Conformational design concepts for anions in ionic liquids

Abstract

The identification of specific design concepts for the in silico design of ionic liquids (ILs) has been accomplished using theoretical methods. Molecular building blocks, such as interchangeable functional groups, are used to design a priori new ILs which have subsequently been experimentally investigated. The conformational design concepts are developed by separately and systematically changing the central (imide), bridging (sulfonyl) and end (trifluoromethyl) group of the bis(trifluoromethanesulfonyl)imide [N(Tf)₂]^- anion and examining the resultant potential energy surfaces. It is shown that these design concepts can be used to tune separately the minimum energy geometry, transition state barrier height and relative stability of different conformers. The insights obtained have been used to design two novel anions for ILs, trifluoroacetyl(methylsulfonyl)imide [N(Ms)(TFA)]^- and acetyl(trifluoromethanesulfonyl)imide [N(Tf)(Ac)]^-. The computationally predicted structures show excellent agreement with experimental structures obtained from X-ray crystallography. [C₄C₁im][N(Tf)(Ac)] and [C₄C₁im][N(Ms)(TFA)] ILs have been synthesised and ion diffusion coefficients examined using pulsed field gradient stimulated echo NMR spectroscopy. Significantly increased diffusion was observed for the more flexible [N(Tf)(Ac)]^- compared with the more rigid [N(Ms)(TFA)]^- analogue. Furthermore, a pronounced impact on the fluidity was observed. The viscosity of the IL with the rigid anion was found to be twice as high as the viscosity of the IL with the flexible anion. The design concepts presented in this work will enable researchers in academia and industry to tailor anions to provide ILs with specific desired properties.

Fig. 1. (a) 1D potential energy surface showing the cis and trans minima and barrier for interconversion for [N(Tf)₂]⁻. The eye indicates the perspective used in the projection in Fig. 2. (b) Minimum geometries for [N(Tf)₂]⁻ and the choice of dihedral angles.

Fig. 2. Reorientation of the CF₃ group in [N(Tf)₂]⁻ compared to the much smaller volume for F in [N(Fs)₂]⁻. The sweep volume is represented by the bright green circles.

Fig. 3. (a) Building blocks used in the design of new anions (b) nomenclature of combinations of bridging group and end group (c) combinations of central group, bridging group and end group that yield well-known anions.

Fig. 4. The upper part of the image is the 3D potential energy surface (PES) of the [N(Tf)₂]⁻ anion and the lower surface the 2D projection. Plus symbols mark selected transition states. The arrows indicate how a local minimum and maximum are mapped on the 2D projection. The bold line at 15 kJ mol⁻¹ will facilitate comparison with other surfaces. The lowest energy conformer is taken as zero reference.

Fig. 5. (a) PES of [N(Tf)₂]⁻, (b) stationary point structures (oxygen atoms not shown). Open circles denote minima, black crosses denote transition states. The dotted line is drawn at 15 kJ mol⁻¹ to facilitate comparison with other surfaces. The lowest energy conformer is taken as the zero reference.

Fig. 6. PES of (a) [CH(Tf)₂]⁻, (b) [N(TFA)₂]⁻, (c) [CH(TFA)₂]⁻, (d) [CH(Tf)(TFA)]⁻, (e) [N(Ms)₂]⁻, and (f) [N(Tf)(TFA)]⁻. Black circles denote minima, black crosses denote transition states. The dotted line is drawn at 15 kJ mol⁻¹ to facilitate comparison with other surfaces. The vertical black line drawn at 90° in (a) indicates the position of the slice used in Fig. 7, 11 and 12. The area enclosed by the bold red line in (d) is sufficient to describe the PES.

Fig. 7. Slices through the 3D PES of the PES of [CH(Tf)₂]⁻, [N(Tf)₂]⁻, and O(Tf)₂. One of the two S–X–S–C dihedral angles is kept at 90°. The geometry of the lowest lying transition state at ∼180° is shown as inset.

Fig. 8. Molecular orbitals showing efficient electronic delocalisation in the trans-[N(Tf)₂]⁻ anion.

Fig. 9. (a) HOMO-7 for [N(Tf)₂]⁻, (b) HOMO-7 for CHTf₂, (c) HOMO-3 for [N(Tf)₂]⁻, (d) HOMO-3 for CHTf₂. All structures are the lowest energy cis–trans TS.

Fig. 10. (a) Schematic representation of the conformational changes in [N(TFA)₂]⁻, (b) the optimised minimum geometries of this anion, (c) the corresponding structures for the non-fluorinated analogue [N(Ac)₂]⁻.

Fig. 11. Slices of the PES of [CH(Tf)₂]⁻, [N(TFA)₂]⁻, and [CH(TFA)₂]⁻. One of the two S–X–S–C dihedral angles is kept at 90°. The global minimum of the whole PES is used as the zero value.

Fig. 12. Slices of the PES of [N(Ms)₂]⁻ and [N(Tf)₂]⁻. One of the C–S–N–S dihedral angles is kept fixed (90° for [N(Tf)₂]⁻, 80° for [N(Ms)₂]⁻).

Fig. 13. Potential energy surfaces of the anions (a) [N(Ms)(TFA)]⁻ and (b) [N(Tf)(Ac)]⁻ in the experimentally investigated ILs. The red crosses correspond to the dihedral angles obtained experimentally from crystal structures of model compounds.

Fig. 14. ORTEP plots of the crystal structures corresponding to the red crosses in Fig. 13. Structures are (a) [K][N(Tf)(Ac)], (b) [H][N(TFA)(Ms)], and (c) [H][N(Tf)(Ac)]. Thermal ellipsoids at 50% probability. Symmetry codes: (i) −x + 1, y, z; (ii) x + 0.5, y, 1 − z.

Fig. 15. Steijskal–Tanner plot and self-diffusion coefficients at 297 K of the ILs studied in this work. The experimental uncertainty in the diffusion values is 4% of the absolute value. The reported values are for the nominal gradient strength of the experimental setup.

Fig. 16. Viscosity of the two ILs in this work as a function of temperature. Drawn lines are the VFT fits according to eqn (1) in the ESI, Section 3. The experimental uncertainty in the viscosity values is 2% of the absolute value. The RMSD between experimental viscosities and the VFT fit was found to be 0.090 mPa s.