Kinetic studies on the electron transfer between azide and nickel(IV) oxime imine complexes in aqueous solution

In aqueous solution at 30 degrees C, azide is oxidized to N-2 by [Ni-IV(L(1))(2)](2+) and [Ni-IV(L(2))](2+) (where HL(1)=6-amino-3-methyl-4-azahex-3-en-2-one oxime and H(2)L(2)=3,14-dimethyl-4,7,10,13-tetraazahexadeca dioxime). The reaction is of overall second order, first order in both [N-3(-)] and [Ni-IV] and exhibits a strong dependence on [H+] of the medium In the range 2.0 less than or equal to pH<6.0 for [Ni-IV(L(1))(2)](2+) and 2.0 less than or equal to pH<4.5 for [Ni-IV(L(2))](2+) Single step two-electron transfer reactions are encountered, whereas a distinct biphasic process with initial faster step of Ni-IV-->Ni-III conversion followed by a second slower step of Ni-III-->Ni-II reduction was observed in the range 6.0 less than or equal to pH less than or equal to 8.0 and 4.50 less than or equal to pH less than or equal to 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 [Ni-IV(L(x))](2+) and N-3(-) follows an outer-sphere mechanism, but the reduction of [Ni-III(L(x))](+) may differ from this path.