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

Abstract

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) Lu₂CeN@C₈₀(I_h) was synthesized and isolated. The electronic and vibrational properties of Lu₂CeN@C₈₀ are characterized by UV-vis-NIR and FTIR spectroscopies and the cage structure of Lu₂CeN@C₈₀ is determined by ¹³C NMR spectroscopy to be C₈₀(I_h). At room temperature the NMR peak positions are shifted from the normal values of the diamagnetic M₃N@C₈₀ NCFs because of the unpaired f-electron localized on the Ce atom in the Ce³⁺ state. The variable-temperature NMR study enabled the estimation of the diamagnetic terms in the ¹³C chemical shifts, which were found to be close to those of diamagnetic M₃N@C₈₀ NCFs. The electrochemical properties of Lu₂CeN@C₈₀ 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 M₃N@C₈₀ NCFs, we conclude that an oxidation of the endohedral Ce occurs with the formation of Lu₂Ce^IVN@C₈₀⁺, the first endohedral metallofullerene species with the tetra-valent cerium atom. This hypothesis is also supported by DFT calculations.