
E-mail: bruno.chaudret@insa-toulouse.fr
Dr. Y. Coppel
Laboratoire de Chimie de Coordination (LCC)
CNRS
205 route de Narbonne, BP44099
F-31077 Toulouse Cedex 4 (France)
Synthesis of dimethyl NHC palladium(II) complexes
Ligand exchange from complexes such as [PdMe2(tmen)2] or
[PdMe2(bpy)] (tmen = N,N,N′,N′-tetramethylethylenediamine; bpy
= 2,2′-bipyridine), which are synthetically accessible and display
a good thermal stability, is the most general route for the
preparation of dimethyl palladium derivatives.[20] Using these
starting materials, several dimethyl NHC Pd complexes have
been synthesized up to date, mainly with chelating
ligands.[21,22,23] Following this methodology, we prepared the
water-soluble complexes 1a−b in moderate to good yields (58-
73%) by treating [PdMe2(bpy)] with the corresponding carbene
obtained in situ by addition of sodium tert-butoxide to the 1-
mesityl-3-(propyl-3-sulfonate)imidazolium
or
sodium
1,3-
Supporting information for this article is given via a link at the end of
the document
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bis(propyl-3-sulfonate)imidazolium proligand, in dmso (Scheme
1a). Complexes 1a−b were obtained as cis stereoisomers as
shown by the chemical non-equivalence observed in the 1H
NMR spectra between the two halves of the mesityl substituents
or the two protons of some methylene groups in the propyl-3-
sulfonate chains (see Scheme 1b). The same configuration was
observed in previously reported [(NHC)2PdMe2] complexes.[23,24]
An isotopically substituted analogue of complex 1a, containing
carbon-13 in the C2 position of the imidazole ring, was also
prepared (13C-1a, Scheme 1a). The new complexes were
characterized by one- and two-dimensional NMR spectroscopy,
FT-IR spectroscopy, mass spectrometry and elemental analysis.
Full details are given in the Experimental section and Supporting
information (SI, Figures S1-S2). The complexes had to be stored
under an inert atmosphere due to the sensitivity of solid samples
to atmospheric water, especially in the case of 1b (see below for
a discussion of the reactivity of 1a-b with water). However, the
complexes cannot be stored for long periods of time because
thermal decomposition is evident after several weeks (especially
in the case of 1b), even when stored at low temperature.
Synthesis of the Pd nanoparticles 2a−b by thermal
decomposition of dimethyl complexes in water
We have recently described that the thermal decomposition of
complex 1a in water affords dark-brown solutions containing
PdNPs (2a) that remain dispersed in the aqueous solution for
indefinite periods of time (Scheme 2).[19] The size of these
PdNPs was notably influenced by the heating ramp used to
decompose the organometallic precursor while other conditions
(temperature, time of heating, or concentration of the metal
precursor) had
a minor effect. The relative rates of the
nucleation and growth steps apparently controlled the
nanoparticle sizes. Thus, the largest nanoparticles (4.6 ± 1.7
nm) were generated raising the temperature of the solution
slowly (at 1 °C per min) from room temperature to 60 °C (the
lowest temperature at which NPs form). The slowest
decomposition favored the growth step in this case. The
smallest PdNPs (3.0 ± 0.8 nm) were obtained when the metallic
precursor was dissolved in water pre-heated to 80 °C, favoring a
quick decomposition and, therefore, the nucleation step. The
organic products formed in this decomposition were removed
from the solution by dialysis. As some coalescence of the
nanoparticles was observed during this process, the time of
dialysis was optimized after recording TEM images and 1H NMR
spectra at regular intervals to find the best compromise between
optimizing the purification (removing as many organic products
as possible) and avoiding coalescence of the nanoparticles.
TEM and HRTEM images of a sample of purified PdNPs (mean
diameter 3.4 ± 0.5 nm) are shown in Figures 1a-b.
(a)
CH3
CH3
N
N
Pd
2
–
+
N
SO3
N
R
a–b
+ 2 tBuONa
dmso, room temp.
cis-[(NHC)2Pd(CH3)2]
1a–b
cis-[(NHC)2Pd(CH3)2]
1a–b
NHC =
Me
H2O
N
NaO3S
N
a
Me
80 °C, overnigth
Me
Me
∗
R =
Me
b
N
N
SO3Na
NaO3S
NaO3S
N
N
β
Me
N
SO3Na
N
(a)
(b)
R
α
γ
n
Me
Me
∗
SO3Na
13C
13C-a
Me
Pd
2a–b
(b)
Scheme 2. Synthesis of water-soluble PdNPs 2a−b by thermal decomposition
of the dimethyl complexes 1a−b.
N
N
NaO3S
N
NaO3S
N
R
R
NHC
CH3
Hb
Hb
Ha
Ha
Pd
Pd
H3C
H3C
NHC
CH3
cis
trans
Scheme 1. (a) Synthesis of the organometallic precursors 1a−b and the
isotopically modified complex 13C-1a with the carbenic carbon substituted by
carbon-13. (b) Protons Ha and Hb are diastereotopic in the cis configuration
(assuming a slow rotation of the NHC ligand around the Pd-Carbene bond)
while they are related by a symmetry plane in the trans configuration.
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(a)
(b)
or undetectable NMR resonances because of the rigidity
induced in the ligands upon coordination to a surface, the
inherent heterogeneity of this surface, and the slow tumbling of
particles that results in rapid T2 relaxation times.[26] However,
relatively narrow proton resonances were observed in the 1H
NMR spectrum of the purified PdNPs 2b in D2O for the b- and g-
methylenes (at 2.15 and 2.85 ppm respectively) of the
propylsulfonate chain (and a broader one at 4.3 ppm related to
the a position of the chain). The similar effect observed in
related NHC-stabilized Pt nanoparticles was rationalized as
resulting from the flexibility of the propylsulfonate chains.[15]
Their conformation with the sulfonate groups pointing away from
the particle was associated with the solubility of the
nanoparticles in water. In contrast, the resonances for the same
protons were hardly observable in nanoparticles 2a despite their
smaller size. The interaction of the mesityl group with the
nanoparticle surface possibly reduces the flexibility of the NHC
in 2a.[8,27]
2a: 3.4 ± 0.5 nm
60
40
20
0
1
2
3
4
5
6
Size (nm)
(c)
2b: 6.3 ± 1.4 nm
80
60
40
20
0
2
4
6
8
10
Size (nm)
Figure 1. (a) TEM image and size distribution, and (b) HRTEM image for
PdNPs 2a after dialysis. (c) TEM image and size distribution for 2b after
dialysis.
NMR spectroscopy is a very useful technique for the
characterization of nanoparticles.[28] Specifically, solid-state 13C
NMR spectroscopy has been used in the last years to support
the coordination of NHC ligands to the surface of metal
Thermal decomposition of complex 1b also affords stable
solutions of PdNPs in water (2b, Scheme 2). These PdNPs are
larger than those obtained from 1a under the same conditions
(6.6 ± 2.0 nm for 2b compared with 3.6 ± 0.6 nm for 2a under
the specific conditions described in the Experimental Section;
see Figure S4 in the Supporting Information). A similar tendency
(NHC ligands with a stronger steric hindrance led to smaller
MNPs) was also observed for Pt nanoparticles prepared recently
in our laboratories employing a parallel methodology.[15] The
nanoparticles were purified by dialysis for 12 h (Figure S3)
without showing significant coalescence or changes in their size
distributions during this process (mean diameters after dialysis:
nanoparticles.[8,17-19]
This
characterization
is
however
complicated by the structural and electronic[29] disorder at the
metallic surface and the presence of conduction electrons.[30] We
have recently shown that the 13C resonance of the carbenic
carbon is shifted to around 600 ppm in NHC ligands coordinated
to the surface of PdNPs.[19] This unusual position for the
carbenic resonance of an NHC ligand is the result of a Knight
shift originated by the presence of conduction electrons in large
metal nanoparticles.[31] We found that this effect was observed
for PdNPs exceeding a critical size of around 2 nm. The Knight-
shifted resonances were so large that they were only observed
in 13C-enriched samples using 13C CPMG or spin-echo MAS
NMR experiments (spectra of two samples of 13C-2a showing
the very broad resonance for the 13C-labeled carbenic carbon at
−600 ppm are showed in Figure S8). This behavior explains the
absence of observable carbenic resonances in the 13C Cross-
Polarization (CP-MAS) spectra of PdNPs 2b (Figure S7).
6.3
± 1.4 nm; Figure 1c). The hydrodynamic diameter
determined by Dynamic Light Scattering (DLS) was 8.1 ± 1.2 nm.
The ability to perform DLS measurements is indicative of the
good dispersion of the nanoparticles in solution. The
hydrodynamic diameter is slightly larger than the TEM diameter
because it includes the ligand and solvation layers. The
difference between TEM and DLS diameters was similar in the
case of 2a (3.4 ± 0.5 nm and 5.2 ± 0.7 nm, respectively), the
DLS diameter being coherent with the hydrodynamic diameter
determined by diffusion-ordered NMR spectroscopy (6.4 ± 2 nm,
Figure S11). After evaporation of the solvent under vacuum,
PdNPs 2b were isolated as a black shiny powder in yields of
19% (based on Pd content). As the metal precursor was
completely decomposed during the formation of 2b, this low
yield is a consequence of metal losses during dialysis. Powder
X-Ray Diffractograms (XRD) showed the formation of crystalline
nanoparticles with a Pd(0) face-centered cubic packing (fcc,
Figure S5). The crystallite size calculated from the XRD data
using the Scherrer equation (2.2 nm) was much smaller than the
mean size obtained by TEM. A similar divergence between TEM
and XRD radii was found in PdNPs 2a,[19] and was ascribed to
the polycrystallinity of the nanoparticles observed in the HR-
TEM images (Figure 1b).[25]
A final comment can be done on the composition of these
PdNPs. The stoichiometric ratio of imidazole versus palladium
can be estimated from the elemental analyses of the samples.
The ratios determined in samples of 2a and 2b after purification
by dialysis are always close to 1:1 (values ranging from 0.8 to
1.1). Therefore, a part of the imidazolium salts formed in the
decomposition of the organometallic precursors was removed by
dialysis as the NHC:Pd proportion was originally 2:1 (this is the
ratio in the organometallic precursors 1a-b). However, these
ratios are yet too high to correspond entirely to NHC ligands
coordinated to the metal surface. In our previous communication,
we estimated by solid-state 13C NMR spectroscopy that only a 3
±
1% of the imidazole species corresponded to NHCs
coordinated to the metal surface in a sample of PdNPs 13C-2a.
The rest of imidazole species are likely in the form of
imidazolium salts and presumably play a role in the stabilization
of the PdNPs (as commented above, some coalescence was
observed when dialysis was prolonged). Indeed, the absence of
The nanoparticles were also characterized by infrared (FT-IR,
Figure S6) and 1H NMR spectroscopy (Figure S3). It is well
known that protons close to MNP surfaces typically give broad
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1
resonances ascribable to these organic residues in the H NMR
spectra of the PdNPs in solution suggests an interaction
between them and the nanoparticle surface. In addition, NOESY
experiments performed before purification by dialysis showed a
transfer NOE effect between the molecular species and the
nanoparticle in the case of 2a (Figure S12). This effect happens
when free molecules in solution are in rapid equilibrium with
molecules coordinated to the surface of large nano-objects. The
negative NOEs of the coordinated molecules are transferred by
the fast exchange phenomena to the free ones resulting in
negative cross peaks for the latter (same sign as the diagonal
peaks).[32,33] This observation evidences the presence of a weak
interaction between the nanoparticles and the decomposition
products. The transfer NOE effect disappears after three hours
of dialysis, as expected from the slower exchange resulting from
the decreased concentration of free imidazolium. Although we
cannot determine the nature of the interaction between the
organic residues and the PdNP surface, we might propose an
organization in multiple layers similar to that observed in ZnO
NPs stabilized by alkyamine ligands.[33]
N
SO3Na
N
R
R
N
Pd
CH3
CH3
N
1a–b
NaO3S
(a)
D2O
NaOD + CH3D
SO3Na
N
N
SO3Na
N
N
R
R
dmso
(b)
H3C
Pd
OD2
R
H3C
Pd
dmso
R
–O3S
N
–O3S
N
N
N
N
N
4a–b
3a–b
SO3Na
R
(c)
CN–
D2O
Mechanism involved in the thermal decomposition of
complexes 1a−b
H3C
Pd
CN
R
80 ˚C
overnight
NaOD
We performed NMR studies to elucidate the intermediates
involved in the thermal transformation of complexes 1a−b in
water. The dissolution of these complexes in deuterated water
was accompanied by immediate formation of the monomethyl
species 3a−b (Scheme 3a). Analysis of the gas composition
produced during the reaction by 1H NMR spectroscopy after
transfer to a tube containing chloroform-d1 showed the exclusive
formation of methane-d1. The selective protonolysis of just one
of the two palladium-methyl bonds was not completely
unexpected considering the reported reactivity of other dimethyl
NHC palladium complexes with Brönsted acids or traces of
water.[22,23] This protonolysis is accompanied by a cis-trans
isomerization around the metal center (only one set of NMR
resonances was observed for both NHC ligands, Figures S13-
S14; the two NHC ligands should be inequivalent in cis isomers).
The coordination vacancy is probably occupied by a molecule of
the deuterated solvent (or deuteroxide anion taking into account
the alkalinity of the resulting reaction medium). This uncertainty
could not be resolved by ESI mass spectrometry where the
heaviest peaks detected corresponded to the tricoordinated
fragments [Pd(NHC)2Me]+ (Figures S15-S16). Addition of dmso
or cyanide to the above solutions resulted in coordination of the
added ligand and clean formation of the expected complexes
4a−b or 5a−b (Scheme 3b) which were characterized by NMR
spectroscopy and mass spectrometry (Figures S17-S18).
Complex 4b was also prepared directly from [PdMe2(bpy)] in
moderate yields (62%) and fully characterized (see Experimental
Section).
–O3S
N
N
5a–b
Pd(0) NPs (2a–b)
+
–
N
N
SO3
40%
R
a-d1 or b-d1
D
+
–
60%
N
SO3
N
R
CH3
6a–b
Scheme 3. Decomposition byproducts observed in the formation of the PdNPs
2a−b. The scheme omits the H/D exchanges observed in step c for clarity.
The D2O solutions were subsequently heated at the
temperature used in the preparation of PdNPs 2a−b (80 °C).
Under these conditions, the resonances corresponding to the
NHC ligand coordinated to palladium(II) in 3a−b disappeared
progressively
while
2-methylimidazolium
(6a−b)
and
monodeuterated imidazolium (a-d1 or b-d1) were formed
(Scheme 3c). No other intermediates were detected in this clean
transformation.
Incidentally, deuteration was found not only at the 2-position
of a−b (as reflected in Scheme 3c) but also at other sites of both
types of imidazolic products (a−b and 6a−b). The degree of
deuteration was marginal at the 4- and 5-imidazole positions but
extensive or complete for the 2-methyl group of 6a−b (see 1H
2
and H NMR spectra in Figures S19-S20 and S23-S24). Handy
and Okello have reported H/D exchange between the 2-Me
group and the deuterated solvent in a series of 2-methyl-
substituted imidazoliums under mildly basic conditions.[34]
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Table 1. Distribution of products (in equiv %) in the decomposition of 1a-b[a]
Control experiments revealed that the deuteration of the 2-Me
group in 6b was insignificant after 24 h under neutral conditions,
whereas it was fast at pH 13 even in the absence of PdNPs (full
deuteration in less than 90 min; Figures S25-S26). This behavior
means that the deuteration observed during the decomposition
of 1a−b is promoted by the alkaline medium generated by the
hydrolysis of the Pd-C bonds, probably without involvement of
Pd species.
+
Me
–
N
SO3
6a–b
N
SO3Na
N
N
R
Me
Me
CH3
H2O
Pd 13C(O)Me
Me
+
N
Me
–
SO3
a–b
N
–
R
N
SO3
N
Me
8a
In light of the sequence of reactions depicted in Scheme 3,
the mechanism of PdNP formation from the dimethyl complexes
1a−b would begin with the hydrolysis of one methyl group
leading to the “cationic” monomethyl bis-carbene 3a−b. We
propose that the next step is a reductive methyl-NHC coupling to
generate a NHC Pd(0) species instead of hydrolysis of a Pd-
NHC bond. This proposal is based on the observation that the
palladium(II)-carbene bonds of water-soluble NHC complexes
are quite stable in alkaline media under conditions similar to
those used here.[35] In addition, seminal work by McGuinness,
Cavell, and coworkers showed that alkyl NHC Pd(II) complexes
can decompose producing 2-methylimidazolium salts.[36-38]
Combined kinetic and DFT studies demonstrated that the
mechanism consists in a concerted reductive elimination, and
not in carbene insertion into Pd-alkyl bonds.[37] These authors
observed that, in coherence with this mechanism, a positive
charge on the metal increased the rate of elimination of 2-
alkylimidalzolium in respect to their neutral counterparts. The
decomposition pathway followed by 1a-b contrasts with the one
observed in the formation of NHC-protected PtNPs from water-
soluble cis-[Pt(NHC)Me2(dmso)] complexes, which mainly
involved the elimination of ethane as reducing step.[15,16] It
should be noted that reductive alkyl-alkyl coupling has been
observed more rarely in Pd(NHC) alkyl complexes than the
reductive coupling of the alkyl and NHC ligands.[24] The
preference for this reductive coupling over hydrolysis of the
remaining Pd-Me bond in 3a−b should be noted. This behavior
is likely to be favored by the cationic nature of the palladium(II)
center in 3a−b. In the last step of the nanoparticle formation, the
Pd(0) monocarbene species could begin aggregation of the
Pd(0) atoms and form nanoparticles, hence liberating a portion
of the NHC ligands in the form of imidazolium salt. The amount
of imidazolium salts a−b measured by 1H NMR integration
during this second step of decomposition was always found
significantly lower than that of the coupling product 6a−b found
in the first step (the ratio determined by 1H NMR integration is
approximately 40:60% in both cases, Table 1a, Figures S19-
S22). This is expected since a fraction of the NHC ligands
remains attached to the surface of the nanoparticles or in a
second coordination sphere.
(a) Decomposition under thermal treatment
Precursor [conditions]
8a
CH313COO–
6a or 6b
58%
a or b
42%
1a
1b
59%
41%
(b) Decomposition under 13CO treatment at 1 bar]
1a [room temp, overnight]
1a [45 °C, 3 days]
79%
69%
--
~10%[b]
~25%[b]
47%
4%
17%
27%
67%
4%
1b [room temp, overnight]
33%
[a] The composition (in equiv %) was determined by 1H NMR integration and is
referenced to 100% for the total of imidazolium-containing species; 1 mmol = 1
equiv (6a, 6b, a, and b) or 2 equiv (8a and acetate). [b] Overlapped with other
resonances; the integral was estimated by deconvolution.
Synthesis of the Pd nanoparticles 7a−b by decomposition
of dimethyl complexes under a 13CO atmosphere
Besides thermal decomposition, the formation of metal
nanoparticles can be achieved by reacting an organometallic
precursor with a reductive gas, in general CO and H2, and using
mild reaction conditions. Here, the organometallic complexes
1a−b were stirred overnight under a 13CO atmosphere of 1 bar in
water at room temperature, leading to dark brown solutions
containing the PdNPs 7a−b (Scheme 4). These solutions were
again highly stable over time and no aggregation was observed
for several months when no purification was performed. The
diameters of the nanoparticles measured by TEM before
purification were of 1.5 ± 0.3 nm for 7a and 1.9 ± 0.6 nm for 7b
(Figures S27 and S28). A clear effect of the decomposition
method was observed, as regards to the nanoparticle size.
PdNPs obtained by decomposition with CO (7a−b) were
significantly smaller than those obtained by thermal
decomposition (2a−b). After purification by dialysis against water
for 24 h, TEM images showed that the diameters of the small
nanoparticles have only slightly increased (to 1.6 ± 0.4 nm for 7a
and to 2.4 ± 0.6 nm for 7b; Figure 3) but also that large
agglomerates were also formed.
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cis-[(NHC)2Pd(CH3)2]
1a–b
are again too high to correspond entirely to NHC ligands
coordinated to the metal surface. The partial retention of the
imidazole salts in the NP solution might suggest some
interaction between them and the PdNP surface. The XRD
diffractograms confirmed the fcc structure of the Pd(0)
nanoparticles (Figure S31). The similarity of the crystallite sizes
obtained from the Scherrer equation (1.5 nm for 7a and 2.3 nm
for 7b) with the TEM diameters proved the monocrystallinity of
the objects.
13CO (1 bar), Ar (1 bar)
H2O, room temp, overnight
Me
Me
∗
R =
N
SO3Na
N
R
Me
n
(a)
(b)
The nanoparticles were further characterized in the solid
state by infrared (FT-IR) and 1H and 13C MAS NMR
spectroscopy (Figure S32-S35). Coordinated 13CO was detected
by 13C CP-MAS spectroscopy in the form of broad resonances at
around 170−180 ppm in 7a. This chemical shift is within the
usual range for metal carbonyls in diamagnetic substances
(typically between 150 and 250 ppm).[39] We recently reported
that carbonyl resonances are expected to move to high
frequencies because of Knight shift, when the PdNP size
exceeds the critical threshold of 2-3 nm.[19] Accordingly to this
hypothesis, two types of 13CO resonances were observed in
NPs 7b (2.4 ± 0.6 nm) after treatment of the powder with 13CO
for 3 days, one centered at around 180 ppm corresponding to
the smaller particles composing the sample, and the other
expanding up to 800-900 ppm and corresponding to the bigger
nanoparticles (Figure S36). The 13C resonances of the
coordinated NHC ligands appeared in the same regions as in
the organometallic precursors, except for the resonances of the
carbenic carbons, which were not observable.
13CO
Pd
∗
SO3Na
O13
C
7a–b
13CO
Scheme 4. Synthesis of water-soluble PdNPs 7a−b by decomposition of the
dimethyl complexes 1a−b under a 13CO atmosphere.
(a)
(b)
7a: 1.6 ± 0.4 nm
7b: 2.4 ± 0.6 nm
80
60
40
20
0
60
40
20
0
1
3
4
5
2
0
1
2
Size (nm)
Size (nm)
Figure 3. TEM images and size distributions for PdNPs 7a (a) and 7b (b) after
dialysis.
Decomposition mechanism under a 13CO atmosphere
As already showed above, the dissolution of the
organometallic precursors 1a−b in water leads to the
monomethyl “cationic” complexes 3a−b, which evolved
differently upon treatment with 13CO. In the case of 3a (Scheme
5), the aqua ligand was progressively replaced by 13CO. The
formation of the trans carbonyl complex 9a was demonstrated
by the observation of doublets for the PdMe group at −0.12 ppm
(1H) and −3.4 ppm (13C) with couplings of 1.6 and 35 Hz,
respectively, to the 13CO resonance at 176.8 ppm (Figures S38-
S39). In a second step, 9a is transformed into the acetyl
complex 8a by 13CO insertion into the PdMe bond. While the
coordination step reverted when the 13CO atmosphere was
evacuated, this insertion was irreversible. The acyl complex 8a
was the main product remaining in the aqueous solution after an
overnight treatment of 1a at room temperature, together with
smaller amounts of (1-13C)acetate (1H doublet at 1.85 ppm with
a coupling constant to 13C of 5 Hz), 2-methylimidazolium (6a),
and imidazolium (a; Table 1b and Figure S29). Complex 8a was
very stable under these conditions (see characterization below).
Even when the reaction was extended to three days at 45 °C,
the concentration of 8a in the solution showed a modest
decrease from 79% to 69% in parallel to the increase from ca.
10% to 25% of the amount of (1-13C)acetate. 2-
Methylimidazolium (6a) was formed in comparatively lower
amounts (around 4%), irrespective of the reaction time and the
temperature (Figure S37). In the case of 3b, the decomposition
under 13CO occurred without the formation of detectable
1
After dialysis, the H NMR spectrum of 7a (Figure S29) still
showed several well-defined 1H resonances that corresponded
to the acyl complex Na[trans-Pd(NHC)2(COMe)(OH2)] (8a). This
complex is an intermediate generated in the synthesis of the
PdNPs (see below) and could not be completely removed during
the purification. The resonances of the NHCs on the PdNP
surface were too broad to be clearly detected in this spectrum.
In the case of 7b, the molecular products were efficiently
removed during the dialysis and the 1H NMR spectrum only
showed the broad signals corresponding to the surface NHCs
(Figure S30). The resonances for the b and g positions of the
alkyl chain (at 2.15 and 2.85 ppm respectively), and for the
positions 4 and 5 of the imidazole ring (around 7.4 ppm) were
better defined than those due to the a protons (around 4.3 ppm),
which were very broadened, as in 2b, due to their proximity to
the particle surface.
The solutions were dried under vacuum to give black shiny
powders in yields of 8% for 7a and 23% for 7b, based on the
metal content. The low yield obtained for nanoparticles 7a is
partly due to the formation of the organometallic complex 8a as
1
the main reaction product (it represented almost 80% of the H
integration of the imidazole-containing species). The NHC:Pd
ratios determined by elemental analysis after purification by
dialysis (1:1.5 for 7a and 1:2.5 for 7b) are lower than in the case
of 2a-b, as it might be expected from the longer dialysis times
(24 h) employed for these nanoparticles. However, these ratios
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10.1002/chem.201702204
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amounts of other NHC-metal intermediates. The organometallic
species decomposed completely after an overnight 13CO
treatment at room temperature, giving imidazolium b, (1-
13C)acetate, and 2-methylimidazolium 6b (Table 1b and Figure
S30).
alcohol.[44] Trans to cis isomerization is more likely in the case of
the less sterically demanding ligand b. A rate-limiting CO
coordination step followed by faster isomerization, insertion and
elimination steps would explain the significant amount of (1-
13C)acetate formed from 3b and the lack of detection of carbonyl
or acyl metal intermediates.
Decomposition of dimethyl complexes under an H2
atmosphere
3a
Me
Me
∗
R =
Me
+
13CO
– H2O
Reduction by H2 is a classical method for the formation of MNPs.
As such, it was of interest to determine whether this
methodology could also be applied to the preparation of water-
stable Pd nanoparticles from complexes 1a−b. When the
organometallic precursor 1a was dissolved in water and stirred
overnight under 1 bar of H2, a dark-brown solution that was
stable over time was obtained (no metallic deposition was
observed after 6 months). This solution contained worm-shaped
nano-objects (10a) with different sizes (see TEM images in
Figures 4 and S45). The 1H NMR spectrum of the solution
showed formation of the imidazolium salt a as the only
decomposition product (Figure S46). In the case of the
organometallic precursor 1b, we did not obtain any stable nano-
object. Metallic palladium precipitated after stirring for 2 hours
under an atmosphere of 1 bar H2. Hydrido palladium–NHC
complexes are often unstable as the attack of the hydride on the
carbene results in reductive elimination to give the imidazolium
salt.[45] In both cases, the coalescence of particles or the
formation of bulk metallic palladium might be a consequence of
the formation of relatively unprotected metal nanoparticles due
to the extensive removal of NHC ligands by carbene-hydride
coupling. Formation of palladium black has sometimes been
observed when NHC complexes are placed under a hydrogen
atmosphere.[46] This method of decomposition seems therefore
not to be adequate for the preparation of well dispersed isolated
Pd nanoparticles.
N
SO3Na
N
SO3N
N
N
R
R
a
H2O
H3C
Pd
13CO
R
H2O
Pd
13COMe
R
–
–
N
N
SO3
N
SO3
N
9a
8a
Scheme 5. NHC complexes observed in the formation of PdNPs 7a.
Complex 8a was characterized by NMR and MS techniques
(Figures S40-S44). The methyl protons were observed as a
doublet at 1.28 ppm with a coupling of 5 Hz to the (13C)-carbonyl
at 234 ppm. The methyl 1H resonance was additionally
correlated with the methyl 13C resonance at 41 ppm by HSQC
1H-13C spectroscopy. These 1H and 13C chemical shifts are
characteristic of acetyl PdII complexes (versus carbonyl-
methyl).[40,41] Thus, for instance, Subramanium and Slaughter
reported resonances at 234.5, 44.3 (13C), and 1.29 ppm (1H) for
the acetyl derivative [Pd(NHC-NHC)(COMe)(CO)]+ while the
resonances of the carbonyl complex [Pd(NHC-NHC)Me(CO)]+
appeared at 183.0, 13.2 (13C), and −0.29 ppm (1H).[40]
The reduction of NHC metal complexes containing alkyl and
carbonyl ligands can involve a priori three different processes:
(a) alkyl-carbene coupling to give 2-methylimidazolium,[36,37,42]
(b) CO insertion into the Pd-alkyl bond followed by OH−
nucleophilic attack to the acetyl ligand to give acetate, and (c)
CO insertion into the Pd-methyl bond followed by acetyl-carbene
coupling to give 2-acetylimidazolium.[36,37,42] We never observed
cis-[(NHC)2Pd(CH3)2]
1a
H
2 (1 bar)
H2O, room temp
Ar (1 bar) overnight
the
formation
of
2-acetylimidazolium,
whereas
2-
10a
methylimidazolium and acetate elimination were much more
favorable in the case of b. The slowing-down of the reductive
methyl-carbene elimination with the most sterically demanding
NHC ligand a was to be expected from previous studies about
the effect of mesityl-substituted NHC ligands on the stability of
methyl Pd derivatives.[40,43] In addition, methyl-carbene coupling
occurs essentially after CO coordination considering that 3b
produced, under the conditions shown in Table 1b, less than
10% of 6b in the absence of carbon monoxide. On the other
hand, the effect of the NHC bulkiness on the formation of (13C)-
acetate is less clear. The stability of the trans acyl complex 8a
might be associated to the finding of Van Leeuwen and
coworkers that alcoholysis of acylpalladium(II) complexes
requires cis coordination between the acyl group and the
Figure 4. TEM image of nano-objects 10a obtained by decomposition of
complex 1a under an H2 atmosphere.
Catalytic hydrogenation of styrene
In order to check the activity of the PdNPs as hydrogenation
catalysts in water, some of the PdNPs synthesized during this
work were tested as catalysts in the chemoselective
hydrogenation of styrene at room temperature in water (see
complete details in the Supporting Information). Nanoparticles
2a (0.46 mol%) gave high conversion values (97%) after 3 hours
of reaction with no precipitation of metallic Pd. In addition, the
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10.1002/chem.201702204
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General procedures and characterization techniques are given in the
Supporting Information.
catalyst could be reused under the same reaction conditions
after extraction of the organic products. The conversions were
almost complete, with yields higher than 80% for at least 10
catalytic cycles (Figure S47) and with no formation of metallic Pd
precipitates. The shape of the particles was checked by TEM
after hydrogenation (Figure S48) and evidenced extensive
coalescence leading to worm-like particles resembling the
PdNPs obtained under hydrogen (10a).
Synthesis and characterization data for palladium(II) complexes
Synthetic procedures and characterization data is given below for
complexes 1a and 1b. Characterization data for some representative
complexes is also given. Information for other complexes is given in the
SI. The imidazolium salts a, b, and 13C-a were prepared as reported
elsewhere.[15,48]
Conclusions
cis-Bis[1-mesityl-3-(3-sodium
sulfonatopropyl)imidazol-2-
ylidene]dimethylpalladium(II) (1a): A solution of sodium tert-butoxide
(0.072 g, 0.75 mmol) in dry dmso (10 mL) was slowly added to a mixture
of [PdMe2(bpy)] (0.100 g, 0.342 mmol; see SI for preparation details) and
imidazolium salt a (0.210 g, 0.683 mmol) under an inert atmosphere. The
reaction mixture was stirred at room temperature for 30 minutes, then the
solution was poured into dry acetone (60 mL). The precipitate was
separated by filtration, washed with dry CH2Cl2 (2 ´ 20 mL), and dried
under vacuum to afford 1a as a white solid (0.188 g, 58%). Anal. Calcd.
(%) for C34H58N4Na2O11PdS3 (1a·dmso·4H2O): C, 43.27; H, 6.15; N, 5.94;
S, 10.19. Found: C, 42.94; H, 5.82; N, 5.97; S, 10.33. 1H NMR (300 MHz,
Herein, we have reported the synthesis of water-soluble PdNPs
stabilized with NHC ligands. These particles are highly stable in
water, and their sizes are dependent upon the organometallic
precursor and the synthetic method employed. The PdNPs are
smaller when the most hindered ligand is employed and upon
decomposition under a CO atmosphere. For nanoparticles 13C-
2a, which were obtained by thermal decomposition of Pd(II)
NHC complex 1a labeled in the carbene position, the 13C
resonance of the carbene carbon coordinated to the
nanoparticle surface is shifted to very high frequencies (600
ppm) as a result of the Knight shift effect. As we argued in our
previous communication,[19] the observation of a Knight-shifted
signal for the carbene carbon demonstrates the coordination of
the NHC ligand to the surface of the particles.
NMR studies in solution helped to elucidate the
organometallic intermediates and organic decomposition
products involved in the reduction process required for formation
of the Pd(0) nanoparticles from their PdII precursors. Key steps
are the reductive coupling between one methyl and one carbene
ligand (upon thermal decomposition), the insertion of CO into a
Pd-methyl bond followed by elimination of acetate (in the
presence of carbon monoxide), or carbene-hydride coupling
(under a hydrogen atmosphere). As expected, steric effects can
stabilize some intermediates, making the activation steps
involved in the reductive couplings or isomerization processes
required to reach the adequate stereochemistry less accessible.
The decomposition pathways have been studied under
conditions that are relevant to many catalytic processes. This
knowledge is therefore important for both the high-yield
synthesis of PdNPs and for homogeneous processes in which
nanoparticle formation must be avoided.
dmso-d6, 2.49 ppm): d 7.26 (d, 1H, Imz-H4, JHH = 1.8), 6.98 (s, 1H, Ar),
3
6.96 (s, 1H, Ar), 6.92 (d, 1H, Imz-H5, JHH = 1.8), 3.70 (broad m, 1H,
3
NCH2), 2.96 (broad m, 1H, NCH2), 2.31 (s, 3H, Ar-p-Me), 2.18 (m, 2H,
CH2SO3), 1.94 (s, 3H, Ar-o-Me), 1.84 (broad m, 2H, CH2CH2CH2), 1.47
(broad s, 3H, Ar-o-Me), −0.89 (s, 3H, PdMe). 13C{1H} NMR (75 MHz,
dmso-d6, 39.0 ppm): d 193.7 (Imz-C2), 136.8 (Ar-C4), 136.5 (Ar-C2), 135.8
(Ar-C1), 133.9 (Ar-C2), 128.1 (Ar-C3), 128.0 (Ar-C3), 121.1 (Imz-C4), 118.5
(Imz-C5), 48.3 (NCH2), 47.5 (CH2SO3), 25.4 (CH2CH2CH2), 20.0 (Ar-p-
Me), 17.6 (Ar-o-Me), 16.8 (Ar-o-Me), –3.1 (PdMe). ESI-MS (negative ion,
MeOH) m/z: 735.2 [M – 2Na – Me]– (calcd. 735.2) 100%.
cis-Bis[1,3-bis(3-sodium
sulfonatopropyl)imidazol-2-
ylidene]dimethylpalladium(II) (1b): Complex 1b (0.249 g, 73%) was
prepared as described above for 1a, starting from sodium tert-butoxide
(0.072 g, 0.75 mmol), [PdMe2(bpy)] (0.100 g, 0.342 mmol) and b (0.228 g,
0.683 mmol). Anal. Calcd. for C22H48N4Na4O17PdS5 (1b·dmso·4H2O): C,
26.44; H, 4.84; N, 5.61; S, 16.04. Found: C, 26.48; H, 4.82; N, 5.60; S,
15.60. 1H-NMR (300 MHz, dmso-d6, 2.49 ppm): d 7.09 (s, 2H, Imz-H4,5),
4.27 (m, 2H, NCH2), 4.03 (m, 2H, NCH2), 2.45 (m, partially under the
solvent resonance, CH2SO3), 1.98 (broad m, 4H, CH2CH2CH2), –0.73 (s,
3H, PdMe). 13C{1H} NMR (75 MHz, dmso-d6, 39.0 ppm): d 192.3 (Imz-C2),
119.4 (Imz-C4,5), 48.5 (NCH2), 48.4 (CH2S), 26.5 (CH2CH2CH2), –3.6
(PdMe). ESI-MS (negative ion, MeOH) m/z: 787.0 [M – 2Na – Me]– (calcd.
787.0) 2%.
Characterization data for 3a: Complex 1a (5.0 mg, 5.3 µmol) was
dissolved in deuterium oxide (0.5 mL) into an NMR tube. Methane
evolution was immediately observed. The 1H NMR spectrum was
recorded at 45 °C because some resonances appeared broadened at
room temperature. 1H NMR (300 MHz, D2O, 4.43 ppm, 318 K): d 7.20 (s,
2H, Imz-H4), 6.97 (s, 4H, Ar), 6.92 (s, 2H, Imz-H5), 4.17 (broad m, 4H,
NCH2), 2.60 (broad m, partially under the residual dmso resonance,
CH2SO3), 2.30 (s, 6H, Ar-p-Me), 2.18 (broad m, 4H, CH2CH2CH2), 1,80 (s,
12H, Ar-o-Me), –0.46 (s, 3H, PdMe). 13C{1H} NMR (75 MHz, D2O, 318 K):
d 183.8 (Imz-C2), 141.1 (Ar-p-Me), 138.9 (Ar-C1), 137.9 (Ar-o-Me), 132.3
(Ar-C3), 131.5 and 124.1 (Imz-C4,5), 51.5 (NCH2), 50.0 (NCH2), 27.1
(CH2CH2CH2), 23.1 (Ar-p-CH3), 20.6 (Ar-o-CH3), –14.5 (PdMe). ESI-MS
(positive ion, H2O/MeOH) m/z: 781.1 [M – OD2 + Na]+ (calcd. 781.1) 52%,
743.1 [M – OD2 – CH3]+ (calcd. 743.1) 51%, 345.1 [6a + Na]+ (calcd.
345.1) 100%, 331.1 [a + Na]+ (calcd. 331.1) 33%. ESI-MS (negative ion,
H2O/MeOH): 735.1 [M – OD2– Na]– (calcd. 735.1) 100%.
Over the last few years nano-chemistry is tending towards
more environmentally friendly conditions in the synthesis and
applications of MNPs.[47] This work constitutes an example of
highly stable MNPs synthesized under green conditions that
might be of potential interest in different catalytic processes
involving palladium in aqueous medium. As an example, we
have shown the efficiency of the PdNPs as recyclable catalysts
for the chemoselective hydrogenation of styrene.
Experimental Section
General procedures and characterization techniques
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10.1002/chem.201702204
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Trisodium
trans-bis[1,3-bis(sulfonatopropyl)imidazol-2-
Acknowledgements
ylidene](dimethylsulfoxide)methylpalladium(II) (4b):
A solution of
sodium tert-butoxide (0.072 g, 0.75 mmol) in moist dmso (10 mL) was
slowly added to a mixture of [PdMe2(bpy)] (0.100 g, 0.342 mmol) and
imidazolium salt b (0.228 g, 0.683 mmol) under an inert atmosphere. The
mixture was stirred at room temperature for 30 min. The solution was
then poured into dry acetone (60 mL) and filtered. The white solid thus
obtained was washed with 2 ´ 20 mL of dry CH2Cl2 and dried under
vacuum (0.213 g, 62%). Anal. Calcd. for C21H46N4Na4O18PdS5
(4b·5H2O): C, 25.17; H, 4.73; N, 5.59; S, 15.99. Found: C, 25.45; H,
4.80; N, 5.71; S, 15.31. 1H NMR (300 MHz, dmso-d6 2.49 ppm): d 7.48 (s,
4H, Imz-H4,5), 4.40 (broad t, 8H, NCH2), 2.50 (overlapped with the solvent
resonance, indirectly observed by HSQC 1H-13C, CH2SO3), 2.17 (broad
m, 8H, CH2CH2CH2), –0.04 (s, 3H, PdMe). 13C{1H} NMR (75 MHz, dmso-
d6, 39.0 ppm): d 176.1 (Imz-C2), 121.5 (Imz-C4,5), 48.7 (NCH2), 47.6
(CH2SO3), 26.2 (CH2CH2CH2), –9.3 (PdMe). ESI-MS (positive ion,
H2O/MeOH): 832.0 m/z [M + Na]+ (calcd. 832.0) 100%. ESI-MS (negative
ion, H2O/MeOH) m/z: 787.0 [M – dmso – Na]– (calcd. 787.0) 51%, 325.1
[6b – Na]– (calcd. 325.0) 68%.
This work was supported by the Spanish Ministerio de
Economía y Competitividad (project CTQ2014-55005-P), the
Centre National de la Recherche Scientifique (CNRS), the
Institut National des Sciences Appliquées (INSA) and the
Agence Nationale de la Recherche (grant ANR-11-INTB-101).
We thank Simon Cayez for recording the HR-TEM images.
J.M.A. is grateful to the Spanish Ministerio de Educación for a
FPU Doctoral Fellowship (AP2012-4745) and S.T. to the
European Commission for a postdoctoral grant (PCIG11-GA-
2012-317692).
Keywords: organometallic chemistry • N-heterocyclic carbene
ligands • nanoparticles • transition metals • palladium • aqueous-
phase organometallic chemistry
Characterization data for 8a. 1H NMR (300 MHz, D2O, 4.69 ppm): d
7.32 (d, 3JHH = 2.0, 2H, Imz-H4), 7.07 (broad s, 4H, Ar), 7.02 (d, 3JHH = 2.0,
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3
12H, Ar-o-Me), 1.25 (d, JHC = 5.4, 3H, COMe). 13C{1H} NMR (75 MHz,
D2O): d 233.7 (COMe), 176.6 (Imz-C2), 139.9 (Ar-C4), 135.6 (Ar-C2),
130.6 (Ar-C1), 128.8 (Ar-C3), 123.4 and 121.3 (Imz-C4,5), 49.3 (NCH2),
48.2 (CH2SO3), 41.2 (COMe), 25.8 (CH2CH2CH2), 20.2 (Ar-p-M), 17.1
(Ar-o-M). ESI-MS (negative ion, H2O/MeOH) m/z: 764.1 [M – H2O – Na]–
(calcd. 764.1) 100%; 735.2 [M – H2O – 13CO – Na]– (calcd. 735.2 m/z)
15%.
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Synthesis of and characterization for PdNPs
PdNPs 2: The corresponding palladium complex 1 (0.500 mmol for 1a
and 1b; 0.200 mmol for 13C-1a) was introduced into a 100-mL pressure
tube and dissolved in deionized water (50 or 20 mL) under an inert
atmosphere. The resulting solution was heated at 80 °C for 15 hours.
The dark-brown solutions formed were left to reach room temperature
slowly, and were dialyzed against water using a cellulose membrane
(MWCO = 14,000 Dalton) for 6 (2a and 13C-2a) or 12 h (2b). The solution
was dried under vacuum to afford the corresponding nanoparticles as a
black shiny powder (0.232 g, 59% based on Pd for 2a; 0.057 g, 19%
based on Pd for 2b; 0.063 g, 39% based on Pd for 3C-2a).
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PdNPs 7: The corresponding palladium complex 1 (0.500 mmol) was
introduced into a 100-mL pressure tube and dissolved in deionized water
(50 mL) under an inert atmosphere. The tube was then pressurized at 1
bar of Ar and 1 bar of 13CO. The solution was kept under stirring for 15
hours at room temperature. The resulting dark-brown solutions were
dialyzed against water using a cellulose membrane (MWCO = 14,000
Dalton) for 24 h. The solution was then dried under vacuum to afford the
corresponding nanoparticles as a black shiny powder (0.039 g, 8% based
on Pd for 7a; 0.102 g, 23% based on Pd for 7b). Characterization data is
given in the ESI.
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FULL PAPER
Entry for the Table of Contents:
FULL PAPER
–
SO3
N
N
R
Decomposition of palladium(II)
NHC complexes leads to the
formation of stable and water-
soluble Pd nanoparticles. The
decomposition pathways have been
elucidated, illustrating the chemistry
of the organometallic precursors in
aqueous phase.
J. M. Asensio, S. Tricard, Y. Coppel,
R. Andrés, B. Chaudret,* E. de Jesús*
R
N
Pd CH3
CH3
N
1a-b
Page No. – Page No.
–O3S
Synthesis of Water-Soluble
Palladium Nanoparticles Stabilized
by Sulfonated N-Heterocyclic
Carbenes
reductive
methyl-carbene
elimination
80 °C,
water
CO (1 bar),
water
acetate
elimination
–
–
SO3
N
SO3
N
N
R
N
R
CO
CO
Pd
Pd
OC
2a (3.4 ± 0.5 nm)
2b (6.3 ± 1.4 nm)
7a (1.6 ± 0.4 nm)
7b (2.4 ± 0.6 nm)
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