Unwilling U-U bonding in U-2@C-80: 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 U<inf>2</inf>@C<inf>80</inf> 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, 7U<inf>2</inf>@C<inf>80</inf>, derived from the C<inf>80</inf>:7 IPR fullerene cage, with six unpaired electrons. Bonding analysis reveals a double ferromagnetic (one-electron-two-center) U-U bond at an r<inf>U-U</inf> 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 U<inf>2</inf>@C<inf>n</inf> (n = 60, 70, 80, 84, 90) series and the U-U bonds are also identified in U<inf>2</inf>@C<inf>70</inf> and U<inf>2</inf>@C<inf>84</inf> systems at r<inf>U-U</inf> ∼ 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. This journal is © the Owner Societies 2015.