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

Abstract

In an unusual reaction of [Pd(L(1))Cl2] (L(1) = 2-(arylazo)pyridine) with amines, a new series of palladium complexes [Pd(L(2•-))Cl] (L(2) = 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 Pd(II) 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).

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

“…Thus, the emerging trend of utilizing “redox-active” or “noninnocent” ligand-derived transition-metal complexes has been expanding at a faster rate primarily because of their multielectron storage capacity . In this regard, metal complexes of azoheteroarenes have played an important role as redox equivalents in catalysis, molecular materials, and formation of bis-stable complexes , besides their fundamental electron-transfer aspects . Although 2,2′-azobis­(pyridine) (abpy) and its analogues are well-illustrated with varying metal precursors and hold a privileged position in the library of redox-noninnocent ligands, the fascinating redox features of its structurally modified form, i.e., azobis­(benzazole), have recently been divulged, which in effect emphasizes its further exploration.…”

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

“…The complex 1a is sparingly soluble in most of the common organic solvents, such as toluene, dichloromethane, acetonitrile, methanol, tetrahydrofuran, etc., but is freely soluble in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Single-crystal X-ray diffraction (XRD) studies 47 reveal that the zinc(II) ion resides in a distorted trigonal bipyramidal coordination sphere in which two N(azo) atoms (N1 and N5) occupy the apical positions. The N1−Zn−N5 bond angle is 141.20 (12)°, while the N3 atom (pyridine nitrogen) and two coordinated chlorides occupy the basal plane (Figure 3).…”

Inner-sphere electron transfer at the ruthenium-azo interface

“…6 The successive electron-uptake processes of the azo functionality ([–NN–] 0 → [–NN–]˙ − → [–N–N–] 2− ) resemble the redox behaviours of isovalent O 2 (O 2 → O 2 ˙ − → O 2 2− ) and aryl nitroso (ArNO → ArNO˙ − → ArNO 2− ) functionalities. 7 The reversible electron transfer properties of the azo moiety support its role as a redox equivalent in catalysis, 8 including redox-driven azo cleavage to generate the corresponding metal-imido moiety, which is an active intermediate in metathesis, cycloaddition, nitrene transfer, and hydroamination reactions. 8 a,b Further, the redox-induced bis-stability of the azo functionality is reported to facilitate its utility when designing memory devices.…”