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H.R. Pawar and R.C. Chikate / Journal of Molecular Structure 1225 (2020) 128985
ported the generation of p-n heterojunction between RuO2 layer
and Ru NPs which is primarily responsible for the excellent cat-
alytic activity of Ru/MMT catalyst towards the synthesis of C-
substituted tetrazoles under ambient conditions [27]. Moreover,
the enhanced oxide layer on the active site of the catalyst is nec-
essary for the sustained catalytic activity with excellent yields in
lesser reaction time and beneficial TOF/TON values. Thus, it is
quite logical to extrapolate this strategy by designing RuO2/MMT
nanocomposite and explore its feasibility for the one-pot synthesis
of N-substituted tetrazole via MCR approach. The objective of this
approach stems from the fact that acidic nature of RuO2 as well
as Lewis and Bronsted acidity of MMT would trigger the conden-
sation of amine with triethyl-ortho-formate while Ru-site would
promote oxidative cyclization step in a concerted manner. Thus,
such a bifunctional nanocomposite may exhibit a synergic effect
towards one-pot three component synthesis of tetrazole in an ef-
ficient manner. Such a methodology is been utilized towards the
synthesis of variety of tetrazoles that has resulted in the excellent
yields within shorter reaction time and under ambient conditions
[29]. Another facet of the present work involves the synthesis of
these bioactive scaffolds under solvent free condition that avoids
usage of toxic solvents, tedious work-up and purification steps in a
greener manner [30]. In the present work, different compositions
of RuO2/MMT catalyst are explored for the one-pot three compo-
nent solvent free MCR type synthesis of N-substituted tetrazoles
with amine, triethyl-ortho- formate and azide.
2.5. General procedure for the synthesis of N-substituted tetrazoles
using RuO2/MMT
To a mixture of primary amine (1.5 mmol), triethyl-ortho-
formate (1.5 mmol) and sodium azide (1 mmol) in a round bottom
flask, 10 mg 10% of RuO2/MMT catalyst was added to this mix-
ture and it was heated at 120°C in oil bath for a desired period of
time. After completion of the reaction, the catalyst was separated
by centrifugation at 2000 rpm and the solid mass was extracted
with 10 ml H2O:ethyl acetate (1:1). The organic layer was sepa-
rated, dried over sodium sulfate, and evaporated in vacuum evapo-
rator so as to obtain tcrude mass. It was then purified by recrystal-
lization in a mixture of EtOAc:MeOH (3:1) to yield pure product. It
was characterized by physical constant, 1H and 13C spectral analy-
sis.
2.6. Characterization
X-ray powder diffraction (XRD) patterns were recorded on a
Shimadzu Lab XRD-6100 X-ray diffractometer using Cu-Kα radia-
tion. FT-IR spectra were recorded on a Shimadzu (model 650 plus)
Infrared spectrophotometer with KBr pellets in the range of 400-
4000 cm−1 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra
.
of tetrazoles in DMSO-d6 were recorded on Bruker AV 500 spec-
trometer. Chemical shifts are reported in ppm and the instrument
wasinternally referenced to tetra methyl silane (TMS) and dimethyl
sulfoxide signals. Reported data is as follows: chemical shift, multi-
plicity (s; singlet, d; doublet, t; triplet, q; quartet, m; multiplet, dd;
doublet of doublet, brs; broad singlet), coupling constants in Hz,
and integration. Melting points were recorded on a Metler Toledo
melting point apparatus and were uncorrected.
2. Experimental
2.1. Chemicals
All chemicals were of analytical grade and used as received.
RuCl3.3H2O (Aldrich), Montmorillonite K-10 (Fluka), triethyl-ortho-
formate (TEOF), NaOH (Loba Chemie, UK), sodium azide and substi-
tuted amines (Aldrich), DMF (S.D. Fine) and pre-coated TLC plates
(silica gel 60 F254, Merck).
3. Result and discussion
3.1. Characterization
The power XRD pattern of RuO2/MMT composite is depicted in
Fig. 1. MMT exhibits a sharp peak around 8.9° corresponding to
(001) phase with a basal interlayer distance of 8.9A0. This spac-
ing is retained even after loading of RuO2 nanoparticles suggesting
that they are present on the surface of MMT [28]. The formation
RuO2 NPs on the MMT surface is reflected from the peaks that are
centered at 27.9°, 34.9° and 54.3° corresponding to (110), (101) and
(211) respectively due to the rutile phase (JCPDS Card No 9007541).
Also, their presence is further confirmed by low angle XRD analysis
(Fig. 1 (ii)) where the peak position of (001) plane remained unal-
tered with progessive enhancement in the intensity pattern, imply-
ing that RuO2 NP’s are not intercalated within the galleries of clay
[29]. The FESEM image of RuO2/MMT composite display spherical
morphology with particle size in the range of 40-50 nm as well
as presence of RuO2 particles on the surface of MMT (Fig. 2). Fur-
thermore, the elemental composition of the composite from EDX
indicates the presence of Ru along with Al and Si from alumino-
silicate structure of MMT.
2.2. Pre-treatment of MMT
The pre-pretreatment of MMT clay was carried out as per the
procedure reported in the literature [27].
2.3. Preparation of RuO2 NPs
2.05 g RuCl3.3H2O was dispersed in 100 mL water and stirred
at room temperature for 4 h. It was then subsequently hydrolyzed
by adding 0.54 g of NaOH pellets and the solution was stirred fur-
ther for 2 h. Afterwards, the powder was filtered and washed thor-
oughly with deionized water to remove the excess salt. It was then
calcined at 700°C in furnace for 3 h and subsequently ground so as
to obtain RuO2 nanoparticles.
2.4. Preparation of RuO2/MMT nanocomposite
0.9 g pre-treated Na-MMT was dispersed in 50 mL water and
stirred for 1 h. To this suspension, 0.205 g RuCl3.3H2O in 10 ml
water was added and the mixture was stirred at room temperature
for another 4 h. It was then subsequently hydrolyzed by adding
0.054 g of NaOH pellets, stirred further for 1 h, the powder thus
obtained was filtered and washed thoroughly with deionized wa-
ter to remove the excess salt. The powder was then calcined at
700°C in furnace for 3 h and subsequently grounded so as to ob-
tain 10% RuO2/MMT composite. Similar procedure was adopted for
the synthesis of 5%, 15% nanocomposites of RuO2/MMT by varying
respective amounts of ruthenium salt and Na-MMT.
3.2. Catalyst screening
3.2.1. Optimization
To optimize the reaction conditions, RuO2/MMT catalyst is eval-
uated towards one pot three component synthesis of N-substituted
tetrazole with 4-chloroaniline, triethyl-ortho-formate and sodium
azide under solvent free condition (Table 1). Inspection of this ta-
ble reveals that the reaction does not proceed in presence of MMT
(Table 1; entry 1) while 38% conversion is observed with RuO2
NP’s (Table 1; entry2) which is further enhanced to 93% and 94%