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, (7)U2@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 rU-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@Cn (n = 60, 70, 80, 84, 90) series and the U-U bonds are also identified in U2@C70 and U2@C84 systems at rU-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.

Crystallographic and Theoretical Investigations of Er2@C2 n (2 n=82, 84, 86): Indication of Distance‐Dependent Metal–Metal Bonding Nature

“…The electron occupancy for its Er−Er bond is only 0.98 e, which, together with the much smaller bond order than the corresponding values of Er 2 @C 82,86 , suggests the possible formation of a weak single‐electron Er−Er bond. This kind of weak Er−Er bond can be named as an “unwilling metal–metal bond” where the metal atoms are strongly bound to the cage, which has been theoretically introduced in the U 2 @C 2 n (2 n =60, 80, 84, 90) series by Straka et al …”

“…The electron occupancy for its ErÀEr bond is only 0.98 e, which, together with the much smaller bond order than the corresponding values of Er 2 @C 82,86 ,s uggests the possible formation of aw eak single-electron ErÀEr bond. This kind of weakE r ÀEr bond can be nameda sa n" unwillingm etal-metal bond" where the metal atoms are strongly bound to the cage, which has been theoretically introduced in the U 2 @C 2n (2n = 60, 80, 84, 90) series by Straka et al [27] Figure 3s hows the corresponding metal bonding orbitals, in which two and one singly occupied orbitals are found for Er 2 @C 82,86 and Er 2 @C 84 ,r espectively.T he metal contributionsi n the molecular orbitals (MOs) indicate that the shared electron betweent he two metal centers of Er 2 @C 84 stems from Er1. Consistently,t he calculated spin densitiess how large distributions on the two Er atoms in each Er 2 @C 2n ,w hereas the spin density distribution is also recognized between the two Er atoms in Er 2 @C 84 (Figure 4a), indicating the existence of the unpaired electron.…”

How Does a Container Affect Acidity of its Content: Charge‐Depletion Bonding Inside Fullerenes

“…It is known that fullerenes can stabilize enclosed metallic clusters by strong charge‐transfer interactions and covalent‐type of bonding with the cage . Recent experiments also revealed that fullerenes, though apolar, strongly interact with external anions, as was predicted from the multicenter covalency model of anion–π bonding .…”

Robust metal‐pentagon interactions in the Th‐based endohedral metallofullerenes revealed by DFT calculations

“…Remarkably, their calculations revealed that the valence state of U changes between 3+ and 4+ in different isomeric cages. They also successfully synthesized and characterized the long‐dreamed U 2 @I h (7)‐C 80 , which features a weak U–U metal bond . Very recently, the first mixed metal actinide‐based EMF Sc 2 CU@I h (7)‐C 80 and the novel uranium carbide UCU@I h (7)‐C 80 were continually prepared and successfully characterized .…”