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Palladium(II) Complexes with Small N‐Heterocyclic Carbene Ligands as Highly Active Catalysts for the Suzuki–Miyaura Cross‐Coupling Reaction
Author(s) -
Szulmanowicz Michał S.,
Gniewek Andrzej,
Gil Wojciech,
Trzeciak Anna M.
Publication year - 2013
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201200428
Subject(s) - carbene , palladium , chemistry , catalysis , steric effects , aryl , suzuki reaction , medicinal chemistry , coupling reaction , ethylene glycol , organic chemistry , polymer chemistry , alkyl
Four new palladium(II) complexes of the type [Pd(NHC) 2 X 2 ] 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) 2 Br 2 ] (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 NaBPh 4 was used instead of PhB(OH) 2 , and the best results (turnover number=580 000) were obtained with [Pd(emim‐y) 2 Cl 2 ] (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) 2 Ph] + and [Pd 3 (NHC) 4 Ph] + —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.