(iron oxide)-supported nanocatalysts: synthesis

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Rakesh K. Sharma,*a Sriparna Dutta,a Shivani Sharma,a Radek Zboril,b. Rajender S. ... Department of Physical Chemistry, Palacky University, Šlechtitelů 11, 783 71. Olomouc ... has published numerous book chapters, reviews and research articles in ...... 116 R. K. Sharma, Y. Monga, A. Puri and G. Gaba, Green. Chem.

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Fe3O4 (iron oxide)-supported nanocatalysts: synthesis, characterization and applications in coupling reactions Rakesh K. Sharma,*a Sriparna Dutta,a Shivani Sharma,a Radek Zboril,b Rajender S. Varma*b and Manoj B. Gawande*b The use of magnetic nanoparticles as a solid support material for the development of magnetically retrievable catalytic systems has led to a dramatic expansion of their potential applications as they enable environmentally-friendly and sustainable catalytic processes. These quasi-homogeneous catalysts possess numerous benefits such as ease of isolation and separation from the desired reaction mixtures using an external magnet and excellent recyclability. Consequently, much effort has been directed towards the synthesis of magnetically isolable nano-sized particles by developing methods such as coprecipitation, thermal decomposition, microemulsion, hydrothermal techniques etc. Further, in order to render them suitable for catalytic applications, several protection strategies such as surfactant/polymer, silica and carbon coating of magnetic nanoparticles or embedding them in a matrix/support have been reported in the literature. This review focuses on the substantial progress made in the fabrication of nanostructured catalysts with special emphasis on the protection and functionalization of the magnetite nanoparticles (Fe3O4). Finally, considering the importance of coupling chemistry in the field of organic

Received 27th March 2016, Accepted 18th April 2016

synthesis, a broad overview of the applications of these magnetite nanoparticle-based catalysts in several

DOI: 10.1039/c6gc00864j

types of coupling reactions has been presented. The future of catalysis lies in the rational design and development of novel, highly active and recyclable nanocomposite catalysts which would eventually pave

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the pathway for the establishment of green and sustainable technologies.

Introduction The role of nanomaterials in modern technologies is becoming increasingly significant because of their unique physicochemical properties and large-scale applicability in the field of catalysis, imaging, photonics, nanoelectronics, sensors, biomaterials and biomedicine.1–6 In recent years, nanomaterials have been widely utilized as a solid support material for the design of environmentally benign heterogeneous catalysts to address various economic and environmental issues.7–16 Such types of quasi-homogeneous catalytic systems possess uniform and precisely engineered active sites similar to those of their homogeneous counterparts, and therefore combine the best attributes of both homogeneous and heterogeneous catalysts. In this context, magnetite nanoparticles supported catalytic

a

Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi-110007, India. E-mail: [email protected] b Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic. E-mail: [email protected], [email protected]

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systems have attracted a considerable amount of attention owing to their exceptional properties such as ease of availability, chemical inertness, high surface area to volume ratio and excellent thermal stability (Fig. 1).17–28 In addition, these magnetic nanoparticles (MNPs) allow facile separation from the reaction mixture through an external magnet that not only eliminates the necessity of cumbersome filtration and centrifugation procedures but also reduces energy consumption, catalyst loss and saves time in achieving catalyst recovery (Fig. 2). Considering the immense benefits of MNPs (i.e. excellent separation properties and low toxicity coupled with better biocompatibility) several research groups have realized their potential as a versatile solid support for the fabrication of novel hybrid photo-catalytic systems which exhibit enhanced catalytic activity.29,30 A variety of suitable methods have been designed for synthesis of MNPs of different shapes, sizes and compositions, yet their successful applications are highly dependent on the stability of these particles under varied reaction conditions.31–33 Magnetic nanoparticles often show a strong tendency to agglomerate as a result of self-interactions and hence suitable protecting agents have been explored for stabilizing them. Surfactant/polymer-, silica- and carbon-

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tically recoverable nanocatalytic systems. Initially, the synthetic methodologies available for the preparation of magnetite nanoparticles are outlined followed by the strategies for their protection against oxidation and acid erosion; often used functionalizing agents for the attachment of catalytically active species are highlighted. Finally, the applications of the magnetite based catalysts employed in various coupling reactions are reviewed. Recent advancements in the synthesis of MNPs

Fig. 1

Attractive features of magnetite nanoparticles (MNPs).

coating or embedding them in a matrix/support are some of the vital strategies that have been adopted for imparting stability to these nanostructured particles.34–36 It is noteworthy that in many cases the protective outer shells formed as a result of coating not only stabilize them but also provide sites for functionalization with other hybrid NPs, and organic groups suitable for specific desired applications. Lately, the focus has been to functionalize MNPs in order to broaden their applicability especially in the field of catalysis as evidenced by the use of MNPs for the fabrication of magnetite based catalysts.37–41 The main focus of this review is to provide a broad overview about the design and development of magne-

Dr R. K. Sharma is a professor and co-ordinator of the Green Chemistry Network Centre (GCNC) at the Department of Chemistry, University of Delhi (D.U.), India. He obtained his doctoral degree from D.U. in the year, 1986. Thereafter, he went to the University of Tokyo and Kumamoto University and worked on metal–bimolecular interactions on a JSPS Post-Doctoral Fellowship. His research Rakesh K. Sharma interests focus on the development of metal selective functionalized silica gels for their applications as scavengers, sensors and catalysts, designing of novel metal-chelating inhibitors of transcription factor NF-κB-DNA binding, chemical speciation, molecular modelling studies etc. He has published numerous book chapters, reviews and research articles in renowned international journals. Besides, he has been acknowledged worldwide for his sincere efforts in bringing about a major reformation in the scientific community by promoting and popularizing Green Chemistry. He is also the Honorary Secretary of the RSC (North India Section) and in charge of an International Chapter of ACS-GCI.

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The synthetic strategies for preparing magnetite nanoparticles play a significant role in tailoring the particle morphology, composition, magnetic and surface properties which greatly expand their application in a wide variety of multidisciplinary fields. Some of the most popular routes that have been commercially utilized to synthesize high quality MNPs include co-precipitation,42–49 thermal decomposition,50–58 microemulsion,59–63 hydrothermal64–67 and solvothermal processes.68,69 The selected strategies mentioned are included in the literature,70,71 hence the prime focus of this review is to only highlight the recent trends and developments in the fabrication of MNPs. Template-mediated synthesis Template-assisted synthesis – “a simple and versatile methodology” has drawn the attention of several chemists and engineers worldwide as it leads to the fabrication of nanomaterials with well-defined structures.72–75 This technique requires a

Sriparna Dutta was born in India in the year, 1990. She received her B.Sc. (Hons.) degree in 2011 and M.Sc. degree (Inorganic specialization) in 2013 from the University of Delhi (D.U.), India (Sri Venkateswara College). She is currently pursuing her doctoral studies under the supervision of Professor R. K. Sharma and is an active member of the Green Chemistry Network Centre (GCNC). Her Sriparna Dutta research efforts are directed towards the fabrication of magnetic silica based nano-catalysts and their use in various organic transformations. She is a recipient of the Prof. K.N. Johri Memorial Gold Medal (awarded by D.U. for being the university topper with inorganic specialization).

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Fig. 2

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Effect of an external magnetic field on the nanoparticles.

sacrificial template of metal/biological/polymeric or chemical origin to shape the initial structure which is eventually removed either partially or completely with an acidic or basic solution to produce free standing nanostructures. Using the template solvothermal technique, six unique shaped iron oxide nanoparticles were obtained.76 Appreciably, all the six shapes, i.e. nanorods, nanohusks, distorted cubes, nanocubes, porous spheres and self-oriented flowers (Fig. 3), could be synthesized by simply changing the precursor iron salts used while keeping the rest of the parameters fixed (i.e. cetyltrimethlammonium bromide (CTAB) as a template, cyclohexane–water– pentanol as a reaction solvent and urea as a hydrolysing agent). A comprehensive characterization of the synthesized nanomaterials provided a deep insight into the shapes, phases and important physico-chemical properties.

netic and surface properties of the developed nanostructures because ultrasonic irradiation creates rather unusual reaction conditions such as a short duration of extremely high temperatures and pressures that cannot be realized by other traditional energy sources.77,78 There have been only a handful of reports in the literature wherein MNPs have been prepared via the sonochemical technique.79–82 Among them, Abu-Much and co-workers carried out the synthesis of magnetite particles and investigated the effect of external magnetic field on the growth of these MNPs.83 The outcome of the studies revealed that the extreme conditions arising due to ultrasonic waves led to the formation of rod-like particles instead of spherically shaped particles.

Use of ultrasonic irradiation for generation of well-tuned MNPs

In recent years, interest has surged in the development of protocols wherein magnetic nanoparticles are synthesized using continuous flow reactors which provide the opportunity to

Sonochemical technique is a new synthetic approach that provides precise control over size, morphology, composition, mag-

Shivani Sharma (born 1988) obtained her B.Sc. (Hons.) degree in 2008 and M.Sc. degree (specialization in Inorganic Chemistry) in 2010 from the University of Delhi, India. She then joined the “Green Chemistry Network Centre” as a Research Scholar under the guidance of Professor R. K. Sharma. Her research work is based on the design and development of silica based organic–inorganic hybrid Shivani Sharma nanomaterials and their applications in the field of sensors and catalysis. She has authored several publications in prestigious journals of the Royal Society of Chemistry (RSC).

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Continuous synthesis of MNPs using flow and microwave reactors

Radek Zboril received his PhD degree at the Palacky University in Olomouc, Czech Republic. After his doctoral studies, he spent a lot of time at universities around the world in locations such as Tokyo, Delaware, and Johannesburg. Currently, he is a professor at the Department of Physical Chemistry and a General Director of the Regional Centre of Advanced Technologies and Materials at the Palacky Radek Zboril University in Olomouc, Czech Republic. His research interests focus on nanomaterial research including iron- and iron oxide-based nanoparticles, silver nanoparticles, carbon nanostructures, and magnetic nanoparticles, including their synthesis, physicochemical characterization, and applications in catalysis, water treatment, antimicrobial treatment, medicine, and biotechnology.

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Fig. 3 SEM images of shaped iron oxides with six different shapes: (a) nanorods, (b) nanohusks, (c) distorted cubes, (d) nanocubes, (e) porous spheres and (f ) self-oriented flowers. Reprinted with permission from ref. 76. Copyright 2015 Nature.

change the way synthetic chemistry is performed at the research and industrial levels.84 The outstanding features of flow reactors have led to their widespread applicability which include improved thermal management, enhanced mass transfer, mixing control, scalability, rapid chemical reactions and ability to withstand harsh reaction conditions. Flow chemistry

promotes efficient mixing and rapid chemical reactions at the nanolitre scale, and thus allows better control over the synthetic parameters which help in tuning the nanoparticle sizes and properties. In a quest to look for better techniques for the fabrication of MNPs, Togashi et al. synthesized water dispersible magnetite (Fe3O4) nanoparticles using a continuous

Prof. Rajender S. Varma was born in India (Ph.D., Delhi University 1976). After postdoctoral research at the Robert Robinson Laboratories, Liverpool, UK, he was faculty member at the Baylor College of Medicine and the Sam Houston State University prior to joining the Sustainable Technology Division at the US Environmental Protection Agency in 1999. He has over 40 years of research experience in Rajender S. Varma the management of multi-disciplinary technical programs and is extensively involved in sustainable aspects of chemistry that includes development of environmentally benign synthetic methods using alternative energy input using microwaves, ultrasound and mechanochemistry and efficient technologies for the sustainable remediation of contaminants, and environmental sciences. Lately, he has focused on greener approaches to the assembly of nanomaterials and sustainable applications of magnetically retrievable nano-catalysts in benign media. He is a member of the editorial advisory board of several international journals and has published over 435 scientific papers and has been awarded 15 US Patents.

Dr Manoj B. Gawande received his Ph.D. degree in Chemistry in 2008 from the Department of Chemistry, Institute of Chemical Technology (formerly UDCT), Matunga, Mumbai, India, with Prof. R.V. Jayaram. After several research stints in Germany, South Korea, Portugal, Singapore, and England, recently, he worked as a Visiting Professor/ Scientist with Prof. Dunwei Wang, Boston, College, Boston, Manoj B. Gawande USA and Prof. Rajender S. Varma at EPA, Cincinnati, USA. Presently, he is working as a Senior Researcher and Head of the Nanocatalysis Research Laboratories at RCPTM, Faculty of Science, Palacky University, Olomouc, Czech Republic. His research interests include nanocatalysis, advanced nanomaterials and their applications. He has published more than 75 scientific papers, including reviews, patents, and editorials.

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tubular flow reactor under high temperature and pressure conditions in the presence of 3,4-dihydroxyhydrocinnamic acid (DHCA);85 the ensuing nanoparticles had superparamagnetic properties at room temperature with a Saturation Magnetization (Ms) nearly identical to that of the bulk nanoparticles. Besides flow, microwave (MW) reactors, based on exploitation of dipolar polarization and ionic conduction effects, are also gaining tremendous popularity in the context of MNPs synthesis owing to their fascinating advantages such as uniform heating, fast reaction under a controlled environment, products with high purity, yield etc.86 The MW-assisted synthesis leads to the formation of spherical nanostructures that possess a narrow size distribution and a higher degree of crystallinity. Kalyani and co-workers have reported the MWassisted synthesis of ferrite magnetic nanoparticles of different particle sizes by controlling the reaction temperature;87 apparently, an increase in the incubation temperature leads to the increase in particle size and saturation magnetization (Ms) values which could be attributed to the decrease in supersaturation at an elevated temperature. Extended LaMer approach for controlled synthesis of MNPs Very recently, Vreeland and co-workers have introduced a new methodology termed, “Extended LaMer approach” for the synthesis of highly crystalline MNPS by establishing steady state growth conditions via the continuous and controlled addition of iron oleate precursors to the reaction solution.88 This method allowed for a high level of reproducibility in particle size from batch to batch by simply varying the reaction duration and volume of the added precursor. It was observed that the blocking temperature of the developed MNPs could be tuned according to the target application because of the exquisite size control property afforded by the extended LaMer mechanism. Greener sustainable protocols Several greener biogenic methods have been reported in the literature for the eco-friendly synthesis of the MNPs89–92 as exemplified by the design of iron oxide nanoparticles from plantain leaf extract.93 This straightforward procedure simply involved mixing of iron(III) chloride hexahydrate and sodium acetate in freshly prepared plantain peel extract solution containing carbohydrates at 70 °C, followed by washing of the resultant solution with ethanol. Plantain peels mainly comprise cellulose, pectin, lignin and most importantly, polyphenols and carbohydrates that can function as both reducing and capping agents thus promoting effective synthesis of MNPs; low cost of synthesis and nontoxicity are the key features that render this an overall effective protocol. Another such green phytosynthesis of ferrite nanoparticles has been carried out by Buazar et al.94 using starch-rich potato extract without any additives such as reducing agents, acids and organic solvents (Scheme 1). The authors reported that the organic functionalities present in the potato extract such as aldehyde groups, hydroxyl groups, and carboxyl groups act as a template for the growth of MNPS due to their ability to undergo intra- and

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Scheme 1 extract.

Synthesis of Fe3O4 nanoparticles using starch-rich potato

intermolecular hydrogen bonding. Further, the synthesized magnetite samples exhibited excellent catalytic activity in the decolorization of aqueous solutions of cationic methylene blue. Bio-inspired co-precipitation methodology The fabrication of new materials with advanced properties and well defined morphologies by incorporating nature’s biomineralization strategies has intrigued scientists for several decades.95,96 Drawing an inspiration from the mineralization tactic adopted by magnetotactic bacteria, Lenders et al. demonstrated an aqueous, room temperature co-precipitation approach for preparing Fe3O4 NPs using a biologically derived M6A peptide additive that worked as nucleation and growth controllers (Fig. 4).97 The method is initiated via slow coprecipitation of FeIII/FeII salts by ammonia diffusion, during which the precipitation of ferrihydrite occurs at low pH which is eventually converted to magnetite crystals with sizes well in the ferrimagnetic regime upon increase in pH. In contrast to the conventional co-precipitation approach, which shows poor control over particle dimensions and morphology, the bioinspired co-precipitation methodology allows tuning of average crystal size by simply manipulating the iron concentration and NH3 influx. A comparative overview of these synthetic approaches has been provided in Table 1. Protective surface coating of MNPs Magnetic nanoparticles possess intrinsic instability and show a high propensity towards agglomeration in order to reduce the energy associated with their large surface area to volume ratio which is a consequence of their nanometre sized dimensions.98 Further, certain undesired interactions due to either oxidative atmospheric conditions or external reagents can result in significant deterioration of the unique properties associated with these valuable magnetically separable par-

Fig. 4 Experimental design of the bioinspired approach for obtaining magnetite from a ferrihydrite precursor (Fe2O3·xH2O). Reprinted with permission from ref. 97. Copyright 2014 American Chemical Society.

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Table 1

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Comparison of various synthetic strategies

Method

Synthetic conditions

Reaction temp (°C)

Reaction period

Size distribution

Shape control

Template assisted synthesis Sonochemical method Flow and MW method Extended LaMer approach Green Method Bio-inspired co-precipitation

Simple Extreme conditions Harsh conditions Complicated Simple, eco-friendly, easy to handle Simple

80–180 Very high Very high 350 70–80 r.t.

Minutes Minutes Hours/minutes Hours Hours/minutes Hours/days

Relatively narrow Narrow Very narrow Very narrow Narrow Very narrow

Very good Good Good Very good Poor Very good

ticles. On oxidation, at times, the formation of a thin layer of oxides is observed which is undesirable as it dramatically changes the inherent properties of these nanoparticles. Synthesis of MNPs while maintaining the long term stability is indeed a key challenge that has captivated the attention of researchers. This issue has been resolved via the design of appropriate coating agents such as surfactants and polymers,99–112 inorganic components like silica,113–127 carbon,128–135 precious metals such as Ag or Au,136–148 metal oxides149–154 and hydroxides.155–158 Recently, Tan et al. accomplished the protection of MNPs using silica as the coating agent (well evident from elemental distributions of the developed core–shell nanostructure as depicted in Fig. 5), following the sol–gel approach.159 The results obtained from the elemental mapping studies clearly revealed that the Fe lies in the core, while the Si element is detected in the shell region. The salient features of all the above mentioned coating strategies have been summarized in Fig. 6.

Surface modification by functionalization Tuning the surface properties of MNPs using appropriate functionalizing agents/linkers represents a very promising approach for the design of advanced catalytic systems. Such functionalized MNPs not only display excellent chemical and thermal stability but also provide the scope for introducing additional functionalities such as organic ligands or metal complexes.32 Literature reports suggest that the surface modification of MNPs can be accomplished in two possible ways: (a) non-covalent adsorption of surfactants, polymers and bifunctional molecules; (b) covalent approach that leads to the formation of a relatively stable linker between hydroxyl groups on the nanoparticle surface and the anchoring agents.27,160 Amongst a plethora of linkers utilized successfully for functionalizing MNPs, 3-aminopropyltrimethoxysilane (APS), 3-aminopropyltrimethoxysilane (APS), (3-Aminopropyl) triethoxysilane (APTES), dopamine, cyclodextrin, sulphonic

Fig. 5 The elemental mapping shows homogeneous dispersion of Fe, Si, C, Ni and O elements in the [email protected]@Ni–L core–shell microspheres. Reprinted with permission from ref. 159. Copyright 2015 Royal Society of Chemistry.

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Fig. 6

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Coating strategies incorporated for stabilizing MNPs.

acid, bisguanide, (S)-α,α-Diphenylprolinol trimethylsilyl ether, imidazole and sulfamic acid have been employed frequently for catalytic applications.161–168 Table 2 provides an illustrative description of the well-known strategies for introducing different types of functional molecules onto the surface of bare or protected MNPs. Characterization The magnetite supported catalysts are characterized using various standard physicochemical techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Powder X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), elemental analysis, inductively coupled plasma spectroscopy, vibrating sample magnetometry (VSM) and thermo-gravimetric analysis (TGA) typically deployed for nanoparticles.182,183 FTIR is an important tool for functional group analysis and is frequently employed to confirm the surface modification of the magnetic nanoparticles. XRD provides information about the crystallographic structure, chemical composition and physical properties of the nanocomposites. TEM and SEM help in analysing the size, shape and morphology of the nanocatalyst that seem to be imperative for tailoring the catalytic properties. The catalytic amount of magnetic catalysts is measured through AAS, ICP or elemental analysis. The XPS technique predicts the oxidation state of the metal present in the final catalyst which is helpful in deducing information regarding the mechanism of a chemical reaction, while TGA is frequently used to investigate their thermal stability. VSM is utilized to examine the magnetic properties of the unmodified as well as modified MNPs.

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Applications as catalysts in various coupling reactions Magnetite NPs have enhanced every aspect of organic chemistry as potentially useful catalysts owing to their fascinating properties. Considering the increasing emphasis on adopting greener pathways for organic synthesis, coupling reactions have gained tremendous significance as they involve the incorporation of all the reactants in the final product leading to overall waste minimization via atom efficiency. They can be briefly classified into two types: (a) homocoupling (C–C) and (b) heterocoupling (C–O, C–S, C–N, C–C) reactions. This review highlights the use of magnetite-based nanocatalysts that have proved their prowess in these reactions. The ease of magnetic separation of the catalyst from the reaction mixture and recyclability are the vital attributes of catalytic systems based on a magnetite nanosupport. C–C coupling C–C coupling is one of the most versatile reactions deployed in synthetic organic chemistry as it leads to the formation of industrially and pharmaceutically significant products;184,185 transition metal catalysts have been developed as excellent reagents for the formation of various C–C simple and stereocontrolled reactions.186–190 Suzuki, Heck, Sonogashira, Stille, Hiyama coupling reactions are amongst the most noteworthy categories of C–C cross coupling reactions that are generally catalyzed by soluble palladium complexes with various ligands.191–194 However, despite the remarkable progress achieved in this area to date, a number of challenges remain. The efficient separation and subsequent recycling of the homogeneous transition metal catalysts continues to present a

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Functionalization strategies for surface modification of MNPs

Functionalization type and description

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Illustration of the functionalization process

Phosphonates Magnetite nanoparticle (MNP)-supported asymmetric catalysts containing phosphonate linkages were synthesized by Hu et al. for the asymmetric hydrogenation of a wide range of aromatic ketones.170

Carboxylates Karaoğlu and co-workers reported the synthesis and application of PPCA functionalized Fe3O4 nanoparticles which was used for catalyzing the Knoevenagel reaction.173

Alkoxyorganosilanes Sharma et al. used APTES functionalized silica coated magnetite nanoparticles for preparing a magnetically retrievable nanocatalyst for reductive amination of ketones.175

Sulphonates Zheng et al. established the synthesis of sulphonic acid functionalized [email protected] nanoparticles which worked as a highly promising catalyst in the acetalization reaction of benzaldehyde and ethylene glycol.177

Sulfamates Kassaee et al. fabricated a novel organic– inorganic hybrid heterogeneous catalyst by grafting chlorosulfuric acid on the surface of amino-functionalized Fe3O4 nanoparticles that afforded sulfamic acid-functionalized magnetic Fe3O4 nanoparticles (SA-MNPs).179

real scientific task. The development of heterogeneous catalysts seems particularly suitable for these reactions due to the wide accessibility and excellent stability of these solid supports. Magnetite nanoparticles are a recent and interesting alternative for these reactions. Suzuki coupling reaction The Suzuki coupling reaction, involving the cross coupling of organoborons and organohalides, has arguably become one of the most powerful synthetic methods for the preparation of a wide range of biaryl compounds which serve as important building blocks and/or intermediates for various natural products, pharmaceuticals, and polymers;195–198 several reasons account for the development of this reaction at both the academic and industrial levels. First, the scope of the protocol

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could be extended for various functional groups due to the mild reaction conditions, which make this an attractive system for the total synthesis of complex drug molecules. Second, boron compounds are readily available, stable, and display relatively low toxicity. Third and most importantly, the reaction works well with a wide range of substrates. Although several homogeneous catalytic systems have already been reported in the literature for effectively catalysing the Suzuki reactions, however they suffer from serious recyclability and reusability issues.199–201 Magnetite nanoparticles manage to overcome such barriers by providing a solid support for immobilizing Pd-based complexes, thus resulting in the formation of recyclable and reusable catalysts. Guo and co-workers designed dipyridyl supported magnetite NPs through click chemistry and evaluated the activity of

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the MNP-supported catalyst in the Suzuki coupling reaction (Scheme 2).202 Firstly, they prepared magnetic nanoparticles by the conventional coprecipitation method and then coated them with a thin layer of silica to prevent the aggregation of the nanoparticles; the ensuing catalyst worked efficiently in the Suzuki coupling reaction and could be readily separated using an external magnet. Attempts have been made towards accomplishing the Suzuki reactions in water as the solvent that would render these cross couplings “cleaner and greener.” Jung and coworkers reported the synthesis of a magnetically retrievable palladium nanocatalyst through the immobilization of Pd (OAc)2 on the surface of a Fe3O4 supported ionic liquid for the Suzuki coupling reaction (Scheme 3);203 Pd(OAc)[email protected] nanocatalyst showed excellent catalytic activity, high stability and recyclability. Despite the success, one of the most serious limitations of the Suzuki–Miyaura coupling reaction has been the poor reactivity of more economical aryl chlorides as compared to aryl bromides and iodides. Consequently, the focus has shifted towards developing such catalytic systems that would efficiently process challenging substrates such as aryl chlorides whilst maintaining relatively mild conditions and low catalyst loadings. Amali et al. developed a methodology to stabilise metallic Pd (0) on the surface-functionalized magnetite NPs leading to a magnetically separable catalyst that would satisfy the requirements of mild reaction conditions and would exhibit an efficient catalytic activity in the ligand-free Suzuki–Miyaura reactions;204 highly branched polyethylenimine (PEI) was used to entrap Pd nanoparticles on the surface of Fe3O4 so that structurally stable catalytic sites could be obtained. For obtaining the PEI functionalized surface, ferrite nanoparticles were first synthesized using the co-precipitation method. Trisodium citrate was then used as the capping agent

Scheme 2 Suzuki reaction catalyzed by a Pd (II) dipyridyl catalyst immobilized on MNP.

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Scheme 3 (a) Synthesis of the Pd(OAc)[email protected] nanocatalyst; (b) TEM image of the catalyst Pd(OAc)[email protected] nanocatalyst. Reprinted with permission from ref. 203. Copyright 2009 KCS publications.

and the ensuing citrate-capped ferrite nanoparticles were functionalized by PEI, metallated using sodium tetrachloropalladate and finally reduced to Pd (0) by the addition of sodium borohydride. The presence of amine groups on the surface of PEI functionalized ferrite nanoparticles prevented metal leaching during the reaction. Besides, the superparamagnetic nature of the support material facilitated the ease of separation of the catalyst and allowed it to be recycled over 5 consecutive runs. Liao and co-workers fabricated a nanofibrous networked metal–organic gel (G1-MNPs) by simply mixing 4,4′,4′′-(1,3,5-triazine-2,4,6-triyl)tris(N-( pyridin-3-ylmethyl)benzamide) (L) and Pd(COD)(NO3)2 in CHCl3–MeOH with a Pd/L molar ratio of 1 : 1 in the presence of the magnetite nanoparticle (Scheme 4).205 The synthesized Pd(II) xerogel efficiently catalyzed the Suzuki coupling reaction and could be easily isolated using an external magnet. Wet impregnation strategies have gained an increasing momentum in the field of catalysis for synthesizing metalFe3O4 nanocatalysts. Cano et al. impregnated palladium on magnetite and used it as a catalyst for the Suzuki Miyaura coupling reaction;206 the reaction could be performed with a broad range of aromatic iodinated substrates. Interestingly, no

Scheme 4 Representation of magnetic gel nanofibers for organic transformation.

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external ligands were required thus eliminating the additional complexities associated with the ligand purification steps etc. Core shell NPs have emerged as an interesting approach for the transition metals owing to their high stability as shown by design of a magnetic NP supported (β-oxoiminato) ( phosphanyl) palladium catalyst via the immobilization of a triethoxysilyl-functionalized Pd complex on the surface of robust SiO2/Fe3O4 under refluxing toluene conditions; the catalyst was used for the Suzuki coupling reaction.207 Reactions of phenylboronic acid with activated aryl chlorides (such as 4-chlorobenzonitrile, 1-chloro-4-nitrobenzene, and 1-chloro-2nitrobenzene, using 0.5 mol% catalyst) led to the desired products with excellent yields after 3 h at 60 °C. The system was particularly efficient for the coupling of phenylboronic acid with deactivated aryl chlorides, including 4-chloroanisole, 4-chlorotoluene, 4-chlorophenol, 2-chloroanisole, and 2-chlorotoluene. The most important attribute of the protocol was that the catalyst could be recovered by an external magnet, and further reused for more than 10 consecutive times without any significant loss in its activity. Zhang and co-workers designed an efficient and recyclable magnetic-nanoparticle-supported Pd catalyst ([email protected]) for the Suzuki coupling reaction of organoboron derivatives with alkynyl bromides. Catalyst preparation was achieved in a stepwise manner beginning with the synthesis of [email protected] nanoparticles using a sol–gel approach. The [email protected] nanoparticles were next functionalized by the (diphenylphosphino)ethyltriethoxysilane ligand and finally metallated using palladium acetate solution (Scheme 5).208 A wide range of organoboron compounds including aryl and alkenyl boronic acids, potassium aryltrifluoroborates and sodium tetraphenylborate reacted efficiently with 1-bromo-2-substituted acetylenes to furnish the corresponding coupling products in good to excellent yields using 0.5 mol% of the [email protected] nanocatalyst. Zhu and Diao developed an efficient magnetic carbon nanocomposite supported Pd catalyst by immobilizing Pd nanoparticles on magnetic [email protected] nanocomposites (MFC) using the precipitation deposition method (Scheme 6).209 To evaluate the activity and the stability of the supported Pd nanocatalysts, the Suzuki reaction was chosen as the model reaction. The results showed that the catalyst could be easily separated

Scheme 5 Preparation of magnetic-nanoparticle-supported palladium catalyst [email protected] and its catalytic application in the Suzuki coupling reaction.

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Scheme 6 Synthesis of the Pd/MFC catalyst and its application in the Suzuki coupling reaction.

from the reaction medium by employing an external magnetic field because of the superparamagnetic behaviour of Fe3O4 and further reused for several cycles with sustained selectivity and activity. Yang and co-workers successfully established one-pot synthesis of magnetic N-heterocyclic carbene-functionalized silica NPs using a reverse micelle strategy through the co-condensation of IPr-bridged organosilane and tetraalkoxysilane (Fig. 7).210 The developed nanocatalyst was then subjected to the Suzuki–Miyaura coupling of aryl chlorides. The ligand N,N′-bis (2,6-diisopropylphenyl)imidazol-2-ylidene, denoted as IPr, displayed good coordination capability towards Pd(acac)2 leading to a higher loading in comparison with magnetic silica NPs. The Pd-loaded material catalyzed the Suzuki–Miyaura couplings of the challenging aryl chlorides under mild reaction conditions. Further, the activity of the functionalized nanoparticles was compared to the mesoporous silica-based catalysts and commercial Pd/C catalysts.211–213 Under similar reaction conditions, the commercial Pd/C (with a Pd loading of 1 wt%) resulted in only 12% conversion, which was found to be much lower than Pd/MSN-IPr (73%) thus highlighting the performances of Pd/MSN-IPr (Fig. 8).

Fig. 7

Structure of a magnetic hybrid silica nanosphere.

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Fig. 8

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The catalytic activity comparison of the different solid catalysts.

Subsequently, Zhou et al. reported the application of [email protected], a core–shell catalyst that had been successfully decorated with spectroscopically detectable metallic Pd domains and used in the aqueous Suzuki–Miyaura reaction at room temperature in air (Scheme 7).214 A series of aryl halides were coupled with aryl boronic acids in the presence of 0.5 mol% [email protected]/Pd in a H2O/EtOH (1 : 1) solvent mixture. The protocol showed a broad functional group tolerance as both aryl bromides and iodides bearing either electron-withdrawing or -donating groups afforded the desired cross-coupling products in considerably high yields. The synthesized core–shell nanocatalyst effectively coupled the advantages of both, the homogeneous (e.g., high yield) and heterogeneous catalytic systems (e.g. low cost, air-stability, easy separation, and good reusability) rendering it a promising material for practical applications. Recently, magnetic mesoporous silica spheres (MMS) have garnered attention as excellent supports for the design and synthesis of heterogeneous catalytic systems. Amongst several mesoporous silica supports, MCM-41 and SBA-15 are the most frequently employed.215 Le et al. used a two-step silica coated process for designing [email protected]@mSiO2 nanoparticles that led to the formation of a core-double-shell structure. While the inner shell played the role of protection, the outer shell was effective in offering a large specific area (Scheme 8).216 Pd(II) species were thereafter immobilized on [email protected]@mSiO2. The resultant [email protected] [email protected](II) catalyst exhibited unprecedented activity in the Suzuki reaction of phenylboronic acid with aryl halides;

Scheme 7 Mechanism for deposition of metallic domains onto the surface of [email protected]

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Scheme 8 Preparation and application of the [email protected]@mSiO2 nanocatalyst in the Suzuki coupling reaction.

the catalyst could be expediently recovered magnetically and was recycled six times without any significant loss of catalytic activity. The exceptional catalytic performance of [email protected] [email protected](II) is attributed to the productive implantation of Pd(II) species on/in both the surface and mesopore channels of the double-shelled structure of silica. Hollow magnetic mesoporous silica nanospheres (HMMS) have emerged as a powerful class of carrier materials as they offer a large number of advantages such as well-defined structures, uniform size, large surface area, and low density. Niu et al. presented a novel strategy for catalyzing the carbonylative Suzuki coupling reaction by synthesizing HMMS via the template assisted method (Scheme 9).217 HMMS displayed good catalyst loading properties for confined co-operative catalysis in comparison with the conventional mesoporous silica materials such as MCM-41, SBA-15,

Scheme 9 Synthesis of the HMMS-SH-Pd catalyst and investigation of its catalytic activity in the Suzuki carbonylative cross coupling reaction.

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FSM-16 and MCF, as they could promote the mass diffusion and transport of the reactant by preventing the aggregation of metallic nanoparticles (Table 3).218–221 Further, compared to Pd/C and Imm Pd–IL catalysts, HMMS-SH–PdII required a shorter reaction time and a milder environment for the carbonylative cross-coupling reaction. Magnetic nanocomposites including dopamine functionalized MNPs, polymer coated MNPs, ionic liquid-modified MNPs, sulfonated graphene(s-G)-decorated MNPs and magnetic [email protected] (MFC) have been used for stabilization of Pd NPs aiming to catalyze Suzuki reactions.222–225 Additionally, ultrasound assisted C–C coupling has been reported by Ghotbinejad and co-workers;226 a novel and highly stable catalyst was synthesized via immobilization of N-methylimidazolium on 1,3,5-triazine-tethered SPIONs (superparamagnetic iron oxide nanoparticles). The resultant complex efficiently catalyzed the Suzuki–Miyaura cross-coupling reactions and generated the targeted cross coupling products at low catalyst loading (0.032 mol% Pd) under both conventional heating as well as ultrasound irradiation conditions. The results indicated that in the presence of ultrasound irradiation, the reaction occurred very fast rendering the desired products in high to excellent yields, while the conventional synthesis required a longer reaction time and gave only moderate yields. Most importantly, the catalyst could be recovered using an external magnetic field and reused for multiple runs without any detrimental loss in its catalytic activity. Niu et al. fabricated an L-dopa-functionalized water dispersible magnetite nano-Pd catalyst using a facile one pot template free approach in combination with a metal absorption reduction procedure for catalyzing Suzuki and Heck coupling reactions in water (Scheme 10).227 The catalyst showed good recycling efficiency as it could be reused for six consecutive cycles without any loss in its catalytic performance (Fig. 9). Long and co-workers synthesized solvent-dispersible dopamine functionalized magnetite nanoparticles (Fe3O4/DA) via a facile one-pot template-free methodology and subsequently immobilized PdII and Pd0 onto it;228 catalytic activities of the developed nanocatalysts were thoroughly assessed using the Suzuki carbonylative cross coupling reaction (Scheme 11) which clearly revealed that PdII exhibited superior catalytic activity in comparison with Pd0. Agglomeration of Pd0 nano-

Table 3 The carbonylative cross-coupling of phenylboronic acid with 4-iodoanisole with different catalystsa

Entry 1. 2. 3. 4. 5.

Catalyst II

MCM-41-2P–Pd Pd/Cb MCM-41-2N–PdII ImmPd–ILc HMMS-SH–PdII

a

Catalyst amount (mol %)

Time (h)

Temp (°C)

Yield (%)

Ref.

2 2 2 2 1.5

5 8 6 8 7

80 100 80 100 80

85 85 86 76 90

218 219 220 221 217

Reaction conditions: CO (1 atm), anisole. b Reaction conditions: CO (200 psi), anisole. c Reaction conditions: CO (1 MPa), toluene.

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Scheme 10 Preparation of Fe3O4-L-dopa functionalized magnetic nanoparticles and the Fe3O4-L-dopa-Pd0 catalyst and its applications in Suzuki and Heck coupling reactions.

Fig. 9 TEM images of (a) Fe3O4-L-dopa (b) The HRTEM image of Fe3O40 L-dopa (c) Fe3O4-L-dopa-Pd (d) The HRTEM image of Fe3O4-L-dopa0 Pd . Reprinted with permission from ref. 227. Copyright 2013 Royal Society of Chemistry.

particles, as evidenced by TEM analysis, was considered to be the primary cause behind the low catalytic performance of Pd0 (Fig. 10). Recently, Azadbakht et al. reported the synthesis of a water stable Pd complex supported onto magnetite nanoparticles and its application in the Suzuki coupling reactions (Scheme 12).229 The synthesized nanocatalyst exhibited exceptional catalytic activity in these C(sp2)–C(sp2) cross coupling reactions in terms of an excellent product yield. Besides, a comparison of the TEM images of the fresh and recovered catalyst clearly showed no change in its morphology thus signifying the intrinsic stability of the synthesized Pd-nanocatalyst (Fig. 11). Heck coupling The Heck reaction represents one of the most noteworthy and comprehensively employed reactions for the formation of carbon–carbon bonds, resulting in the arylation, alkylation, or vinylation of various alkenes by coupling with aryl halides.230–232 Similar to the Suzuki cross coupling, the devel-

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Scheme 12 Synthetic route towards nanocatalysts and their application in the Suzuki Miyaura coupling reaction.

Scheme 11 Preparation and applications of Fe3O4/DA-PdII and Fe3O4/ DA-Pd0 catalysts in the Suzuki carbonylative cross coupling reaction.

Fig. 10 TEM images of (a) Fe3O4/DA-PdII and (b) Fe3O4/DA-Pd0 after the first cycle of Suzuki carbonylative cross-coupling reaction, (c) Fe3O4/DA-PdII and (d) Fe3O4/DA-Pd0 after 5 cycles of the reaction. Reprinted with permission from ref. 228. Copyright 2013 Royal Society of Chemistry.

opment of efficient separable and recyclable catalysts remains a major scientific challenge and an aspect of economic relevance. Wang and co-workers designed [email protected]@[email protected] nanoparticles by a bottom up approach and then investigated the behaviour of the catalyst in the cross coupling of acrylic acid with iodobenzene (Scheme 13).233 It was observed that compared to the traditional palladium catalysts supported on carbon or other supports, this catalyst showed good activity for Heck reaction in the first run (i.e. TOF of 3749 h−1 for NaOAc base). However, the activity of the catalyst in terms of reusability was significantly low and especially influenced by the presence of a base in the reaction. The

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results of Pd leaching analysis and TEM images indicated significant amount of leaching of the metal salt occurred which was the primary reason behind the drop in activity of the catalyst. This prompted the exploration of better strategies for designing magnetite based nanocatalysts that would satisfy all the requirements of a well fabricated nanocatalyst. Zhang and co-workers designed a Pd-based catalyst supported on amine-functionalized magnetite nanoparticles using a facile one-pot template-free method combined with a metal adsorption–reduction procedure;234 the catalyst showed an excellent activity for the Heck reaction, affording the desired products in over 93% yield. The most interesting aspect of the catalyst was that it could be recovered in a facile manner from the reaction mixture and recycled eight times without any appreciable loss in activity. The search for better and more effective nanocatalysts continued and thereafter Yang and coworkers came up with a novel heterogeneous Pd catalyst that was synthesized by anchoring palladium (II) onto a poly(undecylenic acid-co-N-isopropylacrylamide-co-potassium 4-acryloxyoylpyridine-2,6-dicarboxylate)-coated Fe3O4 ([email protected]) magnetic microgel;222 the catalytic behaviour was examined in the Heck coupling reaction besides Suzuki coupling and it was found that [email protected] not only exhibited an impressive catalytic activity, but also possessed exceptional durability. Sonogashira coupling Sonogashira coupling represents an efficient method to construct C–C bonds via the coupling of aryl halides and alkynes and has been frequently reported in organic chemistry, especially for the synthesis of conjugated compounds.235–237 Liu et al. developed a magnetically retrievable Pd catalyst using a wet impregnation methodology that incorporated palladium nanoparticles and superparamagnetic Fe3O4 nanoparticles in KBH4 solution.238 The newly designed nanocatalyst efficiently catalyzed the carbonylative Sonogashira coupling reaction of aryl iodides with terminal alkynes under phos-

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Fig. 11 (a) and (b) TEM images of the nanocatalyst; (c) and (d) TEM images of the nanocatalyst after recycling five times. Reprinted with permission from ref. 229. Copyright 2013 Royal Society of Chemistry.

Scheme 13 Heck coupling of acrylic acid and iodobenzene by core shell nanocatalysts.

Scheme 14 Carbonylative Sonogashira reaction catalyzed by the Pd/ Fe3O4 catalyst.

methodology as the ensuing compounds have interesting applications as antivirals, polymers and ligands.240,241 Anchored ferrite based complexes as catalysts have particularly proved their utility in the homocoupling reactions. Varma and coworkers have reported the aqueous homocoupling of arylboronic acids under MW irradiation using a magnetically separable nanomagnetite-anchored glutathione.242 The catalyst was found to be highly active, stable, magnetically separable and recyclable. The strong anchoring of the glutathione to the magnetite support through the thiol group resulted in high reusability of the catalyst. It provided very good to excellent yields for the preparation of symmetric biaryls. The obvious advantage of incorporating MW irradiation could be recognized from the lesser reaction times. Kaboudin and co-workers have recently designed a novel strategy for the synthesis of such symmetrical biaryl motifs through the preparation of a reusable Fe3O4 magnetic nanoparticle-supported Cu(II)-β-cyclodextrin complex catalyst and then subjected it to the homocoupling of boronic acids (Scheme 15).243 The Fe3O4-supported Cu(II)-β-cyclodextrin nanoparticles were simply prepared by a traditional chemical co-precipitation method. The reusability of the prepared nanocatalyst was successfully examined four times and it showed only a very slight loss in the catalytic activity.

phine-free conditions (Scheme 14). The synthesized Pd/Fe3O4 catalyst, besides solving the catalyst separation and recovery issues, also avoids the use of phosphine ligands compared to the previously reported homogeneous Pd catalytic systems. Recently, immobilized metal complexes on solid supports have aroused a tremendous interest in organic synthesis. Zolfigol and co-workers have developed a novel and efficient Pd-containing phosphorus silica magnetite [email protected] [email protected]@Pd(0) nanocatalyst for the aqueous phase Sonogashira coupling reaction.239 The most appealing features of the proposed methodology were generality, simplicity and great efficiency that led to a high product yield and a cleaner reaction profile. Homocoupling of boronic acids Synthesis of symmetrical biaryl motifs via the homocoupling or self-coupling of aryl boronic acids represents an important

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Scheme 15 Fe3O4-β-CD-Cu2-catalyzed aryl-boronic acids.

homocoupling

reaction

of

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Stille coupling The cross-coupling reaction using organostannane compounds, known as the Stille reaction, has widespread use in synthetic organic chemistry due to the growing availability of organostannanes, and their excellent compatibility with a large variety of functional groups.244 Stille cross-coupling has played a pivotal role in a number of syntheses, including those of rapamycin and dynemicin. The first example of a catalytic system based on Fe3O4 nanoparticles was provided by Jin and colleagues, who reported a heterogeneous Pd-SiO2/Fe3O4 catalyst in the cross-coupling of aryl chlorides with organostannanes (Scheme 16).207 Both electron-deficient and electronrich aryl chlorides reacted with allyltributyltin to give the cross-coupling products in excellent yields including the sterically hindered substrates that could be coupled under the optimized reaction conditions. Prasad and co-workers developed a magnetically recoverable dopamine functionalized Pd/Fe3O4 nanocatalyst for coupling organostannanes with aryl bromides;245 the catalyst could be retrieved easily using an external magnet and the efficiency of the catalyst remained unaltered even after 5 cycles. Hiyama coupling The Pd-catalyzed C–C bond formation between aryl, alkenyl, or alkyl halides or pseudohalides and organosilanes has been commonly referred to as the Hiyama coupling reaction.246 The use of Fe3O4 nanoparticle supported catalysts in such reactions cannot go unnoticed as Zhang and co-workers reported the synthesis of the PFMN-immobilized Pd complex “[email protected]– Pd(OAc)2” which not only worked well in the Heck reaction but also displayed high activity and recyclability in the Hiyama coupling reaction (Scheme 17).247 The recyclability of [email protected]–Pd(OAc)2 was explored in the coupling of 4-iodoanisole and phenyltrimethoxysilane; the nanocatalyst could be recycled at least 10 times with no detectable deactivation. C–O coupling Considering the presence of C–O bonds in a large number of natural products containing ether, ketone or ester functionality, C–O coupling has gained a sizeable interest in the field of synthetic organic transformations.248 Transition metal cata-

Scheme 16 Magnetic nanoparticle-supported (β-oxoiminato)( phosphanyl) palladium complex as a catalyst for the Stille cross coupling reaction.

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Scheme 17 Stepwise preparation of the catalyst and its application in the Hiyama reaction.

lysed coupling reactions of aryl halides and phenols (Ullmantype reactions) represent the most straightforward route for the preparation of diaryl ethers. Magnetically recyclable catalysts that contain transition metal complexes immobilized on the surface of the magnetite core have been developed to catalyze such reactions.249 Zolfigol and co-workers designed a water tolerant novel Pd-containing magnetically separable system ([email protected]@[email protected](0) nanocatalyst) for C–O and C–C coupling reactions including the Sonogashira coupling and the O-arylation of phenols in an aqueous environment (Scheme 18).239 For the C–O coupling reaction, phenol and bromobenzene were chosen as the model substrates. The versatility of the protocol was clearly proved when a range of aryl halides were successfully converted to the target products in very good yields. Naphthols could also be conveniently coupled to bromobenzene and chlorobenzene in good yields. The most significant attributes of the proposed methodology were high efficiency, simplicity and generality, high product yield, cleaner reaction profile and cost effectiveness. Zhang and colleagues developed a Fe3O4 encapsulated CuO nanoparticle via the wet impregnation methodology for the synthesis of diaryl ethers by the cross-coupling reactions of

Scheme 18 Stepwise preparation of the magnetically separable system [email protected]@[email protected](0)) and its application in the O-arylation of phenols.

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Scheme 19 reaction.

CuO encapsulated Fe3O4 catalyst in the C–O coupling

Scheme 20 Nanocat-Fe-CuO catalyzed Ullmann condensation reaction of 4-Methoxyphenol with iodobenzene.

various substituted aryl halides with various substituted phenols (Scheme 19).250 The obvious advantages of the protocol were good recyclability of the catalyst and operational simplicity without any requirement of an additional ligand. The wet impregnation technique has also been utilized by Gawande and co-workers to synthesize magnetite-supported copper nanocatalysts (nanocat-Fe-CuO) in aqueous medium from readily available inexpensive reagents (Scheme 20);251 the ensuing catalyst showed remarkable activity in the Ullmann type condensation reaction in terms of excellent product yields. Recently, Sharma and co-workers have reported the fabrication of a highly efficient and magnetically retrievable catalytic system ([email protected]@Fe3O4) for the C–O coupling of formamides and 2-carbonyl substituted phenols/ β-ketoesters;252 the catalyst was simply prepared via the covalent immobilization of quinoline-2-carboxaldehyde (2QC) on an amine-functionalized silica-coated ferrite nanosupport and subsequently metallated with copper acetate (Scheme 21). A wide range of enol and O-(2-carbonylphenyl) carbamates were obtained successfully with high turnover number and excellent conversion percentage using the newly fabricated magnetic Cu nanocatalyst. Furthermore, in comparison with other known homogeneous (CuBr2, Cu(OAc)2, CuCl) and heterogeneous (Cu-MOF framework) catalysts for the synthesis of carbamates via C–H activation of formamides, the synthesized catalyst, [email protected]@Fe3O4, showed better results such as higher product yield (up to 99%), required less reaction time (15 minutes only) and higher reusability (8 times). C–S coupling The synthesis of aryl sulphides via the coupling of aryl halides and sulphur containing compounds in the presence of a suitable transition metal catalyst commonly known as the C–S coupling has drawn a great deal of interest in the present scen-

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ario as sulphides are found in numerous drugs, with a broad spectrum of therapeutic activities for diverse clinical applications in the treatment of cancer, HIV, Alzheimer’s and Parkinson’s diseases.253 Several homogeneous transition metal catalysts have been reported in the literature for such coupling reactions, however due to the associated drawbacks of no catalyst recyclability and tedious separation procedures, the design of suitable heterogeneous catalysts has become imperative. Baig and Varma successfully fabricated a nano-Fe3O4DOPA-Cu catalyst via a one-pot multi component reaction using MW irradiation and reported its utility in the C–S coupling of thiophenols and several aryl halides (Scheme 22).254 The catalyst was prepared by sonicating nano-magnetite with dopamine hydrochloride (DOPA) in water, followed by addition of CuCl2 at a basic pH; the material with Cu NPs on the DOPA functionalized nano-magnetite was separated using an external magnet, washed with water followed by methanol and dried under vacuum. The C–S coupling reaction proceeded smoothly in the presence of the nano-Fe3O4-DOPA-Cu catalyst with low copper loading (0.82%) and isopropanol as the solvent affording the targeted products in high yield. Further, the absence of copper in the reaction solvent after completion of the reaction confirms the fact that dopamine provides enough binding sites on the surface of Fe3O4 nanoparticles by coordinating with Cu and thereby preventing metal leaching, thus facilitating efficient catalyst recycling. Damodara et al. have designed a mPANI/pFe3O4 nanocomposite from mesoporous polyaniline (PANI) and porous magnetic Fe3O4 for the S-arylation of thiophenol with aryl chlorides and in the C–S bond formation between aryl iodides and thiourea in water.255 It was observed that the mesoporosity of the polyaniline enhances the efficiency and stability of the porous magnetic Fe3O4 nanoparticles in both the coupling reactions. The mPANI/p Fe3O4 nanocomposite could be recovered with an external magnet and reused several times due to the superparamagnetic nature of the porous Fe3O4 NPs. C–N coupling The C–N cross coupling reactions have emerged as an industrially important protocol for the synthesis of a diverse array of products, such as pharmaceuticals and agrochemicals.256 Varma and co-workers have reported a modular approach to aqueous Ullman type amination under MW irradiation conditions using a magnetic silica supported copper catalyst which has been prepared via the one pot synthetic methodology (Scheme 23).257 The synthesis of this nanocatalyst was accomplished by generating magnetic nanoferrite in situ by stirring the solution of FeSO4·7H2O and Fe2(SO4)3 in water in a 1 : 1 ratio at pH 10 (adjusted using 25% ammonia solution) followed by heating in a water bath at 50 °C for 1 h. Tetraethyl orthosilicate (TEOS) was added to this solution under vigorous stirring, which was continued for 18 h under ambient conditions. The supernatant liquid was decanted and fresh water added, then, to this solution CuSO4 was added and the stirring continued for another 24 h. The magnetic silica supported CuSO4 catalyst was separated using an external magnet,

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Scheme 21

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Synthesis of [email protected]@Fe3O4 catalyst and its application in the C–H activation of formamides.

Scheme 23 Synthesis of the magnetic silica supported copper catalyst followed by amination of 4-nitro bromobenzene using [email protected]@Cu.

Scheme 22 C–S coupling between aryl halides and thiols using the nano-Fe3O4-DOPA-Cu catalyst.

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washed with water followed by acetone and dried. The application of the magnetic silica supported copper catalyst was then demonstrated in a heterogeneous catalyzed amination of aryl halides in aqueous medium as a benign solvent under MW irradiation conditions; the reaction proceeded smoothly forming products with good yield. The ease of catalyst separation adds to the significance of this protocol.

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Gawande and co-workers established the immobilization of Pd on the surface of magnetite to form nanocat-Fe-Pd using inexpensive precursors and investigated its catalytic role in the Buchwald–Hartwig amination reaction for arylation of amines and amides (Scheme 24).258 C–N bond formation was achieved in moderate to excellent yields and the catalyst could be separated and recycled up to

Scheme 24 Preparation of the nanocatalyst Fe–Pd and its application in the C–N coupling reaction.

Scheme 25 Three component coupling catalyzed by the [email protected] Fe3O4 catalyst.

Scheme 26

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five cycles by magnetic decantation without a significant loss in yield. This protocol is cost effective and an appealing alternative to the literature precedents as it avoids multistep processes of post synthetic functionalization and the utilization of linkers between the metal and the magnetite. Multicomponent reactions Multicomponent reactions (MCRs) are convergent reactions, in which three or more starting materials react to form a product, where basically all or most of the atoms contribute to the newly formed product.259 These reactions represent one of the most powerful tools for the construction of synthetically useful key intermediates.260 Considering the growing need for more environmentally acceptable processes, a diverse array of magnetically recyclable nanocatalysts have been reported in the literature for the multicomponent reactions.261–264 Zeng et al. reported the Fe3O4 NP catalyzed three-component coupling of aldehyde, alkyne, and amine (A3-coupling). In the presence of Fe3O4 as the catalyst, a wide range of propargylamines could be obtained in moderate to high yields under mild conditions in air; ease of catalyst separation and high recyclability added to the simplicity and economic viability of the protocol.265 Huo and co-workers designed a one-step method for the production of graphene– Fe3O4 composites based on the decomposition of Fe(CO)5 on the surface of graphene oxide and utilized this nanocatalyst for the synthesis of a large number of propargylamines;266 the nanocomposite exhibited excellent catalytic activity in the A3coupling reaction and afforded propargylamines in high yield under mild conditions (Scheme 25).

Preparation of a magnetic nanoparticle-supported palladium catalyst and its application in C-2 arylation of indoles.

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Suzuki coupling

Heck coupling

Sonogashira coupling

Homocoupling of Boronic acids

Stille coupling

Hiyama coupling

C–O coupling

C–S coupling

C–N coupling

1.

2.

3.

4.

5.

6.

7.

8.

9.

10. Multicomponent coupling

Reaction Simple & facile using an external magnet Filtration — Magnetic recovery Centrifugation Filtration — Magnetic recovery Filtration — Magnetic separation Ultra-Centrifugation — Magnetic separation

[email protected]

2.78 1.8 5 5 5 5 10 10 8.5 2.1 5 5 5 1.55 10

— Magnetic recovery Centrifugation — Magnetic recovery Magnetic recovery Centrifugation —

Pd-LHMS-3 Pd(OAc)2 [email protected]@[email protected](0) Pd/ZnO Cu2O mPANI/pFe3O4 Cu-grafted furfural functionalized mesoporous organosilica CuI Nanocat-Fe-Pd Fe/Cg Cu powder Fe3O4nanoparticles Graphene-Fe3O4 NAP-Mg-Au(0)

FeCl3

1 3 0.5

0.75 2 0.1 10 1 2 0.15 0.5 3 10 5 2.5 0.5

0.5

Filtration — Catalyst separation using an external magnet Filtration — Magnetic recovery Centrifugation — Magnetic recovery Filtration

Pd-pol Pd(OAc)2 [email protected](OAc)2

Cell-NHC-Pd Pd(OAc)2 [email protected] Pd/C Diatomite supported Pd Pd(OAc)2 [email protected]@[email protected](0) [email protected] PdCl2(PCy3)2 Fe3O4-Cu2-β-CD [email protected] PdCl2 (rac-BINAP) [email protected]

Catalyst separation

Catalyst

Leaching

No evidence of leaching — Negligible leaching 0.02% leaching — No leaching Negligible leaching — Negligible only (0.00054 mmol of Pd content) Negligible (0.001%) — — Not evaluated Significant leaching (loss of about 8.9% of the initial amount of gold) 34–81 — 81–97 74–95 51–94 45–99 45–86 65–90 45–96

60–90 62–99 60–93 70–98 1–100 61–92 20–88

1.2% of Pd leaching detected by ICP-AES — Negligible Pd leaching (0.47 ppm of Pd) Significant leaching (40 ppm of Pd leached) Leaching of Pd (around 8.48 ppm) — Negligible leaching (