The Current Status of Heterogeneous Palladium Catalysed Heck and ...

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Jul 10, 2018 - heterogeneous catalysts particularly in Suzuki and Heck reactions. Most recent ... Keywords: C–C cross coupling; Heck cross-coupling; Suzuki ...
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The Current Status of Heterogeneous Palladium Catalysed Heck and Suzuki Cross-Coupling Reactions Philani P. Mpungose 1 1 2

*

ID

, Zanele P. Vundla 1 , Glenn E. M. Maguire 2 and Holger B. Friedrich 1, *

Catalysis Research Group, School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban 4000, South Africa; [email protected] (P.P.M.); [email protected] (Z.P.V.) School of Chemistry and Physics, University of KwaZulu-Natal, Varsity Drive, Durban 4001, South Africa; [email protected] Correspondence: [email protected]; Tel.: +27-31-2603107; Fax: +27-31-2603091

Received: 31 May 2018; Accepted: 23 June 2018; Published: 10 July 2018

 

Abstract: In the last 30 years, C–C cross coupling reactions have become a reliable technique in organic synthesis due their versatility and efficiency. While drawbacks have been experienced on an industrial scale with the use of homogenous systems, many attempts have been made to facilitate a heterogeneous renaissance. Thus, this review gives an overview of the current status of the use of heterogeneous catalysts particularly in Suzuki and Heck reactions. Most recent developments focus on palladium immobilised or supported on various classes of supports, thus this review highlights and discuss contributions of the last decade. Keywords: C–C cross coupling; Heck cross-coupling; Suzuki cross-coupling; heterogeneous palladium catalysis

1. Introduction The importance of carbon–carbon bond forming reactions is well documented in literature [1–3]. The industrial importance of Suzuki and Heck cross-coupling reactions along with numerous others in this general field has sparked great interest in C–C bond formation reactions. In addition, the lack of a universal set of reaction conditions for every catalyst system and the drawbacks of industrially employed catalyst systems continue to fuel this interest. Heck and Suzuki reactions are typically catalysed by palladium based homogenous systems that have illustrated high turnover frequency (TOFs). These homogeneous catalysts require the use of ligands (phosphine or N-heterocyclic) to form active catalysts. As a result, separation of these catalysts (the ligand, the Pd metal or the complex) from the reaction media has presented the greatest challenge in this field. Consequently, the catalyst gets incorporated into the final product, resulting in loss of catalyst and a devalued product especially since these reactions are at the forefront of the production in the pharmaceutical industry. Successful separation of these homogeneous catalysts from the product solution requires the use of expensive nanofiltration membranes or an extensive (and usually destructive) column chromatography separation. Table 1 summarises the maximum limits of residual metal catalysts acceptable in the pharmaceutical industry. These low allowed concentrations have prompted the development of recoverable and reuseable catalyst systems to decrease product contamination and alleviate the loss of catalyst. Many attempts have been made over the years to heterogenise homogeneous systems and to develop heterogeneous systems to displace homogenous systems and this review aims at highlighting recent contributions of the last decade at achieving this.

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Table 1. Maximum acceptable concentration limits for the residues of metal catalysts that can be present in pharmaceutical products [4]. Concentration (ppm)

Metal Pd, Pt, Ir, Rh, Ru, Os Mo, V, Ni, Cr Cu, Mn Zn, Fe

Oral

Parenteral

5 10 15 20

0.5 1.0 1.5 2.0

The main challenge in the development of heterogeneous catalytic systems for C–C cross-coupling reactions has been in establishing the nature of the active catalyst species. While literature on homogeneous systems is well developed with the mechanism well understood, the contrary is observed for heterogeneous catalysts. The true nature of the active catalyst when a palladium containing solid material is employed as a catalyst is still a highly debated issue in the field of cross-coupling reactions [5]. In most research papers, the “true nature” of the active form of palladium is ambiguous; claims exist in the literature supporting both soluble molecular and nanoparticle catalysts, as well as truly heterogeneous insoluble Pd catalysts [5–7]. More specifically, the question is whether the oxidative addition of the aryl halide (Ar–X) occurs on the surface of the solid catalyst (heterogeneous catalysis) or on leached metal atoms (homogeneous catalysis). The literature is currently divided on this matter; some researchers recognise the solid pre-catalyst as a “reservoir” of soluble catalytically active palladium species [4,8–10]. Others claim to have developed truly heterogeneous systems, with catalysis taking place on the surface of the solid palladium based heterogeneous catalyst [11,12]. Common heterogeneous cross-coupling catalysts mainly differ in: (i) chemical nature (organic, inorganic, or hybrid organic-inorganic) of the solid matrix entrapping the Pd catalyst and (ii) the nature of the catalyst attachment (chemical or physical entrapment) [13]. There are also examples of efficient catalysts for cross-coupling reactions based on colloidal palladium particles. The usefulness of the developed catalytic system is usually determined by its activity, selectivity and the life-time of the catalyst [14]. The catalytic activity is essentially a measure of percentage of reactants that are converted to product(s); while the catalytic selectivity is a measure of the percentage of the reactants that are converted to useful or desired product(s). The life-time of a catalyst is the time that the catalyst will maintain the required level of activity and selectivity [14]. The life-time of the catalyst can also be estimated by the number of times it can be recovered and reused. Lastly, the efficiency of a catalytic process is measured in turnover number (TON) and turnover frequency (TOF). In the following discussion, the catalytic properties discussed above will be used to determine the efficiency of the selected examples of most common palladium catalyzed heterogeneous catalytic systems for Heck and Suzuki cross-coupling reactions. The aim of the discussion is not to be exhaustive but rather to report on recent and noteworthy research progress aimed at developing heterogeneous palladium based catalytic systems. 2. “Naked” Palladium Nanoparticles as Pre-Catalysts Palladium nanoparticles have become one of the most interesting forms of heterogeneous catalysts because of their size- and shape-dependence, as well as their efficient catalytic activities in cross-coupling reactions [13,15]. The “naked” palladium nanoparticles are the simplest form of heterogeneous catalyst for C–C cross-coupling reactions. However, their use has been limited by the lack of efficient separation procedures; available separation techniques such as filtration and centrifugation are not very effective in achieving complete recovery of the nanoparticles. Additionally, nanoparticles are susceptible to oxidation, resulting in complicated work up procedures in the attempt to avoid deactivation. Furthermore, nanoparticles are also prone to agglomeration or sintering upon heating, leading to formation of insoluble non-catalytic palladium black [13]. Scheme 1 shows that a palladium metal aggregate can serve as catalytically active palladium reservoir. In situ, the palladium

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  stable palladium nanoparticles that facilitate the heterogeneous 3 of 23  metalMolecules 2018, 23, x FOR PEER REVIEW  aggregates produce kinetically catalytic cycle. However, single palladium atoms usually leach out from the surface of the palladium catalytic cycle. However, single palladium atoms usually leach out from the surface of the palladium  nanoparticle and participate in the homogeneous catalytic cycle.

nanoparticle and participate in the homogeneous catalytic cycle. 

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catalytic cycle. However, single palladium atoms usually leach out from the surface of the palladium  nanoparticle and participate in the homogeneous catalytic cycle. 

  Scheme 1. Possible reaction pathways operating in Pd nanoparticle catalysed Suzuki and Heck cross‐

Scheme 1. Possible reaction pathways operating in Pd nanoparticle catalysed Suzuki and Heck couplings [12] (Adapted with permission from American Chemical Society).  cross-couplings [12] (Adapted with permission from American Chemical Society).

Alternatively, supported palladium nanoparticles as catalyst for C–C cross‐coupling reactions 

Alternatively, supported nanoparticles as catalyst C–C cross-coupling reactions are  being  developed.  Most palladium often,  palladium  nanoparticle  become for less  catalytically  active  upon    are being developed. Most often, palladium nanoparticle become less catalytically active upon immobilisation on a solid support. Therefore, an improved catalytic system with desirable aspects of  Scheme 1. Possible reaction pathways operating in Pd nanoparticle catalysed Suzuki and Heck cross‐ immobilisation on a solid support. Therefore, an improved catalytic system with desirable aspects of both systems (high activity and easy recovery) is needed. It is envisaged that better understanding of  couplings [12] (Adapted with permission from American Chemical Society).  the  reactivity  of  palladium  can islead  to  the Itdesign  of  more  efficient  both systems (high activity and nanoparticle  easy recovery) needed. is envisaged that betterheterogeneous  understanding of catalyst for C–C cross‐coupling reactions.  the reactivity of palladium nanoparticle can lead to the design of more efficient heterogeneous Alternatively, supported palladium nanoparticles as catalyst for C–C cross‐coupling reactions  catalyst are  being  developed.  Most  often,  palladium  nanoparticle  become  less  catalytically  active  upon  for C–C cross-coupling reactions. 3. Palladium Nanoparticles on Carbonaceous Supports  immobilisation on a solid support. Therefore, an improved catalytic system with desirable aspects of 

both systems (high activity and easy recovery) is needed. It is envisaged that better understanding of  3. Palladium Nanoparticles on Carbonaceous Supports Palladium supported on carbonaceous supports has been widely applied in C–C cross‐coupling  the  reactivity  of  palladium  nanoparticle  can  lead  to  the  design  of  more  efficient  heterogeneous 

reactions (Scheme 2) [16–18]. Amongst these, palladium on activated carbon (Pd/C) is by far the most 

catalyst for C–C cross‐coupling reactions.  Palladium supported on carbonaceous supports has been widely applied in C–C cross-coupling frequently  used  catalyst  in  heterogeneous  Pd‐catalyzed  coupling  reactions.  This  is  because  of  its  reactions (Scheme 2) [16–18]. Amongst these, palladium on activated carbon (Pd/C) is by far the efficiency  and  commercial  availability  [16–18].  It  can  be  purchased  in  various  quantities  with  a  3. Palladium Nanoparticles on Carbonaceous Supports  most palladium  frequentlycontent  used catalyst in heterogeneous Pd-catalyzed This is because of ranging  from  1–20%.  In  addition,  charcoal  coupling as  a  solid  reactions. support  ensures  a  higher  Palladium supported on carbonaceous supports has been widely applied in C–C cross‐coupling  its efficiency and commercial availability [16–18]. It can be purchased in various quantities with a surface area compared to the corresponding alumina and silica supported catalysts [18]. Palladium  reactions (Scheme 2) [16–18]. Amongst these, palladium on activated carbon (Pd/C) is by far the most  palladium content used  ranging from 1–20%. In addition, charcoal a solidThis  support ensures on activated carbon is reported to be stable in air and water, acids and bases, and it often doesn’t  frequently  catalyst  in  heterogeneous  Pd‐catalyzed  coupling  as reactions.  is  because  of  its  a higher require reactions to be performed under an inert atmosphere [18,19]. As a result, Pd/C catalyzed Heck  surface area compared to the corresponding alumina silica supported Palladium efficiency  and  commercial  availability  [16–18].  It  can and be  purchased  in  various catalysts quantities  [18]. with  a  palladium  content  ranging  from  addition,  charcoal  a  solid and support  ensures  higher  and Suzuki cross‐coupling reactions have been performed under a variety of reactions conditions,  on activated carbon is reported to be1–20%.  stableIn in air and water,as acids bases, anda it often doesn’t surface area compared to the corresponding alumina and silica supported catalysts [18]. Palladium  including in organic media [20–23], aqueous media [24–26], and under microwave conditions [27– require reactions to be performed under an inert atmosphere [18,19]. As a result, Pd/C catalyzed Heck on activated carbon is reported to be stable in air and water, acids and bases, and it often doesn’t  31]. Generally, the Pd/C catalyst can convert a large variety of substrates with good to excellent yields,  and Suzuki cross-coupling reactions have been performed under a variety of reactions conditions, require reactions to be performed under an inert atmosphere [18,19]. As a result, Pd/C catalyzed Heck  and it is recoverable and recyclable over many cycles [16–18].  including and Suzuki cross‐coupling reactions have been performed under a variety of reactions conditions,  in organic media [20–23], aqueous media [24–26], and under microwave conditions [27–31]. Generally,including in organic media [20–23], aqueous media [24–26], and under microwave conditions [27– the Pd/C catalyst can convert a large variety of substrates with good to excellent yields, 31]. Generally, the Pd/C catalyst can convert a large variety of substrates with good to excellent yields,  and it is recoverable and recyclable over many cycles [16–18]. and it is recoverable and recyclable over many cycles [16–18]. 

  Scheme 2. Schematic illustration of palladium nanoparticles supported on carbonaceous material as    efficient catalyst for C–C cross‐coupling reactions [32] (Adapted with permission from John Wiley  Scheme 2. Schematic illustration of palladium nanoparticles supported on carbonaceous material as  and Sons). 

Scheme 2.efficient catalyst for C–C cross‐coupling reactions [32] (Adapted with permission from John Wiley  Schematic illustration of palladium nanoparticles supported on carbonaceous material as efficient catalyst for C–C cross-coupling reactions [32] (Adapted with permission from John Wiley and Sons). and Sons). 

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Molecules 2018, 23, x FOR PEER REVIEW    4 of 23  Jadhav et al. recently (2016) developed a greener, cost effective and operationally convenient Molecules 2018, 23, x FOR PEER REVIEW    4 of 23  Pd/C based catalytic system for Suzuki and Heck cross-coupling reactions [33]. In their setup, the Jadhav  et al. recently (2016)  developed  a greener,  cost  effective and  operationally  convenient  Pd/C catalyst was dispersed in an aqueous hydrotropic (sodium operationally  xylene sulphonate solution) Pd/C based catalytic system for Suzuki and Heck cross‐coupling reactions [33]. In their setup, the  Jadhav  et al. recently (2016)  developed  a greener, medium cost  effective and  convenient  that had surfactant-like properties and thus stabilised the Pd/C catalyst (Figure 1). The hydrotropism Pd/C catalyst was dispersed in an aqueous hydrotropic medium (sodium xylene sulphonate solution)  Pd/C based catalytic system for Suzuki and Heck cross‐coupling reactions [33]. In their setup, the  allows for easy solubility of several types of organic functionalities in water, without the need for that had surfactant‐like properties and thus stabilised the Pd/C catalyst (Figure 1). The hydrotropism  Pd/C catalyst was dispersed in an aqueous hydrotropic medium (sodium xylene sulphonate solution)  allows for easy solubility of several types of organic functionalities in water, without the need for  toxic that had surfactant‐like properties and thus stabilised the Pd/C catalyst (Figure 1). The hydrotropism  and volatile organic co-solvent. In addition, good to excellent yields of the desired product were toxic and volatile organic co‐solvent. In addition, good to excellent yields of the desired product were  allows for easy solubility of several types of organic functionalities in water, without the need for  obtained and the catalytic system was recycled three times without any significant loss in activity. obtained and the catalytic system was recycled three times without any significant loss in activity.  Lastly,toxic and volatile organic co‐solvent. In addition, good to excellent yields of the desired product were  they reported that the palladium metal was “not leached” out into the product solution. Hence, Lastly,  they  reported  that  the  palladium  metal  was  “not  leached”  out  into  the  product  solution.  obtained and the catalytic system was recycled three times without any significant loss in activity.  this system is highly desirable from economical as well as environmental points of view.

Hence, this system is highly desirable from economical as well as environmental points of view.  Lastly,  they  reported  that  the  palladium  metal  was  “not  leached”  out  into  the  product  solution.  Hence, this system is highly desirable from economical as well as environmental points of view. 

  Figure 1. Plausible mechanism of the C–C bond forming reactions in the presence of a hydrotrope 

Figure 1. Plausible mechanism of the C–C bond forming reactions in the presence of a hydrotrope (sodium (sodium xylene sulphonate solution) [33] (Adapted with permission from The Royal Society of Chemistry).  xylene sulphonate solution) [33] (Adapted with permission from The Royal Society of Chemistry). Figure 1. Plausible mechanism of the C–C bond forming reactions in the presence of a hydrotrope  Carbon  nanotubes  (CNTs)  have emerged  as  highly  efficient supports  for  palladium and  they  (sodium xylene sulphonate solution) [33] (Adapted with permission from The Royal Society of Chemistry). 

Carbon nanotubes (CNTs) have emerged as highly efficient supports for palladium and they have have been extensively used as alternatives to activated carbon supports [12,32,34,35]. The CNTs can  be Carbon  evenly  distributed  in  solution  due  to  their as  small  size,  thus  increasing  interaction  between  the  been extensively used as alternatives to activated carbon supports [12,32,34,35]. The CNTs can be nanotubes  (CNTs)  have emerged  highly  efficient supports  for  palladium and  they  reactants  and  the  catalyst.  In  most  cases,  the  activity  of  CNTs  in  C–C  cross‐coupling  reactions  have been extensively used as alternatives to activated carbon supports [12,32,34,35]. The CNTs can  evenly distributed in solution due to their small size, thus increasing interaction between the reactants be depends on their preparation method [35]. In a recent review, by Labulo et al., a wide range of surface  evenly  distributed  in  solution  due  to of their  small  size,  thus  increasing  interaction  between  on the their and the catalyst. In most cases, the activity CNTs in C–C cross-coupling reactions depends functionalization techniques for carbon nanotubes that improve their properties as catalyst supports  reactants  and  the  catalyst.  In  most  cases, by the  activity  of  CNTs  C–C  cross‐coupling  reactions  preparation method [35]. In a recent review, Labulo et al., a widein range of surface functionalization was reported [34]. The resulting CNTs catalysts displayed superior catalytic performance and better  depends on their preparation method [35]. In a recent review, by Labulo et al., a wide range of surface  techniques for carbon nanotubes that improve their properties as catalyst supports was reported [34]. recyclability in C–C cross‐coupling reactions. Scheme 3 shows the graphical representation of surface  functionalization techniques for carbon nanotubes that improve their properties as catalyst supports  The resulting CNTs catalysts displayed superior catalytic performance and better recyclability in C–C functionalization and palladium nanoparticle loading onto carbon nanotubes.  was reported [34]. The resulting CNTs catalysts displayed superior catalytic performance and better  cross-coupling reactions. Scheme 3 shows the graphical representation of surface functionalization recyclability in C–C cross‐coupling reactions. Scheme 3 shows the graphical representation of surface  and palladium nanoparticle loading onto carbon nanotubes. functionalization and palladium nanoparticle loading onto carbon nanotubes. 

  Scheme  3.  Graphical  representation  of  Pd  nanoparticles  supported  on  carbon  nanotubes  [34]  (Adapted with permission from Springer Nature). 

Scheme  3.  Graphical  representation  of  Pd  nanoparticles  supported  on  carbon  nanotubes  [34] 

 

Scheme 3. Graphical representation of Pd nanoparticles supported on carbon nanotubes [34] (Adapted (Adapted with permission from Springer Nature).  with permission from Springer Nature).

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Lakshminarayana et al. [32] extended the study by investigating the influence of various Lakshminarayana  et  al.  [32]  extended  the  study  by  investigating  the  influence  of  various  carbonaceous materials as supports in C–C cross-coupling reactions. They prepared and studied carbonaceous  materials  as  supports  in  C–C  cross‐coupling  reactions.  They  prepared  and  studied  palladium oxide (PdO) nanoparticles impregnated on nanocarbon supports, such as single walled palladium oxide (PdO) nanoparticles impregnated on nanocarbon supports, such as single walled  carbon nanotubes (SWCNT), multiwalled carbon nanotubes (MWCNT), carbon nanofiber (CNF), carbon  nanotubes  (SWCNT),  multiwalled  carbon  nanotubes  (MWCNT),  carbon  nanofiber  (CNF),  graphene oxide (GO), and reduced graphene oxide (RGO) [32]. The prepared catalysts were graphene  oxide  (GO),  and  reduced  graphene  oxide  (RGO)  [32].  The  prepared  catalysts  were  characterized fully and then explored in Heck cross-coupling reactions to assess their catalytic characterized  fully  and  then  explored  in  Heck  cross‐coupling  reactions  to  assess  their  catalytic  activity. Graphene oxide (GO) was found to be the best support for PdO (Table 2). The enhanced activity. Graphene oxide (GO) was found to be the best support for PdO (Table 2). The enhanced  catalytic activity of PdO/GO was believed to be due to the combined effect of a high degree of catalytic activity of PdO/GO was believed to be due to the combined effect of a high degree of surface‐ surface-bound oxygenated moieties, high surface area, electron conductivity and the mesoporous bound oxygenated moieties, high surface area, electron conductivity and the mesoporous nature of  nature of graphene oxide. graphene oxide.  Table 2. Performance of PdO supported carbon materials in the Heck coupling reaction [32] (Adapted Table 2. Performance of PdO supported carbon materials on Heck coupling reaction [32] (Adapted  with permission from John Wiley and Sons). with permission from John Wiley and Sons). 

  Catalyst  a Catalyst



PdO/SWCNT 

PdO/SWCNT PdO/MWCNT  PdO/MWCNT PdO/CNF  PdO/CNF PdO/GO PdO/GO  PdO/RGO PdO/RGO 

Reaction Time (min)  Reaction Time (min) Yield (%) b Yield (%)  20 15 40 5 90

20  15  40  5  90 

72 63 38 80 68

72  63  38  80  68 



−1)  TOF TOF (h (h−1 )

939 

939 1080 1080  274 274  4144 4144  194 194 

a

Reaction Conditions: Iodobenzene (0.25 mmol), ethyl acrylate (0.5 mmol), PdO/support (10 mol %), K2 CO3 a Reaction Conditions: Iodobenzene (0.25 mmol), ethyl acrylate (0.5 mmol), PdO/support (10 mol %),  (0.5 mmol), DMSO (dimethylsuloxide) (1 mL) at 150 ◦ C. b GC yields. b

K2CO3 (0.5 mmol), DMSO (dimethylsuloxide) (1 mL) at 150 °C.   GC yields. 

4.4. Palladium Nanoparticles on Metal Oxide Supports  Palladium Nanoparticles on Metal Oxide Supports A significant number of researchers have devoted themselves to finding a new and a more A  significant  number  of  researchers  have  devoted  themselves  to  finding  a  new  and  a  more  efficient Pd/Mx Oy based heterogeneous catalyst for C–C cross-coupling reactions [36,37]. In this efficient  Pd/MxOy  based  heterogeneous  catalyst  for  C–C  cross‐coupling  reactions  [36,37].  In  this  direction, direction,  Köhler Köhler  and and co-workers co‐workers prepared prepared and and comparatively comparatively studied studied the the catalytic catalytic activity activity of of  Pd/Al O , Pd/TiO , Pd/NaY, and Pd/CeO [38]. The catalysts were able to efficiently catalyse 2 2 Pd/Al22O33, Pd/TiO2, Pd/NaY, and Pd/CeO 2 [38]. The catalysts were able to efficiently catalyse Suzuki  Suzuki cross-coupling reactions and the obtained TOFs for each catalyst were: 2Pd/Al h−1 ,2  2 O3 =−19600 cross‐coupling reactions and the obtained TOFs for each catalyst were: Pd/Al O3 = 9600 h , Pd/TiO − 1 − 1 Pd/TiO 9700 h , Pd/NaY = 4100, and Pd/CeO2 = 4100 h . The Pd/Al2 O3 and Pd/TiO2 catalysts = 9700 h2 −1=, Pd/NaY = 4100, and Pd/CeO 2 = 4100 h−1. The Pd/Al2O3 and Pd/TiO2 catalysts showed very  showed very similar and high catalytic activities, while the activities of Pd/NaY and Pd/CeO2 were similar and high catalytic activities, while the activities of Pd/NaY and Pd/CeO 2 were much lower.  much lower. The TOF values of Pd/NaY2 catalysts are almost half of those obtained with Pd/Al and Pd/CeO2 catalysts are almost half of those obtained The TOF values of Pd/NaY and Pd/CeO 2O3  with Pd/Al O and Pd/TiO . These results clearly suggest that the nature of the support has a great 3 2 and Pd/TiO22. These results clearly suggest that the nature of the support has a great influence on the  influence on the activity of the catalyst. activity of the catalyst.  In Köhler et al. that the Pdthe  concentration in solution during theduring  reaction correlated In addition, addition,  Köhler  et stated al.  stated  that  Pd  concentration  in  solution  the  reaction  clearly with the progress of the reaction (Figure 2). Thus, this clearly suggests that the dissolved correlated  clearly  with  the  progress  of  the  reaction  (Figure  2).  Thus,  this  clearly  suggests  that  the  molecular palladium represents the “true” catalytically active species, and the prepared Pd/M x Oy dissolved molecular palladium represents the “true” catalytically active species, and the prepared  solids are simply precursors to the “true catalyst”. Furthermore, they found that the dissolved “Pd is Pd/MxOy  solids  are  simply  precursors  to  the  “true  catalyst”.  Furthermore,  they  found  that  the  deposited back” onto the support at the end of the reaction. This phenomenon is commonly referred dissolved  “Pd  is  deposited  back”  onto  the  support  at  the  end  of  the  reaction.  This  phenomenon  is  to as the palladium “release-capture or dissolution-redeposition” mechanism [39]. commonly referred to as the palladium “release‐capture or dissolution‐redeposition” mechanism [39]. 

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% Conversion

Temperature 120

90 80

80

60 50 %

60

40 30

40

20

Temperature / C

100

70

20

10 0

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50 60 80 90 100 120 140 200 Reaction Time (min) % Leaching % Conversion Temperature

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90 2.  Time‐dependent  correlation  of  palladium  leaching  with  the  progress  of  the  reaction 120 Figure  [38]  Figure 2. Time-dependent correlation of palladium leaching with the progress of the reaction [38] (Adapted with permission from John Wiley and Sons).  80 with permission from John Wiley and Sons). (Adapted

Temperature / C

%

100 70 In 2013, Amoroso et al. [10] extended the study of Köhler and co-workers [38] to examine the 80 60 catalytic activity of Pd/Mx Oy type catalysts in more detail. More specifically, they wanted to investigate 50 of the “point of zero charge” on the catalytic activity of the Pd/Mx Oy type catalysts. the influence The point of zero charge (PZC) is commonly used in surface science to determine the ability60 of a given 40 solid material to adsorb ions. This concept describes the condition when the electrical charge density 30 of a solid is zero [40]. Generally, the PZC is represented as the pH value at which 40 a solid on a surface submerged 20 in an electrolyte exhibits zero net electrical charge on the surface [40,41]. A given metal oxide (Mx Oy ) surface will have a “positive charge” at solution pH values less than the PZC20 value and   10 thus be a surface on which anions may adsorb. On the other hand, the metal oxide surface will have 2+ from CeO2 and La2O3 surfaces under catalytic  Figure 3. Representation of adhesion and release of Pd a negative PZC value and thus be a surface0on which 0 charge at solution pH values greater than the conditions (pH ~ 10.5) [40] (Adapted with permission from The Royal Society of Chemistry).  cations may adsorb [40,41]. The later statement is more relevant the current 0 10 20 35 40 50 60 80 90 to 100 120 discussion 140 200 since the pH of the reaction mixture in most cross-coupling reactions is around 10, thus it is above the PZC values of Reaction Time (min) In 2013, Amoroso et al. [10] extended the study of Köhler and co‐workers [38] to examine the    the investigated metal oxide supports (Figure 3). This allows one to investigate the degree of adhesion catalytic  activity  of  Pd/MxOy  type  catalysts  in  more  detail.  More  specifically,  they  wanted  to  of investigate the influence of the “point of zero charge” on the catalytic activity of the Pd/M palladium the support, correlation  which in turn speaks to the observed activity and xthe degree Figure onto 2.  Time‐dependent  of  palladium  leaching  with  the catalytic progress  of  the  reaction  [38]  O y type  (Adapted with permission from John Wiley and Sons).  of catalysts. The point of zero charge (PZC) is commonly used in surface science to determine the ability  palladium leaching. of  a  given  solid  material  to  adsorb  ions.  This  concept  describes  the  condition  when  the  electrical  charge density on a surface of a solid is zero [41]. Generally, the PZC is represented as the pH value  at which a solid submerged in an electrolyte exhibits zero net electrical charge on the surface [41,42].  A given metal oxide (MxOy) surface will have a “positive charge” at solution pH values less than the  PZC value and thus be a surface on which anions may adsorb. On the other hand, the metal oxide  surface will have a negative charge at solution pH values greater than the PZC value and thus be a  surface  on  which  cations  may  adsorb  [41,42].  The  later  statement  is  more  relevant  to  the  current  discussion since the pH of the reaction mixture in most cross‐coupling reactions is around 10, thus it  is  above  the  PZC  values  of  the  investigated  metal  oxide  supports  (Figure  3).  This  allows  one  to  investigate  the  degree  of  adhesion  of  palladium  onto  the  support,  which  in  turn  speaks  to  the    observed catalytic activity and the degree of palladium leaching.  2+ Figure 3. Representation of adhesion and release of Pd  from CeO2 and La2O3 surfaces under catalytic  Hence, Amoroso et al. [10] used the PZC values of each support to investigate the role it plays  Figure 3. Representation of adhesion and release of Pd2+ from CeO2 and La2 O3 surfaces under catalytic conditions (pH ~ 10.5) [40] (Adapted with permission from The Royal Society of Chemistry).  in the adhesion of the palladium nanoparticle onto the surface of the support (Figure 3). Thus, they  conditions (pH ~10.5) [42] (Adapted with permission from The Royal Society of Chemistry).

In 2013, Amoroso et al. [10] extended the study of Köhler and co‐workers [38] to examine the  catalytic  activity  of  Pd/MxOy  type  catalysts  in  more  detail.  More  specifically,  they  wanted  to  investigate the influence of the “point of zero charge” on the catalytic activity of the Pd/MxOy type  catalysts. The point of zero charge (PZC) is commonly used in surface science to determine the ability  of  a  given  solid  material  to  adsorb  ions.  This  concept  describes  the  condition  when  the  electrical 

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Hence, Amoroso et al. [10] used the PZC values of each support to investigate the role it plays Molecules 2018, 23, x FOR PEER REVIEW    in the adhesion of the palladium nanoparticle onto the surface of the support (Figure 3). Thus,7 of 23  they prepared and investigated the catalytic activity of Pd/La2 O3 , Pd/Pr6 O11 , Pd/Gd2 O3 , Pd/Sm2 O3 , prepared  and  catalysts. investigated  catalytic  activity  of  Pd/Laof 2O3,  Pd/Pr6O11,  Pd/Gd2O3,  Pd/Sm2O3,  and  and Pd/CeO Thethe  physicochemical properties these materials are tabulated in Table 3. 2 Pd/CeO 2 catalysts. The physicochemical properties of these materials are tabulated in Table 3. The  The study revealed that under basic reaction conditions (pH ~10.5), the surfaces of metal oxides with study revealed that under basic reaction conditions (pH ~ 10.5), the surfaces of metal oxides with high  high PZC values are less negatively charged than the surfaces of metal oxides with low PZC values. PZC values are less negatively charged than the surfaces of metal oxides with low PZC values. Thus,  Thus, the La2 O3 support has the lowest density of negatively charged sites on the surface since it has the La 2O3 support has the lowest density of negatively charged sites on the surface since it has the  the highest PZC value. Hence, the higher reactivity of Pd/La2 O3 is correlated to a marked degree 2+ highest PZC value. Hence, the higher reactivity of Pd/La O3 is correlated to a marked degree of Pd 2+ of Pd leaching from the surface of La2 O3 , favoured by2its less negatively charged surface. On the  leaching from the surface of La 2O3, favoured by its less negatively charged surface. On the contrary,  contrary, the lowest activity observed with Pd/CeO2 is correlated to the higher degree of negatively the lowest activity observed with Pd/CeO  is correlated to the higher degree of negatively charged  charged surface that helps in keeping Pd2+2ions anchored onto the support (Figure 3). surface that helps in keeping Pd2+ ions anchored onto the support (Figure 3).  Table 3. The physicochemical properties of the synthesised Pd/Mx Oy based catalysts and their catalytic Table  3.  The  physicochemical  properties  of from the  synthesised  Pd/MxOy  based  catalysts  and  their  activity results [10] (Adapted with permission Springer Nature). catalytic activity results [10] (Adapted with permission from Springer Nature). 

Compound 

Pd Loading 

Surface Area 

1.76  1.93  1.93  1.84  1.99 

16.6  31.5  22.4  11.9  10.2 

Pore Volume 

Compound Pd Loading Surface Pore Volume PZC 2 (Pd/M xa Oy) a  (wt %) (wt %)  (cm3/g)  (Pd/M Area (m2 /g) (m /g) (cm3 /g) (pH) x Oy )

Pd/La2O3 

Pd/La2 O3 Pd/CeO 2  Pd/CeO 2 Pd/Pr 6 O116O11  Pd/Pr Pd/Sm 2 O3 2O3  Pd/Sm Pd/Gd2 O3 a

Pd/Gd2O3 

1.76 1.93 1.93 1.84 1.99

16.6 31.5 22.4 11.9 10.2

0.24 0.18 0.36 0.21 0.09

0.24 8.8 0.18 6.7 0.36 7.8 0.21 7.4 7.5 0.09 

PZC 

Time 

TOF 

1 Time −1) ) (h− (pH) (min)(min) TOF (h

8.8 13 6.7 260 7.8 17 7.4 21 20 7.5 

13  260  17  21  20 

9130 

9130 450  450 6980 6980  5650 5650  5940

5940 

Reaction conditions: 1-bromo-4-nitrobenzene (0.5 mmol), 4-methylphenylboronic acid (0.6 mmol), K2 CO3  Reaction conditions: 1‐bromo‐4‐nitrobenzene (0.5 mmol), 4‐methylphenylboronic acid (0.6 mmol),  (0.6 mmol), catalyst, 1.5 mL ethanol, 0.5 mL water, temperature = 25 ◦ C. a

K2CO3 (0.6 mmol), catalyst, 1.5 mL ethanol, 0.5 mL water, temperature = 25 °C. 

Figure 4 further shows that there is a linear correlation between the yield of the coupling product Figure 4 further shows that there is a linear correlation between the yield of the coupling product  −PZC ) value of the metal oxide support. Thus, the observed reactivity trend and the PZC ([H3 O++ ] = 10−PZC and the PZC ([H3O ] = 10 ) value of the metal oxide support. Thus, the observed reactivity trend  over the Mx Oy support is related to the tendency of each Pd/Mx Oy system to deliver different amounts over the MxOy support is related to the tendency of each Pd/MxOy system to deliver different amounts  of palladium into the solution. In summary, the above discussion shows that there is a close relation of palladium into the solution. In summary, the above discussion shows that there is a close relation  between the “surface charge” (correlated to the PZC value) and the extent of palladium leaching at the between the “surface charge” (correlated to the PZC value) and the extent of palladium leaching at  pH of catalysis (determined by reaction conditions). Thus, the more positively charged is the surface the  pH  of  catalysis  (determined  by  reaction  conditions).  Thus,  the  more  positively  charged  is  the  (this is the case of Pd/La2 O3 ), the more consistently Pd leaches, and therefore, a higher reaction rate surface (this is the case of Pd/La2O3), the more consistently Pd leaches, and therefore, a higher reaction  is observed. In addition, a much slower reaction is observed when Pd/CeO2 is used as the catalyst, rate is observed. In addition, a much slower reaction is observed when Pd/CeO2 is used as the catalyst,  which shows the lowest PZC value within the series of Pd/Mx Oy . Consequently, prolonged recycling which shows the lowest PZC value within the series of Pd/MxOy. Consequently, prolonged recycling  is possible in the case of Pd/CeO2 due to minimal palladium leaching. Thus, from the studied series is possible in the case of Pd/CeO2 due to minimal palladium leaching. Thus, from the studied series  of Pd/Mx Oy type catalysts, Pd/CeO2 was chosen as superior precatalyst based on the following of  Pd/MxOy  type  catalysts,  Pd/CeO2  was  chosen  as  superior  precatalyst  based  on  the  following  considerations: (i) it can be recycled several times without a significant decrease of activity (prolonged considerations:  (i)  it  can  be  recycled  several  times  without  a  significant  decrease  of  activity  recyclability) and (ii) the organic product is sparsely contaminated since there is minimal leaching of (prolonged recyclability) and (ii) the organic product is sparsely contaminated since there is minimal  palladium into the solution. leaching of palladium into the solution. 

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80 Pd/La2O3

70

Pd/Pr6O11

Product Yield / %

60

Pd/Gd2O3

50

Pd/Sm2O3

40 30 20 10

Pd/CeO2

0 0

50

100

150 H3O+

200

250

(nM)  

Figure 4. Cross‐coupling reaction yield (after 2 min of reaction) vs. proton molar concentration (10 )  Figure 4. Cross-coupling reaction yield (after 2 min of reaction) vs. proton molar concentration (10−PZC ) for Pd/MxOy [10] (Adapted with permission from Springer Nature).  for Pd/Mx Oy [10] (Adapted with permission from Springer Nature). −PZC

Alternatively, the physicochemical properties of the support can be enhanced through substitution of a foreign metal ion in its lattice structure [43]. The substitution of foreign metal ions into the lattice of reducible oxides such as CeO2 and TiO2 to form ionic solid-solution oxides (Ce1-x Mx O2-δ or Ti1-x Mx O2-δ ) has given materials which show high activity for a variety of catalytic reactions, especially in gas-phase oxidation reactions [11,43–48]. However, such catalysts have rarely been applied to C–C cross-coupling reactions [49–51]. Recently, Burange and co-workers [44] reported that the ionic ceria-zirconia (CeZrO4-δ ) solid-solution oxides exhibit high redox properties and thermal stability that make them better catalyst supports than the pure metal oxide counterparts [52]. Hence, CeZrO4-δ solid-solution oxide was used to immobilise palladium nanoparticles and the resultant catalyst (Pd/CeZrO4-δ ) was explored in Suzuki cross-coupling reactions (Figure 5). The mechanistic investigation proved that the redox couple (Ce4+ /Ce3+ ) in the CeZrO4-δ support enhances the catalytic activity through creation of oxygen vacancies. Furthermore, the support displayed strong metal-support (Pd-CeZrO4-δ ) interactions and hence, “no leaching” was observed. Thus, it was concluded that the reactions were “truly” heterogeneous [52]. Lichtenegger and co-workers [53] extended the Burange et al. [52] study by directly incorporating palladium ions into the lattice of the support (ceria), instead of modifying the properties of the support with a foreign metal. In their study, they used CeO2 and SnO2 as supports and prepared a series of corresponding ionic solid-solution oxides: Ce0.99 Pd0.01 O2-δ , Ce0.79 Sn0.2 Pd0.01 O2-δ ,   Ce0.495 Sn0.495 Pd0.01 O2-δ , Ce0.20 Sn0.79 Pd0.01 O2-δ , and Sn0.99 Pd0.01 O2-δ . These catalysts were thoroughly Figure 5. Plausible heterogeneous catalytic reaction mechanism suggested for Suzuki cross‐coupling  explored in Suzuki coupling reactions of phenylboric acid with various bromoarenes (Figure 6). over the Pd/CeZrO4‐δ redox catalyst [43] (Adapted with permission from John Wiley and Sons).  The Ce0.79 Sn0.2 Pd0.01 O2-δ , Ce0.20 Sn0.79 Pd0.01 O2-δ , and Sn0.99 Pd0.01 O2-δ catalysts showed extraordinarily high activities in Suzuki cross-coupling reactions, while the solid-solution Ce0.99through  Pd0.01 O2-δ Alternatively,  the  physicochemical  properties  of  binary the  support  can  be oxide enhanced  proved to be the least active catalyst. Thus, the Sn containing catalysts were shown to be more active; substitution of a foreign metal ion in its lattice structure [44]. The substitution of foreign metal ions  however, the results do not show a clear correlation between Sn loadings and the catalytic activity. into  the  lattice  of  reducible  oxides  such  as  CeO2  and  TiO2  to  form  ionic  solid‐solution  oxides    Furthermore, wasmaterials  achievedwhich  in fiveshow  subsequent reactions all catalysts, except (Ce1‐xMxO2‐δ complete or  Ti1‐xMxconversion O2‐δ)  has  given  high  activity  for  for a  variety  of  catalytic  when Ce0.99 Pd0.01 O2-δ and Ce0.495 Sn0.495 Pd0.01 O2-δ were used. reactions, especially in gas‐phase oxidation reactions [11,44–49]. However, such catalysts have rarely  been  applied  to  C–C  cross‐coupling  reactions  [50–52].  Recently,  Burange  and  co‐workers  [45]  reported that the ionic ceria‐zirconia (CeZrO4‐δ) solid‐solution oxides exhibit high redox properties 

Pd/CeO2 0 0

50

100

150

200

250

H3O+ (nM)   Molecules 2018, 23, 1676 Figure 4. Cross‐coupling reaction yield (after 2 min of reaction) vs. proton molar concentration (10−PZC)  for Pd/MxOy [10] (Adapted with permission from Springer Nature). 

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and thermal stability that make them better catalyst supports than the pure metal oxide counterparts  [43]. Hence, CeZrO4‐δ solid‐solution oxide was used to immobilise palladium nanoparticles and the  resultant  catalyst  (Pd/CeZrO4‐δ)  was  explored  in  Suzuki  cross‐coupling  reactions  (Figure  5).  The  mechanistic investigation proved that the redox couple (Ce4+/Ce3+) in the CeZrO4‐δ support enhances  the  catalytic  activity  through  creation  of  oxygen  vacancies.  Furthermore,  the  support  displayed  strong metal‐support (Pd‐CeZrO4‐δ) interactions and hence, “no leaching” was observed. Thus, it was  concluded that the reactions were “truly” heterogeneous [43].  Lichtenegger  and  co‐workers  [53]  extended  the  Burange  et  al.  [43]  study  by  directly  incorporating  palladium  ions  into  the  lattice  of  the  support  (ceria),  instead  of  modifying  the  properties of the support with a foreign metal. In their study, they used CeO2 and SnO2 as supports  and prepared a series of corresponding ionic solid‐solution oxides: Ce0.99Pd0.01O2‐δ, Ce0.79Sn0.2Pd0.01O2‐δ,  Ce0.495Sn0.495Pd0.01O2‐δ,  Ce0.20Sn0.79Pd0.01O2‐δ,  and  Sn0.99Pd0.01O2‐δ.  These  catalysts  were  thoroughly  explored in Suzuki coupling reactions of phenylboric acid with various bromoarenes (Figure 6). The  Ce0.79Sn0.2Pd0.01O2‐δ,  Ce0.20Sn0.79Pd0.01O2‐δ,  and  Sn0.99Pd0.01O2‐δ  catalysts  showed  extraordinarily  high  activities  in  Suzuki  cross‐coupling  reactions,  while  the  binary  solid‐solution  oxide  Ce0.99Pd0.01O2‐δ  proved to be the least active catalyst. Thus, the Sn containing catalysts were shown to be more active;    however, the results do not show a clear correlation between Sn loadings and the catalytic activity.  Figure 5. Plausible heterogeneous catalytic reaction mechanism suggested for Suzuki cross‐coupling  Figure 5. Plausible heterogeneous catalytic reaction mechanism suggested for Suzuki cross-coupling Furthermore, complete conversion was achieved in five subsequent reactions for all catalysts, except  over the Pd/CeZrO4‐δ redox catalyst [43] (Adapted with permission from John Wiley and Sons).  over the Pd/CeZrO4-δ redox catalyst [52] (Adapted with permission from John Wiley and Sons). when Ce0.99Pd0.01O2‐δ and Ce0.495Sn0.495Pd0.01O2‐δ were used. 

Conversion / %

Alternatively,  the  physicochemical  properties  of  the  support  can  be  enhanced  through  substitution of a foreign metal ion in its lattice structure [44]. The substitution of foreign metal ions  1st Run 2nd Run 3rd Run 4th Run 5th Run into  the  lattice  of  reducible  oxides  such  as  CeO2  and  TiO2  to  form  ionic  solid‐solution  oxides    (Ce1‐x MxO2‐δ  or  Ti1‐xMxO2‐δ)  has  given  materials  which  show  high  activity  for  a  variety  of  catalytic  100 reactions, especially in gas‐phase oxidation reactions [11,44–49]. However, such catalysts have rarely  90 been  applied  to  C–C  cross‐coupling  reactions  [50–52].  Recently,  Burange  and  co‐workers  [45]  reported that the ionic ceria‐zirconia (CeZrO 4‐δ) solid‐solution oxides exhibit high redox properties  80

70 60 50 40 30 20 10 0 Catalyst 1

Catalyst 2

Catalyst 3

Catalyst 4

Catalyst 5

 

Figure 6. The Suzuki coupling of 4‐bromotoluene with phenylboronic acid (after 30 min) using 0.5  Figure 6. The Suzuki coupling of 4-bromotoluene with phenylboronic acid (after 30 min) mol % Pd. Catalyst 1: Ce0.99Pd0.01O2‐δ, Catalyst 2: Ce0.79Sn0.2Pd0.01O2‐δ, Catalyst 3: Ce0.495Sn0.495Pd0.01O2‐δ,  using 0.5 mol % Pd. Catalyst 1: Ce0.99 Pd0.01 O2-δ , Catalyst 2: Ce0.79 Sn0.2 Pd0.01 O2-δ , Catalyst 3: Catalyst  4:  Ce0.20Sn0.79Pd0.01O2‐δ,  and  Catalyst  5:  Sn0.99Pd0.01O2‐δ  [53]  (Adapted  with  permission  from  CeElsevier).  0.495 Sn0.495 Pd0.01 O2-δ , Catalyst 4: Ce0.20 Sn0.79 Pd0.01 O2-δ , and Catalyst 5: Sn0.99 Pd0.01 O2-δ [53] (Adapted with permission from Elsevier).

Thorough catalyst leaching, recovery and recyclability studies were conducted and the results  demonstrate  clear  correlation  between  and reactivity  and  amount  of  were leached  palladium  (Figure  7).  Thorough a  catalyst leaching, recovery recyclability studies conducted and the results Hence, these findings also support the hypothesis that the coupling reaction is catalyzed by small  demonstrate a clear correlation between reactivity and amount of leached palladium (Figure 7). Hence, amounts of leached palladium via a homogeneous reaction mechanism.  these findings also support the hypothesis that the coupling reaction is catalyzed by small amounts of

leached palladium via a homogeneous reaction mechanism.

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Figure  7.  Turnover  frequency  (TOF)  vs.  Pd‐concentration  in  the  solution  (Pd  leached).  Catalyst  1: 

Figure 7. Turnover frequency (TOF) vs. Pd-concentration the solution (Pd leached). Catalyst 1: 0.99Pd 0.01O2‐δ,  Catalyst  2: (TOF)  Ce0.79Sn 0.2Pd 0.01O2‐δ,  Catalyst  in 3: the  Cesolution  0.495 Sn0.495Pd 0.01O 2‐δ,  Catalyst  4:  1:  Ce7.  Figure  Turnover  frequency  vs.  Pd‐concentration  in  (Pd  leached).  Catalyst  Ce0.99Ce Pd O , Catalyst 2: Ce Sn Pd O , Catalyst 3: Ce Sn Pd O , Catalyst 4: 0.20 Sn 0.79 Pd 0.01 O 2‐δ , and Catalyst 5: Sn 0.99 Pd 0.01 O 2‐δ  [53] (Adapted with permission from Elsevier).  0.01 2-δ 0.2 0.01 2-δ 0.495 0.495 0.01 2-δ O2‐δ,  Catalyst  2:  Ce0.79 0.79Sn0.2Pd0.01O2‐δ,  Catalyst  3:  Ce0.495Sn0.495Pd0.01O2‐δ,  Catalyst  4:  Ce0.99Pd0.01 Ce0.20 Sn0.79 Pd0.01 O2-δ , and Catalyst 5: Sn0.99 Pd0.01 O2-δ [53] (Adapted with permission from Elsevier). Ce0.20Sn0.79Pd0.01O2‐δ, and Catalyst 5: Sn0.99Pd0.01O2‐δ [53] (Adapted with permission from Elsevier).  To further explore this concept of “release‐capture mechanism” by ionic solid‐solution oxides  catalysts, we have reported on the application of a highly Pd substituted ceria catalyst (Pd0.09Ce0.91O2‐δ)  To further explore this concept of “release-capture mechanism” by ionic solid-solution oxides To further explore this concept of “release‐capture mechanism” by ionic solid‐solution oxides  in the Heck cross‐coupling reaction [50]. Good to excellent yields of substituted olefins were obtained  catalysts, we have reported on the application of a highly Pd substituted ceria catalyst (Pd0.090.09 Ce O 2‐δ) ) 0.91 catalysts, we have reported on the application of a highly Pd substituted ceria catalyst (Pd Ce 0.91O2-δ with this Pd0.09Ce0.91O2‐δ solid‐solution catalyst. However, our findings also revealed that the catalysis  in the Heck cross-coupling reaction [49]. Good to excellent yields of substituted olefins were obtained in the Heck cross‐coupling reaction [50]. Good to excellent yields of substituted olefins were obtained  occurs over the reduced two phase Pd0/CeO2 and not on the as prepared monophasic Pd0.09Ce0.91O2‐δ  with this Pd0.09 Ce0.91OO solid-solution catalyst. However, our findings also revealed that the catalysis 0/CeO2 formed in situ can be easily recovered and  with this Pd 0.09Ce0.91 2‐δ2-δ  solid‐solution catalyst. However, our findings also revealed that the catalysis  (Scheme 4). In addition, it was observed that the Pd 0 occurs over the reduced two phase Pd /CeO and not on the as prepared monophasic Pd0.090.09 Ce O 2‐δ  0/CeO22 and not on the as prepared monophasic Pd 0.91 reused for several times without any significant loss in efficiency and only a negligible amount of  occurs over the reduced two phase Pd Ce 0.91O2-δ 0 /CeO formed in situ can be easily recovered (Scheme 4). In addition, it was observed that the Pd palladium was detected in the product solution (0.35 ppm). The low level of palladium concentration  0 2 (Scheme 4). In addition, it was observed that the Pd /CeO2 formed in situ can be easily recovered and  in the product solution further highlights the excellent ability of ceria to re‐adsorb leached palladium  and reused for several times without any significant loss in efficiency and only a negligible amount of reused for several times without any significant loss in efficiency and only a negligible amount of  species, as discussed earlier.  palladium was detected in the product solution (0.35 ppm). The low level of palladium concentration

palladium was detected in the product solution (0.35 ppm). The low level of palladium concentration  in the product solution further highlights the excellent ability of ceria to re-adsorb leached palladium in the product solution further highlights the excellent ability of ceria to re‐adsorb leached palladium  species, as discussed earlier. species, as discussed earlier. 

  Scheme 4. Proposed Heck coupling reaction mechanism catalysed by the Pd0.09Ce0.91O2‐δ precatalyst  [50] (Adapted with permission from Elsevier). 

We were also able to show that the reaction conditions greatly affect the catalysis. In this regard,  we investigated the application of a Pd 0.04Ce0.92O2‐δ (PdCuCeO) solid‐solution oxide on Suzuki  Scheme 4. Proposed Heck coupling0.04Cu reaction mechanism catalysed by0.09the Ce0.91 O2-δ 0.09 Scheme 4. Proposed Heck coupling reaction mechanism catalysed by the Pd Ce0.91Pd O2‐δ  precatalyst  cross‐coupling  reactions  (Scheme  5)  [51].  The  precatalyst [49] (Adapted with permission from reactions  Elsevier). were  carried‐out  under  milder  and  more  [50] (Adapted with permission from Elsevier).  environmentally friendly reaction conditions, using water as sole solvent and tetrapropylammonium  bromide (TPAB) as a phase transfer catalyst. The catalytic results displayed good functional group 

 

We were also able to show that the reaction conditions greatly affect the catalysis. In this regard,  We were also able to show that the reaction conditions greatly affect the catalysis. In this regard, we investigated the application of a Pd 0.04Cu Cu0.04 0.04Ce0.92  (PdCuCeO) solid‐solution oxide on Suzuki  we investigated the application of a Pd0.04 O2‐δ2-δ (PdCuCeO) solid-solution oxide on Suzuki 0.92O cross‐coupling  cross-coupling reactions  reactions (Scheme  (Scheme 5)  5) [51].  [50]. The  The reactions  reactions were  were carried‐out  carried-out under  under milder  milder and  and more  more environmentally friendly reaction conditions, using water as sole solvent and tetrapropylammonium  environmentally friendly reaction conditions, using water as sole solvent and tetrapropylammonium bromide (TPAB) as a phase transfer catalyst. The catalytic results displayed good functional group 

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bromide (TPAB) as a phase transfer catalyst. The catalytic results displayed good functional group tolerance  excellent  yields yields  of of  substituted substituted  biphenyls biphenyls  were were obtained. obtained.  However,  tolerance and  and good  good to  to excellent However, catalyst  catalyst leaching and recyclability studies revealed that PdCuCeO acts as a Pd reservoir and that the reactions  leaching and recyclability studies revealed that PdCuCeO acts as a Pd reservoir and that the reactions were essentially quasi‐heterogeneous, occurring over recoverable Pd/TPAB aggregates (generated in  were essentially quasi-heterogeneous, occurring over recoverable Pd/TPAB aggregates (generated situ).  The The PdCuCeO  solid‐solution  catalyst  was was recovered  and and recycled  three  times;  however,  its  in situ). PdCuCeO solid-solution catalyst recovered recycled three times; however, catalytic activity declined with each subsequent recycle.  Molecules 2018, 23, x FOR PEER REVIEW    11 of 23  its catalytic activity declined with each subsequent recycle. tolerance  and  good  to  excellent  yields  of  substituted  biphenyls  were  obtained.  However,  catalyst  Ar1X leaching and recyclability studies revealed that PdCuCeO acts as a Pd reservoir and that the reactions  were essentially quasi‐heterogeneous, occurring over recoverable Pd/TPAB aggregates (generated in  O2-δ pre-catalyst Assitu).  prepared PdPdCuCeO  The  solid‐solution  catalyst  was  recovered  and  recycled  three  times;  however,  its  0.04Cu0.04Ce0.92 Reaction conditions catalytic activity declined with each subsequent recycle.  X O O O O induced Pd/Cu dissolution

Ce3+

Pd2+

C e4+

C u2+

Pd0/TPAB

Pd2+

Ar1X

C e4+

Ar1

Pd/Cu reservoir

As prepared Pd0.04Cu0.04Ce0.92O2-δ pre-catalyst O

Ce3+

O

Pd2+

O

C e4+

Reaction conditions induced Pd/Cu dissolution

O

C u2+

C e4+

Ar1Ar2

Pd0/TPAB

X

Ar2B(OH)2 Pd2+

 

Scheme  5.  A  proposed  reaction  mechanism  for  the  Pd0.04Cu0.04Ce0.92O2‐δ  (PdCuCeO)‐  Ar1 Scheme 5. A proposed reaction mechanism for the Pd0.04 Cu0.04 Ce0.92 O2-δ (PdCuCeO)Pd/Cu reservoir tetrapropylammonium bromide (TPAB) catalysed quasi‐heterogeneous SM (Suzuki‐Miyaura) cross‐ tetrapropylammonium bromide (TPAB) catalysed quasi-heterogeneous SM (Suzuki-Miyaura) coupling reactions [51].  Ar2B(OH)2   Ar1Ar2 cross-coupling reactions [50]. Scheme  5.  A  proposed  reaction  mechanism  for  the  Pd0.04Cu0.04Ce0.92O2‐δ  (PdCuCeO)‐ 

Hence, it tetrapropylammonium bromide (TPAB) catalysed quasi‐heterogeneous SM (Suzuki‐Miyaura) cross‐ can  be  concluded  that  most  of  the  reported  ionic  solid‐solution  oxides  catalysts are  Hence, it can be concluded that most of the reported ionic solid-solution oxides catalysts are simply precatalysts that act as a palladium reservoir for the active palladium species. Therefore, the  coupling reactions [51].  simply precatalysts that act as a palladium reservoir for the active palladium species. Therefore, observed high activity and recyclability of the catalysts could be attributed to a Pd release‐capture  the observed high activity and recyclability of the catalysts could attributed oxides  to a Pdcatalysts are  release-capture Hence, it  can  be  concluded  that  most  of  the  reported  ionic be solid‐solution  mechanism, as discussed earlier.  mechanism, as discussed earlier. simply precatalysts that act as a palladium reservoir for the active palladium species. Therefore, the  observed high activity and recyclability of the catalysts could be attributed to a Pd release‐capture 

5. Palladium Nanoparticle Immobilised on Magnetic Supports  5. Palladium Nanoparticle Immobilised on Magnetic Supports mechanism, as discussed earlier.  Recent studies show that magnetic nanoparticles are excellent supports for various catalysts [54– Recent studies show that magnetic nanoparticles are excellent supports for various 5. Palladium Nanoparticle Immobilised on Magnetic Supports  61].  Additionally,  the  magnetic  properties  can  be  exploited  to  improve  catalyst  separation  from  catalysts [54–61]. Additionally, the magnetic properties can be exploited to improve catalyst separation Recent studies show that magnetic nanoparticles are excellent supports for various catalysts [54– previous  filtration  and  centrifugation  methods.  Supported  palladium  magnetic  catalysts  have  the  from previous filtration and methods. Supported palladium magnetic catalysts 61].  Additionally,  the  centrifugation magnetic  properties  can  be  exploited  to  improve  catalyst  separation  from  have benefits of easy recovery from the reaction media by the application of an external magnetic field  the benefits of easy recovery the reaction media by the application of an external previous  filtration  and from centrifugation  methods.  Supported  palladium  magnetic  catalysts magnetic have  the  field (Figure 8) [57]. Magnetic nanoparticles can be categorized into four main groups, namely; metals (Fe,  (Figure 8)benefits of easy recovery from the reaction media by the application of an external magnetic field  [57]. Magnetic nanoparticles can be categorized into four main groups, namely; metals Co, Ni), alloys (FePt, FePd), ferrites (CoFe 2O4, CuFe2O4), and most notably metal oxides (FeO, Fe2O3,  (Figure 8) [57]. Magnetic nanoparticles can be categorized into four main groups, namely; metals (Fe,  (Fe, Co, Ni), alloys (FePt, FePd), ferrites (CoFe2 O4 , CuFe2 O4 ), and most notably metal oxides (FeO, Fe3O4). Iron oxides are the most widely employed in literature because they have stronger magnetic  2O4, CuFe2O4), and most notably metal oxides (FeO, Fe2O3,  Fe2 O3 , FeCo, Ni), alloys (FePt, FePd), ferrites (CoFe 3 O4 ). Iron oxides are the most widely employed in literature because they have stronger Fe3O4). Iron oxides are the most widely employed in literature because they have stronger magnetic  properties and can be synthesized easily by co‐precipitation methods [62].  magneticproperties and can be synthesized easily by co‐precipitation methods [62].  properties and can be synthesized easily by co-precipitation methods [62].

 

Figure 8. Figure 8. Examples of the use of magnetic separation through the application of an external magnetic  Examples of the use of magnetic separation through the application of an external magnetic   field [63] (Adapted with permission from The Royal Society of Chemistry).  field [63] (Adapted with permission from The Royal Society of Chemistry).

Figure 8. Examples of the use of magnetic separation through the application of an external magnetic  field [63] (Adapted with permission from The Royal Society of Chemistry). 

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Molecules 2018, 23, x FOR PEER REVIEW    12 of 23  A growing number of these magnetic supports have been applied in the development of magnetically recoverable palladium based heterogeneous catalysts for C–C cross-coupling reactions. Molecules 2018, 23, x FOR PEER REVIEW    12 of 23  A  growing  number  of  have these explored magnetic the supports  have ofbeen  applied  in  the  development  of  Nasrollahzadeh and co-workers application Pd/Fe 3 O4 magnetic nanoparticles as magnetically recoverable palladium based heterogeneous catalysts for C–C cross‐coupling reactions.  A catalyst growing for number  of  these  magnetic  supports  applied  in  the  development  of  an efficient C–C cross-coupling reactions [64].have  Thebeen  Pd/Fe 3 O4 magnetic catalyst was found Nasrollahzadeh and co‐workers have explored the application of Pd/Fe 3O4 magnetic nanoparticles as  magnetically recoverable palladium based heterogeneous catalysts for C–C cross‐coupling reactions.  to be stable during the Suzuki coupling reaction and good yields (83–95%) were obtained over a wide an efficient catalyst for C–C cross‐coupling reactions [64]. The Pd/Fe  magnetic catalyst was found  Nasrollahzadeh and co‐workers have explored the application of Pd/Fe3O4 magnetic nanoparticles as  range of substrates. At the end of the Suzuki reaction the catalyst was recovered by the application of an efficient catalyst for C–C cross‐coupling reactions [64]. The Pd/Fe3O4 magnetic catalyst was found  to be stable during the Suzuki coupling reaction and good yields (83–95%) were obtained over a wide  an external magnet, and successively reused five times without significant loss of activity. to be stable during the Suzuki coupling reaction and good yields (83–95%) were obtained over a wide  range of substrates. At the end of the Suzuki reaction the catalyst was recovered by the application  Although bare or “naked” iron oxide nanoparticles have been shown to be efficient supports for range of substrates. At the end of the Suzuki reaction the catalyst was recovered by the application  of an external magnet, and successively reused five times without significant loss of activity.  cross-coupling reactions, the immobilization of palladium directly on the iron oxide occasionally suffers of an external magnet, and successively reused five times without significant loss of activity.  Although bare or “naked” iron oxide nanoparticles have been shown to be efficient supports for  fromcross‐coupling  aggregation or surface The oxidation and agglomeration of magnetic Although bare or “naked” iron oxide nanoparticles have been shown to be efficient supports for  reactions,  oxidation. the  immobilization  of  palladium  directly  on  the  iron  oxide nanoparticles occasionally  cross‐coupling  reactions,  the  of  palladium  directly  on  the  iron  oxide  results in thefrom  loss of their magnetism; thus,oxidation.  it is crucial to develop efficient strategies tooccasionally  strengthen their suffers  aggregation  or immobilization  surface  The  oxidation  and  agglomeration  of  magnetic  suffers  from  aggregation  or  surface  oxidation.  The  oxidation  and  agglomeration  of  magnetic  chemical stability [60]. In this regard, a wide range of stabilizing or coating materials, including organic nanoparticles results in the loss of their magnetism; thus, it is crucial to develop efficient strategies to  nanoparticles results in the loss of their magnetism; thus, it is crucial to develop efficient strategies to  stabilizers (polymers and surfactants) and inorganic stabilizers (silica and carbon materials), have been strengthen their chemical stability [60]. In this regard, a wide range of stabilizing or coating materials,  strengthen their chemical stability [60]. In this regard, a wide range of stabilizing or coating materials,  usedincluding organic stabilizers (polymers and surfactants) and inorganic stabilizers (silica and carbon  to protect magnetic nanoparticles [60,65]. In this direction, Kumar et al. [66] developed efficient including organic stabilizers (polymers and surfactants) and inorganic stabilizers (silica and carbon  C@Fe materials), have been used to protect magnetic nanoparticles [60,65]. In this direction, Kumar et al.  3 O4 magnetic core-shell nano-spheres by coating the Fe3 O4 nanoparticle with a carbon shell to materials), have been used to protect magnetic nanoparticles [60,65]. In this direction, Kumar et al.  [66] developed efficient C@Fe 3O4 magnetic core‐shell nano‐spheres by coating the Fe 3O4 nanoparticle  protect them from being corrupted or oxidized under cross-coupling reaction conditions. Palladium [66] developed efficient C@Fe 3O4 magnetic core‐shell nano‐spheres by coating the Fe3O4 nanoparticle  was with a carbon shell to protect them from being corrupted or oxidized under cross‐coupling reaction  then immobilised on the synthesized C@Fe O magnetic core-shell to form a structurally stable 3 4 with a carbon shell to protect them from being corrupted or oxidized under cross‐coupling reaction  conditions. Palladium was then immobilised on the synthesized C@Fe 3O4 magnetic core‐shell to form  Pd/C@Fe O catalyst (Figures 9 and 10). This catalyst was efficiently used as a heterogeneous catalyst conditions. Palladium was then immobilised on the synthesized C@Fe 3O4 magnetic core‐shell to form  3 4 a structurally stable Pd/C@Fe 3O4 catalyst (Figures 9 and 10). This catalyst was efficiently used as a  for Heck cross-coupling reactions good to excellent yields were obtained. The catalyst was easily a structurally stable Pd/C@Fe 3Oand 4 catalyst (Figures 9 and 10). This catalyst was efficiently used as a  heterogeneous catalyst for Heck cross‐coupling reactions and good to excellent yields were obtained.  heterogeneous catalyst for Heck cross‐coupling reactions and good to excellent yields were obtained.  separated from the reaction mixture through application of an external magnet and the catalyst was The  catalyst  easily  separated  from  the  reaction  mixture  through  application  of external  an  external  The  catalyst  was  easily  separated  from  the  reaction  mixture  through  application  of catalytic an  reused five timeswas  without any significant drop in activity. The authors claim that this system magnet and the catalyst was reused five times without any significant drop in activity. The authors  magnet and the catalyst was reused five times without any significant drop in activity. The authors  is “truly heterogeneous” even though some leaching was observed. They concluded that their catalytic claim that this catalytic system is “truly heterogeneous” even though some leaching was observed.  claim that this catalytic system is “truly heterogeneous” even though some leaching was observed.  system is advantageous because of its heterogeneity, high stability, gram scale applicability, magnetic They concluded  concluded  that  their  because  of  of  its  its  heterogeneity,  high high  They  their catalytic  catalytic system  system is is advantageous  advantageous  because  heterogeneity,  separability, and consequent reusability [66]. stability, gram scale applicability, magnetic separability, and consequent reusability [66].  stability, gram scale applicability, magnetic separability, and consequent reusability [66]. 

 

 

Figure 9. Schematic illustration of synthesis steps for C@Fe 4 core‐shell nano‐spheres [66] (Adapted  Figure 9. Schematic illustration of synthesis steps for C@Fe3O 3O 4 core-shell nano-spheres [66] (Adapted Figure 9. Schematic illustration of synthesis steps for C@Fe 3O4 core‐shell nano‐spheres [66] (Adapted  with with permission from Elsevier).  permission from Elsevier). with permission from Elsevier). 

  Figure 10. Graphical illustration of Pd/C@Fe3O4 catalysed cross‐coupling reactions [66] (Adapted with    permission from Elsevier). 

Figure 10. Graphical illustration of Pd/C@Fe3O4 catalysed cross‐coupling reactions [66] (Adapted with  Figure 10. Graphical illustration of Pd/C@Fe3 O4 catalysed cross-coupling reactions [66] (Adapted permission from Elsevier).  with permission from Elsevier).

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The coated magnetic nanoparticles can be further modified by functionalising their surface with The coated magnetic nanoparticles can be further modified by functionalising their surface with  variousThe coated magnetic nanoparticles can be further modified by functionalising their surface with  functional groups (Figure 11). The functionalization of the surface of magnetic nanomaterials various functional groups (Figure 11). The functionalization of the surface of magnetic nanomaterials  various functional groups (Figure 11). The functionalization of the surface of magnetic nanomaterials  is an important step in the design of supported catalysts, because it significantly improves the physical is  an  important  step  in  the  design  of  supported  catalysts,  because  it  significantly  improves  the  is chemical an  important  step  in  of supports supported  catalysts,  because  it  significantly  improves  the  and properties of the  the design  magnetic [63]. physical and chemical properties of the magnetic supports [63].  physical and chemical properties of the magnetic supports [63]. 

  Figure  11.  Different  approaches  to  functionalize  magnetic  nanoparticles  (MNPs):  (A)  iron  oxide 

  Figure 11. Different approaches to functionalize magnetic nanoparticles (MNPs): (A) iron oxide MNPs; MNPs; (B) silica‐coated MNPs; and (C) carbon‐coated MNPs [63] (Adapted with permission from The  (B)Figure  silica-coated MNPs; and (C) carbon-coated MNPs [63] (Adapted with permission from The Royal 11.  Different  approaches  to  functionalize  magnetic  nanoparticles  (MNPs):  (A)  iron  oxide  Royal Society of Chemistry).  Society of Chemistry). MNPs; (B) silica‐coated MNPs; and (C) carbon‐coated MNPs [63] (Adapted with permission from The  Royal Society of Chemistry).  Heidari  and  co‐worker  [67]  used  isoniazide  to  functionalize  the  surface  of  the  Fe3O4@SiO2  magnetic  nano‐support  12).  The  isoniazide  were  used  linkers  immobilize  Heidari and co-worker(Figure  [67] used isoniazide to groups  functionalize the as  surface ofto the Fe3 O4 @SiO2 Heidari  and  co‐worker  [67]  used  isoniazide  to  functionalize  the  surface  the  Fe3O4@SiO2  palladium nanoparticles and prevent agglomeration on the surface of the Fe 3O4@SiOof  2 magnetic nano‐ magnetic nano-support (Figure 12). The isoniazide groups were used as linkers to immobilize magnetic  (Figure material,  12).  The Fe isoniazide  groups  were  catalyst,  used  as  linkers  to  immobilize  support nano‐support  [67].  The  resulting  3O4@SiO2/isoniazide/Pd  as  a  palladium nanoparticles and prevent agglomeration on the surface of thewas  Fe3 Oexplored  4 @SiO2 magnetic palladium nanoparticles and prevent agglomeration on the surface of the Fe 3O4@SiO2 magnetic nano‐ heterogeneous catalyst in Suzuki coupling reactions and good to excellent yields were obtained. The  nano-support [67]. The resulting material, Fe3 O4 @SiO2 /isoniazide/Pd catalyst, was explored as authors reported that the benefits of their heterogeneous catalyst system were its high efficiency and  support  [67].  The  resulting  material,  Fe3O4@SiO2/isoniazide/Pd  catalyst,  was  explored  as  a  a heterogeneous catalyst in Suzuki coupling reactions and good to excellent yields were obtained. reuse was easily achieved through the use of a magnet. They were able to recover and recycle their  heterogeneous catalyst in Suzuki coupling reactions and good to excellent yields were obtained. The  The authors reported that the benefits of their heterogeneous catalyst system were its high efficiency catalyst six times without any noticeable loss in activity.  authors reported that the benefits of their heterogeneous catalyst system were its high efficiency and 

and reuse was easily achieved through the use of a magnet. They were able to recover and recycle reuse was easily achieved through the use of a magnet. They were able to recover and recycle their  their catalyst six times without any noticeable loss in activity. catalyst six times without any noticeable loss in activity. 

  Figure  12.  Fe3O4@SiO2/isoniazide/Pd  mediated  Suzuki  cross‐coupling  reaction  [67]  (Adapted  with  permission from Elsevier). 

  Figure  12.  Fe3O4@SiO2/isoniazide/Pd  mediated  Suzuki  cross‐coupling  reaction  [67]  (Adapted  with  Figure 12. Fe3 O4 @SiO2 /isoniazide/Pd mediated Suzuki cross-coupling reaction [67] (Adapted with permission from Elsevier).  permission from Elsevier).

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An interesting but slightly different approach was reported by Khalili and co-workers [12], An interesting but slightly different approach was reported by Khalili and co‐workers [12], who  who Molecules 2018, 23, x FOR PEER REVIEW  supported palladium nanoparticles on an amino-vinyl silica functionalized magnetic carbon   14 of 23  supported  palladium  nanoparticles  on  an  amino‐vinyl  silica  functionalized  magnetic  carbon  nanotube (CNT). Their core–shell contains CNT@Fe3 O4 @SiO2 -Pd in which the functionalized SiO2 nanotube  (CNT).  Their  core–shell  contains  CNT@Fe3O4@SiO2‐Pd  in  which  the  functionalized  SiO2  An interesting but slightly different approach was reported by Khalili and co‐workers [12], who  helps with the stability of the Pd nanoparticles and is also responsible for the reduction of Pd(II) to helps with the stability of the Pd nanoparticles and is also responsible for the reduction of Pd(II) to  palladium  on  reducing an  amino‐vinyl  silica  functionalized  magnetic  carbon  Pd(0)supported  without the need for nanoparticles  adding external agents (Figure 13). This catalyst was successfully Pd(0) without the need for adding external reducing agents (Figure 13). This catalyst was successfully  nanotube  (CNT).  Their  core–shell  contains  CNT@Fe3O4@SiO2‐Pd  in  which  the  functionalized  SiO2  applied to Heck and Suzuki cross-coupling reactions and high yields were obtained. In addition, applied to Heck and Suzuki cross‐coupling reactions and high yields were obtained. In addition, the  helps with the stability of the Pd nanoparticles and is also responsible for the reduction of Pd(II) to  the catalyst exhibited good recyclability, and was used for six consecutive recycles without a significant catalyst exhibited good recyclability, and was used for six consecutive recycles without a significant  Pd(0) without the need for adding external reducing agents (Figure 13). This catalyst was successfully  loss in catalytic activity. loss in catalytic activity.  applied to Heck and Suzuki cross‐coupling reactions and high yields were obtained. In addition, the  catalyst exhibited good recyclability, and was used for six consecutive recycles without a significant  loss in catalytic activity. 

  Figure 13. A graphical illustration of the CNT@Fe3O4@SiO2‐Pd catalyst [12] (Adapted with permission 

Figure 13. A graphical illustration of the CNT@Fe3 O4 @SiO2 -Pd catalyst [12] (Adapted with permission from Springer Nature).    from Springer Nature).

Figure 13. A graphical illustration of the CNT@Fe3O4@SiO2‐Pd catalyst [12] (Adapted with permission  There  are  also  numerous  examples  where  organopalladium  complexes,  instead  of  palladium  from Springer Nature). 

nanoparticles,  been  immobilised  magnetic  supports  [68,69].  Recently,  Collinson  co‐ There are alsohave  numerous examples on  where organopalladium complexes, instead ofand  palladium workers [68] prepared a [(NHC)Pd(allyl)Cl] complex, bearing an N‐heterocyclic carbene (NHC) and  nanoparticles, have been immobilised magnetic supports complexes,  [68,69]. Recently, There  are  also  numerous  examples on where  organopalladium  instead  of Collinson palladium  and immobilised it on silica‐coated magnetic nanoparticles (Scheme 6). Their strategy tries to combine the  nanoparticles,  have  been  immobilised  on  magnetic  supports  Recently,  Collinson  co‐ and co-workers [68] prepared a [(NHC)Pd(allyl)Cl] complex, bearing[68,69].  an N-heterocyclic carbeneand  (NHC) high  activity  of  palladium–NHC  catalysts  with  the  facile  separation  and  recyclability  of  a  workers [68] prepared a [(NHC)Pd(allyl)Cl] complex, bearing an N‐heterocyclic carbene (NHC) and  immobilised it on silica-coated magnetic nanoparticles (Scheme 6). Their strategy tries to combine the heterogeneous catalyst. The catalyst was then explored on Suzuki coupling reactions and good to  immobilised it on silica‐coated magnetic nanoparticles (Scheme 6). Their strategy tries to combine the  high activity of palladium–NHC catalysts with the facile separation and recyclability of a heterogeneous excellent  yields  obtained  with catalysts  a  variety with  of  substrates.  However,  the  and  immobilised  palladium  high  activity  of were  palladium–NHC  the  facile  separation  recyclability  of  a  catalyst. The catalyst was then explored on Suzuki coupling reactions and good to excellent yields complex  could  not  be  reused  because  the  catalyst  disassembled  or  decomposed  under  Suzuki  heterogeneous catalyst. The catalyst was then explored on Suzuki coupling reactions and good to  were coupling  obtainedreaction  with a variety of substrates. However, the immobilised palladium complex could not be conditions.  The  recycled  catalyst  only achieved 20%  yield  compared  to  the  90%  excellent  yields  were  obtained  with  a  variety  of  substrates.  However,  the  immobilised  palladium  reused because the catalyst disassembled decomposed under Suzuki coupling reaction conditions. yield that was obtained in its first use. The authors suspected that the base was responsible for the  complex  could  not  be  reused  because or the  catalyst  disassembled  or  decomposed  under  Suzuki  detachment of the magnetic silica linker.  The recycled catalyst only achieved 20% yield compared to the 90% yield that was obtained coupling  reaction  conditions.  The  recycled  catalyst  only achieved 20%  yield  compared  to  the  90% in its first use. The authors suspected that the base was responsible for the detachment of the magnetic yield that was obtained in its first use. The authors suspected that the base was responsible for the  silicadetachment of the magnetic silica linker.  linker.

  Scheme 6. Immobilisation of the [N‐heterocyclic carbene (NHC)Pd(allyl)Cl] complex on silica‐coated  magnetic nanoparticles [68] (Adapted with permission from Elsevier).  Scheme 6. Immobilisation of the [N‐heterocyclic carbene (NHC)Pd(allyl)Cl] complex on silica‐coated 

 

Scheme 6. Immobilisation of the [N-heterocyclic carbene (NHC)Pd(allyl)Cl] complex on silica-coated magnetic nanoparticles [68] (Adapted with permission from Elsevier).  magnetic nanoparticles [68] (Adapted with permission from Elsevier).

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Better results were obtained by Fareghi-Alamdari et al. [69] who supported bis(N-heterocyclic Better results were obtained by Fareghi‐Alamdari et al. [69] who supported bis(N‐heterocyclic  carbene) palladium complex on silica coated magnetic nanoparticles (Figure 14). carbene) palladium complex on silica coated magnetic nanoparticles (Figure 14). 

  Figure  14.  Bis(N‐heterocyclic  carbene)  palladium  complex  supported  on  silica  coated  magnetic  Figure 14. Bis(N-heterocyclic carbene) palladium complex supported on silica coated magnetic nanoparticles [69] (Adapted with permission from John Wiley and Sons).  nanoparticles [69] (Adapted with permission from John Wiley and Sons).

Their catalyst was efficiently used for Suzuki coupling reactions and at the end of the reaction,  Their catalyst was efficiently used for Suzuki coupling reactions and at the end of the reaction, it was separated though application of an external magnet and recycled for six consecutive reactions.  it was separated though application of an external magnet and recycled for six consecutive reactions. Hence, it not easy to identify the reaction parameters that caused the Collinson and co‐workers [68]  Hence, it not easy to identify the reaction parameters that caused the Collinson and co-workers [68] catalytic system to be unrecyclable because the reaction conditions are different (Table 4). However,  catalytic system to be unrecyclable because the reaction conditions are different (Table 4). However, the most obvious difference is the choice of the base; Collinson and co‐workers used NaOH, which is  the most obvious difference is the choice of the base; Collinson and co-workers used NaOH, which is very harsh, while Fareghi‐Alamdari et al. used K 2CO3. Hence, optimisation of reaction conditions is  very harsh, while Fareghi-Alamdari et al. used K CO 2 3 . Hence, optimisation of reaction conditions is crucial in developing efficient and more robust catalytic systems.  crucial developing efficient andshows  more robust catalyticnanoparticle‐based  systems. In in summary,  the  literature  that  magnetic  catalytic  systems  have  a  huge potential in solving the recoverability and recyclability challenges [57]. 

Table 4. The reaction conditions for Suzuki cross-coupling reactions catalysed by supported palladium complexes in references [68,69]. Table  4.  The  reaction  conditions  for  Suzuki  cross‐coupling  reactions  catalysed  by  supported  palladium complexes in references [68,69].  Reaction Conditions

Reaction Conditions  Base Temperature (◦ C) Solvent  Base  Temperature (°C)  Collinson et al. [68] a Isopropanol NaOH 60 b a  Collinson et al. [68]  Isopropanol  NaOH  60  80 DMF/H K2 CO3 Fareghi-Alamdari et al. [69] 2O b  a Reaction Fareghi‐Alamdari et al. [69]  DMF/H2O  acid (1.05 K2CO 3  conditions: Bromotoluene (1.0 mmol), phenylboronic mmol), NaOH (1.1 80  mmol), catalyst  

Solvent

(1 mol % Pd), isopropanol (1 mL). b Reaction conditions: Bromotoluene (1.0 mmol), phenylboronic acid (1.2 mmol), a Reaction conditions: Bromotoluene (1.0 mmol), phenylboronic acid (1.05 mmol), NaOH (1.1 mmol),  K2 CO3 (1.5 mmol), catalyst (0.12 mol % Pd), DMF/H2 Ob (2:1, 3 mL).

catalyst  (1  mol  %  Pd),  isopropanol  (1  mL).    Reaction  conditions:  Bromotoluene  (1.0  mmol),  phenylboronic acid (1.2 mmol), K2CO3 (1.5 mmol), catalyst (0.12 mol % Pd), DMF/H2O (2:1, 3 mL). 

In summary, the literature shows that magnetic nanoparticle-based catalytic systems have a huge potential in solving the recoverability and recyclability challenges [57]. 6. Polymer Supported Palladium Nanoparticles 

Literature  reports Palladium have  repeatedly  shown  that  palladium  nanoparticles  can  be  efficiently  6. Polymer Supported Nanoparticles supported  on  polymer  frameworks  [70–77].  Recently,  Nemygina  and  co‐workers  immobilised  Literature reports have repeatedly shown that palladium nanoparticles can be efficiently various palladium precursors (PdCl2, PdCl2(CH3CN)2, and PdCl2(PhCN)2) on amino‐functionalized  supported on polymer frameworks [70–77]. Recently, Nemygina and co-workers immobilised hyper‐crosslinked polystyrene [78]. They then evaluated the influence of palladium oxidation state  various palladium precursors (PdCl2 , PdCl2 (CH3 CN)2 , and PdCl2 (PhCN)2 ) on amino-functionalized (Pd(II) or Pd(0)) on the rate of Suzuki cross‐coupling of 4‐bromoanisole and phenylboronic acid. Their  hyper-crosslinked polystyrene [78]. They then evaluated the influence of palladium oxidation state results  revealed  that  the  catalyst  impregnated  with  a  PdCl2(CH3CN)2  precursor  resulted  in  better  (Pd(II) or Pd(0)) on the rate of Suzuki cross-coupling of 4-bromoanisole and phenylboronic acid. catalytic activity (Scheme 7). The PdCl2(CH3CN)2 precursor was believed to be more active because it  Their results revealed that the catalyst impregnated with a PdCl2 (CH3 CN)2 precursor resulted in experienced  less  hydrolysis  and  precipitation  in  comparison  with  PdCl2  and  PdCl2(PhCN)2  under  better catalytic activity (Scheme 7). The PdCl2 (CH3 CN)2 precursor was believed to be more active Suzuki  coupling  reaction  conditions.  Lastly,  they  observed  that  unreduced,  Pd(II)  containing  pre‐ because it experienced less hydrolysis and precipitation in comparison with PdCl2 and PdCl2 (PhCN)2 catalyst were more active [78].  under Suzuki coupling reaction conditions. Lastly, they observed that unreduced, Pd(II) containing pre-catalyst were more active [78].

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  Scheme 7. Catalysts of Suzuki cross‐coupling based on functionalized hyper‐crosslinked polystyrene  Scheme 7. Catalysts for Suzuki cross-coupling based on functionalized hyper-crosslinked [78] (Adapted with permission from American Chemical Society).    polystyrene [78] (Adapted with permission from American Chemical Society). Scheme 7. Catalysts of Suzuki cross‐coupling based on functionalized hyper‐crosslinked polystyrene 

Another interesting strategy for immobilising palladium on a polymer was reported by Li and  [78] (Adapted with permission from American Chemical Society).  Another interesting strategy for immobilising palladium on a polymer was reported by Li and co‐workers [79], who synthesised a robust and flexible cellulose sponge using a dual‐cross‐linking  co-workers [79], who synthesised robust and flexible cellulose sponge using and  a dual-cross-linking cellulose  nanofiber  (CNF)  with  a γ‐glycidoxypropyltrimethoxysilane  (GPTMS)  polydopamine  Another interesting strategy for immobilising palladium on a polymer was reported by Li and  co‐workers [79], who synthesised a robust and flexible cellulose sponge using a dual‐cross‐linking  cellulose nanofiber with γ-glycidoxypropyltrimethoxysilane (GPTMS) and polydopamine (PDA) (PDA)  (Scheme  (CNF) 8).  The  characterisation  results  revealed  that  the  palladium  nanoparticles  were  cellulose  nanofiber  (CNF)  with  γ‐glycidoxypropyltrimethoxysilane  (GPTMS)  and  polydopamine  homogeneously dispersed on the surface of the cellulose nanofiber with a narrow size distribution.  (Scheme 8). The characterisation results revealed that the palladium nanoparticles were homogeneously (PDA)  8).  The  results  revealed  the  palladium  nanoparticles  The catalyst was successfully applied to heterogeneous Suzuki and Heck cross‐coupling reactions.  dispersed on(Scheme  the surface of characterisation  the cellulose nanofiber with athat  narrow size distribution. Thewere  catalyst homogeneously dispersed on the surface of the cellulose nanofiber with a narrow size distribution.  Leaching  of  palladium  was  negligible  and  the  catalysts  could  be  conveniently  separated  from  the of was successfully applied to heterogeneous Suzuki and Heck cross-coupling reactions. Leaching The catalyst was successfully applied to heterogeneous Suzuki and Heck cross‐coupling reactions.  products and reused for six times without loss of activity [79].  palladium was negligible and the catalysts could be conveniently separated from the products and Leaching  of  palladium  was  negligible  and  the  catalysts  could  be  conveniently  separated  from  the  reused for six times without loss of activity [79]. products and reused for six times without loss of activity [79]. 

  Scheme  8.  Schematic  of  the  stepwise  formation  of  cellulose  sponge  supported  palladium    Scheme  8.  Schematic  of  the  stepwise  formation  of  cellulose  sponge  supported  palladium  nanoparticles [79] (Adapted with permission from American Chemical Society).  Scheme 8. Schematic of the stepwise formation of cellulose sponge supported palladium nanoparticles [79] (Adapted with permission from American Chemical Society).  nanoparticles [79] (Adapted with permission from American Chemical Society). 7. Palladium Nanoparticles Immobilized on Hybrid Inorganic‐Organic Material  7. Palladium Nanoparticles Immobilized on Hybrid Inorganic‐Organic Material 

The  hybrid  inorganic–organic  materials  are  crystalline  systems  in  which  both  inorganic  and  7. Palladium Nanoparticles Immobilized on Hybrid Inorganic-Organic Material hybrid  inorganic–organic  crystalline  in  which  both  inorganic  and  organic The  structural  elements  co‐exist materials  within  a are  single  phase systems  [80].  These  hybrid  inorganic–organic  organic  structural  elements  co‐exist  within  a  single  phase  [80].  These  hybrid  inorganic–organic  The hybrid inorganic–organic materials are crystalline in which both inorganic and materials  are  gaining  popularity  in  cross‐coupling  reactions systems due  to  their  tunable  pore  size,  high  materials  are  gaining  popularity  in  cross‐coupling  reactions  due  to  their  tunable  pore  size,  high  surface  areas,  high  crystallinity,  and  structural  diversity  Most  hybrid  organic structural elements co-exist within a single phase[81–87].  [80]. These hybridinorganic–organic  inorganic–organic surface  areas,  high  crystallinity,  and  structural  diversity  [81–87].  Most  hybrid  inorganic–organic  materials  are  synthesised  through  functionalization  of due the tosupport  surface  with  materials are gaining popularity in cross-coupling reactions their tunable pore size,appropriate  high surface materials  are  synthesised  through  functionalization  of  the  support  surface  with  appropriate  functional groups. This allows for a better control of the dimensions, dispersion and stability of the  areas, functional groups. This allows for a better control of the dimensions, dispersion and stability of the  high crystallinity, and structural diversity [81–87]. Most hybrid inorganic–organic materials are palladium nanoparticles. Metal‐organic frameworks (MOFs) are one of the most popular classes of  synthesised through functionalization of the support surface with appropriate functional groups. This palladium nanoparticles. Metal‐organic frameworks (MOFs) are one of the most popular classes of  hybrid inorganic–organic materials and they are frequently explored as suitable palladium supports  allows for a better control of the dimensions, dispersion and stability of the palladium nanoparticles. hybrid inorganic–organic materials and they are frequently explored as suitable palladium supports  in heterogeneous cross‐coupling reactions. For example, Shaikh and co‐workers [88] used a zeolitic  in heterogeneous cross‐coupling reactions. For example, Shaikh and co‐workers [88] used a zeolitic  Metal-organic frameworks (MOFs) are one of the most popular classes of hybrid inorganic–organic

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materials and they are frequently explored as suitable palladium supports in heterogeneous Molecules 2018, 23, x FOR PEER REVIEW     17 of 23  Molecules 2018, 23, x FOR PEER REVIEW  17 of 23  cross-coupling reactions. For example, Shaikh and co-workers [88] used a zeolitic imidazolate framework (ZIF) to develop robust ZIF-supported palladium nanoparticles as a heterogeneous catalyst imidazolate framework  framework (ZIF)  (ZIF) to  to  develop  develop  robust  ZIF‐supported  as as  a  a  imidazolate  ZIF‐supported palladium  palladium nanoparticles  nanoparticles  for Heck cross-coupling reactions (Figure 15). The Pd/ZIF catalyst was found to be highly efficient heterogeneous catalyst for Heck cross‐coupling reactions (Figure 15). The Pd/ZIF catalyst was found to  heterogeneous catalyst for Heck cross‐coupling reactions (Figure 15). The Pd/ZIF catalyst was found to  and could be re-used up to four times without any loss in catalytic efficiency [88]. be highly efficient and could be re‐used up to four times without any loss in catalytic efficiency [88].  be highly efficient and could be re‐used up to four times without any loss in catalytic efficiency [88]. 

  Figure 15. A graphical illustration of the Pd/zeolitic imidazolate (ZIF‐8 catalyst) [88] (Adapted with  Figure 15. A graphical illustration of the Pd/zeolitic imidazolate (ZIF-8 catalyst) [88] (Adapted with Figure 15. A graphical illustration of the Pd/zeolitic imidazolate (ZIF‐8 catalyst) [88] (Adapted with  permission from John Wiley and Sons).  permission from John Wiley and Sons). permission from John Wiley and Sons). 

Another  interesting  example  was  reported  by  Kozell  et  al.  [89]  who  immobilised  palladium  Another interesting example was reported by Kozell et al. [89] who immobilised palladium nanoparticles  on  zirconium  glycine  diphosphonate  nanosheets  (ZPGly).  The  Another  interesting  example phosphate  was  reported  by  Kozell  et  al.  [89]  who  immobilised  palladium  nanoparticles on zirconium phosphate glycine diphosphonate nanosheets (ZPGly). The immobilization immobilization of palladium nanoparticles on ZPGly nanosheets provided a stable catalyst with high  nanoparticles  on  zirconium  phosphate  glycine  diphosphonate  nanosheets  (ZPGly).  The  of palladium  palladiumcontent  nanoparticles on wt.  ZPGly nanosheets provided a stable catalyst high palladium (up  to  22  %).  This  Pd@ZPGly‐15  catalyst  proved  to  be with highly  efficient  in  immobilization of palladium nanoparticles on ZPGly nanosheets provided a stable catalyst with high  content (up to 22 wt. %). This Pd@ZPGly-15 catalyst proved to be highly efficient in catalysing Suzuki catalysing Suzuki and Heck cross‐couplings reactions. Surprisingly, the authors reported that their  palladium  content  (up  to  22  wt.  %).  This  Pd@ZPGly‐15  catalyst  proved  to  be  highly  efficient  in  and Heck cross-couplings reactions. Surprisingly, the authors reported that their catalytic system catalytic system operates as an efficient “heterogeneous” system, even though 3–5 ppm of palladium  catalysing Suzuki and Heck cross‐couplings reactions. Surprisingly, the authors reported that their  operates as an efficient “heterogeneous” system, even though 3–5 ppm of palladium leached (Table 5). leached (Table 5). In addition, a “release and catch” mechanism was proposed in which soluble active  catalytic system operates as an efficient “heterogeneous” system, even though 3–5 ppm of palladium  In palladium species are released from the solid Pd@ZPGly‐15 catalyst during the oxidative addition of  addition, a “release and catch” mechanism was proposed in which soluble active palladium species leached (Table 5). In addition, a “release and catch” mechanism was proposed in which soluble active  arethe halide and then re‐deposited on the ZPGly support as a consequence of the reductive elimination  released from the solid Pd@ZPGly-15 catalyst during the oxidative addition of the halide and then palladium species are released from the solid Pd@ZPGly‐15 catalyst during the oxidative addition of  re-deposited on the ZPGly support as a consequence of the reductive elimination step at the end of the step at the end of the reaction [89].  the halide and then re‐deposited on the ZPGly support as a consequence of the reductive elimination  reaction [89]. step at the end of the reaction [89].  Table  5.  Suzuki  cross‐coupling  reaction  between  4‐bromobenzaldehyde  and  phenylboric  acid  catalysed by Pd@ZPGly‐15 (palladium nanoparticles on zirconium phosphate glycine diphosphonate  Table 5. Suzuki cross-coupling reaction between 4-bromobenzaldehyde and phenylboric acid catalysed Table  5.  Suzuki (palladium cross‐coupling  reaction  and  phenylboric  acid  nanosheets) catalyst [89].  by Pd@ZPGly-15 nanoparticles onbetween  zirconium4‐bromobenzaldehyde  phosphate glycine diphosphonate nanosheets) catalysed by Pd@ZPGly‐15 (palladium nanoparticles on zirconium phosphate glycine diphosphonate  catalyst [89]. nanosheets) catalyst [89]. 

  Entry a  Run 1  a  Run 2  Entry  Entry a Run 3  Run 1 

Yield (%) b  98  98  b  Yield (%)  Yield (%) b 98  98 

Pd Leaching (ppm)  5  3  Pd Leaching (ppm) Pd Leaching (ppm)  3  5 

a Reaction conditions: 4‐Bromobenzaldehyde (1.0 mmol), phenylboronic acid (1.2 mmol), K Run 1 98 5 2CO3 (1.1  Run 2  98  3  b GC (gas chromatography) yields.  Run 2 98 3 mmol), catalyst (0.1 mol % Pd), ethanol (2.4 mL).  Run 3  98  3  Run 3 98 3

2CO3 (1.1  a  Reaction conditions: 4‐Bromobenzaldehyde (1.0 mmol), phenylboronic acid (1.2 mmol), K A magnetically separable version of a hybrid organic‐inorganic support was recently reported  Reaction conditions: 4-Bromobenzaldehyde (1.0 mmol), phenylboronic acid (1.2 mmol), K2 CO3 (1.1 mmol), catalyst b GC (gas chromatography) yields.  b mmol), catalyst (0.1 mol % Pd), ethanol (2.4 mL).  (0.1 mol % Pd), ethanol (2.4 mL). GC (gas chromatography) yields. a

by Omar and co‐workers [90]. They supported palladium on magnetically separable organic‐silica  hybrid nanoparticles that were functionalized with ionic liquid groups (Figure 16). The presence of  A magnetically separable version of a hybrid organic‐inorganic support was recently reported  ionic liquid groups within the framework of the hybrid nanoparticles enhanced the stability of the 

by Omar and co‐workers [90]. They supported palladium on magnetically separable organic‐silica  hybrid nanoparticles that were functionalized with ionic liquid groups (Figure 16). The presence of  ionic liquid groups within the framework of the hybrid nanoparticles enhanced the stability of the 

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A magnetically separable version of a hybrid organic-inorganic support was recently reported by Omar and co-workers [90]. They supported palladium on magnetically separable organic-silica hybrid nanoparticles that were functionalized with ionic liquid groups (Figure 16). The presence Molecules 2018, 23, x FOR PEER REVIEW    18 of 23  of ionic liquid groups within the framework of the hybrid nanoparticles enhanced the stability of the palladium nanoparticles on the organic-silica nanoparticles. The resultant Pd/MNP@IL-SiO2 palladium nanoparticles on the organic‐silica nanoparticles. The resultant Pd/MNP@IL‐SiO2 catalyst  catalyst was efficiently applied in Heck and Suzuki cross-coupling reactions and it demonstrated was  efficiently  applied  in  Heck  and  Suzuki  cross‐coupling  reactions  and  it  demonstrated  high  high catalytic activity (Figure 16). The Pd/MNP@IL-SiO2 catalyst was also easily separated from the catalytic  activity  (Figure  16).  The  Pd/MNP@IL‐SiO2  catalyst  was  also  easily  separated  from  the  reaction mixture by application of an external magnetic field and the catalyst could be recycled over reaction mixture by application of an external magnetic field and the catalyst could be recycled over  five times without a significant loss in its catalytic activity. In conclusion, the authors envisaged that five times without a significant loss in its catalytic activity. In conclusion, the authors envisaged that  this and similar catalytic systems could pave a way for the desired bridging of homogeneous and this and similar catalytic systems could pave a way for the desired bridging of homogeneous and  heterogeneous catalysis. heterogeneous catalysis. 

  Figure  16.  Suzuki  cross‐coupling  reactions  catalysed  by  the  Pd/MNP@IL‐SiO2  (palladium  Figure 16. Suzuki cross-coupling reactions catalysed by the Pd/MNP@IL-SiO2 (palladium nanoparticles  on  the  organic‐silica  nanoparticles)  catalyst  [90]  (Adapted  with  permission  from  nanoparticles on the organic-silica nanoparticles) catalyst [90] (Adapted with permission from American Chemical Society).  American Chemical Society).

Another  special  type  of  hybrid  organic‐inorganic  support  was  reported  by  Lin  et  al.  who  Another special type of hybrid organic-inorganic support was reported by Lin et al. who prepared prepared a retrievable Pd/CMC@Ce(OH) 4 catalyst for Suzuki coupling‐reactions [91]. In this system,  aCe(OH) retrievable Pd/CMC@Ce(OH) catalyst for Suzuki coupling-reactions [91]. In this system, Ce(OH)4 4 4  was  coated  with  carboxymethylcellulose  (CMC)  to  form  a  hybrid  organic‐inorganic  was coated with carboxymethylcellulose (CMC) to form a hybrid organic-inorganic CMC@Ce(OH) CMC@Ce(OH)4  support  (Scheme  9).  Carboxymethylcellulose  has  a  large  number  of  inherent 4 support (Scheme 9). Carboxymethylcellulose has a large number of inherent carboxylate (–COO−) and carboxylate (–COO−) and hydroxyl (–OH) groups on its molecular chain. Thus, CMC is an excellent  hydroxyl (–OH) groups on which  its molecular chain. Thus, CMC issupport  an excellent exchange which cation  exchange  agent,  makes  it  an  excellent  for  cation stabilization  of agent, palladium  4+ 3+  makes it an excellent support for stabilization of palladium nanoparticles. While ceria has a to Ce unique nanoparticles. While ceria has a unique ability of forming oxygen vacancies, that reduce Ce 4+ 3+ ability of forming oxygen vacancies, that reduce to Ce create an excess of negative charge and  create  an  excess  of  negative  charge  on  its Ce surface.  The and excess  negative  charge  facilitates  the  on its surface. The excess negative charge facilitates the coordination of palladium nanoparticles to the coordination of palladium nanoparticles to the surface, as discussed earlier. These interactions have  surface, as discussed interactions have been shown to be crucial preventing leaching been  shown  to  be  earlier. crucial These in  preventing  leaching  and  agglomeration  of incatalytic  palladium  and agglomeration of catalytic palladium nanoparticles. Scheme 9 shows an illustrative diagram of nanoparticles. Scheme 9 shows an illustrative diagram of the conceptual mechanism of the Suzuki  3+ on the 3+ on the surface of Ce(OH) the conceptual mechanism of the Suzuki reaction catalyzed by Pd(0)–CMC@Ce(OH) reaction catalyzed by Pd(0)–CMC@Ce(OH) 4. The Ce 4 enables the electronic  4 . The Ce surface Ce(OH)that  the electronic transfer that produces a highly charge of transfer  produces  a  highly  charge negatively  charged  palladium  center. negatively This  then charged makes  4 enables palladium This then makes palladium a oxidative  more electron rich site for facile oxidative addition of palladium center. a  more  electron  rich  site  for  facile  addition  of  aryl  halides,  and  thus  allows  catalysis to commence.  aryl halides, and thus allows catalysis to commence. The Pd/CMC@Ce(OH)4 catalyst was applied to the Suzuki cross‐coupling reaction and good to  excellent  yields  were  obtained.  It  is  believed  that  the  activity  of  the  catalyst  was  enhanced  synergistically by the unique redox properties of Ce(OH)4 (Ce3+/Ce4+) and the coordination with the  functional  (carboxyl  and  hydroxyl)  groups  of  the  hybrid  CMC@Ce(OH)4  support.  Moreover,  the  catalyst could be easily separated by simple filtration and reused at least for five times without losing  its activity. However, it was observed that Pd(0) species leached from the hybrid support during the  reaction and were re–deposited onto the support at the end of the reaction. Therefore, the reaction  mechanism is homogeneous in nature and the leached palladium species are the “true active species”. 

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  Scheme  9.  Illustrative  diagram  of  the  conceptual  mechanism  of  the  Suzuki  reaction  catalyzed  by  Scheme 9. Illustrative diagram of the conceptual mechanism of the Suzuki reaction catalyzed by Pd/CMC@Ce(OH)4 [91] (Adapted with permission from Elsevier).  Pd/CMC@Ce(OH)4 [91] (Adapted with permission from Elsevier).

8. Conclusions  The Pd/CMC@Ce(OH)4 catalyst was applied to the Suzuki cross-coupling reaction and good In conclusion, the literature survey shows that remarkable progress has been made regarding  to excellent yields were obtained. It is believed that the activity of the catalyst was enhanced the  activity  and  of  heterogeneous  based  catalytic  for  C–C  with cross  3+ /Ce 4+ ) and systems  synergistically byrecyclability  the unique redox properties ofpalladium  Ce(OH)4 (Ce the coordination coupling  reactions.  However,  thorough groups leaching  recyclability  tests  are  still  needed  for  most  the functional (carboxyl and hydroxyl) of and  the hybrid CMC@Ce(OH) 4 support. Moreover, reported  heterogeneous  catalytic  systems  to  unequivocally  confirm  that  the  palladium  supported  the catalyst could be easily separated by simple filtration and reused at least for five times without catalysts are responsible for the observed catalytic activity and not the leached material. Hence, at the  losing its activity. However, it was observed that Pd(0) species leached from the hybrid support during moment it is hard to say what the nature of the true catalyst is, due to the dynamic nature of the  the reaction and were re–deposited onto the support at the end of the reaction. Therefore, the reaction reported catalytic systems for cross‐coupling reactions.  mechanism is homogeneous in nature and the leached palladium species are the “true active species”. Author  Contributions:  P.P.M.  and  Z.P.V.  prepared  this  manuscript  under  the  guidance  and  supervision  of  8. Conclusions G.E.M.M. and H.B.F. 

In conclusion, the literature survey shows that remarkable progress has been made regarding the Funding: This research was funded by Mintek.  activity and recyclability of heterogeneous palladium based catalytic systems for C–C cross coupling Acknowledgments: We thorough would  like  to  express  our  gratitude  to tests Mintek  Department  of  Science  and  reactions. However, leaching and recyclability areand  stillthe  needed for most reported Technology South Africa (Advanced Metals Initiative program) for financial support for our work in this area.  heterogeneous catalytic systems to unequivocally confirm that the palladium supported catalysts are responsible for the observed catalytic activity and not the leached material. Hence, at the moment it is Conflicts of Interest: The authors declare no conflicts of interest.  hard to say what the nature of the true catalyst is, due to the dynamic nature of the reported catalytic References  systems for cross-coupling reactions. 1. Martinez,  A.V.;  Garcia,  J.I.;  Mayoral,  J.A.  An  synthesis  of  resveratrol  through  a  highly  Author Contributions: P.P.M. and Z.P.V. prepared thisexpedient  manuscript under the guidance and supervision of recoverable palladium catalyst. Tetrahedron 2017, 73, 5581–5584.  G.E.M.M. and H.B.F. 2. Biajoli, A.F.P.; Schwalm, C.S.; Limberger, J.; Claudino, T.S.; Monteiro, A.L. Recent progress in the use of  Funding: This research was funded by Mintek. Pd‐catalyzed C‐C cross‐coupling reactions in the synthesis of pharmaceutical compounds. J. Braz. Chem.  Acknowledgments: We would like to express our gratitude to Mintek and the Department of Science and Soc. 2014, 25, 2186–2214.  Technology South Africa (Advanced Metals Initiative program) for financial support for our work in this area. 3. Barnard, B.C. Palladium‐catalysed C‐C coupling: Then and now. Platin. Met. Rev. 2008, 52, 38–45.  Conflicts of Interest: The authors declare no conflicts of interest. 4. Pagliaro, M.; Pandarus, V.; Ciriminna, R.; Béland, F.; Demma Carà, P. Heterogeneous versus homogeneous  palladium catalysts for cross‐coupling reactions. ChemCatChem 2012, 4, 432–445.  References 5. Veerakumar,  P.;  Thanasekaran,  P.;  Lu,  K.L.;  Lin,  K.C.;  Rajagopal,  S.  Computational  studies  of  versatile  1. Martinez, A.V.; Garcia, J.I.; Mayoral, J.A. An expedient synthesis of resveratrol through a highly recoverable heterogeneous palladium‐catalyzed Suzuki, Heck and Sonogashira coupling reactions. ACS Sustain. Chem.  palladium catalyst. Tetrahedron 2017, 73, 5581–5584. [CrossRef] Eng. 2017, 5, 8475–8490.  6. Yin, L.; Liebscher, J. Carbon−carbon coupling reactions catalyzed by heterogeneous palladium catalysts.  Chem. Rev. 2007, 107, 133–173. 

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