Isolation of Uranyl Dicyanamide Complexes from N‑Donor Ionic Liquids

An
ionic liquid (IL) approach for soft-donor f-element chemistry has
been demonstrated by the isolation of several new uranyl dicyanamide
complexes through reactions of UO2(NO3)2·6H2O with dicyanamide ([N­(CN)2]−)-containing ILs. The [N­(CN)2]− ions are able to rapidly substitute uranium’s
O-donor ligands, as evidenced by single-crystal X-ray diffraction
studies on two anhydrous adducts of UO2(NO3)2 with [N­(CN)2]− ILs as well as
by IR and NMR spectroscopic studies on solutions of UO2(NO3)2 in these ILs. By contrast, the slow
reaction of UO2(OAc)2·2H2O with
a nitrile-functionalized imidazolium dicyanamide IL in solvent and
the reaction of UO2(NO3)2·6H2O with NaN­(CN)2 at elevated temperature resulted
in irreversible hydrolysis. The reaction of UO2SO4 with [N­(CN)2]− ions in an acidified
aqueous solution resulted in the crystallization of a [UO2]2+ complex with biuret, a N­(CN)2]− hydrolysis product. [N­(CN)2]− ions
in the form of ILs react rapidly with [UO2]2+ at room temperature, allowing ligand substitution with [N­(CN)2]− to out-compete the slower hydrolysis
reaction, enabling the isolation of uranyl dicyanamide complexes and
challenging assumptions regarding the affinity of uranium for O-donors.