A New Mode of Operation of Pd-NHC Systems Studied in a Catalytic Mizoroki-Heck Reaction

Metal complexes bearing N-heterocyclic carbene (NHC) ligands are typically considered the system of choice for homogeneous catalysis with well-defined molecular active species due to their stable metal-ligand framework. A detailed study involving 19 different Pd-NHC complexes with imidazolium, benzimidazolium, and triazolium ligands has been carried out in the present work and revealed a new mode of operation of metal-NHC systems. The catalytic activity of the studied Pd-NHC systems is predominantly determined by the cleavage of the metal-NHC bond, while the catalyst performance is strongly affected by the stabilization of in situ formed metal clusters. In the present study, the formation of Pd nanoparticles was observed from a broad range of metal complexes with NHC ligands under standard Mizoroki-Heck reaction conditions. A mechanistic analysis revealed two different pathways to connect Pd-NHC complexes to "cocktail"-type catalysis: (i) reductive elimination from a Pd(II) intermediate and the release of NHC-containing byproducts and (ii) dissociation of NHC ligands from Pd intermediates. Metal-NHC systems are ubiquitously applied in modern organic synthesis and catalysis, while the new mode of operation revealed in the present study guides catalyst design and opens a variety of novel opportunities. As shown by experimental studies and theoretical calculations, metal clusters and nanoparticles can be readily formed from M-NHC complexes after formation of new M-C or M-H bonds followed by C-NHC or H-NHC coupling. Thus, a combination of a classical molecular mode of operation and a novel cocktail-type mode of operation, described in the present study, may be anticipated as an intrinsic feature of M-NHC catalytic systems. © 2017 American Chemical Society.

Astakhov A.V.1 , Khazipov O.V.1 , Chernenko A.Yu.1 , Pasyukov D.V.1 , Kashin A.S.2 , Gordeev E.G.2 , Khrustalev V.N. 3 , Chernyshev V.M.1 , Ananikov V.P.1, 2
American Chemical Society
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  • 1 Platov South-Russian State Polytechnic University (NPI), Prosveschenya 132, Novocherkassk, 346428, Russian Federation
  • 2 Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow, 119991, Russian Federation
  • 3 Peoples' Friendship University of Russia, RUDN University, Miklukho-Maklay St. 6, Moscow, 117198, Russian Federation
Ключевые слова
Byproducts; Catalysis; Catalyst activity; Catalysts; Chemical reactions; Ligands; Metal complexes; Metal nanoparticles; Metals; Nanoparticles; Organic compounds; Palladium compounds; Catalyst performance; Homogeneous catalysis; Mechanistic analysis; Mizoroki-Heck reaction; Mode of operations; N-heterocyclic carbene ligands; Reductive elimination; Theoretical calculations; Palladium
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