Ruthenium and iron complexes of dipicolinic acid: Synthesis, solution properties and kinetics of electron transfer reactions with ascorbate ions

Abstract

Standard potentials of the redox couples [bis(pyridine-2,6-dicarboxylate)M^III]-^/2- ([M(dipic)₂]-/²-, where M = Fe, Ru, Co) have been determined at 25ºC, and ionic strength 0.1M (NaClO₄ or KNO₃). Kinetics of reduction of the oxidized complexes by ascorbate have also been examined under the same conditions. The [Fe(dipic)₂]-^/2- potential was found to be 355 ± 5 mV. Reduction of [Fe(Fe(dipic)₂]- in the pH range 4-6 was shown to occur by reaction with ascorbate monoanion (HA^-) with a rate constant of (2.2 ± 0.2) × 10³ 1. mol^-1 s^-1, and ascorbate dianion(A²-) with a rate constant of (7 ± 1) × 10⁸ 1. mol^-1 s^-1.

K [Ru(dipic)₂] has been synthesized. Spectroscopic and analytical evidence suggest that it is a simple six-coordinate species in the solid and in non-aqueous solvents, but that in water it exists as an equilibrium mixture of at least two species. The redox potential for this mixture was found to be 270 ± 10 mV. The major component of this mixture is reduced by A²- with a rate constant of (4.7 ± 0.1) × 10⁸1.mol^-1 s^-1. A value of 747 ± 5 mV was measured for the redox potential of the cobalt couple, although equilibration of this system with the inert electrode could be achieved only by using [Fe(bpy)₂(CN)₂] as a mediator. Kinetics of reduction of [Co(dipic)₂]^- by ascorbate were complex and not reproducible.