New Bidentate Trans-Chelating N-Heterocyclic Carbene Ligands for Palladium

Abstract

A family of imidazolium salts of the type [BnN(CH 2 CH 2 CH 2 R Im) 2 ] 3 2[Cl] (Bn = CH 2 Ph; R Im = 1-methylimidazole (1a), 1-tertbutylimidazole (1b), 1-benzylimidazole (1c), 1-methylbenzimidazole (1d)), which contain a tertiary amine linking two imidazolium groups, has been synthesized. These imidazolium salts can be deprotonated with Ag 2 O to generate the Ag carbene complexes 2d)). In the solid state 2d exists as an unusual tetramer, which consists of an [Ag 2 Cl 4 ] 2À core bridging two Ag(NHC) cations. Subsequent reaction of the Ag complexes with PdCl 2 (MeCN) 2 generates Pd species of the type {BnN(CH 2 CH 2 CH 2 R Im) 2 }PdCl 2 ( R Im = 1-methylimidazole (3a), 1-tert-butylimidazole (3b), 1-benzylimidazole (3c), 1-methylbenzimidazole (3d)), which is a rare example of a family of Pd complexes that contain a bidentate trans-chelating N-heterocyclic carbene ligand. Compounds 3a and 3c were crystallographically characterized by X-ray crystallography and contain unusual 12-membered metallacycles. DFT calculations suggest that the preference for trans binding of the ligand is related to conformational effects of the linker. Compound 3b reacts with excess MeI to form {BnN(CH 2 CH 2 CH 2 tBu Im) 2 }PdI 2 (5b), a reaction in which we believe a Pd(IV) intermediate is generated. Compound 5b was crystallographically characterized. Compounds 3aÀd are all active catalysts for the Heck reaction, and 3a can also catalyze the Suzuki reaction.

Group 10 Metal Complexes of a Ferrocene‐Based N‐Heterocyclic Carbene: Syntheses, Structures and Catalytic Applications

“…Complexes of the type [PdCl 2 (NHC) 2 ] are often obtained as mixtures of the cis and trans isomers,20 even with chelating bis(NHC) ligands 21. This is in agreement with results of DFT calculations published by Blakemore et al, which revealed that the trans isomer of [PdCl 2 (NHC) 2 ] is only slightly more stable than the cis isomer 22. The five‐membered NHC complex [PdCl 2 (IBn) 2 ] and the six‐membered analogue [PdCl 2 (THP‐ i Pr) 2 ] adopt a cis and a trans structure, respectively, in the solid state 18,19.…”

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

“…Bulky Mes wingtips prevented associative substitution, which led to the low activity of complex 30 . Later, Hazari and co‐workers prepared four trans ‐chelating complexes, such as 32 , through transmetalation . The trans ‐spanning bis(NHC)‐ligated palladium centers disfavored oxidative addition of the substrate.…”

Recent Advances in the Schiff Bases and N‐Heterocyclic Carbenes as Ligands in the Cross‐Coupling Reactions: A Comprehensive Review

“…Blakemore et al . reported a family of imidazolium salts, which comprise a 3° amine linking two imidazolium groups as shown in Scheme . These imidazolium salts ( L69 ) were deprotonated with Ag 2 O to produce the silver carbene complexes [{BnN(CH 2 CH 2 CH 2 R Im) 2 }Ag]·[AgCl 2 ] ( C113 ), which on reaction with PdCl 2 (MeCN) 2 produces palladium species of the type {BnN(CH 2 CH 2 CH 2 R Im) 2 }PdCl 2 ( C114 ) in 82–96% yields, a rare case of a family of Pd complexes that contain a bidentate trans ‐chelating NHC ligand.…”