Bulk nanostructure of the prototypical 'good' and 'poor' solvate ionic liquids [Li(G4)][TFSI] and [Li(G4)][NO3

Thomas Murphy,Gregory G Warr,Karina Shimizu,José N Canongia Lopes,Nageshwar Yepuri,Tamim Darwish,Sam K Callear,Rob Atkin,Masayoshi Watanabe

doi:10.1039/c6cp00176a

Abstract

The bulk nanostructures of a prototypical 'good' solvate ionic liquid (SIL) and 'poor' SIL have been examined using neutron diffraction and empirical potential structure refinement (EPSR) simulated fits. The good SIL formed by a 1 : 1 mixture of lithium bis(trifluoromethylsulfonyl)imide (Li[TFSI]) in tetraglyme (G4), denoted [Li(G4)][TFSI], and the poor SIL formed from a 1 : 1 mixture of lithium nitrate (Li[NO3]) in G4, denoted [Li(G4)][NO3], have been studied. In both SILs there are strong Lewis acid-base interactions between Li(+) and ligating O atoms. However, the O atoms coordinated to Li(+) depend strongly on the counter anion present. LiO coordination numbers with G4 are 2-3 times higher for [Li(G4)][TFSI] than [Li(G4)][NO3], and conversely the LiO anion coordination number is 2-3 times higher in [Li(G4)][NO3]. In both solvates the local packing of Li around G4 O atoms are identical but these interactions are less frequent in [Li(G4)][NO3]. In both SILs, Li(+) has a distribution of coordination numbers and a wide variety of different complex structures are present. For [Li(G4)][NO3], there is a significant proportion uncoordinated G4 in the bulk; ∼37% of glyme molecules have no LiO contacts and each G4 molecule coordinates to an average of 0.5 Li(+) cations. Conversely, in [Li(G4)][TFSI] only ∼5% of G4 molecules lack LiO contacts and G4 molecules coordinates to an average of 1.3 Li(+) cations. Li(+) and G4 form polynuclear complexes, of the form [Lix(G4)y](x+), in both solvates. For [Li(G4)][TFSI] ∼35% of Li(+) and G4 form 1 polynuclear complexes, while only ∼10% of Li(+) and G4 form polynuclear complexes in [Li(G4)][NO3].

Highlights

1049 001 Lisboa, Portugal e Department of Chemistry and Biotechnology, Yokohama National University, Yokohama 240-8501, Japan f School of Chemistry, The University of Sydney, NSW 2006, Australia † Electronic supplementary information (ESI) available
solvate ionic liquid (SIL) are a sub-class of Ionic liquids (ILs), consisting of a metal cation bound to a stoichiometric quantity of coordinating ligands via strong Lewis acid–base interactions that yield stable complex cations and counter ions in the bulk
The first known examples of SILs consisted of aqua cations in inorganic hydrate melts.[26]

Summary

Introduction

The first known examples of SILs consisted of aqua cations in inorganic hydrate melts.[26] perhaps the most widely studied SILs utilise oligoethers (glymes) with metal salts.[22,23,27] Research into glyme-based SILs owes much of its growth to investigations of Li-salt:polyethylene oxide (PEO) rubbery electrolytes.[28] It was found that particular salt:PEO mixtures produced low melting, highly conductive amorphous phases in the ‘‘crystallinity gap’’ regions of the phase diagrams.[29] Many stoichiometric mixtures of glymes and lithium salts form crystalline complexes, and a variety of crystal structures have been elucidated.[30,31,32,33] Notably, many of these solvate complexes melt near room temperature.[29] These melts

Methods

Results

Conclusion