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

Endohedral actinide fullerenes are rare and a little is known about their molecular properties. Here we characterize the U-2@C-80 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, U-7(2)@C-80, derived from the C-80:7 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 angstrom. 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 U-2@C-n (n = 60, 70, 80, 84, 90) series and the U-U bonds are also identified in U-2@C-70 and U-2@C-84 systems at r(U-U) similar to 4 angstrom. 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.