Electrochemistry and Stability of 1,1'-Ferrocene-Bisphosphonates.

Here, we investigate the electrochemical properties and stability of 1,1'-ferrocene-bisphosphonates in aqueous solutions. 31 P NMR spectroscopy enables to track decomposition at extreme pH conditions revealing partial disintegration of the ferrocene core in air and under an argon atmosphere. ESI-MS indicates the decomposition pathways to be different in aqueous H 3 PO 4 , phosphate buffer, or NaOH solutions. Cyclovoltammetry exhibits completely reversible redox chemistry of the evaluated bisphosphonates, sodium 1,1'-ferrocene-bis(phosphonate) ( 3 ) and sodium 1,1'-ferrocene-bis(methylphosphonate) ( 8 ), from pH 1.2 to pH 13. Both the compounds feature freely diffusing species as determined using the Randles-Sevcik analysis. The activation barriers determined by rotating disk electrode measurements revealed asymmetry for oxidation and reduction. The compounds are tested in a hybrid flow battery using anthraquinone-2-sulfonate as the counterside, yielding only moderate performance.