Redox-Induced Interconversion and Ligand-Centered Hemilability in Ni^II Complexes of Redox-Noninnocent Azo-Aromatic Pincers

Abstract

A series of nickel(II) complexes, namely, [Ni(L)Cl] (1a-c), [Ni(L)](X) {([2a](X), [2b](X)) (X = ClO, I)}, [Ni(L)(OH)](ClO) ([3](ClO)) and [Ni{(L)}] (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], and [3], 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] 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.

Dehydrogenative Synthesis of Quinolines, 2-Aminoquinolines, and Quinazolines Using Singlet Diradical Ni(II)-Catalysts

“…The color of the catalysts 2 and 3 immediately changes upon addition of K t BuO. Because, other than acting as base, alkali metal tertiary butoxides are known to be capable of acting as a reducing agent, the observed color change may be because of simple deprotonation of any of the NH groups of the coordinated ligand or it may be because of the reduction of the coordinated ligands. Therefore, to check whether the origin of the color changes observed upon addition of K t BuO is because of reduction of the catalyst, the reaction mixtures containing K t BuO and catalyst 2 or 3 were studied by EPR.…”

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

“…The >99.99% FeCl 2 , > 99.99% FeCl 3 , and iodobenzenes were obtained from Sigma-Aldrich. L 1a‑c and [Fe II L 1a‑c Cl 2 ] ( 1a – c ) were synthesized following the reported literature methods using >99.99% FeCl 2 obtained from Sigma-Aldrich as the iron source . All other starting materials, chemicals, and reagents were obtained from the available commercial sources and used as received.…”

Neocuproine as a Redox-Active Ligand Platform on Iron and Cobalt

“…Metals complexed to redox-noninnocent ligands have emerged as a promising strategy for facilitating multielectron reactivity. − In particular, this strategy allows first-row transition metals − and base metals , to facilitate redox reactivity more traditionally mediated by precious-metal complexes. An exemplary case is provided by pyridinediimine (PDI) metal complexes, which facilitate an impressive breadth of catalytic and stoichiometric reactivity. ,− …”