Anion radical oxidation of nickel(II) macrocyclic complexes. Pulse radiolysis of (2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene) nickel(II) in sodium bromide solution

Article abstract of DOI:10.1021/ic50219a026

The oxidation of Ni^II(CR+4H) [CR+4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene, α isomer] by Br₂^- and subsequent chemistry of Ni(III) intermediates have been studied by pulse radiolysis. The initial electron transfer is diffusion controlled and results in an initial product which exhibits characteristics of a Ni(III)-Br^- adduct. The stability of this adduct, especially in acidic media, is dependent on bulk bromide concentration, demonstrating an equilibrium process of the form Ni^III(CR+4H)(Br^-)(H₂O) + H₂O ? Ni^III(CR+4H)(H₂O)₂ + Br^-, with K_eq = 360 M^-1. The pK of dissociation for the aquated species is 4.0 ± 0.8. At pH >5 displacement of the bound bromide is first order in [OH^-] with a rate constant of 6 × 10⁹ M^-1 s^-1. Disappearance of the Ni^III(CR+4H)(OH^-) intermediate is also pH dependent, suggesting amine dissociation in the ligand and formation of an unstable ligand radical leading to a final product with increased ligand unsaturation.