Recyclable Pd(0)-Pd(II) composites formed from Pd(II) dimers with NHC ligands under Suzuki–Miyaura conditions

•Pd–NHC dimers catalyze Suzuki–Miyaura reaction in air with TOF up to 106 h−1.•Catalytic activity was independent on the steric hindrance of NHC ligands.•Pd–NHC precursors are transformed to the Pd(0)-Pd(II) composites.•Catalytic system was reused several times with very good activity.•Agglomeration of Pd(0) nanoparticles was not observed.

Dimeric complexes of the type [Pd(μ-X)X(NHC)]2 were employed in the Suzuki–Miyaura cross-coupling leading to 2-methylbiphenyl. The excellent activity of the studied dimers is perfectly illustrated by a TOF of up to 106 h−1. Mechanistic investigations with the application of TEM, XPS, and mercury poisoning tests provided an insight into the nature of the catalytic process. Accordingly, the reduction of the dimeric palladium complex [Pd(μ-X)X(NHC)]2 resulted in the formation of a composite containing Pd(0) nanoparticles protected by a layer of a Pd(II) species such as anions [PdBr4]2− or [PdBr3(NHC)]−. The in situ formation of Pd(0)–Pd(II) composites resulted in the high stability of the catalytic system, which was active in ten subsequent recycles without any additional protection.

Graphical abstractPd–NHC dimers catalyze Suzuki–Miyaura reaction in air with TOF up to 106 h−1. During catalytic reaction Pd–NHC precursors are transformed to the Pd(0)-Pd(II) composites which can be reused several times with very good activity. Agglomeration of Pd(0) nanoparticles was not observed.Figure optionsDownload full-size imageDownload as PowerPoint slide