To Thread or not to Thread: Reaction of Uranyl (UO₂ ²⁺) with two Aza-crown Macrocycles

The complexation of the uranyl ion (UO₂ ²⁺) by macrocyclic ligands is hindered by the rigidly enforced trans orientation of its oxo ligands. To further investigate this coordination chemistry challenge, here we investigate the reactivity of UO₂ ²⁺ with two different 18-membered aza-crown macrocycles, diaza-dibenzo-18-crown-6 (DADBC) and 7,16-bis(N-methylacetamide)diaza-18-crown-6 (BAM). The reaction of uranyl triflate, [UO₂(OTf)₂(THF)₃], with DADBC in toluene afforded [UO₂(DADBC)][OTf]₂, in which the UO₂ ²⁺ ion is fully encapsulated by the macrocycle. This conclusion was supported by NMR, UV-Vis, IR, and Raman spectroscopies, as well as single-crystal X-ray crystallography. Attempts to form the in-macrocycle complex of BAM, however, were unsuccessful. The reaction of BAM with [UO₂(OTf)₂(THF)₃] in THF/CH₂Cl₂ unexpectedly afforded crystals of [H₂ BAM][OTf]₂, where the protons are derived from solvate water in the BAM crystal lattice. In contrast, reaction of BAM with UO₂(NO₃)₂·6H₂O resulted in formation of the coordination polymer, [UO₂(κ²-NO₃)₂(μ-BAM)]_∞. The crystal structure of this compound revealed that the BAM ligand is coordinated to UO₂ ²⁺ via its pendent carboxamide donors, while the donor atoms within the macrocyclic cavity remained unbound. The X-ray crystal structure of the free BAM ligand was also obtained, which showed that it exists in an identical conformation to that seen in the structure of [UO₂(κ²-NO₃)₂(μ-BAM)]_∞, where the pendent carboxamide donors are engaged in an intramolecular hydrogen-bonding interaction with the donor atoms within the macrocycle. These results suggest that the enthalpic penalty of breaking these intramolecular hydrogen bonds prevents UO₂ ²⁺ from "threading the needle." In contrast, DADBC, which lacks such intramolecular hydrogen-bonding interactions, can readily accommodate UO₂ ²⁺. This comparison demonstrates how subtle modifications to the macrocycle, such as cavity size, rigidity, and pendant donor groups, can affect its affinity for UO₂ ²⁺, and provide insights into approaches for macrocycle chelator design for this ion for applications in separations, medicine, and energy.