Selectivity Principles in Anion Separation by Crystallization of Hydrogen-Bonding Capsules

The fundamental factors controlling anion selectivity in the crystallization of hydrogen-bonding capsules [Mg(H2O)6][X ⊂ L2] (X = SO42−, 1a; SeO42−, 1b; SO32−, 1c; CO32−, 1d; L = tris[2-(3-pyridylurea)ethyl]-amine) from water have been investigated by solution and solid-state thermodynamic measurements, anion competition experiments, and X-ray structural analysis. The crystal structures of 1a−d are isomorphous, thereby simplifying the interpretation of the observed selectivities based on differences in anion coordination geometries. The solubilities of 1a−d in water follow the order: 1a < 1b < 1c < 1d, which is consistent with the selectivity for the tetrahedral sulfate and selenate anions observed in competitive crystallization experiments. Crystallization of the capsules is highly exothermic, with the most favorable ΔHcryst° of −99.1 and −108.5 kJ/mol corresponding to SO42− and SeO42−, respectively, in agreement with the X-ray structural data showing shape complementarity between these tetrahedral anions and the urea-lined cavities of the capsules. Sulfite, on the other hand, has a significantly less negative ΔHcryst° of −64.6 kJ/mol, which may be attributed to its poor fit inside the capsules, involving repulsive interactions. The more favorable entropy of crystallization for this anion, however, partly offsets the enthalpic disadvantage, resulting in a solubility product very similar to that of the selenate complex. Because of their very similar shape and size, SO42− and SeO42− have a propensity to form solid solutions, which limits the selectivity between these two anions in competitive crystallizations. In the end, a comprehensive picture of contributing factors to anion selectivity in crystalline hydrogen-bonding capsules emerges.

本研究通过溶液态与固态热力学测量、阴离子竞争实验以及X射线结构分析，探究了水溶液中氢键胶囊（hydrogen-bonding capsules）[Mg(H₂O)₆][X⊂L₂]（其中X分别为SO₄²⁻（1a）、SeO₄²⁻（1b）、SO₃²⁻（1c）、CO₃²⁻（1d）；L为三[2-(3-吡啶脲基)乙基]胺）结晶过程中调控阴离子选择性的核心因素。1a至1d的晶体结构为同晶型，这使得我们可以基于阴离子配位几何构型的差异，更简便地解析所观测到的选择性规律。1a至1d在水中的溶解度遵循1a < 1b < 1c < 1d的顺序，这与竞争性结晶实验中观测到的对四面体构型硫酸根与硒酸根阴离子的选择性结果相一致。该类胶囊的结晶过程为强放热过程，其中对应硫酸根与硒酸根的标准结晶焓ΔH°_cryst分别为-99.1 kJ/mol与-108.5 kJ/mol，为最有利的结晶焓变，这与X射线结构数据吻合——该数据表明这类四面体阴离子与胶囊内壁脲基修饰的空腔具有形状互补性。而亚硫酸根的标准结晶焓仅为-64.6 kJ/mol，负值程度显著更低，这可归因于其无法很好地适配胶囊空腔，存在排斥性相互作用。不过，该阴离子的结晶熵变更为有利，这部分抵消了其焓劣势，最终使其溶度积与硒酸根配合物的溶度积极为相近。由于硫酸根与硒酸根的形状和尺寸极为相近，二者易形成固溶体，这限制了竞争性结晶过程中二者之间的选择性区分。最终，本研究完整阐明了结晶态氢键胶囊中影响阴离子选择性的各类因素。