Phenothiazine radical-cations: electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis

Abstract

The establishment of an equilibrium reaction between iodine radical-anions, I₂^–˙, the protonated form of the phenothiazine, promethazine, PZH⁺, its related radical-cation, PZH²⁺˙, and iodide ions, I^–, has been observed directly by pulse radiolysis [reaction (i)]. From measurements of the radical-anion and radicalcation I₂^–˙+ PZH⁺⇌ PZH²⁺˙+ 21^–(i) equilibrium concentrations at different iodide and promethazine concentrations a value for the equilibrium constant K₃= 94 mol dm^–3 has been attained. A closely similar value K₃= 85 mol dm^–3 has been derived from a kinetic analysis of the radical concentrations as equilibrium was approached. Taking the mean value K₃= 90 ± 5 mol dm^–3 and a reported value of K₈= 8.85 × 10^–6 mol dm^–3 for the equilibrium reaction (ii) the difference in redox potentials given by equation (iii) can be calculated. Values of I₂^–˙⇌ I˙+ I^–(ii), ΔE°=E°(PZH²⁺˙/PZH⁺)–E°(I˙/I^–)=–0.415 V (iii)E°(PZH²⁺˙/PZH⁺)=+0.865 V and E°(I˙/I^–)=+1.31 V have been reported previously. Our experimental result agrees very well with the calculated ΔE°=–0.445 V based on these independently derived redox potentials.

Measurements of absolute rate constants for the reaction of PZH²⁺˙ and related radical-cations from chlorpromazine (3) and metiazinic acid (2) with ascorbate (4)(AH^–) and α-tocopherol (5)(α-T) indicate that such electron transfer equilibria are likely to exist also with other phenothiazine cations and strongly reducing compounds. The following bimolecular rate constants have been obtained: k(PZH²⁺˙+ AH^–)= 8.2 × 10⁸, k(PZH²⁺˙+α-T)= 1.1 × 10⁸, k(MZ^±˙+ AH^–)= 9.0 × 10⁷, and k(MZ^±˙+α-T)= 1.3 × 10⁸ mol^–1 dm³ s^–1.