A Biogenic Green Synthesis Of Palladium Nanoparticles By Sapindus Mukorossi Seed Extract

Undoubtedly, the Suzuki-Miyaura cross-coupling reaction is one of the most elegant and powerful tools for constructing Carbon-Carbon bonds. In the industrial arena, specifically, Suzuki-Miyaura cross-coupling reaction affords biaryls, the structural motif found in many polymers, agrochemicals, and pharmaceutical intermediates.^[1,2]

Compared to the bulk counterpart, nanoparticles (NPs) are nowadays considered to be very attractive catalysts since they have a high surface to volume ratio and their surface atoms are also highly active. Pd-NPs of different sizes and shapes are generally synthesized by various wet synthesis processes such as chemical,^[3] sonochemical,^[4] and polyols reduction.^[5]

Though these wet synthesis processes are simple and provide high growth rate and high yield, these methods are not environmentally friendly and therefore researchers are continually searching for alternative methods in the synthesis of NPs. Biological materials such as plant extracts, microorganisms have the required reduction potential for the synthesis of NPs and additionally, they also act as stabilizing agent for the NPs.^[6] Sapindus mukorossi (S. mukorossi) a locally available plant In the North East part of India, commonly known as ‘monisol’ or ‘ritha’ appears in the months of July-August, and ripens by November-December.

The dried seeds are available in the market for purchase, throughout the year. The aqueous extract of S. Mukorossi fruit pericarp (soapnut shell) composed mainly of saponins (natural surfactants), flavonoids and carbohydrates.^[7] The native people of Asia mainly used soapnut for washing hair. In addition, they are also reported to have some medicinal properties such as anti-inflammatory^[8] and anti-microbial activity.^[9]

At first, soapnut shells were collected, cleaned and shade dried. After complete drying, they were ground using a domestic blender. For the preparation of plant extract, 10g of the finely grounded seeds were stirred in a beaker for about 30 min in 60 ml distilled water at 60 ⁰C and was then filtered with muslin cloth at ambient temperature. The filtrate aqueous extract was used as a reducing agent for the preparation of Pd NPs. The synthesized NPs were characterized using UV-visible spectroscopy, powder XRD, EDX and TEM analyses.

From the XRD pattern of the Pd NPs, three distinct picks are observed at 40.010, 46.040 and 67.790 which correspond to (111), (200) and (220) planes respectively that could be indexed to fcc phase of Pd NPs (JCPDS ≠88-2335). From the broadening of the XRD peak, the synthesized particles are assumed to be in nano range. EDX analysis of Pd NPs reveals the presence of Pd signal devoid of other impurities. The TEM images of the particles indicate that the NPs preferentially crystallize in a spherical shape and most of the particles are fall in the range 3-5 nm size and the average particle diameter was found to be about 3.6 nm.

This methodology follows green chemistry principles as the reaction is performed using biomass (natural feedstock) for the generation of the Pd NPs

The synthesized nanocatalyst was successfully utilized for an efficient Suzuki-Miyaura cross-coupling reaction at room-temperature (Scheme 1). The reaction also performed smoothly over wide varieties of electronically diverse arylhalides with arylboronic acids.

We also examined the reusability of our catalyst and found that the catalyst is reusable up-to 5^th cycle with slight loss of catalytic activity. After 5^th cycle the catalytic activity decreases sharply which may be due to deactivation of the catalyst during the course of the reaction and recovery process.

This methodology offers a green alternative to the existing protocols since the reaction proceeds under ligand-free conditions in an aqueous medium.

References

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4. A. Nemamcha, J. L. Rehspringer and D. Khatmi, J. Phys. Chem. B, 2006, 110, 383-387.
5. Y. Xiang, J. Chen, B. Wiley and Y. Xia, J. Am. Chem. Soc., 2005, 127, 7332-7333.
6. S. Iravani, Green Chem., 2011, 13, 2638.
7. (a) A. Upadhyay, D. K. Singh, Rev. Inst. Med. Trop. Sao Paulo 2012, 54(5), 273-280., (b) A. Iqbal, U. Khan, P. Shatista, S. A. Mohammad, U. A. Viqar. Pak. J. Pharm. Sci., 1994, 7, 33-41.
8. K. Takagi, E. H. Park, H. Kato, Chem. Pharm. Bull. 1980, 28, 1183-1188.
9. R. A. Kamal, J. Radhika, S. Chetan, Ethnobot. Leafl. 2010, 14, 402-412.

The study, A green synthesis of palladium nanoparticles by Sapindus mukorossi seed extract and use in efficient room temperature Suzuki–Miyaura cross-coupling reaction was recently published by Raju Kumar Borah, Abhijit Mahanta, Anurag Dutta, Utpal Bora, and Ashim J. Thakur in the journal Applied Organometallic Chemistry.