Department of Chemistry and Biochemistry, University of North Carolina, Wilmington, North Carolina 28403, United States.
Chemical Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6119, United States.
Inorg Chem. 2022 Jul 4;61(26):9960-9967. doi: 10.1021/acs.inorgchem.2c00775. Epub 2022 Jun 15.
The ligands BDA (2,2'-bipyridyl-6,6'-dicarboxylic acid) and PDA (1,10-phenanthroline-2,9-dicarboxylic acid) are of interest as functional group types for ion-exchange materials for extracting uranium from the oceans, reported in a previous paper for PDA Lashley, M. A. ( 2016 55 10818 10829). Yang, Y. ( 2019, 58, 6064 6074) have published what they claim to be a more accurate result for the formation of the UO/PDA complex of log = 22.84 compared with our reported value of log = 16.5, as well as log = 21.52 for the BDA complex. The determination of log for the PDA and BDA complexes with the UO cation was carried out by Yang et al. using a competition reaction between DTPA (diethylenetriamine pentaacetic acid) and BDA or PDA, monitoring the absorbance due to the BDA and PDA ligands. This competition method using absorbance versus pH titrations was developed for determining the formation constants of the complexes of several polypyridyl ligands plus PDA complexes of metal ions, which were too stable for log determination by competition with protons. A key feature of such titrations is that in the competition reaction, the displacement of the pyridyl donor ligand (e.g., PDA) by the competing ligand (e.g., DTPA), the absorbance spectrum of the displaced pyridyl donor ligand should be observed. Competing ligands used to date have been EDTA (ethylenediamine tetraacetic acid), DTPA, or the hydroxide ion. In the study of Yang et al., no such displaced PDA or BDA was apparent in the absorbance spectra in their titrations so that their reported log values have no validity. Their log values are so much higher than log for the uranyl DTPA complex (∼13.6) that DTPA could not possibly displace BDA or PDA from the uranyl cation, and a competition reaction could not possibly occur. We report the correct value of log = 15.4 (ionic strength = zero) for the uranyl BDA complex, to illustrate the correct determination of such a constant by a competition reaction between BDA and hydroxide, showing how the characteristic absorbance spectrum for a BDA complex, here the UO complex, disappears, and the distinctive absorbance spectrum of the free nonprotonated BDA ligand appears as the pH is increased, and BDA is displaced by the hydroxide ion.
配体 BDA（2,2'-联吡啶-6,6'-二羧酸）和 PDA（1,10-菲咯啉-2,9-二羧酸）是从海洋中提取铀的离子交换材料的功能基团类型，在之前的一篇论文中已经报道过 PDA，Lashley，M. A.（2016 年，55 卷，10818-10829 页）。Yang，Y.（2019 年，58 卷，6064-6074 页）声称他们得到了一个更准确的结果，用于形成 UO/PDA 络合物的 log = 22.84，与我们报道的 log = 16.5 相比，以及 log = 21.52 用于 BDA 络合物。Yang 等人通过 DTPA（二乙三胺五乙酸）与 BDA 或 PDA 之间的竞争反应，监测由于 BDA 和 PDA 配体引起的吸光度，来确定 UO 阳离子与 PDA 和 BDA 络合物的 log 。使用这种竞争方法通过与质子竞争来测定几种多吡啶配体的络合物形成常数和 PDA 络合物的金属离子，这些金属离子太稳定，无法通过与质子竞争来测定 log 。这种吸光度滴定的一个关键特征是，在竞争反应中，取代吡啶供体配体（例如 PDA）由竞争配体（例如 DTPA）取代，取代的吡啶供体配体的吸光度谱应该被观察到。迄今为止，用于竞争的配体一直是 EDTA（乙二胺四乙酸）、DTPA 或氢氧根离子。在 Yang 等人的研究中，在他们的滴定中没有观察到明显的游离 PDA 或 BDA 出现在吸光度谱中，因此他们报告的 log 值没有有效性。他们的 log 值比 UO 与 DTPA 的络合物的 log （约 13.6）高得多，以至于 DTPA 不可能从铀酰阳离子中取代 BDA 或 PDA，并且不可能发生竞争反应。我们报告了正确的 log = 15.4（离子强度为零）对于铀酰 BDA 络合物，通过 BDA 和氢氧根之间的竞争反应来说明这种常数的正确测定，展示了 BDA 络合物的特征吸光度谱如何消失，并且游离非质子化的 BDA 配体的独特吸光度谱随着 pH 值的增加而出现，并且 BDA 被氢氧根离子取代。
