Enhancement of gold catalytic activity and stability by immobilization on the surface of graphene

Article abstract of DOI:10.1016/j.jcat.2017.06.021

The catalytic performance of gold complexes is evaluated at the molecular level and when supported onto reduced graphene oxide (rGO). Gold complexes of general formula [(NHC)AuX] catalyse the synthesis of indoles via intramolecular hydroamination reaction of alkynes. The catalytic properties of the molecular gold complexes are highly improved when supported onto graphene. Faster reaction rates and higher catalyst stability are observed for the immobilized gold complexes. The use of graphene as support of molecular complexes has a positive benefit in the catalytic gold properties in terms of activity and stability.

Full text of DOI:10.1016/j.jcat.2017.06.021

Journal of Catalysis 352 (2017) 498–504
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhancement of gold catalytic activity and stability by immobilization
on the surface of graphene
⇑
David Ventura-Espinosa, Sara Sabater, Jose A. Mata
Institute of Advanced Materials (INAM), Universitat Jaume I, Avda. Sos Baynat s/n, 12006 Castellón, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic performance of gold complexes is evaluated at the molecular level and when supported
onto reduced graphene oxide (rGO). Gold complexes of general formula [(NHC)AuX] catalyse the synthe-
sis of indoles via intramolecular hydroamination reaction of alkynes. The catalytic properties of the
molecular gold complexes are highly improved when supported onto graphene. Faster reaction rates
and higher catalyst stability are observed for the immobilized gold complexes. The use of graphene as
support of molecular complexes has a positive benefit in the catalytic gold properties in terms of activity
and stability.
Received 25 May 2017
Revised 13 June 2017
Accepted 14 June 2017
Keywords:
Gold
Supported catalysis
Graphene
Ó 2017 Elsevier Inc. All rights reserved.
Hydroamination
Cyclisation
1. Introduction
widened as they can be recovered and reused for several catalytic
cycles. In addition, separation of the solid catalyst from reaction
The use of gold as catalyst has acquired a great impact over the
last years [1–3]. Although initially considered as an inert noble
metal, gold has proved his potential application in catalysis [4].
The gold catalytic activity is very high in the form of solid catalysts
[5–7], nanoparticles [8–11], clusters [12], or molecular complexes
[13–19]. Despite the advances of gold in homogeneous catalysis,
one of the main drawbacks that still need to be tackled is stability.
In particular, stability of gold complexes towards decomposition
under common catalytic conditions. Stability issues of gold com-
plexes should address to a rational design of more active and
robust catalysts that inhibit regular decomposition pathways
[20–23]. At the molecular level, decomposition of gold complexes
comes from the nature and number of intermediates involved in
a catalytic cycle and its inherent facile reduction to form inactive
gold metallic mirrors. In order to improve catalytic efficiency and
activity it is important to develop systems in which deactivation
or reduction pathways are minimised or avoided. An alternative
to circumvent this problem consists of the immobilization of
organometallic complexes onto solid surfaces capable of enhancing
their stability [24–28]. Immobilization of molecular complexes
combines the advantages of homogeneous and heterogeneous
catalysis of selectivity and robustness [29–32]. Catalytic applica-
tions of well-defined molecular complexes onto supports are
products is crucial for many applications. The use of solid supports
for immobilization of molecular complexes allows the controlled
incorporation of catalytic active sites. Structural characterization
of the active sites facilitates the understanding of structure-
reactivity relationships that govern the catalytic performance and
stability.
In this work, we aim to study different factors that may play a
role over the control of stability of gold complexes during catalytic
reactions. In this context, we have developed a methodology that
allows the immobilization of well-defined gold complexes onto
the surface of graphene. The structure of the hybrid material is
composed of a regular distribution of active sites completely char-
acterized. Here, we show that the rationale ligand design and
immobilization of organometallic complexes on the surface of gra-
phene enhance the gold catalytic properties.
2. Results and discussion
The gold/graphene hybrid materials were prepared using a gen-
eral procedure previously described in our laboratory that allows
the controlled introduction of molecular complexes onto the sur-
face of graphene [33]. The method consists in the direct immobi-
lization of gold complexes onto the surface of reduced graphene
oxide (rGO) by
p -stacking interactions [34–36]. These types of
⇑
interactions are important in the case of polyaromatic hydrocar-
bons (PAHs) including graphene and related materials [37]. We
Corresponding author.
E-mail address: jmata@uji.es (J.A. Mata).
http://dx.doi.org/10.1016/j.jcat.2017.06.021
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

D. Ventura-Espinosa et al. / Journal of Catalysis 352 (2017) 498–504
499
have used a pyrene tag for the preparation of two imidazolium
salts that serve as NHC ligand precursors and allow the direct
immobilization of the organometallic gold complexes onto the sur-
face of graphene (Scheme 1). The imidazolium salts 1 and 2 con-
taining the pyrene tag were prepared in good yields by alkylation
of methyl (or mesityl) imidazole with 1-bromomethylpyrene.
Deprotonation of the imidazolium salts using a transmetalation
procedure or an external base in the presence of AuCl(SMe2) allows
the formation of the well-defined organometallic gold complexes 3
and 4. The gold complexes were fully characterized by NMR spec-
troscopy, ESI-MS spectrometry and elemental analysis. The ESI-MS
analysis requires the addition of an external sodium or potassium
salt to detect the molecular peak. For instance, in the case of com-
plex 3, the base peak corresponds to an adduct with potassium at
567.03 m/z. The mass/charge relation and the isotopic pattern con-
firm the molecular composition. Single crystal X-ray diffraction
completes the characterization of 3 and 4 (Figs. 1 and 2). The Au-
Ccarbene distance is 1.974(4) Å for 3 and 2.000(3) Å for 4 and both
lye in the expected range for gold NHC complexes. The packing dia-
hydroamination of alkynes as model reaction [39–42]. The cat-
alytic formation of indole derivatives is an elegant approach for
the synthesis of sophisticated organic molecules. The reaction
mechanism is well established and requires the addition of AgBF4
to activate the gold precatalyst. The gold catalysts (3–6) are active
in the cyclisation reactions (Table 1) at low temperatures. Initial
control experiments show that Au is required in the reaction to
proceed, AgBF4 or rGO material is not active. (Data not shown
Table 1, see SI for details). Results show that the catalytic perfor-
mance of 3–6 is particularly different. For instance, the perfor-
mance of catalyst 3 is very low, and only 20% yield is obtained
after 6 h reaction (Entry 1). However, under the same reaction con-
ditions, 4 and 5 afforded 91% and 82% yield respectively (Entries 3
and 8). We observed that the overall turnover was even better in
the case of catalyst 6, which afforded quantitative yields in 1.5 h
with a catalyst loading of 0.08 mol% (Entry 9). These results are
remarkably considering that standard conditions on gold-
catalysed intramolecular hydroamination of alkynes require cata-
lyst loadings in the range of 1–2 mol% [43–46]. The reaction affords
quantitative yields when using acetonitrile or toluene as solvents,
but only 15% yield is obtained when using thf (Entry 6). We have
not observed differences on mass balances in the presence of the
support when monitoring the reactions by gas chromatography.
gram of complex 3 shows an intermolecular p-stacking interaction
between the pyrene groups (Fig. 1). The interplanar distance is
3.5 Å, indicating an interaction between the polyaromatic groups.
The same situation is observed in the case of the gold complex 4.
The packing diagram shows a
pyrene groups with an interplanar distance of 3.6 Å (Fig. 2). We
have previously observed the formation of interactions in pal-
ladium [33] and ruthenium [38] NHC complexes functionalized
with pyrene tags. The formation of these interactions gives
p
-stacking interaction between the
This result suggests that the p-stacking interactions of substrates
with the support are weak and therefore they are not adsorbed.
The use of catalyst 6 allows the reduction in reaction time to min-
utes at a low catalyst loading of 0.56 mol% (Entry 10). These results
show that better outcomes are obtained when using catalyst 6 in
toluene at low temperature.
p-p
p-p
an indication of the tendency of the pyrene tag to interact with
PAHs such as graphene.
A hot filtration experiment was carried out to establish the
heterogeneous nature of catalyst 6 during the intramolecular
hydroamination. We performed the experiment under the reaction
conditions described in entry 9 (Table 1), but after 45 min. (GC
yield 53%), the catalytic hybrid material 6 was separated from
the solution by cannula-filtration at the working temperature
(50 °C). The filtrate was stirred under the same conditions for 5 h
at 50 °C. After this time, GC analysis reveals that there is no more
indole formation. This result suggests that there are not catalytic
active species in solution. On the other hand, the isolated catalyst
6 was treated with toluene and substrate. After 2 h reaction, we
observed the formation of indole in 97% yield. The results suggest
the heterogeneous nature of catalyst 6. The hot filtration experi-
ment confirmed the absence of leached species in solution due to
desorption from the graphene surface.
The catalytic performance evaluation of 3–6 was compared by
monitoring the cyclisation reaction during time (Fig. 4). The cat-
alytic rate of the molecular gold complexes 3 and 4 is remarkably
different (Fig. 4a). Under the same reaction conditions and catalyst
loading, complex 3 affords only 20% yield but complex 4 affords
100% yield. As observed in the monitoring of the reaction, catalyst
3 is completely deactivated after 300 min. The results show that
Gold complexes 3 and 4 are immobilized on rGO at room tem-
perature using ultrasounds (US). The exact amount of gold com-
plex anchored on the surface of graphene was analysed by ICP-
MS. The results accounted for a 3.9 wt% of complex 3 in the hybrid
material 5 and 0.9 wt% of complex 4 in the hybrid material 6. The
hybrid materials were characterized by UV/Vis, FTIR and HRTEM.
Analysis by X-ray photoelectron spectroscopy (XPS) provides evi-
dence of the structure of the molecular complexes on the surface
of graphene. Fig. 3 shows a comparative XPS analysis of the molec-
ular complex 4 and the hybrid material 6. Complex 4 shows three
characteristic peaks corresponding to the core-levels of N1s, Au4f
and Br3d. These peaks are also observed in the XPS of the hybrid
material 6 at the same binding energy. XPS analysis confirms the
presence of the molecular complex 4 on the surface of the reduced
graphene oxide and that the structure of complex 4 is maintained
after immobilization.
2.1. Catalytic properties
We tested the catalytic properties of the molecular gold com-
plexes and the hybrid materials using the intramolecular
Scheme 1. Synthesis of gold complexes and hybrid materials.

500
D. Ventura-Espinosa et al. / Journal of Catalysis 352 (2017) 498–504
3.5 Å
N
Au
Cl
Fig. 1. Crystal packing diagram of compound 3. Ellipsoids are at 50% probability level. Hydrogen atoms have been omitted for clarity. The p-stacking interplanar distance
between the pyrene-tags is 3.5 Å.
Br
Au
3.6 Å
N
Fig. 2. Crystal packing diagram of compound 4. Ellipsoids are at 50% probability level. Hydrogen atoms have been omitted for clarity. The p-stacking interplanar distance
between the pyrene-tags is 3.6 Å.
Fig. 3. Comparative XPS analysis of molecular complex 4 (top) and hybrid material 6 (down) for the core-level peaks of N1s, Au4f and Br3d.
the introduction of an aromatic ring at the nitrogen position of the
NHC ligand affords a higher stability and catalyst performance.
This situation is also observed in the case of the supported cata-
lysts 5 and 6 (Fig. 4b). Catalyst 6, with a mesityl group, affords
quantitative yield after 100 min. at a catalyst loading of only
0.1 mol%. In the case of catalyst 5, the reaction stops after
400 min. reaching an 80% yield but using a tenfold catalyst loading.
These results show that catalyst stability increases by ligand
design. Introduction of aromatic rings at the nitrogen positions of
the NHC ligand increases catalyst stability and activity. More inter-

D. Ventura-Espinosa et al. / Journal of Catalysis 352 (2017) 498–504
501
Table 1
Optimization parameters for the intramolecular hydroamination of alkynes
.
Entry
Cat.
[Au] (mol%)
R
Solv.
t (h)
Yield(%)a
1
2
3
3
4
4
4
4
4
4
5
6
6
1
1
1
2
2
2
2
1
Ph
nBu
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Toluene
‘‘
‘‘
‘‘
‘‘
THF
MeCN
Toluene
Toluene
Toluene
6
24
6
1
1
6
1
6
1.5
0.2
20
71
91
100
100
15
100
82
4
5b
6
7
8
9c
10
0.08
0.56
100
100
a
Reactions carried out at a substrate concentration of 0.0625M. Yields determined by GC analysis using anisole as the internal standard. Three equivalents of AgBF4 respect
to gold catalyst.
b
Reaction carried out under N2 atmosphere.
A hot filtration experiment was carried out under these conditions.
c
a)
b)
100
100
80
60
40
20
0
80
60
Cat. (3) [1 mol%]
Cat. (4) [1 mol%]
40
20
0
Cat. (5) [1 mol%]
Cat. (6) [0.1 mol%]
0
200
400
600
0
200
400
600
t (min)
c)
d)
100
80
60
40
20
0
100
80
60
40
20
0
Cat. (3) [1 mol%]
Cat. (5) [1 mol%]
Cat. (4) [0.5 mol%]
Cat. (6) [0.5 mol%]
0
200
400
600
0
200
400
600
t (min)
t (min)
Fig. 4. Reaction monitoring of the intramolecular hydroamination of alkynes using different catalysts.
esting is the effect observed when the molecular gold complexes 3
rates (activity) and product formation (Fig. 4c). This effect is even
more pronounced in the case of the molecular complex 4 and its
corresponding hybrid material 6 (Fig. 4d). In the case of catalyst
6, quantitative yield is obtained in 12 min. but only 80% is obtained
after 600 min when using the molecular catalyst 4. The reaction
profile for 4 and 6 shows a huge difference in catalyst performance.
Most often, reaction rates are negatively affected by immobiliza-
tion of the molecular complexes due to diffusion problems. This
is especially important when using porous supports. In our case,
we have observed the reversed situation. The reaction rates
increase after immobilization of the molecular complexes on the
and 4 are anchored on rGO. The reaction rates in the case of 3 are
slow, indicating a limited catalytic performance (Fig. 4c). For
instance, the yield after 400 min. is only 19%. After this time, the
reaction practically stops and there is no more indole formation.
The molecular complex is completely deactivated suggesting a lim-
ited catalyst stability. When the molecular complex 3 is supported
onto graphene forming the hybrid material 5, the catalytic proper-
ties are improved. The hybrid material 5 affords 85% yield instead
of 19% after 400 min. Immobilization of the molecular complex 3
on the surface of graphene has a positive effect in terms of reaction

502
D. Ventura-Espinosa et al. / Journal of Catalysis 352 (2017) 498–504
surface of graphene, indicating that there are not diffusion prob-
lems. Although at this point, we do not have a plausible explana-
tion for the increase in reaction rates, the process is general, and
it has been observed for other catalytic systems using graphene
[47,48]. Immobilization onto reduced graphene oxide has a posi-
tive effect on the catalytic properties of gold molecular complexes
by increasing stability and activity.
molecular complex anchored in the surface of graphene is stable
enough to achieve quantitative yields.
The catalytic hybrid material 6 was further analysed by HRTEM
after the recycling experiments. Microscopy evaluation allows
exploring any possible changes in the graphene morphology and/
or the possibility of gold nanoparticles formation (Fig. 6). A com-
parison of the HRTEM images of catalyst 6 before and after the
recycling experiments shows the same morphology for the gra-
phene hybrid material. The only difference is the presence of more
wrinkles, because of the mechanical stirring during the catalytic
reactions. There are not gold metal nanoparticles and the single
layer of graphene is maintained.
2.2. Recycling studies
The stability of the hybrid catalyst (6) was further analysed by
recyclability studies. Recycling experiments were carried out using
the standard conditions described in Fig. 5 at a 0.5 mol% catalyst
loading. The reaction was monitored by gas chromatography
(GC) using anisole as internal standard. After each run, the mixture
was allowed to reach room temperature and the solid catalyst was
separated by decantation, washed with toluene and used in the
next run (Fig. 5). The results show that the hybrid catalyst 6 was
reused without any significant loss of activity for four consecutive
runs. In runs 5 and 6, the catalytic performance decreases and
longer reaction times are required to achieve quantitative yields.
In order to evaluate the deactivation pathways, the gold content
in the solution was analysed by ICP-MS after each run. The results
show that the gold content in the solution corresponding to runs
1–3 is negligible (less than 2 wt%). In contrast, the gold content
after run 6 corresponds to a 50 wt% loss of the initial gold content
of the material. These results suggest that deactivation of the
hybrid material 6 is caused by both, leaching and complex degra-
dation. Despite the low amount of gold content after six runs, the
3. Conclusions
In this manuscript, we have explored the factors that favour the
catalytic performance and stability of gold complexes anchored on
the surface of graphene. Gold complexes containing an N-
heterocyclic carbene ligand functionalized with a pyrene group
are straightforward immobilized on the surface of reduced gra-
phene oxide by
p–p interactions. This methodology grants the
immobilization of well-defined gold molecular complexes onto a
graphene surface and allows the direct comparison in activity of
the molecular and the supported gold complexes. The gold com-
plexes and the corresponding hybrid materials are active catalysts
in the intramolecular hydroamination of alkynes. At the molecular
level, ligand modification increases rates and stability by introduc-
ing an aromatic ring at the nitrogen position of the NHC ligand. We
have observed that the catalytic performance is improved in the
hybrid materials compared to the molecular complexes. Normally,
immobilization of molecular complexes onto supports has a nega-
tive effect in activity due to diffusion problems. The hybrid cat-
alytic materials based on graphene show higher reaction rates
than the corresponding molecular complexes. The results suggest
that there are not diffusion problems when using graphene as sup-
port. In addition to catalyst performance, we have observed that
the hybrid materials are active during more time affording quanti-
tative yields. The use of graphene as support enhances the catalyst
stability most probably by inhibiting decomposition pathways. We
have shown that immobilization of molecular gold complexes on
the surface of graphene improves catalytic performance and stabil-
ity. Future research to establish the role of graphene is ongoing in
our laboratory.
100
80
100
60
40
20
0
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
80
60
40
20
0
Run 1
Run 2
Run 3
0
10
300
20
4. Experimental section
0
100
200
400
500
4.1. Experimental details
t (min)
Anhydrous solvents were dried using a solvent purification sys-
tem (SPS M BRAUN). The imidazolium salt 1 and the substrates
used for catalysis were obtained according to reported procedures
Fig. 5. Recycling experiments on the intramolecular hydroamination reaction using
the hybrid material catalyst (6). Reaction profile for each run determined by GC
analysis.
Fig. 6. HRTEM images of 6 before (a) and after (b) six catalytic runs.

D. Ventura-Espinosa et al. / Journal of Catalysis 352 (2017) 498–504
503
[33,49]. Nuclear magnetic resonance (NMR) spectra were recorded
on Bruker spectrometers operating at 300 or 400 MHz (1H NMR)
and 75 or 100 MHz (13C{1H} NMR), respectively, and referenced
to SiMe4 (d in ppm and J in Hertz). NMR spectra were recorded
at room temperature with the appropriate deuterated solvent.
High-resolution images of transmission electron microscopy
HRTEM and high-angle annular dark-field HAADF-STEM images
of the samples were obtained using a Jem-2100 LaB6 (JEOL) trans-
mission electron microscope coupled with an INCA Energy TEM
200 (Oxford) energy dispersive X-ray spectrometer (EDX) operat-
ing at 200 kV. Samples were prepared by drying a droplet of a
MeOH dispersion on a carbon-coated copper grid. X-ray photoelec-
tron spectra (XPS) were acquired on a Kratos AXIS ultra DLD spec-
washed with dichloromethane (10 mL). The solvent was reduced
to approximately 2 mL and pentane was added, affording a white
solid, which was filtered and dried in vacuum. Yield: 90 mg (66%).
1H NMR (300 MHz, CDCl3)
d 8.35 (d, J = 9.2 Hz, 1H, CHpyr),
8.30–7.98 (m, 8H, CHpyr), 6.97 (s, 2H, CHmes), 6.80 (d, J = 2.0 Hz,
1H, CHimid), 6.73 (d, J = 2.0 Hz, 1H, CHimid), 2.34 (s, 3H, CH3,mes),
2.06 (s, 6H, CH3,mes). 13C NMR (75 MHz, CDCl3)
d 176.10
(Ccarbene-Au), [139.88, 134.92, 134.80, 132.37, 131.33, 130.74,
129.61, 129.52, 129.23, 128.53, 128.02, 127.38, 127.30, 126.60,
126.16, 126.11, 125.35, 125.06, 124.65, 122.52, 122.31, 120.25]
(CPyr, CHimid, Cmes), 53.67 (N-CH2), 21.27 (CH3,mes),18.00 (CH3,mes).
Anal. Calcd for C29H24N2AuBrÁ0.5C5H12: C, 52.98; H, 4.20; N, 3.92.
Found: C, 53.27; H, 3.76; N, 4.15. The calculated elemental analysis
fits with the experimental with the addition of half a molecule
of n-pentane that is also observed in the 1H NMR of complex 4
(Section S3.3). Electrospray MS (Cone 20 V) (m/z, fragment):
763.03 [M-Br + I + K]+. HRMS ESI-TOF-MS (positive mode):
trometer with a monochromatic Al K
a X-ray source (1486.6 eV)
using a pass energy of 20 eV. To provide a precise energy calibra-
tion, the XPS binding energies were referenced to the C1s peak at
284.6 eV.
[M-Br + I + K]+ monoisotopic peak 763.0305; calc. 763.0287, er
:
4.2. Synthesis of 2
2.4 ppm.
To a solution of 1-(Bromomethyl)pyrene (750 mg, 2.75 mmol)
in dry THF (8 mL) was added 1-mesityl imidazole (0.465 mg,
2.5 mmol). The mixture was refluxed under nitrogen for 16 h.
The resulting suspension was filtered off and washed with diethyl
ether (2 Â 10 mL). The imidazolium salt was obtained as a white
solid. Yield: 1.1 g (90%). 1H NMR (400 MHz, CD3OD) d 9.36 (s, 1H,
NCHN), 8.48–7.97 (m, 9H, CHpyr), 7.89 (s, 1H, CHimid), 7.69 (s, 1H,
CHimid), 7.06 (s, 2H, CHmes), 6.34 (s, 2H, CH2, pyr), 2.32 (s, 3H, CH3,
mes), 2.00 (s, 6H, CH3, mes). 13C{1H} NMR (101 MHz, CD3OD) d
142.5 (NCHN), [138.8, 135.7, 133.9, 132.6, 131.9, 130.6, 130.5,
129.7, 129.4, 128.3, 127.7, 127.3, 127.1, 126.9, 126.4, 126.2,
125.8, 125.5, 124.6, 122.5] (Cpyr, CHimid, Cmes) 52.8 (N-CH2), 21.1
(CH3,mes), 17.3 (CH3,mes). Anal. Calcd. for C29H25N2Br: C, 72.34; H,
5.23; N, 5.81. Found: C, 71.89; H, 5.51; N, 6.09. Electrospray MS
(Cone 20 V) (m/z, fragment): 401.3 [M]+.
4.5. Synthesis of 5 and 6
A suspension of 90 mg of rGO in 10 mL of CH2Cl2 was immersed
in an ultrasounds bath for 30 min. Then, 10 mg of 3 or 4 was added
to the mixture. The suspension was stirred at room temperature
for 10 h. The black solid was isolated by filtration and washed with
2 Â 15 mL of CH2Cl2 affording the hybrid material as a black solid.
The exact amount of supported complex was determined by
ICP-MS analysis. The results accounted for a 3.9 wt% of complex
3 in the hybrid material 5 and 0.9 wt% of complex 4 in the hybrid
material 6. The hybrid materials were characterized by UV/Vis,
FTIR, XPS and HRTEM (see Supporting Information for details).
4.6. General procedure for the catalytic experiments
Catalytic experiments were performed under air in a Pyrex
tube, using 0.3 mmol of substrate, 2.4 mL of solvent, catalyst
(0.1–2.0 mol%) and AgBF4 (6 mol%), and heating at 50 °C. Yields
and conversions were determined by GC analysis using anisole as
internal standard. Isolated yields were determined by 1H NMR
spectroscopy using 1,3,5-trimethoxybenzene as the external
standard.
4.3. Synthesis of 3
In a round-bottom flask were mixed, under exclusion of light,
the imidazolium salt 1 (128.3 mg, 0.339 mmol) and Ag2O (78 mg,
0.339 mmol) in 10 mL of acetonitrile. The suspension was heated
at reflux during 5 h. After cooling, [AuCl(SMe2)] (100 mg,
0.339 mmol) and KCl (243 mg, 3.25 mmol) were added and the
reaction mixture was stirred at room temperature for 15 h. The
insolubles were filtered off through a pad of kieselguhr. The filtrate
was concentrated under reduced pressure yielding a white precip-
itate. Recrystallization using dichloromethane/hexane afforded an
analytically pure white solid. Yield: 145 mg (81%). 1H NMR
(300 MHz, CDCl3) d 8.20–7.91 (m, 8H, CHpyr), 7.79 (d, J = 7.8 Hz,
1H, CHpyr), 6.71 (d, J = 1.9 Hz, 1H, CHimid), 6.54 (d, J = 1.9 Hz, 1H,
CHimid), 5.84 (s, 2H, CH2), 3.78 (s, 3H, NCH3). 13C NMR (75 MHz,
CDCl3): d 171.3 (Ccarbene-Au), [131.8, 131.0, 130.4, 128.9, 128.1,
127.5, 127.1, 126.9, 126.3, 125.8, 125.7, 124.8, 124.3] (CPyr, CHimid),
52.9 (CH2), 38.3 (NCH3). Anal. Calcd. for C21H16N2AuCl
(528.78 g/mol): C, 47.70; H, 3.05; N, 5.30. Found: C, 47.87; H,
3.27; N, 5.39. Electrospray MS. (Cone 20V) (m/z, fragment): 567.2
[M+K]+. HRMS ESI-TOF-MS (positive mode): [M+K]+ monoisotopic
Acknowledgements
The authors thank the financial support from MINECO
(CTQ2015-69153-C2-2-R), Generalitat Valenciana (AICO/2015/039)
and Universitat Jaume I (P1.1B2015-09). D. V-E thanks MINECO for
a FPU grant (FPU15/03011). The authors are very grateful to the
‘Serveis Centrals d’Instrumentació Científica (SCIC)’ of the
Universitat Jaume I. We thank S. Martin for XPS analysis.
Appendix A. Supplementary materials
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2017.06.021.
peak 567.0309; calc. 567.0305, er: 0.7 ppm.
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4.4. Synthesis of 4
In a Pyrex tube, imidazolium salt (2) (100 mg, 0.2 mmol), [AuCl
(SMe2)] (60.7 mg, 0.206 mmol), K2CO3 (28.5 mg, 0.206 mmol) and
2.4 mL of acetone were heated at 60 °C for 3 h. Then, the solvent
was removed under vacuum, dichloromethane was added (10 mL)
and the mixture was filtered through silica. The pad of silica was
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