Research Focus 2005-2015 - Ionic Liquids...

For many years we have been involved in the synthesis, understanding and prediction of the fundamental physical properties of Ionic Liquids. The next sections do give an overview on this field of research. Currently, we continue to apply Ionic Liquids with tailored properties for specific applications, i.e. for the 'ideal' Ionic Liquid Salt Bridge, as electrolytes for aluminum batteries or in catalysis to immobilize an ionic catalyst via a SILP-process.

Understanding and Prediction

With the understanding gained from this work, we finally could devise a simple model to predict the melting point of any IL. The first iteration was manual and a bit tedious to operate. J. Phys. Chem. B 2010, 114, 11113-11140.


However, the second iteration was improved and fully automated and tested on a large set of 520 ILs for which the melting point was firmly established.

ChemPhysChem 2011, 12, 2959-2972. 

Our Entry to this field was the development of a suitable Born-Fajans-Haber-Cycle to understand the phase change thermodynamics of ILs. In a series of papers we worked on this. Especially we were interested in lattice energies of IL crystals. J. Am. Chem. Soc.  2006, 128, 13427; J. Am. Chem. Soc. 2007, 129, 11296; Chem. Eur. J. 2011, 17, 6508.

To calculate lattice energies for the above work, we were using the molecular and ion volumes as constituents. Rather early we realized that many properties are linearly correlated to the molecular volume Vm. The first entries were conductivity, viscosity (shown right) and density. 

Angew. Chem. 2007, 119, 5480–5484; Chem. Eur. J. 2010, 16, 13139-13154.

Supported by the DFG-SPP 1191 on ILs, we worked out many more of such relations. Almost all of our prediction schemes for the physical properties of ILs are now included with the IL-Prop-Module of the commercial COSMOthermX program package. This allows non-specialists to predict properties like temperature dependent density, heat capacity, temperature dependent liquid entropy, melting point, temperature dependent viscosity and conductivity as well as critical micelle concentration from simple DFT optimized structures of the isolated ions as input. See:

Aluminate and Borate ILs

Unexpectedly the fluorinated alkoxyaluminates also formed room temperature ILs. Their properties are almost ideally non-interacting and we could learn a lot of first principles from these ideal systems that exclude many otherwise important additional interactions: e.g. dispersion and H-bonding. Chem. Eur. J. 2009, 15, 1966-1976; Chem. Eur. J. 2010, 16, 13139-13154.

ChemPhysChem 2011, 12, 2296; ChemPhysChem 2012, 13, 1802-1805; Dalton Trans. 2011, 1448-1452

Although being 2.5 times as voluminous as the prototypical [NTf2] based ILs, the transport properties of Cat+[Al(ORF)4], e.g. viscosity and conductivity, are better than those of Cat+[Tf2N].

ILs with ideal characteristics. Using the [Al(ORF)4] type of counterions, the first ILs with virtually no ion-pairing were synthesized. This showed from measurements of the ionicity, that was in three cases with these anions 100 % (as opposed to 30 to 70 % for typical good ILs).

Chem. Eur. J. 2014, 20, 9794-9804. Review: Acc. Chem. Res. 2015, 48, 2537–2546.

Boranate Ionic Liquids - BILs...

Boranate ILs:

The first syntheses of pure Ionic Liquids (ILs) with [BH4] and [B3H8] anions were realized as one step reactions. These Boranate ILs are suitable reagents for synthesis and have a potential for redox shuttling and storage/release of H2. Chem. Eur. J. 2012, 18, 2254-2264.