A mineralogically-inspired silver-bismuth hybrid material: An efficient heterogeneous catalyst for the direct synthesis of nitriles from terminal alkynes

Article abstract of DOI:10.1039/c7gc02487h

The synthesis and characterization of a silver-containing hybrid material is reported as a novel heterogeneous noble metal catalyst. In order to eliminate the need for traditional immobilization techniques, and to create a solid material with structurally-bound silver catalytic centers, the layered structure of a naturally occurring mineral served as the basis of the initial catalyst design. The novel material was prepared by means of the urea-mediated homogeneous precipitation of the corresponding metal nitrates, and was fully characterized by means of diverse instrumental techniques (X-ray diffractometry, Raman, IR, UV-Vis, EPR, X-ray photoelectron spectroscopies, thermal methods as well as atomic force, scanning and transmission electron microscopies). The as-prepared material exhibited outstanding activity in silver-catalyzed CC bond activation to yield organic nitriles directly from terminal alkynes with less environmental concerns as compared to the classical synthesis methods. The effects of the reaction time, the temperature, as well as the role of various solvents, nitrogen sources and additives were carefully scrutinized in order to achieve high-yielding and selective nitrile formation. The heterogeneous nature of the reaction was verified and the solid catalyst was recycled and reused numerous times without loss of its activity or degradation of its structure, thereby offering a sustainable synthetic methodology.

Full text of DOI:10.1039/c7gc02487h

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A mineralogically-inspired silver-bismuth hybrid material: An
efficient heterogeneous catalyst for the direct synthesis of nitriles
from terminal alkynes
S. B. Ötvös,a,b*, R. Mészáros,a G. Varga,c,d M. Kocsisa,c,d Z. Kónyae,f, Á. Kukoveczf,
,
P. Pusztai,f P. Sipos,d,g I. Pálinkó,c,d* and F. Fülöpa,b*
a Institute of Pharmaceutical Chemistry, University of Szeged, Eötvös u. 6, H-6720 Szeged, Hungary
b MTA-SZTE Stereochemistry Research Group, Hungarian Academy of Sciences, Eötvös u. 6, H-6720 Szeged, Hungary
c Department of Organic Chemistry, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary
d
Material and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, H-6720 Szeged,
Hungary
e
MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Hungarian Academy of Sciences, Rerrich B. tér 1, H-6720 Szeged,
Hungary
f Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, H-6720 Szeged, Hungary
g Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, H-6720 Szeged, Hungary
*Corresponding authors:
S. B. Ötvös (otvossandor@pharm.u-szeged.hu)
I. Pálinkó (palinko@chem.u-szeged.hu)
F. Fülöp (fulop@pharm.u-szeged.hu)
The synthesis and characterization of a silver-containing hybrid material is reported as a novel
heterogeneous noble metal catalyst. In order to eliminate the need for traditional immobilization
techniques, and to create a solid material with structurally-bound silver catalytic centers, the layered
structure of a naturally occurring mineral served as basics of an initial catalyst design. The novel
material was prepared by means of the urea-mediated homogeneous precipitation of the
corresponding metal nitrates and was fully characterized by means of diverse instrumental
techniques (X-ray diffractometry, Raman, IR, UV-Vis, EPR, X-ray photoelectron spectroscopies,
thermal methods as well as atomic force, scanning and transmission electron microscopies). The as-
prepared material exhibited outstanding activity in silver-catalyzed C≡C bond acꢀvaꢀon to yield
organic nitriles directly from terminal alkynes with less environmental concerns as compared to the
classical synthesis methods. The effects of the reaction time, the temperature, as well as the role of
various solvents, nitrogen sources and additives were carefully scrutinized in order to achieve high-
yielding and selective nitrile formation. The heterogeneous nature of the reaction was verified and
the solid catalyst was recycled and reused numerous times without loss of its activity or degradation
of its structure, thereby offering a sustainable synthetic methodology.
Keywords: nitrile from alkynes, silver, bismuth, heterogeneous catalyst, instrumental characterization
Introduction
Silver-mediated transformations have attained much importance in organic syntheses, which is
exemplified by the large number of related review articles published in the last couple of years.1
Silver-based heterogeneous catalysts play historical role in industrial oxidation processes, and they
are also important in environmental applications (e.g., in catalytic elimination of pollutants).2 Fine-
chemical syntheses (either in the industry or in the academic research) typically rely on Ag(I) salts or
(in situ generated) complexes as homogeneous sources for the catalytically active metal.1, 3
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Because of environmental and economic reasons, there is increasing need for heterogeneous
noble metal catalysts. A literature survey indicates that amongst heterogeneous silver-mediated
processes, the application of supported nanoparticles predominate.4 This can be explained by the
fact that silver nanoparticles can readily be immobilized on various surfaces, such as activated
charcoal,5 silica,6 alumina,7 carbon nanotubes,8 hydrotalcite materials,9 graphene10 or graphene
oxide.11 The main limitation of supported nanoparticles is that the catalytic metal is typically
attached to the surface via weak forces,12 which often involves limited catalyst stability and
robustness, especially under demanding reaction conditions, such as high-temperature, high-
pressure and continuous-flow conditions or in the presence of coordinating ligands and bases.13
Alternatively, there are only few examples for heterogenized silver complexes and silver-exchanged
materials (e.g., heteropoly acids) as reusable silver catalysts.14
Due to the excellent transformability of C≡C bonds and because of their excellent availability,
alkynyl compounds have emerged as versatile intermediates for the atom-economic synthesis of a
wide array of valuable products.15 Alkynes are important components in numerous catalytic
transformations, such as diverse coupling and cyclization reactions16 including the well-established
click chemistry.17 Because of these beneficial features, alkyne-based catalytic reactions are
generating continuously increasing interest.15 Due to its d10 electronic configuration, silver plays
marked role in the activation of C≡C bonds of alkynyl compounds.1g As a consequence of this
pronounced alkynophilicity, large number of practical synthesis methods have been devised via
silver-mediated σ- or π-activation of terminal or internal alkynes.1a, 1b, 1g, 18 Most of these important
reactions rely on homogeneous silver catalysts employing relatively large amounts of the precious
metal. Very recently, Duan and co-workers reported a pioneering methodology for the
heterogeneous catalytic activation of internal alkynes by introducing a porous silver coordination
polymer for the cyclization of propargylic alcohols with CO2.19
Because of their incredible versatility, aromatic and aliphatic nitriles are of outstanding
importance in fine chemical, pharmaceutical and natural product syntheses. Organic nitriles are
essential precursors for amines, amides, aldehydes and various carboxylic acid derivatives,20 and they
are particularly important as intermediates for the production of valuable heterocycles.21 In addition,
the cyano group represents a vital motif in various biologically and medicinally relevant structures,
including numerous FDA-approved drugs like letrozole and rilpivirine.22 Traditional strategies in nitrile
synthesis involve Sandmeyer23 and metal cyanide (or metalloid cyanide)-mediated cyanation
reactions,24 dehydration of amides and oximes,25 ammoxidation of aromatic hydrocarbons26 and the
Schmidt reaction of aldehydes with azides.27 However, these methods suffer from multiple
limitations, including serious environmental concerns (e.g., cyanide salts and toxic metal waste), the
use of expensive catalysts or harsh reaction conditions, selectivity issues and limited scope. To
circumvent these drawbacks, Jiao and co-workers recently demonstrated a silver-catalyzed C≡C bond
cleavage reaction to yield organic nitriles directly from terminal alkynes in the presence of a suitable
nitrogen source and under reasonably mild conditions.28
In chemical industry, the concept of catalytic materials from naturally occurring minerals has
notable historical roots,29 with the family of synthetic zeolites as the best-known examples.30 Due to
economic and environmental reasons, the application of synthetic minerals and mineral-derived
materials as designer catalysts has drawn significant attention in organic syntheses. For example,
layered double hydroxides (LDHs) have emerged in diverse catalytic applications as catalyst supports
or catalyst precursors or as the actual catalysts.31 Unlike silicate-based cationic clays, only relatively
few anionic clays occur naturally;32 however, most of them can easily be synthesized.33 These layered
materials can be comfortably modified, and they frequently offer peculiar catalytic properties due to
the intrinsic characteristics of the layers (surface acid-base character), the interlayer modifier (e.g.,
redox-active intercalated complexes) and the sterically constrained environment, which may give rise
2

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to various selectivity effects.34 Thus, LDHs may be tailored to specific needs with relative ease.35
Traditional immobilization of metal catalysts on various prefabricated supports is often accompanied
by negative effects such as poor accessibility, random anchoring or disturbed geometry of the active
sites, which may involve reduced activity and/or selectivity; and in case of unsatisfactory catalyst‒
support interactions, the possible leaching of the metal component may lead to serious
environmental concerns.36 In contrast, catalytic materials with structurally bound active centers (e.g.
LDHs with layers of active metals) exhibit increased thermodynamic stability and robustness as
compared with the classically immobilized ones.37
In recent years, we have been active in this field concentrating on the synthesis, modification,
structural characterization and catalytic applications of LDHs.38 During this work, we were able to
prepare a thus far unknown Ba(II)Fe(III)-LDH,39 which refuted the generally accepted wisdom that for
the formation of an LDH material, the ionic radii of the cationic components must be similar in size.
As concerns catalytic applications, we not only exploited synthetic LDHs as catalyst carriers
(intercalated catalysts),40 but we also showed that a copper-containing primary LDH can act as a
highly active and robust solid catalyst even under demanding reaction conditions (high-
temperature/high-pressure continuous-flow conditions).41
Although silver-containing minerals are at scarce in nature, we speculated that a mineralogically-
inspired catalyst design may aid the development of a robust heterogeneous silver catalyst. Our aim
was to eliminate the need for classical immobilization techniques, and to prepare a solid catalyst
containing Ag(I) as a constituent. As a proof of the concept, we were intrigued to explore the direct
nitrogenation of alkynes to organic nitriles. To the best of our knowledge, this transformation has not
yet been achieved by using a heterogeneous noble metal catalyst. Thus, we wish to present our
results on the synthesis and characterization of a novel silver-containing hybrid material as a
recyclable and highly selective heterogeneous catalyst for the sustainable synthesis of organic nitriles
from terminal alkynes.
Experimental
Synthesis of the silver-bismuth hybrid material
All chemicals used were of analytical grade Sigma–Aldrich products, and were applied without
further purification.
In a typical synthesis using the urea hydrolysis method,42 appropriate amounts of Bi(NO3)3·5H2O
(5.36 g) and AgNO3 (3.73 g) were dissolved in 50-50 mL 5 wt% nitric acid. After mixing, urea (7.05 g)
dissolved in 100 mL of deionized water was added to the solution and stirred for 72 h at 130 °C. The
obtained colored product was filtered, washed with aqueous thiosulfate solution, water and ethanol
for four times, and dried at 60 °C to obtain the final product (material C). The synthesis of all
byproducts (materials A and B) were carried out the same way except leaving out either the silver or
the bismuth salt.
For control purposes, the catalyst synthesis was also carried out using high purity (99.999%) metal
salts (Sigma-Aldrich).
Characterization of the silver-bismuth hybrid material
The X-ray diffraction (XRD) patterns were recorded on a Rigaku XRD-MiniFlex II instrument applying
CuKα radiation (λ = 0.15418 nm), 40 kV accelerating voltage at 30mA.
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The structure-building inorganic components were identified by Raman microscopy and IR
spectroscopy. Raman spectra were recorded with a Thermo ScientificTM DXRTM Raman microscope at
an excitation wavelength of 635 nm applying 10 mW laser power and averaging 20 spectra with an
exposition time of 6 seconds. The instrument for recording the IR spectra was a BIO-RAD Digilab
Division FTS-65A/896 (mid-range spectra) and BIO-RAD Digilab Division FTS-40 vacuum FT-IR
spectrophotometers (far-range spectra) with 4 cm−1 resolution. The 4000–600 and 600–100 cm−1
wavenumber ranges were recorded, but the most relevant parts are displayed and discussed. 256
scans were collected for each spectrum. The mid IR spectra were recorded in diffuse reflectance
mode, while for the far IR spectra the Nujol mull technique was used between two polyethylene
windows (the suspension of 10 mg sample and a drop of Nujol mull).
UV–vis spectra were registered on Ocean Optics USB4000 spectrometer with a DH-2000-BAL UV–
vis-NIR light source measuring diffuse reflectance and using BaSO4 as reference. The spectra were
analyzed with the SpectraSuite package.
Electron paramagnetic resonance spectra (EPR) were recorded with a BRUKER EleXsys E500
spectrometer (microwave frequency 9.51 GHz, microwave power 12 mW, modulation amplitude 5 G,
modulation frequency 100 kHz) in quartz EPR tubes at room temperature. Approximately 10 mg of
samples were used for each measurement, and the spectra were recorded without any additional
sample preparation. All recorded EPR spectra were simulated with a home-made computer
program.43
X-ray photoelectron spectra (XPS) were recorded using a SPECS instrument equipped with a
PHOIBOS 150 MCD 9 hemispherical electron energy analyzer using AlKα radiation (hν = 1486.6 eV).
The X-ray gun was operated at 210 W (14 kV, 15 mA). The analyzer was operated in the FAT mode,
with the pass energy set to 20 eV. The step size was 25 meV, and the collection time in one channel
was 250 ms. Typically, 5-10 scans were added up to acquire a single spectrum. Energy referencing
was not applied. In all cases the powder-like samples were evenly laid out on one side of a double-
sided adhesive tape, the other side being attached to the sample holder of the XPS instrument. The
samples were evacuated at room temperature, and then inserted into the analysis chamber of the
XPS instrument.
The topology of the as-prepared sample was studied by atomic force microscopy (AFM) on a NT-
MDT SOLVER scanning probe microscope. Measurement was performed in the tapping mode at
room temperature and atmospheric pressure in air. The non-contact silicon cantilevers had typical
force constant of 42 N m–1 and resonance frequency of 278.8 kHz. For imaging an SSS-NCH-10 tip
(nominal radius of curvature: 2 nm, length: 15 μm) manufactured by NANOSENSORS was used with
0.43 kHz scanning rate and 700 x 700 pixel resolution.
The morphology of the as-prepared sample was studied by scanning electron microscopy (SEM).
The SEM image was registered on an S-4700 scanning electron microscope (SEM, Hitachi, Japan) with
accelerating voltage of 10-18 kV.
More detailed images, both on the as-prepared and the used samples, could be captured by
transmission electron microscopy (TEM), and information on the crystal structures could be gathered
by selected area diffraction (SAED). For these measurements, a FEI TecnaiTM G2 20 X-Twin type
instrument was used operating at an acceleration voltage of 200 kV.
The thermal behavior of the hybrid sample was investigated by thermogravimetry (TG) and
differential thermogravimetry (DTG). The silver-containing sample was studied with a Setaram Labsys
derivatograph operating in air at 5 °C/min heating rate. For the measurements, 20-30 mg of the
samples were applied.
The specific surface areas were measured by the Brunauer-Emmett-Teller (BET) method
(adsorption of N2 at 77 K).44 For these measurements, a NOVA3000 instrument was applied
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(Quantachrome, USA). The samples were flushed with N2 at 100 °C for 5 h to clean the surface of any
adsorbents.
The amount of metal ions was measured by ICP–AES on a Thermo Jarell Ash ICAP 61E instrument.
Before measurements, few milligrams of the samples measured by analytical accuracy were digested
in 1 cm3 cc. nitric acid; then, they were diluted with distilled water to 50 cm3 and filtered.
General procedure for the catalytic reactions
All the materials and reagents were of analytical grade and were used as received without
purification.
A typical procedure for the catalytic alkyne‒nitrile transformation is as follows. Dimethyl sulfoxide
(DMSO, 2 mL), the corresponding alkyne (0.5 mmol, 1 equiv.), trimethylsilyl azide (TMSN3, 1 mmol, 2
equiv.) and the silver-containing hybrid material as catalyst (44.2 mg, corresponding to 5 mol% Ag
loading) were combined in an oven-dried Schlenk tube equipped with a magnetic stir bar. The
reaction mixture was stirred for 24 h at 80 °C. After then, the mixture was cooled to room
temperature, and the catalyst was filtered off. Brine (10 mL) was next added, and the resultant liquid
was extracted with CH2Cl2 (3×10 mL). The combined organic layers were dried over Na2SO4, and
1
concentrated under reduced pressure. The crude samples were checked by H NMR and GC-MS (in
order to determine conversion and product ratio) and were purified by chromatographic techniques
to isolate the desired products. Flash column chromatographic purification (performed on Merck
silica gel 60, particle size 63–200 mm) and analytical thin-layer chromatography (TLC, performed on
Merck silica gel 60 F254 plates) were carried out using mixtures of n-hexane/EtOAc as eluent. Upon
using TLC, compounds were visualized by means of UV or KMnO4.
Investigation of the catalyst reusability
For investigation of the catalyst reusability, the reaction of p-methoxy phenylacetylene with TMSN3
was carried out multiple times utilizing a single portion of catalyst. DMSO (5 mL), p-methoxy
phenylacetylene (1.25 mmol, 1 equiv.), TMSN3 (2.5 mmol, 2 equiv.) and the heterogeneous silver
catalyst (110.5 mg, corresponding to 5 mol% Ag loading) were combined in an oven-dried Schlenk
tube equipped with a magnetic stir bar. The reaction mixture was stirred for 24 h at 80 °C. The
mixture was next cooled to room temperature, and the solid material was removed by
centrifugation. The liquid phase was extracted, dried and evaporated as detailed above. The
removed catalyst was washed with DMSO (four times), and was dried in a nitrogen flow before the
next cycle. The conversion and selectivity were determined after each cycle by using 1H NMR.
Characterization of the reaction products
1
The nitrile products were characterized by NMR spectroscopy and mass spectrometry. H NMR and
13C NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer, in CDCl3 as solvent, with
trimethylsilane as internal standard at 400.1 and 100.6 MHz, respectively. GC-MS analyses were
performed on a Thermo Scientific Trace 1310 Gas Chromatograph coupled with a Thermo Scientific
ISQ QD Single Quadrupole Mass Spectrometer using a Thermo Scientific TG-SQC column (15 m × 0.25
mm ID × 0.25 µ film). Measurement parameters were as follows. Column oven temperature: from 50
to 300 °C at 15 °C min‒1; injection temperature: 240 °C; ion source temperature: 200 °C; electrospray
ionization: 70 eV; carrier gas: He at 1.5 mL min‒1; injection volume: 2 µL; split ratio: 1:33.3; and mass
range: 25–500 m/z. The analytical data can be found in the Electronic Supplementary Information.
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Results and discussion
The proof of the concept – application of the AgBi hybrid in direct alkyne‒nitrile transformations
and comparison to other silver-based catalysts
In order to show that it is worthwhile to spend time and energy on the characterization of the
newly developed Ag-Bi hybrid catalyst (AgBi HM), let us show the model reaction (Scheme 1) and its
performance compared to other silver-based catalyst (Fig. 1).
In order to investigate the catalytic capabilities of the heterogeneous AgBi HM, the direct
nitrogenation of p-methoxy phenylacetylene with TMSN3 as nitrogen source was chosen as a model
reaction (Scheme 1).
N
N
N
HN
N3
silver catalyst
+
TMSN3
+
+
OMe
OMe
OMe
OMe
1
2
3
Scheme 1 Model reaction: nitrogenation of p-methoxy phenylacetylene with TMSN3 as nitrogen source.
DMSO was an initial choice of solvent, and the reaction mixture, containing 1 equiv. of alkyne
(0.25 M) and 2 equiv. of TMSN3, was stirred at 80 °C for 24 h. We were delighted to find complete
conversion in the presence of 10 mol% AgBi HM. 1H NMR analysis of the crude material indicated an
excellent selectivity of 91% towards the formation of 4-methoxybenzonitrile (2). The side product
obtained at an extent of 9% was found to be 5-(4-methoxyphenyl)-1H-1,2,3-triazole (3), which was
presumably formed via a thermal azide‒alkyne cycloaddition as a side reaction.45 This was
corroborated by the finding that without the silver catalyst under otherwise identical conditions, a
conversion of merely 6% occurred and triazole 3 was formed exclusively.
The catalytic performance of the AgBi HM was directly compared with commercially available
Ag(I) salts (Fig. 1). Ag2CO3 and AgBF4 gave comparable results with the hybrid material: the
conversion was quantitative in both cases and the nitrile/triazole ratio was 88:12 and 87:13,
respectively. Further Ag(I) salts furnished incomplete conversions and/or lower product selectivities.
Among the commercial catalysts tested, AgOAc and AgOTf proved less effective. In these instances,
traces of 1-(1-azidovinyl)-4-methoxybenzene (1) could also be detected in the crude product mixture.
On the basis of the reaction mechanism proposed by Jiao and co-workers (Scheme 2),28a following a
silver-mediated alkyne activation and the attack of the azide anion, nitrile formation occurs via a
vinyl azide intermediate.46 The presence of the corresponding vinyl azide byproduct in the reaction
mixture therefore indicates the incompleteness of the reaction.
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-
N3
R
H
[Ag]+
H
R
N3
R
H
Silver-mediated
cycle
[Ag]
[Ag]+
H2O
N3
R
H
H
N
N+
N-
H
R
N
N
HN3
R
N
N
HN3
CH2N2
Scheme 2 Possible mechanism of the silver-mediated formation of nitriles from terminal alkynes.28a
As corroborated by a test reaction carried out with 10 mol% of Bi(NO3)3·5H2O as catalyst, the
Bi(III) component of the hybrid material is inactive as concerns nitrile formation. When the AgBi HM-
catalyzed nitrogenation of p-methoxy phenylacetylene was repeated under argon atmosphere, a
significant conversion decrease occurred verifying the oxidative nature of the reaction.
Fig. 1 Investigation of various catalysts in the direct nitrogenation of p-methoxy phenylacetylene with TMSN3 as
nitrogen source (see Scheme 1). Reaction conditions: 1 equiv. (0.25 M) alkyne, 2 equiv. TMSN3, 10 mol%
catalyst, solvent: DMSO, 80 °C, 24 h reaction time.
Synthesis and characterization of a novel silver-bismuth hybrid material
Our previous experiences with LDH-based materials served as basics of the catalyst synthesis. An
LDH-like structure was desired, because organized layers allow easier control over the catalytic
properties, and Ag(I) prefers octahedral coordination in solid materials.47 Although LDH layers
typically consist of di- and trivalent metal cations,31 our initial concept was supported by the fact that
an LDH with monovalent cationic component (instead of the divalent one) is already known.48 Bi(III)
was chosen as possible trivalent cation, since it also prefers octahedral coordination in solids,49 and
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the ionic radii of Ag(I) and Bi(III) are comparable: 129 and 117 pm, respectively.50 (As was referred
above, we were able to prepare LDH with much larger differences in ionic radii.) Moreover, we
identified a mineral called beyerite (CaBi2O2(CO3)2), which has layered structure, and contains Ca(II)
ions fixed among the layers of BiO(CO)3.51 It was reckoned that if we can build in Ag(I) ions instead of
Ca(II), we may well in business, since we have a structure having Ag(I) ions immobilized in a lattice,
which is less ordered than that of beyerite, thus it may have considerable catalytic activity. All these
factors gave enough inspiration to start with the synthesis.
The urea method was chosen for the co-hydrolysis of the metal salts, since in this way the pH can
precisely be controlled through the temperature of hydrolysis.42 Appropriate amounts of
Bi(NO3)3·5H2O and AgNO3 were thus dissolved in nitric acid and urea was added to the solution. After
stirring for 72 h, the precipitate was filtered, washed, dried and then carefully characterized.
The X-ray diffractograms indicated that the suitable temperature for the co-hydrolysis was 130°C.
At this temperature, a novel silver‒bismuth hybrid material (AgBi HM) was formed (Fig. 2, trace C).
The diffractogram obtained was very different from those of the components (Fig. 2, traces A and B),
and it is not their sum either. It is also seen that applying 100 °C for the co-hydrolysis provided with a
material having similar features than Ag2O–Ag2CO3–AgNO3 after hydrolysis (Fig. 2, trace D), i.e., it is
probably an Ag2O–Ag2CO3 mixture supported over material A. For catalytic purposes, especially in the
liquid phase, this substance would be inappropriate, the silver component will surely be leached out,
since it is not a constituent of the crystal lattice of the support.
Fig. 2 The X-ray patterns of the hydrolyzed Bi(NO3)3·5H2O (A),50 the hydrolyzed mixture of Ag2O–Ag2CO3–AgNO3
(B, +: Ag2O, indices Ag2CO3),53 the Bi(NO3)3·5H2O–AgNO3 mixture co-hydrolyzed at 130°C (C)51 and the
Bi(NO3)3·5H2O–AgNO3 mixture co-hydrolyzed at 100°C (D, +: Ag2O, indices Ag2CO3).
Material A was indexed as Bi2O2CO3,52 material B is a mixture of Ag2O and Ag2CO3,53 material D is
Ag2O–Ag2CO3 supported over BiO2CO3. Carbonation is due mainly to urea as well as the airborne CO2.
In LDH chemistry, carbonation is disadvantageous in most cases, since the carbonate ion is firmly
attached to the positively charged layers preventing its modification via anion exchange. In this
instance, however, it is not a problem, since our goal is fixing Ag(I) with keeping its catalytic activity
on one hand and in a position, which is non-leachable, on the other. As it has been mentioned
already, the X-ray pattern of material C differs from those of the constituents, and is not their sum
either. It resembles the X-ray pattern of beyerite (a mineral with various polymorphs but an
approximate formula of CaBi2O2(CO3)2), but it is definitely not the same.
UV–vis–near IR DRS spectra as well as the EPR spectra also revealed that material C is novel
substance, it is not a physical mixture of materials A and B (Figs. S1 and S2). Regarding this work, this
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is the important information, and these spectra are only used as further supports of the decisive X-
ray diffractograms.
Raman spectroscopy is the method of choice to identify the anionic component(s) (Fig. 3). The
observed shifts can be assigned (Table 1). It is seen that the anion is carbonate in the hybrid material,
and there is no nitrate present. One can arrive at the same conclusion from studying the FT-IR
spectra (Fig. S3, Table S1). The IR spectrum of material C reveals that there is no surface OH.
Fig. 3 The Raman spectra of the hydrolyzed Bi(NO3)3·5H2O (A), the hydrolyzed mixture of Ag2O–Ag2CO3–AgNO3
(B), the Bi(NO3)3·5H2O–AgNO3 mixture co-hydrolyzed at 130 °C (C) and the Bi(NO3)3·5H2O–AgNO3 mixture co-
hydrolyzed at 100 °C (D)
Table 1 Observed Raman shifts and their assignations. A: the hydrolyzed Bi(NO3)3·5H2O, B: the hydrolyzed
mixture of Ag2O–Ag2CO3–AgNO3, C: the Bi(NO3)3·5H2O–AgNO3 mixture co-hydrolyzed at 130 °C, and D: the
Bi(NO3)3·5H2O–AgNO3 mixture co-hydrolyzed at 100 °C
Samples
Raman shift (cm–1)
a
b
c
d
e
f
g
h
i
j
k
A
B
C
D
–
312
–
446
–
542
–
–
–
–
–
–
–
–
284
260
260
–
668
–
–
802
–
1038
–
1072
–
1337
–
–
460
470
–
620
–
714
717
–
–
674
810
1034
1064
1339
a: Ag–O(carbonate) bending,54 b: Bi–O–Bi stretching,55 c: bridging Bi–O–Bi bonds in [BiO6],56 d: Bi=O stretching,57 e: Bi–O
stretching,58 f: ν4 in-plane deformation of the nitrate anion,59 g: ν4 in-plane deformation of the carbonate anion,57 h: ν2 out-
of-plane bending mode of the nitrate anion,59 i: ν1 symmetric mode of the nitrate anion,59 j: Ag–O stretching/bending
modes,60 k: ν3 antisymmetric mode of the nitrate anion60
In order to learn more about the novel AgBi HM, elemental analysis was performed by the ICP–
AES method and a formula of Ag0.17Bi0.88O2CO3 was obtained, for the data, see Table S2.
Since the intended use of the hybrid substance is a heterogeneous catalyst, its thermal stability is
an important feature. Thermal analysis revealed that the original structure was kept up to 380 °C,
then, weight losses occurred in three endothermic steps, and no more weight losses could be
observed from 510 °C until 700 °C, the maximum temperature applied (Fig. 4). The three steps
altogether meant 7.5 w% weight loss (4 w% + 2 w% +1.5 w%). Since there is no surface OH, it should
be due the evolving CO2. On a molar basis, it means 22 g CO2, i.e., half of the carbon content
remained in the heat-treated sample. Consequently, the residue should contain a mixture of oxides,
as well as carbonates. Nevertheless, the most important message is that the temperature in the
hybrid material-catalyzed reactions must be under 380 °C. The three-step weight loss may give some
structural information as well.
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Fig. 4 Thermal analytical curve obtained during the heat treatment of the AgBi HM.
As far as the cationic components of material C is concerned, the XPS spectra attest that the silver
and the bismuth components of the hybrid material are in the +1 and +3 oxidation states,
respectively (Fig. S4).60 The shift of ~0.2 eV in the hybrid catalyst relative indicates some electron
transfer towards Ag+; this seems to be advantageous in our probe reaction of oxidative nature, this
might be the reason of outstanding catalytic activity.
The AFM measurement indicates smooth outer surface (Fig. S5), and the SEM image predicts
lamellar morphology (Fig. 5a), which is verified by the high-resolution TEM measurement (Fig. 4b).
On the enlarged version of the TEM image (Fig. S6a), it was possible to measure the basal spacing
(the thickness of one layer plus the interlayer space), which is the d002 value attested by the SAED
image (Fig. S5b). This value was determined, because it can be directly compared to the one
calculated from X-ray diffraction using the reflection at the lowest angle (Table 2). The table also
contains the BET surface areas as well as the particle size. It is seen that the calculated basal spacing
compares well to the one measured on the TEM image.
(a)
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(b)
Fig. 5 The SEM (a) and the high-resolution TEM (b) images of the AgBi HM. (SEM
microscopy, TEM transmission electron microscopy)
–
scanning electron
–
Table 2 The basal spacing (d from the Bragg equation), the particle thickness (from the Scherrer equation) and
the BET surface area of the AgBi HM
Sample
AgBi HM 0.680
d (nm) particle thickness (nm) BET surface area (m2/g)
29.61
3.0
The basal spacing is small (note that it also contains the thickness of one layer) and the low BET
surface indicates that the interlayer space (i.e. the inner surface) is not accessible even for the
nitrogen molecules. In our model reaction, the sizeable organic molecules (Fig. 1 and the compounds
inTable 5) can only use the outer surface of the AgBi HM, and as the activities observed attest, they
can use it efficiently.
Let us sum up what we have learnt from these measurements. In our hands, we have a hybrid
material containing Ag(I) and Bi(III) cationic and carbonate anionic components, the latter in three
possible positions. The silver ion is the minor cationic component, probably a guest in the bismuth-
carbonate lattice. (Indeed, the X-ray diffractogram resembles that of beyerite [CaBi2O2(CO3)2]). This is
a material with layered structure, keeping this structure up to 380 °C. The Bi(III) ions are constituents
of the layers and they are connected via one of the oxygen atoms the carbonate ions. The silver ions
are attached to the carboxylate end of the carbonate ion, thus forming a separate layer residing
among the Bi(III) containing layers.
Optimization and extension of the probe reaction
Encouraged by the promising preliminary results, we set out to investigate the effects of all the
important reaction parameters in order to determine the optimum conditions for a sustainable
methodology. As a next step, the role of the reaction medium was explored. Strong dependence was
registered on the nature of the solvent applied. Besides DMSO, only dipolar aprotic solvents
exhibiting the amide moiety proved acceptable: N,N-dimethylacetamide (DMA) worked similarly well
as DMSO (the conversion was 90% and the nitrile/triazole ratio was 87:13, Table 3, entries 1 and 3),
whereas the application N,N-dimethylformamide (DMF) and N-methylpyrrolidone (NMP) resulted
lower conversions (72 and 41%, respectively) but still good selectivites towards the formation of
nitrile 2 (84 and 76%, respectively, Table 3, entries 2 and 4). In other solvents investigated, including
polar and non-polar ones, only very low conversions were detected even upon prolonging the
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reaction time to 72 h (Table 3, entries 5‒11). This remarkable dependence is possibly related to the
nucleophilicity of the azide anion and the stability of the trans alkenyl silver complex key
intermediate, which are strongly dependent on the solubilizing properties of the solvent applied.61
The choice of DMSO as preferable reaction medium was further corroborated by the fact that,
according to Pfizer’s solvent selection guide for medicinal chemistry, it is a usable solvent, whereas
most amide solvents are designated as undesirable.62
Table 3 Investigation of various solvents in the AgBi HM-catalyzed nitrogenation of p-methoxy phenylacetylene
with TMSN3 as nitrogen source
Product selectivity (%)a
Conv.
#
Solvent
Time (h)
(%)a
1
2
3
1
DMSO
DMF
24
24
24
24
72
72
72
72
72
72
72
100
72
0
0
0
2
91
9
2
84
16
13
22
3
DMA
90
87
4
NMP
41
76
5
EtOAc
THFc
7
n.d.b
n.d.b
n.d.b
n.d.b
n.d.b
n.d.b
-
6
5
7
MeCN
Toluene
CH2Cl2
Acetone
traces
traces
traces
traces
0
8
9
10
11
n
-hexane
aDetermined by 1H NMR analysis of the crude product.
bNot determined.
cTHF= tetrahydrofuran.
Fig. 6 displays the effects of the reaction time on the progress of the model reaction at 80 °C. As
was expected, the total conversion increases gradually with the reaction time reaching completion
after slightly longer than 16 h. In the initial stages of the reaction, substantial amount of vinyl azide 1
can be detected. It is nicely visible that as intermediate 1 is consumed, the amount of the nitrile
product grows continuously. After 6 h, only traces of vinyl azide remains, and the ratio of nitrile 2
becomes constant at around 90%. It was also found that triazole 3 appeared as side product after 1 h
of stirring at 80 °C, and its ratio in the reaction mixture became steady around 10% after 6 h. These
results led us to conclude that the reaction time should be in the range of 16‒24 h to achieve
optimum results.
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Fig. 6 Investigation of the effects of the reaction time on the AgBi HM-catalyzed nitrogenation of p-methoxy
phenylacetylene with TMSN3 as nitrogen source (see Scheme 1). Reaction conditions: 1 equiv. (0.25 M) alkyne,
2 equiv. TMSN3, 10 mol% catalyst, solvent: DMSO, 80 °C.
In attempts to improve the rate and the selectivity of the reaction, the role of the temperature
was examined carefully (see Fig. S7 for the detailed representation of the experimental data).
Elevation of the temperature resulted substantial increase in the reaction rate (the conversion of p-
methoxy phenylacetylene was 91% after 1 h at 130 °C); however, at the expense of the product
selectivity: higher temperatures promoted the formation of the triazole side product via thermal
azide‒alkyne cycloaddition. For example, at 130 °C a nitrile/triazole ratio of 80:20 occurred after 24 h
of stirring, whereas at 50 °C, triazole 3 was barely formed (to an extent of merely 3%, although
conversion was only 79%). 80 °C was picked as a reaction temperature of choice with an excellent
nitrile/triazole ratio of 91:9 and quantitative conversion in 24 h.
According to the suggested reaction mechanism of the direct alkyne nitrogenation (Scheme 2),
trace amounts of H2O are required for the successful nitrile formation.28a First of all, after the attack
of the azide anion, the resulting trans alkenyl silver intermediate should be hydrolyzed to give the
corresponding vinyl azide, and a small portion of H2O may also be crucial to generate some HN3
necessary for the final vinyl azide‒nitrile transformation. We thus speculated that H2O added to the
reaction mixture might have beneficial role on the reaction rate. In contrast to our assumption, the
presence of extraneous H2O (2 equiv.) had no significant effects on the reaction progress. In a 0.5‒72
h reaction time window, similar conversions were measured as earlier without H2O (see Fig. S8 vs.
Fig. 6); however, in a small amount p-methoxyacetophenone appeared among the reaction products
as consequences of competitive silver-catalyzed alkyne hydration.63 These results suggest that none
of the reaction steps involving the presence of H2O is rate determining.
Besides TMSN3, NaN3 and diphenylphosphoryl azide (DPPA) were also investigated as nitrogen
sources. However, no conversion was detected in case of DPPA, and NaN3 was not working well
either resulting in a conversion of merely 8% with triazole 3 as the main product (Table 4, entries 2
and 3). We tried to reduce the excess of TMSN3 applied, but it was found that 2 equiv. were
necessary for the reaction to complete, lower amounts resulted in sharp decrease in conversion
(Table 4, entries 4 and 5). Gratifyingly, under the previously set conditions (80 °C, 24 h, DMSO as
solvent, 2 equiv. of TMSN3 as nitrogen source), we were able to lower the catalyst loading to 5 mol%
without detectable change in conversion or selectivity (Table 4, entry 6).
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It is well-known that Cu(I) compounds are excellent catalysts for the 1,3-dipolar cycloaddition of
organic azides and alkynes to yield 1,2,3-triazoles.17 In order to exclude the possibility of copper
contaminations in the catalytic material contributing consistently to triazole side product formation,
the synthesis of the AgBi HM was repeated using ultra-high purity (99.999%) silver and bismuth salts.
The model reaction was then carried out with the high purity catalyst under optimized reaction
conditions. The amount of triazole present as a side product was exactly the same as with the
ordinary AgBi HM as catalyst (Table 4, entries 6 vs 8). This finding, in combination with the fact that
Ag(I) itself is not active as catalyst in azide-alkyne cycloadditons,64 suggests that the observed triazole
formation (the ratio of triazole 3 was 10% under optimized conditions) can indeed be explained by
thermal cycloadditon as a competing reaction pathway.
Table 4 Investigation of the effects of various nitrogen sources and fine-tuning of the catalyst loading and the
nitrogen source amount in the AgBi HM-catalyzed nitrogenation of p-methoxy phenylacetylene
Prod.
(%)a
selectivity
N. source Catalyst
N.
source
Conv.
(%)a
#
amount
(ekv)
loading
(mol %)
1
2
3
1
2
3
4
5
6
7
8c
TMSN3
NaN3
2
10
10
10
10
10
5
100
8
0
0
91
18
9
2
82
DPPA
2
0
n.d.b
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
1.5
1.1
2
91
80
100
76
100
0
0
0
2
0
87
13
11
10
9
89
90
2
1
89
2
5
90
10
aDetermined by 1H NMR analysis of the crude product.
bNot determined.
cUltra-high purity AgBi HM was used as catalyst (synthesized from 99.999% pure metal salts).
A range of alkynes exhibiting diverse substitution patterns were next submitted to the optimized
reaction conditions. We were delighted to find quantitative conversions and selectivities around 90%
in reactions of phenylacetylene and its substituted derivatives containing methyl, tert-butyl or bromo
moieties (Table 5, entries 1‒5). Even multisubstituted phenylacetylene derivatives and 3-
ethynylthiophene gave outstanding results (Table 5, entries 6‒8), although they are known to be less
reactive in the silver-mediated nitrogenation.28a Phenyl propargyl ether, benzoic acid propargyl ester
and p-nitrobenzoic acid propargyl ester displayed excellent reactivities (conversions were in the
range 95‒100%); however, enhanced triazole formation led to slightly lower selectivities (in the
ranger of 73‒81%, Table 5, entries 9‒11). The nitrogenation of aliphatic alkynes went smoothly, open
chain alkynes with varying chain length and a cyclic one were also transformed into the
corresponding nitriles with excellent conversions (in the range of 96‒100%) and selectivities around
90% (Table 5, entries 12‒15). Reactions of internal alkynes (such as ethyl phenylpropiolate or oct-4-
yne) were also attempted; however, in these cases no nitrile formation occurred.
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Table 5 Substrate scope of the AgBi HM-catalyzed direct alkyne nitrogenation
Prod.
(%)a
selectivity
Conv.
(%)a
#
Alkyne
I
II
III
10
9
1
2
3
4
5
100
100
100
100
100
0
0
0
0
0
90
91
92
90
93
Me
Br
8
10
7
tBu
6
7
100
100
0
0
94
94
6
6
8
9
100
100
0
0
90
81
10
19
10
11
100
95
0
0
74
73
26
27
12
13
14
100
100
96
0
0
0
89
90
93
11
10
7
15
98
0
87
13
aDetermined by 1H NMR or GC-MS analysis of the crude product.
In order to verify the heterogeneous nature of the reaction, the hot filtration test was attempted.
The nitrogenation of p-methoxy phenylacetylene with TMSN3 as nitrogen source was carried out
under the optimized reaction conditions (80 °C, DMSO as solvent, 2 equiv. of TMSN3, 5 mol% catalyst
loading). After 1 h, the AgBi HM catalyst was filtered off, and the filtrate was stirred at 80 °C for
another 23 h. As can be seen in Fig. 7, the conversion of p-methoxy phenylacetylene was 65% after 1
h, and remained the same after 1+23 h. It is worth mentioning that after 1 h, the product distribution
of the reaction was as follows: 47% vinyl azide 1, 49% nitrile 2 and 4% triazole 3. According to our
NMR measurements, after 1+23 h, all vinyl azide intermediate got consumed (as nicely demonstrated
by the disappearance of the alkenyl CH2 signals at 4.9 and 5.3 ppm in Fig. 7) and thermal triazole
formation proceeded somewhat further to yield a final nitrile/triazole ratio of 89:11. These results
are in good accordance with the mechanistic observations of Jiao and co-workers that it is only the
initial hydroazidation step, which is silver-mediated, the final vinyl azide‒nitrile transformation
requires no silver catalyst.28a
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1
Fig. 7 H NMR spectroscopic analysis (400.1 MHz, CDCl3) of the AgBi HM-catalyzed reaction of p-methoxy
phenylacetylene with TMSN3 as nitrogen source (see Scheme 1). Reaction conditions: 1 equiv. (0.25 M) alkyne,
2 equiv. TMSN3, 5 mol% catalyst, solvent: DMSO, 80 °C. After 1 h, the catalyst was removed, and the filtrate
was stirred for another 23 h (hot filtration). Ar-H signals are marked with blue rectangles.
Theoretically, one of the primary benefits of heterogeneous catalysis is the possibility of recycling
and reusing the catalytic material, but, in practice, negative effects such as leaching of the active
components, erosion of the catalyst structure and irreversible substrate deposition may significantly
limit the sustainable application of these materials.29 Although the hot filtration test verified the
predominantly heterogeneous nature of the AgBi HM-catalyzed nitrogenation reaction, we were
intrigued to explore the robustness and reusability of our catalyst. The reaction of p-methoxy
phenylacetylene with TMSN3 was carried out repeatedly (conditions were as follows: 80 °C, DMSO as
solvent, 2 equiv. of TMSN3, 5 mol% catalyst loading) utilizing the same portion of AgBi HM as catalyst.
Between each cycles, the catalyst was removed by centrifugation and reused after washing and
drying. We were delighted to find that no decrease in activity or selectivity occurred even after 10
consecutive catalytic cycles (Fig. 8). The conversion of p-methoxy phenylacetylene was steady in the
range of 97‒100% and the selectivity towards the anticipated nitrile product was found to be around
90% in all reactions. Besides small amounts of triazole 3, no vinyl azide byproduct was detected in
any of the crude product mixtures analyzed.
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Fig. 8 Testing the reusability of the AgBi HM catalyst in the direct nitrogenation of p-methoxy phenylacetylene
with TMSN3 as nitrogen source (see Scheme 1). Reaction conditions: 1 equiv. (0.25 M) alkyne, 2 equiv. TMSN3, 5
mol% catalyst, solvent: DMSO, 80 °C, 24 h reaction time.
The catalyst sample used in repeated runs were structurally characterized by many of the
methods applied for the freshly made material. The results of the most important methods, which
are decisive in determining whether our material is a real efficiently recyclable catalyst with high
stability, are as follows. The post-reaction X-ray pattern (Fig. 9a) looks the same as trace C in Figure 1.
Even indexing may be attempted using that of beyerite as a known close analog. The Raman spectra
of the freshly-made and the used samples are also very similar (compare trace C in Figure 3 to the
spectrum in Figure S9) indicating that the structure did not change, the anionic component (the
carbonate ion), the silver ion attachment to the carbonate oxygen all remained the same. The ICP–
AES measurement performed on the used catalyst gave a formula of Ag0.17Bi0.91O2CO3 (Table S3, last
row), which is practically the same as the freshly-made one, i.e., the sample survived the recycling
test without losing any of its components. The XPS spectra revealed that there was no change in the
oxidation states of any of the cationic components (Fig. S10). The high-resolution TEM image
indicates (Fig. 9b) that the layered structure is kept in the used catalyst sample as well. Taking into
account all the characterization data of the used sample, we can conclude that the hybrid material
proved to be a highly robust heterogeneous catalysts preserving its structure even after extended
use.
(a)
(b)
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Fig. 9 The X-ray pattern (a) and high-resolution TEM image (b) of the used catalyst sample. (TEM
– transmission
electron microscopy)
Conclusions
An Ag(I)Bi(III)-containing hybrid material (Ag0.17Bi0.88O2CO3) was synthesized as inspired by a naturally
occurring mineral and on the analogy of layered double hydroxides. It proved to be a robust,
efficiently recyclable, highly active catalyst in the direct synthesis of organic nitriles from terminal
alkynes. The most important reaction conditions determining the reaction rate and the product
selectivity were carefully optimized in order to achieve high-yielding nitrile formation and minimal
waste generation. A diverse set of aromatic and as well as aliphatic alkynes were successfully
transformed into the corresponding nitriles with small amounts of triazole as the only unwanted
product. The heterogeneous nature of the reaction was demonstrated by the completion of the hot
filtration test. Importantly, the solid catalyst could be recycled and reused ten times without loss of
activity and without detectable structural degradation. To the best of our knowledge, the first
example for a heterogeneous noble metal catalyzed nitrile synthesis via alkyne activation has been
presented herein. Due to its outstanding activity, selectivity reusability and robustness, the catalyst
reported may contribute towards the development of sustainable silver-mediated processes and may
find applications in the synthesis of valuable organic compounds.
Acknowledgments
This work was supported by the National Science Fund of Hungary through grants OTKA NKFI K
115731 and GINOP-2.3.2-15-2016-00013. SBÖ acknowledges the Premium Post Doctorate Research
Program of the Hungarian Academy of Sciences. RM and GV were supported by the UNKP-17-3 New
National Excellence Program of the Ministry of Human Capacities. The financial help is highly
appreciated. Bertalan Oszkó (University of Szeged) and László Korecz (Research Center for Natural
Sciences of the Hungarian Academy of Sciences, Budapest) are thanked for registering the X-ray
photoelectron and the electron paramagnetic spectra, respectively.
Author Information
There are no conflicts to declare.
Notes and references
Electronic supplementary information (ESI) available: UV-vis, IR, EPR, XPS, ICP-AES spectra and
spectral data, AFM, TEM and SAED images on the fresh AgBi HM sample; Raman, XPS and ICP-AES
data on the used catalyst sample; additional reaction optimization data; NMR and MS data of the
reaction products. See DOI:
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Graphical abstract
A silver-containing hybrid material is reported as an effective heterogeneous catalyst for the direct
synthesis of organic nitriles from terminal alkynes.
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