Highly lithium-ion selective ionophores: macrocyclic trinuclear complexes of methoxy-substituted arene ruthenium bridged by 2,3-pyridinediolate

Abstract

Metallacrowns composed of three Ru(alkylbenzene) units bridged by 2,3-pyridinediolate (pyO₂²⁻) are known to act as Li⁺-selective ionophores. In this study, we synthesized analogues with anisole and 3,5-dimethylanisole as the arene ligands, i.e., [{Ru(anisole)(pyO₂)}₃] and [{Ru(3,5-dimethylanisole)(pyO₂)}₃]. Their extraction properties for lithium and sodium picrates were investigated in a dichloromethane–water system at 25 °C. Compared to [{Ru(alkylbenzene)(pyO₂)}₃], the new metallacrowns allow faster and greater extraction of Li⁺ with much higher Li⁺/Na⁺ selectivity. [{Ru(3,5-dimethylanisole)(pyO₂)}₃] is superior to [{Ru(anisole)(pyO₂)}₃] in the Li⁺/Na⁺ selectivity. The extraction equilibrium constants and separation factors were determined for each system. The separation factor between Li⁺ and Na⁺ reaches 7.1 × 10⁴ with [{Ru(3,5-dimethylanisole)(pyO₂)}₃]. The highly Li⁺-selective metallacrown was applied to the extraction separation of Li⁺ from seawater.