Complex Formation of Uranyl Ion with Triphenylphosphine Oxide and Its Ligand Exchange Reaction in 1-Butyl-3-methylimidazolium Nonafluorobutanesulfonate Ionic Liquid

Complex formation of the uranyl ion (UO22+) with triphenylphosphine oxide (OPPh3) in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ([BMI][NfO]) ionic liquid was investigated by means of 31P NMR spectroscopy. In [BMI][NfO], coordination of OPPh3 to UO22+ was found, and its coordination number was 4.1 ± 0.2, indicating UO2(OPPh3)42+. From the [BMI][NfO] solution containing UO22+ and OPPh3, yellow crystals of UO2(OPPh3)4(ClO4)2 deposited, and its molecular and crystal structures were determined by using single-crystal X-ray analysis. An OPPh3 exchange reaction of UO2(OPPh3)42+ in [BMI][NfO] was also examined. The apparent first-order rate constant (kobs) showed the first-order dependence on [OPPh3] (kobs = k4[OPPh3]free), suggesting the “associative” (A) mechanism. Its activation parameters were ΔH4⧧ = 55.3 ± 2.8 kJ mol−1 and ΔS4⧧ = 16.1 ± 7.9 J mol−1 K−1. To compare the reactivity of UO2(OPPh3)42+ in [BMI][NfO] with that in an ordinary organic solvent, the same reaction in CD2Cl2 was studied. In the CD2Cl2 system, an equilibrium between UO2(OPPh3)42+ and UO2(OPPh3)52+ was observed. The kobs values of the OPPh3 exchange reactions in UO2(OPPh3)42+ (k4obs) and UO2(OPPh3)52+ (k5obs) in CD2Cl2 are expressed as k4obs = k4[OPPh3]free and k5obs = k5, respectively, indicating that the exchange reactions in UO2(OPPh3)42+ and UO2(OPPh3)52+ are categorized in A and “dissociative” (D) mechanisms, respectively. The activation parameters of these reactions were also estimated (UO2(OPPh3)42+: ΔH4⧧ = 7.1 ± 0.3 kJ mol−1 and ΔS4⧧ = −122 ± 1 J mol−1 K−1, UO2(OPPh3)52+: ΔH5⧧ = 62.4 ± 1.0 kJ mol−1 and ΔS5⧧ = 68.4 ± 4.2 J mol−1 K−1). A large difference in the reactivity of UO2(OPPh3)42+ was found between [BMI][NfO] and CD2Cl2. This was explained by the formation of a specific solvation barrier of NfO− around UO2(OPPh3)42+ in [BMI][NfO].

本研究采用磷-31核磁共振（31P NMR）波谱法，探究了1-丁基-3-甲基咪唑全氟丁基磺酸盐（[BMI][NfO]）离子液体中，铀酰离子（uranyl ion，UO₂²⁺）与三苯基氧膦（triphenylphosphine oxide，OPPh₃）的配位络合过程。在[BMI][NfO]体系中，观测到OPPh₃与UO₂²⁺发生配位作用，其配位数（coordination number）为4.1±0.2，对应络合物组成为UO₂(OPPh₃)₄²⁺。从含有UO₂²⁺与OPPh₃的[BMI][NfO]溶液中析出了黄色的UO₂(OPPh₃)₄(ClO₄)₂晶体，并通过单晶X射线衍射分析（single-crystal X-ray analysis）确定了其分子与晶体结构。本研究还考察了[BMI][NfO]中UO₂(OPPh₃)₄²⁺的OPPh₃配体交换反应。表观一级速率常数（apparent first-order rate constant，kobs）与游离OPPh₃浓度呈一级依赖关系（kobs = k₄[OPPh₃]free），表明该反应遵循缔合（associative，A）机理。其活化参数（activation parameters）为ΔH₄⧧ = 55.3±2.8 kJ·mol⁻¹，ΔS₄⧧ = 16.1±7.9 J·mol⁻¹·K⁻¹。为对比UO₂(OPPh₃)₄²⁺在[BMI][NfO]与常规有机溶剂中的反应活性，本研究同时探究了其在二氯甲烷-d2（CD₂Cl₂）中的相同反应。在CD₂Cl₂体系中，观测到UO₂(OPPh₃)₄²⁺与UO₂(OPPh₃)₅²⁺之间存在平衡（equilibrium）。UO₂(OPPh₃)₄²⁺（k4obs）与UO₂(OPPh₃)₅²⁺（k5obs）的OPPh₃配体交换反应的表观一级速率常数分别满足k4obs = k₄[OPPh₃]free与k5obs = k₅，表明二者的配体交换反应分别遵循缔合（associative，A）与解离（dissociative，D）机理。本研究还估算了这两类反应的活化参数：UO₂(OPPh₃)₄²⁺的ΔH₄⧧ = 7.1±0.3 kJ·mol⁻¹，ΔS₄⧧ = -122±1 J·mol⁻¹·K⁻¹；UO₂(OPPh₃)₅²⁺的ΔH₅⧧ = 62.4±1.0 kJ·mol⁻¹，ΔS₅⧧ = 68.4±4.2 J·mol⁻¹·K⁻¹。研究发现，UO₂(OPPh₃)₄²⁺在[BMI][NfO]与CD₂Cl₂中的反应活性存在显著差异，该现象可归因于[BMI][NfO]中NfO⁻在UO₂(OPPh₃)₄²⁺周围形成了特异性溶剂化壁垒（solvation barrier）。