Syntheses and Structural Characterizations of Benzoxazolyl N-Heterocyclic Carbene-Palladium Complexes and Their Applications

Six seven-membered benzoxazolyl N-heterocyclic carbene (Benzoxa<^>NHC) palladium complexes with flexible linear alkyl or benzyl linkers were synthesized. Their structures were characterized by NMR, HRMS, and FTIR spectroscopies. The structures of three complexes were further confirmed by X-ray single crystal diffraction. Palladium center adopted slight distorted square planer geometry, and the benzoxazole and NHC planes in its ligand formed a certain angle with each other, not coplanar. The first methylene group connected to a nitrogen-containing heterocyclic carbene fragment exhibited magnetic anisotropy in the complex. The shift difference in the methylene group (Delta delta(g)) changed with the increase of the length of the side chain. When the side chain was a linear chain alkyl group of four carbons, Delta delta(g) reched maximum. Furthermore, these complexes showed excellent catalytic performance in Suzuki-Miyaura coupling, and could be used to synthesize biphenyl gem-difluoroethenes in the manner of two steps in one pot.