Mechanisms of reduction of a nickel(IV) oxime complex by 1,2- and 1,4-dihydroxybenzene compounds in aqueous perchlorate media

Abstract

The kinetics of the reduction of a nickel(IV) oxime complex, [Ni^IVL]²⁺(H₂L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by catechol, hydroquinone, and 2-methylhydroquinone (H₂Q) have been investigated in aqueous perchlorate media in the pH range 3.5–6. Below pH 4.5 the reaction is monophasic with the Ni^IV–Ni^III step rate-determining. Above pH 4.7 a biphasic process is observed with a rapid Ni^IV–Ni^III reduction followed by a slower reaction of the nickel(III) intermediate. A complex pH dependence is attributed to reaction pathways involving several reductant and oxidant species related by proton equilibria. For the principal reaction pathways involving [Ni^IVL]²⁺ and HQ^–, rate constants fall in the range ca. 10⁷–10⁸ dm³ mol^–1 s^–1. With H₂Q as reductant, rate constants are ca. 10⁵–10⁶ times smaller. The specific rate parameters compare favourably with the rate constants determined by Marcus theory calculations.