EXAFS Study into the Speciation of Metal Salts Dissolved in Ionic Liquids and Deep Eutectic Solvents

The speciation of metals in solution controls their reactivity, and this is extremely pertinent in the area of metal salts dissolved in ionic liquids. In the current study, the speciation of 25 metal salts is investigated in four deep eutectic solvents (DESs) and five imidazolium-based ionic liquids using extended X-ray absorption fine structure. It is shown that in diol-based DESs MI ions form [MCl2]− and [MCl3]2– complexes, while all MII ions form [MCl4]2– complexes, with the exception of NiII, which exhibits a very unusual coordination by glycol molecules. This was also found in the X-ray crystal structure of the compound [Ni­(phen)2(eg)]­Cl2·2eg (eg = ethylene glycol). In a urea-based DES, either pure chloro or chloro–oxo coordination is observed. In [C6mim]­[Cl] pure chloro complexation is also observed, but coordination numbers are smaller (typically 3), which can be explained by the long alkyl chain of the cation. In [C2mim]­[SCN] metal ions are entirely coordinated by thiocyanate, either through the N or the S atom, depending on the hardness of the metal ion according to the hard-soft acid-base principle. With weaker coordinating anions, mixed coordination between solvent and solute anions is observed. The effect of hydrate or added water on speciation is insignificant for the diol-based DESs and small in other liquids with intermediate or strong ligands. One of the main findings of this study is that, with respect to metal speciation, there is no fundamental difference between deep eutectic solvents and classic ionic liquids.

溶液中金属的形态分布决定其反应活性，这一点在离子液体溶解金属盐的相关研究中尤为重要。本研究采用扩展X射线吸收精细结构（extended X-ray absorption fine structure），对四种低共熔溶剂（deep eutectic solvents, DESs）与五种咪唑鎓基离子液体中的25种金属盐的形态分布展开了探究。研究表明，在二醇类低共熔溶剂中，一价金属离子（MI）可形成[MCl₂]⁻与[MCl₃]²⁻型配阴离子；二价金属离子（MII）除二价镍（NiII）外，均会形成[MCl₄]²⁻型配阴离子，而二价镍离子会通过乙二醇分子进行配位，该配位模式十分罕见。这一现象在配合物[Ni(phen)₂(eg)]Cl₂·2eg（eg=乙二醇）的X射线晶体结构中得到了验证。在尿素类低共熔溶剂中，可观测到纯氯配位或氯-氧配位模式。在[C₆mim][Cl]体系中，同样可观测到纯氯配位，但配位数较低（通常为3），这一现象可通过阳离子的长烷基链予以解释。在[C₂mim][SCN]体系中，金属离子完全由硫氰酸根配位，根据硬软酸碱原理（hard-soft acid-base principle），配位位点会依据金属离子的硬软特性选择氮原子或硫原子。当使用弱配位阴离子时，可观测到溶剂阴离子与溶质阴离子之间的混合配位模式。对于二醇类低共熔溶剂，水合物或外加水对金属形态分布的影响可忽略不计；在含有中等强度或强配体的其他离子液体中，该影响也相对较小。本研究的核心发现之一是：就金属形态分布而言，低共熔溶剂与经典离子液体之间不存在本质差异。