Structure, magnetic properties and electronic structure of a nickel(II) complex with redox-active 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol

Abstract

We report the preparation and electronic structure of a nickel complex 2 containing ligand 6-(8-quinolylamino)-2,4-bis(tert-butyl)phenol 1. The complex is neutral, six-coordinate (pseudo-octahedral) and homoleptic containing two equivalents of 1 and no counteranions. The bond distances measured by single crystal X-ray diffraction studies suggest that the two equivalents of 1 are bound as mono-anion radicals (iminosemiquinone oxidation state). Variable temperature magnetic susceptibility data indicates a quintet (S=2) ground state for this complex resulting from ferromagnetic coupling between Ni2+ and the coordinated radical anions. Cyclic voltammetry experiments and UV–Vis–NIR spectroscopy indicate the presence of coordinated iminosemiquinone radicals, and these experimental results are corroborated by density functional theory calculations, which support the proposed assignments. Complex 2 is easily oxidized in the presence of ferrocenium hexafluorophosphate or aerial dioxygen to produce a complex with closed shell ligands and spectroscopic features characteristic of ortho-iminoquinone ligands.