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. 2026 Aug 16;21(16):e70952. doi: 10.1002/asia.70952

Improved Synthesis of Sodium and Potassium Salts of Tetrakis (Pentafluorophenyl)Borates

Emily A Murray‐Nobbs 1, Jarrod R Thomas 1, Cameron M Gould 1, John M Cusick 1, Scott A Sulway 1,✉
PMCID: PMC13477992  PMID: 42604830

ABSTRACT

The safe and robust synthesis of sodium and potassium salts of the [B(C6F5)4]− anion (Na[BArF20], 1‐Na and K[BArF20], 1‐K) via an improved method is reported. The implementation of the improved synthetic route to other tetraarylborate salts is carried out, which includes the synthesis of Na[B({CF3}C6H4)4] (Na[BArF12], 2) and Na[B({ t Bu}C6H4)4] (Na[BAr t Bu4], 3). The solid‐state structure of the THF‐solvated adduct of 2 is reported. 1‐Na, 1‐K, 2 and 3 are fully characterized by 1H, 11B, and 19F NMR spectroscopy.

Keywords: borate, counter‐ion, Lewis acid, NMR spectroscopy, WCA

Synthesis of weakly coordinating anions has been achieved whilst avoiding highly toxic and explosive materials, we discuss the 1H, 11B, and 19F NMR spectroscopy along with the single crystal XRD for the compounds.

graphic file with name ASIA-21-e70952-g006.jpg

1. Introduction

Fluorinated counter‐ions have widespread use as weakly coordinating anions (WCAs) [1, 2, 3] in organometallic, main‐group, and catalytic chemistry, due to their ability to stabilise low coordinate cationic metal complexes [4, 5, 6, 7]. In particular, the [B(C6F5)4]− ([BArF20]−) anion has been used as a counter‐ion in many compounds due to the desirable properties of the resultant complexes that form with said anion. Other commonly used WCAs that have been used in similar applications to [BArF20]− include [BArF 4]− (ArF = 3,5‐({CF3}2C6H3)) [8] and [Al(ORF)4]− (RF = fluoroalkyl)) [9, 10, 11]. Having the ability to readily access alkali metal salts of the [BArF20]− anion is of great value as said compounds are a common precursor in the synthesis of compounds that employ the [BArF20]− anion, including pyrophoric and air‐sensitive materials which must be prepared using air‐sensitive techniques to rigorously exclude oxygen and moisture. Alkali metal [BArF20]− salts can also be readily converted to other commonly used salts such as [Ph3C][BArF20], [Et3N][BArF20], and [(Et3Si)2(μ‐H)][BArF20] which can sometimes be more suitable in subsequent reactions due to the higher reactivity of those [BArF20]− salts with other compounds [12, 13]. The [BArF20]− anion is highly sought after in applications which include its use as a WCA in lanthanide cationic complexes [14, 15, 16], in ionic liquids [17], in electrochemistry [18], and in lithium‐ion batteries [19].

The synthesis of the [BArF20]− anion as a lithium salt was first reported by Massey and Park in 1964 with the synthetic procedure involving the initial isolation of the tris‐borane product B(C6F5)3, which can then be reacted with one equivalent of pentafluorophenyllithium to obtain Li[BArF20] (Scheme 1) [20]. Massey and Park's procedure for the synthesis of Li[BArF20] resulted in the product as an addict having either bound water or diethyl ether present, evident from the high hydrogen content found during the analysis of the compound. Massey and Park found that the adduct‐free salt, K[BArF20] (1‐K) could be prepared from a simple salt exchange between Li[BArF20] and KCl, though this still requires first synthesising the lithium salt of the [BArF20]− anion. In 2008, a similar procedure was reported for the synthesis of 1‐K, using KF instead of KCl in a cation exchange with Li[BArF20], though again, this still requires the initial preparation of Li[BArF20] [21].

SCHEME 1.

SCHEME 1

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Massey and Park's synthetic route to Li[BArF20] via the precursor compound B(C6F5)3 [20].

Whilst Massey and Park's method is a reliable method to synthesise alkali metal salts of [BArF20]−, it involves the use of dangerous and highly reactive reagents which makes it unsafe for large scale synthesis. Pentafluorophenyllithium is an unstable and potentially explosive reagent, whereby reactions utilizing said reagent are required to be kept cool throughout the entirety of the reaction, as is the case for Massey and Park's synthesis. Additionally, BCl3 is a highly toxic reagent and exists as a gas so is difficult to handle.

In 2016, it was reported that Na[BArF20] (1‐Na), can be synthesised from the reaction of four equivalents of pentafluorophenyllithium with BCl3, followed by cation exchange to get the sodium salt [22]. Whilst a promising synthetic route that avoids the isolation of B(C6F5)3, the synthesis still involves the use of both explosive and highly toxic reagents.

Improved syntheses of other WCAs have been reported over recent years. In 2019, an improved method to synthesise the [BArF 4]− anion was reported [23], building on the previously reported preparation of Na[BArF 4] in 2005 [24]. An additional double recrystallisation step was employed which allowed for the isolation of the pure solvated salts of the [BArF 4]− anion. Subsequent drying of the solvated salts of the [BArF 4]− anion under dynamic vacuum at 80°C yielded the adduct‐free [BArF 4]− salts. The simple and scalable preparation of the [Ag(C6H6)3][BArF 4] salt was reported in 2025, demonstrating the utility of this salt for halide abstraction and further highlighting the value of having accessible routes to WCAs [25].

In 2020, Krossing and co‐workers reported an improved synthesis for preparing Li[Al(ORF)4] salts which is fast and easily scalable. The cation in Li[Al(ORF)4] salts can subsequently be swapped out for the other cations through salt metathesis reactions, allowing these salts to be used to prepare different materials [26]. In 2024, the upscaled synthesis of salts of the carbaborate ion [CB11H12]− and its undecafluorinated form [CHB11F11]− were reported, providing a useful alternative WCA that can be used in the synthesis of various materials [27]. The synthetic protocols to another interesting WCA was reported in 2025, which is the perfluoropinacol borate‐based anion [B(O2C2(CF3)4)2]−, [FPB]– [28].

Thus, we sought to develop an improved and safer synthetic route to alkali metal salts of [BArF20]− that is reliable and can be carried out on a large scale to prepare bulk material in high yields. Since the [BArF20]− anion is of such importance in numerous areas of research, a relatively simple synthetic route is of great value for being able to reliably source the [BArF20]− anion as alkali metal salts.

Furthermore, we have employed the synthetic route we developed for the [BArF20]− anion in the synthesis of another fluorinated WCA, [B({CF3}C6H4)]− ([BArF12]−). We have also employed the synthetic route, with slight modifications, to synthesise the alkyl tetraarylborate anion, [B({ t Bu}C6H4)4]− ([BAr t Bu4]−).

2. Results and Discussion

Herein, we present a procedure for the synthesis of sodium and potassium salts of the [BArF20]− anion that can be carried out on a large scale (approx. 30 g of product) and with high yields (ca. 60%–70%). Our synthetic route involves the preparation of a Grignard reagent of the pentafluorophenyl substituent (C6F5MgBr), then a subsequent reaction of four equivalents of said Grignard reagent with BF3•OEt2 (which is safer and easier to handle compared with neat BX3 reagents (X═F, Cl, Br), followed by treatment of the [MgBr][BArF20] salt which forms in solution with an aqueous alkali metal chloride solution (MCl(aq), M═Na, K) resulting in the formation of the desired alkali metal salt of the [BArF20]− anion (Scheme 2).

SCHEME 2.

SCHEME 2

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Synthetic route to the sodium and potassium salts of the [BArF20]− anion, compounds 1‐M (M═Na, K).

Standard organic work‐up was employed for the isolation of the [BArF20]− salts, involving extraction of the product into Et2O, drying over MgSO4 and subsequent solvent removal in vacuo which resulted in a tan‐coloured oily product for both 1‐Na and 1‐K. Drying under dynamic vacuum (10−3 mbar) at 115°C for 60 h was required to drive off residual bound Et2O, resulting in 1‐Na and 1‐K as light tan free‐flowing powders. The procedure was repeated multiple times and routinely resulted in material obtained in yields above 60%. Conversion of some of the synthesised alkali metal [BArF20]− salts to the triphenylmethyl (trityl) salt was successfully achieved via reaction with trityl chloride, resulting in the elimination of NaCl or KCl and the formation of [Ph3C][BArF20], following a literature procedure [13].

To see if the procedure could be applied to other fluorinated anions, the [BArF12]− anion was synthesised. First, the Grignard reagent ({CF3}C6H4)MgBr was prepared. Subsequent reaction of four equivalents of ({CF3}C6H4)MgBr with BF3•OEt2, followed by cation exchange using an aqueous solution of NaCl and the same organic work‐up procedure as was used for 1‐Na resulted in the isolation of Na[BArF12] (2) as a light brown, free‐flowing powder. 2 was successfully recrystallised from a THF solution layered with hexane solution (1:10) at −20°C to give the THF‐solvated adduct [Na(THF)6][BArF12] (2‐THF). Filtration and drying of the material in vacuo (10−3 mbar) at 115°C can be carried out to isolate the product after recrystallisation.

Slight modifications can be made to the procedure to synthesise the alkyl tetraarylborate salt Na[BAr t Bu4] (3). Since the [BAr t Bu4]− anion is not soluble in Et2O, the reaction was carried out in THF instead. The Grignard reagent, ({ t Bu}C6H4)MgBr was prepared in THF, before four equivalents of said reagent were reacted with BF3•OEt2. After the reaction was complete, cation exchange was achieved through the addition of an aqueous solution of NaCl, resulting in the formation of 3. Standard organic work‐up was carried out to isolate the product, however DCM was added to the THF/water suspension to separate the organic and aqueous layers. A 1:1 mixture of THF:DCM was required for the extraction of the product from the aqueous layer due to THF being miscible with water and the product not being soluble in Et2O. Removal of DCM left a white residue which was dried in vacuo (10−3 mbar) at 115°C, resulting in the isolation of 3 as a white, free‐flowing powder. 3 can be recrystallised from either a THF/pentane vapour diffusion or a THF/Et2O vapour diffusion, with the product being found to be pure by NMR after drying in vacuo (Figures S7 and S8).

1‐Na and 1‐K were fully characterised via 11B and 19F NMR spectroscopy in DMSO‐d 6 (ATR‐FTIR also collected, Figures S9 and S10). In the 11B NMR spectrums for both 1‐Na and 1‐K, there is a peak observed at ca. −16 ppm, which is consistent with what has been seen in literature [21, 22] and is expected considering said salts are both borate species with electron‐withdrawing fluorine atoms on the phenyl rings (Figures 1, S1, and S2). Interestingly, a peak is also observed at ca. −4 ppm, though this decreases as time spent heating the product in vacuo increases. We suspect this corresponds to a solvated‐adduct [BArF20]− salt, which is perhaps the Et2O adduct ([Na(Et2O)n][BArF20], 1‐Na‐Et2O and [K(Et2O)n][BArF20], 1‐K‐Et2O) (Figure 1), though the two distinct borate peaks may arise due to solvent exchange between a DMSO and H2O solvated‐adduct. Notably, the peak at ca. −4 ppm significantly decreases upon further heating in vacuo, leaving a dominant peak in the spectrum at ca. −16 ppm (Figure 1).

FIGURE 1.

FIGURE 1

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11B NMR spectra of compound 1‐Na in DMSO‐d 6 after drying the ‘crude product’ in vacuo at 115°C for 12 h (blue), 36 h (purple), and 60 h (pink). There are two signal centre at δ(11B) = −4.18 ppm and −16.71 ppm. Samples were prepared in borosilicate NMR tubes.

The 19F NMR spectrums for both 1‐Na and 1‐K have three distinct peaks corresponding to the three fluorine environments in the anion (Figures 2 and S3). A doublet is observed at ca. −135 ppm corresponding to the fluorine atoms in the ortho‐position on the phenyl ring, and two triplets are observed at ca. −162 ppm and ca. −166 ppm, corresponding to the fluorine atoms in the para‐ and meta‐positions on the phenyl ring, respectively.

FIGURE 2.

FIGURE 2

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19F NMR spectrum of Na[BArF20], compound 1‐Na in DMSO‐d 6. Signal centres at δ(19F) = −134.68, −162.42, and −166.03 ppm arise from the ortho‐, para‐ and meta‐fluorine atoms in 1‐Na, respectively. Fluorobenzene (PhF) was used as the standard for calibrating the 19F NMR spectrum.

2 and the THF‐solvated adduct [Na(THF)6][BArF12] (2‐THF) were fully characterised by 1H, 11B, and 19F NMR spectroscopy (Figures S4–S6, ATR‐FTIR Figure S11). The 1H NMR spectrum reveals two doublets in the aromatic region which correspond to the ortho‐ and meta‐hydrogen atoms on the phenyl rings. One signal can be seen in the 11B NMR spectrum at ca. −6.5 ppm, consistent with the compound being a borate species, and one signal can be seen in the 19F NMR spectrum at ca. −62 ppm, which arises from the CF3 substituent in the para‐position on the phenyl rings. 2‐THF results in identical NMR spectra with the exception of the 1H NMR spectrum, whereby two peaks arise due to the presence of the THF adduct.

Characterisation of 3 was achieved via 1H and 11B NMR spectroscopy (ATR‐FTIR Figure S12). The 1H NMR spectrum for 3 revealed two aromatic signals downfield which is consistent with the ortho‐ and meta‐hydrogen atoms on the phenyl rings, as well as a singlet upfield which corresponds to the tert‐butyl substituents in the para‐position on the phenyl rings. There is one signal in the 11B spectrum at ca. −7.5 ppm, which is expected for a borate species.

Large colourless crystals of 2‐THF suitable for single crystal x‐ray diffraction were obtained from a THF solution of said salt layered with hexane (1:10) that was cooled to −20°C (Figure 3).

FIGURE 3.

FIGURE 3

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Solid‐state structure of [Na(THF)6][BArF12], compound 2‐THF, with thermal ellipsoids drawn at a 50% probability. Sodium = teal, fluorine = green, oxygen = red, carbon = grey, boron = yellow; hydrogen atoms and minor disordered components of the THF molecules and ‐{CF3} substituents are omitted for clarity. Atoms in the asymmetric unit are labelled.

2‐THF crystallises in the monoclinic C2/c space group where exactly half of the cation and half of the anion makes up the asymmetric unit. There are four molecules of 2‐THF in the unit cell. The distance between the sodium atom and the boron atom in a molecule of 2‐THF is ca. 11 Å. The THF molecules surrounding the sodium cation exist in an octahedral geometry, causing the fluorine atoms that are part of the closest ‐{CF3} substituent to the sodium cation to be at a large distance from said cation (ca. 6 Å) (Table S1).

Crystallization of 3 as the THF adduct was attempted but unsuccessful. Microcrystalline material obtained from both THF/Et2O and THF/pentane vapour diffusions resulted in 3, as confirmed by 1H NMR spectroscopy (Figure S7).

X‐ray quality crystals grown from a slow evaporation of undried THF yielded the water adduct [Na(H2O)][BAr t Bu4]•THF, 3‐H2O, with the sodium ion being “trapped” by two of the phenyl rings (Figure S13 and Table S2). Crystallization of 3 as the adduct‐free salt was attempted via a slow evaporation of acetone, since it was found in literature that the adduct‐free potassium salt of tetraphenylborate could be recrystallised from a water/acetone mixture [29]. X‐ray quality crystals were obtained but revealed that 3 had recrystallized as the acetone adduct [Na(μ‐Me2CO)(Me2CO)3]2[BAr t Bu4]2, 3‐Me2CO (Figure S14 and Table S3). Both solvated adducts of 3 can be collected by filtration and dried in vacuo (10−3 mbar) at 115°C to isolate the material.

3. Conclusion

In summary, 1‐Na and 1‐K have been prepared in high yields via a safer and more reliable synthetic route that can be carried out on a multigram scale. Drying of the [BArF20]− salts in vacuo results in isolated products as confirmed by 1H, 11B, and 19F NMR spectroscopy. 2 was isolated via the same synthetic route, and recrystallized as the THF adduct, 2‐THF. Furthermore, through slight modifications made to the synthetic route developed, the alkyl tetraarylborate salt 3 was isolated.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: Asia70952‐sup‐0001‐SuppMat.docx

ASIA-21-e70952-s003.docx (1.1MB, docx)

Supporting File 2: Asia70952‐sup‐0002‐SuppMat.cif

ASIA-21-e70952-s004.cif (507.7KB, cif)

Supporting File 3: Asia70952‐sup‐0003‐SuppMat.cif

ASIA-21-e70952-s002.cif (1.2MB, cif)

Supporting File 4: Asia70952‐sup‐0004‐SuppMat.cif

ASIA-21-e70952-s001.cif (2.2MB, cif)

Acknowledgments

This research was supported by the University of New South Wales (UNSW) Faculty Research Grant. The authors would also like to acknowledge the Mark Wainwright Analytic Centre (MWAC) for aid and use of their instrumentation for the collection of single‐crystal X‐ray diffraction data and NMR data at the NMR facility.

Open access publishing facilitated by University of New South Wales, as part of the Wiley ‐ University of New South Wales agreement via the Council of Australasian University Librarians

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article. The authors have cited additional references within the Supporting Information.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File 1: Asia70952‐sup‐0001‐SuppMat.docx

ASIA-21-e70952-s003.docx (1.1MB, docx)

Supporting File 2: Asia70952‐sup‐0002‐SuppMat.cif

ASIA-21-e70952-s004.cif (507.7KB, cif)

Supporting File 3: Asia70952‐sup‐0003‐SuppMat.cif

ASIA-21-e70952-s002.cif (1.2MB, cif)

Supporting File 4: Asia70952‐sup‐0004‐SuppMat.cif

ASIA-21-e70952-s001.cif (2.2MB, cif)

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article. The authors have cited additional references within the Supporting Information.

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