Kinetic Studies on the Electron Transfer between Azide and Nickel(IV) Oxime Imine Complexes in Aqueous Solution

Abstract

Abstract In aqueous solution at 30 °C, azide is oxidized to N2 by [NiIV(L1)2]2+ and [NiIV(L2)]2+ (where HL1 = 6-amino-3-methyl-4-azahex-3-en-2-one oxime and H2L2 = 3,14-dimethyl-4,7,10,13-tetraazahexadeca-3,13-diene-2,15-dione dioxime). The reaction is of overall second order, first order in both [N3−] and [NiIV] and exhibits a strong dependence on [H+] of the medium. In the range 2.0 ≤ pH < 6.0 for [NiIV(L1)2]2+ and 2.0 ≤ pH < 4.5 for [NiIV(L2)]2+ single step two-electron transfer reactions are encountered, whereas a distinct biphasic process with initial faster step of NiIV → NiIII conversion followed by a second slower step of NiIII → NiII reduction was observed in the range 6.0 ≤ pH ≤ 8.0 and 4.50 ≤ pH ≤ 8.0 for the respective oxidants. The bell-shaped pH-rate profile for the reduction of Ni(IV) → Ni(III) step confirms the involvement of an ‘outside’ protonated Ni(IV)-complex as a kinetic intermediate. All the kinetic parameters have been evaluated and the mechanism of the reaction is discussed. Application of Marcus cross-reaction relations shows that the reaction of [NiIV(Lx)]2+ and N3− follows an outer-sphere mechanism, but the reduction of [NiIII(Lx)]+ may differ from this path.