Probing Ni2+ and Co2+ speciation in carboxylic acid based deep eutectic solvents using UV/Vis and FT-IR spectroscopy

In this work, the coordination behaviour of Ni2+ and Co2+ in choline chloride-carboxylic acid (malonic, succinic, glutaric, glycolic and lactic add) deep eutectic solvents (DES's) using UV/Vis and FT-IR spectroscopy is reported. In all systems Co2+ was observed to form the [CoCl4](2-). This species was also observed for solutions of CoCl2 and CoO in a choline chloride-lactic acid deep eutectic solvent. Ni2+ showed more complex chemistry, with specialion being dictated by whether the carboxylic acid was mono- or di-acidic. The [NiCl4](2-) species was dominant in DES's containing dicarboxylic adds, but in DES containing monocarboxyfic adds, an octahedral species was present alongside the tetrachloronickelate ion. FT-IR confirms that the add is involved in coordinating to the metal centre. This is the first time this behaviour has been reported in the literature, and sheds light on the relative strength of the interactions between metals and solvent molecules in DES. The research presented here, in relation to previously reported works, highlights the importance of process conditions on metal speciation in DES. Of particular importance is the presence of water, as this can effect Co2+ and Ni2+ speciation even when the same DES is used. Our data show no difference in Co2+ speciation when using either chloride or oxide as the metal ion source. (C) 2020 Elsevier B.V. All rights reserved.