Unraveling the solvation geometries of the lanthanum(iii) bistriflimide salt in ionic liquid/acetonitrile mixtures

Abstract

A synergic approach combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS) has been used to investigate the structural properties of the La(Tf₂N)₃ salt (where Tf₂N = bistriflimide or bis(trifluoromethansulfonyl)imide) dissolved into several mixtures of acetonitrile and the 1,8-bis(3-methylimidazolium-1-yl)octane bistriflimide (C₈(mim)₂(Tf₂N)₂) ionic liquid (IL), with the IL molar fraction (χ_IL) ranging from 0 to 1. The XAS and MD results show that major changes take place in the La³⁺ first solvation shell when moving from pure acetonitrile to pure C₈(mim)₂(Tf₂N)₂. With increasing the IL concentration of the mixture, the La³⁺ first shell complex progressively loses acetonitrile molecules to accommodate more and more oxygen atoms of the Tf₂N⁻ anions. Except in pure C₈(mim)₂(Tf₂N)₂, La³⁺ is always able to coordinate both acetonitrile and Tf₂N⁻ anions, with a ratio between the two different ligands strongly dependent on the IL content. Moreover, the La³⁺ ion prefers to form a 10-coordinated first shell complex in all the investigated systems, with a slightly different geometry of the cluster depending on the composition of the La³⁺ first solvation shell. In particular, when moving from pure acetonitrile to pure C₈(mim)₂(Tf₂N)₂, the La³⁺ first solvation shell passes from a bicapped square antiprism geometry where all the Tf₂N⁻ anions act only as monodentate ligands, to a “1 + 5 + 4” structure in which the Tf₂N⁻ anion binds La³⁺ both in a monodentate and bidentate fashion. The great adaptability shown by the La³⁺ solvation structure allows it to reach the optimal balance among many different forces at play involving all of the different species present in the mixtures.