Modified Merrifield resin-supported PCP pincer palladium nanoparticles as a new polymeric catalyst for cyanation of aryl iodides

Article abstract of DOI:10.1080/10426507.2015.1031751

A new tridentate PCP pincer palladium (Pd) complex based on cross-linked Merrifield resin containing phosphinite ligand was synthesized and characterized. This polymeric catalyst was used in preparation of benzonitriles from different aryl iodides using potassium hexacyanoferrate (II) as a cyanide source. The presence of active metallic Pd in the catalyst was verified by X-ray power diffraction and X-ray photoelectron spectroscopy techniques. Transmission electron microscopy showed good dispersion of catalytic sites in the range of 30-50 nm. The catalyst is easily separated from reaction mixture and can be used for several times in repeating cycles without considerable loss of activity. The leaching of Pd from the support is negligible, which was confirmed by inductively coupled plasma-optical emission spectrometry and hot filtration test.

Full text of DOI:10.1080/10426507.2015.1031751

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Modified Merrifield resin-supported PCP pincer
palladium nanoparticles as a new polymeric
catalyst for cyanation of aryl iodides
Bahman Tamami, Molki Mohaghegh Nezhad, Soheila Ghasemi & Fatemeh
Farjadian
To cite this article: Bahman Tamami, Molki Mohaghegh Nezhad, Soheila Ghasemi & Fatemeh
Farjadian (2015): Modified Merrifield resin-supported PCP pincer palladium nanoparticles as
a new polymeric catalyst for cyanation of aryl iodides, Phosphorus, Sulfur, and Silicon and the
Related Elements, DOI: 10.1080/10426507.2015.1031751
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1031751
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Date: 10 December 2015, At: 11:42

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Polymeric PCP Pd catalyst for cyanation reaction
Modified Merrifield resin-supported PCP pincer palladium nanoparticles as a new
polymeric catalyst for cyanation of aryl iodides
Bahman Tamamia,*, Molki Mohaghegh Nezhada, Soheila Ghasemia and Fatemeh Farjadiana, b
aDepartment of Chemistry, Shiraz University, Shiraz 7194684795, Iran
bShiraz University of Medical Sciences, Center for Pharmaceutical Nanotechnology and
Biomaterials
*Corresponding author E-mail: tamami@chem.susc.ac.ir
Abstract
A new tridentate PCP pincer palladium complex based on cross-linked Merrifield resin
containing phosphinite ligand was synthesized and characterized. This polymeric catalyst was
used in preparation of benzonitriles from different aryl iodides using potassium hexacyanoferrate
(II) as a cyanide source. The presence of active metallic palladium in the catalyst was verified by
X-ray power diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques.
Transmission electron microscopy (TEM) showed good dispersion of catalytic sites in the range
of 30-50 nm. The catalyst is easily separated from reaction mixture and can be used several times
in repeating cycles without considerable loss in its activity. The leaching of Pd from the support
is negligible which was confirmed by ICP-OES and hot filtration test.
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Keywords
Merrifield resin, pincer palladium complex, cyanation.
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Introduction
1
Aryl nitriles play crucial role in organic synthesis . Products containing the nitrile group are
2,3
biologically important and can be easily transformed into various classes of compounds such
4,5
as nitrogen containing heterocycles, aldehydes, amines, tetrazoles, acids, and acid derivatives
.
Many synthetic methodologies are reported in the literature for the synthesis of benzonitriles
6,7.The traditional method for preparing aromatic nitriles from the corresponding aryl halides is
Rosemund-Von Braun reaction 8,9 which requires stoichiometric amount of copper (I) cyanide at
elevated temperatures. This approach produces equimolar amounts of heavy metal waste. Of
numerous recently reported methods, transitional metal-catalyzed cyanation of aryl halides is one
of the most convenient ones. In this regard, transition metal complex of palladium 10-21, nickel 22
and copper 23-25 have been introduced. Among them, Pd complexes are useful catalysts due to the
tolerance to a variety of functional groups and less sensitivity to air and humidity. In spite of the
fact that many homogeneous active Pd catalysts have been reported successfully for cyanation of
aryl halides 10-21, immobilizations of these catalysts on insoluble support can facilitate the
recovery and reuse of the metal complexes. There are, however, only few examples of
heterogeneous palladium catalysts for the cyanation of aryl halides 26-29
.
In continuation of our previous studies on the introduction of heterogeneous palladium
30
31-33
catalysts based on polyacrylamide support
and polymer grafted silica,
recently we
disclosed the preparation of an active heterogeneous PCP-pincer palladium catalyst containing
phosphinite ligand based on Merrifield resin for carbon-carbon coupling reactions 34. Herein, we
reveal new application of this efficient catalyst in cyanation reaction of aryl iodides using
commercially available potassium hexacyanoferrate (II) as a cyanide source.
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Result and discussion
The design of the Pd catalyst is shown in Scheme 1.
Chloromethylated polystyrene (2% crosslinked) (1) was reacted with excess 5-amino-
isophthalic acid dimethyl ester (2) to form its corresponding ester modified polymer (3). The
ester groups reduced to their corresponding hydroxyl groups with LiAlH4 in dry THF to produce
polymer (4). The FT-IR spectrum of this modified polymer showed a broad band at 3388 cm-1
due to overlapping of stretching frequencies of OH and NH groups (Figure 1). The polymeric
phosphinite ligand (5) was then prepared by the reaction of polymer (4) with ClPPh2 in THF. FT-
IR spectrum of this modified polymer showed no broad band of hydroxyl groups (Figure 1).
Finally, palladium pincer complex (6) was obtained by mixing polymer (5) with PdCl2 in
presence of cesium carbonate as a base in DMF. Determination of Pd content was carried out by
digestion of the catalyst with concentrated H2SO4 and HCl (1/1) followed by ICP analysis using
calibration curve method. ICP analysis revealed that polymer (6) contained an average of 0.36
mmol/g of Pd.
According to the previous studies in the literature 35-37, Pd (II) pincer complexes are simply
precatalysts that decompose under reaction conditions; often basic conditions at elevated
temperatures and releasing colloidal Pd (0) nanoparticles that act as catalytically active species.
Formation of Pd (0) nanoparticles on the catalytic system can be proven using various techniques
34
such as XRD, XPS and TEM . The XRD patterns for the Pd catalyst (6) showed the expected
crystallinity of Pd (0) nanoparticles. The XPS spectrum showed the binding energy appearing in
344 and 349 eV for 3d5/2 and 3d3/2 respectively. The separation of the two peaks to be 5 eV is
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assigned to Pd (0) 38. Furthermore, TEM image clearly showed that Pd nanoparticles were
dispersed well in the polymer matrix with a size in the range of 30–50 nm 34.
Thermogravimetric analysis (TGA) of (6) was carried out to investigate the thermal stability
of the polymer-supported pincer Pd complex (Figure 2). The weight loss of (6) on heating up to
200 °C is negligible and might be attributed to the release of loosely bound volatiles on the
surface. Thermal degradation of (6) started mostly after ~250 °C with three stages of
decomposition and the most weight loss occurred after ~400 °C. The 5.7% decomposition at
temperature range of 250-350 °C could be assign to the loss of organic moiety tethered to
Merrifield resin which loading calculated to be 0.26 mmol/g. Another considerably weight loss
occurred at 350-470 °C which is due to polystyrene backbone decomposition. Lastly, a minor
decomposition at 470-600 °C is assigned to Pd loss and the capacity determined to be 0.31
mmol/g.
The patentability of this polymeric Pd catalyst was checked for the cyanation reaction of
different aryl iodides. To optimize reaction condition, iodobenzene was chosen. Different
cyanide sources such as potassium hexacyanoferrate (II), alkali and transition metal cyanides,
trimethylsilyl cyanide and acetone cyanohydrins are reported in the literature 10-29. Among theses
we select commercially available potassium hexacyanoferrate K4[Fe(CN)6] as a source of
cyanide which combines excellent efficiency with low palladium catalyst loadings. It is non-
toxic and has a low price. Furthermore it is soluble in polar solvents which are suitable for
cyanation reactions. We used different solvents and bases for the cyanation of iodobenzene
(Table 1). Among them DMAc and K2CO3 was the best option. Under this reaction condition,
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100% conversion of iodobenzene to cyanobenzene was achieved within 135 min using 0.5 mol%
of Pd catalyst (Table 1, entry 3).
The optimized reaction condition was applied in cyanation reaction of different aryl iodides
(Scheme 2). Both electron-deficient and electron-rich aryl iodides were transformed to their
corresponding cyanides in moderate to high yields (Table 2). As a general trend, electron-
deficient aryl iodides are more reactive than electron-rich iodides. Ortho substituent was also
tolerated (Table 2, entry 3 and 4).
On the basis of the general mechanism reported earlier in the literature 37, 40, 41, a mechanism
for cyanation reaction is outlined in Scheme 3. After formation of colloidal Pd (0) nanoparticles
from decomposition of Pd (II) pincer complexes as precatalysts, oxidative addition of the aryl
halide to the Pd (0) catalyst gives Pd (II) complex (step 1). Ligand exchange with cyanide ion
(step 2) gives complex which undergoes reductive elimination to produce the benzonitrile (step
3).
We investigated the recoverability and reusability of the supported catalyst in the conversion
of iodobenzene to the cyanobenzene. The catalyst was recovered and reused for 5 cycles. The Pd
catalyst exhibited only a slight loss in its activity and required a bit longer time to achieve full
conversion even after 5 cycles. The results are shown in Figure 3. Furthermore, to probe the
issue of palladium leaching in our system, the filtrate of the reaction mixture of iodobenzene was
analyzed by ICP in 5 repeating cycles. Low palladium contamination was observed during these
experiments. Analysis of the crude reaction mixture for the first reaction indicated a leaching of
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3.2%. A leaching of 8.8% was observed in total for all 5 runs. Furthermore, hot filtration test
proved the heterogeneous character of the catalytically active species 42.
Furthermore, TEM image of the used catalyst after five runs showed the stability of Pd
nanoparticles without agglomeration in the surface of the polymer after recycling (Figure 4).
We provide a comparison of the results obtained for our present catalytic system with those
limited heterogeneous palladium catalysts reported in the literature. From Table 3, it is seen that
present catalyst yielded the cyanation product in a shorter reaction time compared to the other
reported systems 26-29. In other aspects such as solvent, base, cyanide source, amounts of catalyst,
temperature and recyclability the heterogeneous catalyst (6) is comparable with other reported
systems.
Experimental
Materials and techniques
All chemicals were of reagent grade and obtained from Merck, Aldrich and Fluka chemical
1
companies and used without further purification. All products were characterized by FT-IR, H-
13
NMR and C-NMR spectroscopy. 1H-NMR and 13C-NMR were performed on a Bruker Avance
DPX instrument (250 MHz) in CDCl3. FT-IR was performed using a Shimadzu FTIR-8300
spectrophotometer. All yields refer to the isolated products. Progress of reactions was followed
by TLC on silica-gel Polygram SIL/UV 254 plates or by GC on a Shimadzu model GC 10-A
instrument. The Pd analysis and leaching test was carried out by inductively coupled plasma
(ICP) analyzer (Varian, Vista-Pro). TGA thermograms were recorded on an instrument of Perkin
Elmer with N2 carrier gas and the rate of temperature change of 20 ºC/min was used. X-ray
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diffraction data obtained with XRD, D8, Advance, Bruker, axs. X-Ray photoelectron
spectroscopy (XPS) was performed with XR3E2 (VG Microtech) twin anode X-ray source using
AlKα=1486.6 eV). TEM analysis was performed on a Philips model CM 10 instrument. The
Supplemental Materials contains additional characterization of the catalyst (Figures S 1 – S 3)
and sample NMR spectra for the known products (Figures S 4 – S 15)
Preparation of polymer supported PCP pincer palladium catalyst
The Pd polymeric catalyst was prepared as described in our previous paper 34. ICP
determination using calibration curve method revealed Pd content of 0.36 mmol/g.
General procedure for cyanation of aryl iodides
To a mixture of aryl iodide (1.0 mmol), K4[Fe(CN)6].3H2O (1.0 mmol), K2CO3 (1.0 mmol)
and Pd complex (0.5 mol%), DMAc (5 mL) was added. The reaction mixture was stirred at 120
oC for an appropriate time. On completion of the reaction determined by TLC or GC, the mixture
was filtered and the filtrate was poured into water (50 mL) and extracted with EtOAc (3×15 mL).
The combined organic phases were dried over Na2SO4, filtered and evaporated under vacuum.
The pure product was isolated by flash chromatography using petroleum ether or EtOAc or their
mixture as eluents. Characterization of the products was performed by comparison of their FT-
IR, 1H-NMR, 13C-NMR and physical data with those of the authentic samples.
Conclusion
In summary, we have developed a protocol for the cyanation of aryl iodides via a new
recyclable PCP-pincer palladium catalyst supported on Merrifield resin in the presence of
K4[Fe(CN)6] as cyanide source. The catalyst was easily separated from the reaction mixture by
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filtration and re-used without appreciable loss in its activity. Characterization of the catalyst
showed high Pd dispersion and small particle size in nanoscale. This polymeric catalyst is
attractive due to easy recyclability, low Pd leaching and high efficiency in the cyanation reaction
such as acceptable reaction times and moderate to high yields. This supported catalyst is air-
stable and reactions can be conducted in air.
Acknowledgements
The authors gratefully acknowledge the partial support of this study by Shiraz University
Research Council.
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42. Iodobenzene (1.0 mmol), K4[Fe(CN)6].3H2O (1.0 mmol), K2CO3 (1.0 mmol), Pd complex
o
(0.5 mol%) and DMAc (5 mL) were taken in a round-bottomed flask and stirred at 120 C. The
reaction stopped at 25% conversion. After that, the catalyst was filtered off and the experiment
was continued with the filtrate for another 24 h. There was no detectable increase in the product
concentration, as was evident from the GC analysis.
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Table 1. Optimization of base and solvent for catalyzed cyanation reaction of iodobenzene with
K4[Fe(CN)6] a
Time
Entry
Base
Solvent
Conversion (%) b
(h)
4
1
2
3
K2CO3
K2CO3
K2CO3
K2CO3
NMP
C6H5CH3
DMAc
90
50
4
2.15
100
4
5
6
H2O
DMF
4
4
4
30
80
80
K2CO3
Et3N
DMAc
7
NaOAC
DMAc
4
50
8
9
KF
-
DMAc
DMAc
4
4
30
10
a Reaction conditions: iodobenzene (1.0 mmol), K4[Fe(CN)6] (1.0
mmol), base (1 mmol), Pd catalyst (0.5 mol%) and solvent (5 mL) at
120 oC
b Conversions based on iodobenzene
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Table 2. Catalyzed cyanation reaction of aryl iodides with K4[Fe(CN)6] a
Isolated Yield
Entry
Aryl halides
Product
Time (h)
Ref.
b
11, 13,
2
87
85
1
2
39
11, 13,
3
14, 39
13, 39
75
75
3
4
4
4
-
11, 13,
3
90
65
5
6
14, 39
14, 29,
5
39
a
Molar ratio of the reagents ArI: K4[Fe(CN)6]: K2CO3: palladium catalyst =
1.0:1.0:1.0:0.005. Reaction conditions: DMAc, 120 oC
b Isolated yields
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Table 3. A comparison of the catalyst with some previous heterogeneous catalysts reported in
the literature for cyanation of aryl halides
substrate
Catalyst
Reaction conditions
Yield% Ref.
Et3N, DMF, K4Fe(CN)6, 1.0
mol% catalyst, 100 oC, 24 h
98
78
26
27
PS-Pd(II)-anthra complex
Et3N, DMF, K4Fe(CN)6, 500
mg catalyst, reflux, 24 h
Pd containing nanostructured
silica functionalized with
pyridine sites
DMF, K4Fe(CN)6, 0.2 mol%
catalyst, 130 oC, 13 h
ZnO- Pd NPs
81
92
28
29
DMF, Zn(CN)2,0.15 mmol
ligand + 0.07 mmol Pd(OAc)2,
140 oC, microwave, 30 min
polymer-supported triphenyl
phosphine (ligand), Pd(OAc)2
K2CO3, DMAc, K4Fe(CN)6,
0.5 mol% catalyst, 120 oC, 2 h
presen
t
87
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Figure 1. FT-IR spectrum of chloromethylated polystyrene (1), ester modified polymer (3),
hydroxyl modified polymer (4) and polymeric phosphinite ligand (5)
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Figure 2. A typical TGA curve of the polymeric Pd catalyst
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87
100
80
60
40
20
0
72
66
65
65
. Isolated yield (%)
. Time (h)
7
5
5
2.5
2
2
1
3
4
5
Run
Figure 3. Recyclability of polymer supported Pd(0) catalyst in the cyanation reaction. Reaction
condition: iodobenzene (1.0 mmol), K4[Fe(CN)6] (1.0 mmol), K2CO3 (1.0 mmol), Pd cat. (0.5
mol%) in DMAc (5 mL) at 120 oC
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40 nm
Figure 4. TEM image of the recycled PCP polymeric Pd catalyst
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Scheme 1. Synthetic strategy for the preparation of modified Merrifield resin-supported PCP
pincer palladium nanoparticles 34
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Scheme 2. Cyanation reaction of aryl iodides using K4[Fe(CN)6] in the presence of polymeric Pd
catalyst (6)
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Scheme 3. Proposed mechanism for the heterogeneously Pd catalyzed cyanation reaction
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