Electron transfer from thiosulfate to nickel(IV) oxime imine complexes in aqueous solution

Abstract

The kinetics of oxidation of thiosulfate by the complexes [NiIVL32]2+ and [NiIVL2]2+(HL3= 6-amino-3-methyl-4-azahex-3-en-2-one oxime, H2L2= 3,14-dimethyl-4,7,10,13-tetraazahexadeca-3,13-diene-2,15-dione dioxime) was studied spectrophotometrically under pseudo-first-order conditions with an excess of thiosulfate in the range pH 2.50–8.0 with [NiIV]= 5.0 × 10–5 mol dm–3, [S2O32–]=(1.0–10.0)× 10–3 mol dm–3, I= 0.20 mol dm–3(NaClO4) at 20 °C. A monophasic two-electron transfer was found in the regions 3.0 ⩽ pH ⩽ 5.0 and 2.5 ⩽ pH ⩽ 4.0 for reduction of the two complexes respectively. Above these pH regions both reactions showed biphasic decay with an initial faster step NiIV→ NiIII and second slower step NiIII→ NiII, and the general rate law is –d[NiIV(Lx)2+]/dt=kobs[NiIV(Lx)2+]= 2nk[NiIV(Lx)2+][S2O32–] where 2n is a stoichiometric factor. All the kinetic parameters have been evaluated by choosing suitable pH regions. An additional protonation of the nickel(IV) complexes (pKa= 4.91 and 4.28 respectively) has been encountered and a realistic interpretation is achieved by considering a weak acid–base interaction arising out of outer-sphere protonation of the [NiIV(Lx)]2+ complexes. All the reaction steps have been scrutinised. The reduction [NiIV(Lx)]2+→[NiIII(Lx)]+ is assumed to follow an outer-sphere mechanism whereas [NiIII(Lx)]+→[NiII(Lx)] follows an inner-sphere route. The calculated electron-transfer rate constant (k12) obtained through Marcus cross-reaction relations can be considered as a proof of such delineation.