Influence of the Anion on the Stability of Second-Sphere Coordination of Ferrioxamine B with cis-Dicyclohexano-18-crown-6 in Chloroform

The influence of the anion X⁻ on the stability of the supramolecular assembly FeHDFB⁺,CE,X⁻ (I) where FeHDFB⁺ is ferrioxamine B, CE is dicyclohexano-18-crown-6, and X⁻ is ClO4⁻, NO3⁻, Cl⁻, or picrate was investigated. The formation constants for these host-guest adducts in wet chloroform at 25°C were determined as follows: Ka(ClO4⁻) = 1.77 × 10⁴ M⁻¹, Ka(NO⁻) = 1.34 × 10² M⁻¹, Ka(Cl⁻) = 1.00 × 10¹ M⁻¹, Ka(picrate) = 4.68 × 10³ M⁻¹. Distribution constants (Kd) for the ion pairs between water and chloroform, as well as extraction constants (Kex) for the corresponding crown ether separated ion pairs, were determined: Kd(ClO4⁻) = 3.10 × 10⁻⁴ M⁻¹, Kd(NO3⁻) = 2.91 × 10⁻⁴ M⁻¹, Kd(Cl⁻) = 3.00 × 10⁻⁴ M⁻¹,Kd(picrate) = 0.24; Kex(ClO4⁻) = 5.50 M⁻², Kex(NO3⁻) = 3.91 × 10⁻² M⁻², Kex(Cl⁻) = 3.0010⁻³ M⁻², Kex(picrate) = 1.12 × 10³ M⁻². Data are presented which support the assertion that crown ether intercalation into the FeHDFB⁺,X⁻ ion pair to form the supramolecular assembly in I provides a mechanism for the chloroform extraction process which is different from the distribution process. The importance of anion solvation in determining the stability of I is illustrated by a linear plot of log Ka vs anion hydration enthalpy. Comparison with our previously published data enable us to conclude that the stability of I is equally sensitive to hydration of the cation and the anion. We conclude that matching crown ether properties to the {cation.anion} combination in an ion pair may enhance host-guest interactions and optimize aqueous/organic phase extractions. © 1995, American Chemical Society. All right reserved.

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