X-ray crystal structure* and kinetics of oxidation of 1,4,8,11-tetraazaundecanenickel(II), [NiII(2,3,2-tet)]2+ by peroxodisulphate: Formation of a stable non-macrocyclic nickel(III) polyamine

While macrocyclic ligands are well-known to stabilize tervalent nickel, it is very unusual to form stable nickel(III)-amine species in aqueous solution. We describe the kinetics of oxidation of 1,4,8,11-tetraazaundecanenickel(II), [Ni^II(2,3,2-tet)]²⁺ by peroxodisulphate in aqueous solution to the tervalent state. The reaction follows a second order rate law, first order in [Ni^II(2,3,2-tet)]²⁺ and first order in peroxodisulphate. The activation enthalpy and entropy are 55 ± 3 kJ mol^–1 and –38 ± 10 J K^–1 mol^–1 respectively. The X-ray crystal structure of the [Ni^II(2,3,2-tet)]²⁺ cation is also described. The title compound crystallizes in the monoclinic space group P2(1)/n. The cation has a slightly distorted square planar structure, with the bite angle of the 5-membered chelate rings being slightly less than 90°, and that of the 6-membered chelate ring being correspondingly greater than 90°.