Redox Potentials and HPLC Behavior of Cobalt and Iron Complexes with Pyridylazo Compounds

Abstract

The redox potentials of cobalt and iron complexes with ten pyridylazo compounds, E⁰_ML2 (ML₂^+/0; M: Co^III/II, Fe^III/II; L^-: pyridylazo compounds), have been determined in order to explore the difference in their reversed-phase HPLC behavior. The redox potentials of Co complexes were in the range of -0.62 - 0.03 V, while those of Fe complexes were -0.06 - 0.59 V relative to 0.20 V for ferricinium/ferrocene. The redox potentials of both the Co and Fe complexes were linearly correlated to the basicities of the ligands. The correlation was quantitatively explained by a difference in dependence of the stabilities of M^III and M^II complexes on the ligand basicities. The complex of [Co^IIIL₂]⁺ or [Fe^IIL₂] with any compound injected in the reversed-phase HPLC system was detected without any change in the composition. When [Co^IIL₂] was injected, only those complexes having the highest potentials of E⁰_CoL2 ≅ 0.0 V were detected as [Co^IIL₂], while other complexes having lower potentials gave a peak of [Co^IIIL₂]⁺. When [Fe^IIIL₂]⁺ was injected, only complexes having the lowest potentials of E⁰_FeL2 ≅ 0.0 V were detected as [Fe^IIIL₂]⁺, while others having higher potentials gave a peak of [Fe^IIL₂].