Cyclometallation. Palladium 2-arylpyridine complexes

Abstract

A number of new cyclometallated compounds of palladium with 2-arylpyridines have been synthesized and characterized. Palladium acetate has been proven to be a more useful starting material than Li 2PdCl 4, since the resulting acetato-bridged dimers, [Pd(OAc)(2-arylpyridine)] 2, unlike the chloro-bridged dimers, are conveniently soluble in common organic solvents. The effect of varying substituents on the aryl nucleus supports the concept that after initial N-complexation, the 2-position of the aryl nucleus undergoes electrophilic attack by the palladium atom. Detailed NMR studies of the soluble acetato compounds showed that the 6-heteroaryl and “ ortho” to the Pd-C bond protons in the complexes are shifted (≈0.75 and 0.5 ppm, respectively) upfield from the ligand position. These shielding effects are believed to be due primarily to through-space interactions of overlying aromatic rings and secondarily to through-bond (Pd-to-ligand) effects. Both acetato- and chloro-bridged dimers will react with certain ligands to give mononuclear species containing the intact cyclometallated ligand.