Redox reactions of some iron(II), iron(III), and cobalt(II) picolinate complexes

Abstract

The kinetics of reduction of iron(III) in pyridine-2-carboxylate, picolinate (pic^–), solutions by ascorbate ion show a complex dependence upon pH, ligand concentration, and reductant concentration. The data analysis reveals a single redox-active species, [Fe(pic)₂(OH)], which is reduced by ascorbate ion with a second-order rate constant of 6.4 × 10³ dm³ mol^–1 s^–1 at 25 °C and I= 0.10 mol dm^–3. This species is in rapid equilibrium with [Fe(pic)₃], and the equilibrium constant between the two species is evaluated. Oxidations of iron(II) in picolinate solutions by [Co(pic)₃] and [Co(ox)₃]^3–[ox = oxalate(2–)] reveal a dominant pathway involving [Fe(pic)₂] and a minor pathway involving [Fe(pic)3]^–. It is suggested that these are outer-sphere processes and Marcus theory is used to estimate self-exchange rates for the complexes.