Mehio Nada, Ivanov Alexander S, Williams Neil J, Mayes Richard T, Bryantsev Vyacheslav S, Hancock Robert D, Dai Sheng
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA.
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Dalton Trans. 2016 May 31;45(22):9051-64. doi: 10.1039/c6dt00116e.
The design of new ligands and investigation of UO2(2+) complexations are an essential aspect of reducing the cost of extracting uranium from seawater, improving the sorption efficiency for uranium and the selectivity for uranium over competing ions (such as the transition metal cations). The binding strengths of salicylaldoxime-UO2(2+) complexes were quantified for the first time and compared with the binding strengths of salicylic acid-UO2(2+) and representative amidoxime-UO2(2+) complexes. We found that the binding strengths of salicylaldoxime-UO2(2+) complexes are ∼2-4 log β2 units greater in magnitude than their corresponding salicylic acid-UO2(2+) and representative amidoxime-UO2(2+) complexes; moreover, the selectivity of salicylaldoxime towards the UO2(2+) cation over competing Cu(2+) and Fe(3+) cations is far greater than those reported for salicylic acid and glutarimidedioxime in the literature. The higher UO2(2+) selectivity can likely be attributed to the different coordination modes observed for salicylaldoxime-UO2(2+) and salicylaldoxime-transition metal complexes. Density functional theory calculations indicate that salicylaldoxime can coordinate with UO2(2+) as a dianion species formed by η(2) coordination of the aldoximate and monodentate binding of the phenolate group. In contrast, salicylaldoxime coordinates with transition metal cations as a monoanion species via a chelate formed between phenolate and the oxime N; the complexes are stabilized via hydrogen bonding interactions between the oxime OH group and phenolate. By coupling the experimentally determined thermodynamic constants and the results of theoretical computations, we are able to derive a number of ligand design principles to further improve the UO2(2+) cation affinity, and thus further increase the selectivity of salicylaldoxime derivatives.
新型配体的设计以及对UO2(2+)络合作用的研究是降低从海水中提取铀成本、提高铀吸附效率以及提高铀相对于竞争离子（如过渡金属阳离子）选择性的一个重要方面。首次对水杨醛肟-UO2(2+)络合物的结合强度进行了量化，并与水杨酸-UO2(2+)和代表性偕胺肟-UO2(2+)络合物的结合强度进行了比较。我们发现，水杨醛肟-UO2(2+)络合物的结合强度在量级上比其相应的水杨酸-UO2(2+)和代表性偕胺肟-UO2(2+)络合物大2-4个log β2单位；此外，水杨醛肟对UO2(2+)阳离子相对于竞争的Cu(2+)和Fe(3+)阳离子的选择性远大于文献中报道的水杨酸和戊二酰亚胺二肟的选择性。较高的UO2(2+)选择性可能归因于水杨醛肟-UO2(2+)和水杨醛肟-过渡金属络合物中观察到的不同配位模式。密度泛函理论计算表明，水杨醛肟可以作为一种双阴离子物种与UO2(2+)配位，该双阴离子物种是由醛肟的η(2)配位和酚盐基团的单齿结合形成的。相比之下，水杨醛肟通过酚盐和肟N之间形成的螯合物作为单阴离子物种与过渡金属阳离子配位；络合物通过肟OH基团和酚盐之间的氢键相互作用而稳定。通过结合实验测定的热力学常数和理论计算结果，我们能够推导出一些配体设计原则，以进一步提高UO2(2+)阳离子亲和力，从而进一步提高水杨醛肟衍生物的选择性。
