Facile Access to Amides from Oxygenated or Unsaturated Organic Compounds by Metal Oxide Nanocatalysts Derived from Single-Source Molecular Precursors

Article abstract of DOI:10.1021/acs.inorgchem.7b01576

Oxidative amidation is a valuable process for the transformation of oxygenated organic compounds to valuable amides. However, the reaction is severely limited by the use of an expensive catalyst and limited substrate scope. To circumvent these limitations, designing a transition-metal-based nanocatalyst via more straightforward and economical methodology with superior catalytic performances with broad substrate scope is desirable. To resolve the aforementioned issues, we report a facile method for the synthesis of nanocatalysts NiO and CuO by the sol-gel-assisted thermal decomposition of complexes [Ni(hep-H)(H₂O)₄]SO₄ (SSMP-1) and [Cu(μ-hep)(BA)]₂ (SSMP-2) [hep-H = 2-(2-hydroxylethyl)pyridine; BA = benzoic acid] as single-source molecular precursors (SSMPs) for the oxidative amidation of benzyl alcohol, benzaldehyde, and BA by using N,N-dimethylformamide (DMF) as the solvent and as an amine source, in the presence of tert-butylhydroperoxide (TBHP) as the oxidant, at T = 80 °C. In addition to nanocatalysts NiO and CuO, our previously reported Co/CoO nanocatalyst (CoNC), derived from the complex [Co^II(hep-H)(H₂O)₄]SO₄ (A) as an SSMP, was also explored for the aforementioned reaction. Also, we have carefully investigated the difference in the catalytic performance of Co-, Ni-, and Cu-based nanoparticles synthesized from the SSMP for the conversion of various oxygenated and unsaturated organic compounds to their respective amides. Among all, CuO showed an optimum catalytic performance for the oxidative amidation of various oxygenated and unsaturated organic compounds with a broad reaction scope. Finally, CuO can be recovered unaltered and reused for several (six times) recycles without any loss in catalytic activity.

Full text of DOI:10.1021/acs.inorgchem.7b01576

Article
pubs.acs.org/IC
Facile Access to Amides from Oxygenated or Unsaturated Organic
Compounds by Metal Oxide Nanocatalysts Derived from Single-
Source Molecular Precursors
†
‡
†
⊥
,†,‡,§
Akbar Mohammad, Prakash Chandra, Topi Ghosh, Mauro Carraro, and Shaikh M. Mobin*
†
‡
§
Discipline of Chemistry, Discipline of Metallurgy Engineering and Materials Science (MEMS), and Centre for Biosciences and
Bio-Medical Engineering, Indian Institute of Technology (IIT) Indore, Simrol, Khandwa Road, Indore 453552, India
⊥Department of Chemical Sciences, University of Padova and ITM-CNR, Via Marzolo 1, 35131 Padova, Italy
*
S Supporting Information
ABSTRACT: Oxidative amidation is a valuable process for the transformation of oxygenated organic compounds to valuable
amides. However, the reaction is severely limited by the use of an expensive catalyst and limited substrate scope. To circumvent
these limitations, designing a transition-metal-based nanocatalyst via more straightforward and economical methodology with
superior catalytic performances with broad substrate scope is desirable. To resolve the aforementioned issues, we report a facile
method for the synthesis of nanocatalysts NiO and CuO by the sol−gel-assisted thermal decomposition of complexes [Ni(hep-
H)(H O) ]SO (SSMP-1) and [Cu(μ-hep)(BA)] (SSMP-2) [hep-H = 2-(2-hydroxylethyl)pyridine; BA = benzoic acid] as
2
4
4
2
single-source molecular precursors (SSMPs) for the oxidative amidation of benzyl alcohol, benzaldehyde, and BA by using N,N-
dimethylformamide (DMF) as the solvent and as an amine source, in the presence of tert-butylhydroperoxide (TBHP) as the
oxidant, at T = 80 °C. In addition to nanocatalysts NiO and CuO, our previously reported Co/CoO nanocatalyst (CoNC),
II
derived from the complex [Co (hep-H)(H O) ]SO (A) as an SSMP, was also explored for the aforementioned reaction. Also,
2
4
4
we have carefully investigated the difference in the catalytic performance of Co-, Ni-, and Cu-based nanoparticles synthesized
from the SSMP for the conversion of various oxygenated and unsaturated organic compounds to their respective amides. Among
all, CuO showed an optimum catalytic performance for the oxidative amidation of various oxygenated and unsaturated organic
compounds with a broad reaction scope. Finally, CuO can be recovered unaltered and reused for several (six times) recycles
without any loss in catalytic activity.
6
,7
INTRODUCTION
nitriles, aminocarbonylation of aryl halides, rearrangement of
■
8
9
oximes, amidation of aryl halides using isocyanides, carbon-
ylative synthesis from alkanes, primary amines, and CO, and
oxidative amidation of alcohols with amines.
The development of a highly efficient and environmentally
sound methodology for the synthesis of amides, from
inexpensive and abundant feedstocks, is highly desirable both
in academic research and for industrial applications.
Recently, the use of straightforward methodologies for the
The amide functionality is ubiquitous in life and plays a critical
role in virtually all biological processes. The amide bond is
prevalent in most biomolecules, and it is consistently found as a
cardinal structural element in several fine chemicals, pharma-
ceuticals, and agrochemicals. Moreover, polymers based on
the amide linkage, such as polyamides (nylons, Kevlar), play a
pivotal role in a wide range of applications, including drug
10
1
1
1
,2
3
,6,7
3
,4
delivery, wound healing, and adhesive fabrication. Activation
of a carboxylic acid and subsequent coupling of the activated
species with an amine is predominantly utilized for amide
12
13
14
direct amination of nitriles, carboxylic acids, aldehydes,
3
,5
synthesis. However, there are several other methods for the
introduction of an amide functional group, such as hydration of
Received: June 20, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01576
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1
5
8,16
17
methylarenes, aryl esters,
and alcohols, has become a
Scheme 1. Synthesis of SSMP-1 and SSMP-2
topical area of research. The procedures consist of CO-free
oxidative couplings with N,N-dialkylformamides; for example,
N,N-dialkylated amides were synthesized using nitriles and
N,N-dimethylformamide (DMF) or diethylformamide under an
O atmosphere, in the presence of Cu O and 1,10-phenanthro-
2
2
1
8
14
line. Li et al. used a Co-based catalyst on carbonized organic
ligands (Co@C−N) and DMF, obtaining an excellent catalytic
performance for the transformation of aldehydes into N-
alkylated amides in the presence of tert-butylhydroperoxide
16
(
TBHP) as the oxidant. Zhaorigetu et al. reported the
catalytic transformation of aromatic and heteroaromatic esters
to N-alkylated amides using a nanosized palladium catalyst,
N,N-dialkylformamides, and TBHP. However, despite consid-
erable accomplishments in amide formation, all of these
reactions are often associated with some limitations, such as
the use of less available starting materials, expensive noble
metal catalysts or ligands, need for harsh reaction conditions,
12,18,19
use of toxic solvents, and massive production of waste.
thermal decomposition of their respective SSMP-1 and SSMP-
The relatively new concept of using single-source molecular
precursor (SSMP), for the preparation of novel catalytic
species, can be helpful to overcome these issues. In particular,
single-source approaches involve the use of an organometallic
or metal−organic molecule as a precursor for the synthesis of
2
, in an aqueous solution within the pH range of 2.0−2.5, using
citric acid (1.0 M) as the proton source as well as stabilization
of the nanoparticles and control of their nucleation during
thermal decomposition of the molecular precursor (Figure 1).
33
20,21
Molecular Structures of SSMP-1 and SSMP-2. The
molecular structures of SSMP-1 and SSMP-2 were authenti-
cated by their single-crystal X-ray diffraction (XRD) studies.
SSMP-1 crystallizes in the noncentrosymmetric, monoclinic,
transition-metal/metal oxide-based nanoparticles.
SSMPs
provide several key advantages over other routes because they
are air-stable, nontoxic, and easy to handle. Moreover, they
employ clean, low-temperature decomposition routes and yield
crystalline nanomaterials with minimal impurity incorpora-
P2 space group (Table S1). The asymmetric unit of SSMP-1
1
2
2
II
tion.
Transition-metal/metal oxide nanoparticles prepared by
consists of one Ni ion, one bidentate hep-H ligand, and four-
coordinated water molecules. The overall charge on SSMP-1
2
3,24
25
2−
SSMPs have been used in catalysis
as well as in sensing
was balanced by the counterion SO
4
. The N and O atoms
26
and for the preparation of electrochromic materials. For
example, Co O nanorods prepared by the precipitation of
from hep-H and two coordinated water molecules constitute
the basal plane, and the remaining two water molecules occupy
the axial position with elongated bond lengths, forming a
3
4
cobalt acetate using sodium carbonate have been utilized for
27
II
low-temperature CO oxidation. Monodispersed Ni and NiO
nanoparticles synthesized by reduction of the nickel oleylamine
distorted octahedral geometry all over the Ni ion. SSMP-2
̅
crystallizes in the centrosymmetric, triclinic, P1 space group
28
complex were utilized for Suzuki coupling reaction. The
cobalt/cobalt oxide nanocatalyst (Co/CoO or CoNC),
with two half-molecules in an asymmetric unit (Table S1). Each
II
Cu ion in the dimeric unit is in the five-coordinated NO
4
II
synthesized by reduction of the cobalt complex [Co (hep-
environment, where the basal plane consists of one O atom
H)(H O) ]SO (A; hep-H = 2-(2-hydroxylethyl)pyridine)],
from BA, two O atoms [bis(μ -O)], and one N atom from the
hep ligand, while the remaining site is occupied by another O
2
4
4
2
−
was used for the selective reduction of polyaromatic nitro
compounds to their respective amines under ambient
atom of BA with an elongated bond length, thus yielding a
distorted square-pyramidal geometry. The perspective views of
SSMP-1 and SSMP-2 are shown in Figure 1.
Characterization of the Nanocatalysts (NiO and CuO).
The nanocatalysts NiO and CuO were fully characterized by
various physicochemical techniques, such as powder XRD,
transmission electron microscopy (TEM), selected-area elec-
tron diffraction (SAED), field-emission scanning electronic
microscopy (FESEM), energy-dispersive X-ray spectroscopy
2
9
i
conditions, using NaBH as a reducing agent. ( PrO) Ta-
[
4
2
t
t
t
OSi(O Bu) ] and [( BuO) Ti{μ-O Si[OSi(O Bu) ] }] were
3 3 2 2 2
30 31
3 2
IV
utilized as SSMPs to prepare Ta O ·xSiO and Ti /SiO ,
2
5
2
2
respectively, which were used to obtain selective cyclohexene
epoxidation.
Herein, we have investigated the synthesis of N-alkylated
amides starting from alcohols, aldehydes, acids, and unsaturated
compounds using DMF as the solvent as well as the amine
source, in the presence of cobalt, nickel, and copper oxide-
based nanocatalysts, obtained by their respective SSMPs.
(
EDAX), and X-ray photoelectron spectroscopy (XPS).
The crystallinity and phase purity of the NiO and CuO
nanocatalysts were determined by XRD spectra and are shown
in Figure 2. The XRD patterns clearly show the formation of
NiO nanoparticles with 2θ = 37.2°, 43.3°, 62.9°, 75.4°, and
79.3°, which can be readily indexed as the 111, 200, 220, 311,
and 222 crystal planes of bulk NiO, having a face-centered-
RESULTS AND DISCUSSION
Synthesis of SSMPs. The reaction of NiSO ·6H O with
■
4
2
hep-H in methanol (MeOH) and Cu(CH COO) ·H O, hep-H,
3
2
2
and benzoic acid (BA) in acetonitrile at room temperature
II
34
yielded the SSMPs [Ni (hep-H)(H O) ]SO (SSMP-1) and
cubic NiO phase (JCPDS 71-1179). The crystalline nature of
2
4
4
II
[
Cu (μ-hep)(BA)] (SSMP-2) (Scheme 1).
the sample was further confirmed by the sharpness and
intensity of the peaks of the synthesized sample. The average
crystallite size of the nanoparticles was determined using the
2
The synthesis and characterization of CoNC by employing a
29,32
SSMP was recently reported by us.
Two new nanocatalysts,
35
NiO and CuO, have been obtained by the sol−gel-assisted
Scherrer equation.
B
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Figure 1. Preparation of NiO (A) and CuO (B) nanocatalysts from SSMP-1 and SSMP-2. Perspective views show the molecular structures of
SSMP-1 and SSMP-2 (the other molecule in the asymmetric unit of SSMP-2 has been omitted for clarity), while SEM images show the morphology
of the nanocatalysts.
Figure 2. XRD spectra of fresh and spent NiO (A) and CuO (B) nanocatalysts.
Kλ
indicating the crystalline nature (bright spots) of NiO (Figure
D). SEM−EDAX analysis of fresh and spent catalysts showed
not many changes in the morphological characteristics (Figures
S3A−C and S4A−C), while Ni and O atoms in the same
atomic ratio confirmed that there was no leaching of nickel
from the catalyst surface (Figures S3D and S4D).
B(2θ) =
3
L cos θ
The average crystallite size of the NiO nanoparticles was
found to be 20.6 nm. No peak due to Ni(OH) was observed
by XRD. The XRD pattern of the CuO nanoparticles showed
2
monoclinic structures with 2θ = 32.3°, 35.4°, 38.5°, 48.7°,
SEM and HRTEM analysis of the CuO nanocatalyst
confirmed that the rod-shaped morphology may be due to
the agglomeration of several crystallites with widths of
approximately ∼10−15 nm and lengths of ∼100−150 nm
(Figures 4A−C and S5A−C). The SAED pattern obtained
from HRTEM for the CuO nanocatalyst showed a concentric
ring pattern, indicating the crystalline nature of the material
(Figure 4D). Similarly, SEM−EDAX analysis of fresh and spent
CuO catalysts also showed not many changes in the
morphological characteristics (Figures 1 and S5A−C and
S6A−C) and confirmed that there was no leaching of the
metal from the copper oxide nanoparticle surface (Figures S5D
and S6D).
53.4°, 57.9°, 61.5°, 66.1°, 67.9°, 72.1°, and 75.2°, which can be
readily indexed as 110, −111, 111, 20−2, 020, 202, 11−3, 31−
36
1
, 220, 311, and 22−2 planes (JCPDS 48-1548). The average
crystallite size of the CuO nanoparticles was found to be 13.8
nm. Additionally, we have also performed XRD for spent
catalysts (NiO and CuO), which show no additional peaks,
confirming the purity of the catalyst used.
The size and morphology of the NiO and CuO nanocatalysts
were characterized using FESEM and high-resolution TEM
(
HRTEM). FESEM and HRTEM photomicrographs of the
NiO nanocatalyst showed spherical shape having approximately
20−30 nm diameter with smooth and uniform morphology
Figures 1, 3A−C, and S3A−C). The SAED pattern of NiO
obtained from HRTEM showed a concentric ring pattern,
∼
(
Further, XPS analysis was performed to obtain the elemental
composition and chemical and electronic states of the CoNC,
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The Cu 2p peak of the XPS spectrum of CuO is shown in
Figure 6; the two peaks centered at 933 and 953 eV were
assigned to Cu 2p3/2 and Cu 2p1/2 of CuO, respectively. The
presence of two extra shakeup satellite peaks at relatively higher
binding energies, 943 eV, also indicate the existence of CuO on
36,38
the surface.
The XPS spectra of fresh and spent catalysts
were identical, clearly indicating that there was no change in the
oxidation state of Cu2+ even after six recycles. The peak at
∼
530 eV (Figure S7F,H) represents O 1s spectra of the fresh
and spent CuO nanocatalysts. XPS analysis also confirmed the
0
absence of Cu in the fresh and spent CuO nanocatalysts.
Catalytic Activity of the Nanocatalysts. Catalytic
oxidative amidation of oxygenated organic compounds
(
alcohols, aldehydes, and acids) was carried out by the series
of nanocatalysts CoNC, NiO, and CuO employing DMF as the
solvent and TBHP as the oxidative reagent at a temperature of
14
8
0 °C (see Chart 1), as optimized by Li et al. CoNC gave
moderate conversion of benzyl alcohol (78%) with a poor
selectivity (25%) for the desired product, N,N-dimethylbenza-
mide (1). With benzaldehyde, a 77% conversion was observed
without any formation of 1, while N-methylbenzamide (2) was
obtained with 18% selectivity. BA (4) was formed as a major
product with 71% selectivity. Under identical reaction
conditions, without using the catalyst, no reaction was
observed, hence confirming that the reaction was truly catalytic.
Similarly, NiO resulted in 83% conversion of benzyl alcohol,
but with only 8% and 20% selectivity for 1 and 2, respectively,
and 49% selectivity for 4. With benzaldehyde, an improved
catalytic performance was observed, with 92% conversion and
Figure 3. HRTEM images of the NiO nanocatalysts with 200 nm (A),
1
00 nm (B), 50 nm (C) scale bars, and SAED pattern at 2 1/nm (D).
5
0%, 11%, and 21% selectivity for 1, 2, and 4. Both CoNC and
NiO showed poor catalytic performance with BA, with 5% and
% conversion, respectively. On the contrary, the CuO
8
nanocatalyst effectively catalyzed the oxidative amidation of
benzyl alcohol, benzaldehyde, and BA with 92%, 91%, and
1
00% conversion, and their selectivities for 1 were 72%, 89%,
and 100%, respectively (see Chart 1). The isolated yield was
obtained with the help of column chromatography for the
oxidative amidation of benzyl alcohol, benzaldehyde, and BA to
1
. The yields of benzyl alcohol, benzaldehyde, and BA were
found to be 95 mg (64%), 115 mg (77%), and 145 mg (97%),
respectively. The isolated yields obtained from column
chromatography are thus close to those obtained from gas
chromatography−mass spectrometry (GC−MS). Commer-
cially available CuO was also investigated for catalytic
performance for the oxidative amidation of benzaldehyde.
The catalytic reaction results showed 78% benzaldehyde
conversion with 57% selectivity for 1, clearly showing that
CuO prepared from SSMP-2 showed a better catalytic
performance compared to the commercially available CuO.
The order of effectiveness of nanocatalysts in catalyzing the
oxidative amidation of alcohols, aldehydes, and acids was thus
found to be CuO > NiO > CoNC (Chart 1).
A comparative study of Co-, Ni-, and Cu-based nanoparticles
and their respective SSMPs was performed. For benzyl alcohol,
Co- and Ni-based SSMPs showed 75% and 60% conversion of
benzyl alcohol and 45% and 50% selectivity for 1, respectively.
For benzaldehyde, similar catalytic performances were obtained
with both Co- and Ni-based SSMPs. These showed better
catalytic activity for BA with 62% and 57% conversion and 58%
and 70% selectivity for 1, respectively. In this case, the lower
reactivity of Co- and Ni-based nanocatalysts may be due to the
decreased availability of active sites.
Figure 4. HRTEM images of the CuO nanocatalyst with 0.5 μm (A),
0 nm (B), 2 nm (C) scale bars, and SAED pattern at 2 1/nm (D).
2
NiO, and CuO nanocatalysts. All of the binding energies were
calibrated using the C 1s peak at 284.6 eV. The XPS spectra of
29
CoNC were discussed in our previous report. The Ni 2p
region comprises four easily discernible features: the Ni 2p3/2
main peak and its satellites at ∼853 and ∼859 eV and the Ni
3
7
2p
main peak at ∼872 eV (Figure 5). XPS spectra were
1/2
also collected after five catalytic recycles, showing identical
spectra for the fresh and spent catalysts, clearly indicating that
2
+
there was no change in the oxidation state of Ni . The peak at
530 eV (Figure S7B,D) represents O 1s spectra of the fresh
∼
and spent NiO nanocatalysts. XPS analysis also confirmed the
0
absence of Ni in both fresh and spent catalysts.
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Figure 5. XPS spectra of (A) fresh NiO, (B) Ni present in NiO, (C) spent NiO, and (D) Ni present in spent NiO.
Figure 6. XPS spectra of (A) fresh CuO, (B) Cu present in CuO, (C) spent CuO, and (D) Cu present in spent CuO.
SSMP-2 showed 100%, 76%, and 96% conversion of benzyl
alcohol, benzaldehyde, and BA, respectively, with 43%, 69%,
and 72% selectivity for 1. The comparative study for Cu-based
SSMP-2 and CuO nanoparticles shows that the CuO displays
higher conversion of benzaldehyde, benzyl alcohol, and BA and
higher selectivity for 1. The outcome of the comparative
catalytic performance also shows that CuO nanoparticles
outperform the other transition-metal-based nanoparticles for
the catalytic oxidative amidation of oxygenated hydrocarbons,
confirming their superior performance (Chart 2). Further, CuO
shows excellent recyclability, easily separable nature, and higher
selectivity compared to SSMPs and the other nanocatalysts
discussed above.
E
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Chart 1. Comparative Studies for the Reaction of Benzyl
Alcohol, Benzaldehyde, and BA by Employing CoNC, NiO,
and CuO Nanocatalysts
Chart 2. Comparative Studies for the Reaction of Benzyl
Alcohol, Benzaldehyde, and BA by Employing Co-, Ni-, and
Cu-Based SSMPs
a
a
a
Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv; SSMPs as
the catalyst = 10 mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time
=
20 h. The substrate conversion and product selectivity were
determined by GC−MS.
form III; simultaneously, the DMF molecule reacts with I or II
to form IV and V. Finally, III and V react to form the desired
N-alkylated amide. It is worth noting that no intermediate
benzaldehyde can be observed along with the other products. A
similar pathway is also possible when using aldehyde as the
starting material (Scheme 2B).
a
Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv; catalyst =
0 mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time = 20 h. The
1
substrate conversion and product selectivity were determined by GC−
MS.
Carboxylic acid follows a slightly divergent mechanism
compared to the corresponding alcohols or aldehydes. The
experimental results reveal that CoNC and NiO show poor
catalytic performance with carboxylic acid, probably because of
their inefficiency in activating the same to form VI. CuO, on
the other hand, effectively catalyzes the generation of VI from
the carboxylic acid molecule, therefore showing superior
catalytic performance compared to CoNC and NiO. In situ
generated IV and VI react with each other to form VII. In
conclusion, VII loses a carbon dioxide molecule to form the
desired N-alkylated amide (Scheme 2C). The formation of tert-
Probable Mechanism for an N-Alkylated Amide from
Alcohols, Aldehydes, and Acids. According to a previous
report, oxidative amidation in the presence of transition metals
such as Co-, Ni-, or Cu-based oxides proceeds via the radical-
chain mechanism (Scheme 2). Catalytic oxidative amidation of
alcohols or aldehydes begins with the activation of TBHP by
CoNC, NiO, or CuO via the formation of radicals I and II
39
(
Scheme 2A). In the case of alcohol, TBHP in the presence of
13,39
CoNC, NiO, or CuO fosters its oxidation to the aldehyde.
butyl benzoperoxoate as a major intermediate
has been
Further, the resulting aldehyde molecule reacts with I or II to
ruled out by a control experiment, involving the use of this
F
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Scheme 2. Reaction Mechanism for Synthesis of N-Alkylated Amide from Alcohols, Aldehydes, and Acids Using DMF in the
Presence of CoNC, NiO, and CuO Nanocatalysts
peroxoester as a substrate under identical conditions (Chart
S1). This latter, indeed, was quantitively converted to a mixture
containing N,N-dimethylbenzamide (56% selectivity) and N-
methylbenzamide (30% selectivity).
G
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These interesting results further encouraged us to broaden
the substrate scope, including the catalytic oxidative amidation
of substituted derivatives of benzaldehyde and BA and of five-
membered ring reactants (Scheme 3, Tables 1 and 2), under
Table 1. Oxidative Amidation of Substituted Aldehydes and
Acids Using a NiO Nanocatalyst
b
Scheme 3. Catalytic Oxidative Amidation of Various Organic
Substrates Using a NiO or CuO Nanocatalyst in the
Presence of TBHP as the Oxidant and DMF as the Amine
Source
identical reaction conditions, using NiO and CuO nanocatalysts
(Scheme 3). Oxidative amidation of substituted benzaldehydes
in the presence of NiO (Table 1, entries 1−7) showed
moderate catalytic performance with low selectivity for the
tertiary amides. The corresponding carboxylic acids were
indeed formed as major byproducts during the catalytic
reaction. Substituted carboxylic acids (Table 1, entries 8 and
9
) showed poor catalytic performance with low conversions
and moderate selectivity for N,N-dialkylated amides. Five-
membered rings such as furfural (Table 1, entry 10) showed
complete conversion but moderate (40%) selectivity for its
N,N-dialkylated amide. 2-Thiophenecarboxylic acid (Table 1,
entry 11) presented a better conversion (75%) and selectivity
(
51%) for the tertiary amide.
Owing to its better performance, also CuO was investigated
to establish a broader reaction scope, including various
substituted substrates (Scheme 3 and Table 2). Excellent
conversions and selectivity toward tertiary amides were indeed
obtained with BAs substituted with both electron-donating as
well as electron-withdrawing groups attached to the aromatic
ring (Table 2, entries 1−5). A similar trend was observed in the
case of aldehydes and alcohols (Table 2, entries 6−17). Six-
membered heterocyclic N-containing compounds, pyridin-2-
ylmethanol and picolinic acid, gave excellent performance with
CuO with high conversion and selectivity for N,N-dimethylpi-
colinamide (Table 2, entries 18 and 19). Five-membered
heterocyclic compounds containing O and S atoms also gave
promising results: furfural aldehyde presented 92% conversion
and more than 97% selectivity for N,N-dimethylfuran-2-
carboxamide; thiophene-2-carbaldehyde and thiophene-2-car-
boxylic acid showed 100% conversion with 93% and 97%
selectivity, respectively, for N,N-dimethylthiophene-2-carbox-
amide (Table 2, entries 20−22).
After achieving promising catalytic activities of the CuO
nanocatalyst toward alcohols, aldehydes, and acids, we further
explored the catalytic activity of the CuO nanocatalyst toward
unsaturated compounds under identical conditions (Table 3).
To our surprise, phenyl acetylene gave complete conversion
with the formation of N,N-dimethyl-2-phenylacetamide with
36% selectivity and 1 as a major byproduct (16%) via C−C
bond cleavage (Table 3, entry 1). Phenylacetylene substituted
with electron-donor groups at the para position was also
investigated for catalytic transformation into tertiary amides.
With the methyl group, no significant variation in the catalytic
a
%
conversion and % selectivity were determined by GC−MS analysis.
b
Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv; catalyst =
10 mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time = 20 h.
H
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b
Table 2. Oxidative Amidation of Substituted Aromatic Acids, Aldehydes, and Alchols Using a CuO Nanocatalyst
a
b
%
conversion and % selectivity were determined by GC−MS analysis. Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv; catalyst = 10
mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time = 20 h.
performance was observed, but with the methoxy group, 2-(4-
50% selectivity (Table 3, entries 2 and 3). Oxidative amidation
methoxyphenyl)-N,N-dimethylacetamide was obtained with
of cinnamaldehyde gave N,N-dimethyl-2-phenylethanamine as
I
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Article
Table 3. Oxidative Amidation of Unsaturated Compounds
Using a CuO Nanocatalyst
Catalytic oxidative amidation using a series of esters was
performed using CuO nanoparticles prepared from SSMP-2.
Unfortunately, the CuO nanoparticles were ineffective in
catalyzing the conversion of esters to amides. Methylbenzoate
showed 3% conversion with 100% selectivity for 1. Oxidative
amidation of ethyl 2-phenylacetate also showed poor
conversion with 35% selectivity for 2 and 30% selectivity for
N,N-dimethyl-2-phenylacetamide. Methyl 4-chlorobenzoate
was found to be inert toward oxidative amidation under
identical reaction conditions, whereas methyl 4-aminobenzoate
showed 8% conversion with 100% selectivity for N,N-dimethyl-
b
4
-(N-methylformamido)benzamide (see Table S2). In the
presence of CuO nanoparticles, oxidative amidation of
COOMe occurred along with N-formylation and N-alkyation
−
of amine to form N,N-dimethyl-4-(N-methylformamido)-
benzamide. Unfortunately, the product selectivity was low,
and only 8% product was obtained (see Table S2).
Recycling studies were then performed to determine whether
these heterogeneous nanocatalysts can be recycled and reused.
NiO and CuO were thus used to investigate the oxidative
amidation of benzaldehyde along several reaction runs (Figure
7
A,B). Recycling studies clearly showed that NiO gives >95%
benzaldehyde conversion and >68% selectivity for 1 for at least
five cycles. The CuO nanocatalyst was also recyclable for at
least six recycles for BA with >96% conversion and >75%
selectivity for I. XPS experiments (Figures 5 and 6) and XRD
(
Figure 2) on the recovered nanocatalysts confirmed retention
of both the composition and structure of their fresh catalysts,
suggesting that, after recycling of the catalysts, no chemical
modification or surface poisoning occurred (see the discussion
above). Moreover, FESEM results for the spent catalysts also
rule out aggregation of the particles, which retain their
morphology, with only minor product adsorption.
a
%
conversion and % selectivity were determined by GC−MS analysis.
b
Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv; catalyst =
0 mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time = 20 h.
1
a major product (36%) and 1 as a major byproduct (32%)
because of C−C bond cleavage (Table 3, entry 4).
Oxidative amidation of oxygenated organic compounds has
been mostly investigated using homogeneous and heteroge-
neous transition-metal-based catalysts. A comparative study for
the catalytic performance of various homogeneous and
heterogeneous catalysts toward the formation of 1 is presented
in Table S3. The CuO nanocatalyst displays excellent yield of 1,
starting from BA and DMF, using milder conditions and
without additional solvents. The nanocatalysts used display
several advantages with respect to the existing systems (see
Table S3). Although there are several reports on homogeneous
On the basis of previous literature reports on the oxidation of
18,40
alkynes and their tranformation into amides,
we have
proposed a mechanism for the catalytic oxidation of alkynes to
41−43
amides (Scheme S1).
However, we were not able to trap
chetene-based radicals, for example, by using TEMPO. On the
other hand, the occurrence of oxidative cleavage of the −C
C− bond is demonstrated by an extensive amount of 1 as the
major byproduct.
Figure 7. Recycling study for benzaldehyde with NiO (A) and CuO (B) nanocatalysts. Reaction conditions: substrate = 1 mmol; TBHP = 6 equiv;
catalyst = 10 mg; DMF = 2 g (2.12 mL); T = 80 °C; reaction time = 20 h. % conversion and % selectivity were determined by GC−MS analysis; the
catalysts were recoverd via centrifugation of the reaction mixture after every reaction run (see the experimental details).
J
DOI: 10.1021/acs.inorgchem.7b01576
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Inorganic Chemistry
Article
catalysts for oxidative amidation aimed at the synthesis of N-
alkylated amides, these processes suffer from several drawbacks,
such as poor recyclability, difficult catalyst recovery, the need
for additional solvents, and the generation of copious amount
diphenyldimethylpolysiloxane capillary column [30 m in length, 0.25
mm in diameter, and with a df (film thickness) value of 1.0 μm].
X-ray Crystallographic Determination. Single-crystal X-ray
structural studies of compounds were performed on a CCD-equipped
Supernova diffractometer from Agilent Technologies with low-
temperature equipment. The data for SSMP-1 and SSMP-2 were
recorded using a Mo source. The strategy for the data collection was
evaluated using CrysAlisPro CCD software. The data were collected by
the standard ϕ−ω scan techniques and scaled and reduced using
CrysAlisPro RED software. The structures were solved and refined by
full-matrix least squares with SHELXL-97, refining on F . The
positions of all of the atoms were obtained by direct methods. All non-
H atoms were refined anisotropically. The remaining H atoms were
placed in geometrically constrained positions and refined with
isotropic temperature factors, generally 1.2Ueq of their parent atoms.
All molecular drawings were obtained using the program Diamond
4
4
of waste. There are also some reports on heterogeneous
metal-based catalysts for oxidative amidation reactions (Table
S3). Although a heterogeneous Cu-based nanocatalyst was
13
investigated for oxidative amidation, a broad substrate scope
for such a catalyst was not established. Within this scenario, the
CuO nanocatalyst represents an appealing and cost-effective
alternative because it allows one to obtain N-alkylated amides,
starting from several substrates, with higher selectivity, under
milder conditions.
2
45
CONCLUSIONS
(
version 2.1d). The crystal and refinement data are summarized in
Table S1.
Preparation of Single-Source Molecular Precursors (SSMPs):
■
In summary, nickel and copper oxide nanocatalysts featuring
spherical and rod-shaped morphology were procured by
thermal decomposition of their respective SSMPs (SSMP-1
and SSMP-2). Molecular structures of SSMP-1 and SSMP-2
were annotated by X-ray crystallography. We have assiduously
investigated the response of cobalt, nickel, and copper oxide-
based nanoparticles toward oxidative amidation of various
oxygenated and unsaturated organic compounds. Cobalt/cobalt
oxide- and nickel oxide-based nanoparticles showed moderate
catalytic performance for aldehydes and alcohols and poor
catalytic performance for carboxylic acid. On the other hand,
CuO nanoparticles had preponderance over other transition-
metal-based oxide nanoparticles with an excellent catalytic
performance for the oxidative amidation of oxygenates and
unsaturated compounds with a broad reaction/substrate scope,
under moderate reaction conditions. The studied nanocatalyst
can be recycled, retaining high activity up to six runs with an
unaffected performance. Oxidative amidation of various oxy-
genated, as well as unsaturated organic compounds, has been
sparsely investigated at the nanoscale level because very few
literature reports are accessible. The thorough investigation in
the present work can, therefore, serves as a guide in the design
of a catalytic material for the oxidative amidation of oxygenated
and unsaturated organic compounds in the future. The present
work demonstrates a perfect blend of inorganic and material
chemistry by employing inorganic complexes as SSMPs for the
synthesis of their respective metal oxide nanoparticles.
SSMP-1 and SSMP-2. Synthesis of SSMP-1. A methanolic solution
of hep-H (0.123 g, 1.0 mmol, 15.0 mL) was prepared under
continuous stirring conditions. To the aforementioned solution was
added dropwise a NiSO ·6H O (0.262 g, 1.0 mmol, 15.0 mL)
4
2
methanolic solution, and the resultant mixture was stirred magnetically
for 6 h at 298 K. The resulting solution was then passed through filter
paper to remove unreacted solid. The filtrate was allowed to stand at
room temperature for crystallization. Green block-shaped crystals of
SSMP-1 appeared within a week by slow evaporation of the solvent. A
suitable X-ray-quality crystal was then subjected to X-ray analysis,
which confirmed the identity of the crystal as SSMP-1.
−1
SSMP-1. FT-IR (KBr, cm ): 3280(br), 2905(w), 1615(m),
1
7
489(m), 1434(m), 1371(m), 1114(s), 1028(m), 861(m), 766(w),
66(m), 623(m), 577(m).
Synthesis of SSMP-2. A total of 15 mL of a hep-H (0.123 g, 1.0
mmol) and BA (0.122 g, 1.0 mmol) solution was prepared in
acetonitrile. To this was added dropwise a solution of copper acetate
with acetonitrile (0.199 g, 1.0 mmol, 15.0 mL), and the resultant
mixture was stirred magnetically for 8 h at 298 K. The resulting
mixture was then passed through filter paper to remove unreacted
solid. The filtrate was allowed to stand at room temperature for
crystallization. Blue block-shaped crystals of SSMP-2 appeared within
1
0 days upon slow evaporation of the solvent. A suitable X-ray-quality
crystal was then subjected to X-ray analysis, which confirmed the
identity of the crystal as SSMP-2.
−
1
SSMP-2. FT-IR (KBr, cm ): 3425(br), 2924(m), 2802(w),
1615(s), 1562(m), 1395(s), 1257(m), 1111(m), 1066(s), 1030(m),
769(m), 721(m), 682(m), 618(m).
Note: For application purposes, bulk SSMP-1 and SSMP-2 were
also prepared, and the yields (%) were calculated and found to be 91%
and 80%, respectively.
EXPERIMENTAL SECTION
■
Materials. Commercially available starting materials were used as
received. NiSO ·6H O, Cu(CH COO) ·H O, benzoic acid (BA), and
Synthesis of CoNC via a Reduction Method. CoNC was prepared
II
by reduction of the SSMP [Co (hep-H)(H O) ]SO [A; hep-H = 2-
4
2
3
2
2
2
4
4
citric acid were purchased from Merck India. 2-(2-Hydroxyethyl)-
(2-hydroxylethyl)pyridine] using NaBH4 as the reducing agent. A
pyridine (hep-H), tert-butyl hydroperoxide (TBHP; 70 wt % in H O),
detailed synthetic procedure and characterization of SSMP (A) and
CoNC were previously reported.
2
2
9,32
tert-butyl peroxybenzoate, organic aldehydes, acids, and alcohols,
phenylacetylene, and substituted phenylacetylene reagents were
purchased from Sigma-Aldrich and other reagent grade solvents
were used as received.
Physicochemical Characterizations. Powder XRD studies were
carried out on Rigaku SmartLab X-ray diffractometer using Cu Kα
radiation (1.54 Å). IR spectra were performed on a Bio-Rad FTS
Synthesis of a Nanocatalyst (NiO or CuO) via Sol−Gel-Assisted
Thermal Decomposition of Their SSMPs. The synthesis of a
nanocatalyst was carried out using sol−gel-assisted thermal decom-
position of SSMP-1 or SSMP-2. In a typical synthesis of the
nanocatalyst, SSMP-1 or SSMP-2 (10.0 mmol) was dissolved in 100
mL of distilled water under vigorous stirring at 80 °C. The pH of the
reaction mixture was maintained at nearly 2.0−2.5 using citric acid (1.0
M). After complete dissolution of the SSMP, under continuous stirring
and heating, the diluted solution (sol) became a concentrated solution
(gel), which, upon an increase in the temperature at around 140 °C in
45 min, further leads to the formation of a fluffy-like black solid. The
latter was transferred to a 500 °C furnace in order to obtain the pure
and crystalline phase of black NiO or CuO nanocatalysts.
3
000MX instrument using KBr pellets. FESEM in accordance with
EDAX was done using a Supra 55 Zeiss field-emission scanning
electron microscope. TEM was carried out on a FEI Tecnai G2 12
Twin transmission electron microscope. XPS analysis of fresh and
spent catalysts (NiO and CuO) was performed using XPS with an
auger electron spectroscopy (AES) module (model/supplier: PHI
5
000 Versa Prob II, FEI Inc.). Identification of the products was
carried out using a Shimadzu gas chromatograph−mass spectrometer
equipped with a QP2010 mass spectrometer and a RTX-5 tubular
General Procedure for the Catalytic Reactions. The liquid-
phase catalytic oxidations were carried out in a 25 mL two-neck,
K
DOI: 10.1021/acs.inorgchem.7b01576
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
round-bottomed flask equipped with a magnetic stirrer and immersed
in a thermostated oil bath. The flask was charged with a substrate (1
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2
of catalyst, and 2 g (2.12 mL) of the solvent DMF. All of the reactions
were carried out at T = 80 °C. At the end of the reaction, 10 mol %
dodecane was added with respect to the substrate as an internal
standard. Then, the catalyst was separated via centrifugation, and a
sample of the liquid mixture was periodically subjected to GC−MS
analysis. The calibration was done using 10 mol % dodecane, with
respect to the substrate, as the internal standard. After completion, the
reaction mixure was diluted with water and extracted with ethyl acetate
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obtained from GC−MS.
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014, 482, 336−343. (b) Bilyachenko, A. N.; Dronova, M. S.;
Yalymov, A. I.; Lamaty, F.; Bantreil, X.; Martinez, J.; Bizet, C.;
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AUTHOR INFORMATION
■
*
3
Corresponding Author
E-mail: xray@iiti.ac.in. Tel: 91-731-2438 752.
Korlyukov, A. A.; Vologzhanina, A. V.; Kozlov, Y. N.; Shul’pina, L. S.;
Nesterov, D. S.; Pombeiro, A. J. L.; Lamaty, F.; Bantreil, X.; Fetre, A.;
Liu, D.; Martinez, J.; Long, J.; Larionova, J.; Guari, Y.; Trigub, A. L.;
Zubavichus, Y. V.; Golub, I. E.; Filippov, O. A.; Shubina, E. S.;
Shul’pin, G. B. A heterometallic (Fe6Na8) cage-like silsesquioxane:
synthesis, structure, spin glass behavior and high catalytic activity. RSC
Adv. 2016, 6, 48165−48180. (e) Bilyachenko, A. N.; Levitsky, M. M.;
Yalymov, A. I.; Korlyukov, A. A.; Khrustalev, V. N.; Vologzhanina, A.
V.; Shul’pina, L. S.; Ikonnikov, N. S.; Trigub, A. E.; Dorovatovskii, P.
V.; Bantreil, X.; Lamaty, F.; Long, J.; Larionova, J.; Golub, I. E.;
Shubina, E. S.; Shul’pin, G. B. Cage-like Fe,Na-Germsesquioxanes:
Structure, Magnetism, and Catalytic Activity. Angew. Chem., Int. Ed.
ORCID
Shaikh M. Mobin: 0000-0003-1940-3822
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.M.M. thanks SERB-DST (Project EMR/2016/001113), New
Delhi, India, and IIT Indore for financial support. We are also
grateful to Reshma S. Kokane from the CSIR National
Chemical Laboratory, Pune, India, for providing TEM data.
We also acknowledge the Advanced Center for Materials
Science and Material Science & Engineering, IIT Kanpur, for
XPS and microscopic characterization facility, respectively. A.M.
and T.G. are grateful to the MHRD, Government of India, for a
research fellowship and SIC, IIT Indore, for providing the
characterization facility. P.C. is grateful to the MEMS, IIT
Indore, for providing a fellowship.
2
016, 55, 15360−15363. (f) Yalymov, A. I.; Bilyachenko, A. N.;
Levitsky, M. M.; Korlyukov, A. A.; Khrustalev, V. N.; Shul’pina, L. S.;
Dorovatovskii, P. V.; Es’kova, M. A.; Lamaty, F.; Bantreil, X.;
Villemejeanne, B.; Martinez, J.; Shubina, E. S.; Kozlov, Y. N.;
Shul’pin, G. B. Catalysts 2017, 7, 101. (g) Wu, X.-F.; Sharif, M.;
II
Pews-Davtyan, A.; Langer, P.; Ayub, K.; Beller, M. The First Zn -
Catalyzed Oxidative Amidation of Benzyl Alcohols with Amines under
Solvent-Free Conditions. Eur. J. Org. Chem. 2013, 2013, 2783−2787.
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of N, N-dimethyl Benzamide From Nitrile and DMF Under an O2
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acetylacetone Complex Covalently Anchored onto Magnetic Nano-
particles: Synthesis, Characterization and Catalytic Evaluation in
Amide Bond Formation via Oxidative Coupling of Carboxylic Acids
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DOI: 10.1021/acs.inorgchem.7b01576
Inorg. Chem. XXXX, XXX, XXX−XXX