Confining the spin between two metal atoms within the carbon cage: redox-active metal–metal bonds in dimetallofullerenes and their stable cation radicals†

Lanthanide–lanthanide bonds are exceptionally rare, and dimetallofullerenes provide a unique possibility to stabilize and study these unusual bonding patterns. The presence of metal–metal bonds and consequences thereof for the electronic properties of M(2)@C(82) (M = Sc, Er, Lu) are addressed by electrochemistry, electron paramagnetic resonance, SQUID magnetometry and other spectroscopic techniques. A simplified non-chromatographic separation procedure is developed for the isolation of Er(2)@C(82) (C(s)(6) and C(3v)(8) cage isomers) and Sc(2)@C(82) (C(3v)(8) isomer) from fullerene mixtures. Sulfide clusterfullerenes Er(2)S@C(82) with C(s)(6) and C(3v)(8) fullerene cages are synthesized for the first time. The metal–metal bonding orbital of the spd hybrid character in M(2)@C(82) is shown to be the highest occupied molecular orbital, which undergoes reversible single-electron oxidation with a metal-dependent oxidation potential. Sulfide clusterfullerenes with a fullerene-based HOMO have more positive oxidation potentials. The metal-based oxidation of Sc(2)@C(82)-C(3v) is confirmed by the EPR spectrum of the cation radical [Sc(2)@C(82)-C(3v)](+) generated by chemical oxidation in solution. The spectrum exhibits an exceptionally large a((45)Sc) hyperfine coupling constant of 199.2 G, indicating a substantial 4s contribution to the metal–metal bonding orbital. The cationic salt [Er(2)@C(82)-C(3v)](+)SbCl(6)(−) is prepared, and its magnetization behavior is compared to that of pristine Er(2)@C(82)-C(3v) and Er(2)S@C(82)-C(3v). The formation of the single-electron Er–Er bond in the cation dramatically changes the coupling between magnetic moments of Er ions.