Ligand-Directed Actinide Oxo-Bond Manipulation in Actinyl Thiacalix[4]arene Complexes

Understanding the chemistry of the inert actinide oxo bond in actinyl ions AnO₂ ²⁺ is important for controlling actinide behavior in the environment, during separations, and in nuclear waste (An=U, Np, Pu). The thioether calixarene TC4A (4-tert-butyltetrathiacalix[4]arene) binds equatorially to the actinyl cation forming a conical pocket that differentiates the two trans-oxo groups. The 'ate' complexes, [A]₂[UO₂(TC4A)] (A=[Li(DME)₂], HNEt₃) and [HNEt₃]₂[AnO₂(TC4A)] (An=U, Np, Pu), enable selective oxo chemistry. Silylation of the U^VI oxo groups by bis(trimethylsilyl)pyrazine occurs first at only the unencapsulated exo oxo and only one silylation is needed to enable migration of the endo oxo out of the cone, whereupon a second silylation affords the stable U^IV cis-bis(siloxide) [A]₂[U(OSiMe₃)₂(TC4A)]. Calculations confirm that only one silylation event is needed to initiate oxo rearrangement, and that the putative cis dioxo isomer of [UO₂(TC4A)]^2- would be stable if it could be accessed synthetically, at only 23 kcal.mol^-1 in energy above the classical trans dioxo. Calculations for the transuranic cis[AnO₂(TC4A)]^2- (An=Np, Pu) are at higher energies, 30-35 kcal.mol^-1, retaining the U complexes as the more obvious target for a cis-dioxo actinyl ion. The aryloxide (OAr) groups of the macrocycle are essential in stabilizing this as-yet unseen uranyl geometry as further bonding in the TC4A U-O_Ar groups stabilizes the U=O 'yl' bonds, explaining the stability of the putative cis[UO₂(TC4A)]^2- in this ligand framework.