Kinetic studies of the reduction of nickel oxime complexes by tris(1,10-phenanthroline)cobalt(II) and hydroquinone in aqueous media

Abstract

Rate studies have been made on the reduction of a nickel(IV) bis-oxime, bis(6-amino-3-methyl-4-azahex-3-en-2-one oximato)nickel(IV), [Ni(L3)2]2+, and a nickel(III) mono-oxime (15-amino-3-methyl-4,7,10,13-tetra-azapentadec-3-en-2-one oximato)nickel(III), [Ni(L1)]2+, species using [Co(phen)3]2+(phen = 1,10–phenanthroline) and hydroquinone (H2quin), and on the oxidation of [NiII(HL1)]2+ using [NiIII(L4)2]3+(L4= 1,4,7-triazacyclononane). Reactions were carried out in the pH range 4–8, and using the lack of proton dependence on the rate associated with the CoII reductions, it is possible to distinguish pathways involving species derived from proton-related equilibria for both the nickel complexes and hydroquinone. Self-exchange rates have been determined using a Marcus correlation. In the case of the monoxime, for the deprotonated couple, [NiIII(L1)]2+–[NiII(L1)]+, kexc·=(4 ± 2)× 102 dm3 mol–1 s and for [NiIII(HL1)]3+–[NiII(HL1)]2+, kexc·ca. 6 × 102 dm3 mol–1 s–1. For the deprotonated bis-oxime complexes [NiIV(L3)2]2+–[NiIII(L3)2]+ and [NiIII(L3)2]+–[NiII(L3)2]0, kexc·=(8 ± 3)× 104 and (8 ± 4)× 103 dm3 mol–1 s–1 respectively. For the protonated couple, (pK= 4.43 ± 0.04), [NiIII(L3)(HL3)]2+–[NiII(L3)(HL3)]+, kexc·ca. 5 × 102 dm3 mol–1 s–1. However, for the reactions of the nickel(III) species with the quinone and catecholate anions, Hquin– and Hcat–, there is evidence for an inner-sphere process which may be substitution controlled. The rate constants are compared with the few data previously available, and are discussed with regard to the inner-sphere reorganisation required for electron transfer.