(Invited) Metal-Bonding Electrons inside the Fullerene Cage: Electrochemical, Quantum Chemical and EPR Studies

Electronic properties of endohedral metallofullerenes (EMFs) and especially their behavior in the charge transfer processes have been in the focus since the middle of 1990s, when sufficient amounts of EMFs became available. The Closed carbon shell of EMF represents itself as a suitable environment for stabilization of different exotic metal species, including dimetallic clusters with metal-metal bond. Here we report the electrochemical properties of M2@C82 clustersfullerenes (2 cage isomers of C82: with C3v and Cs symmetry; M=Lu, Er, Sc) . UV-Vis and NIR absorption spectroscopy data and theoretical calculations expose the unique structure of M2@C82 EMFs with two M2+ions inside the cage, connected with metal-metal bonding (despite the fact that in majority of EMFs lanthanides demonstrate oxidation state 3+). Electrochemical experiments along with theoretical calculations have proved that metal-metal bonding orbital in EMFs is always one of the frontier orbitals, and its energy to a great extant determines the redox behavior of the molecule. For M2@C82 family metal-metal bonding orbital presents itself as a HOMO. The cyclic voltammetry has revealed the cathodic shift* of the first oxidation potential for Sc2- and Er2-EMFs in comparison with corresponding isomers of Lu2@C82, caused by the significant difference in position of the HOMO. Monocation of Sc2@C82-C3v, generated by chemical oxidation, has been studied with EPR spectroscopy as well. Due to the spd-character of metal-metal bonding orbital, the spin density at the nuclei is extremely high, that provides the second order effects in line position and variations of the signal line width.