Electron transfer between Fe(CN)63- and iodide promoted by supercomplexation with a polyammonium macrocycle

Abstract

Some new properties promoted by the formation of a supercomplex between iron hexacyanometallates and the polyazamacrocycle [32]aneN ₈ (1,5,9,13,17,21,25,29-octaazacyclodotriacontane) are described. In the presence of the polyazamacrocycle, thermal and photoinduced electron transfer from iodide to Fe(CN) ₆ ^3- were observed in moderately acidic media. The thermal reaction is slow (k _obs = 8.9 × 10 ^-4 s ^-1 , at 25 °C) and proceeds to an equilibrium (K = 7 M ^-2 , at 25 °C). The reaction is almost isoergonic, with favorable enthalpy and unfavorable entropy changes (ΔG° = -4.8 kJ mol ^-1 , ΔH° = -160 kJ mol ^-1 , ΔS° = -0.54 kJ mol ^-1 K ^-1 ). A photoinduced electron-transfer process, leading to additional iodide oxidation, was observed upon flash irradiation of equilibrated solutions. Following the photoinduced process, the system reverts to the thermal equilibrium in the dark. The promoting role of the macrocycle is thermodynamic for the thermal process (anodic shift in the Fe ^II/III potential upon supercomplex formation) and kinetic for the photoinduced process [formation of ion-paired species between hexacyanoferrate(III) and iodide upon supercomplex formation]. The thermal reaction is reversible in basic media (where the macrocycle deprotonates and supercomplex formation is prevented), providing an example of on/off switching by pH changes of an electron-transfer reaction.