ELECTRON-TRANSFER BETWEEN FE(CN)(6)(3-) AND IODIDE PROMOTED BY SUPERCOMPLEXATION WITH A POLYAMMONIUM MACROCYCLE

Some new properties promoted by the formation of a supercomplex betwee n iron hexacyanometallates and the polyazamacrocycle [32]aneN(8) (1,5, 9,13,17,21,25,29-octaazacyclodotrane) are described. In the presence o f the polyazamacrocycle, thermal and photoinduced electron transfer fr om iodide to Fe(CN)(6)(3-) were observed in moderately acidic media. T he thermal reaction is slow (k(obs) = 8.9 x 10(-4) s(-1), at 25 degree s C) and proceeds to an equilibrium (K = 7 M-2, at 25 degrees C). The reaction is almost isoergonic, with favorable enthalpy and unfavorable entropy changes (Delta G degrees = -4.8 kJ mol(-1), Delta H degrees = -160 kJ mol(-1), Delta S degrees = -0.54 kJ mol(-1) K-1). A photoindu ced electron-transfer process, leading to additional iodide oxidation, was observed upon flash irradiation of equilibrated solutions. Follow ing the photoinduced process, the system reverts to the thermal equili brium in the dark. The promoting role of the macrocycle is thermodynam ic for the thermal process (anodic shift in the Fe-II/III potential up on supercomplex formation) and kinetic for the photoinduced process [f ormation of ion-paired species between hexacyanoferrate(III) and iodid e upon supercomplex formation]. The thermal reaction is reversible in basic media (where the macrocycle deprotonates and supercomplex format ion is prevented), providing an example of on/off switching by pH chan ges of an electron-transfer reaction.