Analytical Sciences

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

Takashi YASUI, Yoko NUNOME, Takashi OHNISHI, Masayo YAMAGUCHI, Saki TAMAMURA, Hiromichi YAMADA, and Akio YUCHI

Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan

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].

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