MAu2GeS4-Chalcogel (M = Co, Ni): Heterogeneous Intra- and Intermolecular Hydroamination Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.7b01099

High surface area macroporous chalcogenide aerogels (chalcogels) MAu₂GeS₄ (M = Co, Ni) were prepared from K₂Au₂GeS₄ precursor and Co(OAc)₂ or NiCl₂ by one-pot sol-gel metathesis reactions in aqueous media. The MAu₂GeS₄-chalcogels were screened for catalytic intramolecular hydroamination of 4-pentyn-1-amine substrate at different temperatures. 87% and 58% conversion was achieved at 100 °C, using CoAu₂GeS₄- and NiAu₂GeS₄-chalcogels respectively, and the reaction kinetics follows the first order. It was established that the catalytic performance of the aerogels is associated with the M²⁺ centers present in the structure. Intermolecular hydroamination of aniline with 1-R-4-ethynylbenzene (R = -H, -OCH₃, -Br, -F) was carried out at 100 °C using CoAu₂GeS₄-chalcogel catalyst, due to its promising catalytic performance. The CoAu₂GeS₄-chalcogel regioselectively converted the pair of substrates to respective Markovnikov products, (E)-1-(4-R-phenyl)-N-phenylethan-1-imine, with 38% to 60% conversion.

Full text of DOI:10.1021/acs.inorgchem.7b01099

Article
pubs.acs.org/IC
MAu2GeS4‑Chalcogel (M = Co, Ni): Heterogeneous Intra- and
Intermolecular Hydroamination Catalysts
Bambar Davaasuren,*,† Abdul-Hamid Emwas,‡ and Alexander Rothenberger*,†
†Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
‡Imaging and Characterization Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,
Kingdom of Saudi Arabia
S
* Supporting Information
ABSTRACT: High surface area macroporous chalcogenide
aerogels (chalcogels) MAu2GeS4 (M = Co, Ni) were prepared
from K2Au2GeS4 precursor and Co(OAc)2 or NiCl2 by one-pot
sol−gel metathesis reactions in aqueous media. The
MAu2GeS4-chalcogels were screened for catalytic intramolec-
ular hydroamination of 4-pentyn-1-amine substrate at different
temperatures. 87% and 58% conversion was achieved at 100
°C, using CoAu2GeS4- and NiAu2GeS4-chalcogels respectively,
and the reaction kinetics follows the first order. It was
established that the catalytic performance of the aerogels is
associated with the M2+ centers present in the structure.
Intermolecular hydroamination of aniline with 1-R-4-ethynyl-
benzene (R = −H, −OCH3, −Br, −F) was carried out at 100
°C using CoAu2GeS4-chalcogel catalyst, due to its promising
catalytic performance. The CoAu2GeS4-chalcogel regioselectively converted the pair of substrates to respective Markovnikov
products, (E)-1-(4-R-phenyl)-N-phenylethan-1-imine, with 38% to 60% conversion.
applications of this class of materials, and the focus here is on
heterogeneous catalytic hydroamination.
INTRODUCTION
■
Metal chalcogenides are able to form aerogels that are also
called chalcogels. These are predominantly mesoporous
materials constructed from inorganic building blocks to yield
a three-dimensional covalent network comprising high surface
area, surface polarizability, and chemical selectivity.1−7
Chalcogenide-based aerogels are easily accessible using one-
pot sol−gel metathesis reactions.1−3,6 Based on the unique
electronic properties (tunable bandgaps and high surface
polarizability) of the chalcogels, several potential applications
have been proposed and reported including gas separation,1,2,8,9
heavy metal and radioactive element adsorption,10,11 energy
storage materials,12,13 and solar cell.14 Apart from these, due to
its large surface area and porosity chalcogenide based aerogels
are expected and shown to have promising catalytic perform-
ance.5−7,15 The Co−Mo−S-chalcogels were twice as active as
the conventional sulfided Co−Mo/Al2O3 catalyst for the
hydrodesulfurization of thiophene.5 Solid chalcogels containing
FeMoS clusters are capable of photochemical conversion of
nitrogen to ammonia under ambient conditions in aqueous
media.6 Other examples include low-cost pH universal CoMoSx
chalcogel for electrocatalytic hydrogen production7 and
chalcogel supported Pt nanoparticles for catalytic reduction of
4-nitrophenol.15 Chalcogenide aerogels containing late tran-
sition metals are believed to result in further interesting
An atom efficient route for the synthesis of amines,
enamines, and imines is the hydroamination reaction, addition
of amine N−H bonds across unsaturated carbon−carbon
bonds.16,17 N-Heterocycles are of particular importance to the
pharmaceutical, agrochemical, and dye industries.16 The inter-
and intramolecular hydroamination of hydrocarbons can be
efficiently and selectively achieved through metal catalysis.18
Good progress has been made by using metal complex catalysts
under homogeneous conditions.19−25 However, there are few
reports on heterogeneous hydroamination catalysis, such as
zeolites,26,27 metal-cation exchanged zeolites,28 carbon black29
and glassy carbon supported Rh(I) complexes,30 mesoporous
silica encapsulated Rh complex,31 tungstophosphoric acid
(TPA)32 and silver-exchanged tungstophosphoric acid
35
(AgTPA),33 nitrogen-rich mesoporous carbon,34 SiO2 and
TiO2 supported Au nanoparticles,36 ion-exchanged montmor-
illonite, oxide supported Pd37 and Au38 complexes, Al2O3
supported Ni particles,39 sol−gel immobilized Rh complex,40
supported ionic-liquid phase catalysts,41 Cu-exchanged SiO2/
Al2O3,42 polymer supported organo−lanthanide complexes,43
Zn−Co double metal cyanides,44 metal complexes supported in
Received: May 2, 2017
© XXXX American Chemical Society
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a thin film of ionic liquid,45 silica supported Co catalysts,46
transition metal exchanged clays,47 and polymer bound
oxidovanadium complexes.48 However, no work has been
reported on heterogeneous catalytic hydroamination using
chalcogenide based aerogels (chalcogels).
This work is aimed at the preparation of high surface area
porous chalcogels containing catalytically active transition metal
sites confined inside the 3D covalent framework. These types of
chalcogels are expected to have desirable electronic properties,
Lewis acidic, toward the attachment of the substrate molecules.
Herein, (i) the synthesis and characterization of novel
quaternary K2Au2GeS4, (ii) preparation and characterization
of the MAu2GeS4-chalcogels (M = Co, Ni) from K2Au2GeS4
precursor, and (iii) catalytic performance of the MAu2GeS4-
chalcogel heterogeneous catalysts for intra- and intermolecular
hydroamination reactions are presented.
Figure 1. Photographic illustration of the gel formation by metathesis
reaction. The light yellow aqueous solution of K2Au2GeS4 was reacted
with Co(OAc)2 yielding dark brownish solution. CoAu2GeS4-chalcogel
was formed after heating the dark brownish solution at 40 °C for 7
days.
1
the hydroamination reaction was monitored by acquiring H NMR
spectra in situ.50,51 Blank measurements were carried out to record the
NMR spectrum of the pure substrates to evaluate the characteristic
peaks of the substrates as well as the purity of the material. The weak
resonance peaks associated with the impurities were identified and
taken into account for further evaluation of the results (Figures S2−
S6).
EXPERIMENTAL SECTION
■
Synthesis of K2Au2GeS4. The samples were handled in a
nitrogen-filled (99.999%) glovebox due to the air and moisture
sensitivity of the starting materials. Elemental gold (Alfa Aesar 99.5%),
germanium(II) sulfide (Sigma-Aldrich 99.99%), and sulfur (Alfa Aesar
99.99%) were used as received. X-ray single phase K2S was prepared in
liquid ammonia according to a published procedure.49 K2Au2GeS4 was
synthesized from a stoichiometric mixture of elemental Au (2 mmol),
GeS (1 mmol), S (2 mmol), and K2S (1 mmol) by direct combination
reaction. The starting materials were weighed and mixed inside the
glovebox and sealed in fused silica ampules. The sample was heated at
650 °C for 3 days and cooled down to room temperature within 5
days. Phase-pure K2Au2GeS4 was obtained after washing the reaction
products with dimethylformamide (Sigma-Aldrich 99.9%) and diethyl
ether (Sigma-Aldrich 99.9%). The yield was ∼80% based on supplied
Au.
The recoverability of the catalyst was demonstrated on the example
of intramolecular hydroamination of 4-pentyn-1-amine substrate using
CoAu2GeS4 catalyst, at 100 °C. The duration of a catalytic cycle was
48 h. After every cycle, the reaction mixture was cooled down to room
temperature and the liquid phase was decanted. The catalyst was
washed five times with 1,2,4-trichlorobenzene (TCB) to completely
remove the reaction products from the catalyst surface. The
1
conversion rate was determined by measuring H NMR after each
catalytic cycle.
X-ray Diffraction. The powder diffraction patterns were recorded
on a STOE STADI MP X-ray diffractometer, equipped with a
DECTRIS MYTHEN 1 K microstrip solid-state detector, using
monochromatic Cu Kα1 (λ = 1.5406 Å) radiation. The samples were
measured in transmission mode using (2θ + ω) scan type, in the 2° <
2θ < 80° range. The observed and calculated powder diffraction
patterns of K2Au2GeS4 are shown in Figure S7. The colorless needle-
shaped crystals of K2Au2GeS4 were measured on a STOE IPDS2 single
crystal X-ray diffractometer with graphite monochromatized Mo Kα (λ
= 0.71073 Å) radiation at 200 K. The programs X-Area52 and X-
Shape53 were used for data collection, determination, and refinement
of the lattice parameters as well as for data reduction including LP
correction. The crystal structure was solved by direct methods and
refined using SHELX201454 in the WinGX program package.55 The
graphical images of the crystal structure were produced using
CrystalMaker.56
Scanning Electron Microscopy/Energy Dispersive X-ray
Spectroscopy (SEM/EDS). The semiquantitative chemical analyses
were conducted on an Energy Dispersive X-ray (EDX)-equipped SEM
(FEI Quanta 200).
Raman Spectroscopy. Raman spectra were recorded on a
HORIBA JobinYvon ARAMIS Raman spectrometer, using a He−Ne
laser (473 nm) with a resolution of 50 cm−1. The spectra acquisition
time was 10 s. For the liquid sample, ten consecutive spectra were
recorded and averaged to get better signal-to-noise ratio.
Chalcogel Preparation. K2Au2GeS4 dissolves in water forming a
light yellow clear solution. The removal of water from the
K2Au2GeS4(aq) solution, by slow evaporation inside the glovebox,
resulted in the formation of the following solid products:
K3[GeS3(OH)]·H2O, KHS, GeS2, and Au2S (Figure S1). This fact
clearly indicates that the [Au2GeS4]2− anion is only stable in the liquid
phase (Figure 4) and does not recrystallize from the solution. The
hydrolysis of [Au2GeS4]2− cannot be ruled out completely and might
take place slowly with time. Therefore, the M2+ (M = Co, Ni) linker
has to be added straight away after the dissolution of K2Au2GeS4 in
water, to avoid the hydrolysis of [Au2GeS4]2− anion. All divalent third
row transition metals were screened for the gel formation, and only
Co(II) and Ni(II) formed rigid gels. 0.146 mmol of K2Au2GeS4
precursor was dissolved in 7 mL of deionized and degassed water,
inside the glovebox. One equimolar Co(OAc)2(aq) or NiCl2(aq) was
immediately added dropwise under continuous stirring, which resulted
in the formation of dark brownish clear solutions. The black chalcogels
were formed after 7 days by heating the solutions at 40 °C (Figure 1).
The water-soluble products were removed together with the water by
washing the gels with absolute ethanol more than 10 times. The gels
were dried using the CO2 critical point drier to remove the ethanol
and retain the porous nature of the materials.
Hydroamination Catalysis. The 4-pentyn-1-amine (95%, Sigma-
Aldrich), phenylacetylene (98%, Sigma-Aldrich), aniline (99.5%,
Sigma-Aldrich), 1-ethynyl-4-methoxybenzene (97%, Sigma-Aldrich),
1-bromo-4-ethynylbenzene (97%, Sigma-Aldrich), and 1-fluoro-4-
ethynylbenzene (99%, Sigma-Aldrich) were used as received. The
chalcogel catalyzed intra- and intermolecular hydroamination reactions
were carried out, on a small scale, in a NMR tube fitted with Young’s
concentric Teflon valve, at three different temperatures, 60, 80, and
100 °C. All sample preparation and handling was done in a N2-filled
glove box. 0.03 mmol (5 mol % to the substrate) of the catalyst, 0.6
mmol of respective substrates, and 0.7 mL of trichlorobenzene (TCB)
were loaded into a NMR tube. For the intermolecular hydroamination
reactions, the substrates were taken in 1:1 mol ratio. The progress of
UV−Visible Spectroscopy. UV−vis diffuse reflectance spectra
were recorded at room temperature on a Cary 5000 UV−vis−NIR
double-beam, double monochromator spectrophotometer using the
Praying Mantis accessory for solid samples. The measurements were
done in the range 200−800 nm with a scan rate of 200 nm per minute.
Thermal Analysis. The thermal stabilities of the substances were
examined up to 600 °C by a Netzsch STA 449 F3 Jupiter using Al
crucibles (25 μL) with punched lids under nitrogen flow (20 mL·
min−1) with a heating and cooling rate of 10 K·min−1.
Supercritical Drying. Drying of the MAu2GeS4-chalcogels (M =
Co, Ni) was carried out on a Tousimis Autosamdri-815B instrument.
The sample was placed on a small particle holder with 2 μm
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preinstalled particulate screen (mesh) and transferred to the critical
point drier in a sealed container. The chalcogel was soaked and flushed
with liquid CO2 below 10 °C to remove the ethanol completely.
Finally, chalcogenide aerogels were obtained after supercritical drying
at 35 °C.
Adsorption Measurements. The specific surface area of the
aerogels (∼80 mg) was measured on a Micromeritics ASAP 2020 HD
instrument with Kalrez modification. Nitrogen adsorption and
desorption isotherms were recorded at 77 K. The samples were
degassed at 348 K under vacuum (<10−4 mbar) overnight before the
analysis. Low-pressure incremental dosing of 0.1338 mmol/g (STP)
and 30 s equilibration were applied as analysis conditions. BET
transform plots were obtained in the 0.05 to 0.35 relative pressure (P/
Po) region, and the correlation coefficient of 0.999 was obtained in all
of the cases, while the pore size distribution was calculated using the
Barrett−Joyner−Halenda (BJH) model.
S, 2.225(3) Å and 2.213(2) Å; Au−S, 2.302(2) Å and 2.312(2)
Å; and K−S, 3.169(3)−3.284(4) Å) are comparable to the
respective distances observed in K4GeS4,58 K2Au2Ge2S6,59
57
K2Ag2GeS4, K6Ge2S6,60 and K4Ag2Ge3S9·H2O.61 Detailed
crystallographic data is listed in Tables S1−S4. The semi-
quantitative analyses of selected crystals show the presence of
all four elements in the atomic ratio K:Au:Ge:S = 1.7:1.8:1:4.7
(Figure S8), which is in agreement with crystal structure
refinement results.
Characterization of K2Au2GeS4. The solid-state UV−vis
spectrum of the K2Au2GeS4 powder was measured in diffuse
reflectance mode and plotted using the Kubelka−Munk
function F(R) (Figure 3).62 The bandgap was determined as
1H NMR. The NMR spectra were acquired at 60, 80, and 100 °C
using Bruker 600 AVANCE III spectrometer (Bruker BioSpin,
Rheinstetten, Germany) equipped with Bruker 5 mm broadband
observe (BBO) multinuclear probe. The 1H NMR spectra were
recorded by collecting 32 scans using a standard 1D 90 deg pulse
sequence and (zg) program from Bruker pulse library. For quantitative
analysis, the recycle delay time was set to at least 10 s. Chemical shifts
were adjusted using tetramethylsilane (TMS) as internal chemical shift
reference. Exponential line broadening of 1 Hz was applied before
Fourier transformation. All spectra were then visually phased, and the
chemical shifts were adjusted manually to the TMS peak where
necessary. Bruker TopSpin 2.1 software (Bruker BioSpin, Rheinstet-
ten, Germany) was used in all NMR experiments to collect and
analyze the data.
RESULTS AND DISCUSSION
Figure 3. UV−vis spectrum of K2Au2GeS4 measured in diffuse
reflectance mode and plotted using the Kubelka−Munk function.
■
Crystal Chemistry of K2Au2GeS4. K2Au2GeS4 crystallizes
in the monoclinic space group C2/c (15) and is isostructural to
previously reported K2Ag2GeS4.57 The crystal structure
contains two K, one Au, one Ge, and two S sites in the
asymmetric unit. The Ge atom is 4-fold coordinated by S atoms
to form slightly distorted GeS4 tetrahedra (Figure 2). These
tetrahedra are bidentate to Au atoms and form helical chains,
which are charge balanced by the K cations. The K atoms are
surrounded by 6 sulfur atoms. The interatomic distances (Ge−
the intersection point between the energy axis and the line
extrapolated from the linear portion of the absorption edge in
an F(S) vs E plot. The compound exhibits semiconducting
behavior with a band gap of around 2.9 eV.
The Raman spectrum of K2Au2GeS4 possesses several Ge−S
and Au−S characteristic vibrational modes in the 200−500
cm−1 range. Figure 4 (black line) shows the Raman spectrum of
polycrystalline K2Au2GeS4 powder. The modes above 350 cm−1
can be assigned to the Ge−S stretching modes; the peaks at
389 and 414 cm−1 are very close to the vibrational modes
observed in Na4GeS463 and Na6Ge2S6,64 while the bands at 366
and 445 cm−1 are comparable to the terminal Ge−S stretching
vibrations found in Na2GeS3 and Na4Ge4S10.65 Bulk Au2S
Figure 2. Crystal structure of K2Au2GeS4 showing the 1D polyanionic
chains separated by K+ ions. The CIF file can be obtained from CSD
database quoting the CSD number 429757.
Figure 4. Raman spectra of K2Au2GeS4 powder (black line) and
K2Au2GeS4 aqueous solution (blue line).
C
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shows a broad Raman band centered at 332 cm−1, which can be
deconvoluted to 265, 302, and 336 cm−1.66,67 Formation of the
Au−S covalent bond, with Raman mode at 287 cm−1, was
observed on the Au-electrode surface by electrochemical
oxidation using different electrolytes such as tetrathionate
(Na2S4O6), trithionate (Na2S3O6), [Au(S2O3)2]3− complex,66
and thiosulfate (Na2S2O3).66−68 Therefore, the bands at 290
and 337 cm−1 can be assigned to the Au−S vibrational modes.
Stability of K2Au2GeS4. The thermal stability of
K2Au2GeS4 was examined up to 600 °C. No weight loss was
observed based on the TG results, while the DSC curve has a
strong single endothermic effect with onset point of 490 °C
(Figure S9). X-ray phase analysis of the sample after the DSC
shows the presence of GeS2, Au, and K2Au2GeS4. Therefore,
incongruent melting behavior was supposed for K2Au2GeS4.
The comparison of the powder XRD patterns before and after
DSC is depicted in Figure S10.
Figure 5. N2 adsorption and desorption isotherms of CoAu2GeS4-
chalcogel. The SEM image of the chalcogel is shown in the inset.
The K2Au2GeS4 is readily soluble in water forming a light
yellow clear solution. The Raman spectrum of the solution is
illustrated in Figure 4 (blue line). The Raman spectra of
K2Au2GeS4 both in the solid state and in aqueous solution
show practically identical shapes, and the location of the peaks
indicates intactness of the polyanionic chain during the
dissolution and its stability in the solution. Small shifts and
peak broadening might be associated with hydrolysis of the
parent compound, which might be slowly taking place with
time to form thio-hydroxogermanate ions. K2Au2GeS4 is air
sensitive and not soluble in organic solvents such as DMF,
DMSO, ethanol, methanol, THF, and acetone. The synthesized
K2Au2GeS4 was used as a precursor material for MAu2GeS4-
chalcogel (M = Co, Ni) preparation.
MAu2GeS4-Chalcogel (M = Co, Ni). The MAu2GeS4-
chalcogels were synthesized by one-pot sol−gel metathesis
reaction between equimolar K2Au2GeS4 and Co(OAc)2 or
NiCl2 aqueous solutions. The Co(OAc)2(aq) or NiCl2(aq) was
added dropwise right after dissolving the K2Au2GeS4 in water to
avoid the hydrolysis of K2Au2GeS4. The MAu2GeS4-chalcogels
were obtained by CO2 critical point drying of the wet
chalcogels. The BET results and BHJ pore size distribution
analysis reveal that the MAu2GeS4-chalcogels are mainly
macroporous materials with specific surface area of 344 m2/g
and 217 m2/g for CoAu2GeS4 (Co-gel) and NiAu2GeS4 (Ni-
gel), respectively, containing covalently bonded Au2GeS4
building blocks connected through M2+ ions. Figure 5 shows
the BET isotherm curve together with the SEM image of the
Co-gel in the inset (see Figure S11 for Ni-gel).
Scheme 1. Catalytic Conversion of the 4-Pentyn-1-amine
Substrate to 2-Methylpyrroline over MAu2GeS4-Chalcogel
Catalysts
progress of the hydroamination reaction was monitored by
1
acquiring H NMR spectra in situ. The sample was heated and
kept within the magnet during the whole reaction time. Thus,
the first spectrum was measured as soon as after the sample had
been heated to the target temperature. The moment of this first
acquisition was taken to be time zero. Conversion of starting
material to the product was determined by integration of the
product resonances relative to the substrate resonances. The
turnover frequency (TOF) was calculated as the [conversion ×
(mole of substrate/mol of catalyst × time)] and was
determined at 50% conversion. The conversion rate at room
temperature was negligible even after 72 h. Figure S14 shows
the in situ NMR spectra of the hydroamination reaction at 80
°C using Co-gel catalyst.
The catalytic hydroamination of 4-pentyn-1-amine substrate
using Co-gel catalyst with 5 mol % loading was convincing, but
kinetically rather slow, at 60 °C. The conversion of the
substrate reached only 56% after 120 h. As expected, the
reaction rate increased with temperature, and the conversion
reached 66% at 80 °C after 47 h and 87% at 100 °C after 48 h
(Table 1). Figure 6 depicts the conversion rate vs reaction time
The pore size distribution curve is depicted in Figure S12a,b.
The MAu2GeS4-chalcogels are amorphous in nature and
relatively stable at ambient conditions (Figure S12e,f). EDS
analyses of the MAu2GeS4-chalcogels show the presence of M/
Au/Ge/S with compositional ratio of 1.1:2.0:1:5.3, averaged
from 10 different measurement points, which agrees well with
the nominal composition (Figure S13). The MAu2GeS4-
chalcogels are both thermally stable up to 150 °C and
decompose at elevated temperatures forming Au, MS2 (M =
Co, Ni), and germanium sulfides (Figure S12c−f). The
MAu2GeS4-chalcogels were further explored for their catalytic
performance in hydroamination reactions.
Intramolecular Hydroamination. The Co-gel has higher
surface area compared to the Ni-gel. Therefore, the intra-
molecular catalytic hydroamination reaction of 4-pentyn-1-
amine substrate to 2-methylpyrroline was first carried out using
the Co-gel as a catalyst at 60, 80, and 100 °C (Scheme 1). The
Table 1. Kinetic Results of the MAu2GeS4-Chalcogel (M =
Co, Ni) Catalyzed Hydroamination Reaction
a
gel
Co
T (°C)
time (h)
convn (%)
TOF (h−1
)
60
80
120
47
56
66
87
58
0.12
0.44
1.25
0.30
100
100
48
Ni
48
a
TOF was calculated at 50% conversion.
plot at different reaction temperatures. The initial kinetic
studies suggest that the reaction is first order with respect to the
substrate concentration (Figure S15). Presumably, the rate-
determining step of the reaction is the attachment of the
substrate to a high surface area, polarizable chalcogel surface
containing Lewis acidic catalytically active sites. The reaction
D
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promising cobalt aerogel catalyst at 100 °C (Scheme 2).
Intermolecular hydroamination reactions of aniline with (A)
Scheme 2. Intermolecular Hydroamination Reactions Using
Co-Gel Catalyst
phenylacetylene, (B) 1-ethynyl-4-methoxybenzene, (C) 1-
bromo-4-ethynylbenzene, and (D) 1-fluoro-4-ethynylbenzene
were screened for 60 h, 60 h, 57 h, and 60 h, respectively. Table
2 summarizes kinetic data of intermolecular hydroamination
Figure 6. Kinetic data for the CoAu2GeS4-chalcogel catalyzed
conversion of the 4-pentyn-1-amine substrate to 2-methylpyrroline
at 60, 80, and 100 °C.
Table 2. Kinetic Results of the CoAu2GeS4-Chalcogel
Catalyzed Intermolecular Hydroamination Reactions at 100
°C
a
reaction code
R
time (h)
convn (%)
TOF (h−1
)
mechanism might follow the oxidative addition of the amine to
the metal center followed by insertion of the C−C unsaturated
moiety into the metal−nitrogen bond,69,70 or the M2+ π-acid
catalysis route.71,72
The catalytic activity of Ni-gel was measured only at 100 °C,
under the same conditions used for Co-gel. About 58% of the
4-pentyn-1-amine substrate was converted to 2-methylpyrroline
after 48 h, which is rather slow in comparison with Co-gel, 87%
at 100 °C.
Both Co- and Ni-gel contain a combination of metals, Co/
Au and Ni/Au, which are known to be active for catalytic
hydroamination reactions. In order to confirm which metal in
the aerogels is responsible for the catalytic activity, the
following measurements were carried out. Co(OAc)2 was
catalytically inactive for the hydroamination process at room
temperature (Figure S2a). However, at higher temperatures,
Co(OAc)2 started to react with the substrate molecule, which
was evidenced by the color change of the solution as well as by
NMR (Figure S2a). The K2Au2GeS4 precursor did not show
any catalytic performance even at elevated temperature, 100 °C
after 24 h (Figure S2b). Therefore, based on the catalytic
activity results of the Co- and Ni-gels, the catalytic performance
of the MAu2GeS4-chalcogels is believed to be associated with
the M(II) species present in the aerogels. The TOF of the
MAu2GeS4-chalcogel catalysts were calculated at the 50%
conversion and tabulated in Table 1.
A
B
−H
−OCH3
−Br
60
60
57
60
38
60
55
40
0.45
0.24
C
D
−F
a
TOF was calculated at 50% conversion.
reactions that have been examined. All four intermolecular
hydroamination reactions regioselectively converted to the
respective Markovnikov products, which was confirmed by H
1
NMR and HR-MS (Figures S3−S6). The substituted
ethynylbenzene derivatives show higher conversion rate
compared to the phenylacetylene. However, the reaction
kinetics was slower compared to the cyclization of 4-pentyn-
1-amine. The conversions of the educts are comparable to the
other heterogeneous catalysts used for the same reactions. For
instance, the TiO2 supported Au-NP’s showed 79%, 56%, and
80% conversion for reactions A, B, and C, respectively, at 40 °C
after 25 h.36 Several catalysts were screened for intermolecular
hydroamination of aniline with phenylacetylene: Cu-exchanged
K-10 zeolite yielded 93% product after 10 h at 110 °C,47 and
Co/K-10 catalyst was less active with only 7% conversion at
111 °C,45 while 79% yield was achieved using Zn−Co double
metal cyanide catalysts at 150 °C after 24 h.44 Nevertheless,
CoAu2GeS4-chalcogel is catalytically active for both intra- and
intermolecular hydroamination reactions.
The activity of the MAu2GeS4-chalcogel catalysts is
comparable with the other homogeneous and heterogeneous
catalysts studied for intramolecular hydroamination of 4-
CONCLUSION
■
High surface area porous MAu2GeS4-chalcogel (M = Co, Ni)
heterogeneous catalysts were prepared from [Au2GeS4]2−
helical chain precursor and screened for the intra- and
intermolecular hydroamination of various substrates. The
MAu2GeS4-chalcogels were catalytically active for both intra-
and intermolecular hydroamination and showed excellent
recoverability and regioselectivity toward the Markovnikov
products. It was established that the catalytic active site of the
materials is the Lewis acidic M2+ centers. The chalcogenide-
based aerogels are a potentially interesting class of materials
that can find application in the synthesis of industrially
important heterocycles through heterogeneous catalytic hydro-
amination. Preparation of new chalcogels with different catalytic
active centers and higher catalytic performance, as well as
expanding the library of screened substrates, is the subject of
the future studies.
51
pentyn-1-amine substrate. [{CH(PPh2NSiMe3)2}SmCl2]2
shows TOF of 1.54 for cyclization of 4-pentyn-1-amine, and
[Ir(bim)CO2][BPh4] complex has TOF of 5,73 while cis-
[PtCl2(PTA)2]74 is less active with TOF of 0.35. Rh(PyP)-
(CO)(OSO2CF3) and Rh(PyP)(CO)Cl complexes show TOF
of 3.5 and 2.7, respectively.75 Glassy carbon supported
[Rh(bmp)(CO)2] heterogeneous catalyst shows 50% con-
version after 72 h at 60 °C.30
The CoAu2GeS4 catalyst demonstrated excellent recover-
ability after multiple catalytic intramolecular hydroamination
cycles. The conversion of the 4-pentyn-1-amine substrate was
∼87%, ∼86.7%, and ∼86.8% after the first, second, and third
catalytic cycles, respectively (Figure S16).
Intermolecular Hydroamination. Catalytic intermolecu-
lar hydroamination reactions were carried out using the more
E
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01099.
■
S
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diffraction patterns, BET surface area, pore size
1
distribution, linear fit, H NMR spectra, and HR-MS
(PDF)
Accession Codes
CCDC 1546598 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: bambar.davaasuren@kaust.edu.sa.
*E-mail: alexander.rothenberger@kaust.edu.sa.
■
ORCID
Bambar Davaasuren: 0000-0002-0270-3426
Notes
The authors declare no competing financial interest.
(15) Liu, J.; He, K.; Wu, W.; Song, T.-B.; Kanatzidis, M. G. In Situ
Synthesis of Highly Dispersed and Ultrafine Metal Nanoparticles from
Chalcogels. J. Am. Chem. Soc. 2017, 139, 2900−2903.
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Chem. Soc. Rev. 2007, 36, 1407−1420.
ACKNOWLEDGMENTS
■
(17) Yang, H.; Gabbaï, F. P. Activation of a Hydroamination Gold
Catalyst by Oxidation of a Redox-Noninnocent Chlorostibine Z-
Ligand. J. Am. Chem. Soc. 2015, 137, 13425−13432.
This research was supported by King Abdullah University of
Science and Technology (KAUST) baseline funding. We would
like to thank Prof. P. W. Roesky for the advice in catalytic
activity measurement and Dr. E. Ahmed for the assistance in
chalcogel preparation.
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