Organochalcogen ligands and their palladium(II) complexes: Synthesis to catalytic activity for Heck coupling

Abstract

Heck cross coupling reactions have been catalyzed with several palladium(II)-complexes of organochalcogen ligands. The complexes applied in molecular and tethered forms appear to be dispensers of catalytically active species, but this aspect has not been thoroughly looked at in all cases. The thermal stability, air and moisture insensitivity and ease of handling and synthesis have made them viable alternatives to phosphine/carbene based Pd-catalysts. This review covers developments from the design of organochalcogen ligands and their palladium(II)-complexes to the applications of these complexes in catalyzing Heck reactions.

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Arun Kumar

Dr Arun Kumar was born in 1980 in Ghaziabad, India. He did his MSc in chemistry at C. C. S. University, Meerut and PhD in inorganic chemistry at the Chemistry Department, Indian Institute of Technology Delhi, in 2009, under the supervision of Prof. A. K. Singh. His thesis was on chalcogenated Schiff bases. He is presently working as a Research Associate (CSIR) in the same department and handling research projects on the catalysis of C–C coupling reactions and nano-science.

Gyandshwar Kumar Rao

G. K. Rao, born in 1983 in Deoria (India), studied at St. Andrews College, Gorakhpur University, Gorakhpur (India) for his BSc and MSc degrees. Currently he is pursuing research on “Palladium Catalysts for C–C coupling Reaction” for a PhD degree in the Chemistry Department, Indian Institute of Technology Delhi, under the mentorship of Prof. A. K. Singh.

Ajai Kumar Singh

Dr Ajai Kumar Singh is presently a Professor (Higher Academic Grade) in the Department of Chemistry, Indian Institute of Technology Delhi, New Delhi, which he joined in 1982 as a faculty member. He was Head of the Department from Sep 2009 to Aug 2012. A PhD from University of Delhi (1977), got Post Doctoral training in the research group of Professor W. R. McWhinnie, Aston University (U. K.). His research interests are organochalcogen chemistry, organometallics and catalysis. He has nearly 180 research papers in internationally reputed journals and has mentored 30 plus graduates and post doctoral workers. Presently his research group has more than a dozen members. He is an honorary member of the Science Faculty of University of Delhi. He was President of the Indian Council of Chemists (Inorganic Section) in 2011.

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1. Introduction

The olefination of aryl halides (eqn (1)), commonly called the Heck reaction^1–3 is a process generally catalyzed by palladium derivatives. This reaction, discovered independently by Heck and Mizoroki,⁴ is among the most important organic reactions catalyzed by palladium species. It produces chemicals that are important for the synthesis of pharmaceuticals, natural products^5,6 and fine chemicals.⁷ In recent years excellent reviews^2,3,8,9 on the Heck reaction have appeared. The mechanistic approaches plausible for this C–C cross-coupling reaction have also been discussed at length in these reviews.^2,3,8–11 At high temperatures and in a reducing environment the complexes decompose transiently to the homogeneous Pd(0) phase [colloids, nano-particles (NPs) or clusters],¹¹ which are believed to be the pre-catalyst and form heterogeneous Pd(0), which most likely catalyzes Heck coupling via stable surface defect atoms.^10b It appears that the role of ligands around Pd in high temperature Heck reactions is to control the release of Pd(0) species.¹¹ The ligand free Heck coupling catalyzed with Pd(OAc)₂^10a,11 can be cited to support it. The in situ investigations of species involved in the Heck catalytic reactions by FT-NIR¹² and quick scanning EXAFS¹³ have further strengthened the hypothesis regarding the key role of Pd(0) colloids/nano-particles or clusters in Heck's catalytic process. The predictions regarding the activity of ligands and solvents in homogeneous Heck reactions, and the selection of homogeneous catalysts for cross coupling, have also been addressed by computational methods.^14,15

Ar–X + CH₂ = CH–R → Ar–CH = CH–R + HX (1)

The outstanding performance giving palladium based catalytic systems for the Heck reaction has been designed using sterically bulky and electron-rich phosphine ligands^2,3 or phosphine mimics such as N-heterocyclic nucleophilic carbenes (NHCs).^16,17 Other phosphorous compounds (coordinating through P)^2,3 used for this purpose include phosphites (mono- or polydentate) and phosphinites. Palladacycles formed by ligands containing both phosphorus and nitrogen donors are also among the efficient catalysts for this coupling reaction.^18–20 Among the diverse aryl or heteroaryl halides, (for the coupling of which the Heck reaction is important), aryl chlorides are the cheapest but relatively difficult to couple. The activity of aryl halides generally follows the order ArI > ArBr > ArCl. It is not that Pd(II)-complexes of phosphorous as well as other donors do not show any catalytic activity^21–29 for the coupling of ArCl. Several such complexes are active for them also but with low efficiency. However, a large number of conventional catalytic species for Heck coupling, which are generally phosphorous donor based, are moisture and air-sensitive. The strong donor properties of chalcogens have motivated many research workers to design a number of Pd(II)-complexes of organochalcogen ligands for catalyzing³⁰ the Heck C–C coupling reaction. Such complexes have been found to be much more stable in air and moisture in comparison to Pd-complexes of phosphorous donors. In fact, as catalysts for the Heck reaction, they not only show rivalry with their respective phosphorus analogues but, in many cases, outperform them for the same aryl halide. Thus it was thought worthwhile to take stock of the catalytic activities of palladium(II) complexes of organochalcogen ligands for Heck coupling and perform a critical evaluation of them, as no such attempt has been made so far. The present review is therefore focussed on this subject. An overview of ligands and complexes relevant for Heck reactions is also presented.

2. Organochalcogen ligands in Pd-catalyst design

2.1 General

The chemistry of organosulfur ligands has been explored more extensively than that of organo-selenium and -tellurium ligands, in which interest has built up largely in the last 3–4 decades. Some excellent reviews^31–35 covering several aspects of selenides/tellurides and organoselenium/tellurium compounds have been published. The possibilities of using metal complexes of organochalcogen ligands as single source precursors of semiconductors (e.g. II–VI semiconductor) have contributed to strengthening interest in them. We have focussed here only on those organochalcogen ligands which have been used to design palladium based catalysts for the Heck reaction. Such ligands have been described in three groups: (i) pincer type, (ii) chalcogenated Schiff bases and (iii) chalcogenoethers. These ligands are not commercially available and therefore an overview of the synthetic approaches used for them is given here.

2.2 Pincer ligands

The pincer ligand (Fig. 1) can make two stable chelate rings with one metal centre. Such a ligand is referred as a (Y,Z,Y′) pincer. In a large number of cases, Z is C or N and Y = Y′. Varying Y/Y′ may result in many such ligands. In chalcogen donor based pincers, Y or Y′ can be SR, SeR or TeR. The side arms stabilize the complexes of pincer ligands and are reported to affect the electronic properties of the metal center. The interesting changes in catalytic properties^36–38 have been reported when the Pd–C bond is replaced with the Pd–Si or Pd–P bond (i.e. Z = Si or P). Similarly, when Z = N,^39–42 and it is part of an aromatic system, changes in the activity of the resulting Pd(II)–pincer complex occur. The unsymmetrical pincer ligands, having two different donors in the side arms, have also been reported to vary somewhat catalytic activities of the resulting Pd-complexes.^43,44 However, the effect of the ligand is not believed to be direct in most cases. Most likely it affects the availability of the Pd(0) species recognized as the real drivers of catalysis. The most commonly employed pincer ligands have an aryl backbone and symmetrical side arms. One obvious reason for using symmetrical pincer ligands in designing complexes suitable for catalysis is the relative ease of synthesis of such species. The pincer ligands presented here have S or Se donor sites, as success has eluded⁴⁵ all attempts made so far to synthesize Te based pincers. On synthesis, properties and various applications of complexes of pincer ligands reviews have been published earlier,^46–53 but their coverage of organochalcogen pincers is very low.

Fig. 1 Architecture of a pincer ligand.

Chart 1 shows the chalcogenated pincer ligands that are significant in the context of the Heck coupling reaction. The symmetric (S,C,S) type pincer ligands are generally synthesized via nucleophilic substitution reactions of 1,3-bis(bromomethyl)benzene or related compounds with a suitable ArSH or RSH. The reaction is generally carried out either in DMF or in ethanol in the presence of some inorganic base (i.e. Na₂CO₃, K₂CO₃, EtONa or NaOH) at 70–80 °C.⁵⁴ A benzene–water mixture has also been used as a solvent system in the presence of the phase-transfer catalyst ADOGEN–464.^55,56 Generally, thiophenol derivatives are used as the source precursors of sulphur. However, L2 has been synthesized (Scheme 1) using S₈ as a sulphur source precursor.⁵⁷

Chart 1 Pincer ligands.

Scheme 1 Synthesis of a pincer ligand containing a fluorous alkyl chain.

The fluorous (S,C,S) pincer ligand 1,3-C₆H₄[CH₂SCH₂CH₂ (CF₂)₇CF₃]₂ (L3) has been synthesized⁵⁸ (Scheme 2) using 1,3-bis(bromomethyl)benzene as well as dithiol 1,3-C₆H₄(CH₂SH)₂. The latter is reacted with ICH₂CH₂(CH₂)₇CF₃ in the presence of Na₂CO₃, K₂CO₃ or NaOEt (70–78 °C). L4 (Scheme 3), which is suitable for the syntheses of metallopincer–metalloporphyrin hybrid systems,⁵⁹ has been considered an interesting and novel entry into catalyst modulation because of two beliefs: (i) the electronic properties of a porphyrin can be fine-tuned by the introduction of a metal into its cavity and (ii) the (S,C,S) pincer, being a monoanionic, potentially tridentate and chemically robust ligand, is suitable for synthesizing novel peripherally metalated porphyrins.^59,60

Scheme 2 Synthesis of fluorous ligand with C₁₀ alkyl chain.

Scheme 3 Synthesis of tetrakis pincer with porphyrin core.

The reaction of an appropriate halo compound with RSNa (R = alkyl or aryl) without using an external base also results in a pincer ligand. For example, L5²⁹ is readily formed by the reaction of sodium t-butyl sulphide (NaStBu) with 1,3-bis(bromomethyl)-benzene in a benzene–water two-phase system using a phase transfer catalyst.⁵⁶

The presence of substituents on the aromatic ring at a position meta to the pincer arm may have an effect on the C_Ph–H activation during the palladation process. Similarly, substituents at the other phenyl ring attached to chalcogen donor atoms, particularly at a position para to it, also tune the electronic effects. Thus, several substituted (S,C,S) ligands (L6–L13) given in Chart 1 have been designed (Scheme 4).⁶¹ The precursor dichloro compound, N-acetyl-1,3-bis(chloromethyl)aniline, required for their synthesis, has been prepared by a multistep route from an appropriate dicarboxylic derivative (5-aminoisophthalic acid).⁵⁴ The dihalo precursor is reacted with thiophenol, N,N-dimethylaminothiophenol, p-acetamidothiophenol, 2,4-dimethoxythiophenol, 4-tert-butylbenzene-1-thiol, 4-mercapto benzoic acid and 4-mercaptoanisole in order to obtain L6–L9 and L11–L13, respectively. The Na₂S can also be used as a source precursor of sulphur in designing a thio group containing pincer ligands, as in case of the (C,S,C) pincer ligand L14 (Scheme 5).⁶²

Scheme 4 Synthesis of (S,C,S) pincer having a substituent para to C.

Scheme 5 Synthesis of benzimidazolium salt type pincer.

The synthesis of symmetric selenium containing pincer ligands viz (Se,C,Se) pincer/bis-pincer and (Se,N,Se) pincer involves the route of selenylation³⁰ of an appropriate organic halo compound with diphenyl diselenide under reductive conditions (Scheme 6).^30,63,64

Scheme 6 Selenylation via in situ reduction of diphenyl diselenide.

The ditopic pincer ligand L18, which is also macrocyclic in nature and suitable to design a double ditopic pincer-palladium complex, has been prepared by methodology summarized in Scheme 7.⁶⁵ In this case Lawesson's reagent is used to incorporate the sulphur donor into the ligand.

Scheme 7 Synthesis of the polythioamide-based macrocycle.

Undoubtedly, the large number of pincer ligands used for catalyst design are symmetrical, but palladium(II) complexes of the unsymmetrical ones have been not only found to be promising but also to behave somewhat differently in comparison to their symmetric counterparts while catalyzing Heck coupling. The synthesis of the unsymmetrical type of pincer ligands (Y ≠ Y′) is a considerable challenge, as the methodologies involved are usually comprised of a series of steps and separations, which commonly result in low yields.^66,67 Consequently, the development of efficient and powerful methods for the rapid synthesis of a wide variety of tailor-made unsymmetrical pincer ligands has been considered as an area of high importance. The unsymmetrical pincers are made by designing both arms or by starting from a precursor in which one arm already exists. Unsymmetrical pincer ligand 4-(tert-butylthio)-1-(2-pyridinyl)-1-butyne (L21)⁴³ has been synthesized by a route in which substitution of the mesylated hydroxyl group of the corresponding propargyl alcohol with NaSC(CH₃)₃ was carried out.^68,69 However, using this route only a limited number of unsymmetrical pincers may be designed. An efficient strategy, which may used for the facile preparation of a novel and bigger family of tridentate (Y,C,Y′) type pincer ligands, was reported by Gandelman et al.,⁴⁴ as shown in Scheme 8. This route is based on Huisgen dipolar cycloaddition of azides with alkynes to yield triazoles.⁷⁰ The required azide and alkyne were prepared⁴⁴ from PhS–CH₂–Cl and Ar₂PH–BH₃, respectively. This reaction can be carried out under ambient conditions and with exclusive regioselectivity for the 1,4-disubstituted triazole product, when mediated by catalytic amounts of CuI salt.^71,72 This method is to “click” two monomeric groups with coordinating atoms and functionalized with azidomethyl and propargyl units, respectively, under Sharpless conditions to provide a pincer-type ligand framework. The resulting triazole-based ligand possesses two coordinating “arms” in 1,4-positions and the relatively acidic C–H bond in between. Importantly, in contrast to possible traditional synthetic methods, (Y,C,Y′) pincer ligands are selectively obtained in such an approach, since only this covalent assembly is possible under the conditions of the “click” reaction. The application of this “click” strategy to a separately prepared series of monodentate ligand units functionalized with azido and alkynyl groups, respectively, in a systematic combinatorial manner, may result in a wide variety of triazole-based pincer ligands. A number of required azido- and alkynyl-based monomers can be easily prepared, if they are not commercially available, by simple and short protocols.⁴⁴

Scheme 8 Synthesis of unsymmetrical pincer ligands.

2.3 Schiff base ligands

Metal complexes of Schiff bases are well known as catalysts.^73–76 Schiff bases have been prepared following the traditional template procedure as well as via a template-free synthesis.^77,78 The interest in Schiff bases having sulphur, selenium or tellurium donor atoms stems from their possible role in designing a variety of catalysts, as they contain both ‘soft’ and ‘hard’ donor sites and may behave as hemilabile ligands,^79,80 which allow facile reversible generation of a coordinatively unsaturated metal center.^81,82

Schiff bases containing a thioether donor site have been explored more than selenated and tellurated ones, known scarcely before 1980. In fact, they have picked up mostly in the last two decades only. The chalcogenated Schiff bases used to design Pd(II) catalysts for Heck coupling are summarized in Chart 2. They can be prepared either by treating chalcogenated aldehydes/ketones with amines or by treating chalcogenated amines with aldehydes or ketones. The preparation of a wide variety of chalcogenated carbonyl compounds is not easy. If a chalcogenated amine is treated with aldehyde or ketone, then a wider range of Schiff-base ligands may be designed by varying the carbonyl group containing compound.^83–85

Chart 2 Schiff base ligands.

The first such ligand whose Pd(II) complex was explored for the Heck reaction was a thiosemicarbazone, L23.⁸⁶ It was synthesized by direct reaction of salicylaldehyde with N-ethylthiosemicarbazide in ethanol. For the synthesis of Schiff base ligands, both generalized and specific procedures have been reported. The ligands L25 and L26 were made by a non-generalized route (Scheme 9).⁸⁷ Their precursor compound was not available commercially and had to be synthesized.⁸⁸ The C–N bond lengths [1.253(2)–1.257(2) Å] indicate significant double bond character and the bond angles around the C and N atoms are indicative of their sp² hybridized nature. The ligands L27–L38 were synthesized by a general methodology in which chalcogenated amines H₂N–(CH₂)_n–E–Ar (E = S, Se or Te) were reacted with the corresponding carbonyl group containing the compound. The chalcogenated amines are not commercially available but can be easily synthesized^89–93 by reacting the appropriate halogenated alkyl amines with ArE⁻ (E = S/Se/Te). The ArS⁻ may be generated by reacting ArSH with NaOH and ArSe⁻ and ArTe⁻ (in situ) by reduction of Ar₂Se₂ and Ar₂Te₂, respectively, with NaBH₄.

Scheme 9 Synthesis of four-membered heterocyclic ring based diimines.

The reactions of PhSe–(CH₂)₂–NH₂ and ArTe–(CH₂)₂–NH₂ were monitored by ⁷⁷Se{¹H} and ¹²⁵Te{¹H} NMR spectroscopy. Their ⁷⁷Se{¹H}/¹²⁵Te{¹H} NMR signals were found significantly deshielded on the formation of the corresponding Schiff base. These signals, in the case of PhSe–(CH₂)₃–NH₂ and ArTe–(CH₂)₃–NH₂, remain almost unchanged on the formation of Schiff bases.^84,85

2.4 Chalcogenoether ligands

The chalogenoethers, which neither behave as a pincer nor have a >C=N group, but have been used to design palladium(II) complexes for Heck coupling, are summarized in Chart 3. Most of these ligands can be synthesized by chalcogenylation of the appropriate organic halo-compound with suitable diaryldichalcogenide or dialkyldichalcogenide under reductive conditions, which give ArE⁻/RE⁻ (E = S, Se or Te). Most of the organochalcogen precursors needed, Me₂S₂, Me₂Se₂, Me₂Te₂, Ph₂S₂, Ph₂Se₂ and Ph₂Te₂, are commercially available. Thus, L39–L43 and L46–L52 have been synthesized using this methodology.^30,94,95L44 and L45 have been synthesized by the reaction of NaSCH₃ and PhSLi with 2-bromo-3-(bromomethyl)pyridine.⁹⁶ PhSLi was generated in situ by the reaction of BuLi and PhSH at −78 °C in THF. The functionalized diaryldiselenides, required for the synthesis of L46–L52, were not commercially available, and therefore they had to be synthesized by earlier reported procedures.^97,98 Thereafter, chalcogenylation of alkyl halides (CH₂Br₂, BrCH₂CH₂Br and PhCH₂Cl) with appropriate selenolate, generated by the in situ reduction of diaryldiselenide, was adopted for the synthesis of these ligands also.⁹⁹

Chart 3 Chalcogenoether ligands.

Ligands L53–L65¹⁰⁰ were obtained when PyCH₂Li (generated in situ by the reaction of n-BuLi and PyCH₃) was reacted directly with R′′–E–E–R′′ at low temperature.^100–115 Diethylsulfide, L66, can be synthesized easily¹¹⁶ and is also commercially available. The synthesis of L69 involves two steps. Both the steps were carried out at low temperature (−78 °C). In first step, BrC₆H₄–2-(CH₂PPh₂) was synthesized by reacting 2-bromobenzylbromide with LiPPh₂, and in the second step the resulting bromo derivative was converted to L69 by its reaction with n-BuLi and dimethyldisulfide.¹¹⁷

L70 has been synthesized by treating Se powder with t-BuLi and benzyl bromide.³⁰ However, for synthesis of ligand L71, a multistep process (Scheme 10), starting from N,N-dimethyl-o-toluidine was used. The n-BuLi was used to carry out the extension of the side arm (from CH₃) and introduce the PPh₂ group.¹¹⁸ortho-Lithiation of CH₃ of the precursor by n-BuLi, and subsequent reaction with dimethyl disulfide, yielded a thioether, which was again subjected to lithiation at the benzylic carbon with n-BuLi/TMEDA, and subsequently reacted with ethylene oxide to give a functionalised alcohol. The hydrogen abstraction from this alcohol using n-BuLi, and the subsequent reaction of the resulting alkoxide with PPh₂Cl resulted in phosphinite ligand L71. The L72 in good yield was prepared by the same procedure using benzyl methyl sulfide (C₆H₅CH₂–S–Me).

Scheme 10 Synthesis of hemilabile sulfur-containing phosphinite ligands.

L73 was synthesized in good yield by treatment of PhN(PCl₂)₂ with two equivalents of bis(2-hydroxy-1-naphthyl)sulfide, prepared by the reaction of SCl₂ with 2-naphthol in the presence of Et₃N and a catalytic amount of DMAP (4-N,N-dimethylaminopyridine) or a straightforward reaction of aniline with two equivalents of an appropriate cyclic chlorophosphite of thiobis(2,2′-naphthol), {–OC₁₀H₆(μ–S)C₁₀H₆O–}PCl, in the presence of an excess of Et₃N at −15 °C.^119–121

The synthesis of L74, a chiral and fluorous thioether,¹²² has been started from commercially available p-iodobenzaldehyde. In the first step of its synthesis, this aromatic aldehyde undergoes high-yield Wittig reactions with the readily available phosphonium salt CF₃(CF₂)₇CH₂CH₂ PPh₃⁺I^−123–126 to give an alkene, which reacts with i-PrMgCl to effect an iodine/magnesium exchange. The subsequent addition of fluorous aldehyde CF₃(CF₂)₇CH₂CH₂CHO results in the benzylic alcohol. Thiols are known to condense with benzylic alcohols in the presence of Lewis acids.¹²⁷ Thus, fluorous thiol CF₃(CF₂)₇CH₂CH₂CH₂SH,^128,129 synthesized by using thiourea and the fluorous iodide CF₃(CF₂)₇CH₂CH₂CH₂I¹²⁶ was condensed with the fluorous benzylic alcohol (Scheme 11) to give the triply fluoro-pony-tailed chiral fluorous thioether L74. This synthetic route has also been found suitable for other fluorous thiols.¹³⁰

Scheme 11 Synthesis of the triply pony-tailed chiral fluorous thioether ligand.

3. Palladium(II)-organochalcogen ligand complexes and Heck coupling

Palladium(II) complexes of organochalcogen ligands, explored for Heck coupling, are not commercially available. Therefore, it is essential to have an overview of the complexes first. This will also help in understanding the relationship between the catalytic activity and the molecular architecture (if any). The Pd(II) complexes explored for the Heck coupling are listed in Charts 4–7. The significant aspects of their preparation are summarized below. Their characterization has been carried out spectroscopically and by X-ray crystallography whenever single crystals could be grown.

Chart 4 Palladium complexes of pincer ligands.

Chart 5 Non-palladacyclic pincer complexes.

Chart 6 Palladium(II) complexes of Schiff base ligands.

Chart 7 Palladium(II) complexes of chalcogenoether ligands.

3.1 Complexes of pincer ligands

The complexes of palladium(II) with pincer type organochalcogen ligands have got much attention as catalysts for Heck coupling, in spite of the fact that sometimes their synthesis is not easy and desired modifications in the ligand architecture are also difficult. However, the complexes of some of the (S,C,S), (Se,C,Se) and (Se,N,Se) types of pincer ligands are easy to prepare using some of the commonly used palladium precursors for this purpose, such as Pd[OC(O)CF₃]₂, PdCl₂(PhCN)₂, [Pd(CH₃CN)₄][BF₄]₂ or Na₂PdCl₄.

The C–H activation/cyclometallation is a major strategy for the preparation of palladium pincer complexes,^131,132 which is dependent on the electronic character and bulkiness of the side arms of the ligand (i.e. donor atoms Y/Y′). The C–H activation is carried out using Pd(II) species (sometimes in the presence of a base) and is usually dependent on the ancillary ligands present on the metal. Using salts such as PdCl₂(PhCN)₂, Pd(OCOCF₃)₂,¹³³ and Na₂PdCl₄ in the presence of CH₃COONa,^29,55,56 or [Pd(CH₃CN)₄](BF₄)₂^64,134,135 and PdCl₂(TMEDA)⁴⁴ in the presence of NEt₃, an efficient C–H activation can be achieved. All these salts are commercially available but expensive. However, [Pd(CH₃CN)₄](BF₄)₂^136,137 can be easily generated in situ¹³⁴ prior to the C–H activation process. The optimum conditions for a complexation reaction depends on the Pd(II) species as well as the pincer ligand. Some examples are as follows. The reaction of the fluorous (S,C,S) pincer ligand L3 and Pd[OC(O)CF₃]₂ and PdCl₂(PhCN)₂ at 80 °C directly leads to the Pd complexes 1 and 2 (Chart 4), respectively.⁵⁸ The Pd complex 3 has been synthesized by heating the corresponding ligand L2 with [PdCl₂(PhCN)₂] in a mixture of acetonitrile–benzotrifluoride (BTF) for 48 h.⁵⁷ Refluxing the mixture of Na₂PdCl₄ and CH₃COONa with ligand L5 in EtOH for 25 min results in complex 4.^29,55,56 Unsymmetric pincer ligands L19 and L20 on reaction with [PdCl₂(TMEDA)] (TMEDA = tetramethylethylene-diamine) in DMF in the presence of NEt₃ for 12 h at 70 °C gave complexes 9 and 10, respectively.⁴⁴

The synthesis of complex 11 involves reaction of the methanolic solution of Li₂PdCl₄ with unsymmetrical pincer ligand L21.⁴³ The reaction of ligand L18 with two equivalents of K₂PdCl₄ in DMSO in the presence of the base gives the complex ditopic double pincer palladacycle, 25 (Fig. 2), at room temperature.⁶⁵

Fig. 2 Ditopic pincer palladacycle.

Bis-pincer palladium complex 12 has been synthesized⁶⁴ from L17 using C–H activation with [Pd(CH₃CN)₄][BF₄]₂ in the presence of pyridine, which is required in the case of some other pincer ligands also.

Sometimes in an attempt to prepare the Pd(II) complex of a pincer ligand, the Pd–C bond is not formed. For example, the reaction of L1 {a (S,C,S) pincer} with [Pd(PhCN)₂Cl₂] in refluxing acetonitrile results in a complex of PdCl₂ in which the ligand coordinates with its two sulphur donor sites (13 in Chart 4). The pretreatment of [Pd(PhCN)₂Cl₂] with an excess of AgBF₄ facilitates the formation of the Pd–C bond (14).⁵⁴

The quantitative yields of the pure tridentate pincer ligand–Pd(II) complexes were obtained when a substituent in the ligand at a position para to C going to bond with Pd was present. Treatment of the 5-acetamido-(S,C,S) ligands (L6–L13) with [Pd(PhCN)₂Cl₂] or Pd[OC(O)CF₃]₂ in refluxing MeCN easily yielded complexes 15–21 (Chart 4).⁶¹

The C–H functionalization by “transcyclometallation”¹³⁸ gives pincer-Pd(II) complex 22 (Scheme 12).³⁰ The ancillary ligand Cl on palladium can easily be exchanged with other halides, pyridines, phosphines etc. in order to fine-tune the electron density on the metal centre of the pincer complex.

Scheme 12 Transcyclometallation used for the synthesis of 22.

For pincer ligands having nitrogen, sulphur or phosphorous as a central donor atom, the direct reaction of the ligand with Na₂PdCl₄ or K₂PdCl₄ gives a pincer ligand–Pd(II) complex, as shown in Scheme 13 for the first pincer (Se,N,Se) ligand (L16).⁶³

Scheme 13 Synthesis of Pd(II) complexes of the (Se,N,Se) pincer.

A (C,S,C) pincer-type ligand L14, on reaction with an equimolar amount of Pd(OAc)₂, gives a stable complex 26 (Chart 5) only in the presence of a stabilizing anionic ligand like Br, which is added in the form of KBr.⁶² The trans-[PdCl₂(PAr₃)₂] has also been used to synthesize pincer–Pd(II) complexes (27–32, Chart 5) of L22.^139–142 The NEt₃ is added to deprotonate the SH group.

3.2 Complexes of Schiff base ligands

Palladium(II)-chalcogenated Schiff base complexes (Chart 6, 33–50) explored for Heck coupling have been synthesized by the reaction of Na₂PdCl₄,^83–85 K₂PdCl₄, Li₂PdCl₄ or Pd(OAc)₂ with appropriate ligands at room temperature. Generally the reactions were carried out in a water–acetone (1 : 1) mixture and stoichiometric complexes [PdCl(L–H)] were isolated. They were authenticated by crystal structures^83–85 in some cases. The ligands coordinating in a monoanionic tridentate (E, N, O⁻) mode form a six membered chelate ring through O⁻ and N (azomethine) and a five/six membered ring with E and N.

For selenium and tellurium containing derivatives, the ⁷⁷Se{¹H} and ¹²⁵Te{¹H} NMR spectra were found to be interesting. In the ⁷⁷Se{¹H} NMR spectra of those complexes in which one chelate ring formed around the central metal atom is five membered and the other is six membered, the signals show high deshielding (of the order of 158 ppm) relative to those of the corresponding free ligands.^83–85 When both of these rings are six membered, the deshielding is reduced drastically (of the order of only 7–8 ppm) and in the case of 49, the signal shows shielding (16 ppm) rather than deshielding.⁸⁵ A similar trend is observed in the case of tellurium analogues. In the ¹²⁵Te{¹H} NMR spectra of 41 and 47, the signals are deshielded by 298 and 308 ppm, respectively, relative to those of the corresponding free ligands, but in the case of 44 and 50, the magnitude of deshielding is only 24 and 11 ppm, respectively.^84,85

3.3 Complexes of chalcogenoether ligands

The palladium complexes of chalcogenoethers used for Heck coupling are summarized in Chart 7. They include palladacycles (51, 52, 69–71, 75–78 and 81) as well as coordination compounds (53–68, 72–74, 79 and 80).

A simple and efficient way to synthesize palladacycles is based on the oxidative addition of a Pd(0) species, such as Pd(dba)₂, to the carbon–halogen bond of the pincer pro-ligand. For example, bimetallic Pd-complexes 51 and 52 (which convert easily into monomeric species) can be prepared⁹⁶ by this route as yellow solids.

Li₂PdCl₄,¹¹⁶ Na₂PdCl₄,^94,95 PdCl₂(CH₃CN)₂,^99,100 PdCl₂(C₆H₅CN)₂,^73,118 PdCl₂(COD)^117,119 and Pd(OAc)₂^29,30,56,122 are the other palladium precursors used for the synthesis of Pd-complexes (Chart 7) of this type of chalcogenated ligands. The air and moisture stable PdCl₂(SEt₂)₂ complex was prepared in almost quantitative yield by the direct reaction of lithium tetrachloropalladate(II) with the ligand.¹¹⁶ Sodium tetrachloropalladate(II) (Na₂PdCl₄) has been used in the synthesis of 54–57.^94,95 Generally the reaction of Na₂PdCl₄ with an appropriate ligand gives the complex within 30 min at room temperature.^94,95

Complexes 58 and 59 were readily obtained on reacting [PdCl₂(CH₃CN)₂] with an appropriate ligand (L51 and L52, respectively) in dry dichloromethane at room temperature within 2 h.⁹⁹ For complexation reactions of [PdCl₂(CH₃CN)₂] with the ligands, acetonitrile was found to be a most suitable solvent. For example, complexes 60–71 were obtained on the reaction of [PdCl₂(CH₃CN)₂] with the corresponding ligands in acetonitrile for 12 h at room temperature.¹⁰⁰ The treatment of [PdCl₂(PhCN)₂] in dichloromethane with the ligand L71 gives complex 72 in high yield.¹¹⁸ Similarly for synthesis of complex 73, [PdCl₂(PhCN)₂] was reacted with the selenide ligand in acetonitrile.³⁰ Palladium(II) acetate, another precursor employed, forms complexes 76 and 77 on reaction with ligands L67 and L68, respectively.^29,56 The [Pd(COD)Cl₂] also forms complexes readily and is a good Pd(II) precursor. The 1 : 2 reaction of L73 with [Pd(COD)Cl₂] in the presence of H₂O and Et₃N in THF at room temperature gives anionic complex 79.¹¹⁹ The effect of the palladium precursor on the coordination mode of the ligand and the type of resulting complex has been observed in the case of 1,2,4,5-tetrakis(phenylselenomethyl)-benzene (L17), which forms a non-palladacyclic complex 80 with two seven membered chelate rings on reaction with [Pd(η³–C₃H₅)(μ-Cl)]₂, and a bis-pincer palladium complex 12 on reaction with [Pd(CH₃CN)₄][BF₄]₂.⁶⁴ In the ⁷⁷Se{¹H} NMR spectrum of L17, a signal appears at 361.2 ppm. It shifts to higher frequency by ∼33 ppm on the formation of complex 80 and by ∼35 ppm on the formation of 12. The difference between the shifts shown by the two complexes is insignificant with respect to the variation in size of the chelate rings and donor atoms. The synthesis of some of the complexes of Chart 7 requires a higher reaction temperature. The palladacycle 75 of selenium ligand L70 has been synthesized by its reaction with Pd(OAc)₂ in benzene at 70 °C.³⁰ Similarly, palladacycle 81 has been synthesized by the reaction of L74 with Pd(OAc)₂ in AcOH at 95 °C.¹²² The complex 74 has been synthesized by the refluxing ligand with [PdCl₂(COD)] in acetone for 2 h.¹¹⁷

3.4 Catalytic performances of Pd-complexes taken in molecular form for Heck coupling

Undoubtedly all Pd(II)-complexes of organochalcogen ligands explored for Heck coupling are not equally efficient. However some of them definitely outperform phosphine complexes. Hybrid organochalcogen ligands have been generally used to design Pd(II) complexes for the Heck reaction. However, only a few ligands having a P donor atom in combination with chalcogen^44,117,118 have been explored for this purpose. The common observations regarding palladium(II) complexes of organochalcogen ligands investigated for Heck coupling are that they show (i) relatively good stability in air and moisture than phosphine and carbene complexes, (ii) better TON values for reactions of aryl iodides and activated aryl bromides in comparison to those of aryl chlorides and (iii) good catalytic activity for several unactivated bromides and activated chlorides, but at high temperature in the range of 130–180 °C and often in the presence of additional reagents, which probably help in stabilizing small clusters of Pd(0) in solution (believed to be invariably formed at higher temperatures) and prevent their further agglomeration to inactive species.^{29,43,100,116} The most popular of such stabilizing reagents are quaternary ammonium halides.^143–147

The palladacycles and Pd(II) complexes of pincer type organochalcogen ligands generally show good catalytic activity. Among the complexes of pincer type organochalcogen ligands found promising for Heck coupling, complex 22, having a (Se,C,Se)-pincer ligand, is unique as experimental conditions may be so tuned that it shows very high catalytic activity for the Heck coupling of a variety of aryl bromides including activated, deactivated, and heterocyclic ones with n-butyl acrylate. However, aryl chlorides were found to be almost inactive under similar conditions. Thus with complex 22, TON of the order 2.4 × 10⁵ (70 h) has been achieved for the coupling of PhBr with n-butyl acrylate in DMA. It is interesting to note that the TON of 22 is almost twice that of TON (133 000 in 63 h) obtained when Milstein's (P,C,P) pincer palladium complex is used for bromobenzene–methylacrylate coupling in N-methyl pyrrolidone.¹⁴⁸ The catalytic performances reported for complex 22 also appear to be better than those of 12, a homodinuclear Pd(II) complex of a bis-pincer (Se,C,Se) ligand.⁶⁴ However, 12 required in higher mol% than 22 for comparable conversion completes the reaction in less time. The sulphur analogues of 22, i.e.4²⁹ and 15,⁵⁴ the Pd-complexes of (S,C,S)-type pincer ligands are shown to be active for the coupling of aryl iodides with methyl acrylate, but the reported data on them is insufficient for any conclusive comparison with 22.³⁰ However, 4 catalyzes the arylation of methyl acrylate and styrene with C₆H₅I efficiently (TON up to 48 500 and TOF 9.4 to 31.2 min⁻¹) in the presence of NEt₃ as a base.²⁹

The Pd(II) complexes of fluorous (S,C,S) pincer ligands and palladacycles of fluorous ligands have been found to be very interesting for Heck coupling. Among the species 1, 2, 3 and 81 explored for the coupling of iodobenzene with methyl acrylate in solution (Table 1), the mol% needed is the lowest for 81.¹²² This palladacycle and 1–3 are unique as they have poor solubility or are nearly insoluble in many solvents at 20–24 °C, but become soluble in the same ones at elevated temperatures. This enables catalytic reactions in the absence of fluorous solvents. Further, their precipitation as halo-bridged species on cooling makes them recyclable, of course with a somewhat lower activity. Thus, 81 is not only efficient but can be recovered by a liquid–solid phase separation and reused. At the limiting concentration level, recovery may be difficult and the additional advantage of reusability is mitigated. However, purification of 3 prior to reuse on either a small or large scale is relatively easy.⁵⁷ It is the reusability of these catalysts that makes them different.^57,122 The performance of complex 81 (Table 1) is excellent as a catalyst in solution for Heck reactions of aryl iodide (TON 1.51 × 10⁶ for C₆H₅I) as well as activated aryl bromide (TON 3.01 × 10⁵ for OHC–C₆H₄–4-Br).¹²²

Table 1 Catalytic performances of fluorous Pd(II) complexes

	1	2	3	81
Ar–X	Ph–I	Ph–I	Ph–I	Ph–I
Alkene	Methyl acrylate	Methyl acrylate	Methyl acrylate	Methyl acrylate
Mol%	0.22	0.22	3.0	6.22 × 10^−5
Base	i-Pr2NEt	i-Pr2NEt	Bu3N	Et3N
Solvent	DMF	DMA	DMA	DMF
Temperature	125–135	135	140	140
Time	45 min	60 min	30–45 min	18 h
Yield (%)	94	65	76	100
TON	—	—	—	1.51 × 10^6

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The Pd complexes of several other (S,C,S) type pincer ligands are known since 1980,^55,56 but their catalytic potential has been examined in 2004 for Heck coupling.⁶¹ Recently, such species have been used by Reinhoudt and co-workers as scaffolds to build self assembling metallodendrimers.¹⁴⁹ The Pd complexes of (S,C,S) pincers 15–21 have the distinction of being the first chalcogenated pincer–Pd(II) complexes evaluated as catalysts in solution for the Heck coupling of a variety of alkene acceptors and aryl halides.^54,61 In comparison to the analogous (P,C,P)-pincer based palladium catalysts, their preparation did not involve air sensitive phosphine synthesis.¹⁴⁸ They were found to promote Heck reactions of aryl iodides but not of aryl bromides very effectively. For example, for coupling of activated aryl bromides, 17, 18, 19 and 21 gave TON values of the order of 350–500 only. Even for these low values, the presence of the promoter tetrabutylammonium bromide (50 mol %) with K₂CO₃ as a base, higher loadings of (S,C,S) pincer palladium(II) complexes (17, 18, 19 and 21) and a long reaction time were required for complete conversion of even bromobenzene to a Heck product. In the case of deactivated aryl bromides e.g. 3,5-dimethoxybromobenzene, the conversions were very low (e.g. ∼15% with 19).⁶¹

The palladium complexes (9–11)^43,44 of unsymmetrical chalcogenated pincer ligands like (S,C,P), (S,C,N) etc. have been found to show moderate catalytic activity for Heck C–C coupling. For the coupling of methyl acrylate and bromobenzene in DMF in the presence of sodium carbonate at 140 °C, TON values for (S,C,P) pincer based complexes 9 and 10 were found to be 42 000 and 6 900, respectively.⁴⁴ These values indicate that the catalytic activity of the Pd complex of (S,C,P) pincer is better than those of its (S,C,S) counterparts⁵⁴ but is lower than that of the Pd–(P,C,P) pincer complex.¹⁴⁸ The instability of the Pd–(S,C,P) pincer complexes is not an issue of great concern, as the same turnover frequencies are observed for these catalysts after a reaction time of 24 and 48 h.⁴⁴ The potential of Pd-complex (11) of an unsymmetrical (S,C,N) pincer⁴³ as a Heck catalyst has been probed in the coupling of n-butyl acrylate with 2- and 4-bromotoluene and 4-bromoanisole at 150 °C, using DMA as the solvent and NaOAc as the base in the presence of N(n-Bu)₄Br as a salt additive.⁴³ The yields (9–20%) of the cross coupled products as well as the TON values (900–2000) indicate that the (S,C,N) pincer based palladacycle 11 cannot be considered efficient, even for catalyzing the Heck coupling of aryl bromides.

The catalytic activity of ditopic double pincer complex 25 for Heck coupling has been explored for 4-iodotoluene only. It is reported that the catalyst survives in oxygen and at high temperature (165 °C) and gives good a turnover number in a reaction time of 48 h (up to 94 300).⁶⁵ The survival at 165 °C is somewhat surprising. Although for ArI these TON values do not fall in the range of most efficient catalysts known at present, the catalyst is shown to have the advantage of reusability as it works without any evidence of lowering the activity, at least for 10 cycles. However, full characterization of the catalyst before reuse has not been attempted.

The Pd(II) complexes (23, 24, 26–32)^62,63,139 of chalcogenated pincer ligands that are not palladacycles (i.e. no C–Pd bond) have been investigated for Heck coupling but found to show lower catalytic efficiency than that of 22, a palladacycle with a (Se,C,Se) pincer.³⁰ The catalytic activities of 23 and 24, Pd(II) complexes with a (Se,N,Se) pincer, have been tested at a concentration of 1 × 10⁻³ mol% using n-butylamine as a base.⁶³ The 23 having two palladium centers shows a higher catalytic activity than 24. Although the complex of 23 has two Pd per molecule, its TON is not double the value shown by 24. TON and TOF values depend on the reaction conditions (solvent, base, temperature and time). Thus, comparisons of the results of different research groups are not straightforward. In comparison to the Pd complex of the (Se,C,Se) pincer ligand, which is considered an excellent catalyst,³⁰ TON values achieved by employing 23 and 24 within 24 h reactions look promising. For example, for coupling of bromobenzene with n-butylacrylate, the use of 23 and 24 (concentration 1 × 10⁻³ mol%) gives TON values of 43 000 and 40 000, respectively, in 24 h whereas the value of TON for the same Heck reaction catalyzed with 22 [Pd(II) complex of (Se,C,Se) pincer], known to be among the best catalysts, is ∼2.4 × 10⁵ in 70 h (catalyst loading 3.5 × 10⁻⁴ mol%).³⁰ The Pd(II) complexes (27–32) of (S,P,S) pincers¹³⁹ catalyze the Heck coupling reaction of styrene with bromobenzene. At 1.73 × 10⁻¹ mol% loading of the catalyst in N,N′-dimethylformamide (DMF) and in the presence of Cs₂CO₃ as the base, only 65–75% conversion in 4 h at 160 °C could be achieved. The Pd-complex 26 of the benzimidazole-derived (C,S,C) pincer has not been explored to catalyze the Heck coupling of PhBr, but it gives moderate to good yields of the coupled product for deactivated aryl bromides at 1 mol% catalyst loading.⁶² The Heck reaction of 2,6-dibromopyridine is also successful, giving quantitatively the doubly-coupled product. Thus, it may be appropriate to conclude that Pd-complexes 23 and 24 of the (Se,N,Se) pincer ligand are very good catalysts for the coupling of ArBr among the complexes of pincer ligands that are not palladacycles. 26 may also be considered comparable to some extent to 23 and 24; of course the time required for comparable conversion at similar catalyst loading is much more.

The complexes of Pd(II) with Schiff base ligands are much less investigated for Heck coupling than Pd(II) pincer ligand complexes and/or palladacycles. Among Pd-complexes having Schiff base ligands, high efficiency of the diimine palladacycle system⁸⁷ can be easily gauged from the two observations: (i) complex 34 shows catalytic activity for reactions involving both electronically activated and deactivated aryl chlorides at catalyst loadings of 3–5 mol% and (ii) 96–99% conversions in the reactions employing the electronically deactivated aryl bromides occur at 1.0 mol% catalyst loading and 135 °C, within a moderate reaction time of 4 to 12 h. The palladium complex of salicylaldehyde N(4)-ethylthiosemicarbazone (33) is also promising for the Heck reaction (TON 18 000) of aryl bromides with styrene.⁸⁶ In DMF, the reaction of bromobenzene (catalyst 33: substrate ratio of 1 : 1000) for 24 h at 150 °C using AcONa as a base and in the absence of any promoting additive gives a yield of 54%. It is interesting to note that catalysis with 33 can be performed in the presence of air.

The complexes 75–77,^29,30 palladacycles of chalcogenoethers and complexes of type [PdCl₂L₂] (53¹¹⁶ and 73³⁰) appear to be highly active Pd(II) species for catalyzing the Heck reaction of a variety of aryl bromides including activated, deactivated, and heterocyclic ones with n-butyl acrylate. All of them are remarkably air and moisture stable. The [PdCl₂(SeAr₂)₂], 73, acts as an excellent catalyst for the Heck reaction (TON up to 223 000 for PhBr).³⁰ It is more reactive than the PdCl₂(SEt₂)₂ (53),¹¹⁶ which gives a TON value of the order 450–760, only even in the presence of n-Bu₄NBr as an additive. The complex 73 is also better than palladacycle 75 for PhBr (TON ∼ 120 000).³⁰ However, 75 gave a TON of the order 1.7 × 10⁶ for 4-bromobenzaldehyde and also outperformed its sulfur analogue²⁹76 (TON up to 23 500 and 33 000 for PhBr and 4-bromobenzaldehyde, respectively, in the presence of n-Bu₄NBr). Furthermore, 75 is superior to its phosphine analogues, including Herrmann's P-palladacycle¹⁵⁰ and Gibson's P-palladacycle¹⁵¹ for similar Heck reactions. The high activity of palladacycle 75 was also observed for the Heck reaction of styrene with various aryl bromides.³⁰73 and 75, found to be better catalysts for aryl bromides than 53 and 76, are inactive for aryl chlorides, whereas the later ones show low to moderate activity for activated aryl chlorides. For example, the reaction of 4-nitrochlorobenzene with n-butyl acrylate mediated by 76 gave trans-n-butyl cinnamate in 49% yield. The reaction of chlorobenzene with n-butyl acrylate using 76 as a catalyst is very slow, giving less than 1.5% of trans-n-butyl cinnamate after 21 h (TON = 750) at 170 °C. On the other hand, higher concentrations of 53 (≥0.1 mol%) are required to couple electron poor aryl chlorides in good yields within reasonable reaction times.¹¹⁶ For example, in the reaction [catalyzed by 53 (0.1 mol%)] of 4-chloroacetophenone with styrene in DMA and in the presence of NaOAc as a base and additive n-Bu₄NBr, 87% conversion was observed in the presence of air. However, complex 53 fails to catalyze the Heck reactions of electron-neutral or electron-rich aryl chlorides such as chlorobenzene or chloroanisole.¹¹⁶

The palladacycles discussed for Heck coupling so far have shown generally high catalytic activity, but surprisingly there are examples of such derivatives which show very low activity. For example, 69, 70 and 71 are inactive or poorly active (TON < 1000) for Heck coupling of even aryl bromides.¹⁰⁰ The reaction temperature of 120 °C, the addition of sodium formate^152,153 to promote reduction to Pd(0), Na₂CO₃ base,^5,6,144n-Bu₄NCl as a phase transfer reagent^143,144 and stabilizer of nanoparticles, a high [Cl⁻]/[Pd] ratio^154,155 and the use of anhydrous N,N-dimethylacetamide (DMA) as a polar solvent^156–163 significantly improve the performance of 69–71. Similarly, low activity of dimetallic palladacycles 51 and 52 for the arylation of styrene with bromoacetophenone in N,N-dimethylacetamide (DMA) was observed under various experimental conditions.⁹⁶ No conversion of aryl chlorides was observed, even after a prolonged reaction time. For the coupling of aryl bromides employing 51 and 52 as catalysts, a TON more than 480 (conversion ∼95%) could not be achieved even when the temperature was high (140 °C) and the reaction was carried out for 6 h.

54 and 55, which are not palladacycles but complexes of Pd with (N, S/Se) ligands,⁹⁴ have been found to show good catalytic activity for aryl iodides (TON up to 83 000) but only moderate for aryl bromides (TON up to 37 000). The complexes 56 and 57, which are the Pd(II) complexes of selenated and tellurated benzotriazoles, respectively,⁹⁵ are more catalytically active than 54 and 55 for activated aryl bromides. TON values are up to 91 000 when n-butyl amine is the base used. Palladium complexes 58 and 59,⁹⁹ similar to 54–57 in structure, show comparable catalytic activity for the Heck coupling of 4-bromoacetophenone with n-butylacrylate. It is proposed⁹⁹ that the catalytic activities of isolated complexes are higher than those of in situ generated ones. The reason ascribed to it is that during in situ catalyst generation unreacted PdCl₂ is left, resulting in a lower yield. However it does not seem to be very convincing. Further ligands L46–L52 have been tested in the presence of PdCl₂ for the Heck coupling of various aryl bromides under aerobic conditions. Among them, the (Se,N) bidentate ligands L49–L52 were found to be more efficient in comparison to (Se,O) ligands L46–L48.⁹⁹

Among non-palladacycle Pd(II) complexes (60–66) of substituted 2-organochalcogenomethylpyridines, the catalytic activity of 60 for Heck coupling of aryl bromides and chlorides is very good.¹⁰⁰ The TON value is 43 000 for 4-MeC₆H₄Br and 360 000 for PhBr. The complexes 63 and 65 exhibit moderate activities with yields of 65–85% at 0.1 mol% catalyst loading. Surprisingly, complexes 61, 64 and 66, which contain an (N,Se) donor set have been reported to be almost inactive for 4-MeC₆H₄Br.¹⁰⁰ 4-Chloroacetophenone gave a more than >99% yield at a 1 mol% concentration of 60 under non-aqueous ionic liquid conditions,^23,164,165 in which n-Bu₄NCl was used and DMA, Na₂CO₃ and HCOONa were omitted.¹⁶⁵ These results for 60–66 suggest that for small differences in complexes, a large variation in catalytic Heck coupling may be manifested. 67, which is a trimeric Pd(II) complex, and 68 (a dimeric complex) are either inactive or give very low conversions¹⁰⁰ when used. Thus, a higher number of palladium atoms in a molecule does not always ensure a proportionately better catalytic performance. Such an observation has been made for other systems also, e.g. bis-pincer of (Se,C,Se) type.

The catalytic performance of complexes that have P as a donor along with S has also been evaluated. The efficient exclusion of air and the use of a pure and dry solvent and Heck reactants were necessary with 72,¹¹⁸ but not in case of 74¹¹⁷ as it has high thermal and air stability in both the solid state and in solution. Complex 72 can be applied successfully to catalyze¹¹⁸ the Heck reaction of styrene with several 4-substituted aryl bromides (from electron-rich to electron-poor) in DMF at 130 °C. The reaction time of 24 h and the use of AcONa as a base gives good conversion. Any promoting additive such as n-Bu₄NBr is also not required. However, 72 has failed in the case of 4-bromophenol.¹¹⁸ The phosphino–thioether palladium complex 74 catalyzed Heck reaction of styrene with iodo and bromobenzenes not only produced exclusively trans-stilbenes in quantitative yields, but turnover numbers of the order 10⁶ were observed surprisingly for bromobenzenes.¹¹⁷

The anionic palladium(II) complex (79) of heterodifunctional ligands containing O and S donors along with P has been found to be air stable and moderate for the Heck coupling of 4-bromoacetophenone with tert-butylacrylate. The 0.05 mol% loading of the catalyst is optimal, which gives a TON value of 1540.¹¹⁹ The anionic complex generated with such ligand systems having flexible coordination modes is expected to be more efficient than what these observations reveal as the rate of oxidative addition, a vital step in catalysis, increases.

4. Macromolecule–tethered Pd(II)-complexes and Heck coupling

Immobilization of Pd(II) complexes expected to be active for the Heck coupling reaction on a variety of macromolecular supports has been carried out and the resulting materials are explored as catalysts for such a coupling. It has been done in the hope that this may improve the recyclability of the catalyst, purification of products and commercialization of the catalyst. However, such efforts would be less fruitful if it happens at the cost of catalytic activity of the Pd centre, which may be marred by the attachment of its complex to a macromolecular matrix. It is essential for the success of a macromolecule tethered system that the complex is very robust. Undoubtedly, its properties are bound to vary to some extent upon immobilization on a support.

The nature of the support is important as it may influence the properties of the immobilized complex. So far, Pd complexes immobilized on solid matrices for Heck coupling are those of (S,C,S) pincer ligands. To our knowledge, no Se and Te ligand anchored matrix has been explored for the Heck reaction. The macromolecular supports used for anchoring the (S,C,S) pincer are as follows:

Polyethylene glycol (PEG)

Polyethylene oligomers (PE)

Silica and its variants

Poly(ethylene oxide)

Poly(N-isopropylacrylamide)

Polyfluoroacrylates

Polystyrene

Polyisobutylenes

Polynorbornene

In Chart 8, various species having Pd complexes differing in the (S,C,S) pincer ligand tethered to a variety of supports are summarized. Such supports should contain unreactive bonds (except for the anchoring complex) and should be usable for anchoring a wide range of molecular catalysts. Their temperature dependent solubility is a special property, which may be useful in separating/recycling of the catalytic system. Furthermore, due to solubility, these materials can be analyzed by ¹H NMR spectroscopy and thus monitoring of the course of a reaction becomes simplified.¹⁶⁶ The Pd(II)–(S,C,S) pincer complex anchored PEG species 82 has been prepared by reacting [Pd(PhCN)₂Cl₂] with ligand anchored PEG. The first step in this synthesis involved the coupling of the ligand scaffold to a PEG support.

Chart 8 Macromolecule tethered palladium complexes.

Diethyl 5-hydroxyisophthalate was treated with potassium tert-butoxide and the phenolate so formed was allowed to react with mesylate-terminated 5000 molecular weight (Mn) poly(ethyleneglycol) monomethyl ether [CH₃O(–CH₂CH₂O–)_nCH₂CH₂–OMs]. The product of this reaction, a PEG-bound isophthalate diester, was then subjected to reduction with LAH in THF to obtain a diol. Subsequent treatment of the diol with MsCl in the presence of Et₃N, followed by reaction with excess LiBr, resulted in the dibromo compound. The treatment of this dibromo derivative with PhSK in DMF yielded the desired ligand anchored PEG.⁵⁴ The linkage generated between the support and the complex is ether in case of 82. The Pd(II) complex of L10, synthesized by the reaction of [Pd(PhCN)₂Cl₂] with the ligand, has been reacted with PEG having the NH₂ end group [CH₃O(–CH₂CH₂O–)_nCH₂CH₂–NH₂] to prepare 83, which has an amide linkage between the complex and the support. Purification at each step of the preparation was achieved by recrystallization of the polymeric species with 2-propanol.^54,167 Oligo(ethyleneglycol) bound (S,C,S) pincer palladium complex 84, prepared from 1,3-bis(chloromethyl)benzene by its treatment with CH₃O[CH₂CH₂O]₆–CH₂CH₂–S–C(=O)CH₃, followed by cyclometallation with [Pd(PhCN)₂Cl₂], shows better solubility in water or the polar phase than PEG (its high molecular weight analogue). Its thermomorphic behaviour in mixed solvents is suitable for recycling.¹⁶⁸ It has been used both in aqueous (for water soluble aryl halides) and in mixed phase systems (for water insoluble aryl halides). Recycling of the catalyst was feasible thermomorphically in a 1 : 2 v/v mixture of 10% aqueous DMA and heptane. It is claimed that the tethered complex does not exhibit any visual sign of decomposition on heating up to 200 °C in air or organic solvents/water. For reusing the catalyst, a fresh substrate solution only has to be added.

The (S,C,S) pincer palladium complex anchored on poly(N-isopropylacrylamide) (PNIPAM) (85) is air-stable and can be prepared through the route given in Scheme 14.¹⁶⁹ It is used to catalyze Heck C–C coupling reactions in a liquid–liquid biphasic system made of aqueous DMA (90%) and heptane that exhibits an increase in phase miscibility at elevated temperature, together with solubility of 85, which otherwise has a strong preference for the polar phase at ambient temperature. This type of catalysis is known as thermomorphic catalysis and the solvent system used for it is known as thermomorphic catalyst recovery systems.¹⁷⁰ A particular advantage of 85 is its insensitivity to oxygen.¹⁶⁹ No catalyst degradation was observed, even after five cycles. Thus, time-consuming solvent purification and degassing protocols may be avoided. However, the use of 85 is limited when many reaction products or salt byproducts are polar and thus accumulate in the polar, catalyst-containing phase. Such products and byproducts are not separable from a polar polymer-bound catalyst. For such cases, a non-polar polymer support has been designed to stay quantitatively in the non-polar phase of a biphasic mixture (an inverse thermomorphic separation). This will facilitate the easy separation of polar products from the catalyst and its recovery. 86 (Scheme 15) is such a catalyst, having a (S,C,S) pincer–Pd(II) complex loaded on poly(N-octadecyl-acrylamide-co-N-acryloxy-succinimide) (PNODAM), which is soluble in the nonpolar phase of a mixture of polar and non-polar solvents (DMA and heptane) and used for Heck coupling. In 86, PNIPAM's N-alkyl group is a more lipophilic octadecyl group, which inverts the polymer solubility so that the polymer would selectively dissolve in the non-polar phase of a biphasic mixture but stay in solution at elevated temperature.¹⁷¹ Thus, use of 85 and 86 achieves easy separation of the catalysts from products, by tuning the preferential solubility of the catalyst in polar or nonpolar solvent systems, respectively.

Scheme 14 Synthesis of PNIPAM tethered (S,C,S) pincer-palladium complex.

Scheme 15 Methodology for tethering of a pincer complex on PNODAM.

Polyisobutylene (PIB) oligomer based catalyst 87 was synthesized by attaching 3,5-bis{(p-methoxyphenyl)thiomethyl}aniline, a (S,C,S) pincer ligand, with PIB oligomer –(CH₂–CMe₂–)₂₀–CH₂–CH(CH₃)–CH₂CH₂–COOH, via the terminal COOH group followed by cyclopalladation with Pd(TFA)₂. 87 is selectively soluble in the non-polar phase and the oligomer matrix is an alternative to a terminally functionalized PEG oligomer support. The activity of the pincer complex attached to an oligomer is analogous to that of other soluble polymers or low molecular weight catalysts.¹⁶⁶87 is readily recoverable in the non-polar phase by a liquid–liquid separation either after cooling of a thermomorphic solvent mixture or by perturbing a latently biphasic solvent mixture.

88 ¹⁷² is a heterogeneous catalyst, but during catalysis all the Heck activity comes from leached soluble palladium species. However, it is not unambiguously clear whether the leached species result from pincer ligand–Pd bond rupture or via Si–O–Si bond rupture. It has been synthesized (Scheme 16) by covalently immobilizing the (S,C,S) pincer palladium(II) complex functionalized with an Si(OMe)₃ end group onto SBA-15 (mesoporous silica support), utilizing a reaction between the –Si(OMe)₃ group with surface silanols to form Si–O–Si bonds with the concomitant release of methanol (Scheme 16). The remaining accessible surface silanols were capped by reacting at room temperature with excess hexamethyldisilazane. A typical loading in 88 is approximately 0.12 mmol Pd per gram of SBA-15.

Scheme 16 Immobilization of pincer complex on SBA-15.

The (S,C,S) pincer–Pd(II) complex loaded porous silica 89¹⁷³ was prepared by generating an amide or urea linkage. Reaction between a 3,5-bis(phenylthiomethyl)aniline with 3-isocyano-propyltriethoxysilane [(EtO)₃Si–(CH₂)₃–N=C=O] followed by palladation with Pd(PhCN)₂Cl₂ gave an (S,C,S) pincer complex with a Si(OEt)₃ end group in quantitative yield. This complex was covalently immobilized onto SBA-15 (mesoporous silica support), utilizing the reaction between the –Si(OEt)₃ group and silanol groups present on the surface, resulting in Si–O–Si bonds with the concomitant release of ethanol. A typical loading for the SBA-15 supported complex 89 is approximately 0.10 mmol of Pd per gram support. Extensive washing with DMF and CH₂Cl₂ and filtration tests¹⁷² indicated that essentially all the (S,C,S) pincer–Pd(II) complexes in 88 and 89 are covalently bonded to the silica surface and there are no physisorbed species left behind.

Similarly, 90,¹⁷³ which is an insoluble Merrifield resin-supported pincer–Pd complex, has been synthesized by treating 3,5-bis(phenylthiomethyl)aniline with isocyanate modified Merrifield resin, followed by metallation with [Pd(PhCN)₂Cl₂] in refluxing MeCN.

The poly(norbornene) based 91¹⁷³ is a close replicate of PEG supported material such as 83,⁵⁴ as both polymer supports have an amide linkage (in contrast to the urea linkage in 89 and 90). It was synthesized in five steps starting from 3,5-bis(phenylthiomethyl)aniline and cyclobutadiene (Scheme 17). In catalytic reactions, 91 goes into solution at elevated temperature (120 °C). Another polynorbornene-based system is 92, which contains a flexible C₁₁ alkyl chain between the backbone and the terminal palladated (S,C,S) pincer ligand.¹⁷⁴ It is reported that this chain was introduced to decouple the side-chain from the polymer backbone and to increase solubility. During its synthesis, first of all a monomer is synthesized by the reaction of CH₂=CHCOCl and cyclopentadiene. This monomer is condensed with 11-bromo-1-undecanol to obtain a bromoester, which is further reacted with 3,5-bis(phenylthiomethyl)phenol, a hydroxy functionalized (S,C,S) pincer ligand, in the presence of K₂CO₃. Palladation of so formed species with [Pd(CH₃CN)₄][BF₄]₂¹⁴⁹ gives a monomeric cyclopalladated pincer complex. Its polymerization in the presence of a ruthenium catalyst gave 92.

Scheme 17 Synthesis of polynorbornene based precatalyst 91.

All these tethered complexes have been applied to catalyze reactions (Table 2 and 3) of mainly aryl iodides, but TON values are generally ≤1000. The main advantage claimed for using them is that they are recovered quantitatively and can be reused in multiple reaction cycles. However, before reuse it is not fully ensured whether they are in original form or not. 88, 89 and 90 are insoluble in all common solvents and probably function as pre-catalysts only. The Pd(0) released from them has been described as a true catalyst. It is very likely that after the catalytic cycle, Pd(0) is re-deposited on the surface and catalyzes the Heck reaction in the next reuse. The evaluation of (S,C,S) pincer–Pd(II) complexes covalently bound to polymer supports and soluble in water/polar solvents (like DMF, DMA) or in non-polar solvents (like heptane) for Heck coupling reveals that tethering on different supports does not significantly change the % yield and TON values. Similarly, no effect of support on catalytic activity is observed if the catalyst system having loaded the Pd-complex is of an insoluble nature.

Table 2 Catalytic performances under variable reaction conditions

	82	83	84	85	86
a First cycle. b Second cycle. c Third cycle. d Fourth cycle.
Ar–X	Ph–I	Ph–I	Ph–I	Ph–I	Ph–I
Alkene	Methyl acrylate	Methyl acrylate	Methyl acrylate	t-Butyl acrylate	Acrylic acid
Mol%	0.1	0.1	0.1	0.2	0.2
Base	Et3N	Et3N	Et3N	Et3N	Et3N
Solvent	DMF	DMF	DMA	90% aqueous DMA/heptane	Heptane/DMA
T/°C	110	115	150	95	100
Time	4–6 h	6.5 h	10 min	10 h	24 h
Yield (%)	83	85^a	74	80^a	96^a
		87^b		98^b	99^b
		95^c		>99^c	99^c
				>99^d
TON	1000	—	—	—

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Table 3 Catalytic performances under variable reaction conditions

	87	88	89	90	91	92
a First Cycle. b Second Cycle. c Third Cycle.
Ar–X	Ph–I	Ph–I	Ph–I	Ph–I	Ph–I	Ph–I
Alkene	Methyl acrylate	n-Butyl acrylate	n-Butyl acrylate	n-Butyl acrylate	n-Butyl acrylate	t-Butyl acrylate
Mol (%)	1	0.13	—	—	—	—
Base	Et3N	Et3N	Et3N	Et3N	Et3N	Et3N
Solvent	DMA/heptane	DMF	DMF	DMF	DMF	DMF
T/°C	100	120	120	120	120	120
Time	24 h		2 h	25 min	1 h	3.5 h
Yield (%)	63^a	99^a	93^a	92	99	>99
	98^b	97^b	92^b
	99^c	94^c
TON	—	718–764	—	—	—	1000

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5. Propagation of active palladium species from molecular pre-catalysts in Heck coupling

To better appreciate Heck coupling reactions catalyzed with palladium(II)-organochalcogen ligand complexes, it is important to briefly review the functioning of other catalysts known for the Heck reaction, a subject of intense investigation in the past one and a half decades, in which many catalysts have been developed, including palladium–phosphine/carbene complexes. Those showing promise prominently among them are palladacycles, pincer ligand complexes, heterogeneous palladium catalysts, palladium colloids and ligand-free palladium, e.g. Pd(OAc)₂.¹¹ The typical working temperature for many such complexes is between 120 and 160 °C and it has been shown that none of these catalysts is stable at these high temperatures. They all have a tendency to form palladium clusters, colloids or nanoparticles,¹¹ which have the Pd(0) responsible for carrying out the Heck reaction. The attack of the arylating agent on the palladium atoms in the outer rim of these species takes place, setting up the catalytic cycle (Scheme 18). Also, in case of ligand-free palladium catalysts such as Pd(OAc)₂, Pd(0) species have been found important for the Heck catalytic process.^10a,11,175 In the case of heterogeneous catalysts having anchored Pd, the metal is dissolved from the surface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the aryl halide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redeposited onto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution–redeposition processes correlate with the reaction rate and are strongly influenced by the reaction conditions.¹⁷⁶ By monitoring the Heck reactions with FT-NIR spectroscopy and quick scanning EXAFS, the formation and involvement of Pd(0) containing species in the Heck process has been supported.^12,13 Thus, the role of Pd(0) containing nano-particles, clusters or other forms appears to be unequivocally established in the case of high temperature Heck catalytic processes reported using palladium complexes of non-chalcogen ligands.

Scheme 18 Transformations of palladium in Heck reaction of aryl bromides.

The ligand free palladium species, such as Pd(OAc)₂, based cross coupling processes, said to be industrially viable, also need to be mentioned here, as the role of Pd-halide anionic species has also been invoked in their case. For ligand-free catalysis some advantages reported are: (i) possibility of faster catalysis, (ii) easy work-up and (iii) low cost. De Vries and co-workers carried out studies on using ligand free systems such as PdCl₂ and Pd(OAc)₂.^10a,11,175 They have studied the C–C bond formation reaction of butyl acrylate with benzoic anhydride (used as the arylating agent) using PdCl₂ as the catalyst in the presence of sodium halide.¹⁷⁷ At the onset of the reaction, rapid formation of Pd(0) was observed with the help of EXAFS. The presence of a number of anionic monomeric (such as PhPdCl₂⁻) and dimeric palladium species was also noticed during ES-MS analysis.¹¹ Thus a very large amount of the palladium is not only present in the form of soluble nanoparticles, but there is also a role of monomeric or possibly dimeric anionic species in the actual catalysis. The halide salts added have in fact two fold utility. They stabilise the colloids, preventing their further growth to palladium black and provide halide ligands that coordinate with palladium, resulting in anionic intermediates that also drive the actual catalytic cycle.¹¹ Similarly in the course of catalysis of the Heck coupling reaction between iodobenzene and n-butyl acrylate using palladium acetate as the catalyst, the presence of PhPdI₂⁻ and PdI₃⁻ was detected using ES-MS by de Vries and coworkers.¹⁷⁸ Further investigations by Evans et al. using EXAFS showed the presence of large amounts of the dimeric species Pd₂I₆⁻ in this reaction.¹⁷⁹ During the course of such coupling reactions of aryl iodides, hardly any palladium colloid was observed (of course for a lower substrate to catalyst ratio it may appear) because oxidative addition is fast with ArI. However, at the end of the reaction palladium nano-particle formation takes place rapidly, eventually leading to the formation of palladium black.¹¹ In the case of aryl bromides the picture changes dramatically. The oxidative addition of aryl bromides is slower than that of aryl iodides and this now becomes the rate-determining step. As a consequence, most of the palladium will be in the form of Pd(0). It is quite likely that under these circumstances colloid formation will be the faster reaction. If the nanoparticles grow beyond a certain size, palladium black formation will occur and the Heck reaction stops (Scheme 18).

Dupont and co-workers have shown that in the presence of an ionic liquid the sizes of the nanoparticles at the beginning and the end of the Heck reaction are significantly different.¹⁸⁰ To explain this, de Vries¹¹ has suggested that oxidative addition of bromoarene takes place continuously at the outer rim of the nanoparticles and is followed by detachment of the anionic complex from the colloid. Of course it is difficult to pinpoint the stage of detachment from the colloid. It is possible that ArPdBr₂⁻ or its dimer is the leaving species. The detachment may also be assisted by reaction with the olefin.

It also seems very unlikely that the entire catalytic cycle takes place on the surface of the colloids, as not enough coordination sites would be available to complete the cycle. ES-MS study has revealed the presence of minor amounts of PdBr₃⁻ in solution. Thus most of the palladium remains locked up in the colloidal form.¹¹

6. Palladium complexes of organochalcogen ligands: real or pre-catalysts

The mechanistic aspects of Heck reactions catalyzed by palladium complexes of non chalcogen ligands of diverse types have been studied in detail and the role of Pd(0) colloids or nanoparticles is strongly advocated and supported experimentally, as mentioned above. Furthermore, the debate on the actual role of a complex in Heck C–C coupling reactions may be concluded in a large number of cases in the following manner. The real catalysts are the Pd(0) species and the complexes are their dispensers. The ligands appear to play a role in controlling the rate of formation of Pd(0) species. Collaterally to these studies, mechanistic aspects of the Heck reaction catalyzed by Pd(II)-complexes of organochalcogen pincer ligands have been studied, largely in the case of organosulphur ligands. On the basis of several investigations,^{19,58,59,134,172,173,181} a common conclusion for palladium complexes of organochalcogen ligands is that such pincer complexes decompose under the applied (often harsh) conditions, and the catalytic activity arises due to heterogeneous Pd(0), formed probably from homogeneous Pd(0) generated from the complex. Furthermore, it is also believed that involvement of Pd NPs in the Heck reaction may not require such a high temperature as in the Suzuki reaction. Their role is shown at low temperatures also.^10b Thus, for Heck coupling reactions, the efficiently performing pincer complexes are not direct catalysts but dispensers of palladium(0) containing species of high activity. The control of ligand architecture on rate of formation and nature of these small sized species (as mentioned in the previous section) appears to be a reason for the variation in catalytic efficiencies with the ligand. The formation of palladium nano-particles or Pd(0) catalysts can be detected using various methods, which have also been employed several times in the case of organosulphur ligands. However, precipitation of palladium-black (amorphous metallic Pd) may be taken as an indication for the decomposition of the applied pincer complex catalyst. Some observations that throw light on the mechanistic aspects of Pd-complexes of organosulphur ligands are as follows: Rocaboy and Gladysz¹²² have reported that palladacycle 81 acts as a catalyst in the coupling of phenyl iodide with methyl acrylate in the presence of Et₃N in DMF at 140 °C via a Pd(0) species (non recyclable) which bleeds out of the complex, which is found to be recyclable but with diminishing activity. The recycled palladacycle, upon use gives fewer active catalytic species, resulting in lower activity. Gladysz and co-workers,⁵⁸ in the cross-coupling reaction of phenyl iodide and methyl acrylate in the presence of 1, have also supported formation of palladium(0) nanoparticles on the basis of the reddish colour of the reaction mixture and TEM investigations. Curran and co-workers⁵⁷ found that in the reaction of 4-bromoacetophenone and methylacrylate catalyzed by 3 in presence of n-Bu₃N in DMA at 140 °C, traces of palladium (0.45% of the original amount of palladium added as complex 3) were present in the cross coupled product. In the case of 53, the washed reaction vessel and the magnetic stirring bar were found to contain the ultra trace amounts of residual palladium, which has been detected by ICP-MS analysis,¹¹⁶ and therefore this vessel was also found to show some activity for the Heck reaction after the first run. Thus, the organochalcogen ligand assisted Heck reaction may have the role of Pd(0) nanoparticles, and the classical mechanism shown in Scheme 19 may be an operative pathway. Unfortunately, the issue of the real catalyst has not been studied or commented on in the context of a large number of palladium(II)–organochalcogen ligand complexes explored for Heck coupling in solution. They include Schiff base complexes, as well as those designed by using other chalcogenoethers. In the case of Pd(II) complexes of (S,C,S) pincer ligands tethered on thermomorphic polymers, the observations that may lead to a decision about active species for catalysis are of diverse types. Some polymer-supported complexes have been surprisingly reported to be stable at high temperatures and in the presence of oxygen. For example, it has been reported that 85, having a Pd-complex of (S,C,S) pincer ligand linked via an amide group,¹⁶⁹ does not show any sign of decomposition.⁹ It could be recovered (nearly quantitatively) and recycled multiple times. No decrease in catalysis rate was observed during its reuse. This is probably due to the large amount of anchored Pd. On the other hand 82, based on a polymer anchored with (S,C,S) ligand⁵⁴via an ether linkage between the aromatic ring and the polymer, was found to form palladium black⁹ in the course of the reaction. This formation is a tell-tale sign of catalyst decomposition and shows that catalysis probably occurs by leached Pd(0).⁹ Bergbreiter and co-workers^54,61 and Jones and Weck^172,173 have also shown independently that active soluble palladium leaches out to catalyze the Heck reaction when some immobilized (S,C,S) pincer–Pd(II) complexes (82, 86, 88, 89, 90, 91, 92 and 93) were used as the catalyst. The mercury test supported the formation and the direct involvement of colloidal Pd(0) species (and not pincer complexes) in many coupling reactions.^{58,61,172,173,181} Further poly(vinylpyridine) (PVPy) test for the several coupling reactions^{58,172,173,181} also excludes catalysis by palladium complexes tethered on supports. Furthermore, the possibility of a Pd(II)–Pd(IV) catalytic cycle in the case of the Pd(II) complex tethered on supports appears to be unlikely.

Scheme 19 Pd(0)–Pd(II) catalytic cycle for Heck coupling.

Weck, Jones and co-workers^{134,172,173,181} have proposed the role of Et₃N in the formation of the Pd(0) species. The high strain of complexes of palladium(II) with pincer ligands, resulting from their distorted-square-planar configuration makes them prone to cleavage Scheme 20.¹³⁴ Theoretical calculations have revealed that the one-arm-off configuration is only 7.0 kcal mol⁻¹ higher in energy than that of the (S,C,S) pincer–Pd(II) complex in the presence of an amine base. Furthermore, removal of the second arm is calculated to be downhill, relative to the one-arm-off configuration by 0.6 kcal mol⁻¹.¹³⁴ These calculations suggest that the possibility shown in Scheme 20 is feasible. It appears to be favourable in the case of (S,C,S) pincers in comparison to (P,C,P) ones, as palladium black formation occurs by adding only 1 equiv triethylamine to a poly(norbornene)-supported (S,C,S) pincer palladium complex, whereas 7 equiv triethylamine is required for visible decomposition in the case of the (P,C,P) pincer.^47,54 Generally the Pd–P bond is considered stronger than the Pd–S bond, which is consistent with this observation. Most probably the pincer ligand has practically no effect on the Heck reaction, except on releasing the catalytic Pd(0) species.

Scheme 20 Suggested pathway for dissociation of the palladium(II) pincer complex.

Van Koten, Klein Gebbink and co-workers⁵⁹ have shown that pincer–porphyrin complexes 5–8 have different catalytic activities depending on the metal atom (M) coordinated to the porphyrin ligand. Based on kinetic and spectroscopic studies, it has been concluded that the reaction is catalyzed by Pd(0) species arising from decomposition of 5–8. However, the decomposition rate, and thus the amount of catalytically active Pd(0) species, were probably controlled by the electronic effects of the metal atom present in the porphyrin ligand. The catalytic activity of 5–8 was found to increase in the order M = MnCl < 2H < Ni < Mg, which coincides with increasing electron density in the porphyrin ring.

The anionic complex 79 containing O, N, or S donor atoms along with P does not seem to decompose during reduction of palladium(II) to Pd(0). This complex is reported to have a high rate of oxidative addition, a vital step in homogeneous catalysis. A mechanism (Scheme 21) involving the Pd(0)/Pd(II) cycle is postulated in this case, in which the Pd–P bond is preserved. During catalysis the anionic complex 79 exchanges the cation with the base (i.e. K₂CO₃) to form [Pd-chloro complex]⁻[K]⁺ (94), causing faster oxidative addition due to the presence of K⁺.¹¹⁹ The metal cation interacts with the halide anions ligated to palladium by ion pairing, to afford the more reactive Pd(0) complex 96. In this complex, coordination with sulphur provides a temporary platform for the active catalytic species during an oxidative addition step and also extra stability to the catalyst precursor. Addition of a drop of mercury to the reaction mixtures involving 79 did not affect the activity of the catalyst, thus showing them to be truly homogeneous systems.

Scheme 21 Plausible catalytic cycle for the cross-coupling reactions.

It is claimed that the ditopic pincer palladium complex 25 remains active for at least 10 additional cycles, with no evidence of the metallic palladium deposit. Even the influence of elemental mercury, known to poison heterogeneous Pd(0) catalysts, was also not observed on yields and product distributions. On this basis, the author's assertion that colloidal palladium was not involved in catalysis by 25 needs further verification.⁶⁵ Phosphino–thioether palladium complex (74) has also been claimed not to decompose during catalysis on the basis of ³¹P analysis of the reaction mixtures. However, this conclusion will become invalid if only a small fraction of the complex is converted to Pd(0) species; the signal in ³¹P NMR may remain intact. However, the halide of 74 undergoes exchange during the course of catalysis.¹¹⁷ In addition to the Heck reaction, the generation of Pd(0) containing species and their active role has also been noticed in Sonogashira coupling.¹⁸²

7. Conclusion and outlook

The present survey, undertaken to understand Heck reactions assisted by palladium(II) complexes of organochalcogen ligands, suggests the following:

(i) The catalytic efficiency shown by some of the complexes is as good as that of Pd(II)-complexes of phosphines or carbenes. Sometimes it is also better. For example, some molecular species of high promise are 22, 23, 24, 60, 73 and 75.

(ii) Their easy synthesis and better stability can be additional advantages.

(iii) In aerobic conditions, the Heck coupling may be catalyzed with them.

(iv) Generally, the pathway of Heck's catalytic process for Pd(II)-organochalcogen ligands also appears to be through Pd(0) containing species.

(v) These complexes have been found in most cases, not the direct catalysts of the coupling reaction but dispensers of catalytically active palladium(0) containing species, evidenced directly as well as indirectly. Of course, on the basis of just visual observation of precipitation of palladium-black, such inferences sometimes appear premature. For example, in the case of fluorous complexes (1–3 and 81) and [PdCl₂(SEt)₂],¹¹⁶ occurrence of such precipitation has been interpreted in this manner. More investigations to strengthen such conclusions and understand the function of true catalytic species in depth are required.

(vi) In several instances, complexes themselves have been claimed to be catalysts. For example, in the case of 14, 25, 72 and 79, catalysis of coupling reactions is ascribed directly to complexes. Similarly, in the cases of 4 and 76–78,²⁹ though n-Bu₄NBr was used as a promoter, there was no noticeable formation of metallic palladium and the final reaction mixture was pale yellow or almost colourless. However, these cases need further investigation, as such observations may be misleading.

The pathway of Heck coupling, as an alternative to the one based on Pd(0), may involve oxidation of palladium(II) to Pd(IV), but it is believed to be thermodynamically disfavoured in the absence of strong oxidants. However, in the absence of palladium particle formation, the presumption of the Pd(II)–Pd(IV) pathway in some reports^65,117 is not very convincing. It has also been argued that in some cases none of these two conventional possibilities regarding coupling mechanisms is followed.^183–186 Furthermore, if the ligand is separated completely from palladium, when Pd(0) is formed the activity of the palladium catalyst should not be affected by the ligand at all. However, this is not true because the coexisting ligand's architecture mostly affects the performance of its palladium complex as the catalyst. This may be due to its influence on the size and release of Pd(0) containing species.¹⁸⁷ Thus, designing of new efficient catalytic systems through ligand variation continues to be rewarding. At least the catalysis of Heck coupling by palladium(II) complexes of organochalcogen ligands is worth exploring further.

For organochalcogen ligand-assisted Heck coupling at least the following are worth exploring:

(i) Tethered complexes with Se donor atoms, as several molecular species having such donor sites have shown promise.

(ii) Palladium(II) complexes of tellurium ligands, as they are little explored at the moment.

(iii) A generalized route for synthesis of unsymmetrical as well as symmetrical pincer ligands, possibly by a click reaction.

(iv) Mechanistic aspects in detail to understand real catalytic species.

Acknowledgements

The work was supported by Council of Scientific and Industrial Research (CSIR) India through the award of RA/SRF to A. K. and G. K. R., respectively. The financial support from DST (project no. SR/S1/IC–40/2010) is gratefully acknowledged by the authors.

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