Unwilling U–U bonding in U2@C80: cage-driven metal–metal bonds in di-uranium fullerenes

Abstract

Endohedral actinide fullerenes are rare and a little is known about their molecular properties. Here we characterize the U₂@C₈₀ system, which was recently detected experimentally by means of mass spectrometry (Akiyama et al., JACS, 2001, 123, 181). Theoretical calculations predict a stable endohedral system, ⁷U₂@C₈₀, derived from the C₈₀:7 IPR fullerene cage, with six unpaired electrons. Bonding analysis reveals a double ferromagnetic (one-electron-two-center) U–U bond at an r_U–U distance of 3.9 Å. This bonding is realized mainly via U(5f) orbitals. The U–U interaction inside the cage is estimated to be about −18 kcal mol⁻¹. U–U bonding is further studied along the U₂@C_n (n = 60, 70, 80, 84, 90) series and the U–U bonds are also identified in U₂@C₇₀ and U₂@C₈₄ systems at r_U–U ∼ 4 Å. It is found that the character of U–U bonding depends on the U–U distance, which is dictated by the cage type. A concept of unwilling metal–metal bonding is suggested: uranium atoms are strongly bound to the cage and carry a positive charge. Pushing the U(5f) electron density into the U–U bonding region reduces electrostatic repulsion between enclosed atoms, thus forcing U–U bonds.