Wang Xiaoyu, Peroutka Allison A, Kravchuk Dmytro V, Shafer Jenifer C, Wilson Richard E, Servis Michael J
Chemical Sciences and Engineering Division, Argonne National Laboratory 9700 S Cass Ave Lemont IL 60439 USA
Department of Chemistry, Colorado School of Mines 1500 Illinois St. Golden CO 80401 USA.
Chem Sci. 2024 Sep 26;15(40):16494-502. doi: 10.1039/d4sc05061d.
Lanthanide ion solvation chemistry in nonaqueous phases is key to understanding and developing effective separation processes for these critical materials. Due to the complexity and inherent disorder of the solution phase, a comprehensive picture of the solvated metal ion is often difficult to generate solely from conventional spectroscopic approaches and electronic structure calculations, particularly in the extractant phase. In this work, we use classical molecular dynamics (MD) simulation with an advanced sampling technique, metadynamics, supplemented by experimental spectroscopy and speciation analysis, to measure lanthanide solvation free energy landscapes. We define coordination-based collective variables to probe the entire range of solvation configurations in the organic phase of lanthanum (La), europium (Eu), and lutetium (Lu) nitrate salts bound with a commonly used extractant, ,'-dimethyl, ,'-dioctylhexylethoxymalonamide (DMDOHEMA). The known lanthanide extraction trend of La ≈ Eu > Lu is readily explained by the measured free energy surfaces, which show consistent DMDOHEMA coordination from La to Eu, followed by loss of DMDOHEMA coordination from Eu to Lu. These simulations suggest how ligand crowding at the metal center can control selectivity, in this case resulting in the opposite extraction trend as observed with other conventional extractants, where the enthalpic contribution from increasing lanthanide charge density across the series dominates the extraction energetics. We also find that the presence of inner-sphere water, verified by time-resolved fluorescence, diversifies the accessible solvation structures. As a result, understanding solvation requires consideration of an entire thermodynamic ensemble, rather than the single dominant lowest-energy structure, as is often considered out of necessity in interpretation of spectroscopic data or in electronic structure-based ligand design approaches. In general, we demonstrate how metadynamics uniquely enables investigation of complex, multidimensional solvation energetic landscapes, and how it can explain selectivity trends where extraction is controlled by more complex mechanisms than simple charge density-based selectivity.
非水相中镧系离子的溶剂化化学是理解和开发针对这些关键材料的有效分离过程的关键。由于溶液相的复杂性和内在无序性，仅通过传统光谱方法和电子结构计算往往难以全面了解溶剂化金属离子，特别是在萃取剂相中。在这项工作中，我们使用经典分子动力学（MD）模拟和先进的采样技术——元动力学，并辅以实验光谱和物种分析，来测量镧系元素的溶剂化自由能景观。我们定义基于配位的集体变量，以探究镧（La）、铕（Eu）和镥（Lu）硝酸盐与常用萃取剂,'-二甲基,'-二辛基己基乙氧基丙二酰胺（DMDOHEMA）结合时在有机相中的整个溶剂化构型范围。测量得到的自由能表面很容易解释已知的镧系元素萃取趋势La≈Eu>Lu，该表面显示从La到Eu有一致的DMDOHEMA配位，随后从Eu到Lu失去DMDOHEMA配位。这些模拟表明了金属中心处配体的拥挤如何控制选择性，在这种情况下导致与其他传统萃取剂观察到的相反萃取趋势，在其他传统萃取剂中，整个系列中镧系元素电荷密度增加所带来的焓贡献主导了萃取能量学。我们还发现，通过时间分辨荧光验证的内球层水的存在使可及的溶剂化结构多样化。因此，理解溶剂化需要考虑整个热力学系综，而不是像在光谱数据解释或基于电子结构的配体设计方法中通常出于必要考虑的单一占主导地位的最低能量结构。总体而言，我们展示了元动力学如何独特地能够研究复杂的多维溶剂化能量景观，以及它如何解释萃取由比基于简单电荷密度的选择性更复杂的机制控制时的选择性趋势。
