Tailor-Made Tetradentate Bis(Pyridyl Triazole) Pincer Ligands toward Selective Complexation and Extraction of...

Efficient separation of palladium (Pd) from high-level liquid waste (HLLW) is critically important, considering its scarcity in natural resources and associated issues regarding nuclear waste disposal. However, this remains challenging due to the extreme compositional complexity and high acidity of HLLW. Inspired by the square-planar coordination preference of Pd²⁺ with soft donors, we propose a coordination microenvironment-driven strategy to design a series of bis(pyridyl triazole) pincer ligands (LI-LIII). These ligands feature a tetradentate coordination cavity with four inwardly oriented nitrogen donor atoms, enabling exceptional Pd²⁺ extraction across a wide acidity range (0.1-5 M HNO₃) and high selectivity over 16 competing elements (SF_Pd/M > 78) as well as the minor actinide americium (SF_Pd/Am > 793). Varying the alkyl substituents significantly alters the extraction performance, and LIII, bearing the longest alkyl chain, exhibits the highest efficiency. Mechanism studies indicate that extraction proceeds predominantly via a 1:1 metal-ligand complex alongside a minor 1:2 species. Spectroscopic and density functional theory (DFT) results further reveal that in monophasic organic solutions, both 1:1 and 2:1 complexes coexist. The quasi-square-planar coordination geometry of Pd²⁺ is verified by single-crystal X-ray analysis of Pd(LI)·(NO₃)₂·(HNO₃)₂ complex. This work provides a feasible design strategy for advanced extractants through tailoring the metal coordination microenvironment.