Redox-active pH-responsive molecules: ferrocenylphosphonic acid, ferrocenylmethylphosphonic acid and 1,1′-ferrocenylbisphosphonic acid. Structural determination of FcPO3Na2·5H2O

Abstract

The acidity constants of the reduced and oxidized species of ferrocenylphosphonic acids FcPO₃H₂, FcCH₂PO₃H₂ and fc(PO₃H₂)₂ (Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄), fc = (η⁵-C₅H₄)Fe(η⁵-C₅H₄)) in water have been evaluated by potentiometric, ³¹P NMR, and electrochemical methods. The oxidized forms are more acidic than the reduced ones. The interaction between the redox centre and the charged oxygen atoms of the phosphonate group is shown to be electrostatic. The maximum oxidation shift ΔE between the protonated and unprotonated species increases with the number of charges of the substrate and decreases with the increase of the distance between the ferrocenyl centre and the oxygen atoms of the phosphonate group. The structure of FcPO₃Na₂·5H₂O is determined. The compound crystallizes in the monoclinic system. It is lamellar with an inorganic layer formed by tetramers Na₄O₁₄, the ferrocenyl groups occupying the interlamellar space.