Wydział Chemii

Michał S. Szulmanowicz

Andrzej Gniewek

Wojciech Gil

Anna M. Trzeciak

2013

ChemCatChem

5

1152-1160

10.1002/cctc.201200428

Naukowa

Angielski

Artykuł

Four new palladium(II) complexes of the type [Pd(NHC)₂X₂] with N‐heterocyclic carbene (NHC) ligands of relatively small steric hindrance were prepared and characterized by using spectroscopic and X‐ray methods. For [Pd(bmim‐y)₂Br₂] (bmim‐y=1‐butyl‐3‐methylimidazol‐2‐ylidene), crystals of both cis and trans isomers were obtained. All the studied complexes demonstrated very high activity in Suzuki–Miyaura cross‐coupling in ethylene glycol, which yielded turnover numbers of up to 760 000. High activity was also observed if NaBPh₄ was used instead of PhB(OH)₂, and the best results (turnover number=580 000) were obtained with [Pd(emim‐y)₂Cl₂] (emim‐y=1‐ethyl‐3‐methylimidazol‐2‐ylidene). In the reaction mixture, different forms containing [Pd_x (NHC)_y ]⁺ fragments (x =1–4, y =2–5) were identified by using ESI‐MS. In the presence of Suzuki–Miyaura reaction substrates, catalytic palladium intermediates with aryl groups—[Pd(NHC)₂Ph]⁺ and [Pd₃(NHC)₄Ph]⁺—were detected. Additional mechanistic investigations, such as TEM observations and mercury poisoning experiments, substantiated the formation of nanoparticles as a catalyst resting state. These heterogeneous particles serve as a reservoir for soluble palladium species—atoms or clusters that function as homogeneous catalysts for the Suzuki–Miyaura reaction.

carbene compounds, cross-coupling reactions, Heterogeneous catalysis, homogeneous catalysis, palladium

http://dx.doi.org/10.1002/cctc.201200428

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