molecules Review
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.
Molecules 2018, 23, 1676; doi:10.3390/molecules23071676
www.mdpi.com/journal/molecules
Molecules 2018, 23, 1676
2 of 24
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
Molecules 2018, 23, 1676
3 of 24
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.
Molecules 2018, 23, x FOR PEER REVIEW
3 of 23
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).
Molecules 2018, 23, 1676
4 of 24
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).
Molecules 2018, 23, 1676
5 of 24
Molecules 2018, 23, x FOR PEER REVIEW
5 of 23
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
a
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
b
−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].
Molecules 2018, 23, 1676
6 of 24
Molecules 2018, 23, x FOR PEER REVIEW
6 of 23
% Leaching
% Conversion
Temperature 120
90 80
80
60 50 %
60
40 30
40
20
Temperature / C
100
70
20
10 0
Molecules 2018, 23, x FOR PEER REVIEW
0
10
20
35
0 40
50 60 80 90 100 120 140 200 Reaction Time (min) % Leaching % Conversion Temperature
6 of 23
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
Molecules 2018, 23, 1676
7 of 24
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.
Molecules 2018, 23, 1676
8 of 24
Molecules 2018, 23, x FOR PEER REVIEW
8 of 23
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).
Molecules 2018, 23, x FOR PEER REVIEW
9 of 24
9 of 23
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.
Molecules 2018, 23, 1676 Molecules 2018, 23, x FOR PEER REVIEW Molecules 2018, 23, x FOR PEER REVIEW
10 of 24 10 of 23 10 of 23
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
Molecules 2018, 23, 1676
11 of 24
Molecules 2018, 23, x FOR PEER REVIEW
11 of 23
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).
Molecules 2018, 23, 1676
12 of 24
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).
Molecules 2018, 23, 1676
13 of 24
Molecules 2018, 23, x FOR PEER REVIEW
13 of 23
Molecules 2018, 23, x FOR PEER REVIEW
13 of 23
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).
Molecules 2018, 23, 1676
14 of 24
Molecules 2018, 23, x FOR PEER REVIEW
14 of 23
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).
Molecules 2018, 23, 1676
15 of 24
Molecules 2018, 23, x FOR PEER REVIEW
15 of 23
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].
Molecules 2018, 23, 1676
16 of 24
Molecules 2018, 23, x FOR PEER REVIEW
Molecules 2018, 23, x FOR PEER REVIEW
16 of 23
16 of 23
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
Molecules 2018, 23, 1676
17 of 24
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
Molecules 2018, 23, 1676
18 of 24
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”.
Molecules 2018, 23, 1676
19 of 24
Molecules 2018, 23, x FOR PEER REVIEW
19 of 23
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.
Molecules 2018, 23, 1676
2.
3. 4. 5.
6. 7. 8.
9. 10.
11. 12.
13.
14. 15.
16. 17. 18. 19. 20. 21. 22.
23.
20 of 24
Biajoli, A.F.P.; Schwalm, C.S.; Limberger, J.; Claudino, T.S.; Monteiro, A.L. Recent progress in the use of Pd-catalyzed C-C cross-coupling reactions in the synthesis of pharmaceutical compounds. J. Braz. Chem. Soc. 2014, 25, 2186–2214. [CrossRef] Barnard, B.C. Palladium-catalysed C-C coupling: Then and now. Platin. Met. Rev. 2008, 52, 38–45. [CrossRef] 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. [CrossRef] Veerakumar, P.; Thanasekaran, P.; Lu, K.L.; Lin, K.C.; Rajagopal, S. Computational studies of versatile heterogeneous palladium-catalyzed Suzuki, Heck and Sonogashira coupling reactions. ACS Sustain. Chem. Eng. 2017, 5, 8475–8490. [CrossRef] Yin, L.; Liebscher, J. Carbon−carbon coupling reactions catalyzed by heterogeneous palladium catalysts. Chem. Rev. 2007, 107, 133–173. [CrossRef] [PubMed] Chinchilla, R.; Nájera, C. The Sonogashira Reaction: A Booming Methodology in Synthetic Organic Chemistry. Chem. Rev. 2007, 107, 874–922. [CrossRef] [PubMed] Phan, N.T.S.; Van Der Sluys, M.; Jones, C.W. On the nature of the active species in palladium catalyzed Mizoroki–Heck and Suzuki–Miyaura couplings—Homogeneous or heterogeneous catalysis, a critical review. Adv. Synth. Catal. 2006, 348, 609–679. [CrossRef] Ananikov, V.P.; Beletskaya, I.P. Toward the ideal catalyst: From atomic centers to a “cocktail” of catalysts. Organometallics 2012, 31, 1595–1604. [CrossRef] Amoroso, F.; Colussi, S.; Del Zotto, A.; Llorca, J.; Trovarelli, A. Room-temperature Suzuki–Miyaura reaction catalyzed by Pd supported on rare earth oxides: Influence of the point of zero charge on the catalytic activity. Catal. Lett. 2013, 143, 547–554. [CrossRef] Sanjaykumar, S.R.; Mukri, B.D.; Patil, S.; Madras, G.; Hegde, M.S. Ce0.98 Pd0.02 O2-δ : Recyclable, ligand free palladium(II) catalyst for Heck reaction. J. Chem. Sci. 2011, 123, 47–54. [CrossRef] Khalili, D.; Banazadeh, A.R.; Etemadi-Davan, E. Palladium stabilized by amino-vinyl silica functionalized magnetic carbon nanotube: Application in Suzuki-Miyaura and Heck-Mizoroki coupling reactions. Catal. Lett. 2017, 147, 2674–2687. [CrossRef] Veerakumar, P.; Thanasekaran, P.; Lu, K.L.; Liu, S.B.; Rajagopal, S. Functionalized silica matrices and palladium: A versatile heterogeneous catalyst for Suzuki, Heck, and Sonogashira reactions. ACS Sustain. Chem. Eng. 2017, 5, 6357–6376. [CrossRef] Chorkendorff, I.B.; Niemantsverdriet, J.W. Introduction to Catalysis in Concepts of Modern Catalysis and Kinetics; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2003; pp. 1–22. Astakhov, A.V.; Khazipov, O.V.; Chernenko, A.Y.; Pasyukov, D.V.; Kashin, A.S.; Gordeev, E.G.; Khrustalev, V.N.; Chernyshev, V.M.; Ananikov, V.P. A new mode of operation of Pd-NHC systems studied in a catalytic Mizoroki–Heck reaction. Organometallics 2017, 36, 1981–1992. [CrossRef] Felpin, F.X.; Ayad, T.; Mitra, S. Pd/C: An old catalyst for new applications—Its use for the Suzuki-Miyaura reaction. Eur. J. Org. Chem. 2006, 2006, 2679–2690. [CrossRef] Felpin, F.X. Ten years of adventures with Pd/C catalysts: From reductive processes to coupling reactions. Synlett 2014, 25, 1055–1067. [CrossRef] Cini, E.; Petricci, E.; Taddei, M. Pd/C catalysis under microwave dielectric heating. Catalysts 2017, 7, 89. [CrossRef] Wang, Y.; Mao, Y.; Lin, Q.; Yang, J. Preparation of nano Pd/C catalyst and catalysis for Heck reaction. Huagong Xinxing Cailiao 2014, 42, 132–135. Zhou, X.Y.; Chen, X.; Wang, L.G. Recycled Pd/C catalyzed Heck reaction of 2-iodoanilines under ligand-free conditions. Synthesis 2017, 49, 5364–5370. [CrossRef] Seki, M. Practical synthesis of multifunctional compounds through Pd/C-catalyzed coupling reactions. Yuki Gosei Kagaku Kyokaishi 2006, 64, 853–866. [CrossRef] Schils, D.; Stappers, F.; Solberghe, G.; Van Heck, R.; Coppens, M.; Van den Heuvel, D.; Van der Donck, P.; Callewaert, T.; Meeussen, F.; De Bie, E.; et al. Ligandless Heck coupling between a halogenated aniline and acrylonitrile catalyzed by Pd/C: Development and optimization of an industrial-scale heck process for the production of a pharmaceutical intermediate. Org. Process Res. Dev. 2008, 12, 530–536. [CrossRef] Dighe, M.G.; Lonkar, S.L.; Degani, M.S. Mechanistic insights into palladium leaching in novel Pd/C-catalyzed boron-Heck reaction of arylboronic acid. Synlett 2013, 24, 347–350.
Molecules 2018, 23, 1676
24. 25.
26. 27. 28. 29.
30.
31.
32.
33. 34.
35. 36.
37. 38.
39. 40.
41. 42.
43. 44.
21 of 24
Freundlich, J.S.; Landis, H.E. An expeditious aqueous Suzuki–Miyaura method for the arylation of bromophenols. Tetrahedron Lett. 2006, 47, 4275–4279. [CrossRef] Schmidt, B.; Riemer, M. Suzuki–Miyaura coupling of halophenols and phenol boronic acids: Systematic investigation of positional isomer effects and conclusions for the synthesis of phytoalexins from pyrinae. J. Org. Chem. 2014, 79, 4104–4118. [CrossRef] [PubMed] Yuan, Y.Q.; Guo, S.R. Remarkably facile Heck reactions in aqueous two-phase system catalyzed by reusable Pd/C under ligand-free conditions. Synth. Commun. 2012, 42, 1059–1069. [CrossRef] Horikoshi, S.; Serpone, N. Role of microwaves in heterogeneous catalytic systems. Catal. Sci. Technol. 2014, 4, 1197–1210. [CrossRef] Hattori, T.; Tsubone, A.; Sawama, Y.; Monguchi, Y.; Sajiki, H. Palladium on carbon-catalyzed Suzuki-Miyaura coupling reaction using an efficient and continuous flow system. Catalysts 2015, 5, 18–25. [CrossRef] García-Suárez, E.J.; Lara, P.; García, A.B.; Ojeda, M.; Luque, R.; Philippot, K. Efficient and recyclable carbon-supported Pd nanocatalysts for the Suzuki–Miyaura reaction in aqueous-based media: Microwave vs conventional heating. Appl. Catal. A 2013, 468, 59–67. [CrossRef] Giacalone, F.; Campisciano, V.; Calabrese, C.; La Parola, V.; Liotta, L.F.; Aprile, C.; Gruttadauria, M. Supported C60-IL-PdNPs as extremely active nanocatalysts for C-C cross-coupling reactions. J. Mater. Chem. 2016, 4, 17193–17206. [CrossRef] Bowman, M.D.; Schmink, J.R.; McGowan, C.M.; Kormos, C.M.; Leadbeater, N.E. Scale-up of microwave-promoted reactions to the multigram level using a sealed-vessel microwave apparatus. Org. Process Res. Dev. 2008, 12, 1078–1088. [CrossRef] Lakshminarayana, B.; Mahendar, L.; Ghosal, P.; Satyanarayana, G.; Subrahmanyam, C. Nano-sized recyclable PdO supported carbon nanostructures for Heck reaction: Influence of carbon materials. ChemistrySelect 2017, 2, 2700–2707. [CrossRef] Jadhav, S.N.; Kumbhar, A.S.; Rode, C.V.; Salunkhe, R.S. Ligand-free Pd catalyzed cross-coupling reactions in an aqueous hydrotropic medium. Green Chem. 2016, 18, 1898–1911. [CrossRef] Labulo, A.H.; Martincigh, B.S.; Omondi, B.; Nyamori, V.O. Advances in carbon nanotubes as efficacious supports for palladium-catalysed carbon–carbon cross-coupling reactions. J. Mater. Sci. 2017, 52, 9225–9248. [CrossRef] Yang, L.; Cheng, G.; Xing, L.; Cheng, C.; Xia, L. Method for Activating Carbon Nanotube-Supported Palladium Nanoparticle Catalyst and Application Thereof. Patent CN106902817A, 30 June 2017. Hosseini-Sarvari, M.; Razmi, Z.; Doroodmand, M.M. Palladium supported on zinc oxide nanoparticles: Synthesis, characterization, and application as heterogeneous catalyst for Mizoroki–Heck and Sonogashira reactions under ligand-free and air atmosphere conditions. Appl. Catal. A 2014, 475, 477–486. [CrossRef] Del Zotto, A.; Colussi, S.; Trovarelli, A. Pd/REOs catalysts applied to the Suzuki-Miyaura coupling. A comparison of their catalytic performance and reusability. Inorg. Chim. Acta 2018, 470, 275–283. [CrossRef] Köhler, K.; Heidenreich, R.G.; Soomro, S.S.; Pröckl, S.S. Supported palladium catalysts for Suzuki reactions: Structure-property relationships, optimized reaction protocol and control of palladium leaching. Adv. Synth. Catal. 2008, 350, 2930–2936. [CrossRef] Gruttadauria, M.; Giacalone, F.; Noto, R. "Release and catch" catalytic systems. Green Chem. 2013, 15, 2608–2618. [CrossRef] Adair, J.H.; Suvaci, E.; Sindel, J. Surface and Colloid Chemistry in Encyclopedia of Materials: Science and Technology, 2nd ed.; Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., Mahajan, S., Veyssière, P., Eds.; Elsevier: Oxford, UK, 2001; pp. 1–10. Kosmulski, M. Compilation of PZCs/IEPs. In Surface Charging and Point of Zero Charge; Hubbard, A.T., Ed.; Taylor & Francis Group: Boca Raton, FL, USA, 2009; pp. 101–524. Del Zotto, A.; Zuccaccia, D. Metallic palladium, PdO, and palladium supported on metal oxides for the Suzuki-Miyaura cross-coupling reaction: A unified view of the process of formation of the catalytically active species in solution. Catal. Sci. Technol. 2017, 7, 3934–3951. [CrossRef] Bera, P.; Hegde, M.S. Noble metal ions in CeO2 and TiO2 : Synthesis, structure and catalytic properties. RSC Adv. 2015, 5, 94949–94979. [CrossRef] Cwele, T.; Mahadevaiah, N.; Singh, S.; Friedrich, H.B. Effect of Cu additives on the performance of a cobalt substituted ceria (Ce0.90 Co0.10 O2-δ ) catalyst in total and preferential CO oxidation. Appl. Catal. B 2016, 182, 1–14. [CrossRef]
Molecules 2018, 23, 1676
45.
46.
47. 48. 49.
50.
51.
52.
53.
54. 55. 56. 57. 58. 59. 60. 61.
62. 63. 64.
65.
22 of 24
Cwele, T.; Mahadevaiah, N.; Singh, S.; Friedrich, H.B.; Yadav, A.K.; Jha, S.N.; Bhattacharyya, D.; Sahoo, N.K. CO oxidation activity enhancement of Ce0.95 Cu0.05 O2-δ induced by Pd co-substitution. Catal. Sci. Technol. 2016, 6, 8104–8116. [CrossRef] Gupta, A.; Waghmare, U.V.; Hegde, M.S. Correlation of oxygen storage capacity and structural distortion in transition-metal, noble-metal, and rare-earth-ion-substituted CeO2 from first principles calculation. Chem. Mater. 2010, 22, 5184–5198. [CrossRef] Deshpande, P.A.; Hegde, M.S.; Madras, G. A mechanistic model for the water-gas shift reaction over noble metal substituted ceria. AIChE J. 2010, 56, 1315–1324. [CrossRef] Hegde, M.S.; Madras, G.; Patil, K.C. Noble metal ionic catalysts. Acc. Chem. Res. 2009, 42, 704–712. [CrossRef] [PubMed] Mpungose, P.P.; Sehloko, N.I.; Dasireddy, V.D.B.C.; Mahadevaiah, N.; Maguire, G.E.; Friedrich, H.B. Pd0.09 Ce0.91 O2-δ : A sustainable ionic solid-solution precatalyst for heterogeneous, ligand free Heck coupling reactions. Mol. Catal. 2017, 443, 60–68. [CrossRef] Mpungose, P.P.; Sehloko, N.I.; Maguire, G.E.M.; Friedrich, H.B. PdCuCeO-TPAB: A new catalytic system for quasi-heterogeneous Suzuki-Miyaura cross-coupling reactions under ligand-free conditions in water. New J. Chem. 2017, 41, 13560–13566. [CrossRef] Mpungose, P.P.; Sehloko, N.I.; Cwele, T.; Maguire, G.E.M.; Friedrich, H.B. Pd0.02 Ce0.98 O2-δ : A copper- and ligand-free quasi-heterogeneous catalyst for aquacatalytic Sonogashira cross-coupling reaction. J. South. Afr. Inst. Min. Metall. 2017, 117, 955–962. [CrossRef] Burange, A.S.; Shukla, R.; Tyagi, A.K.; Gopinath, C.S. Palladium supported on fluorite structured redox CeZrO4-δ for heterogeneous suzuki coupling in water: A green protocol. ChemistrySelect 2016, 1, 2673–2681. [CrossRef] Lichtenegger, G.J.; Maier, M.; Hackl, M.; Khinast, J.G.; Gössler, W.; Griesser, T.; Kumar, V.S.P.; Gruber-Woelfler, H.; Deshpande, P.A. Suzuki-Miyaura coupling reactions using novel metal oxide supported ionic palladium catalysts. J. Mol. Catal. A Chem. 2017, 426, 39–51. [CrossRef] Karimi, B.; Mansouri, F.; Mirzaei, H.M. Recent applications of magnetically recoverable nanocatalysts in C-C and C-X coupling reactions. ChemCatChem 2015, 7, 1736–1789. [CrossRef] Feng, C.; Liu, J.; Gui, J.; Liu, L. Application of magnetic nanoparticles supported Pd catalysts in C-C bond formation reactions. Yingyong Huaxue 2015, 32, 19–26. Cheng, T.; Zhang, D.; Li, H.; Liu, G. Magnetically recoverable nanoparticles as efficient catalysts for organic transformations in aqueous medium. Green Chem. 2014, 16, 3401–3427. [CrossRef] Baig, R.B.N.; Varma, R.S. Magnetically retrievable catalysts for organic synthesis. Chem. Commun. 2013, 49, 752–770. [CrossRef] [PubMed] Rossi, L.M.; Garcia, M.A.S.; Vono, L.L.R. Recent advances in the development of magnetically recoverable metal nanoparticle catalysts. J. Braz. Chem. Soc. 2012, 23, 1959–1971. [CrossRef] Xiao, C.; Yan, N.; Kou, Y. Quasi-homogeneous catalysis: Towards green and efficiency. Cuihua Xuebao 2009, 30, 753–764. Mulahmetovic, E.; Hargaden, G.C. Recent advances in the development of magnetic catalysts for the Suzuki reaction. Rev. J. Chem. 2017, 7, 373–398. [CrossRef] Elhampour, A.; Nemati, F. Palladium nanoparticles supported on modified hollow-Fe3 O4 @TiO2 : Catalytic activity in Heck and Sonogashira cross coupling reactions. Org. Prep. Proced. Int. 2017, 49, 443–458. [CrossRef] Hudson, R.; Feng, Y.; Varma, R.S.; Moores, A. Bare magnetic nanoparticles: Sustainable synthesis and applications in catalytic organic transformations. Green Chem. 2014, 16, 4493–4505. [CrossRef] Rossi, L.M.; Costa, N.J.S.; Silva, F.P.; Wojcieszak, R. Magnetic nanomaterials in catalysis: Advanced catalysts for magnetic separation and beyond. Green Chem. 2014, 16, 2906–2933. [CrossRef] Nasrollahzadeh, M.; Mohammad Sajadi, S.; Rostami-Vartooni, A.; Khalaj, M. Green synthesis of Pd/Fe3 O4 nanoparticles using Euphorbia condylocarpa M. bieb root extract and their catalytic applications as magnetically recoverable and stable recyclable catalysts for the phosphine-free Sonogashira and Suzuki coupling reactions. J. Mol. Catal. A Chem. 2015, 396, 31–39. [CrossRef] Xie, C.X.; Liu, Y.; Yu, S.T. Research progress of magnetic catalysts. Qingdao Keji Daxue Xuebao Ziran Kexueban 2015, 36, 237–244.
Molecules 2018, 23, 1676
66.
67.
68.
69.
70. 71. 72.
73.
74.
75.
76.
77.
78.
79.
80. 81.
82.
83.
23 of 24
Kumar, B.S.; Anbarasan, R.; Amali, A.J.; Pitchumani, K. Isolable C@Fe3 O4 nanospheres supported cubical Pd nanoparticles as reusable catalysts for Stille and Mizoroki-Heck coupling reactions. Tetrahedron Lett. 2017, 58, 3276–3282. [CrossRef] Heidari, F.; Hekmati, M.; Veisi, H. Magnetically separable and recyclable Fe3 O4 @SiO2 /isoniazide/Pd nanocatalyst for highly efficient synthesis of biaryls by Suzuki coupling reactions. J. Colloid Interface Sci. 2017, 501, 175–184. [CrossRef] [PubMed] Collinson, J.M.; Wilton-Ely, J.D.E.T.; Diez-Gonzalez, S. Functionalised [(NHC)Pd(allyl)Cl] complexes: Synthesis, immobilisation and application in cross-coupling and dehalogenation reactions. Catal. Commun. 2016, 87, 78–81. [CrossRef] Fareghi-Alamdari, R.; Saeedi, M.S.; Panahi, F. New bis(N-heterocyclic carbene) palladium complex immobilized on magnetic nanoparticles: As a magnetic reusable catalyst in Suzuki-Miyaura cross coupling reaction. Appl. Organomet. Chem. 2017, 31, e3870. [CrossRef] Fan, H.; Qi, Z.; Sui, D.; Mao, F.; Chen, R.; Huang, J. Palladium nanoparticles in cross-linked polyaniline as highly efficient catalysts for Suzuki-Miyaura reactions. Chin. J. Catal. 2017, 38, 589–596. [CrossRef] Taher, A.; Choudhary, M.; Nandi, D.; Siwal, S.; Mallick, K. Polymer-supported palladium: A hybrid system for multifunctional catalytic application. Appl. Organomet. Chem. 2017, 32, e3898. [CrossRef] Ashouri, F.; Zare, M.; Bagherzadeh, M. The effect of framework functionality on the catalytic activation of supported Pd nanoparticles in the Mizoroki-Heck coupling reaction. C. R. Chim. 2017, 20, 107–115. [CrossRef] Wang, C.A.; Li, Y.W.; Hou, X.M.; Han, Y.F.; Nie, K.; Zhang, J.P. N-Heterocyclic carbene-based microporous organic polymer supported palladium catalyst for carbon-carbon coupling reaction. ChemistrySelect 2016, 1, 1371–1376. [CrossRef] Liu, X.; Zhao, X.; Lu, M. Novel polymer supported iminopyridylphosphine palladium (II) complexes: An efficient catalyst for Suzuki-Miyaura and Heck cross-coupling reactions. J. Organomet. Chem. 2014, 768, 23–27. [CrossRef] Kodicherla, B.; Perumgani, C.P.; Keesara, S.; Mandapati, M.R. Polystyrene-supported palladium(II) N,N-dimethylethylenediamine complex: A recyclable catalyst for Suzuki-Miyaura cross-coupling reactions in water. Inorg. Chim. Acta 2014, 423, 95–100. [CrossRef] Aravinda Reddy, P.; Babul Reddy, A.; Ramachandra Reddy, G.; Subbarami Reddy, N. Suzuki-Miyaura cross-coupling reaction of naphthyl triflate with indole boronic acids catalyzed by a recyclable polymer-supported n-heterocyclic carbene-palladium complex catalyst: Synthesis of naphthalene-linked bis-heterocycles. J. Heterocycl. Chem. 2013, 50, 1451–1456. [CrossRef] Tamami, B.; Dodeji, F.N. Synthesis and application of modified polystyrene-supported palladium nanoparticles as a new heterogeneous catalyst for Heck and Suzuki cross-coupling reactions. J. Iran. Chem. Soc. 2012, 9, 841–850. [CrossRef] Nemygina, N.A.; Nikoshvili, L.Z.; Bykov, A.V.; Sidorov, A.I.; Molchanov, V.P.; Sulman, M.G.; Tiamina, I.Y.; Stein, B.D.; Matveeva, V.G.; Sulman, E.M.; et al. Catalysts of Suzuki cross-coupling based on functionalized hyper-cross-linked polystyrene: Influence of precursor nature. Org. Process Res. Dev. 2016, 20, 1453–1460. [CrossRef] Li, Y.; Xu, L.; Xu, B.; Mao, Z.; Xu, H.; Zhong, Y.; Zhang, L.; Wang, B.; Sui, X. Cellulose sponge supported palladium nanoparticles as recyclable cross-coupling catalysts. ACS Appl. Mater. Interfaces 2017, 9, 17155–17162. [CrossRef] [PubMed] Rao, C.N.R.; Cheetham, A.K.; Thirumurugan, A. Hybrid inorganic–organic materials: A new family in condensed matter physics. J. Phys. Condens. Matter. 2008, 20, 1–21. Wang, Y.; Dou, L.; Zhang, H. Nanosheet array-like palladium-catalysts Pdx/rGO@CoAl-LDH via lattice atomic-confined in situ reduction for highly efficient heck coupling reaction. ACS Appl. Mater. Interfaces 2017, 9, 38784–38795. [CrossRef] [PubMed] Arsalani, N.; Akbari, A.; Amini, M.; Jabbari, E.; Gautam, S.; Chae, K.H. POSS-Based Covalent networks: Supporting and stabilizing pd for heck reaction in aqueous media. Catal. Lett. 2017, 147, 1086–1094. [CrossRef] Woo, H.; Lee, K.; Park, K.H. Optimized dispersion and stability of hybrid Fe3 O4 /Pd catalysts in water for Suzuki coupling reactions. Impact of organic capping agents. ChemCatChem 2014, 6, 1635–1640. [CrossRef]
Molecules 2018, 23, 1676
84.
85.
86.
87. 88.
89.
90. 91.
24 of 24
Jing, X.; Sun, F.; Ren, H.; Tian, Y.; Guo, M.; Li, L.; Zhu, G. Targeted synthesis of micro-mesoporous hybrid material derived from octaphenylsilsesquioxane building units. Microporous Mesoporous Mater. 2013, 165, 92–98. [CrossRef] Qu, K.; Wu, L.; Ren, J.; Qu, X. Natural DNA-modified graphene/Pd nanoparticles as highly active catalyst for formic acid electro-oxidation and for the Suzuki reaction. ACS Appl. Mater. Interfaces 2012, 4, 5001–5009. [CrossRef] [PubMed] Yang, H.; Ma, Z.; Qing, Y.; Xie, G.; Gao, J.; Zhang, L.; Gao, J.; Du, L. A periodic mesoporous hybrid material with a built-in palladium complex: An efficient catalyst for the Suzuki coupling and alcohol oxidation. Appl. Catal. A 2010, 382, 312–321. [CrossRef] Wang, B.; Gu, Y.L.; Yang, L.M.; Suo, J.S. Organic/inorganic hybrid materials-supported metal complex catalysts: New applications of sol-gel technology. Fenzi Cuihua 2003, 17, 468–480. Shaikh, M.N.; Abdul Aziz, M.; Helal, A.; Kalanthoden, A.N.; Yamani, Z.H. PdNPs@ZIF-8 micro-nanostructured catalyst of regioselective Mizoriki-Heck olefination. ChemistrySelect 2017, 2, 9052–9057. [CrossRef] Kozell, V.; Giannoni, T.; Nocchetti, M.; Vivani, R.; Piermatti, O.; Vaccaro, L. Immobilized palladium nanoparticles on zirconium carboxy-aminophosphonates nanosheets as an efficient recoverable heterogeneous catalyst for suzuki–miyaura and heck coupling. Catalysts 2017, 7, 186. [CrossRef] Omar, S.; Abu-Reziq, R. Palladium nanoparticles supported on magnetic organic-silica hybrid nanoparticles. J. Phys. Chem. C 2014, 118, 30045–30056. [CrossRef] Lin, B.; Liu, X.; Zhang, Z.; Chen, Y.; Liao, X.; Li, Y. Pd(0)-CMC@Ce(OH)4 organic/inorganic hybrid as highly active catalyst for the Suzuki-Miyaura reaction. J. Colloid Interface Sci. 2017, 497, 134–143. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).