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

A new class of ligands, <i>N</i>,<i>N</i>′-dialkyl-2,6-pyridinediamide\n(DRPDA), has been designed with the specific intention of exhibiting\ninterchangeable diversity in coordination modes, including organometallic\ninteractions, for the purpose of solvent extraction of elements relevant\nto the proper treatment of high-level radioactive liquid waste (HLLW)\ngenerated after nuclear fuel reprocessing. Consequently, DRPDA has\nbeen observed to extract Pd(II) and Zr(IV) from HNO<sub>3</sub>(aq)\nto 1-octanol in nearly quantitative yields when the selected ligand\nis sufficiently hydrophobic. However, concomitance of some of other\nHLLW components were also found. The extraction selectivity toward\nPd(II) and Zr(IV) was markedly enhanced by employing <i>n</i>-dodecane instead of 1-octanol as evidenced by good distribution\nratios (<i>D</i><sub>M</sub>) of Pd(II) (<i>D</i><sub>Pd</sub> = 72.5) and Zr(IV) (<i>D</i><sub>Zr</sub> = 12.9), which is several orders of magnitude greater than <i>D</i><sub>M</sub>’s of other HLLW components (10<sup>–3</sup>–10<sup>–2</sup>), where addition of\n20 vol % 1-octanol is still required to accelerate the extraction\nkinetics. Despite direct contact with the highly acidic aqueous phase,\ndeprotonation from one of the amide NH moieties of DRPDA proceeds\nto form [Pd(DRPDA<sup>–</sup>)(NO<sub>3</sub>)] as a good extractables\nin the current biphasic system. This Pd(II) complex with a rather\nunique asymmetric N<sup>–</sup>^N^O tridentate coordination\nwas characterized by SCXRD, elemental analysis and <sup>1</sup>H NMR,\nand theoretically corroborated by DFT calculations and NBO analysis.\nIn contrast, DRPDA also interacts with Zr<sup>4+</sup> in different\ntridentate O^N^O mode without any deprotonation. Based on mechanistic\ndifferences in the extraction chemistry we clarified, Pd(II) and Zr(IV)\ncoextracted to the organic phase were recovered stepwise by using\nappropriate stripping agents such as 1.0 M HCl(aq) and 0.10 M HNO<sub>3</sub>(aq), respectively.