Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for the...

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NANOCATALYSTS FOR THESUZUKI COUPLING REACTIONS

Liane M. Rossi, Natalia J. S. Costa, Jones Limberger,and Adriano L. Monteiro

SUZUKI COUPLING REACTION

Metal-catalyzed Suzuki cross-coupling reaction of aryl, vinyl, or alkyl halide or pseu-dohalides with organoboron reagents is among the most efficient methods for the con-struction of C C bonds.1–22 Thus, this reaction has found widespread use in organicsynthesis. The reaction can be carried out under mild reaction conditions and toleratesa wide range of functionalities. The most common boron partners include boronic acidand its derivative such as boronic esters. These compounds are largely unaffected by thepresence of water and air, are not toxic, and are commercially available. In this context,the Suzuki reaction is nowadays of great industrial significance since many products(e.g., drugs, materials, and optical devices) commercialized or in the development phasehave C C bonds, which can be assembled by catalytic cross-coupling reactions. Theimportance of this powerful synthetic tool culminated in a 2010 Chemistry Nobel Prizeto Professor Akira Suzuki from Hokaido University.23

Palladium is by far the most studied metal for catalyzed Suzuki coupling, andboth Pd zerovalent and divalent compounds can be used as catalysts for the reaction.The simplified classical textbook mechanism for the Suzuki reaction is depicted inScheme 3.1.19 The catalytic cycle starts with a Pd zerovalent species, and when divalentpalladium precursors are used a reduction step is necessary to form the active species.

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

51

52 NANOCATALYSTS FOR THE SUZUKI COUPLING REACTIONS

Scheme 3.1. Simplified catalytic cycle for palladium-catalyzed Suzuki cross-coupling.

The Pd(0) species reacts with the organic electrophile R–X to form the divalent R–Pd–X complex (oxidative addition step). Transmetalation with the organoboron nucle-ophile leads to an R–Pd–R′ complex that by reductive elimination generates the organicproduct R–R′, while at the same time regenerating the zerovalent Pd catalytic-activespecies.

In the last decades, a plethora of efficient Pd catalysts have been developed thatallow the coupling between aryl halides (e.g., iodides, bromides, and chlorides) andpseudohalides (e.g., triflates, tosylates, and alkyl sulfonates) and with aryl boronic acidsunder mild reaction conditions. In the case of more reactive electrophiles such as aryliodides and aryl bromides containing electron-withdrawing groups, even ligandlesspalladium catalysts are shown to be sufficient enough to promote this cross-couplingreaction in high turnovers.24,25 However, in the case of less reactive aryl chlorides andtosylates, the presence of ligands such as bulky and electron-rich phosphines26–28 andcarbenes29 is necessary to effectively promote these cross-coupling reactions. Althoughthese substrates are cheaper than aryl iodides and bromides, in most of the cases theligands that have to be used with them are expensive and/or air-sensitive and requirehigh loading of palladium catalysts. On the other hand, using ligandless palladiumcatalysis has major drawbacks because the “naked” Pd(0) species will agglomerate toform inactive palladium black. However, if agglomeration is avoided by using adequatestabilizers that keep the palladium as small clusters, that is, nanoparticles, the catalystwill remain active.

FROM HOMOGENEOUS TO NANOPARTICLE CATALYSTS 53

FROM HOMOGENEOUS TO NANOPARTICLE CATALYSTS

Homogeneous transition metal catalysts remain the most active catalysts for couplingreactions; however, efficient separation and recycling are still scientific challenges,because catalyst deactivation often occurs during the workup procedures. Generally,homogeneous catalysts have a greater tendency to lead to metal contamination in theend products, which is highly regulated by the pharmaceutical industry. Alternativesto homogeneous catalysts include either the development of ultra-low catalyst load-ing systems making recovery of the metal unnecessary for many purposes,30,31 or theheterogenization of homogeneous catalysts to take advantage of their easy recovery ofthe catalyst from the reaction mixture by simple decantation or filtration.32 The latteralso results in the much desired reusable catalysts. Palladium and other catalysts for theSuzuki reaction have been immobilized on several solid supports such as silica, organicpolymers, and activated carbon.33,34 By using such catalysts, catalyst recovery and recy-cling are also easily achieved in aqueous systems, ionic liquids (ILs), supercritical media,or fluorinated phases.34

Most of the research in the field of supported Suzuki catalysts is based on thestabilization of metal complexes via covalent bonding with the support surfaces eitherby using supports that were previously functionalized to capture metal precursors or bythe design of complexes with functional groups available to bind to the support surface.Many successful examples of very active, easily recoverable, and recyclable supportedcomplexes on surfaces with different functional groups can be found in the literature.Some attempts to prepare anchored metal complexes also resulted the deposition ofsupported active metal cluster catalysts by decomposition of supportedmetal complexes.

In typical homogeneous Suzuki reactions, the agglomeration of Pd(0) species toform inactive palladium black is usually avoided by adding excess of stabilizers. Inthe presence of adequate stabilizers, palladium agglomerates as small clusters, that is,nanoparticles, and the catalyst remain active. Paetzold and coworkers35 prepared pal-ladium catalysts by reacting the precursor PdCl2[Ph2P(CH2)4SO3K]2 with mesoporousmaterials functionalized with sulfonate groups. Although the authors do not clearlyshow the formation of Pd clusters, the results suggest the stabilization of Pd(0) speciessince the catalyst that presented an induction time in the first reaction had no inductionperiod during its reuse. Bedford and coworkers36 have prepared imine palladacycle cat-alysts supported on silica that can act as precursors to active Pd nanoparticles (Pd-NPs),as observed by color changes in the supported catalyst that is still active in succes-sive reactions. Other study shows the formation of Pd-NPs when using Pd(OAc)2 withorganosiloxane-modified silicas containing amine functionalities ranging from primaryto quaternary nitrogen centers.37 Zhang and coworkers38 have also characterized theformation of Pd-NPs by using Pd(OAc)2 with an amino-modified silica. The catalyst ishighly efficient in the Suzuki reaction and could be recovered and recycled by simplefiltration and successfully reused for more than 15 consecutive trials without significantloss of catalytic activity. On the other hand, Paul and Clark39 reported the synthesisof supported Pd complexes by reacting Pd(OAc)2 with silica modified with nitrogen-containing ligandswithout suggesting the formation of Pd clusters in the synthesis. Thereare also examples where the authors notice that the covalently anchoredmetal complexes


decompose during catalysis to form palladium nanoparticles, which were catalyticallyless active than the original complexes and thus not reusable in successive reactions.40

The reports on the formation of active palladium nanoparticle catalysts in situduring Suzuki reactions and the examples where the catalytic activity was improvedand remained stable after successive reactions inspired researchers to develop rationaldesign and synthetic approaches to even better metal nanoparticle catalysts for Suzukicoupling reactions. The observation of in situ generated nanosized Pd colloids obtainedfrom simple Pd salts (i.e., Pd(OAc)2/DMG, where DMG is N,N-dimethylglycine,Pd(OAc)2/R4N+X−, and Pd(OAc)2) is an evidence that other phosphane-free C Ccross-coupling reactions in the presence of Pd salts without special ligands are alsolikely to be catalyzed by nanosized Pd colloids.41 Reetz and coworkers42 described thefirst example of the use of preformed Pd-NPs in the Suzuki reaction in 1996. Pd-NPsand bimetallic Pd/Ni-NPs stabilized by tetraalkylammonium salts were effective in theSuzuki coupling reaction of aryl bromides or the activated 4-nitrochlorobenzene withphenylboronic acid. At that time the catalysts were referred to as Pd clusters or colloids.According to a search in Web of ScienceSM using the words “nanoparticles and palla-dium and coupling” revealed that the term “nanoparticles” became widely employed forPd-catalyzed coupling process mainly after the year 2000.

Supported nanocatalysts are appealing because small metal nanoparticles (NPs�10 nm) have high percentage of atoms on the surface, while keeping one of themost important advantages of the classical heterogeneous catalyst, which is the ease ofseparation and recyclability of the catalysts. Therefore, the use of NPs as catalysts for thecross-coupling reaction has intensively grown in the last few years. In this chapter, wehighlight the use of metal NPs as catalysts in the Suzuki reaction, by using representativeexamples on the synthesis and characterization of nanocatalysts for the Suzuki reaction,especially those involving more challenging substrates and reusable catalysts.

NANOPARTICLE ACTIVITY IN SUZUKIC C COUPLING REACTION

Generally, the performance of metal NP catalysts for many different reactions isstrongly dependent on the shape, size, dispersion, and support and, consequently, on thesynthesis methods, reducing agents, and stabilizers employed to make them. However,only little is known concerning the ideal characteristics of a nanocatalyst for Suzukicoupling. Although there is a huge amount of studies in the literature on the synthesisof catalysts for Suzuki coupling with distinct catalytic activities, few of them discussthe influence of size, shape, and preparation methods on catalytic activity.43–47

Influence of the Preparation Method on the Catalytic Activity

The methods and reaction conditions used for the synthesis of colloidal and supportedmetal NP catalysts (e.g., the stabilizing agent chosen) can affect the size, morphology,and surface properties of the catalysts. Similarly, by using a series of stabilizers

NANOPARTICLE ACTIVITY IN SUZUKI C C COUPLING REACTION 55

composed of different generations of dendrimers, block copolymers, and so on (e.g., lin-ear polymer (hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers (Gn-OH,where Gn represents the nth generation), block copolymer polystyrene-b-poly(sodiumacrylate), and poly(N-vinyl-2-pyrrolidone) (PVP)), NP catalysts with different degreesof stability and catalytic properties for Suzuki reactions were obtained.48 In general,the stability of the Pd-NPs (measured by the tendency of the nanoparticles to givePd black powder after the catalytic reaction) is found to be dependent on the type ofthe stabilizer, the reactant, and the base used in the reaction system. It is important tomention here that different types of Pd-NPs are active for Suzuki coupling reactionsbetween arylboronic acids and activated iodoarenes, and less active for nonactivatedbromoarenes. For example, surfactant-free Pd-NPs were prepared by heating a mixtureof an aqueous solution of PdCl2 with dimethylformamide (DMF) at 140 ◦C. These“naked” Pd-NPs are also active for Suzuki coupling, affording reactant conversion of99% for the coupling reaction between aryl iodide and phenylboronic acid. The Pd-NPswere recycled for five runs without losing catalytic activity and were active for theSuzuki reaction when dispersed in different solvents.49

Influence of the NP Size on the Catalytic Activity

Comparing the catalytic activity of metal NPs exclusively based on their size is a difficulttask because obtaining samples of NPs with different ranges of sizes without changingsynthesis parameters, such as stabilizers, is not straightforward. For example, if onechanges the size of the NPs by changing the stabilizing agent used in the synthesis, thenthe activity of the NPs of different size can also suffer the influence of the ligands ontheir surfaces.

The seed growth of metal NPs is a promising methodology to prepare NPs ofdifferent sizes using the same reagents in successive synthesis. This stepwise growthsynthesis is based on the preparation of a suspension of metal NP seeds that can beadded into a solution containing more metal precursors under the same condition as theone used for preparation of the seeds. This leads to growth of metal NP seeds, withoutchanging the stabilizing agent, solvent, and metal precursor.

The NPs with different sizes synthesized by the seed growth synthetic method canbe expected to exhibit different catalytic activities. Li and coworkers50 studied the effectof the size of PVP-stabilized Pd-NPs prepared by using the stepwise growth reactionin the Suzuki coupling reaction between phenylboronic acid and iodobenzene. Theyspecifically prepared Pd-NPs with approximately 3.0, 3.9, 5.2, and 6.6 nm in diameter.Based on the three-shell cuboctahedron structure of NPs, they determined the totalnumber of atoms, the number of surface atoms, and the number of vertex and edgesurface atoms of the NPs. The comparison of the turnover frequency (TOF) (expressedby moles of biphenyl product per mole of surface Pd atoms per minute) among thePd-NPs with different sizes resulted in a decrease of catalytic activity in the followingorder: Pd (3.9 nm) � Pd (3.0 nm) ∼ Pd (5.2 nm) � Pd (6.6 nm). Considering thatall the surface atoms are active sites, a dependence of the TOF with the particle sizewas observed, but when the TOF was calculated considering only the vertex and edge


atoms, the effect of the size became unimportant to the NPs with approximately 3.9, 5.2,and 6.6 nm in diameter, further indicating that the vertex and edge atoms are the activecenters for the coupling reaction. This reaction is structure-sensitive, which explains thebehavior observed for some Suzuki couplings catalyzed by NPs: the activity increaseswith the decrease of NP size. However, when the NPs are too small, as 3.0 nm NPs thatare less active than the others, it is possible that some reaction intermediates have strongadsorption on the active sites, poisoning the catalyst.50

Piao and coworkers51 prepared Pd-NPs with average diameters of 4.8 ± 0.5 nm,5.4± 0.5 nm, 4.3± 0.5 nm, and 3.8± 0.4 nm, by reducing Pd(II) salts using the triblockPluronic copolymers P123, PL64, P103, and F108, respectively. They also controlledthe particle size and shape by varying the pH of the reaction solution. With the synthesis,uniform Pd-NPs with particle sizes of 5.6 nm (pH 6.4), 6.9 nm (pH 3.8), 8.2 nm (pH3.2), and 10.9 nm (pH 2.8) were produced using P123 (Figure 3.1). Clustered dendriticNPs with much larger overall sizes more than 20 nm were produced when large excessof acid was used. High catalytic activity was obtained when 4.8 nm Pd-NPs (0.5 mol%)were used for iodo- and bromoaryl substrates. Furthermore, the smaller Pd-NPs withlarger surface area were found to exhibit better catalytic performance compared to thelarger ones. The conversion for the coupling reaction between 4-bromoacetophenoneand phenylboronic acid was low (51%) when the reaction was catalyzed by 10.9 nmPd-NPs but increased significantly to 95% when catalyzed by 4.8 nm Pd-NPs under thesame conditions.

Influence of the NP Shape on the Catalytic Activity

Comparing the catalytic activity of NPs with different shapes is as challenging ascomparing the activity of NPs with different sizes. To reliably determine the effect ofshapes of NPs on catalytic activities, it is therefore essential to find synthesis proceduresthat can give NPs with different shapes without changing their stabilizing agent, solvent,or precursor. In one example, Narayanan and El-Sayed52 investigated the influence ofthe shape of NPs in the cross-coupling of phenylboronic acid and iodobenzene. Theyfirst observed that PVP-stabilized spherical-shaped Pd-NPs with 2.1 nm in size weremore active catalysts than PVP-stabilized tetrahedral-shaped Pt-NPs with 5.0 nm in size;however, this result does not mean that the spherical Pd-NPs were more active than thetetrahedral ones for Suzuki coupling because the metal and sizes of the NPs used forcomparison were different. Furthermore, the authors observed that after refluxing thetetrahedral-shaped Pt-NPs in a solvent or under the Suzuki coupling reaction conditions,they were transformed into nearly spherical NPs with similar size (Figure 3.2). Theseresulting nearly spherical Pt-NPs were tested in the Suzuki reaction, and the catalyticactivity was lower than half of the initial activity. The authors thus conclude that, sincethe synthesis of the NPs was the same, tetrahedral Pt-NPs are more active than thecorresponding spherical ones for the Suzuki reaction. The possible explanation for thisbehavior is that nearly spherical Pt-NPs are composed of (100) and (111) facets whiletetrahedral Pt-NPs are exclusively composed of (111) facets, and these facets, that is,(100) and (111), have different electron densities, and thus different surface energy andcatalytic activities.


(a) (b)

(c) (d)

(e) (f)

Figure 3.1. Transmission electron microscopy (TEM) images of the Pd-NPs synthesized in

aqueous solution containing P123 with HCl concentration of (a) 0.1 mM (pH 6.4), (b) 0.5 mM

(pH 3.8), (c) 1 mM (pH 3.2), (d) 2 mM (pH 2.8), (e) 5 mM (pH 2.5), and (f) 10 mM (pH 2.3).

(Reprinted with permission from Ref. 51.)


100

(b)(a)

10 nm

10 nm

10 nm

80

60

40

20

0RT DT S

Shape

Before SuzukiReaction

After First Cycle

After Second Cycle

% o

f Nan

opar

ticle

s

100

(d)(c)

80

60

40

20

0RT DT S

Shape

% o

f Nan

opar

ticle

s

100

(f)(e)

80

60

40

20

0RT DT S

Shape

% o

f Nan

opar

ticle

s

Figure 3.2. TEM images and shape distributions of PVP-stabilized Pt-NPs before the Suzuki

reaction (a and b), after the first cycle of the Suzuki reaction (c and d), and after the second

cycle of the Suzuki reaction (e and f). RT = regular tetrahedral, DT = distorted tetrahedral, and

S = near spherical. (Adapted with permission from Ref. 52, Copyright 2005, American Chemical

Society.)

Chattopadhyay and coworkers53 prepared different types of CuO-doped Pd-NPsstabilized by polyethylene glycol-600 (PEG-600). They changed the structure of the NPsto rod-shaped, a mixture of rod- and oval-shaped, and oval-shaped nanoparticles (with asmall amount of rods in it) by varying the amount of PEG-600. Of these nanoparticles,the oval-shaped ones were found to be the most active catalysts for the Suzuki reactionsbetween different deactivated aryl bromides, giving 70–90% of conversion, whereas therod shape facilitated the cyanation reaction; however, no suggestion was made in the


report about why these types of shaped nanoparticles gave the high catalytic activity inthe reaction.

Pd nanoplates, nanotrees, and nanobelts with size more than 100 nm were synthe-sized at room temperature under aqueous conditions using vitamin B1 as a stabilizingagent and palladium(II) chloride as a precursor. The shape of the Pd-NPs was modifiedby changing the Pd ion concentration; however, their catalytic activity in the couplingbetween iodobenzene and phenylboronic acid was not significantly different.54 The samebehavior was observed for Pd nanorods and branched Pd nanocrystal prepared by a seedgrowth method assisted by copper. The final particle shape changed according to theamount of copper(II) acetate added, but the dimensions of the Pd particles were morethan 20 nm (20 nm for rod diameter and 200–300 nm of rod length), and no considerabledifference in the catalytic activity of these materials in the same coupling model reactionbetween phenylboronic acid and iodobenzene was observed; that is, the Pd nanorodsyielded of 92% in the first reaction cycle and 89% and 85% in the second and thirdreaction cycles, respectively. Branch-shaped Pd nanocrystals yielded 90% in the firstreaction cycle, and 86% and 82% in the second and third cycles.55

Influence of the Support Material on the Catalytic Activity

Silica has been by far the most widely used support material for homogeneous catalystsfor the Suzuki reaction.33,56 The most common method used to support homogeneouscatalysts on silica involves the coordination of Pd complexes on chemically modifiedsilica with ligands such as 3-aminopropyl moiety. The synthesis of supported nanopar-ticles follows a similar strategy, that is, the coordination of Pd complexes on organic-functionalized silica supports, followed by the reduction of the metal ions to producethe supported Pd-NPs.

The behavior of supported metal NPs for the Suzuki reaction is strongly affectedby the support materials, while the presence or type of functional groups on the supportsurface can affect the selectivity and recyclability of metal NPs in the catalytic reaction.Bedford and coworkers37,57 used silica support materials modified with amine function-alities ranging from primary to quaternary nitrogen centers to prepare supported Pd-NPs.The choice of the organic groups grafted on the support materials played crucial role indetermining the activity and recyclability of the catalyst in the Suzuki cross-couplingreaction. An optimum behavior in the catalytic reaction was obtained when chelatingdiamine and triamine groups were used as organic surface modifiers, while the one withquaternary alkylammonium groups showed poor catalytic activities (Table 3.1).

Shin and coworkers58 synthesized Pd-NPs supported on spherical silica particlesmodified with imidazolium groups. To prepare the materials, they first modified thesilica surfaces by grafting NBoc-protected amine and triethoxysilyl bifunctionalized IL.After refluxing, the silica particles were separated and the Boc protection groups wereremoved by treating the material with trifluoroacetic acid. The reaction with Pd(OAc)2in excess of 1,6-diisocyanatohexane (10 equivalent to amine group) for 12 h resulted inPd-NPs supported on organic-modified silica. The imidazolium moiety was responsiblefor not only capturing the Pd species but also stabilizing the Pd-NPs. Consequently, thismaterial was more efficient catalyst than unmodified-silica and amine-modified silica.


TABLE 3.1. Suzuki coupling of aryl bromides with Pd-NPs supported on chemically modifiedsilicaa

R1 R2 R1R2

Br B(OH)2

[cat]tolueneK3PO4

+

Modified silica group R1 R2 Conversion (%)b

−[NBu3]+ 4-OMe H 11−Imidazolium 4-OMe H 40−NH2 4-OMe H 75−NHMe 4-OMe H 82−NEt2 4-OMe H 80−NHPh 4-OMe H 33−NHCH2CH2NH2 4-OMe H 94

4-Me H 904-COMe H 952-Me H 782-OMe H 85H 4-Me 76

−NH(CH2)2NH(CH2)2NH2 4-OMe H 98

Adapted from Ref. 37. Copyright (2005) American Chemical Society.a All reactions were performed in K3PO4/toluene at 110 ◦C.b Conversion to Suzuki product monitored by gas chromatography with a hexadecane standard.

Costa and coworkers59 synthesized Pd-NPs supported on spherical silica particlescontaining magnetic cores and surface modified with iminophosphine moiety. They firstmodified the surfaces of magnetically functionalized silica spheres with amino groups.They then let the amine groups of these spheres react with a palladium complex con-taining 2-(diphenylphosphino)benzaldehyde as a ligand, forming an iminophosphino–palladium complex. Under reflux conditions, the Pd complex decomposed into supportedPd-NPs, leaving the support surface functionalized with pendant phosphine groups (Fig-ure 3.3). The Pd-NPs exhibited better catalytic activity when supported in phosphine-modified silica than in amino-modified silica.

MacQuarrie and coworkers60 compared the stability of Pd supported on differ-ent mercapto-functionalized mesoporous materials, namely KIT-6 and SBA-15, in theSuzuki cross-coupling reaction. The KIT-6-supported catalyst showed a much greaterrecyclability than the corresponding SBA-15-based catalyst. The high stability of theKIT-6 material was attributed to a greater redistribution of Pd to the external surfaceof the material, resulting in Pd-NPs that were not constrained by the size of the pores.In the SBA-15-based catalyst, the Pd was captured inside the pores and the catalyticactivity ceased when the material collapsed.

When the Suzuki reaction occurs in aqueous medium, the polarity of the supportis an important parameter to be taken into account. Soomro and coworkers61 showedthat the activity of 10–15 nm Pd-NPs in the coupling of chloroacetophenone withphenylboronic acid was higher in more hydrophobic supports. They suggested thatthe stronger interaction between the organic phase and the hydrophobic surface of the


OSi

O OEtNH2

O Si

O

EtON

O

SiO

OEt

N

Pd

P Ph

Ph

P PhPh

OSi

O

EtO

NH2

O Si

O

EtON

O

SiO

OEt

N

P Ph

Ph

P PhPh

Pd(0)

O

PPhPh OAc

Stirringin toluene,heating

After 24 h inreflux

Pd

O

Ph

OAc

Fe3O4/SiO2

Fe3O4/SiO2

Fe3O4/SiO2

P Ph

Figure 3.3. Step-by-step synthesis of the Fe3O4–SiO2–iminophosphine–Pd catalyst. (Adapted

with permission from Ref. 59, Copyright 2010, Elsevier.)

support material facilitated the contact of the metal NPs with the substrates present inthe organic phase, consequently increasing the activity of the catalyst (Figure 3.4). Thehydrophobicity of the support was found to be not very important when a large excessof base was added into the reaction mixture though. In strongly basic solution, leachingof metal occurred, forming anionic Pd complexes that were very active catalysts andyielding conversion of 100%, independent of the nature of the support.

Carbon nanomaterials are also studied as support materials for Pd-NP catalystsfor the Suzuki reaction. Different catalytic activities can be obtained by treatment ofcarbon supports, for example by oxidation. Itoh and coworkers62 oxidized an aggregateof carbon nanohorns and used the resultingmaterial as support for Pd-NPs for the Suzukireaction. The activity of the catalyst was compared with Pd-NPs on pristine nanohorns,


Water phase

H2O

Organic phase

SH2O

H2OMOx

H2O

H2O H2OH2O

H2O

PhB(OH)3–

S

PTA

PTA

SS

S

S

S

S

Pd

Pd

Water phaseOrganic phase

SoSi

MOx

H2O

H2O

H2OH2O

H2O

H2O

PhB(OH)3–

S

PTA

PTA

S

P

Salts

S P

S

SS

P

Pd

active Pd species

Pd

o

o

Si

Si

oSi

Figure 3.4. Dynamics of the Suzuki coupling reaction in aqueous solution, according to the

polarity of the support surface. PTA = phase-transfer agent. (Reprinted with permission from

Ref. 61.)

Pd-NPs dispersed in activated carbon (Pd/C), and Pd-NPs tailored pitch-based activatedcarbon fibers. The most active catalyst was found to be the supported Pd-NPs on theoxidized carbon nanohorns.

STABILITY AND REUSABILITY OF NANOCATALYSTS

The most important aspect to discuss about the stability of metal NPs in the Suzukireaction is associated with Ostwald ripening process, which is the main reason behindcluster growth. This mechanism involves the detachment of atoms from smaller clusters

STABILITY AND REUSABILITY OF NANOCATALYSTS 63

160

(b)(a)

120

80

40

01 2 4 60 3 5

Size (nm)

Before Reaction

Num

ber

of N

anop

artic

les

160

(d)(c)

120

80

40

01 2 4 60 3 5

Size (nm)

After First Cycle

Num

ber

of N

anop

artic

les

160

(f)(e)

15 nm

15 nm

15 nm

120

80

40

01 2 4 60 3 5

Size (nm)

After Second Cycle

Num

ber

of N

anop

artic

les

WD = 0.7 + 0.1 nmCD = 1.3 + 0.1 nm

WD = 2.2 + 0.2 nmCD = 2.0 + 0.1 nm

WD = 2.5 + 0.1 nmCD = 2.7 + 0.1 nm

Figure 3.5. Ostwald ripening process after submitting PAMAM-OH dendrimer-capped Pd-

NPs to the Suzuki reaction (a and b), after the first cycle (c and d), and after the second cycle

(e and f). (Reprinted with permission from Ref. 63, Copyright 2003, American Chemical Society.)

and the attachment of these atoms to the larger clusters that have lower surface energy.The result is the formation of monodispersed NPs with larger size when compared to theNPs before the ripening process. As the reaction conditions for Suzuki C C couplingusually involve high temperature (most commonly 100 ◦C) for a long period of time,the growth of the NPs by Ostwald ripening can be expected to continue during andafter recycling of the catalysts. The transformations suffered by PVP-stabilized Pd-NPsunder the cross-coupling reaction conditions were studied by Narayanan and El-Sayed(Figure 3.5).63 The harsh reaction conditions can promote the increase of the NP sizeby Ostwald ripening process and lower the catalytic activity of the NPs. Although the


increase in size of the NPs can be avoided by adding excess of stabilizers, addition ofexcess stabilizers can make the catalyst less active. Furthermore, when excess of thereaction product is added into the reaction medium, the catalytic activity also decreases,indicating that the product of the reaction poisons the active sites of the colloidal Pd-NPs.The same behavior is observed when excess of phenylboronic acid is added because itacts as a capping agent in its deprotonated form. When dendrimer-stabilized Pd-NPswere compared with PVP-stabilized Pd-NPs, similar behavior was observed in excess ofphenylboronic acid. Thus, it is possible to conclude that themechanismof the reaction forPd-NPs is insensitive to the amount of stabilizing agents. Also, the excess of arylboronicacid decreases the Otswald ripening process because it promotes the stabilization ofthe Pd-NPs.64 As discussed previously, the lack of influence of excess of stabilizers ofPd-PVP NPs in the Suzuki reaction was also observed by Li and coworkers50 in theirstudy of the effect of size of NPs.

Pd-NPs supported on silica modified with phosphine moiety were very stable beingrecovered and reused for up to 10 times without losing activity in Suzuki coupling. Toavoid silica corrosion by base, the boronic acid and the base were mixed in equimo-lar quantities before adding the catalyst, and by this way the catalyst structure wasmaintained.59

INSIGHT ON MECHANISTIC ASPECTS

The classical textbook mechanism for the Suzuki coupling is depicted in Scheme 3.2.The key intermediate in the catalytic cycle is a Pd zerovalent species that follows the

or

Reductive

elimination

Discrete Pd(0)

species

+

+

Ar-Ar’

Ar-X

Ar

X

Ar-Ar’

XB(OH)2

Ar’-B(OH)2

Ar

L

LAr’ Pd(II)

“Pd(0)”

Pd(0)

nanoparticleOxidative

insertion

Chemical

etching

Ar

LX

Discrete Pd(II) speciesTransmetalation

L Pd(II)

Scheme 3.2. Mechanism of the Suzuki cross-coupling catalytic cycle proposed by Liu and Hu.

(Reprinted with permission from Ref. 65, Copyright 2005, American Chemical Society.)

INSIGHT ON MECHANISTIC ASPECTS 65

mechanistic course of oxidative addition, transmetalation, and reductive elimination.The involvement of a Pd(0) species in the oxidative addition process is a commonstep for the different Pd-catalyzed cross-coupling reactions. Compared with the Heckcoupling reaction—which is a related reaction to the Suzuki reaction—the mechanismfor the Suzuki reaction using Pd-NPs as catalysts has been less investigated. Therefore,discussion of mechanistic studies of some relevant Heck reactions is included here togive readers some insights about the mechanism of the Suzuki reaction. More detaileddiscussion about the Heck reaction is also available in other chapters in the book.

Despite the importance of the Pd-NPs in the C C coupling reactions, the natureof the true active species is not well established and is a very hard task to determine,whether catalysis occurs on the surface of nanoparticles or by leached Pd species.22 In thelatter case, the nanoparticles may simply act as a reservoir of active, soluble molecularpalladium species.65,66Also, one cannot rule out that both processes can occur at thesame time. Getting the answers about the true active species in palladium-catalyzedcross-coupling reactions is not only important from fundamental scientific point of viewbut also because it helps with the search for stable, solid catalysts that are capableof activating the coupling reaction between organohalides under mild conditions andcan be recycled easily without leaching. The difficulty in unequivocally proving thenature of the true active species is to getting experimental setups that allow monitoringthe structure of the nanoparticles in situ during the coupling reactions, without beingperturbed by invasive chemical/electrical probes.

The phenomenon that strongly contributes to the theory that homogeneous mecha-nism for Suzuki coupling applies also for NPs is without doubts the Ostwald ripeningprocess. Due to this process, there is a considerable degree of atomic dissolution ofmetal atoms from NPs during the reaction that can easily catalyze the C C coupling.Usually this process not only promotes the increase of the particle size but also helpschanges in the morphology of the NPs after the catalyst is stirred in the reactions andafter recycling.67

Morphological changes of the N,N-dihexylcarbodiimide-stabilized Pd-NPs wereexamined by TEM at the end of five runs.68 The Pd-NPs transformed gradually fromspherical shape to larger needle-shaped crystals, and the authors proposed a mechanisminvolving an etching process of NPs, whereby discrete molecular palladium specieswere formed and promoted the Suzuki coupling reaction in the solution (Scheme 3.2).Similar findings were obtained by using quick-extended X-ray absorption fine structurefor in situ analysis in the Heck reaction.69 In addition to the chemical etching process,Pd-NPs with a size of 2 nm were identified during the reaction. They concluded that thereaction was catalyzed by molecular Pd complexes, which were formed in situ, from thePd colloids, which not only served as Pd reservoirs but also were directly involved inthe catalytic cycle. The active catalyst was reduced to NPs at the end of each catalyticcycle (redeposition) and had to be reformed at the beginning of a new catalytic cycleby the interaction of PhBr with the surface of the Pd colloids (dissolution). A mecha-nism for the Pd-NPs-catalyzed Suzuki cross-coupling involving both homogeneous andheterogeneous pathways is depicted in Scheme 3.3.43 It is worthwhile to mention thatPd-NPs were also observed during the Suzuki coupling reaction using palladacycles asmolecular catalyst precursors.70 Poisoning experiment and TEM analysis of aliquots


Ar-Ar’ Ar-Ar’

Ar-X

Ar’-B(OH)2

X-B(OH)2

Reductiveeliminatiion

Reductiveeliminatiion

ArAr-X

Oxidativeinsertion

Pd cluster Pd(0) atom

Oxidativeaddition

(II)

Redeposition

Leaching

X-B(OH)2

Ar’-B(OH)2

Transmetalation TransmetalationChemicaletching

Heter

ogen

eous

Homog

eneo

us

Ar’

Ar

XAr

X

Ar

Ar’

Scheme 3.3. Proposed mechanism for the Pd-NP-catalyzed Suzuki cross-coupling involving

both homogeneous and heterogeneous pathways. (Reprinted with permission from Ref. 43,

Copyright 2012, American Chemical Society.)

taken from the Suzuki coupling between 4-bromoacetophenone and phenylboronic acidconfirmed the presence of Pd-NPs with 3 nm in average size.

There are also some studies indicating that the catalysis in the C C coupling reac-tions occurs on the surface of NPs. Choudary and coworkers71 have prepared and usedlayered double hydroxide (LDH) and Merrifield resin-supported nanopalladium(0) ascatalysts for the cross-coupling process, including the Suzuki coupling of chloroarenes.TEM images of the fresh and used catalyst for the Heck reaction show that the nanos-tructured palladium supported on LDH remains unchanged at the end of the reaction.Also, Ar–Pd–X species on the heterogeneous surface were identified by X-ray photo-electron spectroscopy (XPS) and from the evolved gas detection by thermogravimetricanalysis-mass spectrometry. Li and coworkers50 investigated the effect of the size of NPson the Suzuki reaction between phenylboronic acid and iodobenzene. By studying thecatalytic activity of PVP-stabilized Pd-NPs with four different sizes, they proposed thatthe vertex and edge atoms on the surface were the active sites. Such correlationmay actu-ally simply reflect a greater propensity for low-coordinate Pd to solubilize and therebypromote homogeneous reactivity.22 Similar kinetic findings were described by Ellis andcoworkers (Figure 3.6).72 By using a combination of operando X-ray absorption spec-troscopy, surface-sensitive XPS, and detailed kinetic profiling, they provided the firstdirect evidence that the Suzuki coupling reaction could occur heterogeneously, at thesurface of PVP-stabilized Pd-NPs. Davis and coworkers also described a nice evidencefor the surface-driven Suzuki reaction.73 They used a Pd-modified, catalytically active,atomic force microscopy (AFM) probe to initiate and spatially control surface-confinedSuzuki and Heck C C coupling reactions. These “chemically written reactions” would

ACTIVE Pd-NPs AND OTHER METAL NPs FOR SUZUKI C C COUPLING 67

MeO

Rel

ativ

e TO

F

3.8

3.4

3.0

2.6

2.2

1.8

1.4

1.0

0.61 2 3

Nanoparticle diameter (nm)

Defect site

Defect

Total

4 5

1 2

MeOB(OH)2I +

Figure 3.6. Structure-sensitive Suzuki coupling of 1 and 2 over size-selected, cuboctahedral,

PVP-stabilized Pd-NPs. The TOFs of the catalytic reaction are normalized relative to surface

atom densities of largest nanoparticles: total surface atoms (•) or defect surface atoms (◦). The

normalized cross-coupling rate should be independent of nanoparticle size if the correct active

site has been identified. (Reprinted with permission from Ref. 72.)

be unexpected if coupling were mediated by mobile, solubilized Pd species diffusingfreely throughout the solvent media.

ACTIVE Pd-NPs AND OTHER METAL NPs FORSUZUKI C C COUPLING

The effects of the structural features of nanoparticles on the catalytic behavior have beendiscussed previously. Next, selected examples in the literature describing the synthesis ofvarious catalysts with good catalytic activity for the Suzuki reaction are discussed. Thediscussion includes active palladium and other metal NPs for the Suzuki C C couplingreaction.

Palladium Nanocatalysts

The investigation of Pd-NPs in the Suzuki reaction has intensively grown in the lastyears, and so has the preparation methods for the synthesis of colloidal and supportedmetal NPs that are very active catalysts.74, 75 Regarding the application of Pd-NPs for theSuzuki reaction, it is more appealing to synthesize those that are recyclable and stable


Figure 3.7. Unexpected facilitated activation of bromoanisole by Pd-NPs.

catalysts by facile and reproducible methods. In this regard, Cargnello and cowork-ers76 reported a simple method to synthesize colloidal Pd-NPs by mixing an aqueoussolution of K2PdCl4, acetone, H3PO4, and 11-mercaptoundecanoic acid at low temper-ature. The resulting approximately 2 nm Pd-NPs were employed for C C coupling ofphenylboronic acid with different aryl halides including activated aryl chloride under airconditions. The catalyst could be recovered by centrifugation and reused to five cycleswithout considerable loss of activity and negligible leaching. These NPs also have theadvantage of an amphiphilic nature that allow their solubility in organic solvents andaqueous solutions by simple protonation/deprotonation of their carboxylic acid groupspresent in the edge of their self-assembled monolayer structures.

Dihydroanthracene derivatives have been used as ligands for the stabilization ofPd-NPs starting from Pd0 and Pd2+ precursors. The authors produced Pd-NPs withdiameters ranging from 1.9 to 3.6 nm, which were able to catalyze the Suzuki couplingof aryl bromides. An important feature of this work is that for some nanocatalysts,the activated (4-CF3) and deactivated (4-OMe) bromides showed similar biaryl yields.This unexpected result was attributed to the interaction of the methoxy group with themetallic surface, diminishing its electron donor character, and in consequence favoringthe oxidative addition of the C–Br bond with the metal (Figure 3.7).77

The activity of Pd-NPs in ILs was also explored in Suzuki coupling. Pd-NPsprepared from the reduction of Pd(OAc)2 in 1-n-butyl-3-methylimidazolium hexaflu-orophosphate (BMI·PF6) in the presence of amines, imides, or ammonium ligandsderived from norborn-5-ene-2,3-dicarboxylic anhydride were suggested as the reservoirfor the true catalysts in Suzuki reactions.78 The authors showed that in organic sol-vents the homogeneous catalyst was stabilized in the presence of good donor ligands;otherwise, inactive agglomerates were formed. However, in ILs, the palladium systemwas only active when NPs were formed in situ. Pd(II) complexes stabilized in the ILphase, which prevented the formation of Pd(0) species, were inactive. Pd-NPs werealso prepared by the reduction of Pd(COD)Cl2 (where COD is 1,5-cyclooctadiene) withmolecular hydrogen in the same IL BMI·PF6 at room temperature, and these nanoparti-cles were used as effective catalysts for Suzuki cross-coupling reactions.79 The Pd-NPsin the IL phase were able to catalyze the coupling of bromobenzene and phenylboronicacid at 100 ◦C in a short reaction time; however, the reaction did not proceed with


Figure 3.8. Structure of some ILs synthesized by Zhao and coworkers to stabilize NP catalysts

for the Suzuki reaction.81

chlorobenzene reactant. The amount of palladium leached in the coupling product wasin the range 3–5 ppm, so eight recycles could be carried out without losing activity. Thenanoparticulate nature of the catalyst was proved since mercury completely inhibitedthe progress of the reaction. The authors mentioned that neither the palladium precursorPd(COD)Cl2 nor the isolated palladium powder was active in the reaction. Only themetal NPs stabilized by the IL served as a precatalyst for the Suzuki process. Moreover,Pd-NPs prepared from PdCl2 and Pd2(dba)3·CHCl3 did not show the same satisfac-tory catalytic results. Calo and coworkers80 prepared Pd-NPs by reducing Pd(OAc)2 intetraalkylammonium-based ILs and tested catalytic activity of the nanoparticles for theSuzuki coupling reaction of several aryl chlorides and bromides, with arylboronic acidsin a biphasic medium composed of the IL phase and an aqueous solution containing thebase. Different tetraalkylammonium-based ILs were used and the combination of tetra-heptylammonium bromide IL and tetrabutylammonium hydroxide as base resulted thebest performance, probably due to the stronger stabilization of the Pd-NPs. Aryl chlo-rides bearing electron-donating substituents required higher temperatures and affordedlower conversions and yields. The catalytic system could be reused for three runs anda noticeable decrease of activity was observed in the fourth run. Zhao and coworkersreported the use of a nitrile-functionalized IL (Figure 3.8a) in order to stabilize pal-ladium species in Suzuki and Stille reactions.81 This reaction medium, in conjunctionwith several precatalysts, was able to catalyze the coupling between iodobenzene andphenylboronic acid, giving yields greater than 80% in 12 h. The main advantage of thisIL (Figure 3.8a) in comparison with N-butylpyridinium (Figure 3.8b) is the reusabilityas Figure 3.8a was active in nine recycles without loss of catalytic activity whereasFigure 3.8b was active for only two recycles. However it must be mentioned that onlythe NPs formed in the Stille reaction were characterized. Another CN-functionalized IL(Figure 3.8c), in conjunction with an ionic polymer (Figure 3.8d), was used to stabilizePd-NPs and the resulting NPs showed excellent catalytic activities toward C C couplingreactions, including Suzuki reactions, containing aryl iodides, and activate bromides.Moreover, the nanocatalyst could be stored for 2 years without undergoing aggregationor precipitation.82

Polymeric materials have been used not only to immobilize transition metals ormetal complexes but also to immobilize and stabilize metal(0) nanoparticles for prepa-ration of heterogeneous catalysts. Many catalysts have been developed by coordination


HN

Reduction

Polymer nanoparticle composite

Nano-needle crystalsMicrowave heating

Suzuki coupling reactions

Rigid polymerbackbone

Pd nanoparticlesN n

C6H13

C6H13

HPdCl4

NaBH4

Figure 3.9. Aging of PDHC-stabilized Pd nanocatalyst in the Suzuki coupling reaction.

(Reprinted with permission from Ref. 65, Copyright 2005, American Chemical Society.)

of palladium with nitrogen donor atoms on the polymer matrix. For instance, poly(N,N-dihexylcarbodiimide) (PDHC) is a relatively weak coordinating ligand that was foundto be very effective to immobilize Pd(II) ions and stabilize Pd-NPs.83 The NPs wereprepared by reduction of H2PdCl4 with NaBH4 in a two-phase mixture of the ligand,toluene, and water. The resulting polymer-supported Pd nanocatalyst was active forSuzuki coupling of various aryl bromides and iodides and easily recovered by precip-itation and filtration. The catalyst was active even after five reaction recovery cycles.Aryl chlorides did not undergo catalytic reaction with the materials under the studiedconditions though. The same authors also studied the aging of PDHC-stabilized Pdnanocatalysts under microwave heating during the Suzuki reaction (Figure 3.9).68 Theshape of the NPs gradually changed from spherical shapes, with “blackberry”-like struc-tures, to larger needle-shaped particles upon recycling. The observed ripening processeswere also evident from the observed product yields in each catalyst recycle, whichshowed a steep decline in the catalytic activity of the reactions in the first and secondruns, accompanying the changes in the shapes of the particles. After the decline in thefirst few cycles, the catalytic activity seemed to level off afterward.

Monodisperse poly(vinylpyridine) nanospheres (50 nm) were successfully used assupport for Pd-NPs (1–4 nm). This system was applied to Suzuki coupling of activatedbromides, giving the biaryl product with almost 100% of conversion in 6 h with morethan 0.5 mol% of catalyst.84 Other polymeric nanospheres and microspheres were alsoused as support to Pd-NPs.85,86 Nanospheres that contain a polystyrene core and apH-responsive and chelating shell composed of PGMA-IDA (polystyrene-co-poly[2-methacrylic acid 3-bis-(carboxymethylamino)-2-hydroxypropyl ester]) were utilized asa scaffold to synthesize Pd-NPs embedded within the shell layer of the core–shellnanospheres. This catalyst is highly dispersed in aqueous medium in pH ranging from3 to 11 just like a homogeneous catalyst, and can also be separated and reused like aheterogeneous one by adjusting the pH value of the aqueous medium. This system wasactive in the Suzuki coupling of aryl bromides and iodides even at room temperature,and the catalyst was reused four times, affording a constant yield of 99%.87


Palladium(0) NPs supported on polyaniline were active in the coupling of acti-vated and nonactivated aryl iodides with phenylboronic acid and 4-methylphenylboronicacid.88 The possibility of catalyst recycling was not reported, and modest conver-sions were observed for the coupling of aryl bromides. More effective was thepolymer-supported catalyst resulting from interaction between Pd-NPs and polystyrene-poly(ethylene glycol) resin after cross-linking.89 The catalyst was active in the cou-pling of activated and deactivated aryl bromides with phenylboronic acid and 4-methoxyphenylboronic acid in water, giving moderate-to-excellent yields. Additionally,the catalyst could be recycled six times without losing catalytic activity. Under thereported conditions, however, only low catalytic activity was observed for the couplingof aryl chlorides. Similarly, a polymeric ammonium gel, obtained from the Merrifieldresin and triethylamine, was shown to promote the formation of Pd-NPs during theSuzuki reaction of aryl and heteroaryl bromides with aryl- and heteroaryl boronic acids.The biaryls were obtained in high yields and the catalyst was recycled five times, givingalmost constant activity.90

Dendrimers are particularly attractive hosts for catalytically active NPs, but it hasbeenmuch less studied than polymers for the Suzuki reaction. Li and El-Sayed48 reportedPd-NPs stabilized by a third generation (G3) PAMAM [poly(amidoamine)] dendrimerthat could catalyze the Suzuki reaction. Pittelkow and coworkers91 studied the behaviorof Pd-NPs encapsulated in G4 PAMA-MeOH-terminated dendrimers, but this G4 den-drimer did not avoid the formation of palladiumblack during the Suzuki reaction. Pd-NPsencapsulated in a G3 dendrimer (Frechet-type dendritic polyaryl ether disulfide) resultedin a more stable Suzuki catalyst.92 The catalysts gave high turnover numbers (23,300and 26,100) and TOFs (1942 h−1 and 2175 h−1), but only modest yields of product lessthan 50%. The catalyst could be easily precipitated by the addition of methanol but wasnot used in recycling experiments. Phosphane dendrimer-stabilized Pd-NPs were syn-thesized from Frechet-type polyaryl ether dendrons.93 The catalyst was highly effectivein the Suzuki reaction, even with aryl chlorides, giving the biaryl products in 86–100%yield. Furthermore, the catalyst could be precipitated by the addition of methanol andreused at least eight times with high performance. Astruc and coworkers94,95 reported thedesign of very active “click”-dendrimer-encapsulated and “click”-dendrimer-stabilizedPd-NP catalysts, which catalyzed C C coupling reactions under catalyst loading downto 1 ppm under ambient conditions in organic as well as aqueous solvents. The versatilemolecular structures and specific topology obtained by the dendritic structures gener-ation by generation were considered to be among the advantages of dendrimers overpolymers as support materials for the synthesis of heterogeneous catalysts.

Pd-NPs encapsulated in silica were prepared by mixing Pd(PPh3)4 with a solutionof tetra(ethylene glycol) and tetramethoxysilane. The catalytic activity of the NPs wastested in the Suzuki reaction between phenylboronic acid and several different activatedaryl halides.96 The catalyst was shown to be reusable three times without losing activity,although its application seemed to be limited to aryl bromides and iodides bearingelectron-withdrawing groups. Pd-NPs immobilized onto mesoporous silica, organicallymodified by amine functional groups, proved to be active in the coupling reactionbetween aryl bromides and phenylboronic acid. Although high reaction conversionswere attained, deactivation of the catalyst occurred after three runs for these catalysts.57


CDGGO

R

R

Br

(HO)2B

Figure 3.10. Graphite oxide and graphene used as support for Pd-NP catalysts. (Reprinted

with permission from Ref. 99, Copyright 2009, American Chemical Society.)

Pd and Pd/Au-NPs highly dispersed in functionalized mesoporous silica SBA-15 werealso found to catalyze the Suzuki reaction.97 Other examples include the use of polymer–inorganic hybrid materials as support for Pd-NP catalysts for coupling reactions.98

Pd-NPs supported in tetraalkylammonium salts were shown to activate aryl chlo-rides. Using these Pd-NPs in a mixture of water, long-chain alkylammonium salts, andNBu4OH as base, biaryl products with excellent yields in short times (3–5 h) wereobtained.80 The catalyst was successfully recycled three times. Another remarkablework in terms of activation of aryl chloride was reported by Choudary and coworkers.71

The authors synthesized Pd-NPs supported in layered double hydroxide (LDH) that wascapable to catalyze Suzuki (and another C C coupling) reactions of chloroarenes withgood-to-excellent yields (1 mol% in 10 h), even for the deactivated chloroanisole. Theyalso described that the Pd nanocatalyst was 2.3-fold more active than homogeneous pal-ladacycles. In the reaction between phenylboronic acid and chlorobenzene, it could bereused in five cycles of Suzuki reactions, completing the reactions with the same yield.

Scheuermann and coworkers99 employed Pd(II)-exchanged graphite oxide (GO)and chemically derived graphenes (CDG) as supports for Pd-NPs (Figure 3.10).Different methods of reducing Pd+2-GO to Pd0-CDG were tested. Reduction withH2-generated Pd-NPs (Pd0-CDG-H2) with a diameter of 7 ± 2 nm, reduction withhydrazine hydrate produced 26–82 nm clusters (Pd0-CDG-N2H4), and thermal reductionled to NPs with a narrow size distribution of 3.3 ± 0.7 nm (Pd0-CDG-EXP). BothPd+2-GO and Pd0-CDG catalysts were active in the Suzuki coupling of activatedand deactivated aryl bromides. The highest activity was reached with Pd2+-GO andPd0-CDG-H2, followed by Pd0-CDG-EXP. With the first, a TOF of 39,000 h−1

was attained. Concerning the reusability, although no Pd leaching was detected, theconversion decreased dramatically in the third recycle.

Magnetic NPs have attracted a lot of attention for their appealing features assupport material for metal catalysts. Magnetic separation is a powerful way to achieveseparation of catalysts easily and quickly from reaction media. This, in turn, allowsseparation of the final product and for the catalysts to be recycled and reused. Comparedto the procedures such as filtration, centrifugation, or extraction (liquid–liquid or


TABLE 3.2. Magnetically recoverable Pd-based catalyst for Suzuki coupling of phenylboronicacid and iodobenzene

Catalyst Solvent/base/temperature (◦C) Time (h) Yield (%)

Pd/Mag-MSN CH2Cl2/K2CO3/80 6 85 (Ref. 104)Xerogel gl-MNPs CH3OH/Na2CO3/60 2 99 (Ref. 105)Pd/NiFe2O4 DMF/Na2CO3/90 2 50 (Ref. 106)Fe3O4-Bpy-Pd(OAc)2 Toluene/K2CO3/80 6 �99 (Ref. 107)Fe/FexOy/Pd H2O : EtOH (1 : 1)/K2CO3/rt 2 98 (Ref. 108)Pd-Fe3O4 DME : H2O(3 : 1)/K2CO3/reflux 48 92 (Ref. 109)C/Co/PNIPAM-PPh2-Pd Toluene : H2O(2 : 1)/K2CO3/85 16 99 (Ref. 110)MP/NiSiO/Pd DME : H2O(1 : 1)/Na2CO3/110 – 95 (Ref. 111)Co/C/Pd complex THF : H2O(1 : 2)/Na2CO3/65 2 96 (Ref. 112)Pd/NiFe2O4 NMP : H2O(10 : 4)/K2CO3/80 4 97 (Ref. 113)Pd/MFC EtOH/K2CO3/reflux 1 100 (Ref. 103)Pd/CoFe2O4 EtOH/Na2CO3/reflux 12 81 (Ref. 114)

Reprinted with permission from Ref. 103. Copyright (2011) American Chemical Society.MSN, mesoporous silica nanocomposites., MNPs, magnetite nanoparticles., PNIPAM, poly-N-isopropylacrylamide., MFC, magnetic Fe3O4/C nanocomposites., DMF, N,N-dimethylformamide., DME,1,2-dimethoxyethane., NMP, 1-methyl-2-pyrrolidinone.

chromatographic) used to separate nanoparticles, the magnetic separation avoids theuse of auxiliary substances (e.g., solvents and filtration elements), making the processcleaner, environmentally friendlier, and faster. In addition, magnetic separation doesnot require the catalysts to be removed from inside of the reactors, which is especiallyimportant for air-sensitive catalytic systems. Moreover, the possible loss of catalystsdue to transfer steps can be avoided.

Most of the magnetic recoverable supports for Pd-NPs in Suzuki involve organo-modified magnetite (Fe3O4) NPs or silica-coated Fe3O4 NPs.59,100–102 Other materialsinclude carbon-coated Fe3O4 NPs prepared by in situ carbonization of glucose underhydrothermal condition. After immobilization of Pd-NPs on such support materials viathe deposition–precipitation method, the resulting materials (e.g., Pd/MCF [Pd nanocat-alyst based on magnetic Fe3O4@C nanocomposites as supports])) can be used as easilyrecoverable and very active catalysts for various cross-coupling reactions, including theSuzuki reaction. Pd/MCF was shown to catalyze the cross-coupling reactions involvingnumerous types of deactivated aryl halides, and after the reactions it was successfullyrecycled and reused for five cycles without losing activity and with only 0.2% of leach-ing after the fifth reaction recovery cycle. The activity of Pd/MCF was found to be verygood when compared with other magnetically recoverable catalysts in the literature forthe coupling reaction between iodobenzene and phenylboronic acid (Table 3.2).103

Developing green processes for the synthesis of catalysts as well as green conditionsfor catalytic reactions has received a lot of attention in recent years. For example, thepotential use of ecofriendly biomaterial such as bacterial cellulose (BC) as support hasbeen investigated for Pd-NPs in the Suzuki reaction. BC has a large surface area andstable heterogeneous interface. Zhou and coworkers115 optimized the conditions for


Suzuki coupling for BC-supported Pd nanocatalysts and further analyzed the behaviorof the catalyst using water as the solvent. The catalyst was highly recyclable and showedno Pd aggregation on it after recycling. The most interesting feature is that by usingsuch an ecofriendly support material and water as the solvent for the reaction, it was stillpossible to achieve more than 75% yields for coupling of aryl chlorides with deactivatedboronic acids. Another bioapproach involves the use of microorganisms themselvesas reducing agents of metals during preparation of catalysts. Interestingly, bacterialcells can act as a reducing and stabilizing agent as well as high specific area supportmaterials. So, using this approach, nanocatalysts can be prepared more sustainably andwith less chemicals in the process of synthesis of the NPs. Heugebaert and coworkers116

investigated the use of themetal-respiring bacterium Shewanella oneidensis, a bacteriumcapable of precipitating Pd and Au nanoparticulate species on its cell wall structures,to synthesize Pd and Au/Pd-NPs (bio-Pd and bio-Au/Pd) that were used as catalysts inSuzuki reactions in ethanol/water as the solvent. The activity of the resulting bio-Au/Pdnanocatalysts was higher than that of bio-Pd for most of the substrates; however, it waslower than that of Pd/C.

Søbjerg and coworkers117 also investigated the use of bacteria (Cupriavidus necatorand Staphylococcus sciuri) for the preparation of Pd-NP catalysts for C C coupling reac-tion between iodoanisole and phenylboronic acid. By varying the ratio of biomass–Pd orincreasing of the relative content of the biomass, smaller NPs were formed. However, thesmallest NPs were not active in the Suzuki reaction because the excess of bacteria cellspoisoned the catalysts. This is most likely because the thiol groups in the proteins withinthe bacteria bound to the Pd-NPs, deactivating the catalyst. Despite the advantages asso-ciated with the use of biosupported nanocatalyst, this synthetic method is relative newand the stability of this kind of catalyst may need to be investigated in more detail.

The use ofwater as a solvent for C C coupling reactions under green reaction condi-tions is desirable, but it can also be problematic as most of the reagents are hydrophobic.Some Pd-NPs were shown to be effective in catalyzing the coupling reaction betweenaryl and heteroaryl iodides with arylboronic acids in aqueous media.118,119 For example,Firouzabadi and coworkers120 used agarose hydrogel as an bioorganic stabilizer andsupport for the stabilization of Pd-NPs for the Suzuki coupling reaction of different aryliodides, bromides and chlorides with phenylboronic acid in water and in the absenceof any organic cosolvents or phase-transfer additives. As agarose is a nontoxic, cheap,degradable, and soluble (in hot water) ligand, it is very attractive for reactions conductedin water for transition metal catalysis. Pd-NPs supported on cellulose, the most abundantbiomacromolecule, was also found to be a highly efficient recyclable heterogeneous cat-alyst for the Suzuki coupling between aryl bromides and phenyl boronic acid in water.121

Tsvelikhovsky and Blum122 have proposed a microemulsion/sol–gel system for aqueousC C coupling where an emulsion of hydrophobic substrates is subjected to a metalcatalyst that has been entrapped within silica containing hydrophobic groups preparedby sol–gel. The surfactant in the microemulsion transports the reactants to hydrophobicporous of the silica containing the catalyst where the reaction takes place. The final prod-uct is taken back by the surfactant to the aqueous solution, where it can be separated.Based on this principle, they have shown that Pd-NPs prepared by the entrapment of pal-ladium acetate into a hydrophobic silicamatrix are efficient and highly selective catalysts


100 nm

Silica sphere

(CH3O)3Si(CH2)3SH

Toluene

HFEtching

1) Pd(acac)2 2) Δ

SH

SH SH

SH

SH

SH

HS SH

SH

SH

SH

SH

Figure 3.11. Hollow palladium spheres synthesized by using thiol-functionalized silica tem-

plate. (Reprinted with permission from Ref. 124, Copyright 2002, American Chemical Society.)

for the Suzuki and Heck reaction for different substrates, including deactivated reactants,in water. Sawoo and coworkers123 prepared Pd-NPs in water by reduction of Pd(II) witha Fischer carbene complex using PEG as a stabilizer. This aqueous nano-sized Pd is ahighly efficient catalyst for Suzuki and other coupling reactions in aqueous medium.

Many other types of nanostructuredmaterials containing Pdwith other unique struc-tures for Suzuki coupling reactions have been reported. Recyclable hollow palladiumspheres were synthesized by using thiol-functionalized silica template (Figure 3.11).124

This system was found to be effective for the Suzuki reaction between aryl iodides andbromides. The catalyst was also proven to be reusable several times without loss ofcatalytic activity. However, the stability of the hollow structure of the catalyst, after thereaction, was not shown.

Other Metals and Bimetallic Nanocatalysts

In addition to palladium, a plethora of othermetals have been described as active catalystsfor the Suzuki cross-coupling reaction. Nickel (Ni) NPs synthesized by thermal decom-position of Ni–oleylamine complexes were effective in coupling of aryl bromides.125

However, when the nanoparticles were reused, the yield decreased from 98 to 47%. Thedecrease of the activity was attributed to formation of NiO.

The reactivity and reusability of tetrahedral PVP-stabilized Pt-NPs were comparedwith PVP-stabilized spherical Pd-NPs.52 The Pt-NPs showed one third of the yield ofthe Pd-NPs under the same conditions. Moreover, the yield obtained with the Pt-NPsdecreased dramatically from the first to the second Suzuki cycle. The decrease in catalyticactivity was attributed to a change in the shape of the NPs since spherical nanoparticleswere observed after the reaction.

Thathagar and coworkers126 described the synthesis and activity in the Suzuki reac-tion of Cu-NPs and Cu-NPs modified with Pd, Pt, and Ru. All Cu-based nanocatalystswere active for iodobenzene, giving yields from 62 to 100% in 6 h. The order of activitywas as follows: Cu/Pd � Cu/Pd/Ru � Cu/Pd/Pt � Cu/Pd/Pt/Ru ≈ Cu. The authors


also indicated that the nanocatalyst retained its catalytic activity for at least 2 weeks. Inanother work by the same authors, the bimetallic Cu/Pd-NPs were again the most activesystem. The NPs promoted the coupling of iodides and bromides, giving high yield.For activated chlorides, moderate conversions were observed, even in larger reactiontimes.127

Bimetallic Au/Pd nanoflowers were synthesized by reduction of Pd ions in the pres-ence of Au-NPs and PVP (Figure 3.12). By adjusting the reaction time, the structureof the Pd petals could be controlled and Aucore/Pdshell nanoflowers at different stagesof growth could be identified. The ability of these nanoflowers to catalyze the Suzukireaction between iodobenzene with phenylboronic acid was compared with that of Pdnanocubes, and thin-shelled Au/Pd core–shell NPs. As a result, it was observed thatthe rate of conversion to biphenyl was not dependent upon the surface structure or sub-surface composition of the NPs. Instead, the observed catalytic activity for the Suzukicoupling reaction was attributed to molecular Pd species that leached from the Pd nanos-tructures.128 Active Au–graphene hybrids were produced by reducing chloroauric acidin sodium dodecyl sulfate. By adjusting the Au–graphene ratio, approximately 2.7 or7.5 nm NPs could be obtained. Approximately 2.7 nm NPs gave the highest catalyticconversion and selectivity in Suzuki cross-coupling of iodobenzene. This activity wascomparable with that obtained with Pd(OAc)2 under the same conditions. In addition,the system was recycled four times, giving almost similar conversion (∼75% in 4 h).However, when the methodology was extended to bromobenzene or allyl iodide, theyield decreased considerably.129 The best results with Au-NPs were obtained by Hanand coworkers.130 They produced 1.0 nm poly(2-aminothiophenol)-stabilized Au–NPs,which efficiently promoted the coupling reaction between aryl chlorides and aryl boronicacids. Biaryl products were obtained with remarkable yields, even with 0.05% of cat-alysts, in water. In addition, five recycles were successfully performed, each givingsimilar yield. Furthermore, yield and catalytic activity remained unchanged even whenthe Au nanocatalyst was used after aging for as long as 6 months.

Rh-NPs supported on layered double hydroxide (LDH-Rh0) were also reported asan active nanocatalyst for C C coupling reactions, particularly for aryl iodides andbromides, forming biaryl products.131 By using 3 mol% of the LDH-Rh0, yields rangingfrom 78 to 96%were obtained in 18 h. After the reactions, the catalyst could be recoveredby simple filtration and reused several times without significant loss of catalytic activity.TEM images of the fresh and used catalysts looked similar, corroborating the stabilityof the catalysts.

SUMMARY AND FUTURE OUTLOOK

Impressive progress has been reported during the last few years on the developmentof nanoparticle catalysts for Suzuki coupling reactions. Many advantages have beenreported, such as easy recovery, reusability, and wide accessibility of such catalystssupported on organic, inorganic, or organic–inorganic hybrid solids and liquids (biphasiccatalysis). However, a number of challenges also remain. Most of the nanocatalysts, aswell as homogeneous or supported complexes, are less active for the coupling of the

SUMMARY AND FUTURE OUTLOOK 77

100 5 10 15 20 25 30

15

20

25

Dia

met

er (

nm

)

Time (min)(a) (b)

0 s

1 min10 s

30 min15 min

20 nm

(c) (d)

(e) (f)

30

35

40

Figure 3.12. (a) Diameter of the Au/Pd-NPs versus time, as determined by TEM images.

(b–f) TEM images show of the growth of the Au/Pd nanoflower, obtained by quenching the

reaction with HCl at various time points. The inset of (c) shows clusters, indicated by arrows,

attached to the Au/Pd-NPs. The scale bars in the insets are 5 nm. (Adapted with permission

from Ref. 128, Copyright 2011, American Chemical Society.)


readily available and less costly aryl chlorides. Progress in the research field is hinderedby the lack of consensus on the mechanism of coupling reactions by nanoparticles. Thereare many contradictory results indicating that the nanoparticles serve as reservoirs foractive (soluble) catalyst species and also results that show nanoparticle surface-catalyzedprocesses. The well-known methods usually employed to distinguish homogeneous andheterogeneous catalysts are unfortunately not effective for nanocatalysts. Thus, it is notoften easy to compare the performances of different nanocatalysts, some of which arepredesigned and synthesized to be nanoparticle catalysts and others formed or identifiedonly during the catalytic reactions. One helpful parameter to address the limitationsor potential advantages of a heterogeneous catalytic system is its reusability. Greenmethods for the synthesis of nanocatalysts and greener reaction conditions for catalysis,which have received a lot of attention lately, have also been considered in researchinvolving the Suzuki reactions. The use of ecofriendly biomaterials as catalyst support,stabilizers, and reducing agents has been investigated for Pd-NPs for the Suzuki reaction.Water-soluble Pd-NPs have been reported as highly efficient catalysts for Suzuki andother coupling reactions in aqueous medium, which is consistent with the current trendtoward the replacement of toxic and/or hazardous solvents and reagents with cleaneralternatives.

REPRESENTATIVE EXPERIMENTAL PROCEDURES

Representative Procedures in Terms of Reusability

Supported Pd-NPs—a Reusable Catalysts in Fifteen Cycles.38

Synthesis of Pd-NPs. In a Fischer–Porter bottle, 0.025 mmol of palladiumprecursor—7.1 mg for [PdCl2(cod)], 4.4 mg for PdCl2, 12.9 mg for [Pd2(dba)3 inCHCl3]—in 5 ml of [BMI][PF6] was stirred at room temperature under argon untilcomplete dissolution. The system was then pressurized with 3 bar of dihydrogen andstirred at room temperature for 2 h, leading to a black solution. In the case of [Pd2(dba)3in CHCl3], heating at 60 ◦C was necessary to obtain decomposition. The residual gaswas then released and the volatiles were removed under reduced pressure.

General Procedure for Suzuki Coupling. To 1 ml of a 5 mM solution Pd-NPsin [BMI][PF6] prepared as described above, bromobenzene (0.21 ml, 2 mmol), phenylboronic acid (268 mg, 2.2 mmol, 1.1 equivalent), and Na2CO3 (445 mg, 4.2 mmol,2.1 equivalent) dissolved in 1.5 ml of water were added under argon. The mixture wasthen stirred at 100 ◦C. After 1 h, the solution was allowed to come to room temperature,and biphenyl was extracted with hexane (5 × 5 ml). The combined extracts were thendried over Na2SO4, filtered up, and volatiles removed under reduced pressure, affordinga white crystalline solid. For the recycling experiments, after hexane extraction andremoval of the aqueous phase, the IL solution containing the catalyst was consecutivelywashed with diethyl ether (3 × 5 ml) and water (3 × 5 ml), and dried under vacuum at60 ◦C for 4 h. The successive catalytic experience was then run, adding the reactants asdescribed previously.

REPRESENTATIVE EXPERIMENTAL PROCEDURES 79

Supported Pd-NPs—a Reusable Catalysts in Fifteen Cycles.38

Synthesis of Supported Pd-NPs. In order to prepare the activated silica, 10 gof commercial silica (100–200 mesh), 80 ml of concentrated H2SO4, and 15 ml ofHNO3 were added to a round-bottom flask equipped with a reflux system. The mixturewas kept at 140 ◦C for 24 h and the resulting mixture was filtered. The powder waswashed with distilled water until achieving a neutral pH, and then washed with acetone,methanol, and dichloromethane followed by vacuum drying at 150 ◦C for 48 h. Next,the activated silica was functionalized with amine groups, where a round-bottom flaskwas charged with 20 ml of anhydrous toluene, 3 g of activated silica, and 10 ml of3-aminopropyltriethoxysilane (APTS). This mixture was refluxed for 24 h, filtered, andthe solid was washed with acetone and dichloromethane and vacuum dried at 60 ◦C.Finally, 1 g of silica–APTS in a Schlenk tube was mixed with 0.112 g of Pd(CH3CO2)2in 5ml of dry ethanol. Themixture was stirred under N2 atmosphere at room temperaturefor 4 h, filtered, and the solid (silica–APTS–Pd) was washed with acetone and methanoland dried under vacuum at room temperature for 16 h resulting in a yellow powder.

General Procedure for Suzuki Coupling. In an oven-dried round-bottomflask, 46 mg of silica–APTS–Pd (0.01 mmol of Pd), 276 mg of K2CO3 (2.0 mmol),organoboronic acid (1.0 mmol), organic halide (1.0 mmol), and 3.0 ml of N,N-dimethylacetamide (DMA) were mixed under N2 atmosphere. The reaction mixturewas stirred at 100 ◦C for 2–10 h depending of the substrate. Afterward, the mixturewas cooled down to the room temperature and then vacuum filtrated and washed (2× 5 ml) with CH2Cl2. The organic phase was dried over Na2SO4, filtered, and con-centrated to further purification by flash chromatography on silica gel. For recyclingexperiments, the catalyst was recovered by vacuum filtration followed by wash with3 ml of dichloromethane, 3 ml of diethyl ether, 3 ml of ethanol, and 3 ml of hexane. Thesolid was dried in an oven and then reused in the next reaction.

Representative Procedures in Terms of Chloroarene Activation

LDH-Supported Pd-NPs.71

Synthesis of Pd-NPs. Mg–Al–Cl (LDH) (1.5 g) was suspended in 150 ml ofaqueous Na2PdCl4 (0.441 g, 1.5 mmol) solution and stirred at 25 ◦C for 12 h under N2atmosphere. The solid catalyst was filtered, washed thoroughly with 500ml of water, andvacuum dried to obtain 1.752 g of LDH–PdCl4 (0.86 mmol of Pd/g). Then, LDH–PdCl4(1 g) was reduced with hydrazine hydrate (1 g, 20 mmol) in ethanol (10 ml) for 3 h atroom temperature, filtered, and washed with ethanol to give an air-stable black powder(0.95 mmol of Pd/g).

General Procedure for Suzuki Coupling. Chloroarene (1 mmol), arylboronicacid (1.5 mmol), potassium fluoride (3 mmol), LDH–Pd0 (1 mol %), and 1,4-dioxane/water (5 : 1, 5 ml) were charged in a round-bottom flask. Reactions werecarried out at 100 ◦C for 10 h. After completion of the reaction (monitored by thin layer


chromatography), the catalyst was filtered and the reaction mixture was poured intowater; the aqueous phase was extracted with diethyl ether. After drying, the correspond-ing product was purified by crystallization from diethyl ether/pentane.

Representative Procedure in Terms of Green Synthesis

Pd-NPs Prepared in Water.51

Synthesis of Pd-NPs. Pd-NPs of 4.8 nm were prepared by adding 0.1 ml of0.1 mol l−1 Na2PdCl4 to 10 ml aqueous solution containing 0.2 g of triblock Pluroniccopolymer (P123, PEO19–PPO69–PEO19, 3.4 mmol l−1) at room temperature, and theresulting solution was vigorously stirred. The color of the solution changed from lightyellow to a deep brown, indicating the formation of nanoparticles. The reaction mixturewas kept stirring for 24 h at room temperature. The addition of propanol and hexane(2 : 1 in volume) followed by centrifugation isolated the Pd-NPs in the form of a dark-brown powder. The resulting powder was readily redispersible in water, which afforded atransparent brown suspension of well-dispersed nanoparticles. When the pH was variedby adding the appropriate amount of HCl to the reaction mixture, uniform Pd-NPs withparticle sizes of 5.6 nm (pH 6.4), 6.9 nm (pH 3.8), 8.2 nm (pH 3.2), and 10.9 nm (pH2.8) were produced.

General Procedure for Suzuki Coupling. Reaction conditions were as fol-lows: 1.0 mmol aryl halide, 1.3 equivalent phenylboronic acid, 1.5 equivalent Cs2CO3,5 ml DMF, 0.5 mol% palladium nanoparticles in water at 90 ◦C. The reaction time was8 or 12 h.

Supported Pd-NPs on BC.115

Synthesis of Supported Pd-NPs. In 50 ml of deionized water, 3 g of freshlyprepared bacterial cellulose (BC) nanofibers and 0.1 g of Pd (from PdCl2 or Pd(NO3)2)were added. Themixture was degassed for 0.5 h, and then heated at 140 ◦Cwith vigorousstirring (1200 rpm) and, keeping N2 atmosphere, a solution of potassium borohydride(2 g in 50 ml of water) was added over 5 h. The solid was separated by centrifugationand washed with water.

General Procedure for Suzuki Coupling. In 15 ml of water, 1.2 equivalentof aryl halide, 1 equivalent of boronic acid, and 2 equivalent of K2CO3 were added.The solution was stirred at 60–100 ◦C, and the catalyst was added (0.05 mol%) andthe mixture was stirred for 3.5 h. The mixture was cooled down and the catalyst wasremoved by filtration. The catalyst was washed with ethyl acetate (3 × 30 ml) anddeionized water for recycling. The product was extracted with ethyl acetate and washedwith water followed by drying with MgSO4. The solvent was removed by evaporation,and the final product was recrystallized with petroleum ether.

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