Advances in Cross-Coupling Reactions Catalyzed by

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1. Introduction

Catalysis plays an important role in modern organic chemistry, allowing the efficient and selective synthesis of challenging organic molecules. Among the diverse catalytic methodologies widely used for these purposes, cross-coupling reactions have emerged as powerful tools for the construction of carbon–carbon and carbon–heteroatom bonds. These reactions have revolutionized the field of synthetic chemistry, enabling the synthesis of pharmaceuticals, agrochemicals, and materials with enhanced properties.

Conventionally, noble-metal catalysts such as palladium or platinum have dominated the realm of cross-coupling reactions due to their exceptional reactivity and versatility. However, the scarcity and high cost associated with these metals have prompted significant research efforts to explore the potential of earth-abundant 3d metals as viable alternatives. Therefore, the big challenges in catalysis are focused on the replacement of noble metals with less expensive earth-abundant metals capable of achieving similar or better yields at low loadings [1]. Moreover, the use of palladium and other expensive noble-metal catalysts in cross-coupling reactions in the pharmaceutical industry is raising concerns due to the presence of residues on the final products, which cause health issues to consumers.

Thus, the use of more sustainable alternatives is gaining more attention. In this respect, complexes based on earth-abundant metals, exclusively from the first 3d block series (Mn, Fe, Co, Ni, Cu) have re-attracted the attention as novel catalysts on cross-coupling reactions because they display interesting coordination geometries and multiple spin states, which are useful for the manipulation of the electronic properties required through the catalytic process [2]; however, the design of 3d metal-based catalysts could be difficult due to the reduced covalence of the metal–ligand bonds that causes a fast catalyst degradation and low product selectivity [3]. In this scenario, pincer complexes have gathered significant attention as novel catalysts. These complexes possess a tridentate ligand framework that coordinates the metal center, offering enhanced stability and reactivity. Since their discovery in 1976 [4], pincer complexes have played an important role in organometallic and coordination chemistry, and now, pincer complexes offer a new opportunity as highly reactive and air-stable earth-abundant 3d metal-based catalysts useful for homogeneous catalysis in a wide number of reactions, including cross-coupling. Although in the last few years diverse articles have reviewed the use of pincer complexes based on Ni(II) or other transition metals for catalytic purposes [2,5,6,7,8,9,10,11,12], there are no reviews focused exclusively on the use of earth-abundant metal-based pincer complexes containing aromatic ligands. Recently, our group discussed the application of Co-, Mn-, and Fe-based pincer complexes in hydrogenation and some cross-coupling reactions [13]; nevertheless, although the term “cross-coupling” is mentioned in the review, this section is just briefly discussed and only a few examples of Mn and Co complexes were mentioned. Moreover, the use of Ni(II) pincer complexes was not discussed. Thus, the present review aims to present and discuss the recent and successful use of earth-abundant 3d metal (Mn, Fe, Co, Ni, Cu) pincer complexes containing a pyridine or aryl central structural core in the catalysis of several cross-coupling reactions for C–C, C–S, and C–N bond formation. Also, we discuss the diverse factors affecting the catalytic properties of pincer complexes.

2. Pincer Complexes: An Overview

Pincer complexes are a class of compounds that have a ligand with a pair of donor atoms in their structure, capable of coordinating in a tridentate manner [14]. The complexation of the metal to the pincer ligand results in the creation of two five-membered metalacyclic rings [15].

Pincer complexes can contain either an aliphatic or aromatic backbone. Each one has different features and reactivity; however, the latter is preferred and widely studied in comparison with aliphatic pincers. The reason for this is that aliphatic pincer complexes present a strong σ-donating nature on the C–sp³–M bond, which significantly increases the electron density at the metal center, causing inferior stability on the structure [16]. Moreover, the presence of β-hydrogens in the structure makes them thermally unstable, and the flexibility of ligands obstructs the synthesis of well-defined structures [17]. On the other hand, pincers with an aromatic backbone are based on pyridine or aryl ligands (Figure 1). The variations on the (A) atom in the central structure allow for the attaining of a carbon–metal (C–M) bond when an aryl ligand is used, or an N–M bond when a pyridine ligand is considered. Pincers with an aryl backbone display a stable C–sp²–M as a result of the strong donor character of the central C atom in the structure, whereas pyridine-based backbones display a dearomatization as an inductive effect of the central nitrogen atom [18,19,20].

The donor site (D) implies several atoms such as P, N, O, or S that have side arms (R), which are useful for stabilizing the structure and modulating the steric hindrance and the electronic properties on the complex [6]. Specifically, phosphine ligands are widely used as donor moieties because they provide greater stability, and their P-substituents provide electronic influence and reactivity on pincer complexes [21,22,23]. In addition, substituent groups (W) on the central ring work as anchoring sites and allow functionalization of the ring with specific moieties to provide biological activity. The “Z” atoms between the donors and the central aromatic ring are known as spacers, and their modification leads to pincer systems such as PNCNP, POCOP, and PSCSP [24], and determine the metalacyclic ring size. The different arrangement of the Z and D atoms inside the ligand allows the generation of unsymmetrical pincer structures such as PNCSP, POCNP, etc. Finally, due to the use of metal–halide (MXn = Cl, Br, I) precursors, auxiliary ligands tend to be halogens. Their ionic behavior allows the M–X bond to generate a vacancy useful in catalysis.

Due to the diversity in the structure of pincer complexes, Crabtree and coworkers proposed a classification based on their ionic or neutral nature, and their symmetry (palindromic or non-palindromic) [25]. According to this classification system (Figure 2), the most common pincers containing a central aryl ring and two side arms with P or N donors are classified as monoanionic or palindromic pincers.

Despite the existence of numerous pincer topologies based on alkyl ligands, N-heterocyclic carbenes, and amino acids, among others; the use of aryl pincer ligands is highly reported due to their easy synthesis and widely studied electronic properties [24,26]. As a result, aryl pincer complexes have become important compounds used in a variety of applications such as optics [27,28,29,30], anticancer agents [31,32,33,34], and recently, homogeneous catalysis and electrocatalysis. Although noble metal pincer complexes are continuously used for catalytic purposes, researchers are recently exploring new complexes involving cheaper metal centers for catalytic and electrocatalytic applications [35,36]. As a consequence, pincer complexes based on these metals have emerged as a preferred class of catalysts due to their versatility, high air stability, and their several possibilities to functionalize the ligand environment with multiple donor moieties to increase the catalytic activity [5,22,23].

Hence, in the last decade, the number of reports about earth-abundant 3d metal pincer complexes as catalysts on C–C, C–N, and C–S cross-coupling reactions has considerably increased; however, most review articles debate the application of noble metal-based pincer complexes on these kind of reactions [9,37,38,39,40], and only a few reviews have included a small section to discuss the use of 3d metal-based pincer complexes on cross-coupling reactions.

3. Carbon–Carbon Cross-Coupling Reactions Catalyzed Using Pincer Complexes

Carbon–carbon cross-coupling reactions have emerged as one of the most powerful tools for constructing complex organic molecules [41]. These reactions have revolutionized the field of organic synthesis by providing a versatile and efficient method for the formation of C–C bonds [42]. The development of transition-metal catalysts for carbon–carbon cross-coupling reactions has led to the discovery of many useful and practical methods for the synthesis of a wide range of organic compounds [43]. One such class of catalysts is earth-abundant aromatic pincer complexes. There are reports of a vast diversity of electrophiles and transition-metal complexes employed in cross-coupling reactions, resulting in an extended catalog of synthetic routes for molecular design [44]. In this respect, pincer complexes have been extensively explored as catalysts in these reactions and have presented promising results on the synthesis of a wide variety of alkyl–alkyl, aryl–alkyl, and aryl–aryl products with potential pharmaceutical interest. Therefore, in this section, we summarize the most recent examples of earth-abundant pincer complexes in the catalysis of Kumada–Corriu, Suzuki–Miyaura, and Sonogashira reactions.

3.1. Kumada–Corriu Cross-Coupling

The Kumada–Corriu cross-coupling reaction was independently discovered by its authors in 1972 [45,46]. This reaction catalyzed using Pd or Ni derivatives, involves the use of an organometallic reagent (a Grignard reagent) and aryl or alkyl halides to produce a C–C bond (Scheme 1).

In the last few years, the efforts to apply nickel or palladium complexes as catalysts in cross-coupling reactions with non-activated organic halides have been challenging, especially on those with β-hydrogen atoms [47,48,49,50]. According to Hu [49], the most cited troubles with alkyl halides in cross-coupling reactions are: (1) alkyl halides are less susceptible to oxidative addition with a metal catalyst; (2) alkyl intermediates suffer β-H elimination, and as a result, cross-coupling is inefficient; and (3) alkyl halides are prone to suffer diverse side reactions such as base-promoted halide elimination or hydrohalogenation [51]. However, with an accurate selection of metal ions and reaction conditions, it is possible to avoid such problems, and suppress the β-H elimination to achieve attractive yields [52]. Additionally, the design of the ligands plays a crucial role in catalysis, where tridentate ligands are extensively applied for that purpose because they provide great stability while maintaining electronic tenability to the resulting metal complexes [53]. In this respect, pincer complexes have been successfully explored for the aforementioned applications. Complexes mostly based on Ni with symmetric NNN pincer ligands (Figure 3), or other metals such as iron [54] or copper [55], showed excellent results using low loadings. The catalytic results achieved with these complexes are summarized in Table 1.

Gartia and coworkers reported an Fe–NNN [54] pincer complex (a1) and a Ni–NNN [47] pincer complex (a2) that were applied for the Kumada cross-coupling reaction of polychlorinated solvents (CH₃Cl, CH₂Cl₂, and CCl₄) with diverse inactive alkyl magnesium reagents. The reactions were performed at room temperature in a N₂ atmosphere with low catalyst loading (0.4 mol %), showing percentages of the coupled substrate ranging from 69.1 to 99.91%. As shown in Table 1, reactions carried out in the presence of a2 showed better yields in comparison with a1; however, conversion times were shorter compared with those obtained with a2.

On the other hand, the Kumada–Corriu cross-coupling reaction with aryl halides is widely reported to be catalyzed preferably with palladium complexes [56] due to their high conversion rates and lowest catalyst loading (from ppm to ppb order) [38] compared with 3d metal-based complexes. Moreover, the cross-coupling reaction of aryl halides catalyzed using transition-metal complexes can be difficult due to their low capability to activate the starting compounds [57]. Despite all the advantages of a noble metal catalyst, the increase in the use of nickel and other 3d metal-based complexes is growing. Recently, an important advance was achieved with low-cost transition metals as catalysts in cross-coupling reactions of non-activated alkyl and aryl chlorides [58], which are cheaper in comparison with their bromide and iodide analogous. In this regard, several pincer complexes based on symmetric and asymmetric ligands (Figure 3, complexes a3–a15) with isopropyl or phenyl substituents have been used as catalysts for the high yield synthesis of interesting aromatic products via Kumada cross-couplings of aryl chlorides.

Sanford and coworkers [59] synthesized the complexes a3–a4 and evaluated their catalytic activity in coupling reactions involving aryl chlorides (4-chlorotoluene and 4-chloroanisole) with phenyl magnesium chloride to successfully produce a series of biphenyl derivatives. The complex a4 was found to be catalytically more active in comparison with its analogous a3, with conversions of up to 99% (Table 2). The improvement obtained is a result of the donating ability of the isopropylphosphino groups on the ligand, compared with the bulky phenylphosphino moieties in complex a3. In addition, the strong properties of the ligand decrease the Ni(III)/Ni(II) reduction potential, allowing for effective catalysis. Conversely, an opposite behavior was reported by the Sun group [60] in a series of Ni–PCP pincer complexes (a5–a9) that were also evaluated as catalysts for the cross-coupling of phenyl chloride with o-toluene magnesium bromide. Whereas the spacer changes from C, O to N, the catalytic activity of the complexes containing phenylphosphino (a5, a7, a9) and the reaction yield increased in comparison with their isopropylphosphino analogs, suggesting that high electron density phenyl moieties attached to the PNNP ligand enhance the catalytic activity of their complexes.

Until now, nickel complexes have dominated the scene of alternative catalysts in Kumada–Corriu cross-coupling reactions; however, attempts in the use of pincer complexes based on different transition metals such as chromium or manganese have been presented without a successful result. Recently, Kirchner and coworkers [61] evaluated a series of PNNNP pincer complexes (a12–a15) based on Cr(III), Mn(II) and Fe(II), and Co(II). Although the complexes can catalyze the homo coupling of aryl Grignard reagents in the presence of MeI to produce diverse symmetrical biaryl derivatives in moderate yields, they are unable to catalyze a Kumada cross-coupling.

A selection of the most relevant complexes and their catalytic properties are summarized in Table 2.

Table 2

Examples of earth-abundant pincer complexes applied as catalyst in aryl–aryl and alkyl–aryl Kumada reactions.

Entry	ComplexNumber	Loading(mol %)	Alkyl-Halide	Grignard Reagent	Reaction Condition	Yield(%)	Ref
1	a3	2	o-TolCl	PhMgBr	THF, r.t., Ar, 22 h.	71	[59]
2	a3	2	4-(methoxy)chlorobenzene	PhMgBr		69
3	a4	2	o-TolCl	PhMgBr		99
4	a4	2	4-(methoxy)chlorobenzene	PhMgBr		98
5	a5	2	PhCl	o-TolMgCl	THF, 40 °C, 12 h.	40	[60]
6	a6	2				96
7	a7	2				68
8	a8	2				56
9	a9	2				97
10	a10	3.5	PhI	PhMgBr	THF, 0 °C, 24 h	60	[62]
11	a10	3.5	PhCl	PhMgBr	THF, 0 °C, 24 h	28
12	a11	3.5	PhCl	PhMgBr	THF, 25 °C, 24 h	19
13	a11	3.5	PhBr	PhMgBr	THF, 25 °C, 24 h	51

3.2. Suzuki–Miyaura Cross-Coupling

The Suzuki–Miyaura coupling was discovered by its authors in 1979 [63] and is one of the most frequently used C–C bond-forming reactions because it allows for the formation of an extensive range of C(sp²)–C(sp²) compounds of pharmaceutical interest. This reaction catalyzed using palladium in a basic media involves the use of aryl, vinyl, or alkynyl boronic acids that react with the aryl or vinyl halides to produce conjugated systems (Scheme 2).

As expected, nowadays, palladium complexes still govern the catalysis of Suzuki–Miyaura reactions; however, the trend of reducing the use of noble metals to catalyze them has led researchers to explore new alternatives, where 3d metal-based pincer complexes represent the most suitable options because of their stability and high catalytic activity [64,65]. As shown in Table 3, several Ni(II) and Co(II) PNNNP pincer complexes were reported as promising catalysts with interesting conversion yields at moderate catalyst loadings. Recent examples (Figure 4) include Ni(II) POCOPs, Ni(II) PNNNP, and Co(II) PNNNP complexes.

Early approaches in the use of nickel pincer complexes as catalysts in Suzuki–Miyaura reactions were reported in 2007 and 2009 by Inamoto and coworkers [71,72]. The authors evaluated the catalytic activity of biscarbene-derived CNC nickel pincers on the cross-couplings of aryl bromides, alkenyl, and aryl tosylates with aryl borates. The reactions were performed under mild conditions with conversion yields of about 60–80% in diverse reactions of industrial and pharmaceutical interest, which suggested the potential application of nickel pincers in Suzuki–Miyaura reactions. Regarding pincers with aromatic ligands, in 2012, our group [73] reported an air-stable Ni(II)–POCOP pincer complex (b1), with notable catalytic activity for activated bromo-aryls in coupling reactions at 110 °C. The obtained yields (entries 1–5, Table 3) surpassed 97%, except for the reaction involving 4-bromoaniline. The results were very close to those achieved with its Pd pincer analogous under similar reaction conditions. Our group also reported the catalytic evaluation of a series of non-symmetric Ni(II) POCOP complexes (b2–b4) [67] in the presence of bromobenzene and phenyl-boronic acid at a 1 mol % loading. In this respect, complex b4 showed the best activity with 53% conversion in a conventional heating reaction; nonetheless, using microwave irradiation (90 °C, 150 W, 20 min), the reaction yield decreased to 19%. The opposite effect was obtained with its Pd analog. The complex was evaluated as a catalyst in the same reaction using conventional heating, where only a 40% yield was obtained, whereas by using microwave irradiation, the yield increased to 70%. According to the authors, the efficiency presented by complex b5 could be associated with steric effects of the P-substituents rather than electronic effects as presented by most pincer complexes containing –P(iPr)₂ or –P(tBu)₂ groups.

On the other hand, the catalytic activity of cobalt pincer complexes in Suzuki–Miyaura reactions is scarcely reported. The reason is the difficulty of the transmetalation from boron to cobalt, which causes low or null yields. To overcome this problem and facilitate the transmetalation with cobalt, organoborons are activated with organolithium or organomagnesium species [74]; nevertheless, this methodology could be difficult, hence, it is necessary to understand the transmetalation to successfully apply cobalt as a catalyst. In this respect, Chirik and coworkers [68] synthesized a Co(I)–PNP pincer complex (b5) that was used for the first time as a catalyst for the cross-coupling of aryltriflates with arylboronic acids. The catalysis was carried out via transmetalation. Surprisingly, the obtained conversion rates were between 75 and 90% (Table 3). According to the authors, the key role of the cobalt catalyst was to optimize the complex activity by using 2-benzofuranilboronic-pinacolate and stabilize reactivity and electronic properties with small substituents such as iPr. Other examples are the series of complexes b6–8 [69] that were employed to catalyze the cross-coupling between halogenated benzonitriles and phenylboronic acid in MeCN at 0.5% mol loadings. The highest catalytic activity was presented by b8, which has a PNNNP ligand in comparison with their analogs b6 and b7, which have different ligands and presented slightly lesser activity. This effect is due to the increased number of “N” atoms in the side arm of the ligand, which enhanced the donating ability and increased the catalytic activity of the complex.

Regarding the use of iron pincer complexes, the papers reported are scarce. In this respect, only Bhat and coworkers [70] reported the application of complexes b9 and b10 as catalysts on the cross-coupling reaction of halogenated benzonitrile with phenylboronic acid using a loading of 0.4 mol %, exhibiting moderate yields.

Overall, the catalytic activity presented by Ni or Co pincer complexes in Suzuki–Miyaura reactions demonstrates their viability on the production of small biaryl molecules reaching reaction yields around 90% using up to 5.0 mol% catalyst loading. Despite having results slightly close to those obtained with Pd(II) analogs (>94 mol%) [75,76], the reduction in the catalyst loading remains the main challenge for researchers. Therefore, efforts have been focused on understanding the reaction conditions and the electronic properties of ligands and their effect on the catalytic properties of the resulting pincer complexes with the purpose of reducing the catalyst loading in an efficient way.

3.3. Sonogashira Cross-Coupling

The Sonogashira cross-coupling reaction is widely employed to synthesize enynes, enediynes, and ynones, which are highly valued functionalities for preparing natural products, electronic and electro-optical molecules, and nanostructures [77]. In this reaction, a sp²–sp coupling is made between aryl or alkenyl halides and terminal alkynes by using a Pd(0)/Cu(I) system (Scheme 3). A Cu(I) cocatalyst would be necessary to generate in situ an acetylide intermediate, which is then transmetalated to a Pd center [78]. For industrial purposes, these catalytic systems are disfavored due to the environmental issues related to their use. Hence, there is a growing interest in developing palladium-free Sonogashira methodologies.

At present, efforts are being made to use other more abundant metals like Ni and Cu in this catalysis (Figure 5). In this respect, pincer complexes are still explored as catalysts in Pd-free Sonogashira cross-coupling, where only a few articles are available in the literature. The first report about the use of aromatic Ni-pincer complexes was presented by Xu and coworkers in 2011 [79]. In their work, they evaluated a Ni-POCOP ([NiCl{C₆H₃-2,6-(OPPh₂)₂}] as a novel catalyst in the Sonogashira cross-coupling of alkyl halides with lithium acetylides, obtaining yields ranging from 30 to 99% with a 0.5 mol% catalyst loading. In a recent work, Gallego and co-workers [80] evaluated the catalytic activity in Sonogashira couplings of Ni pincers bearing silylenes (c1) and germylenes (c2) in their structure. They were able to isolate a Ni–Cu adduct that would generate the nickel acetylide intermediate. Further mechanistic studies (Scheme 4) showed that this intermediate would decompose to Ni(0) by a bimolecular reaction at high concentrations, leading to the undesired acetylene homocoupling (Glaser coupling).

Although Cu pincer compounds are less explored in Sonogashira couplings, they have shown high yields in this catalysis. For example, the Kirchner group [81] evaluated the catalytic activity of complex c3 in aryl-alkyne cross-couplings, showing yields of up to 93% (5 mol%, 16 h). Interestingly, Domyati and co-workers have shown that oxygen in air played an important role in the catalysis using complexes c4–c7, since a Cu–O₂ adduct would be present in the reaction mixture and activate the substrates [82].

Despite the scarce number of reports presented in the literature, 3d metal-based pincer complexes discussed in this section displayed similar results to those obtained with Pd(II) analogs [83,84] at 1 mol% in the same reactions, which demonstrates the interesting advance in the application of 3d metal-based pincers in Sonogashira Pd-free reactions.

4. Carbon–Sulfur Cross-Coupling Reactions Catalyzed Using Pincer Complexes

The synthesis of diarylthioethers and alkylthioethers in the last decade has been of great importance in various fields, such as organic synthesis, biological chemistry, and material science [85]. Moreover, the presence of thioethers in pharmaceutical products for the treatment of different diseases has led to the search for new synthetic strategies that are more efficient for obtaining this type of compound [8,86].

The use of metal complex-based catalysts has been one of the most efficient tools in C–S bond synthesis, where complexes based on Pd, Rh, and Pt have received much attention during the last few years [87], and recently, new alternative complexes based on Ni (Figure 6) are being explored as catalysts for C–S coupling reactions [88]. Among these complexes, the first POCOP–Ni (d1) applied to this reaction was presented by our group [89]. The complex was employed as a catalyst for the thiolation of iodobenzene with different disulfides in the presence of metallic zinc, with yields ranging from good to excellent. During the reactions, it was observed that the yields decreased due to electronic factors, as in the case of the phenyldisulfide derivative (86%), and steric factors for tert-butyl disulfide (49%). In both cases, the formation of biphenyl was observed as a homocoupling product with yields ranging from low (8%) to considerable (35%). Other reducing agents such as magnesium and tin powder were also used; however, the results were not as efficient as with zinc. Due to this, the group proposed a mechanism that proceeds through the formation of Ni(0)/Ni(II) intermediates. This mechanism consists of two pathways, one which explains the formation of thiolated compounds, and the other which explains the formation of biphenyl (Scheme 5).

Asymmetric pincer complexes POCSP–Ni(II) were also explored for C–S cross-coupling reactions. In an initial work, the catalytic activity of the d2 pincer complex in C–S couplings was explored [90] (Scheme 6). First, the optimal conditions to perform the catalytic reactions were determined, using iodobenzene and diphenyl disulfide as the starting substrates. It was established that the best catalysis conditions were using a catalyst loading of 0.3 mol% at 140 °C for 16.5 h. From these established parameters, studies were conducted on the steric and electronic factors and their effect on the reaction yields. The electronic effects were compared with substituents in the para position of iodobenzene derivatives (–H, –NH₂, –COCH₃) in terms of the Hammett parameter. On the other hand, the steric effects were evaluated on different substituents in the disulfide substrates. Regarding electronic factors, it was observed that the iodobenzene substrate with the –COCH₃ substituent showed the best conversions (92%). With this substrate, the steric factors were evaluated with the disulfide substrates, where it was established that the substrate with the least steric hindrance, dimethyl disulfide, had the best conversion percentage (91%), while the conversion of the substrate with the most steric hindrance, in this case, di-tert-butyl disulfide, was the lowest (10%). This suggests that the greater the steric hindrance of the substituents in the disulfide substrates, the lower the conversion percentage will be (Table 4).

In a second study, the catalytic activity of complex d3 [91] was investigated in C–S couplings. In these reactions, a study was made of the steric factors of the disulfide substrates. As in the previous case, the substrates with lower steric hindrance showed a better conversion percentage. Phenyl disulfide (88%) and methyl disulfide (60%) showed the best conversion percentages (Table 4).

5. Carbon–Nitrogen Bond-Forming Reactions Catalyzed Using Pincer Complexes

C–N bond-containing compounds are an important basis in organic chemistry due to their abundance in various natural products and their significant role in biochemical and bio-inspired reactions and medicine [92]. Moreover, a great variety of C–N bond-containing aromatic compounds has great industrial applicability as agrochemicals, pesticides, dyes and pigments, and medicine [93]. In this respect, diverse methodologies for direct coupling are highly used in laboratories and industries to obtain the desired products. To achieve excellent yields, these reactions are catalyzed with transition metals under harsh conditions. As an attempt to reduce the reaction time, the catalyst loading, and employ mild conditions, the design of novel catalysts based on earth-abundant metals for efficient C–N coupling reactions has increased during the last few years. In this regard, pincer complexes have garnered significant attention due to their ability to catalyze a wide range of C–N bond-forming reactions with high efficiency and selectivity. Therefore, in this section, we discuss the recent uses of earth-abundant pincer complexes as catalysts in C–N cross-coupling reactions, acceptorless dehydrogenation couplings (ADC), and the borrowing-hydrogen (BH) strategy.

5.1. C–N Cross-Coupling Reactions

The development of efficient pathways to obtain alkyl and arylamines took a century to obtain remarkable results, and nowadays, the syntheses of C–N bonds can be achieved using the Ullmann reaction [94], and Buchwald–Hartwig amination (BHA) reaction [95,96]. These processes involve the formation of a new C–N bond through the coupling of a nitrogen-containing substrate with an organic halide or pseudohalide.

The Ullmann reaction (Scheme 7a) was the first reported C–N coupling, which employs stoichiometric amounts of copper salts for the activation of aryl halides that react with amine nucleophiles. Despite its great versatility, the stoichiometric generation of copper salts as a residue and the harsh conditions required represent the main problems in this reaction [97]. On the other hand, the Buchwald–Hartwig coupling (Scheme 7b) employs palladium complexes as catalysts with conditions that are much milder compared to Ullmann couplings [93].

In this line, only a few aromatic pincer complexes were reported as catalysts for C–N couplings (Figure 7), and as expected, copper complexes dominate the scene.

In a recent work, the group of Tahsini and coworkers [98] reported the synthesis of a CNC–Cu(I) pincer complex (e1) that was used as a catalyst on Ullmann couplings using imidazole and 4-iodoacetophenone as a reaction model (Scheme 8). It was observed that the best reaction conditions occurred in DMF at 120 °C for 24 h, with yields of 82%. By varying the iodo aryl substrates, the conversion percentages for these couplings ranged from good to excellent. An advantage of using complex e1 on Ullmann reactions is its capability to facilitate the coupling of activated aryl halides without an inert atmosphere.

On the other hand, Kirchner et al. [81] presented a PNP–Cu(I) pincer complex (e2) that was tested as a catalyst in C–N cross-coupling reactions. To identify the optimal conditions, a model reaction with iodobenzene and phenylalanine was performed, finding the best results to be obtained in THF at 110 °C, using tBuOK as a base, 3 mol% of catalyst in a reaction time of 16 h. Subsequently, to study the scope of the catalyst e2, different aryl and heteroaryl halides of anilines were employed. They observed that, in all cases, the isolated yields of the products were from 73 to 93% (Scheme 9).

Likewise, an NNN–Ni(II) pincer complex (e3) was employed as a catalyst for the formation of C–N bonds by the group of Ghosh [97]. To establish the optimal reaction conditions, the catalyst was tested with chlorobenzene and propylamine. This reaction proceeded efficiently using DMSO as a solvent, and tBuOK as a base, at a temperature of 110 °C, and using a catalyst loading of 0.2 mol%. The TON (turnover number) under these conditions was the highest, resulting in a value of 243 ± 29. Employing these conditions, primary and secondary amines were used to carry out C–N couplings (Scheme 10), where primary amines showed a higher TON than secondary amines; however, sterically hindered amines showed lower TONs. In this work, the authors proposed a possible catalytic mechanism by which the reaction could proceed. Suggesting that the intermediates formed are paramagnetic Ni(I-III) species, whose mechanism proceeds through oxidative additions by the aryl halides followed by reductive elimination with the formation of the desired product (Scheme 11).

5.2. Acceptorless Dehydrogenation Coupling (ADC) and Borrowing Hydrogen (BH) Strategy

One of the strategies currently employed to carry out coupling reactions is the so-called acceptorless dehydrogenation coupling (ADC) (Scheme 12a), which consists of a direct coupling between two X–H bonds (X = C, N, O, P, etc.) from different substrates [99,100]. This type of coupling has emerged as an attractive tool for the formation of C–C, C–heteroatom, or heteroatom–heteroatom bonds, since, unlike conventional coupling reactions, the byproducts generated are H₂ and H₂O, making the process an efficient and environmentally friendly pathway [101,102].

The ADC reactions for the formation of imines from primary alcohols and primary amines were demonstrated to be an efficient pathway using catalysts based on metal complexes [103]. Diverse types of transition-metal complexes have been widely explored in this area, and the first papers related to catalysis with pincer complexes were released by Milstein’s and Beller’s groups by using aliphatic Mn(I) complexes [104]. Based on previous results, several examples of aromatic pincer complexes applied as catalysts on ADC reactions are available in the literature (Figure 8).

In 2016, Milstein and coworkers [105] reported the synthesis and application of a PNP–Mn(I) pincer (e4) for the direct dehydrogenative coupling of alcohols with amines. In this work, they performed the coupling of benzyl alcohol with benzylamine under an N₂ atmosphere at 135 °C as a model system (Scheme 13), where using a catalysis loading of 4 mol% resulted in a 76% conversion of the starting materials after 30 h. Under optimized conditions, yields ranging from 86 to 92% were obtained after 60 h.

Inspired by the previous work, the Kirchner group [106] reported the use of the Mn(I) complex e5 also stabilized using a PNP ligand coupled to PiPr₂ moieties. The complex was initially evaluated as a catalyst (3 mol%) for the coupling reaction of 4-fluorobenzyl alcohol with p-toluidine in toluene at 140 °C and compared with its Fe(II) analogous (e10). When the reaction was catalyzed using e5, the resulting imine was obtained in an 84% yield, whereas with e10, the resulting amine was isolated in an 81% yield. According to the authors, this difference is due to the presence of an inert CO co-ligand and a hydride ligand that participate in the catalytic reaction; therefore, complex e10 can carry out both alcohol dehydrogenation and imine hydrogenation, whereas complex e5 is only able to oxidize alcohol with an irreversible H₂ release.

Multi-component C–N coupling was also reported employing Mn pincer compounds via ADC. For example, Kirchner et al. [107] demonstrated an aminomethylation of several aromatic alcohols with methanol and secondary amines, using the previously presented complex e5. The three-component catalysis performed selective C–C and C–N bond formations, with yields of up to 91% (Scheme 14).

On the other hand, the Borrowing Hydrogen Strategy (BH) (Scheme 12b) is a methodology that allows for the direct synthesis of amines using alcohols as alkylating reagents. Like ADC, this method catalyzed using transition metals involves the transfer of hydrogen from an alcohol or other hydrogen donor to a nitrogen-containing substrate, in this case, the imine formed during the condensation step, then mediates a reduction to generate the amine [102]. The key advantage of this method is that it avoids the use of toxic and expensive reagents, such as hydrides or boranes, which are commonly used in traditional methods for the synthesis of amines. Instead, the hydrogen atom is borrowed from a cheap and readily available alcohol, which acts as a hydrogen donor, and the byproduct is typically water, making the process greener and more sustainable [108].

The application of the BH strategy on the direct methylation of amines with methanol represents a challenge due to the high activation barrier on the dehydrogenation step of methanol in comparison with other aliphatic alcohols [109,110]. This methodology was initially catalyzed with ruthenium or iridium species [111,112,113,114]; however, researchers are looking for cheap and highly active catalysts to be used for these purposes. In this regard, the first report about using pincer complexes was presented by Beller [115], who reported a PNP Mn(I) pincer complex (e6) based on lutidine, which catalyzes the N-methylation of aniline via BHS at 100 °C. The reaction exhibited an excellent conversion (>99%) after 6 h with a 2 mol% catalyst loading. The complex was also evaluated on the mono-methylation of diverse aromatic amines producing yields ranging from 65 to 93%. In addition, Sortais and coworkers [116] also tested the catalytic activity of complex e7 in the mono-methylation of aniline at 120 °C in the presence of tBuOK, with a catalyst loading of 5 mol%. After 24 h, a yield of 94% was obtained. The evaluation with a variety of aniline derivatives showed that complex e7 is sensitive to steric hindrance; therefore, the yields obtained with 2-methylaniline or 2-phenylanlynine as starting materials were 10 and 35%, respectively.

Hultzsch and coworkers [117] proposed a bipyridine-based asymmetric Mn(I) pincer complex (e8) to implement milder reaction conditions with low catalyst and base loadings. For this purpose, they observed that with a catalytic amount of 0.5 mol% at temperatures of 80 °C using DME as a solvent, conversion percentages of 80% were obtained. Similarly, the N-alkylation reactions using benzyl alcohol and different aniline substrates afforded yields that reached 99%.

Mn(I) pincer complexes have demonstrated high applicability in catalysis for the formation of different types of C–N bonds. In this regard, Milstein and coworkers [118] reported the first hydrogen-borrowing dehydrogenative coupling between alcohols and hydrazine to yield N-substituted hydrazones. In this work, they used an NNP–Mn pincer catalyst (e9) and performed various reactions using different phenolic substrates, obtaining yields ranging from good to excellent (Scheme 15). Additionally, they proposed a catalytic mechanism based on isolatable intermediates, which involves both a dehydrogenative coupling of alcohols without acceptor and a borrowing-hydrogen process.

On the other hand, the amination of alcohols catalyzed using Co(II) pincer complexes is scarcely reported. At the time of writing this review, only a few examples could be found in the literature. For example, Kirchner and coworkers [119] developed the complex e11 stabilized using a PCP ligand. The complex was evaluated as a catalyst for the direct alkylation of aniline with benzyl alcohol in toluene at 80 °C. By using complex e11 (2 mol%) as a catalyst, the resulting amine was obtained in 93% yield after 16 h. With a low catalyst loading (1 mol%) and short reaction times (4–8 h), the produced yields ranged from 65 to 84%. In addition to this work, Ghosh and coworkers [120] reported the complex e12 catalyzes the same model reaction. Unlike other complexes previously presented, complex e12 is based on a 2,6-bis(2(4-ethoxybenzylidene)-1-phenylhydrazineyl)pyridine scaffold. The reaction performed at 110 °C in toluene, tBuOK and 4 mol% catalyst loading afforded an 82% yield after 24 h. The authors evaluated the solvent effect on the reactions and obtained yields ranging from 30 to 78% when DMSO, xylene, or benzene were used. Notably, the reaction conditions presented by Ghosh [120] are harsher compared with the reaction conditions evaluated by Kirchner [119], which suggests that bulky ligands do not improve catalysis.

6. Conclusions

The utilization of pincer complexes in catalysis is a well-established concept and the recent strides made in this area represent a transformative paradigm shift in the realm of chemical synthesis. As stated, the purpose of this review was to demonstrate the applicability of catalysts based on earth-abundant metals such as Mn, Fe, Co, Ni, and Cu, with the aim of exploring the efficiency and selectivity compared with noble metal-based analogs.

The last few years have witnessed remarkable progress, particularly in the exploration of aromatic pincers based on earth-abundant 3d metals. Despite the continued dominance of noble metals in catalyzing cross-coupling reactions, 3d metal-based pincer complexes have emerged as promising catalysts, demonstrating not only attractive conversion rates and high yields but also remarkable efficiency at remarkably low catalyst loads, up to 0.2 mol% in some cases. Future applications of 3d metal-based pincer complexes not only address the issue of sustainability and cost-effectiveness but also offer unique reactivity profiles, expanding the synthetic toolbox for cross-coupling reactions. Although these catalysts were only evaluated in the synthesis of simple compounds, there is a promising application in the design of difficult/complex molecules due to the studies performed considering diverse factors such as solvent, type of base, and catalyst load.

Looking ahead, further advancements in this field are anticipated. One area of opportunity is the exploration of different substituents on the pincer structure designs to enhance the catalytic activity, stability, and selectivity of the proposed complexes. Moreover, expanding the substrate scope to more challenging functional groups and exploring new cross-coupling methodologies would broaden the applicability of these catalysts in organic synthesis. For now, efforts should be directed towards understanding the mechanistic aspects of these catalytic systems.

As we reflect on these advancements, it becomes increasingly evident that the application of earth-abundant pincer complexes in catalysis holds the promise of overcoming traditional limitations and opening new avenues in the synthesis of complex molecules. This optimistic outlook is not merely speculative but is grounded in the tangible achievements observed in recent studies. The ongoing endeavors in this field, coupled with the demonstrated catalytic prowess of these 3d metal-based pincer complexes, collectively advocate for their continued exploration and adoption as catalysts of choice in advancing the frontiers of chemical synthesis.

Author Contributions

Writing—original draft preparation, J.A.C.-N., A.S.-M., J.S.S.-G., A.A.-F. and D.M.-M.; execution and drawing, J.A.C.-N., A.S.-M., J.S.S.-G., A.A.-F. and D.M.-M.; writing—review and editing, J.A.C.-N., R.C.-P., V.R.-M. and D.M.-M.; visualization and supervision, J.A.C.-N. and D.M.-M.; Funding acquisition, D.M.-M. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures, Schemes and Tables

Figure 1. General structure of pincer complexes based on aryl ligands.

Figure 2. Classification of pincer complexes in function of their ligands. Readapted from reference [25].

Scheme 1. General schematic presentation of Kumada–Corriu reaction.

Figure 3. Examples of earth-abundant pincer complexes used for Kumada–Corriu cross-coupling of alkyl halides.

Scheme 2. General schematic presentation of Suzuki–Miyaura reaction.

Figure 4. Examples of catalyst applied for Suzuki–Miyaura aryl–aryl cross-coupling.

Scheme 3. General schematic presentation of Sonogashira cross-coupling reaction.

Figure 5. Selected examples of Ni pincers used in Sonogashira catalysis.

Scheme 4. Proposed catalytic cycle for Sonogashira reaction catalyzed using [ECE]-Ni pincers, E = Si, Ge.

Figure 6. Example of pincer complexes used as catalysts in C–S cross-coupling reactions.

Scheme 5. Proposed reaction mechanism for thiolation catalyzed using a POCOP–Ni complex.

Scheme 6. General C–S cross-coupling reaction catalyzed with d2 or d3.

Scheme 7. C–N couplings of the Ullmann type (a) and Buchwald–Hartwig (b).

Figure 7. Examples of pincer complexes used as catalysts in C–N cross-coupling reactions.

Scheme 8. Reaction of iodinated aryls with imidazole using e1 as a catalyst.

Scheme 9. C–N cross-coupling catalyzed using catalyst e2.

Scheme 10. Reaction of C–N coupling using e3 as catalyst.

Scheme 11. Proposed reaction mechanism for C–N coupling using the e3 complex as catalyst.

Scheme 12. (a) Synthesis of imines via ADC reactions. (b) Synthesis of amines via BH strategy.

Figure 8. Examples of pincer complexes used as catalysts in ADC and BH reactions.

Scheme 13. Deshydrogenative coupling of benzylamine with benzyl alcohol catalyzed using complex e4.

Scheme 14. Multi-component N-alkylation via ADC using complex e6.

Scheme 15. N-alkylation from hydrazine using e9 as a catalyst.

Table 1

Examples of earth-abundant pincer complexes applied as catalysts in alkyl–alkyl Kumada reactions.

Entry	Complex Number	Loading(mol %)	Alkyl-Halide	Grignard Reagent	Reaction Condition	Yield(%)	Ref
1	a1	0.2	CH₃Cl	EtMgCl	THF, N₂, r.t., 5 min	99.5	[54]
2			CH₂Cl₂	nBuMgCl		99.9
3			CCl₄	HexMgCl		97.2
4	a2	0.4	CH₃Cl	nBuMgCl	THF, N₂, r.t., 20 min	99.9	[47]
5			CH₂Cl₂	HexMgCl		97.5
6			CCl₄	EtMgCl		99.9

Table 3

Examples of earth-abundant pincer complexes applied as catalysts in aryl–aryl Suzuki–Miyaura reactions.

Entry	Complex Number	Loading(mol %)	Alkyl-Halide	Boronic Acid	Reaction Condition	Yield(%)	Ref
1	b1	1.0	4-bromobenzaldehyde	PhB(OH)₂	DMF, Na₂CO₃, 110 °C, 15 h	99	[66]
2			4-bromoacetophenone			99
3			4-bromobenzonitrile			99
4			4-bromonitrobenzene			97.1
5			4-bromoaniline			42.4
6	b2	1.0	4-bromobenzonitrile	PhB(OH)₂	DMF, Na₂CO₃, 90 °C, 12 h	1	[67]
7	b3	1.0				3
8	b4	1.0				53
9	b5	5.0	4-(trifluoromethyl)phenyl triflate	Benzofuran-2-boronic acid, pinacol ester	THF, NaOCH(Ph)Me, 60 °C, 24 h	90	[68]
		5.0	phenyl triflate			85
		5.0	3-(acetyl)phenyl triflate			80
		5.0	4-(methoxy)phenyl triflate			75
10	b6	0.5	4-bromobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C, 16 h	81	[69]
11	b7	0.5				83
12	b8	0.5				89
13	b6	0.5	4-iodobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C 16 h	87
14	b7	0.5				90
15	b8	0.5				92
16	b9	0.4	4-bromobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C, 14 h	85	[70]
17	b10	0.4	4-bromobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C, 14 h	89
18	b9	0.4	4-iodobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C, 14 h	89
19	b10	0.4	4-iodobenzonitrile	PhB(OH)₂	MeCN, Cs₂CO₃, 100 °C, 14 h	94

Table 4

C–S coupling reactions using POCSP–Ni(II) pincer complex catalysts.

Entry	Complex Number	Loading(mol %)	Disulfide	Aryl-Halide	Reaction Condition	Yield(%)	Ref
1	d2	0.3	diphenyl disulfide	iodobenzene	Zn(0), DMF, 140 °C, 16.5 h	58	[90]
2			diphenyl disulfide	4-iodoaniline		69
3			diphenyl disulfide	4-iodoacetophenone		89
4			dimethyl disulfide	4-iodoacetophenone		89
5			n-dibutyl disulfide	4-iodoacetophenone		71
6			n-dibutyl disulfide	4-iodoacetophenone		50
7	d3	0.2	diphenyl disulfide	iodobenzene	Zn(0), DMF, 110 °C, 4 h	88	[91]
8			dimethyl disulfide	iodobenzene		60
9			diisopropyl disulfide	iodobenzene		20
10			n-dibutyl disulfide	iodobenzene		1
11			n-dibutyl disulfide	iodobenzene		14
12			ditertbutyl disulfide	iodobenzene		2

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The increase of noble-metal-free catalysis in organic chemistry is a trending topic in constant growth due to the price increase of noble metals and their scarce abundance. As a result, many earth-abundant transition-metal complexes containing nickel, iron, or cobalt have been successfully applied in the homogeneous catalysis of a wide number of cross-coupling reactions, and the use of pincer complexes based on these earth-abundant metals was recently explored, affording interesting results. Thus, this review provides a general overview of earth-abundant 3D pincer complexes and their application during the last decade as catalysts in cross-coupling reactions such as Kumada–Corriu, Suzuki–Miyaura, Sonogashira, C–S cross-coupling, and C–N bond-forming reactions.

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Advances in Cross-Coupling Reactions Catalyzed by Aromatic Pincer Complexes Based on Earth-Abundant 3d Metals (Mn, Fe, Co, Ni, Cu)

Author

Cruz-Navarro, Jesús Antonio¹

; Sánchez-Mora, Arturo¹

; Serrano-García, Juan S¹

; Amaya-Flórez, Andrés¹

; Colorado-Peralta, Raúl²

; Reyes-Márquez, Viviana³

; Morales-Morales, David¹

¹ Instituto de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Circuito Exterior s/n, Ciudad de Mexico C.P. 04510, Mexico; [email protected] (A.S.-M.); [email protected] (J.S.S.-G.); [email protected] (A.A.-F.)
² Facultad de Ciencias Químicas, Universidad Veracruzana, Prolongación de Oriente 6, No. 1009, Orizaba C.P. 94340, Veracruz, Mexico; [email protected]
³ Departamento de Ciencias Químico-Biológicas, Universidad de Sonora, Luis Encinas y Rosales s/n, Hermosillo C.P. 83000, Sonora, Mexico; [email protected]

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2024

Publication date

2024

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MDPI AG

e-ISSN

20734344

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.3390/catal14010069

ProQuest document ID

2918529507

Advances in Cross-Coupling Reactions Catalyzed by Aromatic Pincer Complexes Based on Earth-Abundant 3d Metals (Mn, Fe, Co, Ni, Cu)

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