Interfacial Electron Transfer to the Zeolite-Encapsulated Methylviologen Acceptor from Various Carbonylmanganate Donors. Shape Selectivity of Cations in Mediating Electron Conduction through the Zeolite Framework

The series of (one-electron) reductions of methylviologen (MV 2+ ) intercalated into zeolite- Y by various carbonylmanganate donors [C + Mn(CO) 4 L -, L = CO, P(OPh) 3 ] are very selective and highly dependent on the size/shape of the counterion C +, although the same electron transfers carried out (homogeneously) in solution always occur spontaneously, irregardless of C + . For example, the complete reduction of MV 2+ extensively doped into zeolite- Y proceeds rapidly and quantitatively when the Na + salts of the carbonylmanganates are employed as the reductants, but only to a very limited extent (1%) when the large PPN + [bis(triphenylphosphine)iminium] salts of the carbonylmanganates are employed. The medium-size tetraethylammonium (TEA + ) salt of Mn(CO) 4 P(OPh) 3 - slowly effects an intermediate conversion (80%). Based on the fact that the large phosphite-substituted Mn(CO) 4 P(OPh) 3 - donor cannot enter the supercage of zeolite- Y, we propose interfacial electron transfer from the carbonylmanganate to the MV 2+ acceptor to occur only at the zeolite periphery. Importantly, the strong dependence of the further progress of the redox reaction with decreasing size of the cation C + ( i.e., shape selectivity) predicts that electron conduction throughout the zeolite framework requires the simultaneous transport of these cations in order to effect the complete reduction of all the encapsulated MV 2+, as presented in Chart 5.