Palladium(II) Complexes with Small N‐Heterocyclic Carbene Ligands as Highly Active Catalysts for the Suzuki–Miyaura Cross‐Coupling Reaction

Abstract

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 [Pdx(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.

Suzuki–Miyaura and Hiyama coupling catalyzed by PEPPSI-type complexes with non-bulky NHC ligand

“…Two complexes were selected for these experiments, Pd(bmim-y)Br 2 (CN-py) and Pd(bmim-y)Br 2 (Clpy), and in both cases only palladium fragments without the pyridine ligand were detected. In fact, the ESI-MS patterns obtained in these studies were very similar to those measured earlier for PdBr 2 (NHC) 2 complexes under the Suzuki-Miyaura conditions [31] . Accordingly, it can be assumed that catalytically active intermediate palladium species do not contain N ligands.…”

“…Considering the fact that the original PEPPSI catalysts contained bulky NHC ligands, we focused on the smaller bmim-y (1-butyl-3-methylimidazol-2-ylidene) NHC ligand, which was coordinated to palladium in the catalyst precursors. We were encouraged by very good results obtained in the Suzuki-Miyaura reaction for Pd(NHC) 2 Br 2 and [Pd(NHC)Br 2 ] 2 precursors with small NHC ligands [31,32]. It was supposed that the bmim-y ligand would be able to efficiently stabilize catalytically active forms of palladium, either molecular complexes or nanoparticles.…”

Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

“…As imilar mechanism was derived for a[ Pd II NHC] catalyst system for Suzuki-Miyaura cross-coupling reactionp roposed for catalysts. [107] In this publication the NHC ligand is still proposed to be part of the molecular active speciest hat is in equilibrium with the NPs, however,t he substrate to Pd ratios of such catalyst systems are generally orders of magnitude highert han for our system. [4,108] Efficient well-defined molecular catalysts ystems are expected to give values greatert han 40 %o fa ctive metal.However,i f also inactive NPs are formed, ap art of the added palladium is deactivated, thus leadingt oal ower percentage of active Pd atoms.…”

Synthesis, characterization, photophysical properties, and catalytic activity of an SCS bis(N-heterocyclic thione) (SCS-NHT) Pd pincer complex

“…In the Sonogashira reaction, diphenylacetylene was obtained in 58% yield in 18 h (Table 1, entry 5). [67][68][69][70][71][72][73] It is possible that compound 3 acts as a pre-catalyst as has been found for many palladium complexes. Explicitly, the catalytic trials did not become black.…”