Thermally Driven Catch-and-Release of CoCl₂

A heat-driven catch-and-release strategy for CoCl₂ capture is described. It is based on the use of an immobilized neutral dicyclohexylacetamide-based receptor L supported on polystyrene (PS-L). An X-ray diffraction analysis of a single crystal of L·CoCl₂ revealed an ion-pair complex comprising a hexacoordinated cobalt cation [L·Co]²⁺ and a tetrachlorocobaltate anion [CoCl₄]^2-. Temperature dependent binding was seen, as inferred from UV-vis spectroscopic studies. Fits to the van't Hoff equation yielded values of ΔH° = 12.4 kJ/mol and ΔS° = 56.0 J/K·mol for L + CoCl₂, and ΔH° = 16.5 kJ/mol and ΔS° = 85.0 J/K·mol for PS-L + CoCl₂ in 95% ethanol. Consequently, cobalt capture and release are mediated by heating and cooling, respectively. The material PS-L exhibits a preference for binding cobalt over manganese and nickel as inferred from Langmuir-Freundlich isotherm analyses that revealed binding constants of K_LF = 88.5 M^-1 for CoCl₂, 52.7 M^-1 for MnCl₂, and 49.7 M^-1 for NiCl₂. In a simulated ion mixture containing equimolar CoCl₂, MnCl₂, and NiCl₂, ICP-MS analyses served to confirm that cobalt was selectively enriched to 52 mol % (from an initial level of ca. 32 mol %) after one catch-and-release cycle and 76.6% after three cycles. Our experimental results were validated by density functional theory calculations, which also show stronger binding of Co over Mn and Ni to L.