Synthesis and catalytic applications of heterobimetallic complexes involving bis-N-heterocyclic carbenes

In the past few decades, metal-mediated catalysis has become an indispensable tool in the world of homogeneous catalysis and organic synthesis. Transition metal-based catalysts dominate in this field because of their high activity. Most, however, are utilised for single organic transformations.¹  The evolving socioeconomic demands require continuous modification and upgrading of the existing catalysts to overcome their limitations and to produce better systems.¹  Moreover, to meet the demands, researchers are endeavouring to streamline multiple organic transformations into a one-pot tandem catalytic protocol and this challenging process necessitates the development of advanced catalysts with improved designs.^1–4  One approach is to combine two independent single metal-based catalysts, where each metal performs a different and independent catalytic transformation. Since each metal catalyses individual catalytic conversions separately, synergism or cooperativity among the metals involved in these tandem catalytic processes is not possible as they are two separate entities.¹  Thus, it is commonly acknowledged that the single ligand frame catalysts containing multiple metal centres in close proximity exhibit better performance than the equivalent mixtures of the individual monometallic counterparts due to the synergistic action of the catalytically active metal fragments.^2a,4  Additionally, multimetallic catalysts offer higher nanolocal concentrations of the catalytically active sites in comparison to the individual analogous catalysts.⁵  In this chapter, we will discuss the synthetic strategies towards the development of various N-heterocyclic carbene (NHC)-based heterobimetallic complexes and their applications in tandem catalytic processes.

NHCs belong to an elite group of ligands with great topological diversity, exemplary σ-electron donation capability, and the ability to coordinate with almost all the transition metals in different oxidation states. They have established themselves as versatile supporting ligands for the generation of diverse homogeneous catalysts, including catalysts containing more than one metal ion.⁶  Polymetallic homogeneous catalysts are generally supported by poly-NHC systems, commonly involving bis- or tris-NHC ligands.⁷  We will be mostly focussing on the bis-NHC systems as they are majorly employed in tandem catalysis. Mainly, two types of bis-NHC ligands are employed for these purposes, one with aliphatic and the other with phenyl linkers. The bis-NHCs separated by the aliphatic linkers are reported to generate either monometallic chelated or bimetallic complexes depending upon the metal centres and applied reaction conditions during metalations.⁷  On the other hand, the Janus type bis-NHC ligands are usually good candidates for supporting heterobimetallic complexes, as their geometry forces the two coordinated metals to be spatially well separated.⁸  Janus di-NHC ligands are a subclass of poly-NHC ligands that feature coordination to two transition metals in a facially opposed manner.⁷  As compared to the bis-NHC systems bridged via aliphatic linkers, Janus-type systems are preferred for the bimetallic complex syntheses because of the well-defined metal coordination and control over their spatial arrangement.^3,8  As synthetic routes to bimetallic complexes continue to evolve, several modifications have been made with the NHC ligands employed for this purpose. These include examples such as incorporating abnormal mesoionic carbenes (MICs)⁹  and introducing both normal and abnormal carbenes in single ligand framework¹⁰  to achieve better selectivity during bimetallic complex synthesis. During the past couple of decades, the concept of heterobimetallic complex-based homogeneous catalysis has been developing rapidly, which is evident from the number of reports on NHC-supported transition metal-based heterobimetallic complexes with applicability in various beneficial tandem organic transformations.^1,9,10 

The synthesis of heterobimetallic complexes and its application in tandem catalysis may appear simple, but in practice, it faces numerous challenges^1a  as follows:

1. For the synthesis of heterobimetallic complexes as tandem catalysts, proper combination of two different metals with distinct catalytic properties is needed. Metals showing similar reactivity patterns need to be avoided.

2. The proper design of the ligand is crucial for stabilising the initial mono-metallic complex during sequential metalation.

3. The design of a bimetallic catalyst should involve two metals that do not mutually deactivate during catalytic processes.

4. High stability of the heterobimetallic catalysts should be ensured to sustain the different reaction conditions which are employed for various catalytic processes in tandem catalysis.

5. During the tandem catalytic process, two or more catalytically active sites of the catalyst should be compatible with the residual products generated during each catalytic step.

These points are to be kept in mind while designing heterobimetallic catalysts or planning tandem catalysis. Variation with the probable combinations of transition metals for the synthesis of tandem catalysts is restricted, however, numerous alterations in the surroundings of these metals are possible by modifying the ligand systems.^1a  Hence, with time, there is a continuous progress in the design of NHC-based ligand templates for coordinating to two or more transition metals and categorically, there have been four general types of NHC-based ligand reported so far (Fig. 1).

• 1,2,4-Trimethyltriazol-3,5-diylidene (ditz) – Janus head type ligand (A)

• Bis-imidazole (bis-ImNHC) and poly-(benz)imidazole (poly-Bz/ImNHC)-based ligand systems (B, D and H)

• 1,2,3-Triazole (1,2,3-TzNHC)-based ligands – “click”-derived systems (F)

• Mixed-NHC systems – where two different types of NHC donors are combined together in single ligand framework (C, E, and G).

The subheadings below will provide detailed descriptions on how the above mentioned NHC-based ligand systems are utilised for the preparation of heterobimetallic complexes and their application in tandem catalysis.

Heterobimetallic complexes of 1,2,4-triazol-3,5-diylidene (ditz), have attracted considerable interest owing to their distinctive electronic properties and possible utility across diverse domains such as catalysis, materials science, and bioinorganic chemistry. The distinctive coordination environment facilitated by the ditz ligand allows the presence of two metal ions in close proximity, which provides a solid ground for investigating various chemical and physical properties.

In this area, Peris and coworkers in 2007 were the first to report effective preparation of Rh^I/Ir^I-based homobimetallic (2 and 3) and heterobimetallic (5) complexes utilising the ditz ligand, starting from the triazolium salt 1 (Scheme 1).^3a  Initially, the homobimetallic complexes (2) were prepared by heating the dicationic ligand 1 and the respective Ir^I- and Rh^I-metal precursors ([M(COD)Cl]₂, M = Rh/Ir, COD = 1,5-cyclooctadiene) in the presence of KO^tBu. These complexes were well-characterised by multinuclear NMR spectroscopy and single crystal X-ray analysis. As an additional step, the COD ligands in complex 2a were substituted to yield the tetracarbonyl homobimetallic complex, 3, by treating with CO. On the other hand, the heterobimetallic complex Ir^I/Rh^I, 5, was synthesised following a two-step metalation process: firstly, the mono-Ir complex, 4, was obtained via selective metalation, which was then utilised for the synthesis of desired complex 5. To prevent the formation of a homobimetallic complex, the ditz ligand precursor (1) was first reacted with the Ir^I-metal precursor and KO^tBu at a low temperature of −40 °C, leading to the generation of a monometallic complex 4. This was further treated with the Rh^I-metal precursor and KO^tBu at room temperature to afford the heterobimetallic Ir^I/Rh^I complex, 5, as validated by ¹³C NMR spectroscopy. In this case, two distinct metal-bound carbene signals, at δ = 191.4 and 187.4 ppm were found for the Rh- and Ir-bound C_NHC, respectively.

Later in 2008, Peris and co-workers utilised the same ditz ligand for the synthesis of an Ir^III-based homobimetallic complex, 6, as well as a heterobimetallic Ir^III/Rh^I (8b) and a homobimetallic Ir^III/Ir^I (8a) complex (Scheme 2A).¹¹  Initially, the monometallic complexes, 2 and 7, were obtained by reacting the compound 1 with the corresponding Ir^I/III-metal precursor ([Ir(COD)Cl]₂ or [Ir(Cp*)Cl₂]₂) in presence of NaH at 0 °C to avoid dimetalation of the ditz ligand. The resulting complexes, 2 and 7, respectively, were then treated with Ir^III- or Ir^I/Rh^I-metal precursors leading to the formation of the homobimetallic Ir^I/Ir^III complex, 8a, or the heterobimetallic Ir^III/Rh^I complex, 8b. Molecular structures of the complexes revealed that the metal-to-metal separations fall in the range of 6.7–7.0 Å, highlighting the inherent rigidity of the ditz ligand. The obtained complexes, 6 and 8a/8b, were then evaluated as catalysts in a tandem catalytic reaction that involves oxidative cyclisation of 2-aminophenyl ethyl alcohol (Ia) and the alkylation of the resultant indole moiety with primary alcohols (IIa) (Scheme 2B) to form IIIa and IVa. The catalytic outcome demonstrates that the heterobimetallic complex 8b is less catalytically active as compared to the homobimetallic complexes (6 and 8a) in the present process.

The same ligand precursor 1 was further used for the preparation of a heterobimetallic Ir^III/Ru^II complex 10 (Scheme 3A).¹²  As stated previously, the ditz ligand precursor 1 in presence of NaH at 0 °C generated a carbene centre via abstraction of an acidic triazolium proton, which further reacted with Ir^III- and Ru^II-metal precursors ([Ir(Cp*)Cl₂] and [Ru(p-cymeme)Cl₂]), respectively, to produce the monometallic Ir^III- and Ru^II-complexes, 7 and 9. These monometallic complexes were then reacted with Ir^III- or Ru^II-metal precursors in presence of NaOAc to generate the heterobimetallic complex 10. In a parallel fashion, the respective homobimetallic complexes, 6 and 11, were also synthesised to compare their activity with the heterobimetallic complex 10.

Next, these complexes were evaluated as catalysts in various tandem processes and again, the homobimetallic ruthenium complex 11 was found to be superior to the heterobimetallic complex 10 in all the tandem reactions studied (Scheme 3B). Firstly, in the chelation-assisted arylation of 2-phenylpyridine (Ib) with chlorobenzene (IIb), the bisarylated product, IVb, was produced in 99% yield using complex 11 as catalyst, and only in 18% yield using complex 10 as catalyst. The homobimetallic Ir^III complex 6 was found to be a more efficient catalyst than complexes 10 and 11 in the Oppenauer oxidation of 1-phenylethanol (Vb) providing 94% of 1-phenylethanone (VIb). Finally, the homobimetallic Ru^II-complex 11 exhibited greater efficiency compared to heterobimetallic (Ir^III/Ru^II) complex 10 in the tandem oxidation of 1-(4-halophenyl)ethanols (VIIIb) and arylation of arylpyridines (VIIb). These results uncovered that installing two different metal centres in a single ligand framework does not always lead to better catalytic systems for tandem reactions as several other factors need to be in line, as delineated in the introduction section.

With these understandings, to create heterobimetallic catalysts which are capable of facilitating two fundamentally distinct reactions, combining iridium and palladium emerges as a meaningfully more favourable choice. This preference is rooted in the inherent differences in catalytic behaviour exhibited by these metals, and moreover, both the metals exhibit activity across a broad range of transformations. Along this line, in 2009, Peris et al. reported the preparation and characterisation of two ditz ligand bridged Ir^III/Pd^II-based heterobimetallic complexes, 12 and 13 (Scheme 4A).¹³  The previously reported ditz-iridium complex 7 was treated with Pd(OAc)₂ in acetonitrile under refluxing conditions, which led to the formation of the heterobimetallic Ir^III/Pd^II complex, 12. Similarly, when complex 7 was heated with PdCl₂ and pyridine in presence of K₂CO₃, the heterobimetallic Ir^III/Pd^II complex, 13, was generated. The corresponding homobimetallic Pd^II-complex, 14, was also synthesised from the ditz ligand precursor 1 by reacting with PdCl₂ and K₂CO₃ in pyridine. Formation of the heterobimetallic complexes, 12 and 13, was supported by NMR spectroscopy, which showed two distinct metal-bound carbene carbon resonances (Ir–C_NHC: δ = 168.4 and 169.0 ppm and the Pd–C_NHC: δ = 158.7 and 163.4 ppm). Finally, these complexes were employed as catalysts in two different tandem catalytic processes. The first was dehalogenation/transfer hydrogenation of acetophenones Ia ₁ and Ia ₂ (Scheme 4A) and the second was Suzuki coupling of p-bromoacetophenones Ia ₂ with aryl boronic acid Vc in different types of alcohols VIc (Scheme 4B).¹³  Notably, the heterobimetallic complex, 13, was detected to be more efficient than the equivalent catalytic systems generated from the combination of their respective homobimetallic counterparts (6 and 14) for all the catalytic processes studied here. For example, complex 13 exclusively produced the secondary alcohol IVc from Ia ₂ in more than 99% yield (Scheme 4B). Later, investigation was conducted into the catalytic efficiency of complex 13 in preparing imines (IId) from nitroarenes Id following a tandem pathway (Scheme 5).¹⁴  In this case, the nitroarene is first reduced to the corresponding anilines and then a condensation reaction occurs between the aniline and various aldehydes, which are produced from the corresponding alcohols by oxidation, leading to the formation of imines IId. Again, the Ir^III/Pd^II heterobimetallic catalyst 13 was noted to be more efficient when compared to its homobimetallic complexes, 6 and 14.

Furthermore, in 2010, the same group reported the effective preparation of two bimetallic complexes, one of which was the Pt^II-based homobimetallic complex, 15, and the other one the heterobimetallic Ir^III/Pt^II complex, 16, (Scheme 6A).¹⁵  First, the reaction of the ditz ligand precursor (1) with PtI₂ in presence of K₂CO₃ and pyridine resulted in the formation of homobimetallic complex, 15, as indicated by the characteristic Pt^II-bound carbene resonance in the ¹³C NMR spectrum at δ = 164.1 ppm. Next, the heterobimetallic Ir^III/Pt^II complex 16 was synthesised in a similar way from the already reported monoiridium complex 7. Its formation was suggested by two separate ¹³C NMR resonances at δ = 155.5 (Ir^III–C_NHC) and 164.0 (Pt^II–C_NHC) ppm.

Complex 16, along with the corresponding homobimetallic complexes 6 and 15, were then employed in a tandem catalytic process involving the conversion of 2-(ortho-aminophenyl)ethanol (Ie) into functionalised indole, IVe (Scheme 6B). This reaction was chosen with the understanding that the presence of a Ir^III-fragment in complex 16 would facilitate the oxidative cyclisation step to form indole IIe, while the Pt-centre would mediate the functionalisation of indole with alkynyl alcohols to form IVe. Thus, this would expand the reaction pathway by allowing the straightforward conversion of Ie into functionalised indole IVe (Scheme 6B). As per the expectation, complex 16 provided higher catalytic conversions than the combinations of homobimetallic catalysts (15 and 6), implying catalytic cooperativity between the Ir and Pt centres in 16. These findings emphasise the broad utility of ditz-based heterobimetallic complexes in intricate tandem processes, which might find the potential industrial applications in future.

The monometallic Ir^III complex 7 has also been utilised to synthesise the Ir^III/Au^I heterobimetallic complex, 19 (Scheme 7A)¹⁶  via initial reaction with Ag₂O followed by treatment with [Au(SMe₂)Cl]. Furthermore, the Au^I-based homobimetallic complex, 18, was obtained by reacting the ditz ligand precursor, 17, with 2 equiv. of AgOAc and the Au^I precursor [Au(SMe₂)Cl]. The formation of 18 was indicated by a distinct ¹³C NMR Au–C_NHC resonance at δ = 174.5 ppm, as compared to δ = 177.0 ppm for 19. Next, the complexes 18 and 19 were utilised in tandem reactions involving nitrobenzene and benzyl alcohol (analogous to those in Scheme 5). It was found that Ir^III/Au^I complex 19 produces the imine (IVf) in higher amounts than that provided by the equivalent catalyst system obtained from the mixture of homobimetallic Ir^III and Au^I complexes, 6 and 18 (Scheme 7B). These findings add to the knowledge that organogold complexes may also be exploited in tandem catalysis.

Although the ditz-based heterobimetallic complex containing Ir and Pd metals (complex 13) had been previously synthesised,¹³  related heterobimetallic complexes containing a chiral ligand had not been reported. So, to obtain a chiral ditz ligand supported heterobimetallic catalyst, and to check the ability for inducing chirality in catalysis, in 2013, Peris et al. synthesised the chiral heterobimetallic Ir^III/Pd^II complexes, 20a, 20b and 21a, 21b, from the monometallic Ir^III-complex 7 by reacting with the chiral N,N-dimethylbenzylaminate palladacyclic complexes, A1′ and A2′ (Scheme 8A).¹⁷  For the both palladacycles A1′ and A2′, an external base was needed to deprotonate the acidic triazolium proton of 7, while for palladacycles B1′ and B2′, which contain an acetate bridging ligand, these reacted directly with 7 to generate the corresponding heterobimetallic complexes, 21a and 21b. As a result of the restricted rotation about the M–C_NHC bond, these complexes exhibit distinct atropisomerism. Next, heterobimetallic complexes 20–21 were evaluated in a tandem reaction consisting of the isomerisation and asymmetric hydrophosphination of propargyl alcohol (Ig) derivatives (Scheme 8B). It was found that these complexes showed similar catalytic efficiency in the formation of the isomerised product, IIg. Complexes 20b and 21a exhibited better efficiency in the hydrophosphination reaction than complexes 20a and 21b. However, all four of the employed complexes failed to induce substantial chirality in the product (IIIg), as confirmed by conversion to the corresponding phosphine oxide (IVg) product.

Again in 2013, the same group used another N,N-dimethylbenzylaminate palladacycle dimer, A3′, to synthesise heterobimetallic Ru^II/Pd^II complex, 23, from the monometallic Ru^II complex 9 along with the homobimetallic Pd^II-complex, 22 (Scheme 9A).^3b  These were obtained from the ditz precursor 1 following a similar protocol as used previously.¹³  Formation of complex 23 was manifested by the observation of two distinct resonances in the ¹³C NMR spectrum of the complex at δ = 186.4 (Ru–C_NHC) and 182.4 (Pd–C_NHC) ppm. The complexes were screened as catalysts for various reactions as shown in Scheme 9B. The Ru^II/Pd^II complex 23 outperformed both the homobimetallic Pd^II complex 22 and the Ru^II complex 11 when used separately (Scheme 9B). Firstly, for the catalytic dehalogenation of aryl halides and hydrodefluorination (HDF) of fluoroarenes, complex 23 delivers significantly better activity than the homobimetallic complexes, as well corresponding activities when used in combination (Scheme 9B,b). Encouraged by these findings, it was thought that the Ru^II/Pd^II complex 23 may act as an effective catalyst in challenging HDF reactions of α,α,α-trifluorotoluenes (Vh) (Scheme 9B). These compounds are known for their inertness among the organofluorine compounds, and at that time, there were no reports for the HDF of such benzylic C–F bonds. Intriguingly, complex 23 efficiently hydrodefluorinated the trifluorotoluene derivatives, providing good to excellent yields (up to 99%). In the catalyst system utilising 23, the Pd^II-centre is thought to facilitate the C–F bond activation, whilst the Ru^II-centre enables hydrogenation. Thus, a synergistic action between the ruthenium and palladium centre within a single catalyst confers significant advantages as evident from the comparison of activity between the complex 23 and combined 11 and 22, driving this challenging HDF reaction forward.

In the same year, Straub et al. reported a two-step approach to produce additional examples of heterometallic NHC complexes. The heterobimetallic Pd^II/Au^I complexes 26 and 27, and a heterotrinuclear Pd^II/Cu^I complex, 28, were synthesised using a thiol-functionalised unsymmetrical bis-1,2,4-triazolium salt, 24 (Scheme 10).¹⁸  The ligand precursor was reacted with PdCl₂ and pyridine in presence of a mild base K₂CO₃ to produce the monometallic Pd^II-complexes, 25a and 25b, as a mixture of cis- and trans-isomers. Interestingly, the first metalation for this ligand happened at the thiol rather than the N-aryl substituted triazolium unit due to the chelating effect of the thiol group. From the NMR spectroscopic analysis, the ratio of the two diastereomers was found to be 2 : 1 with the trans-isomer dominating the mixture. Furthermore, the second metalation was achieved after ligand deprotonation followed by reaction with the Au^I metal precursor, [Au(SMe₂)Cl]. This gave access to the heterobimetallic Pd^II/Au^I complexes, 26 and 27, again as a mixture of cis- and trans-isomers, whose stability was enhanced via salt metathesis using K[PF₆]. An additional complex containing copper was also synthesised. Copper was incorporated as a second metal by reacting the mixture of 25a and 25b with Cu^IOAc in acetone and finally, by exchanging the counterion with [PF₆]⁻. In this case however, unlike the Pd^II/Au^I complexes 26 and 27, a heterotrinuclear Pd^II/Cu^I complex was generated as confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallographic analysis. No catalytic study involving these heterometallic complexes was conducted.

Along with the exploration of the ditz ligand, imidazole-based ligand systems (bis-/poly-NHCs) were also well-explored for the generation of transition metal-based bimetallic complexes. This subsection will cover various ligand template designs where imidazole-based NHC donors (ImNHCs) were employed for anchoring two or more metals in a single ligand framework.

In this area, Braunstein and coworkers reported in 2009 an ImNHC-based ligand of C_NHC^C_H^C_NHC type (30, Scheme 11). The precursor bis-imidazolium salt, 29, was prepared via coupling of N-ethyl imidazole with 1,3-bis(bromomethyl)benzene. The corresponding dicarbene species, 30, was generated by treating the bis-imidazolium salt with two equivalents of LiN(SiMe₃)₂. The in situ generated dicarbene was reacted with different Ir^I-metal precursors to provide a mixture of homobimetallic, 31 and 32, and monometallic, 33 and 34, complexes, dependant on the metal precursor (Scheme 11).¹⁹  These cationic complexes were separated using column chromatography after exchanging the chloride counterions with [PF₆]⁻. The formation of the complexes was supported by NMR spectroscopy e.g. a singlet at δ = 21.0 ppm (³¹P NMR) for the equivalent trans-PPh₃ and a triplet at δ = 174.1 ppm (² J_C–P = 13.7 Hz, ¹³C NMR) for the Ir–C_NHC moiety were observed for complex 31. Furthermore, upon treatment of the mono-Ir^I complex 34 with [M(COD)Cl]₂ (M = Ir/Rh) in presence of Cs₂CO₃ at room temperature, either the homobimetallic complex 32 or the heterobimetallic Ir^I/Rh^I complex 35, were formed. Interestingly, in solution, complex 35 exists as an equimolar mixture of two diastereoisomers, as evidenced by the presence of four AB spin systems for the CH₂ protons connected to phenylene spacer at δ = 5.28 and 6.00 (² J_H–H = 14.7 Hz, 2H, CH₂), 5.43 and 5.86 (² J_H–H = 14.8 Hz, 2H, CH₂), 5.43 and 6.11 (² J_H–H = 14.8 Hz, 2H, CH₂), 5.56 and 5.98 (² J_H–H = 14.8 Hz, 2H, CH₂), in the ¹H NMR spectrum and two distinct doublets for the rhodium-bound carbene carbons at δ = 182.3 ppm (¹ J_Rh–C = 51.1 Hz) and δ = 182.4 ppm (¹ J_Rh–C = 50.3 Hz) in the ¹³C NMR spectrum for the 1 : 1 mixture of the diastereoisomers.

Later in 2012, Peris and coworkers used a Y-shaped tris-NHC ligand system to synthesise bimetallic complexes, which could potentially ensure short distances between the metal centres, in a similar way as found in the case of the ditz ligand (Fig. 2). However, unlike the ditz ligand, the Y-shaped tris-NHC ligand affords two different coordination environments to the metals that are coordinated to it.^20a  In this work, they utilised the Y-shaped tris-imidazolium salt, 36, for accessing a series of homo- and heterobimetallic complexes (Scheme 12).^20b  Initially, they synthesised the monometallic complexes of Pd^II (37), Ir^I (39), and Rh^I (41), wherein selective metalation is driven by the chelation of two NHC ligands to the metal. The initial coordination at the chelating segment of the ligand simplifies the design of complex heterobimetallic structures since the sequence of installing appropriate metal centres can be adjusted as required. Using this approach, the Ir^I/Pd^II heterobimetallic complexes, 38 and 40, were synthesised from the monometallic complexes, 37 and 39, respectively, by reacting them with the appropriate metal precursors ([Ir(COD)Cl]₂ and Pd(OAc)₂) under suitable conditions. Additionally, the monometallic Rh^III-complex 46 was further exploited to synthesise the heterobimetallic Rh^III/Ir^I complex, 42a, and homobimetallic Rh^III/Rh^I complex, 42b, by treating with the respective [M(COD)Cl]₂ (M = Ir/Rh) precursors.

These well-characterized complexes were found to serve as effective catalysts in various tandem organic transformations (Scheme 12B). For example, the heterobimetallic complex 40 (1 mol% loading), more effective than 38, delivered 99% yield of the final product, 2-phenyl ethanol (IV) for the tandem dehalogenation/transfer hydrogenation of 4-bromoacetophenone. Activity of these complexes were also compared with the previously reported ditz-based homobimetallic Ir^III (6) and Pd^II (14) complexes (Scheme 4A), and it was found that the heterobimetallic complex 40 offers better results than this combination of homobimetallic complexes. Further, for a tandem combination of Suzuki coupling and transfer hydrogenation of 4-bromoacetophenone, complex 40 provided a descent 72% yield (after 24 h) of the product VIIIc, which is again significantly higher than the combination of homobimetallic complexes 6 and 14. These complexes were further utilized in two different reactions which are generally catalyzed by Ir (cyclization of 2-aminophenyl ethyl alcohol to generate indole) and Pd (acylation of bromobenzene with n-hexanal) complexes. For indole synthesis, the heterobimetallic Ir–Pd complexes (38 and 40) supplied >99% yield of IIe, while the corresponding homobimetallic Pd complex (43) gave only 56% yield. On the other hand, full conversion to the acylated product (XIVj) was observed in case of the heterobimetallic complex Ir–Pd (38 and 40) whilst the related Ir–Rh complex (42a) was found to be ineffective, suggesting a major role of the Pd fragment (in the heterobimetallic complexes 38 and 40) in this reaction.

Later in 2013, Hahn et al. introduced a unique one-pot procedure to generate heterobimetallic complexes using a different tris(ImNHC) ligand which contained an unsymmetrical (1,2,4 pattern) substitution of the central phenyl ring.²¹  The tris-imidazolium salt of ligand 44 was treated in one pot synthesis with 1 equiv. of Pd(OAc)₂ and 0.5 equiv. of [M(Cp*)Cl₂]₂ (M = Rh/Ir) in presence of NaOAc as base in acetonitrile (Scheme 13). The heterobimetallic complexes 46a and 46b were obtained selectively in moderately high yields of 53% and 40%, respectively. Intriguingly, formation of the other possible regioisomeric complexes was not observed. In these complexes, the Pd^II centre was regioselectively coordinated by the chelating 1,2-bis-ImNHC groups, whereas the M^III ion (M = Rh/Ir) was selectively coordinated to the single ImNHC donor and orthometalates at the least sterically hindered side of the phenyl ring adjacent to the bound ImNHC moiety. The heterobimetallic complexes, 46a and 46b, could also be accessed in stepwise manner from the tris-imidazolium salt 44. The initial reaction with 1 equiv. of Pd(OAc)₂ in dimethylformamide (DMF) generated the bis-NHC-coordinated monometallic Pd^II chelate complex, 45, which contains one free imidazolium unit. Thus, when complex 45 was further treated with 0.5 equiv. [M(Cp*)Cl₂]₂ (M = Rh/Ir) using Cs₂CO₃ as base, it yielded the same heterobimetallic complexes, 46a and 46b in 69% and 68%, respectively. This study highlights the importance of proper choice of ligand systems as well as the metalation procedure to access heterobimetallic complexes.

After employing a tris(ImNHC) ligand for the purpose of heterobimetallic complex synthesis, Hahn et al. in the same year utilised a bis-imidazolium salt, 47, which featured a bridging 1,4-phenylene group, for the same purpose (Scheme 14). They observed that the type of metal precursor used had an impact on the formation of the targeted bimetallic complexes.^22a  When the bis-imidazolium salt, 47, was treated with 0.5 equiv. of [Pd(allyl)Cl]₂ using Cs₂CO₃ as base, a dicationic dinuclear Pd^II-complex, 48, involving two bis-ImNHC ligands was obtained. However, treatment of 47 with 0.5 equiv. of [Ir(Cp*)Cl₂]₂ in the presence of a mixture of Cs₂CO₃ and NaOAc exclusively afforded the mononuclear orthometalated complex 50. No indication of the formation of a bimetallic complex was obtained even when excess amount of [Ir(Cp*)Cl₂]₂ was used. In contrast, the reaction of 47 with 1 equiv. of [Rh(Cp*)Cl₂]₂ generated the dinuclear Rh^III complex, 49. Complex 50 could be further metalated with 0.5 equiv. of [Rh(Cp*)Cl₂]₂ in presence of Cs₂CO₃ and NaOAc to yield the doubly orthometalated Ir^III/Rh^III heterobimetallic complex, 51. No catalytic studies employing these complexes were available.

Later, Maity et al. anticipated that the introduction of two methyl groups at the 2,3-position of the central phenylene bridge as in 52 (Scheme 14)^22b,c  would restrict the anti-orientation of the metal centres in the heterobimetallic complexes and hence, might facilitate the combined action of the metal centres during tandem catalysis. Accordingly, the bis-imidazolium salt 52 was reacted with PdCl₂ in presence of K₂CO₃ in pyridine and a bis-ImNHC to form the bimetallic complex 53. The presence of pyridine and NHC units in 53 resembles the mono-ImNHC ligand system utilised in PEPPSI-type complexes (PEPPSI standing for pyridine-enhanced precatalyst preparation, stabilisation and initiation).^22d  Furthermore, when the salt 52 was treated with metal precursors, ([M(Cp*)Cl₂]₂, M = Ir/Rh), only mono-cyclometalated complexes 54a and 54b containing one free imidazolium unit were formed. Upon further metalation of 54a and 54b with PdCl₂ in pyridine using K₂CO₃ as base, the Ir^III/Pd^II heterobimetallic complexes, 55a and 55b were cleanly obtained. The catalytic activity of the M^III/Pd^II (M = Ir/Rh) heterobimetallic complexes 55a and 55b was evaluated using 0.5 mol% of the complexes in Suzuki–Miyaura coupling/transfer hydrogenation reactions.^22c  Whilst 55b (Rh/Pd) exhibited negligible activity, 55a (Ir/Pd) showed high activity (65–85% isolated yield of the desired product in four examples).^22b  In this case, equimolar combinations of their monometallic Ir^III and Pd^II counterparts were also studied in Suzuki–Miyaura coupling and transfer hydrogenation tandem catalysis giving only moderate results (38–41% isolated yield of product).^22b  Complex 55a turned out to be a far better catalyst than 55b. Furthermore, the pyridine ligand in complex 55a was exchanged for PPh₃ and the resulting complex also showed high activity in this tandem catalysis.^22c 

Polydentate macrocyclic ligands that incorporate NHC donor groups exhibit a wide range of coordination chemistry. Along this line, Hahn et al. in 2015 reported trinuclear heterobimetallic complexes via stepwise and selective metalation process involving a macrocyclic tetra-imidazolium salt 56 (Scheme 15).²³  At first, reaction of 56 with Ag₂O, followed by transmetalation with [Au(tht)Cl] (tht = tetrahydrothiophene) in acetonitrile, furnished the mononuclear Au^I complex 57 (Scheme 15), whose formation was supported by NMR analysis (¹H NMR, resonance at δ = 9.28 ppm for the two chemically equivalent imidazolium C2 protons and ¹³C NMR resonance at δ = 183.2 ppm for the Au^I–C_NHC). Complex 57 was further explored in a sequential metalation process to incorporate additional metal centres (Ru^II or Rh^III). Accordingly, the reaction of 57 with K₂CO₃ generated a carbene species, which reacted with either [Ru(p-cymene)Cl₂]₂ or ([M(Cp*)Cl₂]₂; M = Rh, Ir) to produce the corresponding heterotrimetallic complexes, 58 and 59. The ¹³C NMR of all three complexes revealed two distinct resonances for the two types of NHC carbene carbons at δ ≈ 186 ppm for Au^I–C_NHC, 176.1 ppm for Rh^III–C_NHC (¹ J_Rh–C = 60.1 Hz), 163.3 ppm for Ir^III–C_NHC, and 178.3 ppm for Ru^II–C_NHC.

Later in 2018, Huynh et al. reported a potentially pentadentate ligand scaffold 60 _Bn/Me featuring two benzimidazolium units bridged via a tridentate dipropyl-pyridine-2,6-dicarboxamide moiety (Scheme 16).²⁴  First, the monometalation of this ligand was performed with [Pd(CH₃CN)₂Cl₂]. Interestingly, different products were observed depending on the reaction conditions. In particular, there was a dependence on the base employed. When Ag₂O was used as base, trans-bis-NHC complex, of type [Pd(NHC)₂Cl₂] (62 _Bn/Me ), which contains the uncoordinated pyridine carboxamide unit. This was obtained via transmetalation of the corresponding Ag^I-NHC complexes (61 _Bn/Me ) in presence of NMe₄Cl. On the other hand, complexes of type [Pd(N′,N,N′)Cl]BF₄ (63 _Bn/Me ) were formed when Et₃N was used as base. In both the cases, the reaction was found to be selective for a particular coordination pocket. Furthermore, these monometallic Pd^II complexes were explored to synthesise bimetallic Pd^II complexes. Accordingly, the benzyl substituted monometallic complexes, 62 _Bn and 63 _Bn , were first treated with excess Nme₄Cl and Pd(Oac)₂ which furnished the tetrapalladium complex 64, instead of the desired bimetallic palladium complex, 65. Nevertheless, such hloride-bridged dimeric complexes are well known to get easily cleaved in presence of pyridine. Indeed, the dipalladium complex, 67, was obtained when 64 was reacted with pyridine. To explore a potential pathway leading to the formation of the tetranuclear complex 64, the pincer complex 63 _Bn was reacted with Ag₂O (0.6 equiv.) at room temperature to give another pincer Pd^II-complex, 66, which contains an uncoordinated benzimidazolium moiety, in high yield. It is plausible that an intermediate silver-NHC species was initially generated, which transferred the carbene centre to Pd^II-pincer unit. Subsequently, complex 66 was subjected to a reaction with Pd(Oac)₂ and Nme₄Cl. This led to the formation of the tetranuclear complex 64 with an enhanced yield of 86%, suggesting that the generation of 64 from complex 63 _Bn involves a two-step process: (i) formation of the carbene and its coordination to the Pd^II-pincer unit, and (ii) palladation of the unbound benzimidazolium moiety and subsequent dimerisation. Finally, a heterobimetallic (Pd^II/Au^I) complex, 68, was also obtained by reacting complex 66 with Ag₂O (0.6 equiv.) in the presence of NMe₄Cl, resulting in the formation of an Ag^I-NHC intermediate, followed by transmetalation to a Au^I centre upon addition of [Au(tht)Cl].

In 2014, Cossairt et al. reported a tridentate ligand containing two NHCs connected to a phosphine to create a ligand framework that can support two different metal centres (Scheme 17).^25a  The bis-benzimidazole-phosphine ligand PhP[(CH₂)₂benz]₂ (70) was synthesised via a three-step process starting from the 5,6-dimethylbenzimidazole, which first involved the synthesis of N-vinyl benzimidazole followed by final reaction with one equiv. of phenylphosphine in presence of KO^tBu in THF at −35 °C. The formation of 70 was confirmed by ³¹P{¹H} NMR, showing a distinct singlet at δ = −35 ppm (free PhPH₂ appears at δ = −120 ppm). The ligand was then treated with [Ru(Cp*)(μ³-Cl)]₄ at ambient temperature to produce the homobimetallic Ru^II complex, 71, where the metal is coordinated via the benzimidazole imine and phosphine units. This underwent tautomerisation of the benzimidazole units when heated at 100 °C in DMSO for 30 h, leading to the formation of a mononuclear Ru^II-biscarbene complex, 72. Interestingly, this complex features an NHC ligand where one of the benzimidazole nitrogen atoms possess a proton rather than the commonly reported alkyl/aryl groups. Coordination of the phosphine unit to the Ru centre is evident from a ³¹P{¹H} NMR resonance at δ = 38 ppm and from the corresponding ¹H NMR spectrum which showed a broad N–H resonance at δ = 14.5 ppm. Additionally, a doublet at δ = 198.5 ppm (² J_C–P = 17.5 Hz) in the ¹³C{¹H} NMR spectrum supported the formation of a protic biscarbene^25b  ruthenium complex. Moreover, the N–H protons present in complex 72 provided the opportunity for further metalation and in fact, these protons could be abstracted by n-BuLi to form the corresponding lithiated complex, 73, (the deprotonation of the N–H proton being supported by ¹H NMR spectroscopy). Furthermore, appearance of a doublet at 195.8 ppm (² J_C–P = 20 Hz) in the ¹³C{¹H} NMR spectrum suggests that the NHC-ruthenium containing core structure remained intact. Satisfyingly, complex 73 could be readily utilised to synthesise heterobimetallic Ru^II–Fe^II/Co^II complexes, 74a and 74b) by reacting with metal halides (MCl₂; M = Fe, Co) in acetonitrile at room temperature, via salt elimination (Scheme 17).

Later in 2021, Hahn et al. reported a biphenyl spacer containing symmetrical bis-(2-chlorobenzimidazole), 75, and an unsymmetrical 2-chlorobenzimidazolium/2-chlorobenzimidazole, 76. The authors smartly demonstrated that, unlike symmetrical bis-NHC precursor 75, the unsymmetrical bis-NHC precursor 76 could be metalated stepwise via a site selective oxidative addition, leading to a single frame bis-NHC-heterobimetallic complex, 80, (Scheme 18).²⁶  Even when attempting monometalation with the symmetrical 75, by controlling the amount of [Pd(PPh₃)₄] (1 equiv.) in the presence of a proton source such as pyridinium tetrafluoroborate (py·HBF₄), it always led to the formation of the bimetallic complex, 77, via double oxidative addition at both of the benzimidazole sites. Thus, the synthesis of the heterobimetallic complex was not possible from 75. However, a monometallic Pd^II-complex, 78, was easily accessible when the unsymmetrical bis-NHC precursor 76 was reacted with 1 equiv. of [Pd(PPh₃)₄] via oxidative addition involving the cationic benzimidazolium moiety, leaving the 2-chlorobenzimidazole group uncoordinated. Complex 78 was then metalated via oxidative addition at the uncoordinated 2-chlorobenzimidazole moiety to give the corresponding bimetallic complexes. Accordingly, the homobimetallic Pd^II-complex, 79, was generated by reaction with [Pd(PPh₃)₄] in presence of py·HBF₄. Similarly, the heterobimetallic complex, 80, was synthesised via oxidative addition with [Pt(PPh₃)₄] (Scheme 18). The metal-carbene bonds were assigned as two triplet signals in the ¹³C NMR spectra due to coupling to the phosphine ligands. For the Pd centre connected to the N–Et substituted BzNHC carbon (C_a), this appeared at δ = 173.2 ppm (² J_C–P = 7.9 Hz) while the one connected to the N–H substituted BzNHC carbon (C_b) appeared at δ = 171.4 ppm (² J_C–P = 8.8 Hz) confirming the complexation of the two NHC sites in complex 79 with trans-[PdCl(PPh₃)₂] groups.

Since their discovery, 1,2,3-triazolylidenes have been enormously applied as fascinating ligand motifs for accessing various transition metal complexes with widespread applications in material sciences and synthetic chemistry, as well as in homogeneous catalysis.²⁷  The prime reasons behind these growing demands are their simple access via the click-inspired [3+2] Huisgen reaction between an alkyne and an azide, and their superior donor properties over their classical analogues.²⁸  Among their widespread applications, 1,2,3-triazolylidene-based ligands have also been utilised for the generation of various heterobimetallic complexes along with their homometallic counterparts, which will be discussed in this section.

In 2023, Maity et al. reported the synthesis of the heterobimetallic complex, 83, containing a cyclometalated Ir^III centre and a PEPPSI-type Pd^II-centre attached to a naphthalene-based bis-mesoionic carbene (MIC) containing ligand template, 81 (Scheme 19A). Furthermore, they have also prepared the analogous homodinuclear Pd^II-complex, 84.^9a  Importantly, the interplay between regiospecific vs. regioselective C–H bond activation was showcased by applying a similar type of naphthyl-based mono-ImNHC/1,2,3-TzNHC ligand with two potential C–H bond activation sites (C2– and C8–H). Interestingly, for the ImNHC case, a regioisomeric mixture of the cyclometalated complexes, 86 and 87, was obtained via metalation with [Ir(Cp*)Cl₂]₂ whereas, in the case of 1,2,3-TzNHC, the regiospecific formation of a sterically tuned six-membered ring containing complex 89 was preferred over a five-membered ring-based complex. Furthermore, the Ir^III/Pd^II heterodinuclear complex, 83, was successfully employed as a precatalyst in the tandem Suzuki–Miyaura/transfer hydrogenation reaction (Scheme 19B), which displayed superior catalytic activity when compared to the combined activity of an equivalent catalytic system, generated from a mixture of Ir^III (89) and Pd^II (84) complexes. Moreover, the complexes 83 and 84 were also applied as catalysts in the Sonogashira coupling and α-arylation of 1-methyl-2-oxindole tandem reaction. Homobimetallic complex 84 exhibited a better catalytic performance than the heterobimetallic complex 83 in the case of Sonogashira coupling and α-arylation tandem reaction.

In the same year, Rit et al. demonstrated a method for the easy access to a variety of heterobimetallic complexes using the readily accessible ‘click’ derived C-unsubstituted 1,2,3-TzNHC salts, 90, featuring a 4-halophenyl group (Scheme 20).^9b  This ligand system includes two distinct coordination sites: metalation can be achieved at one site via oxidative addition, whilst the other, via a classical deprotonation cum metalation pathway. This C-unsubstituted 1,2,3-TzNHC ligand was exploited for the synthesis of different triazole C5-coordinated orthometalated complexes, 91–93, and Pd^II-complexes, 94 and 95, and finally, the heterobimetallic complex, 96. Initially, the mononuclear orthometalated complexes (91–93) were investigated to access the intended heterobimetallic complexes via an oxidative addition pathway, but the heterobimetallic complex formation was not observed. Similarly, no heterometallic complex could be synthesised when the oxidatively added Pd^II-complex, 94, was used for the sequential metalation via the transmetalation and concerted deprotonation metalation pathway. It was then contemplated that a chelating ligand DPPE (1,2-bis(diphenyl-phosphino)ethane) may provide extra stability via chelation and additionally, can reduce the steric congestion by directing the trans-oriented phosphines at the Pd^II-centre (in 94) to cis-directed phosphines (in 95) allowing access to the desired heterobimetallic complexes. Indeed, the Ru^II/Pd^II heterobimetallic complex, 96, was accessed via the transmetalation route from complex 95 with DPPE (Scheme 20c). The coordination of two metals (Ru^II/Pd^II) in this complex was insinuated by the Ru^II–C_TzNHC carbene carbon resonance at δ = 165.1 ppm (¹³C NMR) and the presence of two doublets at δ = 53.36 and 34.17 ppm in the ³¹P NMR spectrum corresponding to the Pd–DPPE moiety. Furthermore, a detailed 2D NMR spectroscopic analysis established that the Ru^II-centre was connected at the rarely observed triazole C4-site (Scheme 20), which was further supported by DFT calculations.

Using only one type of NHCs for the synthesis of heterobimetallic complexes generally confer difficulties in selective metalation. Thus, after exploring various types of normal and abnormal NHCs separately for this purpose, researchers have started probing the possibilities of mixing different NHCs into a single ligand framework to achieve better selectivity during sequential metalation.

Along this line, Cowie et al. in 2012 used a heteroditopic ligand featuring the mixed ImNHC/1,2,3-TzNHC donors bridged by a CH₂ unit for the synthesis of heterobimetallic complexes (Scheme 21).^10a  Accordingly, the mixed pre-carbene salt, 97, was initially metalated selectively at the ImNHC C2 centre with metal precursors, [Pd(OAc)₂] and [Rh(OMe)₂(COD)]₂, to afford complexes 98 and 101. This could be achieved due to comparatively higher acidity of the ImNHC C2 proton over the 1,2,3-TzNHC backbone proton. However, the longer reaction times required for palladation or the acetate analogue of rhodium precursor [Rh(OAc)₂(COD)]₂, induced cyclometalation and generated the corresponding chelate complexes, 99 and 100. Nevertheless, Pd complex 98 could be utilised for heterobimetallic complex synthesis. For this purpose, it was further reacted with a phosphine ligand, which led to the formation of complex 102 featuring a trans-oriented phosphine and ImNHC donors. Furthermore, as the cyclometalation was hindered in case of 102, the intact 1,2,3-TzNHC site could be metalated with [Rh(OMe)₂(COD)]₂ to provide the heterobimetallic complex 103.

Later in 2019, Hahn et al. prepared the mixed NHC ligand precursor, 104, possessing an imidazolium unit connected to a benzimidazolium core for the generation of bimetallic complexes (Scheme 22A).²⁹  Initially, the authors attempted the monometalation of the bis-azolium salt, 104, with Rh^III- and Ir^III-metal precursors ([M(Cp*)Cl₂] type). In the case of iridium, a mixture of monometallic complexes was obtained via metalation at both the BzNHC and ImNHC sites. However, in the case of rhodium, a single monometallic complex, 105, containing an uncoordinated BzNHC unit was obtained via selective metalation at the ImNHC site followed by orthometalation. The presence of orthometalated ImNHC rhodium moiety in complex 105 was supported by the ¹³C NMR spectrum which showed two doublets for the rhodium linked C_NHC and C_phenyl carbon atoms at δ = 180.3 ppm (¹ J_C–Rh = 54.2 Hz) and 157.5 ppm (¹ J_C–Rh = 36.7 Hz), respectively. Next, complex 105 was applied as a synthon and underwent further metalation at the BzNHC site, allowing for the preparation of a range of heterobimetallic complexes. Thus, four different heterobimetallic Rh^III/M complexes [M = Pd^II (106), Ir^I (107), Au^I (108), Ru^II (109)] were generated in moderate yields following a one-pot transmetalation pathway mediated by Ag₂O.

After characterising all of the aforementioned complexes via various analytical techniques including multinuclear NMR spectroscopy and X-ray crystallographic studies, the authors employed complex 106 as a potent catalyst for tandem reactions involving Suzuki–Miyaura coupling and transfer hydrogenation (TH), and found that this heterobimetallic complex has superior activity than that displayed by a comparable system produced by mixing the related monometallic analogues 110 and 111 (Scheme 22A) which had been synthesised separately. This suggests the possible role of cooperativity between the metals when installed at single ligand framework (Scheme 22B).

Later, Rit et al. in 2021 reported another type of heterobimetallic Ir^III–M complex, [M = Pd^II (114), Au^I (115)], with similar ligand system, 112, following a different sequential selective metalation strategy (Scheme 23A).³⁰  Contrary to the previous report by Hahn et al., in this case the first metal centre (Ir^III) was selectively attached to the BzNHC moiety, instead of the imidazole fragment leading to the monometallic iridium complex, 113. This complex served as an excellent synthon for the generation of heterobimetallic complexes, 114 and 115. Furthermore, the corresponding homobimetallic (both Ir^III and Pd^II) complexes, 116 and 117, were also synthesised. Next, the heterobimetallic Ir^III–Pd^II complex 114 was investigated as a catalyst for various one pot tandem catalytic reactions such as Suzuki–Miyaura coupling/hydrodefluorination (HDF) and transfer hydrogenation (TH) of ketones, hydrodehalogenation and transfer hydrogenation of imines (Scheme 23B). Along with the Suzuki–Miyaura coupling/TH, complex 114 also displayed superior catalytic activity for the HDF reaction and for the first time, a tandem catalysis of HDF and TH of ketone was achieved (up to 98% yield) using a heterobimetallic catalyst. Furthermore, the same complex (114) was also found to be an efficient catalyst for the hydrodehalogenation/TH of imines to obtain amine derivatives in near quantitative yields. It is important to note that the catalyst 114 demonstrated a clear advantage over the homobimetallic counterparts, 116 and 117, in the catalytic studies of the above-mentioned tandem reactions, which strengthen the fact that catalytic activity of heterobimetallic complexes in complex organic transformations is enhanced by some degree of cooperativity between two different catalytically active metal centres, Ir and Pd, when connected by a single frame ligand.

In continuation of their investigations, Rit et al. reported another mixed NHC-based ligand system, wherein a 1,2,4-triazole moiety is connected to a benzimidazole core (Scheme 24).^10b  Using this mixed donor bis(NHC) ligand, the heterobimetallic complexes, Ir^III–M [(M = Pd^II (121) Au^I, (122) and Pd^II–Ir^III (123)], were successfully synthesised following a sequential selective metalation strategy (Scheme 24). In this case, it was observed that the initial metalation occurred exclusively at the 1,2,4-triazolium site because of the higher acidity of the 1,2,4-triazolium proton than the benzimidazolium C2-proton. If one looks closely at complexes 121 and 123, the positions of the metal centres (Ir^III and Pd^II) are simply exchanged in these complexes. This is rarely reported in literature, and it might have a significant impact on their catalytic performances. The respective monometallic, 126 and 127, and the homobimetallic complexes, 124 and 125, were also synthesised to enable a comparison of their activity with the heterobimetallic ones.

After completely characterising via standard analytical methods, these well-defined complexes were next evaluated as catalysts in several one-pot tandem catalytic reactions: hydrodechlorination and dehydrogenation (DH) of secondary alcohols/transfer hydrogenation (TH) of ketones (Scheme 25). Encouragingly, the heterobimetallic catalysts 121 and 123 exhibited clear advantages over the equivalent catalytic systems (maintaining the same concentration of the catalytically active metal centres) generated by combining either the homobimetallic Ir^III (124) and Pd^II (125) complex or the monometallic analogues Pd^II (126) and Ir^III (127), respectively. These results again validate that when two different metal centres (in this case, Ir and Pd) are connected by a single ligand framework, it enhances the catalytic activity of the complexes in tandem organic transformations in comparison to the cases where two separate complexes are used. Furthermore, upon closer examination of the heterobimetallic complexes 121 and 123, it was found that complex 121 showed better communication between the Ir^III and Pd^II centres based on detailed NMR, electrochemical, and theoretical studies. Thus, complex 121 was detected to be a better catalyst than 123 in the tandem reactions. This is likely due to direct connection between the metal centres because of orthometalation of the 1,2,4-tzNHC coordinated Ir^III centre at the benzimidazole core.

In 2022, Rit et al. studied the influence of initial metalation on the sequential metalation of a heteroditopic bis-NHC ligand, 128, for the synthesis of heterobimetallic complexes (Scheme 26).^10c  These comprised of normal (ImNHC) and abnormal carbene precursors (C-unsubstituted 1,2,3-TzNHC) bridged by a phenyl spacer. Intriguingly, it was observed that the mode of metal coordination, i.e. either orthometalation or non-orthometalation, during the initial metalation, forming either complex 130 or 129, has a significant impact on the coordination behaviour of the second metal centre (Pd^II/Au^I) towards the C-unsubstituted 1,2,3-TzNHC salt. Thus, the Ir^III/Rh^III bound ImNHC moiety in orthometalated complexes, 130a and 130b, led to the selective activation of the C4–H bond of the 1,2,3-TzNHC moiety, generating the C4-coordinated heterobimetallic complexes, 133 and 134. On the other hand, the Pd complex, 129, directed the second metal towards the C5-position of C-unsubstituted 1,2,3-TzNHC motif resulting in the formation of C5-coordinated homo- (131) and hetero-bimetallic (132) complexes. To understand this difference in reactivity, detailed NMR analysis of the monometallic complexes was performed, and it was noted that the 1,2,3-Tz C4–H is more downfield shifted, essentially more acidic, than the C5–H in complex 129, whereas the reverse is observed for 130. This might be one of the major factors for the regioselective 2nd metalation discussed above as the %V_bur (percentage buried volume)³¹  calculations disregard any significant steric contributions towards this process. Preliminary DFT studies further supported these experimental results.

After the introduction of the ditz ligand system for synthesis of transition metal-based bimetallic complexes, this ligand system has been successfully employed as the standard template for the generation of numerous homo- and hetero-bimetallic complexes. These well-defined bimetallic complexes have been evaluated for various beneficial tandem catalytic processes. The high catalytic activity of these bimetallic catalysts is believed to originate from the presence of two catalytically active transition metal centres in close proximity, facilitating cooperativity between them. The evolution of bimetallic complexes has been further enriched by the use of distinctly designed bis-/poly-imidazole-based ligand systems. Later, researchers have explored the use of ‘click’-derived 1,2,3-triazolium salts for the synthesis of bimetallic complexes due to their ease of synthesis. However, using only one type of NHC donor to access heterobimetallic complexes can be challenging because of the formation of homobimetallic complexes during selective metalation. As a result, researchers have explored the possibility of using a single ligand framework that contains a mixture of different types of NHCs to overcome this obstacle. Indeed, this approach was successful and the heterobimetallic complexes generated from these ligand systems have shown their efficacy in various tandem organic transformations. Generally, these heterobimetallic complexes exhibited enhanced catalytic efficiency and selectivity when compared to the analogous catalytic systems generated from the homobimetallic and/or monometallic complexes as observed for the ditz ligand-based systems. Furthermore, it has been observed that certain combinations of metals like Ru/Pd and Ir/Pd perform better in one-pot multiple catalytic transformations due to their mechanistically discrete activity profile e.g. for hydrodefluorination reactions catalysed by Ru/Pd systems, the Pd fragment catalyses the C–F bond activation, while the Ru metal centre is involved in the hydrogenation process. For the combination of Ir/Pd, the Ir and Pd centres are involved in the (de)hydrogenation and hydrodehalogenation/C–C coupling, respectively.

The synthesis of bimetallic complexes is an evolving area that requires further improvement. There is a lot more ground to cover regarding (a) understanding the mechanism of such bimetallic catalysis, (b) the types of organic transformations that could be combined in tandem catalytic processes, (c) expanding this concept of bimetallic cooperativity towards non-precious metals, and (d) utilising such bimetallic complexes in other scientific research disciplines beyond homogeneous catalysis. We hope that this chapter will provide the reader with a useful overview of the “synthesis and catalytic applications of heterobimetallic complexes involving bis-N-heterocyclic carbenes” and motivate them towards this fascinating area of organometallic chemistry.

We gratefully acknowledge SERB, India and IIT Madras (Grant No. CRG/2020/000780 and RF22230090CYRFIR008699, respectively) for the financial support. C. S. T. gratefully acknowledges IIT Madras and A. D. thanks UGC, India, for research fellowships.