Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction

Article abstract of DOI:10.1515/chem-2018-0065

We successfully synthesized Pd@MMT clay using a cation exchange process. We characterized all the synthesized Pd@MMT clays using sophisticated analytical techniques before testing them as a heterogeneous catalyst for the Mizoroki - Heck reaction (mono and double). The highest yield of the Mizoroki-Heck reaction product was recovered using thermally stable and highly reactive Pd@ MMT-1 clay catalyst in the functionalized ionic liquid reaction medium. We successfully isolated 2-aryl-vinyl phosphonates (mono-Mizoroki-Heck reaction product) and 2,2-diaryl-vinylphosphonates (double-Mizoroki-Heck reaction product) using aryl halides and dialkyl vinyl phosphonates in higher yields. The low catalyst loading, easy recovery of reaction product and 8 times catalyst recycling are the major highlights of this proposed protocol.

Full text of DOI:10.1515/chem-2018-0065

Open Chem., 2018; 16: 605–613
Research Article
Open Access
Vivek Srivastava*
Synthesis and Characterization of Pd exchanged
MMT Clay for Mizoroki-Heck Reaction
https://doi.org/10.1515/chem-2018-0065
received February 26, 2018; accepted April 24, 2018.
of dispersion in reaction mass and the large surface area
[4-8] when applying Pd NPs as nanocatalysts for different
Abstract: We successfully synthesized Pd@MMT clay organic transformations. In some of the reports, the
using a cation exchange process. We characterized all the extraordinary nature of these Pd NPs has been recorded
synthesized Pd@MMT clays using sophisticated analytical for different types of C-C coupling reactions like Suzuki,
techniques before testing them as a heterogeneous catalyst Mizoroki-Heck and Sonogashira reaction [6-8]. Although,
for the Mizoroki - Heck reaction (mono and double). The Pd NPs appeared as an important form of Pd catalyst,
highest yield of the Mizoroki-Heck reaction product was unfortunately, they also suffer with some unique problems
recovered using thermally stable and highly reactive Pd@ of nanoparticles like stability, agglomeration and uneven
MMT-1 clay catalyst in the functionalized ionic liquid size distribution of Pd NPs during the chemical reaction.
reaction medium. We successfully isolated 2-aryl-vinyl In some of the scientific reports, Pd NPs were supported
phosphonates (mono-Mizoroki-Heck reaction product) on different organic/inorganic supports like polymer,
and 2,2-diaryl-vinylphosphonates (double-Mizoroki-Heck ionic liquid, activated carbon, silica, MCM-41, clays. Al2O3
reaction product) using aryl halides and dialkyl vinyl and Zeolite get agglomeration free and stable Pd NPs, but
phosphonates in higher yields. The low catalyst loading, the curse of a costly starting material, tedious synthetic
easy recovery of reaction product and 8 times catalyst protocol and, most important, reproducibility of synthetic
recycling are the major highlights of this proposed protocol. protocol collectively hurts the supported synthesis of Pd
NPs [8-12].
Keywords: Mizoroki-Heck reaction; MMT clay; Palladium;
Na-Montmorillonite clay (MMT) is one of the
important materials which is used as a support for various
transition metals, mainly because of its exceptional
physiochemical properties such as high cation exchange
capacity and good swelling properties [13]. Additionally,
MMT supported catalytic systems also offer, a high
C-C coupling; Ionic liquids; Nanoparticles.
1 Introduction
Palladium metal is considered one of the most well- degree of thermal stability, provides protection to metal
investigated materials among the series of transition against air/moisture, easy catalyst isolation and recycling.
metals for C-C coupling reactions [1-3]. Several forms The interlayer spacing of aluminosilicates in MMT is
of palladium metal, such as Pd metal complex with filled with several Na+, K+ and Ca2+ ions, these ions are
carbenesand phosphines as well as palladium salts considered as exchangeable cations in MMT, which are
have been utilized as a catalyst [3-5]. Palladium metal mainly responsible for the metal ion exchange reaction
was also supported on different organic and inorganic [13-15]. MMT supported metal has been tested as a
supports to make the Pd metal catalyst more active and catalyst for several reactions like oxidation, reduction,
stable in terms of air and moisture issues, substrate scope, transesterification and coupling reactions, but to achieve
catalyst leaching and catalyst loading. Recently, different maximum metal exchange as well as high catalyst loading
synthetic protocols have been introduced to synthesize Pd are still a challenge [13-18].
nanoparticles (NPs) which utilize the advanced Physio-
Ionic liquids (ILs) are organic salts that are liquid
chemical properties of nanoparticles such as high degree below 100˚C and have received considerable attention
as substitutes for volatile organic solvents. Since they
are nonflammable, non-volatile and recyclable, they
*Corresponding author: Vivek Srivastava, NIIT University, NH-8
are classified as green solvents. Due to their amazing
Jaipur/Delhi Highway, Neemrana, Rajasthan, Pin Code: 301705,
properties, such as outstanding solvating potential,
India, E-mail: vivek.shrivastava@niituniversity.in
Open Access. © 2018 Vivek Srivastava et al., published by De Gruyter.
Attribution-NonCommercial-NoDerivatives 4.0 License.
This work is licensed under the Creative Commons
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thermal stability and their tunable properties by suitable the registered product name Cloisiite Na and it was used
choices of cations and anions, they are considered as a for tetraamminepalladium(II) chloride monohydrate
favorable reaction medium over conventional solvent intercalation without going to any further purification
systems for chemical synthesis [4]. ILs are mainly made of pretreatment process. The cation exchange capacity
up of cationic and anionic components that can be (CEC) of Na-montmorillonite was 92.6 mequiv/100 g [13-
arranged to achieve a specific set of properties. In this 15]. Philips X’Pert MPD instruments were used to record
context, the term “designer solvents” has been used to all small and wide-angle X-Ray Powder Diffraction (XRD)
illustrate the potential of this environment-benign ILs in data using an X-ray tube voltage of 30.0 kV and current
chemical transformations. Being designer solvents, they of 15.0 mA, scan rate of 5/min and step size of 0.05. The
can be modified as per the specific requirement of the fine powder of Pd@MMT clay was mixed with vacuum
reaction conditions, therefore they also name as “task grease and fixed on the glass substrate. A flat surface was
specific ILs (TSILs).” Since these liquids, can dissolve obtained by pressing the mixed powder between two flat
a series of transition metal complexes, they have been glass sheets. The diffraction pattern was obtained after
utilized as an alternative for conventional solvent systems subtraction of the powder spectrum from the background
in many transition metal complex catalyzed organic of a glass substrate plus the vacuum grease.
transformations to get enhanced reaction rates, selectivity
The Pd@MMT clay material was characterized by
and catalyst recycling with easy isolation of the reaction TEM (Hitachi S-3700N). The TEM sample was prepared
product [13-20]. The present study is aimed to explore the by mixing the Pd@MMT sample with epoxy resin
application of ionic liquid as TSIL to avoid the use of a distributed between two silicon wafer pieces at 50oC for 30
toxic base in the Mizoroki - Heck reaction.
minutes. Cross section of TEM samples were pre-thinned
In this paper, we are presenting the controlled mechanically followed by ion milling. Particle size
synthesis of Pd NPs within the interlayer spacing of MMT distribution was determined once the original negative
followed by cation exchange method, where Pd metal had been digitalized and expanded to 500 pixels per cm
(using tetraamminepalladium(II) chloride monohydrate for more accurate resolution and measurement.
as substrate) ions were exchanged by interlayer cations
EDX spectra were measured with a Si(Li) EDS detector
(Na+, K+ or Ca2+) of MMT. We utilized Pd@MMT clay as (Thermo Scientific) or an XFlash® SDD detector (Bruker),
a catalyst for the Mizoroki - Heck reaction in TSILs (1,3- both having an active area of 10 mm2. A large-area SDD
di(N,
N-dimethylaminoethyl)-2-methylimidazolium EDS detector (Bruker) capable of high-sensitivity detection
bis (trifluoromethylsulfonyl) imide ([DAMI][NTf2])) to has been used for some specific samples.
synthesize 2-aryl and 2,2-diary vinylphohonates. It is well
The specific surface area (BET) of the catalyst
documented that ionic liquid interacts very effectively was determined on a Micrometrics Flowsorb III 2310
with transition metal catalysts. In addition, we will also instrument. The samples were dried with nitrogen purging
investigate the Pd@ MMT/ionic liquid recycling test. or in a vacuum applying elevated temperatures. We used
High reaction rate, base free reaction process, stable P/P0 of 0,1, 0,2 and 0,3 as standard measurement points.
nano-catalytic system, low catalyst loading, easy product The volume of gas adsorbed to the surface of the particles
isolation and catalyst recycling are the most interesting is measured at the boiling point of nitrogen (-196°C). The
features of this proposed protocol.
amount of adsorbed gas is correlated to the total surface
area of the particles including pores in the surface. The
calculation is based on the BET theory. Traditionally
nitrogen is used as adsorbate gas. Gas adsorption also
enables the determination of size and volume distribution
2 Experimental
All the reagent grade chemicals were purchased from of micropores (0.35 – 2.0 nm).
1
Sigma Aldrich and SD Fine chemicals. All the H and
Fourier transform infrared spectrophotometer (FTIR)
13C NMR spectra were recorded with 400 MHz Bruker analysis of all the samples were studied with Bruker
spectrometer with the CDCl3 solvent system. The internal Tensor-27. Elemental analysis was conducted in a Perkin
standard for 1H and 13C NMR spectra were kept at 7.26 and Elmer Optima 3300 XL. All the solid samples were
31
7.36 ppm respectively (supporting information). P NMR prepared by using KBr pellets while liquid samples were
spectra of all the unknown compounds were recorded prepared by using nujol as an internal reference.
Varian 400 NMR spectrometer (85% H3PO4 as an external
ICP-OES (inductively coupled plasma atomic emission
standard) (supporting information). The Na MMT was spectroscopy) was applied to determine the metal Pd
supplied from Southern Clay Product, Texas-USA with and phosphorus content. A sample for ICP-OES analysis
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was prepared by accurately weighing 2.00 g of Pd@MMT catalysts in a solvent system with or without a base (as per
and placing in a pre-cleaned 100 mL volumetric flask. An Table 2, 3 and 4). The combined reaction mass was heated
optimized amount of extractant solution (10 mL Aqua Regia) at the 80oC for one hour. After the completion of the
was then added and the resulting mixture irradiated at the reaction, the reaction product was, then, recovered with
optimum sonication time of 120 min to guarantee maximum diethyl ether (5 x 2 mL) and further purified by column
sample irradiation, and the volumetric flasks were kept chromatography. Isolated ionic liquid immobilized Pd@
stationary at selected positions in the bath with only four MMT-1 clay was further dried in high vacuum at 50oC for
samples used for simultaneous sonication. The resulting 0.5 hours to evaporate all the volatile impurities. After the
supernatant liquid was separated from the solid phase by vacuum treatment, all the reactants were added as per the
centrifugation at 2500 rpm for 15 min, after which diluted up above-mentioned protocol with ionic liquid immobilized
to 40.0 g with Milli-Q water before going to ICP-OES analysis. Pd@MMT-1 clay to recycle the catalytic system.
1,3-di
(N,
N-dimethylaminoethyl)
Ethical approval: The conducted research is not
-2-methylimidazolium
bis
(trifluoromethylsulfonyl) related to either human or animals use.
imide ([DAMI] [NTf2]) and 1-butyl-3-methylimidazolium
bis (trifluoromethylsulfonyl) imide ([Bmim] [NTf2]) were
synthesized as per reported procedure [4 and 17].
3 Results and Discussion
Task-specific [DAMI][NTf2] ionic liquid was synthesized
as per our previously reported procedure. We synthesized
two different types of Pd@MMT clay 1 & 2, followed by
mixing the neat Na-MMT clay with aqueous solution
tetraamminepalladium(II) chloride monohydrate in
acidic medium for 24 hours at room temperature to ensure
complete exchange of palladium metal ion with the
exchangeable cation of MMT clay. After the exchange, the
Pd@MMT clay was washed several times deionized water
and dried under lyophilizer. We obtained Pd@MMT clay-
1 while mixing MMT clay with 6 mM aqueous solution
of tetraamminepalladium(II) chloride monohydrate.
Pd loading on MMT clay was determined by calculating
the change between the concentrations of palladium
metal ion in initial tetraamminepalladium(II) chloride
monohydrate solution with respect to the mother liquor
recovered after the filtration of Pd@MMT clay followed by
inductively coupled plasma emission spectroscopy (ICP-
OES). We obtained good palladium metal ion loading over
Pd@MMT-1 clay (0.95% w/w Pd metal) in comparison with
Pd@MMT-2 clay (0.51 % w/w Pd metal, obtained by mixing
3mM aqueous solution of tetraamminepalladium(II)
chloride monohydrate with MMT clay).
2.1 Synthesis of Pd@MMT Clay
A perfectly cleaned and dried 250 mL round bottom
flask was charged with the suspension of Na MMT clay
(5 g) with 100 mL water. The aqueous solution of 50 mL
of tetraamminepalladium (II) chloride monohydrate (6
mM) solution was added in dropwise manner into the
homogeneous slurry of Na MMT clay within 2 hours by
maintaining the pH of the solution at 5.5 (using 0.1N HCl).
The combined reaction was stirred at 25-30oC for the next
24 hours to obtain a uniform dispersion. Later, [Pd (NH3)4]
-MMT exchanged clay was stirred with NaBH4 (5 g) for
the next 5 hours at room temperature. The color change
from brown to black confirmed the complete reduction
of Pd2+ to Pd0. Further, Pd exchanged clay was washed
with double distilled water using centrifugation. Washing
was stopped as soon as we got chloride free supernatant
(monitored with silver nitrate solution). Finally, chloride
free Pd exchanged clay was dried under lyophilizer for
12 hours. At last, we obtained free-flowing black colored
powder as Pd@MMT-1 clay (4.8 g, 3w/w % Pd). In the
same pattern, we also prepared Pd@MMT-2 clay (4.3 g,
1.01 w/w % Pd) using tetraamminepalladium(II) chloride
monohydrate solution (3 mM) with 5 g Na MMT clay. pH
of the above-mentioned reaction mass was also controlled
by the addition 0.1 N HCl.
The change in the basal spacing of Pd @MMT clay
with respect to neat MMT clay was studied using small to
medium angle X-ray scattering (SAXS) analysis (Figure 1,
supporting information). In SAXS study, an increase in the
basal spacing of neat MMT clay was (d001=12.95 Å) recorded
after the intercalation of palladium metal ion within the
interlayer spacing of MMT clay. After the intercalation,
the basal spacing of Pd@MMT-1 clay was increased up to
2.2 Experimental Procedure of Mono or
Double Mizoroki-Heck Reaction
50 mL glass-made reaction vessel was charged with aryl d001=15.86 Å. Such significant increase in d001 spacing of
halides and vinylphosphonate with Pd@MMT clay -1 or 2 MMT clay confirms the presence of palladium metal ion
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Vivek Srivastava
between the interlayer spaces of MMT clay. Sharp 001 XRD MMT-2 clay was also recorded where the basal spacing has
peaks gave a clear indication of parallel arrangements of increased from 14.15 Å to 15.01 Å.
clay sheets and the uniform addition of Pd NPs (Figure
The textural properties of Na-MMT clay and Pd@
2 supporting information) [13-15]. Therefore, we can MMT clays represented them as mesoporous solid
conclude the presence of an ordered lamellar structure of (Table 1, Figure 6).
Pd metal loaded MMT with the face-to-face arrangement
of MMT clay sheets. We obtained a small characteristic MMT, Pd@MMT clay 1 and Pd@MMT clay 2 gave typical
signal as a peak in XRD data of Pd@MMT clay which type IV with hysteresis loop at P/P0
0.4 to 0.9 (Figure 6
The nitrogen adsorption-desorption isotherm of neat
̴
confirms the presence of Pd NPs in MMT clay. The XRD supporting information). This data reveals no change
signals of Pd NPs appeared as sharp peaks near to 40, 46 in the porous structure of Na-MMT clay even after the
and 68 degrees respectively, representing the (111), (200) insertion of Pd NPs. A drop in the BET surface area and
and (220) Bragg reflection. The XRD pattern of Pd metal total pore volume after the intercalation Pd NPs was
was found in good agreement of JCPDS standard (#05- recorded mainly due to the capping of some pores with
0681) and confirmed the synthesis of Pd NPs within the metal NPs. The data shows the development of adsorbent
interlayer spacing of MMT clay. Face-centered cubic (fcc) multipliers’ and weak adsorbate-adsorbent connections.
crystal structure was used to calculate the size of Pd NPs The different shapes of the adsorption-desorption
using peak broadening profile of (111) signal at 40˚ using isotherms were due to the configuration and size
the Scherrer equation. The calculated crystallite size of distribution of Pd NPS. Absenteeism of any unexpected
the Pd NPs was found in 6.5 nm size for Pd@MMT-1 clay changes in the surface area and pore volume supported
and 8.75 nm size for Pd@MMT-2 clay. We also studied the the no sign of agglomeration.
effect of the ionic liquid on the basal spacing of MMT clay.
Energy dispersive X-ray spectroscopy (EDS) was
The sign of an increase in basal spacing of Pd@MMT -2 applied to understand the chemical composition of Pd@
clay was also noticed and it was found near to d001=14.15Å. MMT clay to better know the structure and homogeneity
The morphology of MMT clay with and without Pd of prepared materials (Figure 7 supporting information).
metal exchange and the particle size of Pd NPs as well The presence of characteristic signals of Pd metal in EDS
as the presence of Pd NPs within the interlayer spacing spectraconfirmedthepresenceofPdNPs. TheXPSanalysis
was further confirmed by performing the high-resolution of Pd metal in Pd@MMT clays gave two characteristic
transmission electron microscopy (HRTEM) (Figure 3 peaks for Pd 3d5/2 and 3d3/2 at 335.3 eV and 340.5 eV
supporting information). We obtained the agglomeration respectively (Figure 7 supporting information). This data
free uniform distribution of Pd NPs within the interlayer confirmed the presence of well reduced Pd (0) NPs within
spacing of MMT clay. In Pd@MMT-1 clay, the particle size of the interlayer spacing of MMT clay. Heterogeneous nature
Pd was recorded near to 7.5 nm with a standard deviation of catalysts was examined by catalyst poising experiment
of 0.15 nm while some increase in the particle size of (supporting information).
Pd metal was observed near to 12.5 nm with a standard
deviation of 0.21 nm with Pd@MMT-2 clay.
After the completion of the careful physiochemical
analysis of Pd-MMT clay, we further used them as a catalyst
During the reaction, Pd@MMT clay was used as a for two very important reactions usingthe Mizoroki-Heck
catalyst with functionalized ionic liquid used as a reaction reaction.
medium as well as an effective substitute of amine.
Presence of functionalized ionic liquid with Pd@MMT clay
creates a chance of cation exchange between the cationic 3.1 Mizoroki-Heck Reaction with Pd@MMT
part of ionic liquid with remaining unexchanged Na+ ions
in the Pd@MMT clay (Figure 4 supporting information) [14,
Clay as Catalyst
15]. This type of exchange was confirmed by further SAXS Alkenylphosphonates are considered an important
analysis and FT IR analysis. The presence of characteristic chemical in medical science, material science and
bands near to 1650, 1543, 843 cm-1 both in [DAMI][NTf2] as polymer additives [18]. Various protocols such as
ionic liquid and [DAMI]+ ion exchanged Pd@MMT clay the Suzuki - Miyaura coupling of boronic acids with
confirmed the exchange of remaining Na+ ions with vinylphosphonate,
a Mizoroki-Heck reaction using
cations of [DAMI][NTf2] ionic liquid (Figure 5 supporting vinylphosphonate with different aryl derivatives, aldehyde
information). This exchange was also supported by SAXS insertion into zirconacycle phosphonates and Olefin cross-
analysis and d001 spacing was reaching up to 16.76 Å from metathesis have been reported using numerous transition
15.86 Å in Pd@MMT-1 clay. The same change in Pd@ metal catalytic systems under a toxic conventional solvent
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Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
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O
system [18-22]. The reaction is primarily suffering from
catalyst recycling and the requirement of the toxic base for
the successful completion of the reaction. In this report, we
are using our well characterized Pd@MMT clay catalysts
to improve the reaction kinetics of vinyl phosphonates
synthesis followed by Mizoroki -Heck reaction. Also, we
are exploring the synthesis of a unique double-Mizoroki-
Heck reaction using our developed MMT supported Pd
catalytic system.
Initially, we allowed Pd @MMT clay 1 to catalyze a
model Mizoroki- Heck reaction between iodobenzene and
diethyl vinyl phosphonate under functionalized ionic
liquid medium (instead of using the toxic conventional
solvent system and toxic base) at 80oC for 1 hour
(Scheme 1, Table 2). We successfully obtained the
I
O
Solvent
P(OEt)2
P(OEt)2
1-5 h, 60-100oC
Scheme 1: Model Mizoroki- Heck reaction.
Table 1: Surface properties of Na-MMT clay and Pd@MMT clays.
Samples
BET surface
area (m2/g)
Average Pore Pore volume
size (nm)
(cm3/g)
Na-MMT
385
371
379
3.48
3.32
3.39
0.362
0.211
0.209
Pd@MMT clay-1
Pd@MMT clay-2
Table 2: Reaction optimization of Mizoroki Heck reaction.
Entry
Catalyst (0.01 g)
SolvInent (0.150 g)
Base (1 mmol)
Time (h)
Temperature (oC) Yield (%)
1.
Pd@MMT clay-1
Pd@MMT clay-2
Pd@MMT clay-1 (0.02g)
Pd@MMT clay-1 (0.005g)
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd@MMT clay-1
Pd(NH3)4Cl2·H2O
Pd(OAc)2
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2] (0.200 g)
[DAMI][NTf2] (0.100 g)
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[bmim][NTf2]
[bmim][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
DMF
-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
30
1
1
1
1
1
1
80
80
80
80
80
80
80
80
80
80
80
80
50
92
55
92
52
91
72
90
87
91
90
35
73
34
91
91
42
66
59
45
65
61
-
2.
-
3.
-
4.
-
5.
-
6.
-
7.
KOH
K2CO3
iPr2NH
Et3N
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
iPr2NH
-
-
100
100
100
80
80
80
80
80
80
-
-
iPr2NH
THF
iPr2NH
CH3CN
iPr2NH
[DAMI][NTf2]
[DAMI][NTf2]
[DAMI][NTf2]
-
-
-
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O
O
corresponding reaction product with good yield. The
same reaction condition was also tested with Pd@MMT
clay-2 catalyst unfortunately, lower yield was recorded
due to low Pd metal loading (Table 2, entry 2).
Pd@MMT-1 clay
ArI
P(OR)2
P(OR)2
[DAMI][NTf2], 1h, 80oC
Ar
R= Et, iPr
Scheme 2: Mono Mizoroki-Heck reaction.
No sign of drastic change in the yield of Mizoroki-
Heck reaction was observed when elevating the reaction
temperature or time and quantity of ionic liquid or
catalyst. But, as expected, while lowering the reaction
condition, a clear drop in reaction yield was recorded.
Replacement of functionalized ionic liquid with a series
of base and non- functionalized [bmim][NTf2] ionic
liquid showed the importance of [DAMI][NTf2] ionic
liquid which works as active bases (due to the presence
of two -NH functional group) and effective reaction
medium. We also received lower catalytic activity of
conventional Pd catalysts over our developed Pd@MMT
clay catalytic system in ionic liquid medium (Table 2,
entry 20 and 21). No sign of reaction product was recorded
in absence of Pd@MMT clay 1 catalyst. After reaction
completion, the product was effortlessly isolated via
diethyl ether extraction and further purified using column
chromatography. We successfully recycled our ionic
liquid immobilized Pd@MMT-1 clay up 8 cycles without
showing any significant loss in catalytic activity in terms
of reaction yield. Surprisingly, no signature of catalyst
leaching was recorded while performing a filtration test
during recycling experiments. All the solid material was
isolated with 0.45 mm polytetrafluoroethylene (PTFE)
filter and recovered liquid was mixed with the reactants of
the model Mizoroki heck reaction. No product formation
was recorded during this reaction which confirmed the
zero leaching of Pd metal from MMT clay. These filtration
experiments were also supported by ICP-OES analysis of
above-mentioned filtrate, where no signal from Pd metal
was received. A sign of agglomeration and low reaction
yield was recorded in TEM image analysis after 8 times
recycling of Pd@MMT clay-1 catalyst (Figure 3 and 8
supporting information). Due to agglomeration, particle
size increase in Pd NPs was observed from 7.5 nm to 19.5
nm (with a standard deviation of 0.75 nm). In some of the
reports, the formation of palladium black was reported
due to high reaction temperature [1-3]. In our case, no
such observation was found mainly due to the extended
thermal stability of Pd NPs because of thermally stable
MMT clay.
Table 3: Pd@MMT-1 clay catalyzed mono Mizoroki-Heck reaction.
Entry
Aryl halide
(1mmol)
Phosphonate
(2 mmol)
Yield
(%)
O
I
1.
89
83
86
81
92
91
91
93
84
86
87
91
92
91
86
88
P(OEt)2
O
2.
P(OiPr)2
O
I
3.
P(OEt)2
O
O
4.
P(OiPr)2
O
I
5.
P(OEt)2
F
O
6.
P(OiPr)2
O
I
7.
P(OEt)2
Cl
O
8.
P(OiPr)2
O
I
9.
P(OEt)2
Br
O
10.
11.
12.
13.
14.
15.
16.
P(OiPr)2
O
I
P(OEt)2
F3C
O
P(OiPr)2
I
O
P(OEt)2
O
P(OiPr)2
O
We applied the optimized reaction conditions for
the variance of aryl halides and with different types of
dialkylvinylphophonates. All the results were summarized
Table 3, Scheme 2.
We tested a variety of aryl iodide (electron rich
and electron poor) with two different types of vinyl
O
I
P(OEt)2
NC
O
P(OiPr)2
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ContinuedTable 3: Pd@MMT-1 clay catalyzed mono Mizoroki-Heck reaction.
O
Ar
O
Pd@MMT-1 clay
ArI
P(OR)2
P(OR)2
[DAMI][NTf2], 1h, 80oC
Entry
Aryl halide
(1mmol)
Phosphonate
(2 mmol)
Yield
(%)
Ar
R= Et, iPr
Scheme 3: Double Mizoroki-Heck reaction.
O
I
17.
85
83
66
67
59
55
P(OEt)2
O
18.
19.
20.
21.
22.
phosphonate derivatives. We obtained good to excellent
yield in all cases. Surprisingly, lowering of reaction yield
was not reported with sterically hindered aryl halides
(Table 3, entry 15-18). We easily performed typical oxidative
addition of aryl bromide derivatives with MMT supported
Pd metal without increasing the reaction temperature,
reaction time and catalyst loading (Table 3, entry 19-30).
This outcome represented the high catalytic performance
of Pd@MMT -1 clay catalyst in the ionic liquid medium.
We obtain the slow reaction rate with electron-rich aryl
bromide derivatives than electron deficient composition.
The presence of steric effect on aryl bromide derivatives
as low reaction yield was recorded in their corresponding
reaction products (Table 3, entry 31-34.).
P(OiPr)2
O
Br
P(OEt)2
O
P(OiPr)2
O
Br
P(OEt)2
O
O
P(OiPr)2
O
Br
23.
24.
25.
26.
27.
28,
29.,
30.
31.
70
69
70
76
76
77
73
80
74
75
62
63
P(OEt)2
F
O
P(OiPr)2
3.2 Double Mizoroki-Heck Reaction
O
Br
In our study, we also extended the application of Pd@
MMT-1 clay catalysts for double-Mizoroki-Heck reaction
while changing the limiting reagent from aryl halides
to vinyl phosphonates (Scheme 3, Table 4, Entry 1-14).
We obtained the corresponding double-Mizoroki-Heck
reaction products with average to good yield. A series of
electron rich and electron poor aryl halides were easily
converted to their reaction products (Table 4, Entry 1-10).
Sterically hindered aryl iodides were easily converted
to 2, 2,-diary vinyl phosphonates with good yields (Table
4, entry 11-12). This result confirmed no effect of steric
hindrance on double Mizoroki-Heck reaction. In addition,
brominated aryl halides were allowed to react with vinyl
phosphonates without changing any reaction condition
via double Mizoroki-Heck reaction and their matching
reaction products were nicely isolated in good yield.
P(OEt)2
Cl
O
P(OiPr)2
O
Br
P(OEt)2
F3C
O
P(OiPr)2
Br
O
P(OEt)2
O
O
P(OiPr)2
O
Br
P(OEt)2
NC
O
32.
33.
34.
P(OiPr)2
4 Conclusions
O
Br
The cation exchanged method was used to prepare two
different types of Pd@MMT clays with different Pd loading.
All the catalysts were properly analyzed through advanced
analytical techniques including FTIR, ICP-OES, SAXS,
XRD, FTIR, N2 physisorption, EDS and TEM. High Pd NP
P(OEt)2
O
P(OiPr)2
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612ꢀ
Vivek Srivastava
Table 4: Pd@MMT-1 clay catalyzed double Mizoroki-Heck reaction.
we also investigated the benefits ionic liquids, other
than reaction medium, and concluded that the exchange
of ionic liquid cations with the remaining Na+ cations
of Pd@MMT clay has expanded the interlayer spacing
to accommodate better reaction between Pd NPs and
reactants. Such feature of ionic liquid found responsible
for the highly selective Mizoroki-Heck reaction. Pd@MMT
clayundercatalystrecyclingexperimentswasfoundhighly
active under thermal heating and successfully recycled
the Pd@MMT-1 clay catalyst up to 8 runs. Surprisingly,
no catalysts leaching was recorded during catalysts
recycling experiments. Along with mono Mizoroki-Heck
reaction, we also obtained double Mizoroki-Heck reaction
with a good reaction yield without increasing reaction
time/temperature and catalyst loading. In addition, we
developed an operationally simple catalytic recycling
system for Mizoroski-Heck reaction without using any
toxic ligands and harmful conventional solvent systems.
Conflict of interest: Authors state no conflict of interest.
Entry
Aryl halide
(2 mmol)
Phosphonate
(1 mmol)
Yield (%)
O
I
1.
72
P(OEt)2
O
2.
66
69
64
75
74
74
76
70
74
75
74
75
74
75
74
P(OiPr)2
O
I
3.
P(OEt)2
O
O
4.
P(OiPr)2
O
I
5.
P(OEt)2
F
O
6.
P(OiPr)2
O
I
7.
P(OEt)2
Cl
O
8.
P(OiPr)2
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O
I
9.
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Synthesis and Characterization of Pd exchanged MMT Clay for Mizoroki-Heck Reaction
ꢀ613
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