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
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The oxidation of Ni/sup II/(CR + 4H) (CR + 4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo(11.3.1)heptadeca-1(17),13,15-triene, ..cap alpha.. isomer) by Br/sub 2//sup -/ 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/sup -/ adduct. The stability of this adduct, especially in acidic media, is dependent on bulk bromide concentration, demonstrating an equilibrium process of the form Ni/sup III/(CR + 4H)(Br/sup -/)(H/sub 2/O) + H/sub 2/O in equilibrium Ni/sup III/(CR + 4H)(H/sub 2/O)/sub 2/ + Br/sup -/, with K/sub eq/ = 360 M/sup -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/sup -/) with a rate constant of 6 x 10/sup 9/ M/sup -1/ s/sup -1/. Disappearance of the Ni/sup III/(CR + 4H)(OH/sup -/) 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.