Characterizing pressure-induced uranium C-H agostic bonds

Under pressure: The diuranium(III) compound [UN''2]2(μ-η6:η6-C6H6) (N''=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional quantum chemistry. In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metalCH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal-metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between different ligand CH bonds and the U centers. Although X-ray crystallographic software suggests agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, at the highest pressure studied (3.2 GPa), interactions with the U centers, assigned as agostic, are evidenced by QTAIM.