Highly Active and Magnetically Recoverable Pd-NHC Catalyst Immobilized on Fe₃O₄ Nanoparticle-Ionic Liquid Matrix for Suzuki Reaction in Water

Abstract

Pd-NHC-ionic liquid matrix was immobilized into ionic liquid layers coated on the surface of Fe₃O₄. The immobilized Pd-NHC was used as a catalyst for Suzuki coupling. The catalyst exhibited both high catalytic activity and stability for the coupling between aryl bromide and arylboronic acid in water. This catalyst was simply recovered by an external permanent magnet and recycled without a significant loss in the catalytic activity.

Key words

heterogeneous catalysis - Suzuki reaction - palladium - magnetic nanoparticle - ionic liquids

References and Notes

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12

Surface Modification of Nano-Fe ₃ O ₄
To a solution of 1-(3-trimethoxysilylpropyl)-3-methyl-imidazolium chloride (0.28 g, 1.0 mmol) in toluene was added nano-sized Fe₃O₄ (Aldrich, 1.0 g). The mixture was stirred at 100 ˚C for 10 h. After cooling, the nano-Fe₃O₄ was magnetically separated from reaction mixture. Modified Fe₃O₄ 2 was washed with CH₂Cl₂ several times and dried at 60 ˚C under vacuum. Elemental analysis and weight gain showed that 0.67 mmol of 1-(3-trimethoxysilylpropyl)-3-methylimidazolium chloride was anchored on 1.0 g of 2.

13

Immobilization of NHC-Pd onto Modified Nano-Fe ₃ O ₄ 2 To a solution of Pd-NHC 1 (195 mg, 0.29 mmol) and
1-butyl-3-methylimidazolium hexafluorophosphate (165 mg, 0.58 mmol) in CH₂Cl₂ (2 mL) modified Fe₃O₄ 2 (1.0 g) was added. The mixture was sonicated for 15 min at r.t., and then CH₂Cl₂ was slowly removed under reduced pressure. The resulting powder was washed with Et₂O and dried under vacuum at 60 ˚C to give Pd-NHC@Fe₃O₄-IL 3 (1.21 g). The Pd content of 0.17 mmol/g was measured by inductively coupled plasma (ICP) analysis.

16

General Procedure for the Suzuki Coupling Reaction
Aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K₃PO₄ (424 mg, 2.0 mmol), TBAB (161 mg, 0.5 mmol), dodecane (40 mg, internal standard), and catalyst 3 (30 mg, 0.5 mol%) were mixed in H₂O (2.0 mL). The mixture was stirred at 75 ˚C in an air atmosphere. The reaction was periodically monitored by GC. After magnetic separation of the catalyst, the organic material was twice extracted with Et₂O. The organic phase was dried over MgSO₄, and the solvent was evaporated under reduced pressure. The crude was analyzed by GC/GC-MS. The product was purified by short column chromatography on silica gel.

17

Reuse of Pd-NHC@Fe ₃ O ₄ -IL 3 In the recycling experiment the reaction was performed by using a mixture of 4-bromoanisole (187 mg, 1.0 mmol), phenylboronic acid (134 mg, 1.2 mmol), K₃PO₄ (424 mg, 2.0 mmol), TBAB (161 mg, 0.5 mmol), and catalyst 3 (30 mg, 0.5 mol%) in H₂O (2.0 mL) at 75 ˚C for 0.7 h. After completion of the reaction, the catalyst was magnetically separated from the solution. The solution was worked up as described above. The separated catalyst was successively reused for the next reaction without any pre-treatment.