Reactivity of gold nanoparticles towards N-heterocyclic carbenes

Article abstract of DOI:10.1039/c3dt53579g

The reaction of gold nanoparticles with benzimididazol-2-ylidene ligands leads to the formation of well-defined bis-carbene gold(i) complexes, as shown by characterization techniques such as powder XRD and solid state NMR. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53579g

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Reactivity of gold nanoparticles towards
N-heterocyclic carbenes†
Cite this: Dalton Trans., 2014, 43,
5978
María Rodríguez-Castillo,a Danielle Laurencin,a Frederik Tielens,b Arie van der Lee,c
Received 20th December 2013,
Accepted 6th January 2014
Sébastien Clément,a Yannick Guaria and Sébastien Richeter*a
DOI: 10.1039/c3dt53579g
www.rsc.org/dalton
The reaction of gold nanoparticles with benzimididazol-2-ylidene
ligands leads to the formation of well-defined bis-carbene gold(I)
complexes, as shown by characterization techniques such as
powder XRD and solid state NMR.
Gold nanoparticles (AuNPs) are widely used in the field of bio-
medical sciences for applications such as imaging or drug
delivery.1 They are also extensively studied for catalytic appli-
cations.2 Thus, it is of crucial importance to have a good
understanding of interfacial phenomena occurring at AuNP
surfaces, in order to control their morphology (size and
shape), stability, and properties, depending on the nature of
the coating agents at their surface.3
N-heterocyclic carbene (NHC) ligands have found appli-
cation in many research areas, ranging from catalysis4 to the
discovery of new metallodrugs.5 The use of NHCs in materials
science is also of growing interest, notably for the preparation
of light-emissive materials6 and the coating of planar gold sur-
faces.7 Given that NHCs are strong σ-donor ligands, they have
also been used as stabilizing agents for metallic nanoparticles,
such as Ru,8 Pd,9 and Ir.10 The synthesis of NHC-coated AuNPs
was also recently reported. These were formed either by redu-
cing Au–NHC complexes11,12 or by replacing labile ligands at
the AuNP surface by NHCs.13 Results show that the size, size
distribution, shape and stability of NHC-coated AuNPs strongly
depend on the structure of the NHC ligands and the reaction
conditions used for their synthesis. In particular, it was found
that the stability of NHC-coated AuNPs obtained by ligand
Fig. 1 Structures of the molecules and gold(I) complexes studied.
exchange reactions is surprisingly low, despite the formation
of strong NHC–Au bonds.13 Thus, it appeared to us that the
reactivity of NHC ligands towards AuNPs deserved further
investigation. Since most studies described so far have used
imidazol-2-ylidene derivatives as NHC stabilizers,8–13 we
decided to study mostly the reactivity of benzimididazol-
2-ylidene ligands NHCa and NHCb towards AuNPs (Fig. 1).
Below, we show how the reaction between these ligands and
the Au surface leads to the formation of structurally well-
defined Au(I) complexes.
The starting nanoparticles used here were spherical
thioether-coated AuNPs (thioether = di-n-dodecylsulfide), with
a mean size of 5.8 ( 1.2) nm (Fig. 2, top), and a surface
plasmon resonance (SPR) centered at λmax = 510 nm (see ESI,
Fig. S23†).14 Thioether molecules are weak stabilizing agents,
which can be easily removed from the AuNP surface.10 Thus,
aInstitut Charles Gerhardt Montpellier ICGM, UMR5253 CNRS-ENSCM-UM2-UM1,
Place E. Bataillon, CC1701 34095 Montpellier Cedex 05, France.
E-mail: sricheter@univ-montp2.fr; Fax: +33 4 6714 3852; Tel: +33 4 6714 3971
bUPMC Université Paris 06, UMR 7574, Laboratoire Chimie de la Matière Condensée
de Paris, Collège de France, 11 place M. Berthelot, 75231 Paris Cedex 05, France
cInstitut Européen des Membranes, UMR 5635, CNRS, UM2, ENSCM, CC047,
Place E. Bataillon, 34095 Montpellier Cedex 05, France
†Electronic supplementary information (ESI) available: Synthetic procedures,
characterization techniques and data, cif file of complex 3a and calculations
details. CCDC 965132. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt53579g
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Fig. 4 13C CPMAS NMR spectra of compounds 1a, 2a and 3a, and
crude AuNPs(A) before washing with acetonitrile.
observations made by Chechik and coworkers upon removal of
weakly coordinated ligands from AuNP surfaces.13 Compared
to the starting thioether-coated AuNPs, the obtained AuNPs(A)
are two times smaller, with a mean size of 2.8 ( 0.6) nm
(Fig. 2, bottom), and are more aggregated, as shown by trans-
mission electronic microscopy (TEM) and by the broad
SPR observed at λmax ∼ 552 nm. This evolution of the AuNP
morphology suggests that ligand exchange is not the only
process occurring at the AuNPs surface, and that the reaction
with NHCa also leads to etching.
Fig. 2 TEM images of di-n-dodecylsulfide-coated AuNPs (top) and
AuNPs(A) isolated after reaction with 1a + tBuONa (bottom).
Since the isolated crude AuNPs(A) were not fully soluble in
organic solvents, they were analyzed by 13C solid state NMR
(Fig. 4). On the 13C CPMAS (cross-polarization magic angle
spinning) NMR spectrum of AuNPs(A), no signal corres-
ponding to thioether was observed, confirming that it had
been totally removed from the surface of the AuNPs. The 13C
resonances are consistent with the presence of NHC–Au
species, but the signals are strikingly sharp and intense. This
is in contrast with the broad 13C NMR resonances usually
observed when monolayers of molecules are grafted on met-
allic NP surfaces,16 and strongly suggests that a significant
quantity of molecular species involving NHCs was generated,
which are not grafted on the AuNP surface. To try to identify
these species, Au(I) complexes 2a and 3a were synthesized fol-
lowing literature procedures17 and characterized by solid state
13C CPMAS NMR (see ESI†). As shown in Fig. 4, there is a good
match between the spectra of the crude AuNPs(A) and the bis-
carbene Au(I) complex 3a. This is particularly true for the
signal at δ ∼ 190 ppm, which can be unambiguously assigned
to the carbene C(2) atom, which is directly linked to Au(I).
Fig. 3 Different conditions tested for this study.
benzimidazol-2-ylidene ligands were expected to easily dis- Powder X-ray diffraction (XRD) analyses were then performed
on the crude AuNPs(A) and complex 3a, showing that their
XRD powder patterns are very similar (see ESI, Fig. S26†). As a
place the thioether molecules. The N,N-dimethylbenzimid-
azolium tetrafluoroborate salt 1a was used as an NHC precursor
(∼3.8 eq. vs. Au),15 and the corresponding carbene NHCa was matter of fact, the diffraction peaks corresponding to the bis-
carbene Au(I) complex are so intense in the diffractogram of
generated in situ by adding tBuONa leading to the immediate
AuNPs(A) that the signals coming from the remaining AuNP
formation of a precipitate of AuNPs(A) in the reaction medium
(Fig. 3, first reaction). This is in agreement with the cores are barely observable. Indeed, only a slight increase in
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precipitation of AuNPs(C) and (ii) the removal of thioether
ligands from the AuNPs surface.
Previous studies on NHC-coated AuNPs had identified bis-
carbene Au(I) complexes as decomposition compounds, and
their formation was considered as an indirect proof of the pres-
ence of NHCs at the surface of AuNPs.11,13 Our preliminary
results presented here tend to show that bis-carbene Au(I) com-
plexes may rather be formed in the course of the reaction
between the NHCs and the AuNP surface. Further experiments
were carried out in order to see whether these observations
could also be generalized to other AuNPs/NHC systems (see
Fig. 3 and ESI† for experimental details). First, the nature of
the NHC was varied by reacting the in situ generated imidazol-
2-ylidene NHCc (imidazolium salt 4 used as NHC precursor,
Fig. 1) with thioether-coated AuNPs (Fig. 3, third reaction, syn-
thesis of AuNPs(D)). It was found that the corresponding bis-
carbene Au(I) complex 5 could also be isolated from crude
AuNPs(D), meaning that our observations on NHCa cannot
be simply related to the specific reactivity of benzimidazol-
2-ylidenes (see ESI†). Second, the nature of the starting AuNPs
was varied: instead of thioether-coated AuNPs, n-dodecyl-
amine-coated AuNPs were directly reacted with the benzimid-
azolium salt 1a (Fig. 3, fourth reaction, synthesis of AuNPs(E)).
In this case, no additional base was added to generate NHCa,
since the amine could also play the role of a base. As pre-
viously, the bis-carbene Au(I) complex 3a was formed,
suggesting that the nature of the base is not of importance for
the formation of complex 3a. As a matter of fact, even when
LiBEt3H (a reducing agent also commonly used to generate
AuNPs from NHC–Au–X complexes) was used as the base, the
bis-carbene Au(I) complex 3a also formed (Fig. 3, fifth reaction,
synthesis of AuNPs(F)). Thus, all in all, the formation of bis-
carbene Au(I) complexes seems to be a common feature of the
reactivity of NHCs at AuNP surface.
For further identifying the species remaining at the surface
of the AuNPs, a series of washings were performed to remove
ungrafted molecular species, and 13C CPMAS solid state NMR
characterizations were then carried out. Unfortunately, despite
several attempts, washed AuNPs(A) proved to be highly
challenging to characterize by NMR, due to the small amount
of residual AuNPs, the small amount of ligand remaining at
their surface, and in some cases, their tendency to aggregate
under MAS (causing difficulties in tuning the probe and/or
spinning the sample). Based on the different observations
presented above, a second system was looked into to try to
prevent the aggregation of the grafted AuNPs in view of solid
state NMR characterizations. Thus, thioether-protected AuNPs
were reacted with N,N-dihexylbenzimidazol-2-ylidene ligands
NHCb (generated in situ from 1b) to afford AuNPs(B). These
were prepared following a similar synthetic strategy to that
described for AuNPs(A). The crude AuNPs(B) were character-
ized by powder XRD, showing again the quantitative formation
of crystalline molecular species. The 13C CPMAS NMR spec-
trum of the crude AuNPs(B) displayed sharp and well-resolved
signals belonging to two different phases (see ESI, Fig. S29†).
Based on a comparison with 13C CPMAS NMR spectra of Au(I)
Fig. 5 X-ray structure of the complex 3a isolated from AuNPs(A).
intensity between 36 and 40° in 2θ is observed, due to the
(111) Bragg reflection of the AuNPs. Thus, powder XRD and
13C solid state NMR analyses show that the AuNPs(A) sample
mainly contains a crystallized bis-carbene Au(I) complex
similar to 3a, and that the AuNPs initially observed by TEM in
Fig. 2 only represent a small fraction of the sample.18 This was
subsequently confirmed by scanning electronic microscopy
(SEM, see ESI, Fig. S24†).
By washing the crude AuNPs(A) with acetonitrile, the bis-
carbene Au(I) complex was isolated in 45% yield. Single-
crystals were grown from a CH3CN–Et2O mixture, and X-ray
diffraction analysis unambiguously established its structure
to be identical to 3a (Fig. 5, see ESI†). The C(2)–Au distance
of 2.004 Å and the C–Au–C angle of 180° were in agreement
with other bis-carbene Au(I) complexes reported in the
literature.17,19 The counter-anion was found to be BF4−, as
confirmed by 11B and 19F solid state NMR (see ESI,†
Fig. S30†), and to exhibit crystallographic disorder over some F
atoms.
One possible interpretation of the quantitative formation of
the bis-carbene Au(I) complex 3a would be that the initially
generated NHC–Au containing species are weakly bound to the
Au surface, and may be released from the surface of AuNPs.
This would be consistent with previous studies on PPh3-
capped AuNPs releasing (PPh3)AuCl complexes.20 The released
NHC–Au species would then further react with the remaining
NHC generated in situ (or with the corresponding enetetr-
amines NHCvNHC)21 to form bis-carbene Au(I) complexes.
Although the exact mechanism leading to the obtention of 3a
is unclear at the moment, this process could explain its
formation and the progressive erosion of the AuNP surface. It
should be noted that despite NHCa being in excess with
respect to Au in the reaction medium, no complete consump-
tion of the thioether-coated AuNPs was observed, even after
longer reaction times (up to 8 days). This could be due to the
quick initial precipitation of AuNPs(A), which are no longer
stabilized by thioether ligands, but by NHC ligands and/or
perhaps even by bis-carbene Au(I) complexes through aurophi-
lic interactions. Indeed, recent studies have shown that such
aurophilic interactions between Au(I) complexes and AuNPs
surfaces can occur.22 Furthermore, we noticed that the reac-
tion of thioether-coated AuNPs with 3a also leads to the direct
formation of a precipitate similar to AuNPs(A) (Fig. 3, second
reaction, synthesis of AuNPs(C)). Thus, we presume that auro-
philic interactions between 3a and AuNPs would explain (i) the
5980 | Dalton Trans., 2014, 43, 5978–5982
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computational models shed light on possible modes of graft-
ing, which is useful for understanding the reactivity of AuNPs.
Further computational studies of such systems will be reported
in a forthcoming paper.
In summary, the reactivity of AuNPs towards NHC ligands
was investigated in detail, showing (i) the formation of bis-
carbene Au(I) complexes, and (ii) the presence of residual NHC
ligands at the AuNPs surface. The use of several characteriz-
ation techniques in the solid state (including 13C solid state
NMR and powder XRD) was found to be key to ensure a proper
interpretation of the reactivity of AuNPs. As an extension of
this work, we are now exploring, in a similar way, the reactivity
of NHC towards other metallic NPs.
Fig. 6 (a) 13C CPMAS NMR spectrum of the washed AuNPs(B). (b) DFT
model of NHCa chemisorbed onto the Au(111) surface.
complexes 2b17 and 3c,19 which were synthesized following
This work was supported by the Université Montpellier 2 and
previously described procedures, one set of signals was un- the CNRS. We are grateful to the Labex “CheMISysT” program
ambiguously ascribed to the corresponding bis-carbene Au(I)
for financial support of Dr M. R.-C. The calculations were per-
complex (resonance of the C(2) at δ = 190 ppm), and its pres- formed using HPC resources from GENCI-[CCRT/CINES/IDRIS]
ence in the crude AuNPs(B) was further confirmed by ESI-TOF (grant 2013-[x2013082022]) and the CCRE of Université Pierre et
mass spectrometry (signal at m/z = 769.45). The counter-anion
Marie Curie. We are also grateful to Dr David J. Watkin and
–
was identified as BF4 by 11B solid state NMR, and shows that Richard Cooper (University of Oxford) for fruitful discussions.
it is 3b which has formed in the course of the reaction
between thioether-coated AuNPs and NHCb. The other set of
signals was ascribed to the remaining tetra-n-octylammonium
Notes and references
cations.23
By washing the crude AuNPs(B) several times with EtOH, all
residual crystalline phases were removed, as shown by powder
XRD (see ESI, Fig. S27†). The remaining AuNPs were then
successfully characterized by 13C CPMAS NMR spectroscopy
(Fig. 6a). Broad 13C NMR resonances were observed, as
expected for organic ligands grafted at the surface of nano-
particles, and the chemical shifts are consistent with NHCb–Au
species. Two main resonances are present in the carbene C(2)
region (centered at δ ∼ 181 and 170 ppm), which may suggest
that there is a heterogeneity in NHCb local environments at
the AuNPs surface. However, further experiments would
need to be performed to confirm this point, using NHC
ligands purposely 13C-labeled on C(2), because it has been
shown that significant shifts of NMR resonances generally
occur upon grafting onto metallic nanoparticles.20
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5982 | Dalton Trans., 2014, 43, 5978–5982
This journal is © The Royal Society of Chemistry 2014