The electrochemical ion-transfer reactivity of porphyrinato metal complexes in 4-(3-phenylpropyl)pyridine | water systems

Abstract

The transfer of ions between an aqueous and an organic phase is driven electrochemically at a triple phase junction graphite | 4-(3-phenylpropyl)pyridine | aqueous electrolyte. Tetraphenylporphyrinato (TPP) metal complexes (MnTPP⁺, FeTPP⁺, CoTPP) and hemin readily dissolve in the organic 4-(3-phenylpropyl)pyridine phase and undergo oxidation/reduction processes which are coupled to liquid | liquid ion transfer. In order to maintain charge neutrality, each one-electron oxidation (reduction) process is coupled to the transfer of one anion (here PF₆⁻, ClO₄⁻, SCN⁻, NO₃⁻, OCN⁻, or CN⁻) from the aqueous (organic) into the organic (aqueous) phase. The range of anions studied allows effects of hydrophobicity and complex formation (association of the anion to the metal center) to be explored. A preliminary kinetic scheme is developed to quantify complex formation, facilitated anion transfer, and competing cation transfer processes. The effects of the organic solvent on the ion transfer processes are explored. Very strong binding and specific effects are observed for the reversible cyanide transfer process.