New tetrabutylphosphonium organic ionic plastic crystals incorporating borate anions

Abstract

Organic plastic crystals have seen rapid growth in their development and potential application as solid-state electrolytes in electrochemical devices. Two tetrabutylphosphonium ([P₄₄₄₄]⁺) salts incorporating non-halogenated bis(butane-1,4-dioxy)borate ([B(bdo)₂]⁻) and tetrakis(pyrazolyl)borate ([B(pyr)₄]⁻) anions have been synthesised and their physicochemical properties characterised. Both [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] are solid under ambient conditions and display solid–solid thermal phase transitions suggesting the formation of plastic crystal phases, which was confirmed by observation of significant ionic conductivity and ion mobility in the solid state. [P₄₄₄₄][B(pyr)₄] showed more extensive polymorphism and greater ionic conductivity, and from the ³¹P linewidth NMR the phosphonium cation exhibited diffusion at lower temperatures. Therefore, these are of interest as potential new halogen-free solid-state electrolyte materials.

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Introduction

Organic ionic plastic crystals (OIPCs) consist of organic cations and usually symmetric anions.^1,2 OIPCs ideally incorporate desirable electrolyte properties and high ionic mobility coupled with high thermal stability, non-volatility, and non-flammability.³ Lithium-metal cells containing OIPCs have been reported to have cycling stabilities and current density of around 0.05 and 0.5 mA cm⁻² over more than 2000 cycles.⁴ However, restricted ion transport and electrode compatibility are challenges that still remain and drive current research.⁵

The potential for structural modification in OIPCs mimics that of ionic liquids (ILs) in that both OIPCs and ILs exhibit local structural disorder, and this can, in principle, be modified or influenced by the nature of the ions incorporated into the materials. In the case of ILs, local structural disorder and poor solid-state packing of ions leads to suppression of melting points and, for OIPCs, to positional and rotational defects and disorder within a solid phase providing plasticity and enhanced diffusional vectors for ionic conductivity.² Indeed, OIPCs can be considered as solid state analogues of ILs⁶ and salts incorporating N- and P-containing cations including ammonium,⁷ pyrolidinium,⁸ imidazolium,⁹ and phosphonium^10–12 have been extensively studied.

The formation of an OIPC phase and its characteristics can, in principle, be modified through selective changes to the cation and anion to reduce specific cation–anion interactions, as with ILs. However, the link between ion size, shape, temperature-dependent ion dynamics, physical, and electrochemical properties is still unclear.^11,13 The formation of an OIPC is reliant on achieving a balance between a coulombic attraction that is sufficient to allow retention of a long-range crystalline lattice and minimisation in site-specific associations to allow reorientation and disordering of ions within the lattice.^11,14 It should also be noted that many IL materials can also crystallise with significant degrees of structural disorder within their unit cells (for example, see Dymon et al.¹⁵).

Incorporating boron atoms as components of electrolyte anions can provide a central site for functionalisation allowing new anion motifs to be developed,¹⁶ and can actively contribute to advantageous SEI (solid electrolyte interphase) chemistry through formation of thin, boron-rich stable layers that increase interface stability^17,18 However, despite the desirability of borate anions as contributors towards high performance electrolytes and enhanced SEI chemistry, most work to-date on OIPCs and non-aqueous organic electrolytes in general¹⁹ has focused on materials containing fluorinated-borate anions, including the ubiquitous tetrafluoroborate^19,20 and bis-fluoro and trifluoromethyl-sulfonylimide²¹ both of which may be used with dopants that facilitate ion conduction.²²

With the global effort to transition away from fluorine-containing electrolytes and other materials in batteries, membranes and polymeric materials due to environmental impact, sustainability, and safety of fluorinated components, new halogen-free electrolytes are increasingly sought after for both liquid and solid-state applications.²³

A goal of our on-going work exploring the utility of fluorine-free anions in ionic liquid applications,²⁴ has been to explore whether new fluorine-free functional ionic materials incorporating functionalised borate anions can be developed, taking inspiration most significantly from the utility of bis(oxalato)borate ([BOB]⁻) and related anions.¹⁶ We describe here the synthesis and characterisation of two new tetrabutylphosphonium salts containing fluorine-free borate anions; tetrabutylphosphonium bis(butane-1,4-dioxy)borate ([P₄₄₄₄][B(bdo)₂]), the 1,4-butanediol derived analogue of [P₄₄₄₄][B(edo)₂], tetrabutylphosphonium bis(ethane-1,4-dioxy)borate ([P₄₄₄₄][B(edo)₂]), recently described by Xu et al.¹² that has an ortho-borate anion derived from ethylene glycol/boric acid and exhibits OIPC behaviour over an uncommonly wide temperature range (−59 to 158 °C), and tetrabutylphosphonium tetrakis(pyrazolyl)borate ([P₄₄₄₄][B(pyr)₄]) that incorporates an N4–coordinated borate anion (Fig. 1). The thermal properties, solid-state conductivity, and ion dynamics determined via ¹H, ³¹P and ¹¹B solid-state wideline NMR have been investigated for these new organic ionic plastic crystals that have the potential to be used as ambient temperature solid-state electrolytes.

Fig. 1 Chemical structure of the two tetrabutylphosphonium salts under investigation, tetrabutylphosphonium tetrakis(pyrazolyl)borate ([P₄₄₄₄][B(pyr)₄], left) and tetrabutylphosphonium bis(butane-1,4-dioxy)borate ([P₄₄₄₄][B(bdo)₂], right).

Experimental

Materials and methods

Acetonitrile (99.5%), acetone (99.5%), toluene (99.5%), absolute ethanol (99.5%), chloroform (99.5%), DMSO-d₆ (99.5 atom% D), CDCl₃ (99.8 atom% D), 1,4-butanediol (99%), sodium borohydride (98%), hexagonal boron nitride powder (98%) and boric acid (99%) were purchased from Sigma-Aldrich. Anhydrous sodium carbonate (99.5%) was purchased from Fisher Scientific, ammonium dihydrogen phosphate (NH₄H₂PO₄) (98%) was purchased from Alfa Aesar, 1H-pyrazole (99%) was purchased from Doug Discovery (Fluorochem), and tetrabutylphosphonium chloride (95%) was purchased from TCI. All chemicals were used as received.

Na[B(bdo)₂], first reported as a crystalline salt by Gainsford and Kemmitt²⁵ was prepared using a modification of the method described by Chiappe et al.²⁶ for sodium bis(glycerol)borate using sodium carbonate rather than NaOH, and removing co-formed water by azeotropic distillation to inhibit competing hydrolysis of labile B–O bonds in the orthoborate anion. Na[B(bdo)₂] is insoluble in toluene, allowing facile isolation. Na[B(pyr)₄] was prepared from sodium borohydride and 1H-pyrazole as described by Chao and Moore²⁷ using excess pyrazole as both solvent and to shift the substitution equilibrium to four pyrazole ligands on boron forming the tetrahedral [B(pyr)₄]⁻. The two salts [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] were prepared from the corresponding sodium salts Na[B(pyr)₄] and Na[B(bdo)₂] by cation exchange with tetrabutylphosphonium chloride ([P₄₄₄₄]Cl).

Sodium bis(butane-1,4-dioxy)borate, (Na[B(bdo)₂]). Boric acid (5.30 g, 80.93 mmol), butane-1,4-diol (14.5 ml, 161.73 mmol), and Na₂CO₃ (8.57 g, 80.93 mmol) were combined in toluene (100 ml). The mixture was then heated at 125 °C for 8 h., collecting evolved water in a Dean–Stark condenser. The remaining toluene was then removed via rotary evaporation. The crude product was dispersed in acetonitrile, stirred for 1 h. at 60 °C and collected by vacuum filtration, washed with cold ethanol (3 × 30 ml) and then dried overnight in vacuo at 80 °C, to give a waxy solid, yield 13.38 g, 78.7%. ¹H (400 MHz; DMSO-d₆; SiMe₄) δ 3.38 (8H, m, O–CH₂), 1.43 (8H, m, –CH₂–); ¹³C{¹H} (100 MHz; DMSO-d₆; SiMe₄) δ 60.69 (O–CH₂), 29.15 (CH₂–CH₂); ¹¹B (128 MHz; DMSO-d₆; 15% BF₃·OEt₂) δ 3.09. Calc. for C₈H₁₆BNaO₄: C, 51.38; H, 8.62. Found: C, 51.38; H, 8.63.

Sodium tetrakis(pyrazolyl)borate, (Na[B(pyr)₄]). Sodium borohydride (3.00 g, 79.30 mmol) and pyrazole (27.00 g, 400 mmol) (1 : 5 ratio) were placed into a round bottom flask under nitrogen and heated to 125 °C. The evolution of H₂ was monitored and, after 4 h., evolution of gas ceased and the reaction mixture was allowed to cool to ambient temperature. The excess pyrazole was extracted with acetone (100 ml) and the product was collected by filtration and dried overnight in vacuo at 80 °C, to give a white powder (yield 15.71 g, 65.6%).

¹H (400 MHz; DMSO-d₆; SiMe₄) δ 7.10 (8H, s, N–CH), 6.68 (4H, s, C–CH); ¹³C{¹H} (100 MHz; DMSO-d₆; SiMe₄) δ 143.59 (N–CH), 125.23 (CH); ¹¹B (128 MHz; DMSO-d₆; 15% BF₃·OEt₂) δ −6.68. Calc. for C₁₂H₁₂BN₈Na: C, 51.64; H, 4.33; N, 40.15. Found: C, 51.62; H, 4.35; N, 40.17.

Tetrabutylphosphonium bis(butane-1,4-dioxy)borate, ([P₄₄₄₄][B(bdo)₂]). [P₄₄₄₄]Cl (2.95 g, 10 mmol) and Na[B(bdo)₂] (2.73 g, 13 mmol) (1 : 1.3 ratio) were combined in CHCl₃ (50 ml) and stirred for 24 h under ambient conditions. Deionised water (20 ml) was then added. mixed and the organic layer was removed and washed with deionised water (5 × 20 ml) to remove traces of Na⁺ and Cl⁻. The solvent was removed under reduced pressure by rotary evaporation followed by drying in vacuo at 80 °C to give [P₄₄₄₄][B(bdo)₂] as pale yellow crystals (yield 2.87 g, 64.2%). ¹H (400 MHz; CDCl₃; SiMe₄) δ 4.43 (8H, m, O–CH₂), 2.93 (8H, m, –CH₂–), 1.65 (8H, m, P–CH₂–), 0.95 (16H, m, –CH₂–), 0.34 (12H, t, –CH₃); ¹³C{¹H} (100 MHz; CDCl₃; SiMe₄) δ 60.44 (O–CH₂), 27.86 (OCH₂–CH₂), 22.43 (d, J_CP = 22 Hz, P−CH₂), 17.39 (PCH₂–CH₂), 16.92 (–CH₂–), 12.10 (–CH₃); ¹¹B (128 MHz; CDCl₃; 15% BF₃·OEt₂) δ 2.15. ³¹P{¹H} (162 MHz; CDCl₃; 85% H₃PO₄) δ 32.99. MS (ESI) calcd for [C₁₆H₃₆P]⁺ m/z 259.3; found m/z 259.2; calcd for [C₈H₁₆BO₄]⁻ m/z 187.1; found m/z 187.1; Chloride content determined by XRF analysis was 219 ± 7 ppm.

Tetrabutylphosphonium tetrakis(pyrazolyl)borate, ([P₄₄₄₄][B(pyr)₄]). [P₄₄₄₄][B(pyr)₄] was prepared from [P₄₄₄₄]Cl (2.95 g, 10 mmol) and Na[B(pyr)₄] (3.93 g, 13 mmol) (1 : 1.3 ratio) in CHCl₃ and produced the product as a cloudy soft solid (yield 4.15 g, 77.1%). ¹H (400 MHz; CDCl₃; Me₄Si) δ 7.51 (8H, s, –N–CH=), 7.46 (4H, s, =CH–), 1.93 (8H, m, P–CH₂–), 1.41 (16H, m, –CH₂–), 0.92 (12H, t, –CH₃); ¹³C{¹H} (100 MHz; CDCl₃; SiMe₄) δ 138.46 (–N=CH–), 134.50 (–N–CH = ), 103.28 (CH=CH–), 24.15 (d, J_CP = 12 Hz, P−CH₂), 18.95 (PCH₂–CH₂), 18.48 (–CH₂–), 13.90 (–CH₃); ¹¹B (128 MHz; CDCl₃; 15% BF₃·OEt₂) δ −6.58. ³¹P{¹H} (162 MHz; CDCl₃; 85% H₃PO₄) δ 32.86. MS (ESI) calcd for [C₁₆H₃₆P]⁺ m/z 259.3; found m/z 259.2; calcd for [C₁₂H₁₂BN₈]⁻ m/z 279.1; found m/z 279.2; Chloride content determined by XRF analysis was 203 ± 9 ppm.

¹H, ¹³C, ¹¹B, and ³¹P NMR spectra and electro-spray mass spectrometry for Na[B(bdo)₂], Na[B(pyr)₄], [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] are shown in Fig. S1–16 in the ESI.†

Characterisation and analysis

NMR spectroscopy. Solution state ¹H, ¹³C, ¹¹B, and ³¹P NMR spectra for the characterisation of materials were recorded on a Bruker Avance III 400 MHz spectrometer at ambient temperature in either CDCl₃ or DMSO-d₆ in standard 5 mm id. borosilicate glass NMR tubes. Additional peaks observed in the ¹³C NMR spectrum are attributed to ¹³C–³¹P coupling. The unassigned peak at 37 ppm in the ³¹P spectrum is likely due to the presence of tributylphosphine oxide, a known impurity from the synthesis of the starting material [P₄₄₄₄]Cl. Solid-state ¹¹B and ³¹P NMR spectroscopy was performed on a Bruker Avance III 300 MHz spectrometer (operating at 96.3 MHz for ¹¹B and 121.5 MHz for ³¹P) in 10 °C increments between −30 to 60 °C. All experiments were run and processed in TopSpin, the line widths were extracted using Dmfit.²⁸ Single pulse excitation was used for all experiments without any ¹H decoupling. Chemical shift referencing was performed using secondary references: hexagonal boron nitride powder for ¹¹B (30.4 ppm) and NH₄H₂PO₄ for ³¹P (0.9 ppm).

Thermal characterisation. Thermogravimetric analysis (TGA) was performed using a Trios TGA 5500 instrument. Sample sizes ranged between 10–50 mg, platinum pans were used, and a temperature gradient of 10 °C min⁻¹ from 30–1000 °C under a constant nitrogen flow of 20 ml min⁻¹. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q2000 DSC. The sample size varied between 5–30 mg and sealed Tzero aluminium pans were used under a constant flow of nitrogen (50 ml min⁻¹). During DSC analysis, each sample was cooled and then heated at a rate of 10 °C min⁻¹ between −90 and 150 °C (−90 and 80 °C for [P₄₄₄₄][B(bdo)₂]) and cycled three times. Transition temperatures in the DSC profiles were determined using the peak maximum of the transition, and entropies of fusion were calculated from the melting endotherm area (ΔH) according to eqn (1)where T_m is the melting temperature in K.

Elemental analysis (CHNS) was also performed on the sodium salts using a Perkin Elemer 2400 series 2 elemental analyser. Mass spectrometry measurements were performed using a Waters LCT Premier Instrument equipped with an Advion Nanomate Infusion system. All reported mass spec data are accurate within the range of 0–10 ppm. Chloride content in the [P₄₄₄₄]⁺ salts was measured directly by quantitative energy dispersive X-ray fluorescence analysis (Rigaku NEX QC + QuantEZ) with analysis carried out in triplicate with an error range of ± 9 ppm.

Ionic conductivity. Electrochemical impedance spectroscopy (EIS) experiments were performed using a Biologic MTZ-35 AC impedance analyser across the frequency range from 1 Hz to 10 MHz with a peak-to-peak voltage amplitude of 0.01 V. The samples were placed in an air-tight barrel-cell fitted with Pt electrodes, designed in-house. The instrument temperature was controlled by a Eurotherm 2204e thermostat with analysis in 10 °C increments from 20 to 70 °C (with equilibrium time of 20 minutes between measurements). The conductivity of the solid electrolytes was estimated by relating the touch-down (TD) point of the Nyquist plot (see ESI†) with the following equation.

Results and discussion

Two new tetrabutylphosphonium borate salts containing O–O chelated [B(bdo)₂]⁻ and N-coordinated [B(pyr)₄]⁻ anions (Fig. 1) have been prepared from [P₄₄₄₄]Cl by anion metathesis with the sodium salts of the respective anions. Na[B(bdo)₂] was first reported as a crystalline polymeric salt by Gainsford and Kemmitt²⁵ with the Na⁺ and K⁺ salts subsequently described as additives in polyether acid gas (CO₂) scrubbing applications,²⁹ while butyrolactone solutions of the methyltriethylammonium ([N₁₁₁₂]⁺) salt has been reported as a non-aqueous electrolyte formulations³⁰ in the patent literature.

We provide here a brief description of the thermal properties of the two [P₄₄₄₄]⁺ salts, highlighting the thermal characteristics to initially lead to the potential identification of OIPC behaviour. We then describe the ionic conductivities of the two salts, [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] and correlation of cation and anion dynamics determined by NMR spectroscopy, demonstrating a much more extensive region of conductivity in the solid (plastic) state than should be expected from the initial DSC thermal profile. This further suggests that, when searching for potential OIPC materials, using DSC as an initial screen may lead to many ‘misses’, particularly if the plastic crystal domain is large and extends to very low temperatures as was described for [P₄₄₄₄][B(edo)₂],¹² the ethylene glycol-chelated analogue of [P₄₄₄₄][B(bdo)₂].

Thermal properties

TGA profiles, shown in Fig. 2, indicate that both salts are thermally stable to >150 °C. In comparison, [P₄₄₄₄][B(edo)₂] reported by Xu et al.¹² shows similar stability profile with weight loss starting around 150 °C and complete decomposition by 400 °C.

Fig. 2 TGA profiles from 50–600 °C of [P₄₄₄₄][B(pyr)₄] (red) and [P₄₄₄₄][B(bdo)₂] (blue).

DSC reveals the presence of a number of transitions in the heating and cooling cycles for both [P₄₄₄₄]⁺ salts, and these were repeatable across multiple thermal cycles. Data is shown from the second cycle of each sample in Fig. 3. For both salts, multiple endothermic transitions are observable, with peak positions determined from onset of each peak on heating before the solids finally melt. [P₄₄₄₄][B(pyr)₄] shows three transitions on heating as a broad, unresolved endothermic peak between ca. 50 °C (first onset) to 86 °C. In the cooling cycle, the reversible crystallisation and subsequent solid–solid transitions resolve into three sharp, and distinct exothermic events. [P₄₄₄₄][B(bdo)₂] melts around 60 °C with a broad, transition that appears to contain two peaks at 58 and 60 °C, and on cooling crystallises after extensive super-cooling to <10 °C with at least two distinct events evident in the DSC profile.

Fig. 3 DSC scan of [P₄₄₄₄][B(pyr)₄] (red) and [P₄₄₄₄][B(bdo)₂] (blue) with a scan rate of 10 °C min⁻¹ (endothermic up).

The entropy of fusion (ΔS_fus) for each phase transition was calculated using eqn (1) and is summarised in Table 1, where T_m are 69 °C and 86 °C for [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] respectively. The solid–solid (II–I) transition of [P₄₄₄₄][B(bdo)₂] occurs at 58 °C (ΔS = 5 J K⁻¹ mol⁻¹) and is followed closely by melting (I–liquid) at 60 °C (ΔS = 7 J K⁻¹ mol⁻¹). [P₄₄₄₄][B(pyr)₄] undergoes a first solid–solid transition (III–II) at the slightly higher temperature of 69 °C (ΔS = 2 J K⁻¹ mol⁻¹). The small ΔS suggests only small changes in molecular or ionic reorientation and motion occur. The second transition (II–I) at 79 °C has a more significant change in entropy associated with it (ΔS = 23 J K⁻¹ mol⁻¹) and is followed by melting (I–liquid) at 86 °C (ΔS = 5 J K⁻¹ mol⁻¹) where the change in entropy is of similar magnitude to the phase change from II–I.

Table 1 DSC transitions on heating

Sample	Transition	Peak (onset)/°C	ΔSfus/J K^−1 mol^−1
[P4444][B(bdo)2]	II–I	58 (48)	5
	I–liquid	61 (60)	7
[P4444][B(pyr)4]	III–II	69 (66)	2
	II–I	79 (74)	23
	I–liquid	86 (83)	5

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The transition shown for [P₄₄₄₄][B(pyr)₄] in Fig. 3 (and listed in Table 1) are all broad and poorly resolved merging over the range 58–86 °C, however, on cooling, these resolve into two distinct, well separated events at 68 °C and 57 °C, followed by broader and less exothermic transitions at 47 °C and finally at 40 °C.

For both salts, the DSC melting profiles show a long broad onset to melting which is characteristic of a gain in rotational or translational disorder prior to melting of the lattice and is consistent with plastic behaviour, as is the observation of multiple phase transitions. The melting entropy (ΔS_melt) for [P₄₄₄₄][B(bdo)₂] (7 J K⁻¹ mol⁻¹) and [P₄₄₄₄][B(pyr)₄] (5 J K⁻¹ mol⁻¹) are significantly lower than Timmermans' threshold (<20 J K⁻¹ mol⁻¹)³¹ thereby classifying these materials as plastic crystals.

Compared to [P₄₄₄₄][B(edo)₂] (mp 158 °C), the mps of [P₄₄₄₄][B(pyr)₄] (86 °C) and [P₄₄₄₄][B(bdo)₂] (60 °C) are significantly reduced. In addition, on further cooling in the DSC, no further low temperature thermal events were observed. This contrasts to [P₄₄₄₄][B(edo)₂] where an exceptionally low temperature first-order solid–solid transition at −59 °C was reported giving an OIPC phase range of almost 240 °C between ca. −59–158 °C.

However, it is important to note that multiple phase transitions alone are not definitive indicators of OIPC formation. While these transitions suggest the potential for orientational flexibility – a hallmark of OIPC behaviour – they are not the sole markers of plastic crystal characteristics. Additional factors, such as the material's ability to retain a pseudo-crystalline state across transitions and structural disorder at the molecular level are also critical in establishing a true OIPC.

Conductivity

The conductivities of [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] were determined by EIS between 20–70 °C in order to scan across the phase transitions observed by DSC. The samples were not heated above their respective melting points due to the barrel-cell not being designed for liquid samples. The solid state conductivity of [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] as a function of temperature is shown in Table 2 and in Fig. 4 as an Arrhenius plot (eqn (3)).where σ is conductivity, E_a is the activation energy, R is the molar gas constant and A is a coefficient constant. E_a can be determined from the slope of the line.

Table 2 Conductivity σ (S cm⁻¹), activation energy E_a (kJ mol⁻¹) and the quality of fit (R²) to eqn (3) for [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] over the temperature range 20–70 °C

Temperature/K	σ/S cm^−1
	[P4444][B(bdo)2]	[P4444][B(pyr)4]
a Between 303–323 K.b Between 313–343 K.c Between 293–313 K.
293	1.55 × 10^−7	8.85 × 10^−7
303	5.20 × 10^−7	1.57 × 10^−6
313	9.13 × 10^−7	3.07 × 10^−6
323	1.87 × 10^−6	4.57 × 10^−6
333	—	6.87 × 10^−6
343	—	8.89 × 10^−6
Ea	51.9^a	32.2^b, 47.4^c
R^2	>0.9996	>0.996

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Fig. 4 Arrhenius plot of the conductivity of [P₄₄₄₄][B(pyr)₄] (red) and [P₄₄₄₄][B(bdo)₂] (blue). Dashed lines represent fits to the Arrhenius equation with activation energy, E_a, of 51.9 kJ mol⁻¹ for [P₄₄₄₄][B(bdo)₂] between 303–323 K, 32.2 kJ mol⁻¹ for [P₄₄₄₄][B(pyr)₄] between 312–343 K and 47.4 kJ mol⁻¹ for [P₄₄₄₄][B(pyr)₄] between 293–313 K (all with r² > 0.996).

The temperature-dependent conductivity measurements (Fig. 4) show that both salts are conductive at temperatures below the respective endothermic transitions observed by DSC (Table 1) at 50–86 °C for [P₄₄₄₄][B(pyr)₄] and 50–60 °C for [P₄₄₄₄][B(bdo)₂]. This suggests a high degree of disorder or defects enabling conductivity in the solid state at, and below, ambient temperature. Plastic crystal formation is supported by the ion mobilities determined from wide-line NMR spectroscopy described below and, with the absence of a large (ΔS ≫ 20 J K⁻¹ mol⁻¹) solid–solid transition below the II-I transition for [P₄₄₄₄][B(bdo)₂] and below the III–II transition for [P₄₄₄₄][B(pyr)₄] allows the tentative conclusion that these materials are also plastic crystals at lower temperatures with a possible plastic → ordered crystal transition below the lower limit of scanning on the DSC, e.g. below −75 °C.

It is important to note that while the wideline NMR data were collected down to 243 K (−30 °C), the conductivity measurements were limited to 293 K (20 °C). This temperature range was chosen to focus on the practical operating conditions relevant to solid-state electrolyte applications. However, extending conductivity measurements to lower temperatures in future studies would provide further insight into the ionic transport mechanisms across a broader temperature range.

[P₄₄₄₄][B(pyr)₄] showed higher conductivity than [P₄₄₄₄][B(bdo)₂] and, for both salts, the conductivities increase by an order of magnitude from 8.85 × 10⁻⁷ S cm⁻¹ to 8.89 × 10⁻⁶ S cm⁻¹ for [P₄₄₄₄][B(pyr)₄] between 293–343 K, and from 1.55 × 10⁻⁷ S cm⁻¹ to 1.87 × 10⁻⁶ S cm⁻¹ between 293–323 K for [P₄₄₄₄][B(bdo)₂].

It is notable that the conductivities of both [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂] in the solid state are comparable to the reported values for other tetraalkylphosphonium-based OIPCs.^10,12 For example, the conductivity of [P₄₄₄₄][B(bdo)₂] at 293 K (1.55 × 10⁻⁷ S cm⁻¹) is the same as that reported by Xu et al.¹² for tetrabutylammonium bis(ethylene-dioxyl)borate. At 293 K, [P₄₄₄₄][bis(glycolato)borate] was reported to have a higher conductivity of 2.3 × 10⁻⁶ S cm⁻¹, however for the corresponding tetrabutylammonium analogue, [N₄₄₄₄][bis(glycolato)borate], conductivity is reduced to 1.0 × 10⁻⁷ S cm⁻¹. It is worth noting that [P₄₄₄₄][BOB], an IL at 293 K is 2.8 × 10⁻⁵ S cm⁻¹. Compared to OIPCs containing smaller asymmetric tetraalkylphosphonium cations,¹⁰ [P₄₄₄₄][B(pyr)₄] displayed greater conductivity than [P₁₄₄₄][BF₄] and [P₁₂₂₄][BF₄], while the less conductive [P₄₄₄₄][B(bdo)₂] was intermediate between [P₁₂₂₄][BF₄] and [P₁₄₄₄][BF₄].

From the Arrhenius plot of log(conductivity) vs. reciprocal temperature shown in Fig. 4, both [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] show similar changes in conductivity with temperature. A small change in slope appears for [P₄₄₄₄][B(pyr)₄] at 313 K which may be due to a change in the dominant ion transport mechanism or subtle phase behaviour changes within the plastic crystal domain. Fitting to the Arrhenius equation between 313–343 and 293–313 K, activation energies (E_a) of 32.2 and 47.4 kJ mol⁻¹ respectively, with r² > 0.996. For [P₄₄₄₄][B(bdo)₂], a drop in conductivity can be seen at the lowest temperature measured (293 K, σ = 1.55 × 10⁻⁷ S cm⁻¹) and E_a, determined between 303–323 K, was 51.9 kJ mol⁻¹. The lower E_a of [P₄₄₄₄][B(pyrazole)₄] compared to [P₄₄₄₄][B(bdo)₂] is likely due to the larger, more bulky borate anion that will show a greater sensitivity to temperature change in comparison to the smaller [B(bdo)₂]⁻.

Despite the absolute conductivity values of [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] (1.55 × 10⁻⁷–8.89 × 10⁻⁶ S cm⁻¹ between 293–343 K) being greater than some analogues, they remain less conductive than fluorinated OIPCs (e.g., [P₁₄₄₄][FSA] > 1.70 × 10⁻³ S cm⁻¹ at 313 K).¹⁰ This limitation reflects the trade-off between eliminating fluorine (for safety and sustainability) and achieving the highest ionic transport rates. Future work will focus on molecular and structural modifications to bridge this gap.

Wideline solid state nuclear magnetic resonance (NMR) spectroscopy

Cation and anion dynamics were investigated over the temperature range 243–343 K using static (wideline) ³¹P and ¹¹B NMR spectroscopy, with spectral analysis modelled using Dmfit.²⁸ For ³¹P spectra, with spin , the chemical shift anisotropy (CSA) model was applied. Whereas for ¹¹B nuclei where quadrupolar interactions have a more significant influence on signals and the CSA model is less effective, dynamics were assessed by examining the full width at half maximum (FWHM) and linewidths of ¹¹B signals.

Wideline ³¹P and ¹¹B NMR signals from [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] over the temperature range 243–343 K are shown in Fig. 5. At 243 K, the ³¹P NMR signal from [P₄₄₄₄][B(bdo)₂] presents as a broad band (line width >5000 Hz) with a secondary sharp component, with both signals centred at the same chemical shift suggesting that this reflects dynamics rather than formation of different chemical species and is characteristic of a material containing a bulk of static components (the broad signal) and a small concentration of mobile species. On increasing temperature, from 243–343 K, the broad peak reduces in intensity and the sharp signal (line width <1000 Hz), representing mobile dynamic cations, becomes dominant from 303 K. For [P₄₄₄₄][B(pyr)₄], the ³¹P signal is far less broad at 243 K with a line width of ca. 2000 Hz that sharpens with increasing temperature to <1000 Hz by 283–293 K mapping to the line width seen for [P₄₄₄₄][B(bdo)₂].

Fig. 5 Stacked wide line static solid state ³¹P (top) and ¹¹B (bottom) NMR signals from at various temperatures [P₄₄₄₄][B(bdo)₂] (a, left) and [P₄₄₄₄][B(pyr)₄] (b, right) at T = 243 K (green), 273 K (purple), 303 K (blue), and 343 K (black).

The ³¹P signals for [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] were deconvoluted into the broad and sharp peak components using dmfit. The contributions of the two components from [P₄₄₄₄][B(bdo)₂] at 253 K are shown in Fig. 6 and the line width of the dominant species (static or mobile) is plotted as a function of temperature in Fig. 7.

Fig. 6 Solid state, wideline ³¹P (top) and ¹¹B (bottom) NMR spectrum of [P₄₄₄₄][B(bdo)₂] at 253 K (−20 °C) showing deconvolution into the broad (static) contribution in green and sharper (mobile) contributions in purple.

Fig. 7 Line width (Hz) of ³¹P (top) and ¹¹B (bottom) static NMR signals from [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] over the temperature range 243–343 K.

For the [P₄₄₄₄]⁺ cation in both [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄], a reduction in line width of the broad component as the temperature is raised from 243–293 K can be clearly seen, and from around 303 K, both solid salts contain dynamic [P₄₄₄₄]⁺ cations characterised by the sharp ³¹P NMR signal (line width <1000 Hz). The ³¹P NMR signal for [P₄₄₄₄][B(bdo)₂] shows a large reduction in line width between 243–293 K, with a transformation of the NMR signal from static to almost wholly dynamic character. For [P₄₄₄₄][B(pyr)₄], the change in line width is smaller, due to the presence of a larger dynamic component in the salt even at 243 K. It is notable that this increase in the cation dynamics and the switch to a single, sharp signal above 303 K for both salts does not appear to be associated with any of the thermal events observed from DSC (Fig. 3), but is consistent with the conductivity measured even below the transition temperatures.

Dynamics of the [B(bdo)₂]⁻ and [B(pyr)₄]⁻ anions were examined using the corresponding ¹¹B NMR data shown in Fig. 5. Compared to the cation ³¹P NMR signals, there is less evidence for large changes in the ¹¹B signals with temperature. For both [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄], the ¹¹B NMR presents a broad signal (ca. 500–1000 Hz) with a shoulder at higher frequency. For [P₄₄₄₄][B(pyr)₄], the ¹¹B NMR signal also contains some unresolved additional structure within the major signals. The asymmetric shape of the signal may be due to the sensitivity of the quadrupolar ¹¹B to magnetic anisotropy from the solid state environment and from the quadrupolar N-atoms coordinating to the B-centre.

Deconvolution of the dominant symmetric signal within the composite ¹¹B signal using dmfit allows separation of the broad and sharper peak components. These are shown independently in Fig. 6. The broader peak, corresponding to static solid boron nuclei, is the most dominant component at this temperature (253 K), this is repeated at each temperature between 243–343 K for ¹¹B for [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄] as shown in Fig. 5. The line width of the more dominant species (static or mobile) is plotted against temperature, as shown in Fig. 7.

The line width of the ¹¹B signals in both OIPCs (Fig. 5) show very little change. The signal for [P₄₄₄₄][B(pyr)₄] has a line width at half height of ca. 4500 Hz and is retained between 243–273 K before becoming sharper and narrower between 273–343 K with a monotonic reduction in line width of ca. 50% to 2500 Hz, which is of the same order as that reported from solid state studies of a range of boronate esters³² and it is unlikely that the borate anion is mobile so any mobile component from the interpretation of the conductivity results above could reflect rotation¹¹ that would yield a more isotropic magnetic environment. For [P₄₄₄₄][B(bdo)₂], the ¹¹B NMR signal shows little variation with temperature, with the linewidth decreasing from ca. 4250–3750 Hz between 243–343 K. Again, this suggests that the borate anions are not dynamic and so are not the predominant charge carriers for the two OIPCs.

Conclusion

This work has introduced two new fluorine-free borate anion motifs through the synthesis of Na[B(pyr)₄] and Na[B(dbo)₂] and subsequent transformation to tetrabutylphosphonium salts [P₄₄₄₄][B(pyr)₄] and [P₄₄₄₄][B(bdo)₂]. The thermal behaviour of the [P₄₄₄₄]⁺ salts was studied using DSC, and an initial identification of potential organic ionic plastic crystal (OIPC) phases in both salts was made based on observation of multiple low-energy, reversible solid–solid transitions were observed between 50–90 °C. Conductivity measurements further supported this observation, revealing unexpectedly high ionic conductivities (1.57 × 10⁻⁶ S cm⁻¹ for [P₄₄₄₄][B(pyr)₄] and 5.20 × 10⁻⁷ S cm⁻¹ at 303 K) comparable to other phosphonium-based OIPCs, even below the observed phase transition temperatures. These findings suggest a high degree of disorder in the room-temperature phase, confirmed by wideline solid-state NMR and linewidth analysis, which showed increased cation mobility between 240–293 K and indicate the potential to incorporate these and related¹² new fluorine-free O–O chelated and N-coordinated borate anions into the development of advanced ionic electrolytes both as OIPCs and potentially as ILs.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Australian Research Council for support and funding through the IC180100049 (storEnergy).

Notes and references

1. (a) M. L. Thomas, K. Hatakeyama-Sato, S. Nanbu and M. Yoshizawa-Fujita, Energy Adv., 2023, 2, 748–764; (b) L. Jin, P. C. Howlett, J. M. Pringle, J. Janikowski, M. Armand, D. R. MacFarlane and M. Forsyth, Energy Environ. Sci., 2014, 7, 3352–3361; (c) Y. Abu-Lebdeh, P. Alarco, A. Abouimrane, L. Ionescu-Vasii, A. Hammami, M. Armand and J. New Mat, Electr. Sys., 2005, 8, 197; (d) Y. Abu-Lebdeh, A. Abouimrane, P.-J. Alarco and M. Armand, J. Power Sources, 2006, 154, 255–261; (e) Y. Abu-Lebdeh, A. Abouimrane, P.-J. Alarco, I. Dividson and M. Armand, J. Power Sources, 2006, 159, 891–893; (f) T. Rüther, J. Huang and A. F. Hollenkamp, Chem. Commun., 2007, 5226–5228; (g) Z.-B. Zhou and H. Matsumoto, Electrochem. Commun., 2007, 9, 1017–1022; (h) Y. Abu-Lebdeh, E. Austin and I. J. Davidson, Chem. Lett., 2009, 38, 782–783.
2. F. Makhlooghiazad, R. Yunis, D. Mecerreyes, M. Armand, P. C. Howlett and M. Forsyth, Solid State Ionics, 2017, 312, 44–52.
3. P. C. Howlett, J. Sunarso, Y. Shekibi, E. Wasser, L. Jin, D. R. MacFarlane and M. Forsyth, Solid State Ionics, 2011, 204, 73–79.
4. (a) P. C. Howlett, Y. Shekibi, D. R. MacFarlane and M. Forsyth, Adv. Eng. Mater., 2009, 11, 1044–1048; (b) A. Lennert, K. Wagner, R. Yunis, J. M. Pringle, D. M. Guldi and D. L. Officer, ACS Appl. Mater. Interfaces, 2018, 10, 32271–32280.
5. H. Zhu, D. R. MacFarlane, J. M. Pringle and M. Forsyth, Trends Chem., 2019, 1, 126–140.
6. D. R. MacFarlane, M. Forsyth, P. C. Howlett, M. Kar, S. Passerini, J. M. Pringle, H. Ohno, M. Watanabe, F. Yan and W. Zheng, et al., Nat. Rev. Mater., 2016, 1, 1–15.
7. (a) C. Shi, S. Li, W. Zhang, L. Qiu and F. Yan, J. Mater. Chem. A, 2013, 1, 13956–13962; (b) W. A. Henderson, D. M. Seo, Q. Zhou, P. D. Boyle, J.-H. Shin, H. C. De Long, P. C. Trulove and S. Passerini, Adv. Energy Mater., 2012, 2, 1343–1350.
8. D. R. MacFarlane, J. Huang and M. Forsyth, Nature, 1999, 402, 792–794.
9. M. Lee, Y.-H. Lee, J. H. Park and U. H. Choi, Org. Electron., 2017, 48, 241–247.
10. V. Armel, D. Velayutham, J. Sun, P. C. Howlett, M. Forsyth, D. R. MacFarlane and J. M. Pringle, J. Mater. Chem., 2011, 21, 7640–7650.
11. L. Jin, K. M. Nairn, C. M. Forsyth, A. J. Seeber, D. R. MacFarlane, P. C. Howlett, M. Forsyth and J. M. Pringle, J. Am. Chem. Soc., 2012, 134, 9688–9697.
12. Y. Xu, A. Reinholdt, O. N. Antzutkin, M. Forsyth, P. Johansson and F. U. Shah, Cryst. Growth Des., 2024, 24, 8989–8998.
13. J. Adebahr, F. C. Grozema, S. W. Deleeuw, D. R. MacFarlane and M. Forsyth, Solid State Ionics, 2006, 177, 2845–2850.
14. (a) D. R. MacFarlane and M. Forsyth, Adv. Mater., 2001, 13, 957–966; (b) H. Zhu, F. Chen, L. Jin, L. A. O'Dell and M. Forsyth, ChemPhysChem, 2014, 15, 3720–3724.
15. J. Dymon, R. Wibby, J. Kleingardner, J. M. Tanski, I. A. Guzei, J. D. Holbrey and A. S. Larsen, Dalton Trans., 2008, 2999–3006.
16. (a) W. Xu and C. A. Angell, Electrochem. Solid-State Lett., 2001, 4, L3; (b) K. Xu, S. Zhang, T. R. Jow, W. Xu and C. A. Angell, Electrochem. Solid-State Lett., 2001, 5, A26.
17. (a) Z. Liu, A. Patra and N. Matsumi, ACS Appl. Energy Mater., 2025, 8, 3360–3368; (b) J. Li, Y. Long, L. Li, F. Pu, W. Liao, X. Yu, H. Liao and X. Hu, New J. Chem., 2025, 49, 1755–1762; (c) N. R. Park, M. Zhang, B. Han, W. Li, K. Qian, H. Nguyen, S. Kumakura and Y. S. Meng, Adv. Energy Mater., 2024, 14, 2401968.
18. (a) D. Zhu, J. Xu, K. Ding, Q. Xu, P. Shi and Y. Min, Adv. Funct. Mater., 2023, 33, 2213822; (b) L. Ma, J. Tan, Y. Wang, Z. Liu, Y. Yang, T. Gray, X. Zhang, M. Ye and J. Shen, Adv. Energy Mater., 2023, 13, 2300042; (c) C. Bodin, J. Forero Saboya, P. Jankowski, K. Radan, D. Foix, C. Courrèges, I. Yousef, R. Dedryvère, C. Davoisne, M. Lozinšek and A. Ponrouch, Batteries Supercaps, 2022, 6, e202200433; (d) J. Sheng, Q. Zhang, M. Liu, Z. Han, C. Li, C. Sun, B. Chen, X. Zhong, L. Qiu and G. Zhou, Nano Lett., 2021, 21, 8447–8454; (e) Q. Xie, W. Li, A. Dolocan and A. Manthiram, Chem. Mater., 2019, 31, 8886–8897.
19. (a) K. Xu, Chem. Rev., 2004, 104, 4303–4418; (b) K. Xu, Chem. Rev., 2014, 114, 11503–11618.
20. (a) C. S. M. Kang, O. E. Hutt and J. M. Pringle, ChemPhysChem, 2022, 23, e202200115; (b) F. Nti, L. Porcarelli, G. W. Greene, H. Zhu, F. Makhlooghiazad, D. Mecerreyes, P. C. Howlett, M. Forsyth and X. Wang, J. Mater. Chem. A, 2020, 8, 5350–5362; (c) R. Yunis, J. M. Pringle, X. Wang, G. M. A. Girard, R. Kerr, H. Zhu, P. C. Howlett, D. R. MacFarlane and M. Forsyth, J. Mater. Chem. A, 2020, 8, 14721–14735; (d) N. Iranipour, D. J. Gunzelmann, A. J. Seeber, J. Vongsvivut, A. F. Hollenkamp, M. Forsyth and P. C. Howlett, J. Mater. Chem. A, 2017, 5, 24909–24919; (e) T. J. Stockmann and Z. Ding, J. Phys. Chem. B, 2012, 116, 12826–12834.
21. (a) J. Sun, C. S. M. Kang, G. Huang, F. M. F. Vallana, A. Kumar, L. A. O'Dell, M. Galceran, O. Hutt, P. C. Howlett, M. Forsyth and J. M. Pringle, J. Mater. Chem. A, 2023, 11, 22329–22339; (b) Y. Garcia, L. Porcarelli, H. Zhu, D. Mecerreyes, M. Forsyth and L. A. O'Dell, J. Mater. Chem. A, 2024, 12, 4146–4158.
22. (a) J. Luo, O. Conrad and I. F. J. Vankelecom, J. Mater. Chem. A, 2013, 1, 2238–2247; (b) M. Forsyth, T. Chimdi, A. Seeber, D. Gunzelmann and P. C. Howlett, J. Mater. Chem. A, 2014, 2, 3993–4003; (c) X. Chen, H. Tang, T. Putzeys, J. Sniekers, M. Wubbenhorst, K. Binnemans, J. Fransaer, D. E. De Vos, Q. Li and J. Luo, J. Mater. Chem. A, 2016, 4, 12241–12252; (d) X. Li, Z. Zhang, S. Li, K. Yang and L. Yang, J. Mater. Chem. A, 2017, 5, 21362–21369.
23. (a) Y. Xu, A. Filippov, S. Bhowmick, P. Johansson and F. U. Shah, Energy Adv., 2024, 3, 564–573; (b) G. Hernández, R. Mogensen, R. Younesi and J. Mindemark, Batteries Supercaps, 2022, 5, e202100373; (c) R. Mogensen, A. Buckel, S. Colbin and R. Younesi, Chem. Mater., 2021, 33, 1130–1139; (d) L. O. S. Colbin, R. Mogensen, A. Buckel, Y. Wang, A. J. Naylor, J. Kullgren and R. Younesi, Adv. Mater. Interfaces, 2021, 8, 2101135.
24. (a) A. McGrogan, E. L. Byrne, R. Guiney, T. F. Headen, T. G. A. Youngs, A. Chrobok, J. D. Holbrey and M. Swadźba-Kwaśny, Phys. Chem. Chem. Phys., 2023, 25, 9785–9795; (b) L. Wang, G. A. Nelson, J. Toland and J. D. Holbrey, ACS Sustainable Chem. Eng., 2020, 8, 13362–13368; (c) L. Moura, L. C. Brown, M. Blesic and J. D. Holbrey, ChemPhysChem, 2017, 18, 3384–3389.
25. G. J. Gainsford and T. Kemmitt, Acta Crystallogr., Sect. C, 2005, 61, m136–m138.
26. C. Chiappe, F. Signori, G. Valentini, L. Marchetti, C. S. Pomelli and F. Bellina, J. Phys. Chem. B, 2010, 114, 5082–5088.
27. S. Chao and C. E. Moore, Anal. Chim. Acta, 1978, 100, 457–467.
28. D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calvé, B. Alonso, J. Durand, B. Bujoli, Z. Gan and G. Hoatson, Magn. Reson. Chem., 2001, 40, 70–76.
29. C. R. Laroche, Composition comprising organoborates and physical solvents and use thereof for the removal of acid gases from hydrocarbon fluid streams, WO Pat. App. WO2017180285, 2017.
30. Y. Washio, N. Takeishi, H. Shimamoto, K. Mori, N. Ushio, K. Shiono, T. Kishi and H. Samura, An electrolyte for use in electrolytic capacitors, Eur. Pat., App. EP381936, 1990.
31. J. Timmermans, J. Phys. Chem. Solids, 1961, 18, 1–8.
32. J. W. Weiss and D. L. Bryce, J. Phys. Chem. A, 2010, 114, 5119–5131.

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C, ¹¹B, and ³¹P NMR spectra and electro-spray mass spectrometry for Na[B(bdo)₂], Na[B(pyr)₄], [P₄₄₄₄][B(bdo)₂] and [P₄₄₄₄][B(pyr)₄]. See DOI: https://doi.org/10.1039/d5ta00562k

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