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

A series of nickel­(II) complexes, namely, [Ni^II(L^a–c)₂Cl₂] (<b>1a</b>–<b>c</b>), [Ni^II(L^a,b)₃]­(X)₂ {([<b>2a</b>]­(X)₂, [<b>2b</b>]­(X)₂) (X = ClO₄, I₃)}, [Ni^II(L^c)₂­(OH₂)₂]­(ClO₄)₂ ([<b>3</b>]­(ClO₄)₂) and [Ni^II{(L^a,b)^·–}₂] (<b>4a</b>, <b>4b</b>) featuring the redox-active tridentate azo-aromatic pincer ligand 2-(arylazo)-1,10-phenanthroline (<b>L</b>) 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 <b>L</b> 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 [<b>L</b>]^·– binds the nickel­(II) atom in a tridentate fashion. In complexes <b>1</b>, [<b>2</b>]²⁺, and [<b>3</b>]²⁺, the 1,10-phenanthroline moiety of the neutral unreduced azo-aromatic ligand <b>L</b> binds the central nickel­(II) atom in a bidentate fashion, while the azo-chromophore remains pendent. The complex <b>4</b> is a singlet diradical species, where two monoanionic azo-anion radical ligands [<b>L</b>]^·– are bound to the central nickel­(II) center in a tridentate fashion. Redox-induced reversible hemilability of the coordinated azo-aromatic ligand <b>L</b> was revealed from the interconversion of the synthesized complexes upon reduction and oxidation. Complex <b>1</b> upon reduction transformed to complex <b>4</b> with the loss of two chlorido ligands, whereas the complex <b>4</b> upon oxidation in the presence of excess chloride (LiCl) source transformed back to <b>1</b>. Similarly, the complexes [<b>2</b>]²⁺ and <b>4</b> 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 <b>L