Azo-containing pyridine amide ligand. A six-coordinate nickel(II) complex and its one-electron oxidized species: structure and properties

Abstract

A new potentially tridentate ligand HL¹¹ consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L¹¹(-) affords a bis-complex [Ni^II(L¹¹)₂] (1). In 1, the two L¹¹(-) ligands bind to the Ni^II center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The Ni^IIN₂(pyridyl)N'₂(amide)N"₂(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni-N_am (am = amide) bond lengths are the shortest and the Ni-N_azo bond lengths are the longest. Complex 1 exhibits a quasireversible Ni^III/Ni^II redox process. Moreover, the complex displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results.