An endohedral redox system in a fullerene cage: the Ce based mixed-metal cluster fullerene Lu2CeN@C80.

Redox reactions of endohedral fullerenes, and especially their oxidation, usually result in a change of the redox state of the carbon cage. Here we demonstrate that an oxidation of the endohedral species is possible bypassing the fullerene cage in an unprecedented reversible cascade electron transfer under anodic conditions. The first Ce-based non-scandium mixed-metal nitride clusterfullerene (NCF) Lu(2)CeN@C(80)(I(h)) was synthesized and isolated. The electronic and vibrational properties of Lu(2)CeN@C(80) are characterized by UV-vis-NIR and FTIR spectroscopies and the cage structure of Lu(2)CeN@C(80) is determined by (13)C NMR spectroscopy to be C(80)(I(h)). At room temperature the NMR peak positions are shifted from the normal values of the diamagnetic M(3)N@C(80) NCFs because of the unpaired f-electron localized on the Ce atom in the Ce(3+) state. The variable-temperature NMR study enabled the estimation of the diamagnetic terms in the (13)C chemical shifts, which were found to be close to those of diamagnetic M(3)N@C(80) NCFs. The electrochemical properties of Lu(2)CeN@C(80) were investigated by cyclic and square wave voltammetry, revealing two electrochemically irreversible but chemically reversible reduction steps and one reversible oxidation step. As the oxidation potential is significantly less positive than that in all other M(3)N@C(80) NCFs, we conclude that an oxidation of the endohedral Ce occurs with the formation of Lu(2)Ce(IV)N@C(80)(+), the first endohedral metallofullerene species with the tetra-valent cerium atom. This hypothesis is also supported by DFT calculations.