Synthesis and characterization of PdII-methyl complexes with N-heterocyclic carbene-amine ligands

Abstract

A number of palladium(ii) complexes with a heteroditopic NHC-amine ligand and their precursor silver(i) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.

Crystal structure of di-μ-chlorido-bis[1-(2-(4-nitrophenyl)amidoethyl)-3- (2,4,6-trimethylphenyl)imidazol-2-ylidene-κC]di(dichlorido)dipalladium( II)) (Pd–Pd) dichloromethane solvate, C43H46Cl6N8O6Pd2

“…It was shown thatakinetically stable trans-isomer was obtained when two equivalents of NHC were coordinated to palladium [3].When only one equivalent is used, the dinuclear species is formed preferentially over thee xpectedm ononuclear species in which the ligand coordinates bidentate [4][5][6]. However, theamide does play arole, in contrast to earlier reports [3].…”

Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes

“…Subsequently, the carbenes are generated from the imidazolium bromides by reaction with silver(I) oxide. [5,6] This reliable protocol is routinely applied and the method of choice to stabilize reactive NHCs, while simultaneously giving access to a variety of TM complexes by transmetallation. [17,18] Additionally, the silver complexes are able to transfer the NHC to another transition metal, which obviates the need for generation of the free carbene.…”

“…In previous publications, we have shown the effect of various nitrogen donors on the coordinative properties of the ligands and the catalytic activity and selectivity of their zero-and divalent palladium complexes. [5][6][7]26] The use of picoline as donor in combination with NHC ligands is also known, and the coordination chemistry of these flexible aromatic have been studied by NMR and X-ray diffraction, confirming their bidentate nature. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the picolyl substituent to a high degree.…”

An Alternative to Nitromethane as Solvent: The Promoting Influence of Nitro‐Functionalized Imidazolium Salts for Synthesis and Catalysis

“…Under the reaction conditions, a carbene fragment might be somehow formed. Because of the presence of a tertiary amino group, the newly formed species might act as a chelating NHC-N donor hybrid ligand [17] to coordinate with the copper ion center, generating the Cu-carbene complex (II) that might possess a moderate activity for catalyzing the model Glaser coupling reaction. It should be noted that coordination of the diethylamino group to a metal center can form a six-membered ring that might allow an easy construction of the intermediate (II).…”