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.通过六水合硝酸铀酰（UO₂(NO₃)₂·6H₂O）与含二氰二胺阴离子([N(CN)₂]⁻)的离子液体（ionic liquid, IL）反应，分离得到多种新型二氰二胺合铀酰配合物，证实了离子液体路径可用于软给体型f区元素配位化学研究。单晶X射线衍射对两种由UO₂(NO₃)₂与[N(CN)₂]⁻离子液体形成的无水加合物的表征结果，以及该离子液体中UO₂(NO₃)₂溶液的红外（IR）与核磁共振（NMR）光谱分析，均证明[N(CN)₂]⁻阴离子可快速取代铀中心的氧给体配体。与之形成对比的是，二水合乙酸铀酰（UO₂(OAc)₂·2H₂O）与腈基功能化咪唑鎓二氰二胺离子液体在溶剂中的缓慢反应，以及六水合硝酸铀酰与NaN(CN)₂在高温下的反应均发生了不可逆水解。硫酸铀酰（UO₂SO₄）在酸化水溶液中与[N(CN)₂]⁻阴离子反应，得到了含缩二脲——[N(CN)₂]⁻的水解产物——的[UO₂]²⁺配合物晶体。离子液体形式的[N(CN)₂]⁻阴离子可在室温下与[UO₂]²⁺快速反应，使得二氰二胺配体取代反应的速率优于更缓慢的水解过程，从而成功分离得到二氰二胺合铀酰配合物，同时对“铀对氧给体配体具有更高结合亲和性”这一传统认知提出了挑战。