A thermophysical investigation of weakly coordinated metals in ionic liquids

Abstract

Ionic liquids can solvate metals without strongly coordinating them, which gives a rare opportunity to probe the complexity of weakly coordinated metals through characterisation of liquid properties. In this work we use bis(trifluoromethanesulfonyl)imide (i.e. bistriflimide; [NTf₂]⁻) anions to prepare weakly coordinated metal containing ionic liquids (MILs) that are highly versatile because they are reactive with readily substituted ligands. Weakly coordinated metals are more than highly active catalysts. They are primed to create dynamic systems that are useful in other areas such as battery electrolytes, soft materials, and separations. However, very little is known about the properties of ionic liquids with weakly coordinated metals, so we present a wide scope analysis of nineteen 1-alkyl-3-methylimidazolium bistriflimide ILs with five different M[NTf₂]_n salts (M = Li, Mg, Zn, Co, Ni) in variable concentration to understand how metal cations influence thermophysical properties. We investigate short- and long-term thermal stability, decomposition kinetics, and decomposition mechanisms which provides operating windows and knowledge on how to improve stability. In particular, we find that all metals catalyse the elimination decomposition process, which severely compromises thermal stability. Alongside this, we present a detailed analysis of viscosities, densities, and heat capacities, the latter of which revealed that bistriflimide metal ILs are prone to drawing water from the air to form strong hydration spheres. Thermal parameters are affected to varying degrees, but desorption is possible under elevated temperatures — further justifying the need to know upper temperature limits. Altogether, this work provides a broad and methodical study to help understand solvent–solute interactions and thus design better systems for emerging applications that utilise weakly coordinated metals.