Thioether–NHC‐Ligated Pd^II Complex for Crafting a Filtration‐Free Magnetically Retrievable Catalyst for Suzuki–Miyaura Coupling in Water

Abstract

A new benzimidazolium salt, 1‐[benzylaminocarbonylmethyl]‐3‐(2‐phenylsulfanylethyl)‐1H‐benzimidazolium chloride (L), which is a precursor of a novel thioether‐functionalized NHC, has been synthesized by subjecting 1H‐benzimidazole to a sequence of reactions with 1,2‐dichloroethane, sodium thiophenolate, and N‐benzyl‐2‐chloroacetamide, successively. The moisture/air‐insensitive complex [Pd(L–HCl)Cl2] (1) was prepared by the reaction of L with PdCl2. The molecular structure of 1, established by X‐ray crystallography, revealed a square‐planar geometry around Pd. Complex 1 was screened for Suzuki–Miyaura coupling reactions of various aryl/heteroaryl bromides (yields of up to 94 % in 2 h) in water at room temperature. Furthermore, complex 1 was immobilized onto the surface of aminopropyl‐functionalized silica‐coated magnetite nanoparticles [NPs, Fe3O4@SiO2‐(CH2)3‐NH2] by a stepwise modification strategy to develop the heterogeneous magnetically retrievable catalyst Fe3O4@SiO2‐1′, in which the amide functionality present in the side‐arm of the NHC within the NHC–PdII complex serves as linker. This magnetic nanosupport, bearing palladium complexes incorporating a novel thioether‐based NHC ligand that functions in aqueous aerobic medium and can be easily separated, renders Fe3O4@SiO2‐1′ a most desirable catalyst for the Suzuki–Miyaura coupling reaction. It was also observed that the catalyst was effective for up to seven cycles and was easily separated from the reaction medium by the use of an external magnet, further increasing its appeal.

Recent Advances in Benzimidazole Based NHC‐Metal Complex Catalysed Cross‐Coupling Reactions**

“…Joshi et al . synthesized a moisture and air insensitive thioether functionalized NHC Pd(II) complex for the coupling of phenylboronic acid with aryl/hetero‐aryl bromides possessing electron‐donating and withdrawing substituents in water at rt [51] . The catalytic system consisting of 0.01 mol% of the catalyst and K 2 CO 3 base gave the desired products in high yields (up to 94 % in 2 h) with activated aryl bromides at rt and inactivated aryl bromides requiring temperature up to 55 °C.…”

Advances in Magnetic Nanoparticles-Supported Palladium Complexes for Coupling Reactions

“…Sharma and co-workers synthesized a novel homogeneous Pd(II) catalyst incorporating a chelating thioether-NHC ligand. Moreover, to overcome the issue of the reusability of the catalyst and study the catalytic activity of the homo- and heterogeneous versions of the catalyst, the Pd complex was supported on aminopropyl-functionalized silica-coated magnetite NPs ( Scheme 77 ) [ 62 ].…”

“… Strategy used for the preparation of Pd(II)-NHC complex and solid-supported magnetically retrievable Fe 3 O 4 @SiO 2 -Pd(II)-NHC catalyst. Reproduced with permission from [ 62 ]. …”

“… Catalytic investigation of Fe 3 O 4 @SiO 2 -Pd(II)-NHC catalyst in the Suzuki‒Miyaura coupling of aryl/heteroaryl bromides with arylboronic acids [ 62 ]. …”

PdII on Guanidine-Functionalized Fe3O4 Nanoparticles as an Efficient Heterogeneous Catalyst for Suzuki–Miyaura Cross-Coupling and Reduction of Nitroarenes in Aqueous Media

“…The utilization of appropriate supported ligands and the immobilization of ligands on heterogenized metals give a steady and sustainable catalyst framework, resulting in minimal leaching and metal aggregation issues. , Of such ligands, nitrogen-containing ligands and their derivatives have been considerably used as potent ligands, such as Schiff bases, − N-heterocyclic carbenes (NHC), − NNN-pincer ligands, pyrazoles, pyridines, DABCO, N-containing dendrimers, and thiols . Guanidine is a significant class of generally occurring compounds frequently used in natural science and is extensively exploited as catalysts .…”