Catalytic application of ligand protected metal nanoparticles
Palladium catalysed cross coupling reactions have developed as a central tool in organic chemistry for the synthesis of a large number of organic compounds that have a wide range of applications as pharmaceutics, and in material sciences and various other fields. In these reactions, palladium salts are mainly used as catalysts but these cannot be separated from solution and reused because of their high solubility in the reaction mixture used for these coupling reactions. The recovery of palladium becomes essential because it is an expensive metal and will be used in large quantities especially when these reactions are carried out on large scale. Heterogeneous supported Pd nanoparticles (NPs) as catalysts is a good alternative because they can be easily separated from the solution, but these are not solvent dispersible which decreases their catalytic activity and efficiency. Further, once the reactants are absorbed on the surface of these heterogeneous catalysts, the reaction does not proceed further until the product formed leaves the surface. Ligand capped metal NPs form stable dispersions in non-polar organic solvents because the ligand involved interacts with the surface of these metal NPs as well as the surrounding solvent molecules. This could bridge the gap between homogeneous and heterogeneous catalysis giving rise to a new platform for catalysis. It is generally perceived that the ligand protected metal NPs do not show any catalytic activity as these ligands occupy the surface of these metal NPs. But recently several researchers reported in literature that this ligand protected metal NPs also show excellent catalytic activity in many coupling reactions that have potential application in many fields. Following this hypothesis, in the present work, monodispersed amine capped Pd NPs were synthesized by using Digestive Ripening (DR) process and the catalytic activity of these Pd NPs was tested in coupling reactions. Our results demonstrate that these amine capped Pd NPs exhibit catalytic activity for Buchwald Hartwig reaction involving C-N coupling between aryl halides and aryl/alkyl halides in the presence of base at a temperature of 110 °C-150 °C.
Keywords: cross coupling reaction, homogeneous catalysis, heterogeneous catalysis, amine capped Pd NPs, buchwald-hartwig reaction
|DDAB||Didodecyldimethyl ammonium bromide|
|NMR||Nuclear magnetic resonance|
|GCMS||Gas chromatography-mass spectrometer|
|TLC||Thin layer chromatography|
|TEM||Transmission electron microscope|
Nanoparticles (NPs) are those particles in which at least one of the dimensions is in the range of nanometer scale. These NPs display unique optical, magnetic, electrical, catalytic properties and many more because of its nanoscale size and the presence of large percentage of surface atoms compared to their corresponding bulk form. Because of these unique properties NPs find application in various fields such as electronics ( Li, et al, 2005 ), quantum dots ( Yi, 2005 ), medicine ( Salata, et al, 2004 ), superhydrophobic ( Bravo, et al, 2007 ), agriculture ( Ingale, et al, 2013 ), solar cells ( Wang, et al, 2003 ), cosmetics ( Lu, 2015 ) etc. These nanoscale properties are size dependent as these are determined by the percentage of surface atoms.
Catalysis can be classified into two types, homogeneous and heterogeneous catalysts, based on the physical state of the catalyst used. Homogeneous catalysts are those catalysts that have the same physical state as that of the reactants while heterogeneous catalysts have different physical state from the reactants.
In present work, amine capped Pd NPs were synthesized by DR process and their catalytic activity were explored for Buchwald-Hartwig coupling reaction.
In our study monodispersed amine capped Pd NPs were used as catalysts for Buchwald Hartwig reaction, these amine capped Pd NPs have not been reported as catalyst in literature as per our knowledge. These monodispersed amine capped Pd NPs can be easily prepared by DR process and can be separated from the reaction mixture by centrifugation and reused.
The most innovative and novel aspect of our work is that the capping agents (amine) of Pd NPs acts as reactants for Buchwald coupling reaction. Hence, there is no requirement for adding another amine for carrying out the coupling reaction, thereby minimizing the number of reactants and side products that can form in case of additional reactant. Furthermore, once the amine that are involved as capping agents participate in these coupling reactions the Pd NPs can aggregate, thereby losing its catalytic activity. Nevertheless, these can be prevented by adding excess amine to the reaction mixture so that the required amount of amine is available for both capping as well as for the coupling reaction.
METHODS AND TECHNIQUES
Thin Layer Chromatography (TLC)
Thin layer chromatography is a unique technique that is used for monitoring the progress of organic reactions, it gives a qualitative idea about the conversion of the starting material and indicates the formation of products. TLC is based on the principle of chromatography i.e. it contains two phases stationary phase (silica gel) and mobile phase (volatile organic solvents). The mixture of compounds is separated based on the subsequent adsorption and desorption in accordance with the polarity of the compound.
For performing TLC at first spot of the reaction mixture along with the starting material are made at the bottom of the TLC plate by using a capillary tube. Then the TLC plate is placed in the developing chamber containing the mobile phase. During the solvent run the lid of the developing chamber is closed because the mobile phase contains organic solvents which are volatile in nature, so they could escape in the form of gas from the chamber. The mobile phase travels up the TLC plate, once it reaches the top, the TLC plate is taken out from the chamber and viewed under ultraviolet light in an U.V. lamp.
Column Chromatography (CC)
Column chromatography is based on the similar principle as thin layer chromatography but instead of qualitative it gives a quantitative idea of the product. CC is mainly used in pharmaceutical industries for purification of the desired product on a large scale.
The first step involved in CC is column packing which is carried out by selecting a column of desired length and then a small cotton is placed at the bottom of the column; the cotton should not be packed tightly as it will affect the rate of flow of mobile phase. Then slurry of silica gel is prepared by adding mobile phase to dry silica gel in a conical flask, which is poured quickly but carefully into the column. The silica gel is allowed to settle at the bottom of the column while the mobile phase is allowed to flow through the column which is collected in another conical flask.
Once all the silica gel has settled down at the bottom and the mobile phase has drained out from the column the sample is loaded carefully in such a manner that it forms a fine layer on top of the silica gel layer. Then a layer of sodium sulphate is added on top of the sample layer to avoid disturbing the sample layer while adding the mobile phase. Then the mobile phase is added from the top of the column and polarity of the mobile phase increases gradually according to the polarity of the product and collected in test tubes, subsequent TLC of the collected mobile phase is taken with reaction mixture to know about whether these test tubes contain the desired product or not.
Rotavapor (rotavap) is generally used for drying samples/reaction mixtures by removing organic solvents (ethyl acetate, DCM, pet ether, hexane etc.) from the sample under reduced pressure.
Rotary vapor consists of a evaporation flask that contains the sample to be dried, a vacuum system that reduces the pressure above the sample allowing the solvents present in it to boil at a lower boiling point, a water bath that is used for heating the sample, a motor unit that rotates the evaporation flask so that heat is distributed uniformly throughout the sample, a condenser that contains a cooling fluid such as crude methanol which condenses the vapours of the solvent form the sample and a receiving flask that is used for collecting the condensed solvent.
Palladium (II) acetate, Didodecyldimethyl ammonium bromide (DDAB), Sodium borohydride, Toluene, Dimethyl sulfoxide (DMSO), Aniline, Dioctyl amine (DOA), Iodo benzene, 4-Bromo biphenyl, Potassium tert-butoxide (K-tBuO) and Sodium hydroxide (NaOH).
Preparation of Pd NPs by DR process
Pd NPs were prepared by dissolving 11 mg of Pd (II) acetate (0.01 M) and 50 mg of long chain surfactant DDAB (Didodecyldimethyl ammonium bromide) in 5 mL toluene by continuous stirring. Then, to the resulting wine-red coloured solution freshly prepared aqueous NaBH4 (40 µl of 9.4 M) was added and it was stirred for 1 hr for complete reduction of Pd (II) to Pd (0). This results in the formation of polydispersed Pd NPs. (Fig 5)
Then the entire solution was transferred to another round bottom flask to remove any of the unreacted material. Then to this solution excess of DR or capping agents or ligands (Aniline, DOA) were added, keeping the metal to ligand ratio as 1:20 and it was stirred for 5 mins at R.T. This results in the exchange of weakly bound surfactant molecules with DR or capping agents and the formation of ligand capped Pd NPs as shown in Fig 6.
Then the solution is refluxed at 110 °C for 1 hour. The schematic diagram of the three processes that occur during refluxing are demonstrated above (Fig 8)
When an equilibrium is established between all the three processes demonstrated above it results in the formation of monodispersed ligand capped Pd NPs (Fig 9). Then these monodisperse ligand capped Pd NPs are precipitated by addition of excess ethanol (10 mL) to the solution followed by centrifugation at 9000 rpm.
Typical Procedure for Buchwald-Hartwig Coupling Reaction
Firstly, amine capped Pd NPs prepared by DR process were dispersed in DMSO (5 mL) by sonication in a round bottom flask containing a magnetic stir bar. Then, 1 mmol of halobenzene (bromine or iodine), 1-2 mmol of amine and 1 mmol of base were added to the round bottom flask. Then the reaction mixture was heated to 110 ⁰C-150 °C under continuous stirring by using a magnetic stirrer. The reaction progress was monitored by TLC. The reaction was carried out until the starting material was consumed which would be observed from the intensity of the spot of the starting material obtained on the TLC plate for the reaction mixture.
When all the starting material was consumed the workup of the organic reaction was carried out by using water and ethyl acetate to separate the reaction mixture from DMSO and unreacted base. Then the organic layer was collected in a conical flask and Na2SO4 was added for absorption of any water present in the organic layer. Then the organic layer was filtered in a round bottom flask and the reaction mixture was dried by evaporating the solvent (ethyl acetate) using a rotary vapor.
Then the required product was separated from the reaction mixture by using column chromatography and its mass was measured by using a weighing machine to calculate the percentage yield. For column chromatography pet ether and silica gel were used as mobile and stationary phase respectively.
The purified product isolated using column chromatography or the reaction mixture obtained by drying the solvent after workup was characterized by using GCMS and NMR. For GCMS minute amount of sample was dissolved in methanol and it was analysed by the GC (Gas Chromatogram) and MS (Mass Spectrum) that were obtained by GCMS characterization. For NMR the sample preparation was carried out by dissolving 10 mg of sample in chloroform-D by sonication. Then the prepared sample was taken in an NMR tube and submitted for NMR characterization. NMR data was analysed by using 1D NMR processor software to predict the exact structure of the molecule.
RESULTS AND DISCUSSION
TEM characterization of DOA capped Pd NPs prepared by DR process.
TEM images (A) and (B) show the narrow size distribution i.e. monodispersity of the DOA capped Pd NPs. The average size of these NPs is 4.5 nm that is obtained by calculating the size of 300 NPs in TEM image (B) using ImageJ software and then plotting a histogram representing the size distribution of these 300 NPs using OriginPro software.
Catalytic activity of amine capped Pd NPs.
The catalytic activity of amine capped Pd NPs prepared by DR process was explored for the Buchwald-Hartwig coupling reaction of alkyl / aryl amines and aryl halides. Initially, the reaction of aniline with iodo benzene and 4-bromo biphenyl with aniline capped Pd NPs as catalyst was chosen as the model reaction. The reaction was first tested using K2CO3 as base in DMSO at 110 °C but formation of product was not observed even after 12-24 hrs of reaction (Table 1, entry 1). The conversion of starting material (4-bromo biphenyl) was observed when alkoxide such as K-tBuO (Table 1, entry 2) was used as base in DMSO at 110 °C and R.T. But most of the starting material remained intact and very faint spots for product were obtained.
For increasing the reaction productivity, the amount of base added was increased by three times. Upon increasing the amount of K-tBuO by three times significant conversion of starting material was obtained, but many products were formed (Table 1, entry 3 and 4). For improving the selectivity of the reaction, the base added was varied from K-tBuO (strong base) to triethyl amine and sodium acetate (weaker base). (Table 1, entry 5 and entry 6)
By using weak bases like triethyl amine and sodium acetate with starting material as 4-bromo biphenyl no reaction was observed. With iodo benzene and K-tBuO the selectivity of the coupling reaction was improved but only a 16 % yield of the desired Buchwald coupling product (biphenyl amine) was obtained by analysis of GCMS data (Table 1, entry 7). Triphenyl amine with 39 % yield was obtained because of the further reaction of biphenyl amine with iodo benzene. On changing the base to NaOH only a trace amount of biphenyl amine formed while the Ullmann coupling product i.e. biphenyl was the major product with a yield of 51 %. (Table 1, entry 8)
With aniline low yield of Buchwald coupling product was obtained, which could have happened because of polymerization of aniline to form polyaniline in the presence of base, causing a decrease in the reactivity of aniline towards coupling reaction. For improving the yield, alkyl amines such as DOA were used along with 4-bromo biphenyl and iodo benzene as reactants (Table 2). Initially the reaction was carried out with K2CO3 as base and starting material as 4-bromo biphenyl but no conversion was observed (Table 2, entry 1). The desired C-N cross coupling product was obtained with a yield of 33 % when K-tBuO was used as a base (Table 2, entry 2). Temperature strongly affects the kinetics and efficiency of coupling reactions as an increase in temperature increases the reaction productivity. For the reaction in Table 2, entry 2 the yield of the desired product increased from 33 % to 70 % (Table 2, entry 3) on increasing the temperature from 110 °C to 150 °C. The desired product was isolated by column chromatography and was characterized by GCMS (GCMS data 4) and NMR (Fig. 12)
The effect of base on the reaction outcome of DOA and iodo benzene as substrates was studied using a strong base like K-tBuO and a comparatively weaker base like NaOH. On using K-tBuO as base the C-N cross coupling product (N, N-dioctyl aniline) was obtained (Table 2, entry 4), while with NaOH as base the C-C cross coupling product (biphenyl) was obtained (Table 2, entry 5). The same type of results was also obtained with aniline (Table 1, entry 7 and 8). These clearly indicates that strong base like K-tBuO favours Buchwald Coupling reaction while comparatively weaker base like NaOH favours the Ullmann coupling reaction.
Table 1: Results of Buchwald-Hartwig reaction of Aniline with iodo benzene & 4-bromo biphenyl under various conditions[a].
Table 2 : Results of Buchwald-Hartwig reaction of DOA with iodo benzene & 4-bromo biphenyl under various conditions[a].
As per as our literature survey this study is the first report in which monodispersed amine capped Pd NPs, prepared by Digestive ripening (DR) process, that form stable dispersions in organic solvents were used as catalysts in the Buchwald-Hartwig amination reaction of aryl halides and aryl / alkyl amines. These amine capped Pd NPs perform the dual role of acting as catalyst and substrate in these Pd catalysized C-N cross coupling reaction. For maintaining the stable dispersions of Pd NPs in organic solvents excess amount of amine was added during reaction, so that these Pd NPs retain their catalytic activity and efficiency throughout the reaction. These catalysts can be easily separated from the solution by precipitation/centrifugation and can be reused for the next cycle of reaction. Under optimized reaction conditions good amount of yield was obtained with 4-bromo biphenyl in the presence of K-tBuO as base with DOA capped Pd NPs dispersed in DMSO as both catalyst and reactant. Further investigation should be carried out for improving the yield of this reaction by varying the solvent used to other organic solvents such as toluene, trifluoro toluene, tert-butyl toluene and so on. The catalytic activity of these amine capped Pd NPs should also be explored for Ullmann Coupling, Sonogashira coupling, Heck Coupling and other coupling reactions.
I would like to thank Indian Academy of Sciences, Bangalore for giving me the opportunity to work at National Chemical Laboratory, Pune, one of the most prestigious research institutions of our country.
I would like to express my heartiest gratitude to my guide, Dr. B.L.V. Prasad for his stimulating suggestions, enthusiastic discussions, immense knowledge, patience, motivation and constant encouragement. I am extremely fortunate and privileged to have got the opportunity of working under his supervision and guidance for this period of two months.
I would like to extend my sincere thanks to Mr. Jayesh R Shimpi for providing his valuable guidance and assistance throughout the period of this project. Without him, it would have been difficult for me to complete the work within the stipulated time, I would like to thank him for his inspirational guidance, timely help, valuable suggestions and discussions throughout my project.
I would like to express my generous thanks to all the delightful members of the SAAM group, for their constant encouragement, guidance, assistance, support, advice and providing an ideal atmosphere for working in this span of two months, Dr. Shankar Dalavi, Ashish Kawale, Pooja Deshpande, Mayur Baravkar, Dr. Vijay Chaudhari, Abhijit Bera and Dr. Kaustav Bhattacharjee. I wish you all happiness in life and best of luck for your future endeavors.
I would like to acknowledge all these people for making these research work possible, an experience that I will cherish forever.
I am grateful to Dr. Saroj L Samal, Assistant Professor, Department of Chemistry, National Institute of Technology, Rourkela for his guidance in applying for these internship and recommendation letter. I would like to thank all the faculty members of the Department of Chemistry, National Institute of Technology, Rourkela.
I thank all the supporting staff of AuthorCafe, for providing such an excellent space to write my report and to collaborate with my guide.
Most importantly, I am deeply obliged to my family, teachers and friends for their love care and everlasting support towards me.
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