2-Hydroxyaryloximes as Tunable Extractants for Selective First–Row Transition Metal Liquid–Liquid Extraction: Dimerization Coefficients, p<i>K</i>_a, and pH_0.5

As critical minerals such as Ni and Co are essential for modern technologies and clean energy solutions, there is an urgent need to develop sustainable extraction methods to meet rising demand and mitigate environmental impacts of existing mining practices. 2-Hydroxyaryloximes are promising candidates for metal separation as they form stable size-selective pseudomacrocyclic dimers through hydrogen-bonding networks. These ligands exhibit pH-dependent coordination properties, influenced by the protonation state of the phenolic oxygen. Herein, structure–property relationships of seven 2-hydroxyaryloximes and their efficacy as ligands in liquid–liquid extraction schemes for Ni and Co are explored to provide a framework for designing ligands that could lead to more effective and sustainable solvent extraction processes. A generalized multigram synthesis for 2-hydroxyaryloximes is reported along with concentration- and solvent-dependent characterization of the complexes. Steric and electronic effects induced by modifications of peripheral substituent groups on dimerization constants and pKa values of the complexes were systematically investigated using NMR and potentiometric titrations. Our findings demonstrate the potential to tune the extractive properties of 2-hydroxyaryloximes, allowing for optimization of pH windows for selective metal recovery. Solvent extraction experiments with Ni and Co systems further illustrate that extraction efficiency and selectivity can be altered through rational ligand design.