Dicyanamide Ions as Complexing Agents of Co(II): From Weak Ligands in Water to Strong Ones in an Ionic Liquid

Abstract

ABSTRACTThe complexation of cobalt cations (Co(II)) by dicyanamide (DCA–) in water and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C1C4Pyrr][NTf2]) has been investigated by recording UV–vis spectra at 20 °C for solutions containing different ratios for the concentrations of DCA– vs. Co(II) (R = [DCA–]/[Co(II)]). The variation of the absorbance at a fixed wavelength as a function of R was expressed using the Beer Lambert law, based on a chemical model assuming the formation of up to four successive cobalt–dicyanamide complexes. This allowed us to determine the speciation of cobalt and the complexation constants of all complexes in both aqueous and ionic liquid media. In water, a unique complex of [Co(H2O)5(DCA)]+ exhibiting a weak complexation constant (K = 20.5) is observed. In the ionic liquid, cobalt is shown to coordinate with up to four dicyanamide anions. All species coexist until the ratio R reaches 5000, where more than 90% of the fourth and last complex [Co(NTf2)(3–x)(DCA)4](x–5) is formed. In addition, the extraction of cobalt towards hydrophobic ionic liquids trihexyltetradecylphosphonium dicyanamide ([P66614][DCA]) and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2]) was carried out using various concentrations of DCA both in the aqueous and ionic liquid phases. Results were found to correlate very well with the speciation of Co(II) obtained in this work.