Imidazolio-substituted secondary phosphine oxides as potential carbene reagents

New pre-ligands of 1-aryl-1H-imidazol-2-yl substituted secondary phosphine oxides (HRP(O)-Im:SPO-Im) were prepared and characterized by spectroscopic means. The SPO-Im pre-ligands are able to tautomerize to their corresponding genuine ligands, the 1-aryl-1H-imidazol-2-yl substituted phosphinous acids ((HO)RP-Im:PA–Im) while the condition permitted. Further reactions of selected SPO-Im pre-ligands toward Pd(COD)Cl2 or PdBr yielded bis-(PA–Im)-coordinated cis-palladium dichloride or dibromide complexes, respectively. These cis-palladium dihalide complexes are all having an intramolecular hydrogen bond between two coordinated PA–Im ligands (R(Im(H)+)P–O−⋯HO–PRIm) by transferring a proton from one of the two P–OH groups to its free nitrogen site of imidazolyl substituents; and, therefore, imposes the formation of zwitterionic cis-form palladium complexes. Three crystal structures of these square–planar zwitterionic palladium complexes were determined by the X-ray diffraction methods. Additionally, two novel NHC carbene coordinated palladium complexes were obtained from the crystal-growing process of the above-mentioned zwitterionic cis-palladium dihalide complexes. The crystal structures of two NHC carbene coordinated palladium complexes were also successfully determined. Density-functional theory (DFT) calculations were carried out to substantiate the feasibility of the proposed reaction pathways for the formation of NHC carbene coordinated palladium complexes. Finally, applying selected SPO-Im pre-ligands to Heck reactions with pre-optimized reaction conditions exhibited satisfactory performances.

The reaction of SPO-Im 1h with Pd(COD)Cl2 yielded a bis-1h′ (PA–Im) coordinated cis-palladium dichloride complex, 2Ah′. It subsequently converted to a carbene coordinated palladium complex, 3Ah, in the presence of moisture, and eventually to 4Ah. The P–C(imidazole) bond of 1h′ was dissected and the dissociated species acted as a NHC carbene ligand towards palladium metal.Figure optionsDownload as PowerPoint slide