X-ray absorption studies of poly(vinylferrocene) polymers for anion separation

X-ray absorption spectroscopy (XAS) was used to study the electrochemical incorporation of perrhenate anion, from a 0.1 M NH₄ReO₄ solution, into poly(vinylferrocene) (PVFc) and a modified PVFc (a copolymer of 30% t-butyl acetylferrocene and 70% t-butyl divinyl ferrocene). The polymers were deposited on a carbon cloth current collector from a solution of the polymers in CH₂Cl₂. In situ XAS measurements were done at the Fe K edge on the reduced polymers and at 0.9 V vs. Ag/AgCl. Ex situ XAS was done at the Re L₃ edge after oxidation of the polymers at 0.9 V. The oxidized electrodes were washed in water to remove dissolved NH₄ReO₄ in the electrode pores. XAS was done both on wet-washed electrodes and on electrodes that were dried. XAS showed that at 0.9 V the Fe was oxidized from a ferrocene to a ferrocenium moiety and the Fe-C bond distance increased from 2.05 to 2.08 Å. Both the X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure (EXAFS) results are consistent with having >75% of the ferrocene moieties in the polymer oxidized at 0.9 V. The Fe K-edge EXAFS showed no direct indication of interaction of Fe with ReO₄^-. At the Re L₃ edge the only indication of interaction of ReO₄^- with the polymers was a slight change in the XANES features.