Iron-Catalyzed/Mediated C-N Bond Formation: Competition between Substrate Amination and Ligand Amination.

Iron catalyzed carbon-nitrogen bond formation reactions of a wide variety of nucleophiles and aryl halides using well-defined iron-complexes featuring redox noninnocent 2-(arylazo)-1,10-phenanthroline (L1) ligands are reported. Besides substrate centered C-N coupling, C-N bond formation reactions were also observed at the ortho- and para-positions of the phenyl ring of the coordinated azo-aromatic scaffolds affording new tetradentate ligands, 2-N-aryl-(2-arylazo)-1,10-phenanthroline (L2), and tridentate ligands, 4 -N-aryl-(2-arylazo)-1,10-phenanthroline (L3), respectively. Control experiments and mechanistic studies reveal that the complex [FeL1Cl2] (1) undergoes in situ reduction during the catalytic reaction to produce the monoanionic complex [1]-, which then acts as the active catalyst. The metal (iron) and the coordinated ligand were found to work in a cooperative manner during the transfer processes involved in the fundamental steps of the catalytic cycle. Detailed experimental and theoretical (DFT) studies were performed to get insight into the competitive substrate versus ligand centered amination reactions.

Redox-Induced Interconversion and Ligand-Centered Hemilability in NiII Complexes of Redox-Noninnocent Azo-Aromatic Pincers.

A series of nickel(II) complexes, namely, [NiII(La-c)2Cl2] (1a-c), [NiII(La,b)3](X)2 {([2a](X)2, [2b](X)2) (X = ClO4, I3)}, [NiII(Lc)2(OH2)2](ClO4)2 ([3](ClO4)2) and [NiII{(La,b)·-}2] (4a, 4b) featuring the redox-active tridentate azo-aromatic pincer ligand 2-(arylazo)-1,10-phenanthroline (L) were synthesized. The coordinated azo-aromatic ligand showed reversible hemilability depending on its formal oxidation state. On the one hand, in its native state, the unreduced ligand L shows bidentate coordination; the 1,10-phenanthroline moiety binds the central Ni(II) atom in a bidentate fashion, while the azo-chromophore remains pendent. On the other hand, the one-electron reduced ligand [L]·- binds the nickel(II) atom in a tridentate fashion. In complexes 1, [2]2+, and [3]2+, the 1,10-phenanthroline moiety of the neutral unreduced azo-aromatic ligand L binds the central nickel(II) atom in a bidentate fashion, while the azo-chromophore remains pendent. The complex 4 is a singlet diradical species, where two monoanionic azo-anion radical ligands [L]·- are bound to the central nickel(II) center in a tridentate fashion. Redox-induced reversible hemilability of the coordinated azo-aromatic ligand L was revealed from the interconversion of the synthesized complexes upon reduction and oxidation. Complex 1 upon reduction transformed to complex 4 with the loss of two chlorido ligands, whereas the complex 4 upon oxidation in the presence of excess chloride (LiCl) source transformed back to 1. Similarly, the complexes [2]2+ and 4 were also found to be interconvertible upon reduction and oxidation, respectively. Thorough experimental and density functional theory studies were performed to unveil the electronic structures of the synthesized complexes, and attempt was made to understand the redox-induced hemilability of the coordinated azo-aromatic ligand L.

Deprotonation Induced Ligand Oxidation in a Ni(II) Complex of a Redox Noninnocent N(1)-(2-Aminophenyl)benzene-1,2-diamine and Its Use in Catalytic Alcohol Oxidation.

Two nickel(II)-complexes, [Ni(II)(H3L)2](ClO4)2 ([1](ClO4)2) and [Ni(II)(HL)2] (2), containing the redox-active tridentate ligand N(1)-(2-aminophenyl)benzene-1,2-diamine (H3L) have been synthesized. Complex [1](ClO4)2 is octahedral containing two neutral H3L ligands in a facial coordination mode, whereas complex 2 is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom with two pendant NH2 side arms from each of the coordinated ligands. Both complexes are found to be chemically interconvertible; complex [1](2+) gets converted to complex 2 when exposed to base and oxygen via simultaneous deprotonation and oxidation of the coordinated ligands. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory. The observed dissociation of a ligand arm upon oxidation of the ligand was exploited to bring about catalytic alcohol oxidation using coordinatively saturated complex [1](ClO4)2 as a catalyst precursor. Both the complexes [1](ClO4)2and 2 were tested for catalytic oxidation of both primary and secondary alcohols.