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

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 (χIL) 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.