Utility of Interchangeable Coordination Modes of <i>N</i>,<i>N</i>′-Dialkyl-2,6-pyridinediamide Tridentate Pincer Ligands for Solvent Extraction of Pd(II) and Zr(IV) from High-Level Radioactive Liquid Waste

A new class of ligands, <i>N</i>,<i>N</i>'-dialkyl-2,6-pyridinediamide (DRPDA), has been designed with the specific intention of exhibiting interchangeable diversity in coordination modes, including organometallic interactions, for the purpose of solvent extraction of elements relevant to the proper treatment of high-level radioactive liquid waste (HLLW) generated after nuclear fuel reprocessing. Consequently, DRPDA has been observed to extract Pd(II) and Zr(IV) from HNO₃(aq) to 1-octanol in nearly quantitative yields when the selected ligand is sufficiently hydrophobic. However, concomitance of some of other HLLW components were also found. The extraction selectivity toward Pd(II) and Zr(IV) was markedly enhanced by employing <i>n</i>-dodecane instead of 1-octanol as evidenced by good distribution ratios (<i>D</i>_M) of Pd(II) (<i>D</i>_Pd = 72.5) and Zr(IV) (<i>D</i>_Zr = 12.9), which is several orders of magnitude greater than <i>D</i>_M's of other HLLW components (10^-3-10^-2), where addition of 20 vol % 1-octanol is still required to accelerate the extraction kinetics. Despite direct contact with the highly acidic aqueous phase, deprotonation from one of the amide NH moieties of DRPDA proceeds to form [Pd(DRPDA^-)(NO₃)] as a good extractables in the current biphasic system. This Pd(II) complex with a rather unique asymmetric N^-^N^O tridentate coordination was characterized by SCXRD, elemental analysis and ¹H NMR, and theoretically corroborated by DFT calculations and NBO analysis. In contrast, DRPDA also interacts with Zr⁴⁺ in different tridentate O^N^O mode without any deprotonation. Based on mechanistic differences in the extraction chemistry we clarified, Pd(II) and Zr(IV) coextracted to the organic phase were recovered stepwise by using appropriate stripping agents such as 1.0 M HCl(aq) and 0.10 M HNO₃(aq), respectively.