Reaction and Mechanistic Studies of Heterogeneous Hydroamination over Support-Stabilized Gold Nanoparticles

Article abstract of DOI:10.1002/cctc.201600762

Highly stable gold nanoparticles (GNPs) around 5–6 nm have been prepared by in situ reduction of chloroauric acid on the surface of nitrogen-rich mesoporous carbon (MCN) without adding any extra stabilizing agent. The synthesized materials have been effic

Full text of DOI:10.1002/cctc.201600762

DOI: 10.1002/cctc.201600762
Full Papers
Reaction and Mechanistic Studies of Heterogeneous
Hydroamination over Support-Stabilized Gold
Nanoparticles
Manideepa Sengupta,[a] Arijit Bag,[b] Subhasis Das,[a] Astha Shukla,[a] L. N.
Sivakumar Konathala,[a] C. A. Naidu,[a] and Ankur Bordoloi*[a]
Highly stable gold nanoparticles (GNPs) around 5–6 nm have
been prepared by in situ reduction of chloroauric acid on the
surface of nitrogen-rich mesoporous carbon (MCN) without
adding any extra stabilizing agent. The synthesized materials
have been efficiently utilized as a catalyst for the truly hetero-
geneous hydroamination of phenylacetylene with aniline.
Large turnover numbers (42ꢀ106) were achieved by suitably
adjusting the gold/support (w/w) ratio, time, temperature, and
solvent, leading to 98% selectivity towards the Markovnikov
product. Density functional theory (DFT) studies have been
performed to predict the mechanistic pathway of hydroamina-
tion with Au0 in GNP@MCN. To understand the structure–activi-
ty relationship, the catalyst was characterized by using differ-
ent techniques such as X-ray diffraction (XRD), scanning elec-
tron microscopy (SEM), transmission electron microscopy
(TEM), nitrogen physisorption studies (BET), X-ray photoelec-
tron spectroscopy (XPS), and Fourier transform infrared (FTIR)
spectroscopy.
1. Introduction
Catalysis with gold nanoparticles (GNPs) is a subject of sub-
stantial current interest and use of GNP-based catalysts has
been widely explored in recent years.[1] However, gold was tra-
ditionally viewed as catalytically inactive for a long time;[2] this
changed after Bond et al.,[3] in 1973, reported gold was active
for olefin hydrogenation if dispersed as small nanoparticles.
More than a decade later, another milestone in heterogeneous
catalysis was established when Haruta et al.[4] demonstrated
the low-temperature oxidation of carbon monoxide and
Hutchings[5] showed the extraordinary catalytic activity of gold
for the hydrochlorination of ethyne to vinyl chloride. Since
then, interest in gold catalysis has grown exponentially and
over the past few years gold has emerged as a source of effec-
tive homogeneous[6] and heterogeneous[7] catalysts. Although
the bulk metal is largely non-active in catalysis,[8] GNPs, howev-
er, exhibit high catalytic activity, high reaction selectivity, and
also provide low reaction temperatures, which depend mainly
on the particle size and the support used.[2,9]
compounds such as enamines, imines, and alkylated amines,
which are widely present as chemical intermediates in the field
of natural products, pharmacological agents, polymers, dyes,
agrochemicals, cosmetics, and fine chemicals.[11] Both the Mar-
kovnikov and anti-Markovnikov addition products provide bio-
logical and pharmaceutical activity. Even if hydroamination
merges thermodynamic feasibility along with 100% atom
economy owing to no formation of any byproducts such as
water and salts, the reaction still has a considerable kinetic
barrier and hence the use of a catalyst is the desired option.
The nucleophilic addition of amines across CÀC multiple
bonds is kinetically difficult owing to the repulsion between
the high electron density of the amine and the p-electrons of
the multiple bonds.[12] Although considerable progress has
been made by using metals and metal complex catalysts
under homogeneous conditions, nevertheless, only a few het-
erogeneous catalysts[13] are reported in the literature. Recently,
Rodionov et al.[14] reported the hydroamination of alkynes with
anilines by using Au@SiO2 as a catalyst. Zhu et al.[15] reported
photolytic hydroamination over Au/TiO2, however, the turnover
numbers were found to be quite low.
Hydroamination, the addition of NÀH bonds across CÀC
multiple bonds[10] is of considerable interest in organic chemis-
try as the reaction enables the synthesis of organo-nitrogen
Herein, gold nanoparticles have been incorporated on nitro-
gen-rich mesoporous carbon (GNP@MCN), which acts as a sta-
bilizing and size-controlling agent and thus gold nanoparticles
of particle size 5–6 nm have been synthesized without the ad-
dition of an external stabilizing agent. The inner surface of the
mesoporous walls of MCN provides nitrogen functionalities,
which stabilize GNPs[16] (Scheme 1) and with this synthesized
catalyst, GNP@MCN, the hydroamination reaction of phenyla-
cetylene with aniline was performed. Moreover, a computation-
al approach has been used to understand the reaction path-
[a] M. Sengupta, S. Das, A. Shukla, L. N. S. Konathala, C. A. Naidu,
Dr. A. Bordoloi
Refinery Technology Division
CSIR-Indian Institute of Petroleum
Dehradun-248005 Uttarakhand (India)
E-mail: ankurb@iip.res.in
[b] Dr. A. Bag
Chemical Science Division
IISER Kolkata
Mohanpur, Nadia, West Bengal (India)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600762.
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Scheme 1. Incorporation of gold nanoparticles over nitrogen-rich meso-
Figure 1. Low- and wide-angle powder XRD patterns of MCN and
porous carbon (GNP@MCN).
GNP@MCN.
ways of selective heterogeneous hydroamination over an Au0
catalyst system for the first time.
indexed to the (002) plane of graphitic carbon.[17] The peaks
recorded from wide-angle powder XRD of both the fresh and
spent catalysts were also the same, which implies that there
was no alteration of the morphology of the catalyst and hence
it was stable and active for hydroamination.
The catalyst system (GNP@MCN) exhibits high reactivity and
over 98% selectivity towards the Markovnikov addition
product, phenyl-(1-phenylethylidine) amine A (Scheme 2).
To understand the morphology and structure, scanning elec-
tron microscopy (SEM) images of the catalyst GNP@MCN were
taken. Energy-dispersive X-ray spectroscopy (EDX) was used in
connection with SEM for the elemental analysis. White dots
corresponding to gold nanoparticles are distributed uniformly
over the mesoporous nitrogen-rich carbon; no agglomeration
of the gold nanoparticles was seen, which also confirms that
the size of the gold nanoparticles was controlled and stabilized
by the ÀNH and ÀNH2 units of MCN (Figure 2e–f). SEM images
for the template SBA-15 (Figure 2a–b) and support MCN (Fig-
ure 2c–d) were also taken, which reveal that the typical rod-
like morphology of SBA-15 was well replicated in the support
MCN and also in the catalyst GNP@MCN, even after encapsula-
tion of gold nanoparticles within the nanochannels of MCN.
Thus, the successful replication and formation of ordered mes-
ostructures were also confirmed by SEM. The presence of
peaks only for C, N, and Au and the absence of peaks for Cl
and Na in EDX spectroscopy (Figure S1, in the Supporting In-
formation) signify the purity of the catalyst.
2. Results and Discussion
2.1. Characterization of catalyst
2.1.1. Structure and morphology of GNP@MCN
The low-angle XRD patterns for both MCN and GNP@MCN ex-
hibit a sharp peak within the range 0.5–68 (indexed as the
(100) reflection of the hexagonal p6mm space group), indicat-
ing that the hexagonal ordered porous structure of MCN re-
mains intact even after incorporation of gold nanoparticles.
The wide-angle XRD pattern of GNP-loaded MCN displays five
peaks, which could be indexed as the (111), (200), (220),
(311), and (222) reflections of the face centered cubic struc-
ture of crystalline Au0 (JCPDS Card No. 89-3697; Figure 1,
inset). The weak and broad peaks also indicate the formation
of small gold nanoparticles within the mesoporous channels of
the nitrogen-rich carbon. The absence of any sharp peaks con-
firms no aggregation of the GNPs and no large particles were
formed on the surface of the MCN. In the XRD pattern, the
(111) peak is the strongest, indicating that the (111) plane is
the predominant crystal facet. Both the low- and wide-angle
XRD patterns show a broad peak around 268, which could be
The particle size and GNP distribution within the nanochan-
nels of MCN were analyzed by transmission electron microsco-
py (TEM). High-resolution (HR)-TEM images of GNP@MCN
(2 wt% Au loading) are shown in Figure 3b–d, f. The black
Scheme 2. Supported Au catalysis of phenylacetylene with aniline.
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dots corresponding to gold nanoparticles; the average size of
the particles obtained from HR-TEM was around 5–6 nm. How-
ever, on increasing the amount of gold loading, agglomeration
of gold nanoparticles over the surface of the support was
found. The field emission (FE)-SEM image of GNP@MCN
(4 wt% Au loading, Figure 3a) exhibits particles of 15–25 nm
originating from the aggregation of the same over the surface
of the support and particles of sizes less than 10 nm are pres-
ent within the mesochannels of MCN. Elemental mapping
shows that the Au nanoparticles are well implanted in the
MCN (Figure 3e).
The XPS survey scan of GNP@MCN (Figure 4a) is composed
of peaks at 85.08, 88.08, 285.08, 399.08, and 533.08 eV, which
correspond to Au4f7/2, Au4f5/2, C1s, N1s, and O1s, respectively,
which confirms the presence of gold, carbon, nitrogen, and
some surface oxygen.[18] Higher resolution XPS spectra of C1s,
N1s, and Au4f are also shown in Figure 4 and the spectra
were deconvoluted. The C1s spectrum was deconvoluted into
four components at binding energies of 284.3, 285.5, 286.7,
and 288.3 eV (Figure 4b).
Figure 2. FE-SEM images of (a, b) SBA-15, (c, d) MCN, and (e, f) GNP@MCN.
Figure 4. (a) XPS survey scan and XPS curves of (b) C1s, (c) N1s, and
(d) Au4f5/2, Au4f7/2 of GNP@MCN.
The two main peaks at 284.3 and 285.5 eV correspond to
sp2-hybridized graphite-like carbon (CÀC sp2) and sp3-hybrid-
ized diamond-like carbon (CÀC sp3), respectively. However, the
peaks at 286.7 and 288.3 eV are a result of the presence of
oxygen groups (as CÀO, C=O, and ÀCOO) on the surface.[19]
The N1s area (Figure 4c) reveals three major peaks at 398.0,
399.4, 400.6 eV, which relate to pyridinic, pyrrolic, and quater-
nary nitrogen, respectively, whereas the minor peak at
402.5 eV originates from the N-oxide of the pyridinic nitroge-
n.[19a,b] The Au4f scan (Figure 4d) exhibits two sharp peaks at
83.6 eV (Au4f7/2) and 87.3 eV (Au4f5/2), which confirms the
presence of gold in the 0 oxidation state in GNP@MCN.[18]
The specific surface area of MCN before and after incorpora-
tion of gold nanoparticles was evaluated by measuring nitro-
Figure 3. (a) FE-SEM image, (b, c, d, f) TEM images, and (e) elemental
mapping of GNP@MCN.
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gen adsorption–desorption isotherms and was found to de-
crease from 400 m2 gÀ1 to 360 m2 gÀ1. At the same time, a de-
crease in the specific pore volume from 0.39 to 0.36 cm3 gÀ1
and an increase in specific pore diameter from 3.5 nm to
5.6 nm was found, which could a result of the formation of
gold nanoparticles inside the pores of MCN (Table 1). The ab-
Table 1. Specific surface area, specific pore volume, and specific pore
diameter for MCN and GNP@MCN.
Entry
Sample
Specific surface
area
Specific pore
volume
Specific pore
diameter
[nm]
[m2 gÀ1
]
[cm3 gÀ1
]
Figure 5. FTIR spectra of MCN and GNP@MCN.
1
2
MCN
GNP@MCN
400
360
0.39
0.36
3.5
5.6
2.2. Hydroamination of phenylacetylene
To explore the potential of the catalyst GNP@MCN, hydroami-
nation of phenylacetylene was carried out under reflux condi-
tions under a nitrogen atmosphere and the reaction conditions
were optimized. The effect of catalyst, substrate concentration,
amount of solvent, temperature, and pressure on the hydroa-
mination of phenylacetylene with aniline over GNP@MCN was
studied:
sence of any drastic change in the specific surface area and
pore volume also supports the idea that gold nanoparticles
were formed within the pores of the MCN and the pores of
the MCN are not blocked by gold particles, which reveals that
there was no agglomeration of the gold particles.
The existence of nitrogen functionalities was also confirmed
by performing FTIR spectroscopy. The FTIR spectra (Figure 5) of
the GNP@MCN and MCN samples showed similar vibration
peaks. Both the samples showed broad peaks around 3000–
3500 cmÀ1, which are assigned to the stretching vibration
mode of ÀNH and ÀNH2 units. The infrared absorption bands
around 1415 cmÀ1 and 1614 cmÀ1 represent the CÀN and C=N
stretching frequencies, respectively.[20] However, a minor shift
in absorption bands in GNP@MCN compared with that of pure
MCN was observed, which may arise from the incorporation of
gold nanoparticles within the mesochannels of MCN.
(i) The temperature was varied between 90 and 1308C, while
keeping the amount of catalyst (2 wt% with respect to (w.r.t.)
phenylacetylene, 0.1ꢀ10À2 mmol of Au) and ratio of phenyla-
cetylene and aniline (1:2) constant (Figure 6a). A linear increase
in the rate of phenylacetylene conversion was observed with
increasing temperature from 90 to 1308C over a period of
24 h. After this, a steady state was observed for 1308C with
a maximum conversion of 95% and almost 100% selectivity,
whereas a further increase in conversion was observed for
other reaction temperatures (908C and 1108C).
Figure 6. Effect of (a) temperature variation, (b) catalyst concentration variation, (c) molar ratio variation of the reactants, and (d) amount of solvent on the hy-
droamination reaction of phenylacetylene with aniline by using GNP@MCN as the catalyst.
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(ii) While keeping the reaction temperature at 1108C and the
mole ratio of phenylacetylene and aniline (1:2) constant, the
catalyst amount was varied from 1 wt% to 4 wt% w.r.t. phenyl-
acetylene. A linear increase in the conversion of phenylacety-
lene was observed on increasing the catalyst amount from
1 wt% to 4 wt% over a period of 24 h. After 24 h, an almost
steady state was observed for 4 wt% catalyst, whereas with
other catalysts amounts (1 wt% and 2 wt%) a further rise in
conversion was noticed (Figure 6b).
Table 3. Hydroamination of different alkynes with amines by using
GNP@MCN.[a]
Entry
Rl
R
Selectivity
[%]
Yield
[mol%]
(iii) The molar ratio of phenylacetylene to aniline was altered
from 1:1 to 1:3 while keeping the temperature (1108C) and the
amount of catalyst (2 wt% w.r.t. phenylacetylene) constant. On
increasing the amount of aniline, it was noticed that after 48 h
the conversion of phenylacetylene for the ratios of 1:3 and 1:1
remained almost the same. A linear increase in the rate of con-
version over a period of 24 h was observed for the entire ratio
range (Figure 6c).
1[b]
2
C4H9
C6H5
4-CH3C6H4
1-ClC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
4-CH3C6H4
2-ClC6H4
4-O2NC6H4
99
98
98
99
99
99
–
20
75
52
29
56
45
NR
3
4[c]
5
6
7[c]
C6H5
C6H5
[a] Conditions: amine/alkyne=2:1, 2 wt% catalyst (w.r.t. alkyne, 0.1ꢀ
10À2 mmol of Au), temperature=110 8C, time=24 h, without solvent.
Yields were determined by GC and GC-MS analysis. Selectivity for the
Markovnikov product was observed in each case. [b] Reaction tempera-
ture=758C. [c] Reaction was performed in 3 mL toluene.
(iv) The amount of solvent (toluene) was also varied from
0 mL to 6 mL while keeping the temperature (1108C), catalyst
amount (2 wt% w.r.t. phenylacetylene, 0.1ꢀ10À2 mmol of Au),
and ratio of phenylacetylene and aniline (1:2) constant. It was
found that on increasing the solvent amount (6 mL) conversion
decreases owing to the decrease in the concentration of phe-
nylacetylene (Figure 6d).
effect on hydroamination. Alkynes and anilines with electron-
donating substituents (ÀCH3) provided better yields whereas
analogs with electron-withdrawing substituents (ÀCl, ÀNO2)
possessed less reactivity. Owing to the strong electron-with-
drawing effect of the ÀNO2 group, no reaction was observed
for 4-nitroaniline. The lower yield in the case of both the reac-
tants possessing electron-donating substituents with respect
to the reactants itself (entries 2, 3, and 5) might be a result of
the steric hindrance between the substituted reactants and
the gold supported on the nitrogen-rich mesoporous carbon.
In all cases, selectivity for the Markovnikov product was
observed.
(v) The effect of solvent on the rate of conversion of phenyl-
acetylene was also studied by keeping the temperature
(1108C), catalyst amount (2 wt% w.r.t. phenylacetylene, 0.1ꢀ
10À2 mmol of Au), and ratio of phenylacetylene and aniline
(1:2) constant. It was found that the conversion was lower in
polar solvents and in the absence of solvents the conversion
reached a maximum (Table 2). Polar solvents retard the reac-
tion rate owing to the strong adsorption between solvent and
support, which hinders the availability of the reactant.
(vi) The effect of pressure was also studied on the rate of
conversion of phenylacetylene and it was found that on apply-
ing a pressure of 10 bar, conversion increases; however, the se-
lectivity of the product decreases, which leads to the formation
of coupling between phenylacetylene units.
Different weight percentages of gold were loaded onto the
supporting MCN and the variation in conversion is given in
Table 4. It was noticed that with an initial increase in the
amount of gold, conversion increases; however, on further in-
creasing the amount, the conversion decreases owing to the
agglomeration of gold nanoparticles on the surface of the sup-
To investigate further the scope and limitations of our cata-
lyst GNP@MCN, the reactions of different alkynes with different
aniline derivatives (Table 3) were examined. Aromatic alkynes
were more reactive than aliphatic alkynes. The nature of the
substituents on both alkynes and anilines had a significant
Table 4. Hydroamination of phenylacetylene with aniline by using differ-
ent wt% of gold loading on the support (Th=Theoretical and Exp=Ex-
perimental from ICP-AES.)[a]
Table 2. Effect of solvent variation on the hydroamination reaction of
phenylacetylene with aniline by using GNP@MCN as the catalyst.[a]
Entry
Gold loading
[wt%]
Conversion
[mol%]
Particle size of Au
in GNP@MCN
[nm]
TON
Entry
Solvent
Conversion
[mol%]
Selectivity
[%]
Yield
[mol%]
Th
Exp
1
2
3
4
No solvent
Toluene
Benzene
87
66
52
1
98
98
99
100
85
65
51
1
1
2
3
1
2
4
0.7
1.8
3.6
67
77
71
5
93ꢀ106
42ꢀ106
19ꢀ106
5–10
15–25
Acetonitrile
[a] Conditions: amine/alkyne=2:1, 2 wt% catalyst (w.r.t. phenylacetylene),
temperature=110 8C, time=24 h, without solvent. Yields were deter-
mined by GC and GC-MS analysis. Selectivity for the Markovnikov product
was observed in each case. TON=turnover number; TON=moles of
desired product formed/number of moles of active sites.
[a] Conditions: amine/alkyne=2:1, 2 wt% catalyst (w.r.t. phenylacetylene,
0.1ꢀ10À2 mmol of Au), temperature=110 8C, time=48 h. Yields were
determined by GC and GC-MS analysis. Selectivity for the Markovnikov
product was observed in each case.
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port. Thus, an increase in gold loading results in an
increase in the particle size (Figure S2, in the Sup-
porting Information), which affects the activity of
the catalyst. It has been found that the catalyst pos-
sessing a particle size of around 5 nm is highly
active towards hydroamination, whereas an increase
in particle size results in lower activity for the same.
The vital role of the nitrogen functionalities within
the nitrogen-rich mesoporous carbon (MCN) has also
been explained through the MCN acting as a size-
controlling and stabilizing agent for the controlled
growth of gold nanoparticles. A similar control ex-
periment with phenylacetylene and aniline over
gold nanoparticles embedded on mesoporous
carbon (MC) has also been performed and the con-
version was found to be 28% after 24 h, which re-
veals that despite the formation of gold nanoparti-
cles on the surface of the support, the absence of ni-
trogen functionalities led to uncontrolled growth
and agglomeration of the particles. Separate experi-
ments were performed in the absence of a catalyst
and with Au-free MCN. The reaction did not proceed
without catalyst and yields were very low (around
2%) with Au-free MCN.
2.3. Mechanistic study
Scheme 3. A plausible mechanism for hydroamination of an alkyne with an amine by
using GNP@MCN as a heterogeneous catalyst.
In principle, there are several mechanisms for NÀH
bond addition to CÀC multiple bonds and the cata-
lyst plays a vital role for selectivity and efficacy, how-
ever, catalysis in hydroamination can be divided
roughly into four sorts: (a) CÀC multiple bond activation
through the formation of a p-coordinated Lewis acidic metal
complex followed by nucleophilic addition by the N-nucleo-
phile moiety; (b) activation of amine to generate a metal–
amide complex monitored by insertion of olefin; (c) initial for-
mation of a metal–hydride followed by migratory insertion of
olefin into the MÀH bond; and (d) rearrangement of initially
formed metal–alkyne complex followed by nucleophilic attack
by the nucleophile.[12]
elimination of enamine 5 from the vinyl aurium species to re-
produce the Au0 cluster; (iv) finally, enamine 5 tautomerizes to
give the Markovnikov product 6 (Scheme 3). To understand
and verify the proposed mechanism, density functional theory
(DFT)-based calculations were performed by using the Gaussi-
an 09 package.[22] All the minimum energy structures for the re-
actants, products, and intermediates were confirmed as the
harmonic vibrational frequency without any imaginary mode.
Transition states were confirmed as having one imaginary
mode of vibration along the suspected bond breaking or bond
making direction. Activation energies were calculated from the
free energy differences between the transition states and reac-
tants. The convergence thresholds were set to 0.000015 Har-
tree/Bohr for the forces, 0.00006 ꢂ for the displacement, and
106 Hartree for the energy change. All calculations were per-
formed with DFT with the unrestricted Beck’s three-parameter
hybrid exchange functional[23] combined with the exchange
component of Perdew and Wang’s 1991 functional,[24] abbrevi-
ated as B3PW91. The Lanl2dz basis set has been used for all
calculations, which is available in the Gaussian 09 package and
this basis set has been used because for gold (Au), only this
series of basic functions are available.
Several studies have reported the hydroamination of al-
kynes, allenes, and alkenes by using Au+1 complexes, which
follow the route of CÀC multiple bond activation by p-coordi-
nation followed by addition of the nucleophile;[21] however, it
has not yet been established that the mechanistic pathway is
the same for Au0 complexes used as a heterogeneous catalyst.
As part of the continuing interest in the development of hy-
droamination catalysts, in this article a plausible mechanistic
pathway of addition of a N-nucleophile across the CÀC multi-
ple bond is presented for the Au0 catalyst GNP@MCN, which
involves: (i) oxidative addition of the NÀH bond of amine 1 to
Au0 from the gold nanoparticles supported on MCN, giving
the amido–aurium complex 2; (ii) coordination of the p-elec-
trons of alkyne 3 to the Au center of the amido–aurium com-
plex, which forms an intermediate that rearranges through
subsequent nucleophilic attack by the N atom to the second
alkyne C atom, giving vinyl aurium species 4; (iii) reductive
The stabilization of the GNP catalyst over the MCN support
has been studied. As the particle size is nearly 5 nm, for the
theoretical study a tetranuclear gold cluster (Au4) on the MCN
support (Figure 7) has been used. The absorption energy of
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Figure 7. Tetranuclear gold cluster on the MCN support.
Au4 on the MCN surface is À14.32 kcalmolÀ1. Thus, there is no
need for an extra stabilizing agent for GNP stabilization.
The first step of the reaction pathway considering the ani-
line activation leads to the formation of intermediate-
1 (Figure 8), which involves AuÀH and AuÀN bond formation
via the formation of transition state-1 (Figure S3, in the Sup-
porting Information).
Figure 9. Intermediate-2.
Figure 10. (a) Target enamine and (b) target imine.
tion energies with and without catalyst shows the importance
of the catalyst GNP@MCN.
2.5. Catalyst recyclability and stability
Figure 8. Intermediate-1.
To study the recyclability and stability of the catalyst
GNP@MCN, the reaction was performed at 1108C with 25 mg
(2 wt% w.r.t. phenylacetylene) catalyst and using a phenylacety-
lene to aniline molar ratio of 1:2 (Table 5).
This is similar to that hypothesized in alcohol oxidation by
gold catalysts, in which the reaction occurs on two adjacent
gold atoms of the nanoparticle surface.[25] In intermediate-1, it
is observed that the nitrogen atom attaches to one Au atom
and the hydrogen atom bonds to the adjacent Au atom. From
natural bond orbital (NBO) analysis[26] of this complex, it was
found that lone pair (LP) of the nitrogen atom is donated to
BD* of the Au atom. The activation energy for this step is
The fresh catalyst gave 77% conversion of phenylacetylene.
For the recyclability test, the catalyst was separated by centri-
fuging the reaction mixture after the first run, it was then
washed several times with acetone and dried at 1008C under
vacuum for 5 h. The catalyst was used with a fresh reaction
mixture in the second run and the procedure was repeated for
four runs. The results show that the conversion of phenylacety-
lene was nearly the same in all four runs with ultimately con-
46.35 kcalmolÀ1
.
Intermediate-1 reacts with the alkyne to form intermediate-2
(Figure 9) via transition state-2 (Figure S4, in the Supporting In-
formation). This involves coordination of the p-electrons of the
CꢀC bond of alkyne to the Au atom that is attached to the ni-
trogen atom. As a result of this electron co-ordination, the
(N)AuÀAu(H) bond opens up and a new AuÀN bond is formed.
Table 5. Recyclability tests of GNP@MCN in the hydroamination of phe-
nylacetylene with aniline, showing selectivity for the desired product,
turnover numbers (TONs) of the catalyst, and wt% of gold loaded on
support (ICP-AES).
The activation energy of this process is 58.39 kcalmolÀ1
.
On reduction, intermediate-2 rearranges to form Au0 and
the enamine (Figure 10a). This rearrangement passes through
transition state-3 (Figure S5, in the Supporting Information),
which requires an activation energy of 87.20 kcalmolÀ1. The
total process is presented in an energy profile diagram
(Figure 11). The enamine finally tautomerizes to give the tar-
geted imine (Figure 10b). Calculation of the activation energy
for this process without catalyst has also been performed and
was found to be 159.81 kcalmolÀ1. The difference in the activa-
Run
Gold loading
[wt%]
Selectivity
[%]
TON
1
2
3
4
1.8
1.8
1.8
1.8
99
99
99
99
42ꢀ106
40ꢀ106
38ꢀ106
37ꢀ106
Reaction conditions: phenylacetylene/aniline (1:2 molar ratio), catalyst
2 wt% (w.r.t. phenylacetylene), temperature=110 8C. TON=moles of
desired product formed/number of moles of active sites.
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Figure 11. Energy profile diagram for different intermediates and transition states in the hydroamination of phenylacetylene with aniline by using GNP@MCN
as the catalyst.
stant selectivity (99%; Figure 12), suggesting that GNP@MCN
Conclusions
can be used in repeated cycles without any loss in activity. A
nominal decrease in conversion occurred after each cycle,
which mainly resulted from handling losses and was not
a result of leaching of the active catalyst into the reaction
medium. The catalyst stability was also verified by performing
a leaching test, in which GNP@MCN was heated at reflux with
phenylacetylene and aniline (1:2 molar ratio) at 1108C for 12 h,
then the mixture was cooled and the catalyst was separated
by filtration. The filtrate (reaction mixture) was again heated at
reflux at 1108C for another 12 h, which gave no additional con-
version and ascertained the absence of metal leaching. The
mother liquor was also analyzed by using inductively coupled
plasma atomic emission spectroscopy (ICP-AES) to check for
metal leaching. Thus, the catalytic activity of studied catalyst
system is found to be truly heterogeneous and at the same
time the probability of catalyst deactivation is also very low.
Gold nanoparticles (GNPs) have been synthesized and stabi-
lized by an in situ process and incorporated on nitrogen-rich
mesoporous carbon (MCN). The catalyst system exhibits very
good catalytic activity and above 98% selectivity towards the
Markovnikov addition product in the hydroamination of phe-
nylacetylene with aniline with high turnover numbers. The ni-
trogen functionalities of MCN play a significant role in the con-
trolled growth of the GNPs and stabilization of the zero oxida-
tion state of gold. The optimum reaction conditions were eval-
uated by varying the temperature, solvent, molar ratio of reac-
tants, and the catalyst concentration. It was observed that
conversion of phenylacetylene increases on increasing the
temperature and catalyst concentration. The best yield was
achieved in the solvent-free system. Characterization tech-
niques such as SEM and TEM show that particles are present in
two different environments; some particles are within the mes-
ochannels whereas others are on the surface of the MCN. To
predict the mechanistic pathway, a DFT study has been per-
formed by using a tetranuclear gold cluster (Au4), which ties in
with the size of the GNPs (5–6 nm) stabilized on the MCN sup-
port. Recyclability tests up to four cycles have been performed,
which show a low probability for deactivation and leaching.
Hence, GNP@MCN has been found to be a stable, very effi-
cient, and truly heterogeneous catalyst system for selective
hydroamination.
Experimental Section
Figure 12. Recyclability test of GNP@MCN for hydroamination of phenylace-
tylene with aniline showing the conversion of phenylacetylene, correspond-
ing selectivity, and yield of desired product. Reaction conditions: phenyl
acetylene/aniline (1:2 molar ratio), catalyst 2 wt% (w.r.t. phenylacetylene),
temperature=110 8C.
General considerations
All reactions were performed under an inert atmosphere of nitro-
gen by using Schlenk techniques. Ethylenediamine, carbon tetra-
chloride, sodium hydroxide, chloroauric acid (99.999%), sodium cit-
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rate (>99%), sodium borohydride (98%), phenylacetylene, aniline,
anhydrous toluene, benzene, and acetonitrile (HPLC grade) were
purchased from Sigma–Aldrich. All the chemicals and solvents
were used without further purification.
dispersion of MCN. The obtained mixture was then transferred to
a boiling flask followed by addition of H[AuCl]4 (2 mL, 2 mm) and
a
freshly prepared aqueous solution of sodium borohydride
(112 mm, 100 mL). Sodium borohydride is a strong reducing agent,
which enhanced the full reduction of H[AuCl]4. The mixture was
then heated at reflux at 1008C for 12 h, cooled to room tempera-
ture, centrifuged, and washed several times with an acetone/water
(1:1) mixture. Finally, the obtained GNP@MCN was dried under
vacuum at 708C for 1 h and stored in a dark and cool place. The
gold content in the supporting MCN was verified by ICP-AES analy-
sis and is around 2 wt% (0.4ꢀ10À2 mmol of Au).
The X-ray diffraction (XRD) patterns of GNP@MCN were collected
with a Bruker D8 advance X-ray diffractometer fitted with a Lynx
eye high-speed strip detector and a CuKa radiation source using
CuKa radiation with a wavelength of 1.5418 ꢂ. Diffraction patterns
in the 0.58–858 region were recorded at a rate of 0.5 degrees (2q)
per minute. The resulting XRD profiles were analyzed to check the
structure of MCN before and after incorporation of gold nanoparti-
cles. SEM images of GNP@MCN were taken with an FEI Quanta
200F, using a tungsten filament doped with lanthanum hexaboride
(LaB6) as an X-ray source, fitted with an ETD detector with high
vacuum mode using secondary electrons and an acceleration ten-
sion of 10 or 30 kV. Samples were analyzed by spreading them on
carbon tape. TEM images were recorded with a JEM 2100 (JEOL,
Japan) microscope, and samples were prepared by mounting an
ethanol-dispersed sample on a lacey carbon formvar coated Cu
grid. Elemental mapping was collected with the same spectropho-
tometer. X-ray photoelectron spectroscopy (XPS) was performed
with a Thermo Scientific K-Alpha XPS instrument and binding ener-
gies (Æ0.1 eV) were determined with respect to the C1s peak at
284.8 eV. The porous properties of MCN before and after incorpora-
tion of GNPs were examined by nitrogen adsorption–desorption
isotherms at 1808C (Belsorbmax, BEL, Japan) using the Brunauer–
Emmett–Teller (BET) equation. Pore size distributions were deter-
mined by using the Barrett–Joyner–Halenda (BJH) cylindrical pore
approximation. The percentage of gold loading in MCN was con-
firmed by inductively coupled plasma atomic emission spectrosco-
py (ICP-AES) by using a PS 3000 UV (DRE), Leeman Labs Inc. (USA).
Fourier transform infrared (FTIR) spectra were recorded with
a Thermo Nicolet 8700 (USA) instrument with the following operat-
ing conditions: resolution=4 cmÀ1, scans=36, operating tempera-
ture=23–258C, and frequency range=4000–500 cmÀ1. Spectra in
the lattice vibrations range were recorded for wafers of sample
mixed with KBr.
Catalysis with GNP@MCN (hydroamination)
Typically, GNP@MCN (10 mg, 2 wt% w.r.t. phenyl acetylene, 0.1ꢀ
10À2 mmol of Au) was added to a mixture of phenylacetylene and
aniline (1:2 molar ratio). The resulting mixture was bath-sonicated
for 5 mins for efficient dispersion of the catalyst and then heated
at reflux at 1108C with stirring under an N2 atmosphere over
a period of 24 h. At the end of the reaction, the catalyst was sepa-
rated by filtration and the product was analyzed by gas chromato-
graph (GC, Agilent 7890) with an HP5 capillary column (30 m
length, 0.28 mm id, 0.5 mm film thickness) and flame ionization de-
tector (FID). Phenylacetylene conversion and the selectivity of the
product were calculated by using calibration curves (obtained by
manual injection of authentic standard compounds). The product
identification was performed by injecting authentic standard sam-
ples in GC and GC-MS. To study the effect of pressure, the reaction
was performed in a 50 mL stainless steel bomb with nitrogen gas
applying a pressure of 10 bar.
Acknowledgments
A.B. gratefully acknowledges CSIR, India for funding CSC-0125,
the Director, CSIR-IIP for his help and encouragement. S.D. thanks
UGC for a fellowship. The authors also thank the analytical sci-
ence division, Indian Institute of Petroleum, for analytical serv-
ices.
Synthesis of nitrogen-rich mesoporous carbon (MCN)
In a typical synthesis, calcined SBA-15 synthesized as reported by
Zhao et al.[27] was added to a mixture of ethylenediamine and
carbon tetrachloride. The mixture was heated at reflux at 908C
with stirring for 6 h. The acquired dark-brown solid was dried in an
oven at 1008C for 12 h and then ground into a fine powder fol-
lowed by heat treatment in a nitrogen flow of 50 mL per minute at
6008C for 5 h with a heating rate of 0.58C per minute and kept
under 6008C for carbonization of the polymer. Finally, the MCN
was recovered after removal of the silica framework by heating at
reflux in 2m sodium hydroxide solution followed by filtration,
several washings with ethanol, and drying at 1008C.
Keywords: density functional calculations
hydroamination · mesoporous carbon · nanoparticles
·
gold
·
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Received: June 24, 2016
Published online on && &&, 0000
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Au hydroamination catalyst: The in
situ synthesis of gold nanoparticles in
the surface cavity of mesoporous nitro-
gen-rich carbon (MCN) allowed them to
be utilized efficiently as a catalyst in the
hydroamination of phenylacetylene with
aniline, with more than 98% selectivity
towards the Markovnikov addition prod-
uct. The mechanistic pathway has been
studied with DFT-based calculations,
which predict the stabilization of tetra-
nuclear Au clusters over the MCN sup-
port.
M. Sengupta, A. Bag, S. Das, A. Shukla,
L. N. S. Konathala, C. A. Naidu,
A. Bordoloi*
&& – &&
Reaction and Mechanistic Studies of
Heterogeneous Hydroamination over
Support-Stabilized Gold Nanoparticles
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