Palladium(II) Complexes of N-Heterocyclic Carbene Amidates Derived from Chalcogenated Acetamide-Functionalized 1H-Benzimidazolium Salts: Recyclable Catalyst for Regioselective Arylation of Imidazoles under Aerobic Conditions

The chalcogenated acetamide-functionalized 1H-benzimidazolium salt precursors [1-(CH2C(O)NH(CH2)(2)S/ SePh)-3-R-C7H5N2](+) (X) over bar (LI-L4; R = Me, CH2Ph; X = Br, I) to C,N,S/Se ligands were synthesized by reaction of 1Hbenzimidazole with 2-bromo-N-(2-phenylthio/seleno)ethyl)acetamide (Al/A2), followed by treatment with methyl/benzyl bromide/iodide. The reaction of Ll-L4 with Ag20 followed by treatment with [Pd(CH3CN)(2)Cl-2] resulted in complexes [Pd(C,N,E)Cl] (C1-C4), where the C,N-,E pincer ligand was derived from L1-IA. Ll-L4 and C1-C4 were characterized by elemental analyses, HR-MS, and H-1, C-13{H-1}, and Se-77{H-1} NMR spectroscopy. Single-crystal structures of L2 and CI-C3 established with X-ray diffraction reveal the geometry of palladium as being nearly square planar and alignment of NHC rings as being nearly perpendicular to the coordination plane of Pd. In C1-C3 the Pd-C bond distances are in the range 1.974(5)1.982(2) angstrom. The Pd-S bond lengths for Cl and C3 are 2.369(10) and 2.357(7) angstrom, respectively. In C2, the Pd-Se bond distance is 2.464(7) angstrom. C1-C4 can be stored under ambient conditions for several months without decomposition, indicating their air and moisture insensitivity. Complexes C1-C4 were all found to be efficient for regioselective C-5 arylation of imidazoles under aerobic conditions. The optimum catalyst loading for good conversion was found to be 0.5-1 mol %. A wide range of ArCl/Br can be used with the present catalysts. Complexes C3 and C4 appear to be more efficient than C1 and C2. Similarly the complexes of selenium-containing ligands are more efficient as catalysts in comparison to their sulfur counterparts. C1-C4 were found to be recyclable up to six times for regioselective arylation of imidazole with very little decrease in efficiency. The products of regioselective arylation of imidazole with few ArX were identified by X-ray diffraction on their single crystals.