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

Abstract

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 M₂@C₈₂ (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 Er₂@C₈₂ (C_s(6) and C_3v(8) cage isomers) and Sc₂@C₈₂ (C_3v(8) isomer) from fullerene mixtures. Sulfide clusterfullerenes Er₂S@C₈₂ 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 M₂@C₈₂ 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 Sc₂@C₈₂-C_3v is confirmed by the EPR spectrum of the cation radical [Sc₂@C₈₂-C_3v]⁺ generated by chemical oxidation in solution. The spectrum exhibits an exceptionally large a(⁴⁵Sc) hyperfine coupling constant of 199.2 G, indicating a substantial 4s contribution to the metal-metal bonding orbital. The cationic salt [Er₂@C₈₂-C_3v]⁺SbCl₆^- is prepared, and its magnetization behavior is compared to that of pristine Er₂@C₈₂-C_3v and Er₂S@C₈₂-C_3v. The formation of the single-electron Er-Er bond in the cation dramatically changes the coupling between magnetic moments of Er ions.