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 [Ni^II(HL¹)]²⁺ using [Ni^III(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 Co^II 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, [Ni^III(L¹)]²⁺–[Ni^II(L¹)]⁺, k_exc·=(4 ± 2)× 10² dm³ mol^–1 s and for [Ni^III(HL¹)]³⁺–[Ni^II(HL¹)]²⁺, k_exc·ca. 6 × 10² dm³ mol^–1 s^–1. For the deprotonated bis-oxime complexes [Ni^IV(L³)₂]²⁺–[Ni^III(L³)₂]⁺ and [Ni^III(L³)₂]⁺–[Ni^II(L³)₂]⁰, k_exc·=(8 ± 3)× 10⁴ and (8 ± 4)× 10³ dm³ mol^–1 s^–1 respectively. For the protonated couple, (pK= 4.43 ± 0.04), [Ni^III(L³)(HL³)]²⁺–[Ni^II(L³)(HL³)]⁺, k_exc·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.