Liquid Structure of Ionic Liquids with [NTf2]− Anions, Derived from Neutron Scattering

The liquid structure of three common ionic liquids (ILs) was investigated by neutron scattering for the first time. The ILs were based on the bis(trifluoromethanesulfonyl)imide anion, abbreviated in the literature as [NTf2]− or [TFSI]−, and on the following cations: 1-ethyl-3-methylimidazolium, [C2mim]+; 1-decyl-3-methylimidazolium, [C10mim]+; and trihexyl(tetradecyl)phosphonium, [P666,14]+. Comparative analysis of the three ILs confirmed increased size of nonpolar nanodomains with increasing bulk of alkyl chains. It also sheds light on the cation–anion interactions, providing experimental insight into strength, directionality, and angle of hydrogen bonds between protons on the imidazolium ring, as well as H–C–P protons in [P666,14]+, to oxygen and nitrogen atoms in the [NTf2]−. The new Dissolve data analysis package enabled, for the first time, the analysis of neutron scattering data of ILs with long alkyl chains, in particular, of [P666,14][NTf2]. Results generated with Dissolve were validated by comparing outputs from three different models, starting from three different sets of cation charges, for each of the three ILs, which gave convergent outcomes. Finally, a modified method for the synthesis of perdeuterated [P666,14][NTf2] has been reported, with the aim of reporting a complete set of synthetic and data processing approaches, laying robust foundations that enable the study of the phosphonium ILs family by neutron scattering.


Materials
Trihexyl(tetradecyl)phosphonium chloride, [P 666,14 ]Cl, was kindly provided by Solvay.Perdeuteriated 1-hexanol and 1-tetradecanol as well as perdeuteriated [C 2 mim][NTf 2 ] and [C 10 mim][NTf 2 ] were provided from the deuteration facility at ISIS Neutron and Muon source.Unless otherwise stated, all other chemicals were purchased from Sigma-Aldrich and used as received.XRF analysis was performed on a Rigaku NEX QC+ QuantEZ High-Resolution Energy Dispersive X-ray Fluorescence (EDXRF) Spectrometer.NMR spectra were recorded on either a Bruker Avance III 400 MHz spectrometer or a Bruker Avance II DPX 600 MHz spectrometer.Quantitative 1 H NMR was recorded on a Bruker Avance III 400 MHz spectrometer, with benzene as an internal standard and CD 3 OD as NMR solvent.

Safety
In the case of deuteration of P 666 and [P 666,14 ]Cl, Pd/C and/or Pt/C catalysts are used.These catalysts are highly flammable (particularly after the reaction) and proper protocols and precautions should be used to avoid any potential fires.This involves keeping the catalyst waste wet and safe disposal after the reaction.

Attempted synthesis of D 68 -trihexylphosphine
Magnesium turnings (0.54 g, 1.20 eq.) were transferred into an oven-dried two-necked roundbottomed flask (100 ml) equipped with a reflux condenser with argon gas inlet, a septum and a PTFE coated magnetic stirring bar.Anhydrous diethyl ether (10 cm 3 ) was then added, followed by a crystal of iodine, and the flask was heated to 30 °C, with stirring.Subsequently, a small portion of D 13 -1chlorohexane (2.51 g, 1.0 eq.) was added dropwise via a syringe.After approximately 50% was added, the yellow iodine colour disappeared, with the solution becoming a grey colour, and the commencement of gentle refluxing.The remainder of the solution was added and the reaction was allowed to proceed (35 °C, overnight), before being cooled back to room temperature.
In another oven-dried two-necked round-bottomed flask (100 ml), equipped with a stirring bar and connected to an argon filled Schlenk line, phosphorus trichloride (0.51 g, 12.35 mmol), lithium bromide (0.03 g, 1.24 mmol) and copper(I) iodide (0.07 g, 1.24 mmol) were added to degassed, dry diethyl ether (15 ml).The flask was placed in an acetone-dry ice bath (-78 °C), and the mixture was allowed to cool, with vigorous stirring.
The solution containing the Grignard reagent was transferred via a cannula filter into the PCl 3 solution, and stirred at -78 °C.The dry ice-acetone bath was then removed, and the reaction mixture was brought to ambient temperature and left to react for a further 2 h with vigorous stirring on reaching this temperature.The solvent was removed under reduced pressure (25 °C, 10 -2 bar) and the product was dissolved in pentane (25 ml).Degassed water (25 ml) was subsequently added, and the flask was vigorously shaken by hand, the organic layer was removed via cannula transfer into an oven dried flask (100 ml) and again washed with degassed water (25 ml).This was transferred via cannula into an oven dried flask (100 ml).Finally, the organic phase was dried using sodium sulfate and the liquid phase was transferred via cannula filtration into an oven dried flask (100 ml).The solvent was removed under reduced pressure in an ice bath (0 °C, 10 -2 bar), to give a colourless liquid. 31P NMR showed that multiple phosphorus environments were present (Figure S19).

Attempted synthesis of D 68 -trihexyltetradecylphosphonium chloride
D 39 -P 666 (1.0 eq.) and D 29 -1-tetradecylchloride (1.3 eq.) were added to a flask in acetonitrile and heated to reflux (60 °C) under argon for 1 week. 31P NMR showed that multiple phosphorus environments were present.

Figure
Figure S19. 31P NMR spectra of a) attempted synthesis of D 39 -P 666 showing the presence of multiple species and b) alkylation with D 29 -1-tetradecylchloride after one week.

Figure
Figure S26. 1 H NMR of D 68 -[P 666,14 ]Cl in CDCl 3 Figure S38.Total structure factors F(Q) (top), and the corresponding Fourier transform to real space G(r) radial distribution functions (bottom) showing experimental data (red symbols) and Dissolve modelled (blue solid line) for [C 2 mim][NTf 2 ] for left: CLP charges, middle: ESP charges and right: LPG charges sets.

Figure S39 .
Figure S39.Total structure factors F(Q) (top), and the corresponding Fourier transform to real space G(r) radial distribution functions (bottom) showing experimental data (red symbols) and Dissolve modelled (blue solid line) for [C 10 mim][NTf 2 ] for left: CLP charges, middle: ESP charges and right: LPG charges sets.

Figure S40 .
Figure S40.Total structure factors F(Q) (top), and the corresponding Fourier transform to real space G(r) radial distribution functions (bottom) showing experimental data (red symbols) and Dissolve modelled (blue solid line) for [P 666,14 ][NTf 2 ] for left: CLP charges, middle: ESP charges and right: LPG charges sets.

Table S1 .
Simulation box size parameters.

Table S2 .
Interatomic distances (taken from the first peak maximum in the site-site radial distribution functions) and relative coordination numbers (CN, calculated to the minimum after the first peak in the radial distribution functions) between different atom types for [C 2 mim][NTf 2 ] and [C 10 mim][NTf 2 ].H CW1 refers to H CW beside the alkyl chain and H CW2 refers to H CW beside the methyl group.

Table S3 .
Interatomic distances (taken from the first peak maximum in the site-site radial distribution functions) and relative coordination numbers (CN, calculated to the minimum after the first peak in the radial distribution functions) between different atom types for [P 666,14 ][NTf 2 ].

Table S4 .
Three sets of charges used for the Dissolve model for [C 2 mim][NTf 2 ].

Table S5 .
Three sets of charges used for the Dissolve model for [C 10 mim][NTf 2 ].

Table S6 .
Three sets of charges used for the Dissolve model for [P 666,14 ][NTf 2 ].