Effect of ligand structures on oxygen absorbability and viscosity of metal-containing ionic liquids

Abstract

Metal-containing ionic liquids (MCILs) composed of a cobalt(II) Schiff base complex and ionic-liquid-based axial ligands (ligand ILs) are potential O2 absorbents. To determine the design criteria of MCILs with both highly selective O2 absorbability and low viscosity, the relationship between the physicochemical properties and chemical structure of the MCILs was investigated. The measurement of the amount of O2 absorbed in MCILs with various ligand ILs indicated that O2 reactivity is determined by the electron density of the Co atom of the MCILs. The electron density of the Co atom could be controlled by the σ electron donation ability of the ligand ILs. Moreover, the viscosity of the MCILs was strongly affected by the interaction among the MCIL molecules caused by the π electron system. This interaction was weakened by the equatorially coordinating Schiff base and the ligand ILs within the chemical structure. Therefore, to develop MCILs with both high O2 reactivity and low viscosity, suppression of the interaction caused by the π electron system without decreasing the electron density of the Co atom is important.