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

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posted on 07.12.2015, 00:00 by Debabrata Sengupta, Nabanita Saha Chowdhury, Subhas Samanta, Pradip Ghosh, Saikat Kumar Seth, Serhiy Demeshko, Franc Meyer, Sreebrata Goswami
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) (1a1h) 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 monoradical complexes. Molecular and electronic structures of the isolated complexes were further scrutinized by various spectroscopic techniques including cyclic voltammetry, variable temperature magnetic susceptibility measurements, electron paramagnetic resonance, and UV–vis spectroscopy. Finally the electronic structure was confirmed by density functional theory calculations. The isolated monoradical complexes adopt an unusual π-stacked array, which leads to a relatively strong antiferromagnetic interaction (J = −40 cm–1 for the representative complex 1c).