Palladium Complexes with Pyrimidine-Functionalized N-Heterocyclic Carbene Ligands: Synthesis, Structure and Catalytic Activity

Abstract

A series of novel pyrimidine functionalized palladium(II)(NHC) complexes with aryl and alkyl substituents [1-(2-Pyrimidyl)-3-(aryl or alkyl)imidazoline-2-ylidene palladium(II) chlorides] was synthesized by transmetalation via the corresponding silver complexes. All compounds have been fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and in two cases by X-ray single crystal structures. Different solid state structures were observed for aryl and alkyl substituted ligands: for the sterically less demanding methyl substituent, a complex is formed, where two ligands are coordinated to one metal center [Pd(L)2Cl]+, while in the mesityl case a [Pd(L)Cl2] structure was observed. They show good catalytic activity in the CH activation of methane as well as in the Mizoroki-Heck reaction, where especially the methyl substituted complex shows a remarkably high activity combined with a very high selectivity.

Activity and Recyclability Improvement Through Adjusting the Tethering Strategy for Pd-Catalyzed Suzuki–Miyaura Coupling Reaction of Aryl Chlorides

“…In addition to these, the signals near 120 ppm were attributed to the unsaturated carbons of imidazole ring, and the resonances of the pyrimidine were at 120-160 ppm (Fig. 2b), which was in good agreement with the Palladium complexes in Thomas's work [55]. The spectra of catalysts 6 and 7 both showed the signals at 15 ppm due to carbons linked to SBA-15, and the resonances at 20-30 ppm were attributed to the methyl and methylene group unlinked to the nitrogen atom.…”

“…For catalyst 5 with less sterically demanding pyrimidine group [55], the yield was only 35 %, and no byproducts were observed. These results are consistent with the previous findings that the steric bulk of the substituents surrounding the metal centre enhances catalytic activity [15].…”

Preparation of Mononuclear, Homodinuclear, and Heterotrinuclear Complexes by Salicylaldiminato‐Functionalized Imidazolium Salt: Approach to Multifunctional Catalysts

“…[8] In the reaction between ligand 1 a and [{Cp*IrCl 2 } 2 ] (Cp*= pentamethylcyclopentadiene), we also isolated a minor product 2 a', which could arise from intramolecular C À H activation. All four compounds were characterized by NMR spectroscopy, elemental analysis, and in the case of the complex 2 a, by X-ray diffraction studies.…”

Development of Molecular and Solid Catalysts for the Direct Low‐Temperature Oxidation of Methane to Methanol

“…Recently, Meyer et al reported improved Pd complexes based on pyrimidine-functionalized NHC ligands, reaching TONs of 41 within 17 h reaction time. [14] Systematic complex and ligand design appear to open the way for further improvements. A superior system for methane oxidation in sulfuric acid has been reported by Periana et al [15] They identified a platinum-bipyrimidine complex, [PtA C H T U N G T R E N N U N G (bpym)], reaching TONs of around 300 at 81 % selectivity to methyl bisulfate as a highly promising system with the highest activity described thus far.…”