Regioselective ortho Amination of Coordinated 2-(Arylazo)pyridine. Isolation of Monoradical Palladium Complexes of a New Series of Azo-Aromatic Pincer Ligands

In an unusual reaction of [Pd(L1)Cl2] (L1 = 2-(arylazo)pyridine) with amines, a new series of palladium complexes [Pd(L2•–)Cl] (L2 = 2-((2-amino)arylazo)pyridine) (1a–1h) were isolated. The complexes were formed via N–H and N–C bond cleavage reactions of 1°/2° and 3° amines, respectively, followed by regioselective aromatic ortho-C–N bond formation reaction and are associated with ortho-C–H/ortho-C–Cl bond activation. A large variety of amines including both aromatic and aliphatic were found to be effective in producing air-stable complexes. Identity of the resultant complexes was confirmed by their X-ray structure determination. Efforts were also made to understand the mechanism of the reaction. A series of experiments were performed, which point toward initial ligand reduction followed by intraligand electron transfer. Examination of the structural parameters of these complexes (1) indicates that the in situ generated ligand coordinated to the PdII center serves as the backbone of these air-stable monor...