Chiral Induction in Electron-Transfer Reactions of Iron(II) and Iron(III) Bis(oxime-imine) Complexes

The kinetics and mechanisms of reductions of [Co(edta)]⁻ and [Co(phen)₃]³⁺ by the bis(oxime-imine) reagent [Fe^IIMe₂L(1)] (Me₂L(1)H₂ = C₁₄H₂₈N₆O₂) are reported over the pH range 6-9 at 0.10 M ionic strength and 25.0 °C. The reaction rates are dependent on pH, and pathways involving [Fe^IIMe₂L(1)] dominate with second-order rate constants 86.5 M⁻¹ s⁻¹ for [Co(edta)]⁻, and 8.3 × 10⁵ M⁻¹ s⁻¹ for [Co(phen)₃]³⁺. In the pH range 1-4, oxidation of [Co(phen)₃]²⁺ by [Fe^IIIMe₂L(1)]⁺ has been examined, and in this instance, it is the protonated form, [Fe^IIIMe₂L(1)H]²⁺, with a rate constant of 1.09 × 10⁵ M⁻¹ s⁻¹, which is more reactive than the unprotonated form. The reactions are outer sphere in nature, and the reactivity patterns reflect the thermodynamic driving forces for the individual pathways. The synthesis and characterization of a chiral iron(II) derivative, [Λ-Fe^II(Me₂L(2))] (Me₂L(2)H₂ = C₁₆H₃₂N₆O₂), allows for investigations of chiral induction in these reactions and for comparisons with stereoselectivities obtained for the isostructural nickel(IV/III) oxime-imine complexes. Trends in the data indicate that where electrostatic interactions in the ion-pair precursor complex are favorable, they govern the magnitude and sense of the observed stereoselectivity. However, when the electrostatic interactions are less important, or repulsive, electronic factors may be predominant in determining the stereoselectivity.