Palladium Nanoparticles as Efficient Catalysts for ... - ACS Publications

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Palladium Nanoparticles as Efficient Catalysts for Suzuki Cross-Coupling Reactions Moisés Pérez-Lorenzo* Department of Physical Chemistry, Universidade de Vigo, 36310 Vigo, Spain ABSTRACT: In this Perspective, we discuss some of the most significant aspects in the development of nanosized catalysts for Suzuki cross-coupling reactions. Thus, the effect of the size and shape of Pd nanoparticles on the catalytic activity, together with their stability, recycling ability, and the influence of different reaction parameters, will be brought up for consideration. Furthermore, a comprehensive discussion on the homogeneous or heterogeneous nature of the mechanism for Pd-catalyzed carbon−carbon bond-forming reactions will be conducted. Understanding where the active sites are and how the reaction takes place at those sites is the key for the design of new and more effective nanocatalysts.

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alladium-catalyzed carbon−carbon bond-forming reactions developed by Heck, Negishi and Suzuki, among others (Scheme 1) have made a critical impact on synthetic organic chemistry.1,2 In this regard, coupling reactions present wide applications in the production of polymers, agrochemicals, pharmaceutical intermediates, and high-tech materials.3,4 This widespread use is mainly due to the mild conditions associated with these reactions together with their tolerance to a wide variety of functional groups. However, the significance of carbon−carbon coupling reactions for both academic and industrial research is far from settled down. The recent methods for the development of new catalysts5,6 have broadened enormously the potential uses of these processes. Thus, if palladium has been considered as one of the most efficient metals in catalysis of carbon−carbon bond formation in the last decades, Pd nanoparticles (PdNPs) have also attracted great interest recently. Their high surface-to-volume ratio and their highly active surface atoms compared to those of the bulk catalysts establish PdNPs as a promising alternative in the search for milder reaction conditions as well as more environmentally friendly methods. Hence, if coupling reactions have been traditionally catalyzed by PdII/Pd0 complexes in the presence of phosphines or other ligands, the use of PdNPs allows “ligandfree” synthesis, which reduces costs, simplifies workup procedures, and facilitates the separation of the final product. In addition, the use of colloidal-suspended or solid-supported PdNPs also allows the easy recovery of the nanocatalysts, considering their multiple recycling and continuous processing. It is also essential to mention that a critical issue in PdNP-catalyzed cross-coupling reactions is the mechanistic nature of these processes. Thus, authors diverge as to whether the PdNP catalysis arises from leached metals or nanoparticles themselves. This has been the subject of intense debate in the past decade, and recent progress in the field confirms that this question is far from being free of controversy. In this Perspective, we examine some of the most significant features in the development of nanosized catalysts for Suzuki cross-coupling reactions. Although several methods © 2011 American Chemical Society

are available for the construction of carbon−carbon bonds, Suzuki cross-coupling has proven to be one of the most popular in the last years. Besides the mild reaction conditions associated with this method, one of the key assets for this preference is the availability of diverse boronic acids and the easy handling and removal of boron-containing byproducts when compared to other organometallic reagents. Thus, the effect of the size and shape of NPs on the catalytic activity of these reactions will be assessed. Likewise, the stability and recycling ability of PdNPs together with the influence of different reaction parameters will also be brought up for consideration. In this respect, two review articles regarding this matter have been recently published. Balanta et al.7 have outlined a general and objective description of PdNP-catalyzed carbon− carbon bond-forming processes, highlighting recent work in this area considering the stabilizing agents, catalytic results, and recycling possibilities. Likewise, Fihri et al.8 have presented a broad overview of nanocatalysts for Suzuki cross-coupling reactions, emphasizing their performance, stability, and reusability. However, not much attention has been paid in these reviews to the mechanistic aspects of the reaction. In the present Perspective, a comprehensive discus-

Understanding where the active sites are and how the reaction takes place at those sites is the key for the design of new and high-performance nanocatalysts. Received: October 18, 2011 Accepted: December 28, 2011 Published: December 28, 2011 167

dx.doi.org/10.1021/jz2013984 | J. Phys. Chem. Lett. 2012, 3, 167−174

The Journal of Physical Chemistry Letters

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Scheme 1. Most Representative Pd-Catalyzed Carbon−Carbon Bond-Forming Cross-Coupling Reactions (where X = Cl, Br, or I)

sion about the homogeneous or heterogeneous nature of the mechanism for PdNP-catalyzed Suzuki cross-coupling will be presented. Parameters Af fecting Catalytic Activity. Effect of Nanoparticle Size and Shape. In a long-term study, El-Sayed and co-workers9 investigated the catalytic activity of poly(vinylpyrrolidone) (PVP)-capped PdNPs in the Suzuki reaction between iodoarenes and phenylboronic acids. In a first approach, they reported that the initial rate depends linearly on the concentration of palladium in the system, and that was interpreted as evidence for a heterogeneous mechanism, that is, a process occurring on the nanoparticle surface. Furthermore, the effect of particle size was studied by this group. While monitoring the reaction between phenylboronic acid and iodo-

widely reported in different studies, decreasing the size of the particle is not always the key for better performance of the catalyst. In this regard, there are examples in the literature9 where the smallest nanoparticles are found to be less catalytically active toward Suzuki cross-coupling than expected (Figure 1). This has been attributed to surface poisoning caused by the stronger adsorption of the reaction intermediates to the nanoparticle. Moreover, the formation of biphenyl derivatives in the Suzuki reaction could also act as a surface poison given the affinity of aromatic groups for metallic surfaces. Concerning the nanoparticle size, another factor that must be taken into account for the catalytic activity of PdNPs for the Suzuki reaction is the size distribution before and after every catalytic cycle. According to the works of El-Sayed on the reaction between phenylboronic acid and iodobenzene in the presence of PVP-capped and PAMAM−OH generation 4 dendrimer PdNPs, after the first cycle, the initial mean diameter shifts toward larger values for both catalysts, which is accounted for by Ostwald ripening. This spontaneous phenomenon occurs because larger nanoparticles are more energetically favored than smaller nanoparticles due to their greater volume-to-surface ratio. Atoms on the surface of small clusters are less stable than those packed in the interior, and therefore, they tend to attain a lower energy state diffusing to larger nanoparticles. This interfacial-energy-driven dissolution and reprecipitation mechanism becomes a significant problem for some nanoparticle formulations. After the second catalytic cycle, it is observed that when PVP-capped PdNPs are used as catalysts, the larger PdNPs (generated after the first catalytic cycle) precipitate out of solution, and only the smaller remain. Conversely, if PAMAM−OH generation 4 dendrimer PdNPs are employed, the size of the PdNPs continues to grow larger. That points to a higher stability of this catalyst due to a more efficient encapsulation of the nanoparticles. As a deeper knowledge about the nature of the active species involved in PdNP-catalyzed cross-couplings was acquired,10 El-Sayed and co-workers11 opened the door to a homogeneously operated catalysis in the Suzuki reaction. The fact that Ostwald ripening takes place during the reaction points to the existence of an important degree of atomic dissolution. Thus, nanoparticles, especially their low-coordination sites (corners and edges), would act as a reservoir of active soluble molecular palladium (a more comprehensive discussion on this topic will be given below). It is also true that in some cases, no large

Figure 1. Turnover frequency as a function of particle size for four different-sized Pd catalysts in the Suzuki cross-coupling reaction of iodobenzene (1.1 mmol) and phenylboronic acid (3 mmol) in the presence of sodium acetate (6 mmol) in 150 mL of a 3:1 CH3CN/ H2O solution, 156 mL of total reaction volume, and Pd colloidal solution containing 3.5 × 10−4 μmol of Pd. The TOF is calculated on the basis of the (a) total number of surface atoms and (b) total number of vertex and edge surface atoms. Reprinted from ref 11 with permission from Springer, copyright 2008.

benzene to form biphenyl, they found an increase of the catalytic activity as the nanoparticle size was decreased (Figure 1), in other words, as the fraction of low-coordination-number vertex and edge Pd atoms was increased compared to the fraction of face atoms. At that time, this behavior was considered by the authors as an indication of a “structuresensitive” process where the active catalytic sites were represented by the vertex and edge atoms on the nanoparticle surface. Even though this kind of anticorrelated trend has been 168



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variations in the PdNP size distribution before and after the reaction have been reported. This result may be attributed to the efficient redeposition mechanism onto the support with the completion of the reaction. Effect of Stabilizer. In line with the above-mentioned findings, El-Sayed also observed that the addition of an excess of PVP to this reaction inhibited the catalytic activity as well as the Ostwald ripening process. At this point, it is clear that there is a close relationship between both phenomena, or in other words, there is a balance between the protection of PdNPs against further agglomeration and either the Pd leaching or the free access for the reactants to the PdNP surface. Accordingly, the more stable the nanoparticle, the less catalytically active. Polymer-stabilized metal nanoparticles such as PVP-PdNPs have attracted much attention as a new research direction in the precise modulation of their catalytic properties and their easy shape and size control.12 Thus, polymer matrixes serve as the supporting material for preventing nanoparticle aggregation, provide the desired chemical interfaces between the nanoparticles and the reaction media, and, most importantly, enhance the reuse of catalysts. It is also worth mentioning that two, three, or even more metals in the same nanoparticle have also been used in order to improve the catalytic efficiency of PdNPs. Thus, Toshima’s group developed a method to synthesize PVP-capped Au-cored PdNPs, that is, NPs in which the core is Au whereas Pd atoms are located in the shell. Those bimetallic nanoparticles were found to be more active than the simple PVP-stabilized PdNPs. However, there is a large choice of organic and inorganic stabilizers that has been used in order to synthesize PdNPs with catalytic applications.13 In this regard, besides polymers and copolymers, other homogeneous systems such as NPs supported on macromolecules (especially dendrimers), micelles, microemulsions, surfactants, inorganic salts (in particular, tetraalkylammonium salts), ligands, mesoporous materials, zeolites, and ionic liquids have been successfully applied in Suzuki cross-coupling reactions. Because of their green chemical nature, the latter have attracted much attention in the last years. Bulky species such as imidazolium salts favor the electrostatic stabilization of PdNPs and, depending on their structure, also have an important influence on crucial factors such as the size and solubility of nanoparticles. Likewise, catalytic studies involving heterogeneous systems such as NPs supported on oxides (silica, alumina, or other metal oxides) and different forms of carbon supports, including carbon nanotubes, prevent NPs from aggregation and

allow an easy separation (by precipitation, filtration, magnetic separation, etc.) toward a multiple recycling of the catalyst. Recycling Potential. Given their continuous recovery and reuse, the recycling potential of the PdNP-based catalysts is also a critical aspect in the development of new and more efficient strategies in cross-coupling reactions. Traditionally, high yields in a few repeated runs have been taken as evidence to confirm the high recycling performance of a catalyst. However, achieving consistently high conversions does not necessarily guarantee high efficiency of the catalyst in further uses. In fact, it is often the case where a sudden drop in yields takes place after the fifth or sixth catalytic cycle (mostly due to the aggregation of nanoparticles, leaching, or poisoning phenomena). In an outstanding review article,14 Molnár proposes the criteria for selection of good performance catalysts in recycling studies for Heck and Suzuki cross-coupling reactions. In this regard, the following requirements are set by the author: (i) constant yields or only a 1% decrease (in 5−9 cycles) or a maximum loss of 2% in at least 10 cycles or (ii) a minimum total turnover number (TON) of about 100 000. Additionally, the catalysts must exhibit adequate stability. Thus, efficient catalyst performance (i.e., high yields and high stability) usually requires careful selection of the inorganic or polymeric support together with appropriate functionalization (such as nitrogencontaining groups or anchored ionic-liquid-like units) or the introduction of electron-donating ligands. On the other hand, catalysts affording high cumulative TONs in cross-coupling reactions, especially in Suzuki, are less numerous than those showing good recyclability. In other words, catalysts with consistently high yields are not necessarily effective to provide high TON values. However, from an application-oriented point of view, a high cumulative TON is more crucial than good recycling ability of the catalyst. Accordingly, total TON values and not the number of recycles should be considered as the true measure of the recycling performance. Effect of Reaction Parameters. Recent investigations on the Suzuki reaction performed by Köhler and co-workers15 show that catalytic activity is largely dependent on the chosen reaction parameters. This influence is attributed to the influence of the selected synthetic conditions on the leaching phenomena. In this regard, these authors show that under the working conditions, substantial amounts of palladium (>50 mol %) have been found to be temporarily dissolved during the reaction (Figure 2a). Because this great deal of atomic dissolution is

Figure 2. (a) Correlation of Pd leaching and conversion in the Suzuki cross-coupling reaction of bromobenzene with phenylboronic acid. Reaction conditions: bromobenzene (116 mmol, 1 equiv), Na2CO3 (128 mmol, 1.1 equiv), PhB(OH)2 (128 mmol, 1.1 equiv), NMP water 250:100 mL, 0.1 mol % of Pd (1% Pd/Al2O3), and T = 65 °C. (b) A presentation of Pd leaching and conversion in Suzuki cross-coupling reactions at different temperature ranges: (A) Pd leaching at 120 °C; (B) conversion of arylbromide at 120 °C; (C) Pd leaching at 65 °C; and (D) conversion of arylbromide at 65 °C (a). Reprinted from ref 15 with permission from Elsevier, copyright 2010. 169



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Scheme 2. Proposed Mechanism for the PdNP-Catalyzed Suzuki Cross-Coupling Involving Both Homogeneous and Heterogeneous Pathwaysa

a

Note that even though only redeposition is displayed in the scheme, reclustering to form (new) smaller particles and/or formation of palladium black have also been reported, depending on the reaction conditions.

found to be directly correlated with the progress of the reaction, precise control of the parameters that influence leaching will have a direct effect on the catalytic activity. In this respect, the amount of palladium in solution depends in a complex manner on a variety of factors. Thus, the ability of the chosen solvent to “extract” palladium from the nanoparticle surface will have a direct impact on the leaching process. Moreover, the presence and the nature of bases, functional groups, and additives (such as tetraalkylammonium salts) will also play an important role in the stimulation of this phenomenon and, therefore, in the efficiency of the reaction.16,17 In a similar way, the introduction of boronic acid in solution will involve an important Pd atom escape from the surface, even forming Pd black. Boron NMR studies have been carried out by this group to understand the role of boronic acid in Pd leaching. The simultaneous presence of aryl halide and base has also been explored, finding a more favorable contribution to leaching than aryl halide alone. It is obvious that temperature also constitutes a critical parameter for the leaching issue. However, the exact effect of temperature is not clear. While an increase in temperature may facilitate leaching, a further rise will force reduction to Pd0 as well as palladium precipitation. As a result, higher temperatures will inhibit Pd dissolution (Figure 2b). Other factors that can influence leaching are the atmosphere under which the reaction takes place, the catalyst pretreatment, and Pd loading. The latter has a crucial influence. On one hand, high Pd loadings give rise to larger amounts of leached palladium into the solution. On the other hand, this high Pd concentration may lead to a less-controlled redeposition process or, in other words, a decrease in the catalytic activity. In summary, it seems evident that proper control of the different reaction parameters can be used to optimize the action of the nanocatalyst. In addition, it must be noted that for a heterogeneously Pdcatalyzed Suzuki cross-coupling, the chosen synthetic conditions may also significantly influence the reaction rate. In this respect, literature on effects of reaction parameters on coupling

processes catalyzed by nonleached Pd species is scarce. Nevertheless, it may be assumed that conditions favoring atomic dissolution, as the ones described above, may be also accompanied by a decrease in the metal nanoparticle surface area due to aggregation and even precipitation phenomena,9 consequently giving rise to a less efficient process. Because synthetic conditions favoring homogeneous and heterogeneous mechanisms seem to be opposed to one another, precise knowledge of the mechanistic aspects of PdNP-catalyzed Suzuki cross-coupling appears to be essential in order to select the most appropriate reaction conditions.

Additional evidence for the mechanistic nature of the Suzuki crosscoupling may be extracted by exploring the selectivity of this reaction in the presence of nanoparticles.

Mechanism for the PdNP-Catalyzed Suzuki Cross-Coupling: Homogeneous versus Heterogeneous Catalysis. Understanding the nature of the active sites that drive certain chemical transformations has been an important but often elusive goal in catalytic studies. In recent years, a great body of reviews and original papers has been published related to the homogeneous or heterogeneous nature of the mechanism of PdNP-catalyzed carbon−carbon bond-forming reactions. As a matter of fact, this topic constitutes the key issue in order to comprehend and control the operational properties of Pd nanocatalysts. Among the many synthetic strategies to produce carbon− carbon bonds, Suzuki cross-coupling reactions have been 170



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calculation of the final yields of successive runs and the comparison between the initial and final size distribution appear to be, in some cases, inconclusive in the absence of further evidence. In this regard, the introduction of solid insoluble poisons for extinguishing a potential solution-phase catalysis5 constitutes an interesting alternative to the previously described methods. Poisoning tests have been widely applied to different Suzuki reactions as a tool to determine whether the carbon− carbon formation occurred through a supported or leachingbased operating mechanism. Different conclusions regarding the nature of the catalysis have been obtained depending on the system and the working conditions.22,23 The application of

especially investigated given the small loading of metal required by these processes to achieve very high TONs, the so-called “homeopathic” catalysis. However, the nature of the true catalytic species in this reaction has somehow remained controversial. On one hand, a substantial number of reports have been published pointing toward a soluble Pd speciesoperated catalysis (Scheme 2) analogous to the one proposed by de Vries for all high-temperature Heck reactions.18 On the other hand, many authors have claimed a heterogeneous nature for the Suzuki reaction mechanism. In this case, both aryl halide and boronic acid would come into contact by colliding on the nanoparticle surface and not within the solution (Scheme 2). The correlation of Pd content in solution with the reaction yield as a function of reaction time is considered to be a reliable and unambiguous test in order to establish the mechanistic nature of cross-coupling reactions. In the first reports on PdNPcatalyzed carbon−carbon bond formation, many authors checked for metal leaching only after a reaction had been completed. However, substantial evidence showed later that soluble Pd species could redeposit on the support fast enough so that leaching could be entirely missed. In this regard, continuous monitoring of Pd content in solution becomes essential. Thus, this methodology has been applied to the Heck reaction, confirming a leaching-based mechanism.19 The same approach was used for the Suzuki reaction, finding that the dissolved molecular palladium appears to be the catalytically active species.15,20,21 In line with the search of soluble active Pd, many authors have resorted to hot filtration tests. This protocol consists of filtering the PdNPs out of the reaction and applying the filtrate for the reaction. A lack of catalytic activity of the filtrate has been reported on many occasions as proof of heterogeneous catalysis for the Suzuki reaction. However, some authors attribute this absence of catalysis to a deactivation of the leached palladium due to a disruption of the filtration process itself. Such deactivation may be operated through the redeposition of soluble Pd on the solid support, the overcoordination of the active Pd due to the presence of added species, and even the formation of palladium black, which could lead to misleading results regarding the absence of Pd species in solution. In addition, because only extremely

Scheme 4. Photograph and Schematic of the Two-Compartment Membrane Reactor Used in the Nanoparticle-Exclusion Experimentsa

Scheme 3. Representation of the Three-Phase Testa a

Reprinted from ref 25 with permission from Wiley-VCH, copyright 2008.

three-phase tests (Scheme 3) has also been applied in order to unravel this disjunction. This protocol consists of two different steps. First, one of the reactants (either the aryl halide or the boronic acid) is covalently attached to a solid surface. Then, the reaction between the aryl halide and the boronic acid is performed in solution in the presence of the Pd catalyst and the functionalized solid. If leaching occurs, in addition to the free (in solution) final product, the anchored reactive group will be transformed into the corresponding diaryl compound. Once again, different conclusions on the nature of the Suzuki reaction have resulted from this test.23,24 Additional methods to assess the homogeneous or heterogeneous character of PdNP-catalyzed cross-coupling reactions have been recently developed. By using a simple approach based on physical separation (Scheme 4), Rothenberg and co-workers25 have shown that Pd atoms and ions leach

a

(A) The Pd catalyst remains site-isolated, allowing only solutionphase reagents to participate in the reaction; the resin-bound reagent does not react. (B) The Pd catalyst leaches out of the heterogeneous support, allowing both the solution-phase and the resin-bound reagents to react. Reprinted from ref 24.

small amounts of palladium (on the order of ppm) are needed for the reaction to take place, additional methods such as the 171



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Figure 3. Operando Pd K-edge XAS fits of 1.8 nm PVP-stabilized Pd nanoparticles during the Suzuki coupling of iodoanisole and phenylboronic acid; error bars indicate the standard deviation. The normalized, time-dependent EXAFS spectra and actual/simulated radial distribution functions shown in the inset for the prepared (236 atom) and leached (188 atom) Pd cubeoctahedra provide evidence for the preservation of the initial nanoparticle structure throughout the coupling reaction. Reprinted from ref 26 with permission of Wiley, copyright 2010.

Scheme 5. Suzuki Cross-Coupling Reaction Catalytic Nanolithography Reaction Schemea

a

A SAM-phase aryl halide reagent couples with solution phase boronic acid at the contact point of a palladium catalytic probe. Reacted regions are associated with negligible (within RMS roughness) topographic contrast but strong (dark) contrast in frictional imaging. Reprinted from ref 28 with permission of IOP, copyright 2010.

of iodoanisole with phenylboronic acid. Monitoring the local coordination environment, these authors found that the observed mean first-shell Pd−Pd coordination number remains constant during the reaction, in excellent agreement with the calculated values (Figure 3). The great stability of these particles would point to stable surface defect Pd atoms as the active catalysts. Poisoning tests combined with XPS and a detailed kinetic profiling provide further evidence for a heterogeneous Suzuki cross-coupling reaction (Scheme 2). Through a creative approach, Davis and co-workers28,29 have introduced the use of a PdNP-coated AFM tip in order to carry out a spatially controlled carbon−carbon formation process. With this aim, a surface-tethered aryl bromide is placed into a methanolic solution of 3-aminophenylboronic acid and sodium acetate. After the immersed monolayer is scanned with the catalytic probe, a highly localized reaction is observed (Scheme 5). These results are considered to be inconsistent

from Pd clusters under Suzuki coupling conditions. The leaching mechanism would depend on the reaction conditions. In the absence of any oxidizing agent, Pd0 atoms would leach out from the surface into the solution entering into the crosscoupling cycle. Accordingly, a homogeneous mechanism for the Pd-catalyzed carbon−carbon forming reaction is proposed. However, in the presence of iodobenzene, PdII complexes would be formed by the oxidative addition of the aryl halide to Pd atoms, either on the cluster surface (quasi-homogeneous mechanism; Scheme 2) or previously leached into the solution (homogeneous mechanism; Scheme 2). Thus, the formed PdII complex would enter into the catalytic cycle either directly or after being leached out into the solution. According to these works, no catalytic activity would be found on the Pd cluster surface. Fairlamb and Lee26,27 reported a meticulous study where X-ray absorption spectroscopy (XAS) was used to quantitatively monitor the structure of PdNPs during the reaction 172



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by the authors seem reasonable based on the data presented. Unfortunately, a direct comparison of the results is difficult due to variations in reaction conditions from author to author. One may also consider the possibility that Suzuki cross-coupling reactions may operate under homogeneous or heterogeneous catalytic conditions depending on a variety of factors such as the nature of the PdNPs and their stabilizers as well as the properties of the reactants involved and the working conditions. In this case, rather than mutually exclusive, both mechanisms should be considered as complementary and dependent on the immobilization degree of PdNPs. Having a deep understanding of the nature of this mechanism, and therefore about where the active sites are and how the reaction takes place at those sites, is the key for the design of new and high-performance nanocatalysts for the Suzuki cross-coupling reaction. Further reports shedding light on this issue are expected to come in the near future.

with an active Pd-soluble species diffusing freely throughout the solvent. Recently, Amatore and co-workers30 have implemented a strategy relying on the use of Au−Pd core−shell NPs in order to assess the nature of the Pd-catalyzed Suzuki cross-coupling of 4-bromoanisole with phenylboronic acid. In this case, the presence of the gold core allows a precise estimation of the status of the Pd surface after the catalytic reaction by surfaceenhanced Raman scattering (SERS) spectroscopy. The high sensitivity of this method together with a series of sequential reactivity experiments to pinpoint the origin of the catalytic activity suggests that this is primarily due to Pd centers leaching away from the NPs. In this regard, the leaching is mostly attributed to the synergistic action of the arylboronic acid and the base and, surprisingly, not to the oxidative insertion by the aryl halide.17 It seems reasonable to assume that additional evidence for the mechanistic nature of the Suzuki cross-coupling may be extracted by exploring the selectivity of this reaction in the presence of nanoparticles. Although this property alone is not a conclusive diagnostic, one should expect rather significant differences between reactant activation on a solid surface and that on a single metal complex in solution. However, to date, there have been fewer systematic studies concerning this issue. The recent significant improvements in the development of nanosized catalysts have brought PdNPs to the forefront of carbon−carbon bond syntheses, and a large number of applications are expected in the near future given the good yields obtained with these systems together with the fact that PdNPs allow “ligand-free” cross-couplings, thereby reducing costs, simplifying workup procedures, and facilitating the separation of the final product.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Biography Moisés Pérez-Lorenzo (www.webs.uvigo.es/coloides) obtained his Ph.D. from the University of Vigo. After a postdoctoral stay at the University of California at Santa Cruz with C. F. Bernasconi, he joined the Department of Physical Chemistry of the University of Santiago de Compostela. Since 2009, he has been a research associate at the University of Vigo. His current research interest focuses on the use of plasmonic nanoreactors in different organic transformations.

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ACKNOWLEDGMENTS The author thanks Prof. Luis M. Liz-Marzán for critical reading of this manuscript and insightful discussions. Financial support from Isidro Parga Pondal Program (Xunta de Galicia, Spain) is acknowledged.

Rather than being mutually exclusive, both mechanisms should be considered as complementary and dependent on the immobilization degree of PdNPs.

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REFERENCES

(1) Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-I., de Meijere, A., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (2) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (3) Torborg, C.; Beller, M. Recent Applications of PalladiumCatalyzed Coupling Reactions in the Pharmaceutical, Agrochemical, and Fine Chemical Industries. Adv. Synth. Catal. 2009, 351, 3027− 3043. (4) Magano, J.; Dunetz, J. R. Large-Scale Applications of Transition Metal-Catalyzed Couplings for the Synthesis of Pharmaceuticals. Chem. Rev. 2011, 111, 2177−2250. (5) Nanoparticles and Catalysis; Astruc, D., Ed.; Wiley-VCH: New York, 2008. (6) Zahmakıran, M.; Ö zkar, S. Metal Nanoparticles in Liquid Phase Catalysis: from Recent Advances to Future Goals. Nanoscale 2011, 3, 3462−3481. (7) Balanta, A.; Godard, C.; Claver, C. Pd Nanoparticles for C−C Coupling Reactions. Chem. Soc. Rev. 2011, 40, 4973−4985. (8) Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J.-M.; Polshettiwar, V. Nanocatalysts for Suzuki Cross-Coupling Reactions. Chem. Soc. Rev. 2011, 40, 5181−5203. (9) Narayanan, R.; El-Sayed, M. A. Some Aspects of Colloidal Nanoparticle Stability, Catalytic Activity, and Recycling Potential. Top. Catal. 2008, 47, 15−21. (10) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. On the Nature of the Catalytic Species in Palladium Catalyzed Heck and Suzuki

The catalytic activity of PdNPs in the Suzuki reaction depends on the size and shape of the nanoparticles. Thus, the catalytic activity has been found to increase as the nanoparticle size is decreased, in other words, as the fraction of low-coordinationnumber Pd atoms is increased. These defect atoms have been reported to act either as a reservoir of active soluble molecular palladium or as the real catalyst. Surface poisoning caused by the adsorption of reaction intermediates to the nanoparticle may be found for the smallest sizes. Regarding the stabilizer, there is a compromise between the protection of PdNPs against further agglomeration and either the Pd leaching or the free access for the reactants to the PdNP surface. The precise control of the different reaction parameters such as temperature, solvent, reactants species, base, additives, and Pd loading appears to be a critical issue in the performance of the nanocatalyst. Conclusions in the literature differ as to whether the PdNP catalysis of the Suzuki reaction arises from leached metals or nanoparticles themselves. In each case, the conclusions drawn 173



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