2‑Hydroxyaryloximes\nas Tunable Extractants\nfor Selective First–Row Transition Metal Liquid–Liquid\nExtraction: Dimerization Coefficients, p<i>K</i><sub>a</sub>, and pH<sub>0.5</sub>

As critical minerals such as Ni and Co are essential\nfor modern\ntechnologies and clean energy solutions, there is an urgent need to\ndevelop sustainable extraction methods to meet rising demand and mitigate\nenvironmental impacts of existing mining practices. 2-Hydroxyaryloximes\nare promising candidates for metal separation as they form stable\nsize-selective pseudomacrocyclic dimers through hydrogen-bonding networks.\nThese ligands exhibit pH-dependent coordination properties, influenced\nby the protonation state of the phenolic oxygen. Herein, structure–property\nrelationships of seven 2-hydroxyaryloximes and their efficacy as ligands\nin liquid–liquid extraction schemes for Ni and Co are explored\nto provide a framework for designing ligands that could lead to more\neffective and sustainable solvent extraction processes. A generalized\nmultigram synthesis for 2-hydroxyaryloximes is reported along with\nconcentration- and solvent-dependent characterization of the complexes.\nSteric and electronic effects induced by modifications of peripheral\nsubstituent groups on dimerization constants and p<i>K</i><sub>a</sub> values of the complexes were systematically investigated\nusing NMR and potentiometric titrations. Our findings demonstrate\nthe potential to tune the extractive properties of 2-hydroxyaryloximes,\nallowing for optimization of pH windows for selective metal recovery.\nSolvent extraction experiments with Ni and Co systems further illustrate\nthat extraction efficiency and selectivity can be altered through\nrational ligand design.