Potassium Trifluorotris(pentafluoroethyl)phosphate

Abstract

A transparent white crystalline product, K[(C₂F₅)₃PF₃] (KFAP), has been synthesized via a one-pot route. Difluorotris(pentafluoroethyl)phosphorane was reacted first with aqueous HF then with K₂CO₃ or KHCO₃ in a solution. Isolation of the intermediate, tris(pentafluoroethyl)trifluorophosphoric acid H[(C₂F₅)₃PF₃] (HFAP), was not carried out. The salt has been characterised by powder and single crystal X-ray diffraction. The single-crystal structure shows a monoperiodic coordination polymer involving K∙∙∙F bridging interactions to be present. The thermal characteristics and behaviour of KFAP have been established by simultaneous TGA-DSC measurements.

1. Introduction

Difluorotris(pentafluorethyl)phosphorane, (C₂F₅)₃PF₂, is a strong Lewis acid known to be an active catalyst of various organic syntheses [1,2,3] and to react with aqueous HF [4] to give the strong acid H[(C₂F₅)₃PF₃] (HFAP), a source of various salts used in ionic liquids, electrolytes, lubricants and membrane modifiers [5,6,7,8,9]. The weakly coordinating anion [(C₂F₅)₃PF₃]⁻ is more stable towards hydrolysis than [PF₆]⁻, and this property, along with its greater hydrophobicity, has provided a wide range of applications for its salts [10,11,12,13,14,15,16,17]. The potassium salt, KFAP, in particular, has been commonly used [11,12,13,14,15,16], although a detailed synthesis of this material is not available in the open literature and its crystal structure is unknown. The present work provides a simple synthesis, crystal structure and measurements of the thermal stability of KFAP.

2. Results and Discussion

Potassium trifluorotris(pentafluorethyl)phosphate, KFAP, was prepared in a two-step, one-pot manner according to Scheme 1 by initially reacting (C₂F₅)₃PF₂ with aqueous HF to give a solution of HFAP which was then neutralised by the addition of either K₂CO₃ or KHCO₃. Evaporation of the final solution provided a colourless solid that could be conveniently crystallised from either water, propan-2-ol or 1,4-dioxane. A crystal obtained from water was used for an X-ray structure determination which showed the material to be anhydrous, crystallising in the polar space group Cmc2₁. The [(C₂F₅)₃PF₃]⁻ unit has the meridional form seen in other salts of this anion [13,14,15] and the K(I) countercation has 10 interactions exceeding dispersion (K∙∙∙F < 3.15 Å) involving both PF and CF donors (Figure 1); these interactions generate monoperiodic coordination polymer units lying along [001] (Figure 2). PXRD measurements on the bulk product of the synthesis provided a pattern in good agreement with that predicted from the single-crystal structure determination, indicating that a single phase was present.

Thermogravimetry of KFAP (Figure 3) showed mass loss to be negligible up to 230 °C. Above that temperature, C₂F₄ could be detected by mass spectrometry as a decomposition product, though its exact mechanism of formation remains to be established.

DSC measurements provided evidence of a phase change at 106–108 °C (ΔH 5.8 J g⁻¹) and the melting of the solid at 181–183 °C (ΔH 51 J g⁻¹).

3. Materials and Methods

3.1. General Information

All reagents were purchased from commercial sources and used without further purification.

The NMR spectra were recorded for 5% DMSO-d6 solutions using a 400 MHz AVANCE III HD instrument (Bruker, Billerica, MA, USA).

The thermal behaviour and properties of the substances were studied using an STA 449 F1 Jupiter^® instrument (Netzsch-Geratebau GmbH, Selb, Germany) coupled with a QMS 403 C Aëolos^® mass spectrometer and a DSC 214 Polyma instrument (Netzsch-Geratebau GmbH, Selb, Germany). The sample was placed in platinum crucible and subjected to a heating rate of 10 K/min in a dynamic inert argon atmosphere within the temperature range of 40–320 °C. Before the experiment, the furnace atmosphere was evacuated, the calibration was performed with calibrated reference standards, and the baseline was corrected according to the method supplied with the device. The mass spectra of degradation products were recorded in the Bargraph mode. The products were identified using the NIST 2008 database.

The unit cell parameters and X-ray diffraction intensities were measured using an Xcalibur Ruby (Bruker, Billerica, MA, USA, diffractometer. The empirical absorption correction was introduced by a multi-scan method using a SCALE3 ABSPACK algorithm [18]. The structure was solved using an OLEX2 [19] and SUPERFLIP [20] software and refined by the full-matrix least-squares method in the anisotropic approximation for all the atoms using the SHELXL [21] program.

3.2. Synthesis

A 1% aqueous solution of HF (18.1 mL, 10 mmol) was added to difluorotris(pentafluoroethyl)phosphorane (C₂F₅)₃PF₂ (4 g, 10 mmol) in a fluoroplastic reactor to give a colourless solution. KHCO₃ (0.94 g, 10 mmol) was then added with stirring as CO₂ evolution rapidly occurred. The final solution was evaporated to dryness, giving a white powder. The product was recrystallized from water or iso-propyl alcohol. Substitution of K₂CO₃ (0.65 g, 5 mmol) for the KHCO₃ gave the same result.

3.3. NMR Data

¹⁹F NMR (377 MHz, DMSO-d6) δ −44.10 (dm, J = 889.8 Hz, PF). −79.56 (m, CF₃), −81.64 (dm, 2CF₃), −86.07 (dm, PF₂) −115.42 (dm, CF₂), −115.98 (dm, 2CF₂). ³¹P NMR (162 MHz, DMSO-d6) δ −147.66 (qseptq, J = 901.9, 901.9, 889.8, 98.0, 97.0, 96.9, 86.07, 82.6, 82.4, 82.0, 29.4, 13.7 Hz). ¹³C NMR (300 MHz, DMSO-d6) δ 121.53 (qm, J = 286.2, CF₂), 118.61 (mt, J = 238.8 Hz, CF₃) (see supplementary materials).

3.4. Crystal Data of KFAP

C₆F₁₈KP, M = 484.13, orthorhombic, space group Cmc2₁, a = 12.045(3) Å, b = 14.505(3) Å, c = 7.6950(14) Å, V = 1344.4(5) ų, T = 295(2) K, Z = 4, μ(Mo Kα) = 0.731 mm−1. The final refinement parameters: R₁ = 0.0304 [for observed 1554 reflections with I > 2σ(I)]; wR₂ = 0.0792 (for all independent 1554 reflections, R_int = 0.0346), S = 1.029; Flack parameter 0.04(4). Largest diff. peak and hole 0.353 and –0.244 ēÅ⁻³.

4. Conclusions

Potassium trifluorotris(pentafluorethyl)phosphate can be obtained in high purity and, essentially, a quantitative yield from a very simple one-pot reaction as a stable solid up to 230 °C. Its crystal structure shows the anion to be involved in multiple weak coordinative interactions with the cation, leading to monoperiodic polymer units.

5. Patents

Application (19) RU (11) 2023 107 468 (13) A 28.03.2023.