Formation and Role of Palladium Chalcogenide and Other Species in

Review pubs.acs.org/Organometallics

Formation and Role of Palladium Chalcogenide and Other Species in Suzuki−Miyaura and Heck C−C Coupling Reactions Catalyzed with Palladium(II) Complexes of Organochalcogen Ligands: Realities and Speculations Arun Kumar, Gyandshwar Kumar Rao, Satyendra Kumar, and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India ABSTRACT: Palladium(II) complexes of organochalcogen ligands have emerged as viable alternatives to complexes of phosphine/carbene ligands for Suzuki−Miyaura and Heck C−C cross coupling reactions, as thermal stability and air and moisture sensitivity are not impediments with many of them. Sometimes they outperform their phosphorus analogues. The ease in handling and synthesis of palladium(II) complexes of organochalcogen ligands has made them attractive. For such complexes in situ generation of palladium containing nanoparticles (NPs) or palladium(0) species and their role in catalysis have been reported. However, in a large number of reports such species are ill defined or speculated, particularly due to limited experimental investigations. Recently in some cases in situ generated palladium species have been studied in detail to establish their identities, which appear to be palladium chalcogenides phases or Pd(0) protected with organochalcogen fragments. In this review the current status regarding in situ generated such species is summarized and critically analyzed to demarcate realities from speculative propositions.

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INTRODUCTION C−C bond formation reactions, viz. Heck, Suzuki, Sonogashira, Tsuji−Trost, Stille, Negishi, and Hiyama reactions, are powerful synthetic tools in organic chemistry.1−5 The 2010 Nobel Prize in chemistry awarded to Professor Akira Suzuki jointly with Professors Richard F. Heck and Ei-ichi Negishi6 is an acknowledgment of the great importance of C−C coupling. Palladium species are versatile and useful catalysts for such organic transformations, and therefore Pd is at center stage in the group of transition-metal catalysts.7 The facile oxidation of Pd(0) to Pd(II) or Pd(II) to Pd(IV) and the tolerance of palladium compounds to many functional groups present on the substrate under mild reaction conditions of the catalytic process are responsible for making Pd a unique catalytic center. Suzuki−Miyaura and Heck C−C coupling reactions (Schemes 1 and 2), the focus of this review, have been explored more extensively than the others. The common palladium(II) salts catalyze the coupling of aryl iodides, but for bulky or electronically deactivated ArBr/Cl, specifically

Scheme 2. Heck C−C Coupling Reaction

designed Pd complexes may be required. For example, complexes of Pd(II) with bulky and electron-rich phosphines8 and carbenes9 and palladacycles10−13 are efficient as catalysts and are easy to modify. The phosphorus ligand based Pd complexes have been found many times to be sensitive to moisture and air. Hence, an argon or nitrogen atmosphere14−18 is required to protect them from deactivation when they are used as catalysts. However, stable phosphine/carbene complexes of Pd are not unknown.19−23 The thermally stable and aerial oxidation resistant Pd(II) organochalcogen ligand complexes (due to the strong donor properties of S and Se),17,18 when used as catalysts, may avoid the requirement of an inert atmosphere. The Heck coupling of various aryl iodides and alkenes, catalyzed with a Pd(II) complex of an (S,C,S) pincer ligand, was probably the first report24 involving a chalcogen-based ligand in the coupling reaction. Thereafter, applications of palladium complexes of organosulfur and

Scheme 1. Suzuki−Miyaura C−C Coupling Reaction

Received: July 22, 2013

© XXXX American Chemical Society

A

dx.doi.org/10.1021/om4007196 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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precatalysts on the basis of limited experimental observations.44,89−117 The appearance of black particles in the reaction89−91 and the effect of Bu4NBr44,92−101 on catalytic efficiency are among the observations which have led authors to propose that the complexes are the reservoirs of Pd(0). Generation of Pd(0) nanoparticles (NPs) or ill-defined Pd(0) species has been proposed when phosphine addition modifies the behavior of the organochalcogen-based catalyst.111,112 The propagation from a palladium complex of an organochalcogen ligand (i.e., organosulfur, -selenium, and -tellurium compounds) to final real catalytic species has been dealt with in different reports to different levels. Attempts made to understand the propagation in depth are very limited, and generally confusion persists. To understand the propagation from a molecular complex to Pd(0) is important for the rational design of Pd(II) organochalcogen ligand complexes suitable as coupling catalysts. Recently the generation of NPs of known palladium chalcogenide phases and nanosized species containing both Pd and chalcogen have been reported118−123 in the catalysis of Suzuki coupling with Pd(II) complexes of organochalcogen ligands. The issues related to the generation and catalytic activity of such NPs and the propagation of true catalytic species from them in coupling reactions have been addressed in slightly more detail in the recent past, and attempts have been made to give experimental support to the proposed species and pathways. The present review is mainly focused on the analysis of the real catalytic species in Heck and Suzuki coupling. The current understanding of their propagation and tools used for this purpose have been noted. The better analyzed cases have been segregated from the poorly analyzed ones. Further comments are also made about the prospects in this context. Thus, in the present review reports on (i) in situ generated Pd-containing species and (ii) catalytic pathways proposed through them have been examined critically. An overview of a variety of Pd chalcogenide NPs, Pd and chalcogen containing NPs, and Pd NPs formed along with experimental supports in their favor is given here. The issues of independent catalysis by these isolated nanospecies and the generation of real catalytic species from them are addressed. The review has been divided into the following sections: (i) Introduction, (ii) Nanoparticles of Palladium Chalcogenides, (iii) Nanoparticles Containing Palladium and Organochalcogen Species, (iv) Miscellaneous Palladium Species, (v) Insight into the Role of Nanoparticles in Catalysis, (vi) Miscellaneous Palladium Species: Attempts at Insight, and (vii) Comments and Outlook.

-selenium ligands in the catalysis of C−C coupling gained momentum and the complexes of several variants of these ligands for catalysis of Suzuki/Heck coupling have been reported. The chalcogenide derivatives of triphenylphosphine, which are relatively robust and insensitive to air due to the pentavalent nature of phosphorus, have also been used to design catalytic Pd systems for coupling reactions.25 In the last decade and a half the catalytic potential of metal complexes of organochalcogen ligands has been explored not only for C−C coupling but also for various other chemical transformations.26−38 However, a large number of reports continue to appear on C−C coupling reactions,36−44 particularly Heck and Suzuki reactions. Some recent reviews have covered the synthesis and catalytic applications of Pd complexes of organochalcogen ligands36−38 to these coupling reactions. It has been found that as catalysts for Suzuki/Heck C−C coupling they are not only rivals of their respective phosphorus analogues but, in some cases, also outperform them for the same aryl halide.43,44 The coupling of aryl chlorides, the cheapest and most abundant among the aryl halides, has been addressed in the recent past using such complexes as catalysts.37,38 True catalytic species in these coupling reactions have been discussed in some reviews in the past45−50 and the catalytic cycle of the Heck reaction has also been highlighted in some reviews and a paper.51−53 In a comprehensive review on Pdcatalyzed Heck and Suzuki reactions it has been mentioned that, regardless of the catalyst used, a Pd(0)−Pd(II) cycle is involved in the coupling.54 These reviews have emphasized the involvement of Pd(0) in the catalysis but are silent on its generation pathway: i.e., whether it is directly dispensed from the complex or via some intermediate. As an alternative to molecular complexes which dispense Pd NPs or some other Pd(0) species during the course of catalysis, preformed Pd NPs have emerged as promising selective and reusable catalysts. Various reviews on the use of palladium nanoparticles for C−C coupling reactions have been published.55−57 In 2007 Astruc presented a unifying view about Pd NPs and catalysis of the C− C coupling and reported that Pd NPs are precatalysts which function as precursors of catalytically active Pd species.55 The preparation of palladium nanoparticles has also been reviewed.58−60 In 2011 a review dealing with the synthetic advantages and disadvantages of various nanocatalysts immobilized on supports, such as high-surface-area silica, carbon nanotubes, polymers, metal oxides, and double hydroxides, was published.61 This review also covered nanocatalysts immobilized on nonconventional supports, such as dendrimers, cyclodextrin, and magnetic nanomaterials, and nanocatalyst systems in nonconventional media (i.e., fluorous media and ionic liquids). However, these reviews have not dealt much with Pd complexes of organochalcogen ligands. In many research papers on the catalysis of the two coupling reactions with Pd(II) organochalcogen ligand complexes, the active form of palladium has not been discussed.17,18,40−43,62−77 In some papers on Heck/Suzuki reactions, the formation of Pd(0) has not been accepted24,78−86 and the stability of the catalyst justified due to two reasons: (i) their solutions in organic solvents at elevated temperatures were found to be stable and (ii) the presence of air did not affect their ability to promote catalysis effectively. In some reports the generation of Pd(0) is presumed or proposed without any experimentation.87,88 There have been several reports which describe the complexes as

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NANOPARTICLES OF PALLADIUM CHALCOGENIDES The preformed chalcogenides of platinum-group metals, including those of palladium, have been used to catalyze several reactions such as C−O coupling,124 hydrodesulfurization,125 hydrodenitrogenation,125 hydrogenation,125 isomerization and acetoxylation,125 and dehydrogenation reactions.125 Though these chalcogenides can be prepared using various methods, the single source precursors (such as palladium organochalcogenolates) have been paid lot of attention.126 The reports on in situ generation of NPs of various palladium chalcogenide phases in the course of Suzuki Miyaura C−C coupling reactions catalyzed by palladium complexes of various organochalcogen ligands, are recent. The structures of complexes known at present as the dispensers of nanosized palladium chalcogenides are given in Chart 1. The formation of B


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Chart 1. Complexes Dispensing NPs of Palladium Chalcogenide Phases

Figure 1. HRTEM images of Pd16S7 NPs before annealing.

chemical composition did not change on annealing. The size became 6 nm (spherical) in the case of 1 and 100 nm (rice shaped) in the case of 2. The formation of small-sized NPs results in more efficient catalysis. Thus, the higher efficiency of 1 relative to 2 has been somewhat rationalized in terms of the size of the NPs. Complex 3 has been used to catalyze the Suzuki coupling reactions of electronically activated as well as deactivated aryl bromides. The catalytic activity of this complex is promising, as its low loading (∼0.01 mol %) has been found to be sufficient for good conversion in the case of several aryl halides. The highest yield (90%) was obtained when 3 was used as a catalyst in the coupling of both 4-bromobenzonitrile and 1-bromo-4nitrobenzene. When the black precipitate, formed in the course of catalysis of coupling of 4-bromobenzaldehyde with phenylboronic acid under optimum reaction conditions (DMF−water, K2CO3, 100 °C, 2 h) with 3, was characterized by powder XRD (X-ray powder diffraction), SEM (scanning electron microscopy), TEM, and SEM-EDX (energy dispersive X-ray spectroscopy), it was found to be made of nanoparticles of Pd4S, playing an active role in the catalysis.119 TEM studies have revealed that these nanosized (∼19 nm) particles are spherical (Figure 2). The catalytic activity of complex 4 has been found

NPs of phases Pd16S7,118 Pd4S119 and Pd5S2119 occurs in the course of Suzuki-Miyaura coupling catalyzed with 1−4. Similarly Pd17Se15,120 Pd2Se119 and PdSe119 are palladium selenide phases which appear as NPs in the catalysis of Suzuki Miyaura coupling with complexes 5−7 respectively. The formation of NPs of Pd3Te2 has been reported in the catalysis of same coupling with complex 8.127 Generally the nanoparticles on isolation were found to be amorphous in nature and hence they were annealed in argon atmosphere at high temperature (∼380−450 °C), so that resulting crystalline phase may be subjected to powder X-ray diffraction for identification. Further details about these NPs are as follows. Nanoparticles of Palladium Sulfides. It has been reported that complexes 1−4 are only precatalysts and in the course of catalytic C−C coupling reactions they dispense nanoparticles of palladium sulfides, which play a role in the catalysis. When complexes 1 and 2 are used particularly to catalyze the Suzuki−Miyaura C−C coupling reactions of aryl halides, including aryl chlorides, the formation of Pd16S7 nanoparticles has been reported.118 This was the first report of its kind. In the course of representative reactions between 4bromonitrobenzene and phenylboronic acid using 1 and 2 as catalysts, Pd16S7 NPs have been isolated and characterized with powder XRD, SEM, SEM-EDX, and TEM. The phase Pd16S7 has a body-centered cubic structure with 46 atoms in the cubic cell and is structurally related to the γ-brass structure.128 The efficiency of 2 in carrying out the coupling was found to be significantly lower than that of 1. This has been correlated with the characteristics of NPs formed in situ. HRTEM (highresolution transmission electron microscopy) studies have revealed (Figure 1) that the Pd16S7 NPs are spherical in shape, highly uniform, and monodisperse but different in size (∼2 nm for 1 and 6 nm for 2) (Figure 1). The variation in their size further increased on annealing at 380 °C for 4 h under an argon atmosphere to make the sample crystalline. However, the

Figure 2. HRTEM images of Pd4S (a) and Pd5S2 (b) NPs.

to be marginally higher than that of 3. The black precipitate formed in the Suzuki coupling of 4-bromobenzaldehyde catalyzed with 4 has been found to be made of NPs of Pd5S2 on the basis of SEM, SEM-EDX, and TEM.119 The shape of NPs dispensed by 4 (Figure 2) is similar to that of Pd4S NPs. However, they are smaller in size (∼5 nm), which has been suggested as a possible reason for the observation that 4 is more efficient than 3. Nanoparticles of Palladium Selenides. The catalysis of Suzuki−Miyaura coupling with complexes 5−7 has been reported to proceed via NPs of the palladium selenide phase, generated in situ in the course of catalysis: i.e., 5−7 are precatalysts dispensing NPs of palladium selenides. The NPs C


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NPs depends on the precursor along with the reaction conditions. Pd2Se NP formation has been reported in the course of catalysis of Suzuki coupling reactions of several aryl halides (i.e., electronically activated as well as deactivated aryl bromides) with complex 6.119 The promising catalytic activity of 6 at ∼0.01 mol % loading has been attributed to the formation of such NPs. The yield (94%) of cross-coupled product was highest in the case of 4-bromobenzonitrile. The NPs of Pd2Se were isolated as black precipitates in the coupling of 4bromobenzaldehyde with phenylboronic acid catalyzed with 6 under optimum reaction conditions and characterized with HRTEM (Figure 4). The catalytic activity of complex 7 has

have been characterized with powder XRD, SEM, TEM, and SEM EDX. It has been reported that 5 functioning as a precatalyst dispenses Pd17Se15 NPs proposed to participate in catalyzing Suzuki−Miyaura coupling of aryl bromides and chlorides (having the substituents NO2, −CN, −NH2, −OH, −OCH3, −CHO, −COCH3, −COOH), including heteroaryl halides with phenylboronic acid.120 The precatalyst 5 has been described as one of the best complexes because coupling of aryl chlorides, considered to be one of the most unreactive substrates due to high C−Cl bond dissociation energy, can be achieved successfully at low concentration. This has been attributed to the high catalytic activity of the in situ generated NPs of Pd17Se15. Graphine oxide grafted with these preformed NPs have been reported recently to catalyze C−O coupling reaction with high efficiency at room temperature.124 To authenticate the identity of the black residue formed in the coupling reaction of 4-chloronitrobenzene with phenylboronic acid (catalyzed with 5 in DMF−water containing K2CO3 at 100 °C) as Pd17Se15 NPs, the residue was separated, annealed under an argon atmosphere at 450 °C for 5 h to get a crystalline phase, and characterized using various techniques. The HRTEM has indicated that these NPs are highly uniform, monodispersed and spherical. Their average size was found to be ∼8 nm (Figure 3) before annealing. The SEM−EDX studies

Figure 4. HRTEM images of palladium selenide NPs.

been tested for Suzuki coupling reactions of those aryl bromides studied with 6. The efficiency of 7 for catalysis was found to be marginally higher than that of 6. The black precipitate appearing in the course of the catalysis of coupling between 4-bromobenzaldehyde and phenylboronic acid with 7, under reaction conditions similar to those of 6, was found to be made up of PdSe nanoparticles.119 The sizes of the Pd2Se and PdSe NPs have been reported to be ∼2 and ∼3 nm, respectively, on the basis of HRTEM studies (Figure 4). The compositions of Pd2Se and PdSe nanoparticles dispensed from complexes 6 and 7, respectively, have been supported by SEM− EDX studies. Nanoparticles of Palladium Telluride Phase. Complex 8 is the first and only example of a palladium complex of an organotellurium ligand, which on use as a catalyst in Suzuki coupling reaction, has been found to give a palladium telluride phase.127 The Pd3Te2 NPs are formed when Suzuki−Miyaura coupling reactions of several aryl bromides are catalyzed with complex 8, a tellurium analogue of complexes 1 and 5. Complex 8 has been found to show less efficiency than 1 and 5. Though the NPs appear to be formed as black precipitates in the course of all Suzuki reactions catalyzed with 8, their characterization and other studies were carried out using the black residue formed in the catalysis of coupling of 1-bromo-4nitrobenzene with phenylboronic acid under optimum reaction conditions. HRTEM indicates that black residue is made of highly uniform, monodispersed, and spherical nanoparticles of average size ∼1−2 nm (Figure 5). The amorphous black residue, when annealed under an argon atmosphere at 450 °C for 5 h, led to the formation of crystalline Pd3Te2 nanorods (36 nm × 730 nm; Figure 5). TEM and SEM−EDX studies have revealed that the composition of these nanoparticles before annealing (in terms of wt %) is 56 for Pd and 44 for Te and after annealing is 54 for Pd and 46 for Te. The powder X-ray diffraction pattern

Figure 3. HRTEM images of Pd17Se15 NPs.

have supported that the composition of the nanoparticles (in wt %) on annealing (Pd, 57.42; Se, 42.58120) does not change significantly from that of Pd17Se15. The powder X-ray diffraction pattern of these nanoparticles matches with that of JCPDS No. 73-1424 (based on a primitive cubic unit cell with the refined lattice parameter 10.60 Å).120 On the basis of HRTEM images, the crystalline NPs were found to be agglomerates (average size ∼20 nm), appearing to be assembled from unannealed spherical NPs of ∼8 nm. The composition of these NPs inferred on the basis of TEM−EDX analysis is similar to that of SEM−EDX. Pd17Se15, a phase known for a long time and characterized by single-crystal X-ray diffraction, has four crystallographically different palladium atoms.125 One of them is surrounded by a regular octahedron of selenium atoms (Pd−Se distance 2.58 Å). Each of the remaining three palladium atoms is coordinated to four selenium atoms. There is a flattened-tetrahedral geometry for one Pd with an average Pd−Se distance of 2.48 Å and a squareplanar geometry for the remaining two atoms with Pd−Se distances of 2.53 and 2.44 Å. The square-planar palladium atoms are involved in Pd···Pd interactions (distance 2.78 Å).125 In a subsequent study, when the same molecular complex 5 was used as a single-source precursor, Pd7Se4 NPs (capped with TOP) were obtained.129 This implies that the composition of D


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complexes. The ratio of palladium and chalcogen in nanoparticles varies (Table 1) with their precursor complex. Table 1. Ratio of Palladium and Chalcogen in NPs

Figure 5. HRTEM images of Pd3Te2 nanoparticles.

entry

complex originating NP

chalcogen in NP

Pd:Se/S

1 2 3 4 5 6 7

9 12 13 14 15 16 17

Se Se Se Se Se Se S

51:49 40:60 3:2 4:5 1:1 38:62 43:57

The common feature in complexes 9−12121 and 13−15122 is the presence of alkyl chains of different lengths in the framework of the ligands. The complex 12 among 9−12 and 15 among 13−15 have shown the highest catalytic activities in their groups. The variation in the activity of complexes with the length of the alkyl chain has been attributed to the difference in the nanosized particles generated in situ in the course of catalysis by 9−15.121,122 The presence of alkyl chains of different lengths in the complexes probably causes the difference. The length of the pendant alkyl chain probably controls the dispersion and composition of in situ generated NPs and consequently the catalytic activity. Energy dispersive X-ray spectroscopy (SEM and TEM EDX) has revealed their chemical compositions (palladium to chalcogen ratio). The size and shape of NPs were found to differ with the complex of their origin. Spherical nanoparticles of average size ∼3 nm were formed in the case of complexes 9 and 12.121 The NPs formed in case of 12 (Figure 6) were

of these nanorods matches with that of the phase JCPDS No. 65-2511 having an orthorhombic unit cell.127 Pd3Te2 is one of the eight binary phases reported to be formed so far by Pd with Te (Pd17Te4, Pd20Te7, Pd8Te3, Pd7Te3, Pd9Te4, Pd3Te2, PdTe, and PdTe2).130 Matkovic and Schubert131 reported its crystal structure. Pd3Te2 is isotypic with Rh3Te2, which belongs to the NiS.r family.131 The binding is of indium type. It is compatible with the bindings reported for Pd20Te7, PdTe, and PdTe2.131 Pd3Te2 has been prepared132 by milling a mixture of Pd and Te in a 3:2 ratio with a planetary-type ball mill (Fritsch P-7) in an aerial atmosphere at room temperature. Its preparation has also been reported by treating a xylene solution of [Pd2(μ-Cl)2(η3-C4H7)2] with solid Hg(TePh)2 at 0 °C under a nitrogen atmosphere.133 However, the NPs of this phase have not been reported so far and hence their formation in catalytic reactions is very important.

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NANOPARTICLES CONTAINING PALLADIUM AND ORGANOCHALCOGEN SPECIES In the recent past some complexes (Chart 2) behaving as precatalysts and dispensing NPs made up of palladium and chalcogen in the course of catalysis of Suzuki−Miyaura C−C coupling have been reported.121−123 These particles are reported to contribute ultimately to the activity of the Chart 2. Complexes Dispensing NPs Containing Palladium and Chalcogen

Figure 6. HRTEM images of nanoparticles generated from 9/12.

found to be much more uniformly dispersed than those formed from 9 (Figure 6). The black NPs formed in the reactions of different aryl halide substrates (Scheme 3) catalyzed with 9 and Scheme 3. Representative Catalytic Reactions for the Isolation of Nanoparticles

E


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and TEM) revealed that they are composed of palladium and chalcogen. The Pd:S ratio in nanoparticles generated from 17 is 43:57, while in 16, the Pd:Se ratio is 38:62. The complex 17, being a palladacycle, shows higher efficiency. It is also somewhat rich in Pd.

12−17, under almost similar reaction conditions, were isolated and analyzed. The size of 80−85% of the NPs obtained from 13−15122 was found to be between 2 and 5 nm (Figure 7). The

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MISCELLANEOUS PALLADIUM SPECIES The generation of Pd NPs and other Pd species has been reported in the case of many S-ligated Pd(II) complexes which have been used as catalysts for Heck and Suzuki−Miyaura C−C coupling reactions in the last 15 years. However, the nature of these species is not well substantiated in any case (Chart 3) reviewed in this section. In the recent past there have been reports on efficient catalysis of the Suzuki reaction by Pd(II) complexes of tridentate [C,N,S] thiosemicarbazone, but a real catalytically active Pd species has not been noted. It appears that Pd(0) is taken for granted in some unspecified form.77 The sulfur-ligated palladacycles are important among Pd(II) complexes of sulfur ligands used as catalysts for the two coupling reactions. For these complexes in situ generation of either Pd NPs or some other Pd species appears to carry forward the actual catalysis. The chemical nature of such species has not been investigated fully, and the formation has been speculated mainly on the basis of limited observations. The occurrence of a catalytic reaction in the presence of n-Bu4NBr, an additive thought to stabilize NPs,134 or lesser conversion of substrates into the cross-coupled products in its absence is one of the main arguments used to propose the formation of Pd NPs or Pd(0) species. Bromobenzene gave a 60% yield of transmethyl cinnamate after a Heck reaction with methyl acrylate for 2 h at a high concentration of catalyst 18 (TON, 60). Interestingly, the use of tetrabutylammonium bromide (TBAB) as promoter increased the rate of this reaction and a TON of 23500 was achieved.92 However, the authors have not attempted to find reasons for this increase in efficiency in terms of the formation of Pd NPs, unlike some others who did so later. The catalytic efficiency of 18 for Suzuki coupling has also been reported to be improved in the presence of the same additive.117 The use of complex 18 as a catalyst in Suzuki coupling was revisited in 2008,117 when aliquots taken from the Suzuki coupling occurring between 4-bromoacetophenone and phenylboronic acid under the optimum reaction conditions (0.5 mol % of 18, DMF, 80 °C, 66% conversion) were analyzed by transmission electron microscopy, and the presence of palladium NPs of average size 3 nm has been reported (Figure 8). However, the chemical nature of the NPs was not clarified. PdCl2(SEt2)2 (19), insensitive to oxygen and water, has been described as a reservoir of catalytically active Pd(0) nano species, when it was used for Heck coupling, because the use of the additive TBAB promoted its catalytic efficiency.94 The activity of these Pd(0) species was so high that even a washed reaction vessel that contained ultratrace amounts of Pd due to previous catalytic reactions was able to catalyze the Heck coupling of PhI with alkenes. The additive TBAB has been reported to be essential to obtain almost complete arylation of the alkenes in the presence of catalytic amounts of 19. For example, in the reaction of iodobenzene and n-butyl acrylate, under the same reaction conditions, only 44% conversion of iodobenzene was observed in the absence of the ammonium salt, while >99% conversion was achieved in the presence of this salt. Thus, it has been proposed that Pd(0) is stabilized as the anionic palladium species45 [Br−Pd−ligand]−. However, the catalytic behavior of 19 for Suzuki coupling is not affected

Figure 7. HRTEM images of nanoparticles obtained from 13−15.

NPs formed in the case of 15 were much more uniformly dispersed than those resulting from 13 and 14 (Figure 7). The catalytic efficiency of a complex catalyst was found to be high when it generated small and more uniformly dispersed NPs. Thus, the higher catalytic activity of 15 relative to those of 13 and 14 may be attributed to the long alkyl chain present in its (SeCNHC) ligand, as this is the only difference between them and may cause better dispersed NPs. A similar reason has been attributed to the better catalytic activity of 12 relative to that of 9, as the linear −C18H37 group is present in the ligand of 12. These NPs have an organochalcogen ligand or its fragment, as supported by weight losses in TGA recorded up to 500 °C (22.56, 47.97, and 67.20% for NPs obtained in the catalysis of Suzuki coupling by 9, 10, and 12, respectively).121 However, the nature of the organic entity has not been established unequivocally by spectroscopic methods. Similarly weight losses up to 200 °C have been observed122 (Table 2) in the Table 2. TGA and Elemental Analyses of Isolated NPs122 analysis (%) complex originating NP

Pd (wt %)

C

H

N

wt % loss in TGA (up to 200 °C)

13 14 15

63.6 54.9 45.2

4.26 6.36 7.55

1.36 1.39 1.36

2.06 1.80 1.82

9.98 14.52 16.73

case of NPs obtained from 13−15, when they were subjected to TGA. These weight losses in TGA and C, H, N analysis results (Table 2) indicate that the NPs are associated with organic matter (most likely the fragment of ligands). The percentage of C in NPs follows the order 15 > 14 > 13,122 suggesting that ligand fragments associated with NPs probably have the effect of the alkyl chain (full or partial) of the original ligand. This corroborates with the effect of the alkyl chain on the characteristics of NPs. The influence of alkyl chain length on the catalytic activity and NP dispersion indicates that the real catalysts are NPs or originate from them. In the course of the Suzuki−Miyaura coupling reaction catalyzed with 16 and 17, black particles appear which suggest that these complexes are probably also precatalysts and dispense real catalyst during the reaction.123 Such species obtained from 16 and 17 during catalysis of the coupling reaction of 4-bromobenzonitrile with phenylboronic acid under optimum reaction conditions were isolated and studied to understand their nature. When they were subjected to SEM, SEM-EDX, and HRTEM they were found to be nanosized and spherical. The size of the NPs obtained from complex 17 was ∼1−2 nm, whereas 16 gave NPs of size ∼1 nm.123 EDX (SEM F


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Chart 3. Complexes Showing the Effect of TBAB on Catalytic Performance

Figure 8. TEM image of nanoparticles formed in a Suzuki reaction catalyzed with 18.

catalytic species.95 In a similar manner complex 21 has also been described as a catalyst precursor, as it catalyzes the Heck coupling of n-butyl acrylate with 4-bromotoluene and 4bromoanisole using relatively harsher reaction conditions (performed at 150 °C using DMA as solvent and NaOAc as the base) in the presence of TBAB as an additive.96 The Suzuki coupling of chloroacetophenone with phenylboronic acid at 1 mol % loading of 22 as catalyst resulted in only a 10% yield after 24 h. However, with the addition of 1 equiv of TBAB to the reaction mixture quantitative results were obtained after 1 day at 0.5 mol % catalyst loading.97 Colloidal Pd species, not properly defined, have been invoked to explain such an increase. 23 is a PdII−(C,S,C) pincer complex bearing two carbene moieties. It catalyzed the Heck coupling reaction of 4-bromotoluene and 4-bromoanisole in the presence of TBAB very efficiently.99 Therefore, in this case also it has been anticipated that the complex decomposes under harsh conditions to palladium NPs responsible for the catalytic action. In the Mizoroki−Heck reaction of n-butyl acrylate/ styrene with 4-bromotoluene catalyzed with complexes of type 24 and 25 in the presence of TBAB, a presumption of decomposition of 24 and 25 at 120 °C to generate NPs has been made.100 The catalytic behavior of unsymmetrical pincer palladacycles 26−33 (Chart 3) for Suzuki C−C coupling reactions presents a conflicting picture98,101,136 regarding the role of TBAB, a salt said to stabilize palladium(0) NPs. For 0.3−3 mol % of 26 and 27, in the absence of the ammonium salt and in its presence the conversions were the same. When these catalysts were used in lower amounts, yields were somewhat lower in the absence of the salt.98 At the same time, TBAB has been found to slightly inhibit Suzuki reactions catalyzed with complexes 28−33.101,136 For example, in the coupling reactions of 4-bromoanisole at a catalyst loading of 0.1 mol % the conversion increased from 77% up to 99% for 28 and from 89% up to 99% for 29 in the absence of TBAB.136 Though complexes 30−33 show high efficiency for Suzuki cross coupling, the yield of cross-coupled product decreases on increasing the catalyst loading beyond a limit.101 The complexes are said to serve as a reservoir of catalytically active colloidal Pd(0) particles which at a high level

significantly by this additive.135 In some cases just the presence of TBAB (also called Jeffery conditions) during the C−C coupling reaction (such as a Heck reaction between 4bromonitrobenzene and n-butyl acrylate/styrene in the presence of 20, 0.2 equiv of Bu4NBr in DMA, and NaOAc as the base) has been presumed self-explanatory regarding the real G


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of catalyst loading stick together (Ostwald ripening), resulting in a decrease in their effective surface area and thereby catalytic activity. On the basis of 31P spectral studies of decomposition of complexes 28 and 29 including the hydrolysis of P−O and P− NH bonds and possible rupture of the Pd−C bonds, the pincer palladacycles have been suggested as mere depots of zerovalent palladium NPs.136 A similar study regarding the decomposition of complexes, carried out in the case of 30−33 using 31P NMR spectroscopy, has been used to propose that they serve as precatalysts of zerovalent low-ligated palladium.101 On the other hand, complexes 26 and 27 have been reported to be unchanged after the cross-coupling reactions, on the basis of monitoring of the reaction mixtures by 31P NMR, excluding the possible exchange of the Cl anion with a bromide anion in the presence of TBAB as a component of the reaction mixture.98 The possibility of a Pd(II)/Pd(IV) catalytic cycle, proposed in such a case, is not convincing in the absence of required support such as detection and isolation of Pd(IV) intermediates. The report that palladacycles 28−33 serve merely as precatalysts and reservoirs of Pd(0) has merit but needs further substantiation, as a decrease in the catalytic efficiency of these complexes at lower concentration (0.001 mol %) is not strong supporting evidence.101,136 34−37 (Chart 4) have been described as precatalysts for the Heck coupling of iodobenzene and styrene, and release of free Pd(0) or its cluster has been described as affected by intramolecular interactions and metal at the center of a porphyrin.104 They exhibited sigmoidal kinetics. As shown by the Weck group,114 the sigmoidal curves observed from (S,C,S)-pincer PdCl-catalyzed Heck reactions can be fitted to a modified version of Finke’s model for transition metal nano cluster formation.137,138 In this model, a key notion is that the rate of product formation will follow the kinetic model for metal particle formation. Thus, sigmoidal kinetics has been employed to support the role of 35−37 as precatalysts.104 Complexes 38−40 have been described to catalyze the Heck coupling between styrene and 4-bromoacetophenone via a heterogeneous mode of action, as elemental mercury stops the reaction.105 The prime function of the pyridylidene ligand in these palladium complexes seems to be the release of Pd, perhaps via reductive elimination. It has been reported that reduction to palladium(0) may be more facile in the monomeric complex 40 than in the dimers 38 and 39, perhaps because the ligand in complex 40 contains a methylated nitrogen, which ensures a more pronounced pyridylidene-type ligand bonding mode. The decomposition of complexes 41 and 42 to Pd(0) during the catalysis of Suzuki coupling has been assumed on the basis of an observation that [Cy3PPdCl2]2 shows higher activity for the same coupling reaction in comparison to its PPh3 analogue.102 The PCy3 ligand is more labile than PPh3 and hence undergoes faster decoordination to form Pd(0) species. A series of palladium(II) thiolate complexes (43−47) were reported for Heck coupling in 2004.66 These species, having a smaller P−Pd−P bite angle, are said to be more reactive. This has been considered enough regarding the understanding of the catalytic cycle. Generally in reports made before 2004 on the catalytic activity of Pd(II) chalcogen ligand complexes the issue of real catalytic species was not vigorously probed. In the case of complex 48,106 no decomposition of the catalyst was observed during catalytic reactions and the performance of the catalyst in a control experiment in the presence of elemental mercury did not change. In spite of all these observations, this complex has been

Chart 4. Complexes Implied as Precatalysts by Kinetic Experiments and Hg Poisoning

described as the dispensor of Pd(0) species due to the deligation of thiols. Thus, the TMEDA ligand in 48 serves as a stabilizing ligand and allows this species to be persistent through the catalysis to perform the C−C coupling efficiently, via a Pd(0)/Pd(II) catalytic cycle.106 H


Organometallics

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49−53 (Chart 5) have been reported as truly homogeneous catalysts for C−C coupling reactions.83,84

Chart 6. Palladium Complexes of Fluorous Organochalcogen Ligands

Chart 5. Complexes Implied as Homogeneous Catalysts

particles with TEM in some cases, it has been proposed that 54−59 are catalyst precursors. They are recyclable and steadystate sources of Pd NPs or ligandless elemental palladium. Complexes 54 and 55 have been reported to catalyze the Suzuki coupling of aryl bromides at room temperature in a solvent mixture of aqueous DMF and CF3C6F11.107 They were found to be reusable up to three times. No black precipitate or other sign of palladium(0) formation was observed during the course of reaction in any cycle. A progressive loss of activity took place, as evidenced by the slower precipitation of biaryl products and, in most cases, lower conversions and yields even after reaction times distinctly longer than those of the first cycle. However, these catalysts have been specifically assayed as catalyst precursors for the Suzuki reaction on the basis of the following: (i) the appearance of the characteristic gray color of palladium black when any given run was allowed to proceed significantly past complete conversion and (ii) an 19F NMR spectroscopic analysis of the catalytic system after the reaction, which suggests that the free ligand, i.e. fluorous sulfide, was present after the catalytic process. The eventual appearance of metallic palladium has envisaged two possibilities: (i) generation of a nonrecyclable heterogeneous or metallic catalyst at a slow rate from a homogeneous precursor, with recycling of the remaining precursor until it becomes exhausted, or (ii) the generation of a fluorous heterogeneous or metallic catalyst that is not very stable or efficiently recycled. The markedly attenuated catalytic activity of the product containing organic phases suggests lower levels of active catalyst, although palladium in catalytically inactive forms may of course be present. The fluorous palladacycle complex 56 has been described as a catalyst precursor for the Heck reaction.108 It is an example of a thermomorphic fluorous compound, with little or no solubility in organic solvents at room temperature but significant solubility at elevated temperatures, thus enabling homogeneous reactions in the absence of fluorous solvents. Its turnover number exceeded 106 for the Heck coupling of phenyl

The anionic complexes 51−5383,84 containing O and S donor atoms along with P, which are known to have flexible coordination modes, may also be placed among the few complexes which do not decompose under the harsh conditions, resulting in reduction of palladium(II) to Pd(0). Addition of a drop of mercury to the reaction mixtures involving 51−53 did not affect the activity. This observation has been made the basis of describing them as truly homogeneous systems.83,84 During catalysis with these complexes, an exchange (Scheme 4) of cation with the base Scheme 4. Formation of K+−Pd− Species83,84

(i.e., K2CO3) to form a K+−Pd− system has been proposed to occur, and it is believed that this system acts like a true catalyst.83,84 The high rate at which these anionic complexes catalyze the Suzuki coupling reaction may also be due to the interaction between K+ and Pd(0) via the halide anion ligated with the latter, as proposed by Jutand and co-workers.45 However, further investigations to understand the effect of such interactions on active catalyst formation in the course of catalysis by such anionic species are required. Fluorous complexes 54−59 (Chart 6) were introduced mainly by Gladysz and co-workers.107−110 These types of complexes constitute a class of recyclable catalysts for one or both of the C−C coupling reactions. On the basis of the reduced activity of complexes in recycling experiments, the appearance of palladium black in the course of the catalytic reaction, the presence of trace palladium metal in cross-coupled product, and the identification of generated palladium nanoI


Organometallics

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iodide and methyl acrylate under homogeneous conditions in DMF at 140 °C. 4-Bromoacetophenone was also coupled successfully, giving a TON value on the order of 105. This palladacycle has been found to be recyclable four times. It precipitates as a bridged halide on cooling after the reaction and can be recovered by liquid/solid phase separation. An induction period in both the first and second cycles of 56 and progressively lower activities have been noted. On the basis of TEM studies (indicating the formation of soluble Pd NPs) of similar catalysis by an imine palladacycle108 and the results of recycling experiments, it has been proposed that soluble colloidal palladium or NPs are the active catalysts in the case of the so-called recyclable palladacycle 56, which is a steady-state source of NPs until exhausted.108 The NPs are nonrecyclable and are generated by the decomposition of some amount of total palladacycle used. Then the remaining palladacycle would be recycled (as the bridged halide), with the diminishing activity representing progressively smaller quantities available for real catalyst generation. This results in an illusion about the recyclability of 56. Complex 57, a light fluorous palladium complex, can be recovered largely intact by solid phase extraction after Heck reactions and reused either as the crude species or after recrystallization109 in contrast to heavy fluorous catalysts (such as 55 and 56) and PEG/resin-bound catalysts. 57 promotes rapid reactions when added in small amounts. For example, it catalyzes Heck reactions of various aryl halides, including aryl iodides and bromides, and a triflate successfully over 30−45 min at 140 °C with yields of cross-coupled products in the range 76−94%. It has been reported that 57 is a precatalyst, releasing ligandless active palladium(0) (real catalytic species) on the basis of results of recovery experiments (Table 3) and

Figure 9. TEM image taken from the DMF phase of the Heck reaction catalyzed by 58.

In the case of a number of soluble and insoluble supported pincer palladium complexes synthesized (Charts 7 and 8) by Chart 7. Immobilized Palladium(II) Complexes of (S,C,S) Pincer and Other Organosulfur Ligands

Table 3. Recovery of 57 in Recycling109

cycle

amt of 57 used (mg)

yield of cross-coupled product (%)

amt of recovered 57 (mg)

1 2 3

41.5 31.4 26.0

94 97 98

31.4 26.0 19.6

the presence of trace palladium (0.45% of the original amount of palladium) in cross-coupled product obtained in the catalytic reaction on a large scale (substrate, 5 mmol; catalyst, 3 mol %). However, the diminishing recovery of 57 and the presence of Pd in the product as a support for catalysis by active palladium(0) appear to be somewhat questionable. Complexes 58 and 59 are effective catalyst precursors for the Heck reaction of iodobenzene and methyl acrylate under aerobic conditions (0.21−0.23 mol % of catalyst, i-Pr2NEt, DMF, 100−125 °C).110 These complexes have been said to dispense palladium NPs in the catalytic coupling process. These NPs are active catalysts and may be reused. They are not fluorous in nature and do not exhibit any fluorous phase affinity. The DMF reaction mixture has been found to be reddish, a characteristic color of colloidal palladium NPs, which are responsible for catalytic activity and have been visualized (Figure 9) with transmission electron microscopy.

immobilizing (S,C,S)-pincer-type Pd(II) complexes onto polymeric and mesoporous silica support through covalent bonds, when used in Heck/Suzuki coupling, palladium(0) formation has been claimed.113−116 Weck and Jones have discussed (for 64−69 and 71; Chart 8) such aspects116 in detail. Palladacycles of (S,C,S) pincer ligands supported on thermomorphic polymers (61 and 62) were found to be stable J


Organometallics

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as supports, proving the methodology based on immobilization of a molecular Pd(II) complex to be very versatile.18,62,65,103 However, the invisibility of elemental palladium in the case of 61 and 62 may be due to its adsorption on the solid support and it is not leached out or remains soluble in the form of palladium halides, such as the bridged [Pd2I6]2− anion in the case of ArI. The formation of such an anion was reported by Evans in 2002140 in the case of the Heck reaction of aryl iodides. The formation of such species was also supported later on with the help of in situ X-ray absorption spectroscopic (XAS) investigations under Heck reaction conditions on palladium-(S,C,S) pincer complexes immobilized on both polymer and silica. The XAS data also do not support the formation of metallic palladium in the case of ArI substrates.115 In 2004, Weck and Jones claimed that tethered Pd-(S,C,S) pincer complex species (64−66) did not directly catalyze Heck couplings of iodobenzene and n-butyl acrylate113 but that all catalysis was due to leached Pd(0) via a Pd(0)−Pd(II) cycle. The various experiments used by them are discussed in the section “Insight into the Role of Nanoparticles in Catalysis”. However, the definitive nature of the soluble Pd(0) species was not established. For amide-linked PdII-(S,C,S)114 pincer ligand complexes (67−69) also, Weck and Jones have reported that they are not recyclable for Heck catalysis and soluble Pd(0) species liberated from them are responsible for their catalytic activity. This is in contrast to reports made on 61 and 62, which are said to be very stable due to an amide linkage. However, regarding the origin of soluble Pd(0) (directly from the complex or from in situ generated NPs) there is no clarity. The basis of soluble Pd(0) as the active species is discussed in the section “Insight into the Role of Nanoparticles in Catalysis”. Tethered Pd(II) complexes of (S,C,S) pincer ligands as reservoirs of catalytically active palladium(0) have also been suggested by Bergbreiter and co-workers112 but without definitive formulation for Pd(0) species. In the case of 70, recyclability for five times with decreased catalytic activity and the results of ICP analysis of the filtrate (after removal of the heterogeneous catalyst) have been considered enough to conclude that a leached Pd (0.2% of original) species is responsible for the catalysis of Suzuki coupling. However, clarity about the Pd species is missing.141 Addition of phosphine to palladacycle 71 (Scheme 5) has increased its catalytic activity for Heck reactions of iodobenzene or 3,5-dimethylbromobenzene with methyl acrylate.112 Two inferences made from this observation are (i) (S,C,S)-Pd complexes are catalyst precursors and there is possibly in situ

Chart 8. Immobilized Complexes Described as Precatalysts

at high temperatures and in the presence of oxygen.17,24 Under high-temperature Heck conditions, these palladated polymers do not decompose and effectively promote the coupling of iodoarenes with olefins. Similar stability has been observed in the case of the heterogenized catalyst 63, and leaching of Pd species is ruled out on the basis of the results of atomic absorption measurements on the filtrate obtained after removing the catalyst from the reaction mixture.139 With palladated polymer 60, on the basis of an (S,C,S) ligand anchored via an ether linkage between the aromatic ring and the polymer, the formation of palladium black has been reported during the Heck reaction, a tell-tale sign of catalyst decomposition, making catalysis by leached Pd possible.24 However, Pd complexes of the (S,C,S) ligand anchored on a support via an amide group (61 and 62) appear to be more stable and catalyst decomposition was not detected.17,24 These seemingly stable polymeric catalysts could be recovered (nearly quantitatively) and recycled multiple times, without any decrease in catalysis rate.17 Several polymers have been used

Scheme 5. Effect of Phosphine Addition on 71: Coordination with the Pd Center by Displacing the Sulfur Atoms

K


Organometallics

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contain polymer-bound palladacycle. It may be due to soluble Pd species.112 Thus, a presumption that it is either colloidal Pd or Pd NPs has been made. However, the identification of palladacycles as the actual catalysts is definitely in question. A palladium deposited single crystal Si(100) surface oxidized terminated with mercaptopropyltrimethoxysilane (MPTMS) has been used as a Suzuki−Miyaura catalyst and examined by XPS before and after catalysis. The catalyst exhibited the highest turnover numbers when Pd(II) species dominated. The most effective catalyst is that which is least prone to the undesirable leaching of Pd: i.e., in which the formation of metallic Pd in the form of nanoparticles is minimized and the number of S-bound Pd species on the surface is maximized.144 In the presence of PCy3, complex 76 shows excellent catalytic activity comparable to that of 71 for Suzuki coupling of a range of deactivated, nonactivated, and activated aryl chloride substrates using 1,4-dioxane as solvent and Cs2CO3 as base.111 In order to gain insight into the process, the reaction between PCy3 and 76 (Scheme 6) was carried out. Complex 77

generation of Pd(0) and (ii) Pd(0) is trapped by phosphine, probably in the form of a complex such as Pd(PPh3)4. Alternately, phosphine may modify (Scheme 5) the Pd-(S,C,S) pincer ligand complex to give 72, which plays the role of catalyst. This alternative is based on the observation that PPh3 coordinates to the palladium center of an (S,C,S) complex.142 However, Pd(0) colloids143 or some other Pd species have not been unambiguously identified as the actual catalysts. It is also possible that 72 reductively eliminates the (S,C,S) ligand to form a phosphine-ligated Pd(0) species that in turn converts 3,5-dimethylbromobenzene to Heck products. The catalytic activity of 73−75 (Chart 9) has been examined using protocols of biphasic thermomorphic reactions.103,112 Chart 9. Complexes Described as Precatalysts on the Basis of Thermomorphic Catalytic Reactions

Scheme 6. Reaction of an Excess of PCy3 with 76 and Formation of a Mixture of Bis- and Mono-Phosphine Adducts

predominated in the mixture of products and has been characterized crystallographically. To understand more about the generation of the truly active catalyst, the reaction of a 76/ PCy3 mixture with phenylboronic acid was examined in the absence of aryl halide (Scheme 7) and it was found that complex 76 rapidly generates Pd(0). This happens via nucleophilic attack of the phenylboronic acid at the palladium center, followed by reductive elimination (Scheme 7). The resulting Pd(0) is trapped by PCy3 as Pd(PCy3)n, which is probably an active catalyst for Suzuki coupling. No activity of palladacycle 76 for coupling of phenylboronic acid with aryl chloride in the absence of added PCy3, even when the catalyst loading is increased to 1 mol % of Pd, has been rationalized in terms of formation of this low-coordinated Pd(0) species.111 A process similar to that shown in Scheme 7 has been reported to occur with the sulfur-containing palladacycle 18 not only in a model reaction which is carried out without aryl halide (Schemes 8 and 9) but also in a catalytic reaction (Scheme 10).117 The formation of palladium black was observed (Scheme 8) within a few minutes even at room temperature. After 20 h at 80 °C, the o-phenyl thioether 1-(1-tertbutylsulfanylethyl)-2-phenylbenzene (79) was formed as the main product (70%), while the thioether (1-tertbutylsulfanylethyl)benzene (80) was obtained as a byproduct

The presence of catalytic activity in the phase that does not contain a polymer-bound palladacycle has been made the basis to propose that real catalytic species are leached from (S,C,S)Pd(II) complexes in the course of catalysis. It is not clear what they are. 73 is an oligo(ethylene glycol)−(S,C,S) pincer− Pd(II) complex which is an air-stable and water-soluble palladacycle type catalyst103 and catalyzes Heck couplings carried out with microwave irradiation over a reaction time of less than 1 h. Recycling of the catalyst was accomplished using a 10% aqueous DMA−heptane thermomorphic system that was advantageously homogeneous during the microwave-promoted reactions and biphasic during the catalyst recovery step. Inductively coupled plasma emission mass spectrometric (ICP-MS) analysis of the heptane phase for each cycle indicates leaching of Pd from the polar phase. The amounts of Pd detected in an aliquot of the heptane phase for cycles 1−4 are 0.133, 0.075, 0.080, and 0.064 ppm, respectively.103 These observations have been used to support the precatalyst nature of 73. 75 has shown residual activity in the phase that does not L


Organometallics

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Scheme 7. Reaction of a 76/PCy3 Mixture with PhB(OH)2 in the Absence of Aryl Halide Leading to the Formation of the Presumed Active Catalyst and Pd Black

Scheme 10. 19F NMR Monitoring of Suzuki Coupling

Scheme 8. Reaction of Palladacycle 18 with PhB(OH)2

on the aromatic ring (Scheme 10) by 19F NMR, for the coupling of 4-trifluoromethylbromobenzene with phenylboronic acid.117 4-Trifluoromethylbiphenyl (cross-coupled product) was detected within the first 0.5 h (δ −64.9 ppm). After 4.5 h the signal for the palladacycle 81 (δ −120.2 ppm) disappeared and a new signal at δ −122.0 ppm appeared due to o-phenyl thioether 82, formed due to coupling between 81 and phenylboronic acid (Scheme 10). tert-Butyl 1-(4′fluorophenyl)ethyl sulfide (analogous to the thioether 80) also formed in a small amount. After the disappearance of the signal of the palladacycle the reaction mixture was transferred to a resealable Schlenk flask and stirred for 3 h at 130 °C. The conversion improved to 70%, implying that the catalytically active species retained activity after the palladacycle was completely consumed. Thus, the palladacycle served as a reservoir of real catalytic species.117 The active Pd(0) species generated from the sulfur-containing palladacycle (18 or 81) could be a soluble palladium(0) or colloidal species. However, in the case of 18 it is suggested that Pd(0) is in the form of NPs, as the aliquots taken from the Suzuki coupling going on between 4-bromoacetophenone and phenylboronic acid in the presence of 0.5 mol % of 18 in DMF at 80 °C have been analyzed by transmission electron microscopy (TEM) and the presence of palladium NPs (average size 3 nm) has been detected (Figure 8). Thus, it appears that sulfur-containing palladacycles decompose to Pd NPs, and an equilibrium between palladium NPs and the most likely active species, Pd(0), exists during the catalytic reaction. Formation of soluble palladium iodide species, i.e. [Pd2I6]2− anion140 (which acts as the primary Pd species and reservoir of Pd(0)), and not metallic palladium has been reported by Weck, Jones, and co-workers with the help of an in situ XAS investigation during the catalysis of Heck coupling of ArI with both polymer and silica immobilized palladium-(S,C,S) pincer complexes.115 However, it is interesting to note that metallic palladium black formation was observed when no aryl iodide was present during the experiment. Although the XAS studies indicated that the primary Pd species in solution was a Pd(II) moiety, the nature of the true zerovalent catalyst was not apparent. There are a number of observations which imply the involvement of molecular species in catalyzing the Heck reaction. Such observations include the observed reactivity and the high concentration of [Pd2I6]2− species in ligand-free Pd catalysts, 46 XAS observations, kinetic and other data

Scheme 9. Pathways for the Formation of Active Pd(0)

(17%). The presence of triphenylphosphine has been found to improve the yield of 79 (92%), and 80 was still formed, but in a lower yield (2%).117 Two possible processes (Scheme 9) for the generation of Pd(0) species from the palladacycle 18 have been proposed. One is similar to that assumed in the case of 76, while the other represents a pathway involving a reduction process and regeneration of the organic thioether ligand 80. These activation mechanisms (i.e., two possible processes) were supported by following the catalytic activity of an air- and water-stable palladacycle (81) containing a fluorine substituent M


Organometallics

Review

Table 4. Suzuki Coupling by Palladium Chalcogenide NPs CH3OC6H4‐4‐Br + PhB(OH)2 → CH3OC6H4‐4‐Ph freshly isolated NPs Pd16S7 size of NP (nm) catalyst loading T (°C) t (h) yield (%)

obtd from 1

obtd from 2

Pd4S

Pd5S2

Pd17Se15

Pd2Se

PdSe

Pd3Te2

∼2 0.1 mol % 100 24 84

∼6 1.0 mol % 100 24 80

∼19 0.01 ga 90 12 85

∼5 0.01 ga 90 12 95

∼8 0.5 mol % 110 10 89

∼2 0.01 ga 90 12 87

∼3 0.01 ga 90 12 97

∼1−2 2.0 mol % 100 15 −

a

Scale of the reaction: 1.0 mmol of 4-bromoanisole, 1.5 mmol of phenylboronic acid, 2 mmol of K2CO3, mixture of 3 mL of DMF and 2 mL of H2O as solvent, temperature of bath 90 °C.

presented,145,146 and the fact that enantioselective Heck reactions are possible with chiral ligands.147 If Pd(0) colloids had been the primary active species, none of these observations would have been expected. It is generally agreed upon that ultimately catalytically active Pd species leach out from palladium complexes whether they are tethered or not. A theoretical study148 has also been carried out on the Heck reaction of 4-bromoacetophenone with styrene catalyzed with a polymer-supported, sulfur-containing palladacycle. It has been derived that in DMF/NEt3 large leaching of palladium species from catalyst would occur and there is no observable leaching in a dioxane/NaOAc system.

Table 5. Suzuki Coupling by Palladium Chalcogenide NPs OHCC6H4‐4‐Br + PhB(OH)2 → OHCC6H4‐4‐Ph NPs (freshly isolated) reaction conditions size of NP (nm) catalyst loading T (°C) t (h) yield (%)

■

Pd4S

Pd5S2

Pd2Se

∼19

∼5

∼2

∼3

PdSe

∼1−2

Pd3Te2

0.01 ga 90 2 92

0.01 ga 90 2 95

0.01 ga 90 2 92

0.01 ga 90 2 96

2.0 mol % 100 15 86

a

Scale of the reaction: 1.0 mmol of 4-bromobenzaldehyde, 1.5 mmol of phenylboronic acid, 2 mmol of K2CO3, mixture of 3 mL of DMF and 2 mL of H2O as solvent, temperature of bath 90 °C.

INSIGHT INTO THE ROLE OF NANOPARTICLES IN CATALYSIS The NPs of palladium chalcogenides, palladium NPs stabilized with organochalcogen moiety or with other species dispensed from precatalyst complexes, may be characterized by TEM, SEM, powder XRD, EDX, etc. and their presence established. However, their role in the course of catalysis is not revealed by these instrumental techniques. For this purpose the independent catalytic activity of isolated NPs has to be monitored and to answer the question of catalytic pathway, i.e. to gain insight into the catalytic process, experiments such as mercury poisoning tests, PPh3 poisoning tests, CS2 poisoning tests, PVPy [poly(vinyl)pyridine] tests, hot filtration tests, and two-/ three-phase tests are important. Activities of Freshly Isolated Nanoparticles. The relationship of activity of the palladium complexes (Charts 1 and 2) with in situ generated NPs of palladium chalcogenides or palladium NPs stabilized with organochalcogen moieties has been established by testing the catalytic activities of many of the isolated NPs independently for selected coupling reactions. They have been found somewhat deactivated in comparison to those generated in situ. The NPs of Pd16S7, Pd17Se15, Pd4S, Pd5S2, Pd2Se, PdSe, and Pd3Te2 isolated fresh were independently examined (Tables 4−6) for Suzuki coupling of aryl bromides under optimum conditions in which the corresponding molecular complex was tested. The results of successful catalysis (Tables 4−6) have substantiated the role of NPs dispensed from complexes 1−8 in the catalytic process of C−C coupling but does not rule out the role of 1−8 as molecular species. However, there are some noticeable differences between the activities of isolated NPs and those of corresponding molecular complexes. The Pd16S7 NPs obtained from 1 and 2 were found to be unable to catalyze the Suzuki−Miyaura coupling reaction of ArCl, while molecular complexes 1 and 2 gave the same catalysis.118 Similarly, yields

Table 6. Suzuki Coupling by Palladium Chalcogenide NPs RC6H4‐4‐Br + PhB(OH)2 → RC6H4‐4‐Ph NPs (freshly isolated) R=H

R = NO2 Pd16S7 reaction conditions catalyst (mol %) T (°C) t (h) yield (%)

Pd16S7

obtd from 1

obtd from 2

Pd17Se15

obtd from 1

obtd from 2

Pd3Te2

0.1

1.0

0.5

0.1

1.0

2.0

100 24 88

100 24 81

110 10 92

100 24 81

100 24 69

100 15 69

of cross-coupled products in the reaction of 4-chloronitrobenzene and 4-chloroanisole with PhB(OH)2 catalyzed with Pd17Se15 NPs were not good. However, the corresponding molecular complex 5 catalyzed these reactions very efficiently with the in situ generation of the same types of NPs.120 The aggregation occurring in the isolation process may decrease the catalytic efficiency of these NPs in comparison to those generated in situ during C−C coupling reaction. The Pd4S, Pd5S2, Pd2Se, and PdSe NPs have also been reported to be somewhat deactivated in comparison to those generated in situ, as their amounts needed for comparable conversions are more than those of the corresponding molecular complexes.119 The reaction of 4-bromoanisole with PhB(OH)2 did not result in the formation of cross-coupled product using Pd3Te2 NPs as catalyst. However, the molecular catalyst 8 has catalyzed the same reaction successfully.127 If the catalytic activities of isolated nanoparticles of palladium chalcogenides (generated from structurally similar molecular N


Organometallics

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(Table 8) indicate that the catalytic efficiency increases with the chain length of the pendant alkyl group. A similar observation was made in the case of NPs obtained from 13−15.122 The NPs formed in the case of 15 are much more uniformly dispersed than those from 13 and 14 (Figure 7). The −CH3 group is present on the ligand framework of 13 and the linear C18H37 group on that of 15. The large size of the alkyl chain in the case of 15 appears to have made the NPs more uniformly dispersed (Figure 7), resulting in its higher catalytic activity. Almost the same correlation exists between 9 and 12. The carbon analysis results of 13−15 have also suggested that ligand fragments associated with NPs appear to have the influence (full or in part) of the length of the alkyl chain of the original ligand.122 This corroborates the influence of the alkyl chain on the characteristics of NPs. The influence of alkyl chain length on the catalytic activity and NP dispersion both suggest that the real catalysts are NPs or originate from them. The lower activity of isolated NPs in comparison to those generated in situ may be attributed to the fact that some Pd may be leached in the reaction mixture before isolation of the NPs. This may also result in lowering of reactivity, which may be further reduced due to some aggregation in the course of isolation. However, the isolated NPs have not been found to be recyclable.122 This may be due to loss of their protection. The catalytic role of in situ generated palladium chalcogen NPs in the case of 16 and 17 has been corroborated by the catalysis of Suzuki−Miyaura coupling (Table 8) with isolated NPs, but with lower efficiency in comparison to those generated in situ.123 4-Bromobenzaldehyde and 4-bromonitrobenzene react smoothly with PhB(OH)2 in aqueous DMF in the presence of these NPs. The NPs obtained from the S analogue 17 considerably convert 4-bromoanisole to the coupled product, whereas in the presence of NPs generated from 16 (i.e., Se-ligated mononuclear complex) no crosscoupled product has been obtained.123 Mercury Poisoning Test. The methodologies/tests for distinguishing between types of catalysis by metal NPs and metal complexes when starting with metal complexes of easily reducible metals such as Pd, Pt, Ru, etc. have been reviewed by Finke.149 By utilizing a combination of such tests, including kinetic investigations, filtration tests, and poisoning studies, in many cases it is possible to have somewhat convincing evidence regarding the nature of the true active species/catalytic process: i.e., homogeneous or heterogeneous catalysis. The mercury poisoning experiment is one of these important tests, and it is easy to perform but is not very definitive and is not universally applicable. No conclusive comment about the nature of the catalytic activity is possible with the help of the Hg poisoning test. The ability of Hg(0) to poison metal particle heterogeneous catalysts, by amalgamating the metal or adsorbing on the metal surface, has been known for many years. Hg(0) is especially effective in poisoning Pt, Pd, and Ni metals by forming an amalgam.150 However, the poisoning effect on molecular homogeneous organometallic complexes containing metals in high oxidation states that are tightly bound by protective ligands is not shown. When a C−C coupling reaction is performed in the presence of excess Hg(0) using a molecular complex, no activity is observed if catalysis is carried out either by soluble Pd(0) species formed by decomposition of the molecular complex under the reaction conditions or heterogeneously by metal particles. Hg(0) interactions with these Pd(0) moieties completely kill the reaction. This test is performed by carrying out a catalytic reaction in the presence of

complexes such as 1, 2, 5, and 8) are compared (Table 7), it is noticed that the difference in catalytic activities cannot be Table 7. Comparison of Catalytic Activities of Palladium Chalcogenide NPs MeOC6H4‐4‐Br + PhB(OH)2 → MeOC6H4‐4‐Ph

a

reaction conditions

Pd16S7 NPsa

Pd17Se15 NPs

Pd3Te2 NPs

catalyst loading (mol %) T (°C) t (h) yield (%) TON

0.1 100 24 84 840

0.5 110 10 89 178

2.0 100 15 none

NPs obtained from complex 1.

attributed only to the size of nanoparticles alone, of different chalcogenides. The activity of Pd17Se15 and Pd16S7 nanoparticles is higher than that of Pd3Te2 NPs. Pd17Se15 and Pd16S7 nanoparticles are able to catalyze the reactions of both electronically activated and deactivated aryl halides, while Pd3Te2 nanoparticles catalyze the reactions of only electronically activated aryl halides such as 4-bromobenzaldehyde and fail in the case of electronically deactivated aryl halides such as 4-bromoanisole. The size of nanoparticles of the three types of palladium chalcogenides follows the order Pd17Se15 (8 nm) > Pd16S7 (2 nm) > Pd3Te2 (∼1−2 nm). Though the Pd3Te2 nanoparticles are the smallest, they are the least active. The isolated nanoparticles of palladium stabilized with organochalcogen fragments obtained from 9 and 12−18 have also been found to show catalytic activity for the Suzuki coupling reaction. The NPs obtained from 12, having the longest alkyl chain (−C18H37) among 9−12, are richer in Se than those obtained from 9.121 The independent catalytic activity of NPs obtained from 12 for Suzuki coupling (Table 8) is higher than that of NPs obtained from 9. However, to reach the efficiency level shown by in situ generated NPs, the quantity of isolated NPs (i.e., mol % of Pd) needed is higher and is expected to some extent to be due to their possible aggregation in the process of isolation. The percent yields Table 8. Suzuki Coupling by Palladium−Chalcogen NPs RC6H4‐4‐Br + PhB(OH)2 → RC6H4‐4‐Ph complex originating NPs

size of NPs (nm)

9 12 13 15 16a 17a

∼3 ∼3 2−5 2−5 ∼1 ∼1−2

9 12 13 15 16a 17a

amt of catalyst

R = CHO 2.0 mol % 2.0 mol % 2.98 mol % 2.13 mol % 50 mg 10 mg R = OMe ∼3 2.0 mol % ∼3 2.0 mol % 2−5 2.98 mol % 2−5 2.13 mol % ∼1 50 mg ∼1−2 10 mg

T (°C)

t (h)

yield (%)

100 100 80 80 90 90

8 8 2 2 3 3

30 94 55 93 89 94

100 100 80 80 90 90

8 8 24 24 3 3

25 91 45 91 75

a

Scale of the reaction: 1.0 mmol of aryl bromide, 1.5 mmol of phenylboronic acid, 2 mmol of K2CO3, mixture of 3 mL of DMF and 2 mL of H2O as solvent, temperature of bath 90 °C. O


Organometallics

Review

Suzuki coupling between 4-bromoacetophenone and phenylboronic acid (0.5 mol % of 18, DMF, 80 °C) has also been studied for the same reaction in presence of excess elemental mercury (100 equiv of Hg(0) relative to 18).117 It was found that the reaction (which gives complete conversion to the coupling product under Hg-free conditions) was terminated immediately after the addition of mercury and only ∼20% conversion was observed. The same experiment was also carried out for the reaction of 4-bromonitrobenzene with phenylboronic acid (in the presence of K3PO4, 0.5 mol % of palladacycle 18, and 100 equiv of elemental Hg). The reaction mixture was stirred at 30 °C but no conversion was observed even after 20 h. A similar reaction without Hg furnished the coupled product in quantitative yield. The suppression of catalysis by elemental mercury has also been used as evidence to propose the NPs (characterized by TEM in the case of 58; Figure 9) as catalysts or catalyst precursors in case of fluorous complexes 58 and 59.110 The nanoparticles have been suggested to serve as steady-state sources of low concentrations of nonfluorous molecular (colloidal) catalysts, which are no longer generated in the presence of mercury. There are some extreme conclusions which have been drawn by different authors on the basis of the results of the mercury poisoning test. A ditopic pincer palladacycle has been reported to function via a mechanism involving Pd(II)/Pd(IV) couples because of the fact that the presence of excess elemental mercury does not influence the yields and product distributions in Heck coupling.80 Complexes 49−53 have been assumed to be truly homogeneous catalytic systems,83,84 as addition of a drop of mercury to the reaction mixtures does not affect the activity of the catalyst. A similar behavior has also been observed in the case of complex 48. Its performance in a control experiment in the presence of elemental mercury did not change. In spite of this observation, 48 has been described as the dispenser of Pd(0) species which are stabilized by the TMEDA ligand. The catalytic cycle of 48 probably goes through a Pd(0)/Pd(II) couple.106 PPh3 Poisoning Test. This test has been reported for both Suzuki155 (catalyzed by a palladium-containing perovskite, LaFe0.57Co0.38Pd0.05O3) and Heck156 (catalyzed by ligand-free palladium acetate) coupling reactions. In the case of catalysis of Suzuki coupling by various Pd(II) complexes of organochalcogen ligands and NPs of Pd16S7/Pd3Te2, it was applied in a manner similar to that of the Hg poisoning test using identical substrates except for the amount of PPh3, which is not taken in as much excess as elemental mercury in the case of the Hg poisoning test. In this test, the yields of cross-coupled products followed the same pattern as in the Hg poisoning test. The complexes 1−4, 6−8, and 13−17 were poisoned (Table 10) by PPh3 during catalysis, and they failed to catalyze the coupling reactions. The same observation was made in the case of NPs of Pd16S7 and Pd3Te2 NPs. These results indicate either that catalysis takes place at the surface of NPs or that the Pd atoms leached into the solution from the rim of NPs catalyze the reaction, and both may be prevented in the presence of PPh3. These results are similar to those of the Hg test and may be explained in a similar fashion. However, the catalysis by 12 could not be killed by PPh3.121 The cross-coupled product biphenyl-4-carboxyldehyde was obtained in ∼100% yield (Table 10) after 24 h of reaction between 4-bromobenzaldehyde and phenylboronic acid catalyzed with complex 12 under optimum conditions. However, with 15, which also has an alkyl

an excess of elemental mercury, and the suppression of catalysis by Hg(0) is usually considered as evidence for heterogeneous catalysis149 or catalysis via a Pd(0) intermediate.54,113 However, recently it has been reported that the efficient inhibition of catalysis by Hg may be caused not only by the amalgamation of heterogeneous or soluble Pd(0) species but also by the decomposition of the homogeneous Pd(II)-containing catalyst, due to its interaction with Hg(0). A palladacycle has been found151 to lead to the formation (due to this interaction with Hg) of the redox-transmetalation product [{κ2(N,C)-L}HgCl], which has been characterized by spectral and X-ray diffraction studies. This test has been used to gain insight into the catalytic process of Suzuki152,153 and Heck coupling154 reactions catalyzed by some non-organochalcogen systems, and the results have been used to conclude that catalysis is carried out neither by leached metal particles nor by soluble Pd(0) species. This result can be regarded as (negative) evidence for reactions involving nanoparticles or soluble palladium species as the real catalysts. In the case of complexes 1−4 and 6−8 the expected crosscoupled products were not obtained (Table 9) in significant Table 9. Mercury Poisoning Tests RC6H4‐4‐Br + PhB(OH)2 → RC6H4‐4‐Ph R

catalyst

OMe

1/2 Pd16S7 NPs 3/4/6/7 8 Pd3Te2 NPsa 12 NPs obtained from 12 13/14/15 16 17

CHO Me CHO

OMe

catalyst loading 0.1 mol 1.0 mol 1.0 mol 0.1 mol 0.02 g 2.0 mol 4.0 mol 1.0 mol

% % % % % % %

t (h)

yield (%)

24 24 2 12 12 24 24 5 3 3

none none none none none >99 >99 5−10 none none

a

Scale of the reaction: 1.0 mmol of aryl bromide, 1.3 mmol of phenylboronic acid, 2 mmol of K2CO3, DMF/H2O (3 mL/1 mL) as solvent, temperature of bath 100 °C.

yields in the presence of Hg. In case of catalysis by of Pd16S7 and Pd3Te2 NPs, the yield of cross-coupled product was negligible (Table 9) in the presence of Hg. Similar observations were made in the case of 16 and 17. However, the results were surprising when this test was performed on a reaction between 4-bromobenzaldehyde and phenylboronic acid catalyzed with complex 12 and isolated Pd−Se NPs (obtained from 12) under optimum conditions. The cross-coupled product biphenyl-4-carboxyldehyde was obtained in ∼100% yield (Table 9) after 24 h even in the presence of excess of Hg.121 If the formation of the product is attributed to the protection of in situ generated and isolated NPs with an organoselenium ligand fragment, the same is expected in case of 15, having an alkyl chain of same length. However, in the case of 13−15 only 5−10% conversion was observed after 5 h of reaction.122 Therefore, it can be assumed in the case of 12 that the atoms at the rim of NPs are involved in catalyzing the reaction and the surface of the NPs is protected by fragments of organoselenium ligands in such a way that the aryl halide molecules can access the surface atoms but elemental mercury does not poison them. The catalytic behavior of 18 which has been reported to dispense the palladium nanoparticles of 3 nm size during the P


Organometallics

Review

protected with organochalcogen moieties may contribute in a homogeneous as well as heterogeneous fashion (Scheme 11).

Table 10. PPh3 Poisoning Test RC6H4‐4‐Br + PhB(OH)2 → RC6H4‐4‐Ph R

catalyst

OMe

1/2 Pd16S7 NPs 3/4/6/7 8 Pd3Te2 NPsa 12 13/14/15 16 17

CHO Me CHO

OMe

catalyst loading 0.1 mol 1.0 mol 1.0 mol 0.1 mol 0.02 g 2.0 mol 1.0 mol

% % % % % %

t (h)

yield (%)

24 24 2 12 12 24 5 3 3

none none none 99

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