Thermally Driven Catch-and-Release of CoCl₂

Abstract

A heat-driven catch-and-release strategy for CoCl 2 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•CoCl 2 revealed an ion-pair complex comprising a hexacoordinated cobalt cation [L•Co] 2+ and a tetrachlorocobaltate anion [CoCl 4 ] 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 + CoCl 2 , and ΔH°= 16.5 kJ/mol and ΔS°= 85.0 J/K•mol for PS-L + CoCl 2 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 CoCl 2 , 52.7 M −1 for MnCl 2 , and 49.7 M −1 for NiCl 2 . In a simulated ion mixture containing equimolar CoCl 2 , MnCl 2 , and NiCl 2 , 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.