Chaumont Alain, Wipff Georges
Laboratoire MSM, UMR CNRS 7551, Institut de Chimie, 4 rue B. Pascal, 67 000 Strasbourg, France.
J Phys Chem B. 2008 Sep 25;112(38):12014-23. doi: 10.1021/jp8031447. Epub 2008 Aug 28.
The stepwise addition of 1 to 4 Cl(-) anions to the uranyl cation has been studied via potential of mean force (PMF) calculations in the [BMI][Tf 2N] ionic liquid based on the 1-butyl-3-methylimidazolium cation (BMI(+)) and the bis(trifluoromethylsulfonyl)imide anion (Tf2N(-)). According to these calculations, the four Cl(-) complexation reactions are favored and UO2Cl4(2-) is the most stable chloride complex in [BMI][Tf2N]. The solvation of the different chloro-complexes is found to evolve from purely anionic (ca. 5 Tf2N(-) ions around UO2(2+)) to purely cationic (ca. 8.5 BMI (+) cations around UO2Cl4(2-)), with onion-type alternation of solvent shells. We next compare the solvation of the UO2Cl4(2-) complex to its reduced analogue UO2Cl4(3-) in the [BMI][Tf2N] and [MeBu3N][Tf2N] liquids that possess the same anion, but differ by their cation (imidazolium BMI(+) versus ammonium MeBu3N(+)). The overall solvation structure of both complexes is found to be similar in both liquids with a first solvation shell formed exclusively of solvent cations (about 9 BMI(+) cations or 7 MeBu3N(+) cations). However, a given complex is better solvated by the [BMI][Tf2N] liquid, due to hydrogen bonding interactions between Cl(-) ligands and imidazolium-ring C-H protons. According to free energy calculations, the gain in solvation energy upon reduction of UO2Cl4(2-) to UO2Cl4(3-) is found to be larger in [BMI][Tf2N] than in [MeBu3N][Tf2N], which is fully consistent with recent experimental results (Inorg. Chem. 2006, 45, 10419).
通过基于1-丁基-3-甲基咪唑鎓阳离子（BMI⁺）和双（三氟甲基磺酰基）亚胺阴离子（Tf₂N⁻）的[BMI][Tf₂N]离子液体中的平均力势（PMF）计算，研究了向铀酰阳离子逐步添加1至4个Cl⁻阴离子的过程。根据这些计算，四个Cl⁻络合反应是有利的，并且UO₂Cl₄²⁻是[BMI][Tf₂N]中最稳定的氯化物络合物。发现不同氯络合物的溶剂化从纯阴离子型（UO₂²⁺周围约5个Tf₂N⁻离子）演变为纯阳离子型（UO₂Cl₄²⁻周围约8.5个BMI⁺阳离子），溶剂壳呈洋葱型交替。接下来，我们将UO₂Cl₄²⁻络合物与其还原类似物UO₂Cl₄³⁻在具有相同阴离子但阳离子不同（咪唑鎓BMI⁺与铵MeBu₃N⁺）的[BMI][Tf₂N]和[MeBu₃N][Tf₂N]液体中的溶剂化情况进行了比较。发现两种液体中两种络合物的整体溶剂化结构相似，第一溶剂化壳仅由溶剂阳离子形成（约9个BMI⁺阳离子或7个MeBu₃N⁺阳离子）。然而，由于Cl⁻配体与咪唑环C-H质子之间的氢键相互作用，给定的络合物在[BMI][Tf₂N]液体中的溶剂化更好。根据自由能计算，发现将UO₂Cl₄²⁻还原为UO₂Cl₄³⁻时，[BMI][Tf₂N]中的溶剂化能增益比[MeBu₃N][Tf₂N]中的大，这与最近的实验结果（《无机化学》2006年，45卷，10419页）完全一致。
