Physical and Electrochemical Analysis of N-Alkylpyrrolidinium-Substituted Boronium Ionic Liquids

In this work, a series of novel boronium-bis(trifluoromethylsulfonyl)imide [TFSI–] ionic liquids (IL) are introduced and investigated. The boronium cations were designed with specific structural motifs that delivered improved electrochemical and physical properties, as evaluated through cyclic voltammetry, broadband dielectric spectroscopy, densitometry, thermogravimetric analysis, and differential scanning calorimetry. Boronium cations, which were appended with N-alkylpyrrolidinium substituents, exhibited superior physicochemical properties, including high conductivity, low viscosity, and electrochemical windows surpassing 6 V. Remarkably, the boronium ionic liquid functionalized with both an ethyl-substituted pyrrolidinium and trimethylamine, [(1-e-pyrr)N111BH2][TFSI], exhibited a 6.3 V window, surpassing previously published boronium-, pyrrolidinium-, and imidazolium-based IL electrolytes. Favorable physical properties and straightforward tunability make boronium ionic liquids promising candidates to replace conventional organic electrolytes for electrochemical applications requiring high voltages.

was stopped, and the pale yellow-white precipitate was separated from the liquid by vacuum filtration using a Buchner funnel.While still on the funnel, the solid was washed with two 50 mL portions of toluene, followed by one 50 mL portion of diethylether.The solid was transferred to a round-bottomed flask and dried overnight by rotary evaporation.Yield: 109.2 g (83%).Formation of the desired cation was verified by 1 H-, 13 C-, and 11 B-NMR.
Next, 50.0 g (0.17 mol) of the iodide salt (above) was dissolved in 250 mL of water.While stirring, 64 g of K [TFSI] was added in portions.As the latter dissolved, the solution became cloudy, and as addition continued a clearly defined, pale-yellow second liquid phase became apparent.After stirring overnight, stirring was stopped and the phases were allowed to separate.
The lower (product) phase was then washed and separated twice using 100 mL portions of water, after which it was dissolved in CH2Cl2, and anhydrous MgSO4 was added to dry the solution.After removing the MgSO4 by filtration, the CH2Cl2 was removed with a rotary evaporator.The remaining clear, colorless ionic liquid was then dried for 12 h at 10 mbar while being heated to 50 °C.Accounting for slight mass losses from transferring the liquid product 1, the yield of the ion metathesis step was quantitative.NMR (Figure S15-17) (500 mHz, DMSO-d 6 ). 1 H: δ 1.17 (t, 6H, CH3-CH2), δ 1.9 (very broad, B-H), δ 2.66 (s, 12H, N-CH3), δ 3.04 (4H, CH3-CH2). 13C: δ 8.25 (CH3-CH2), δ 48.8 (CH3-CH2), δ 57.8 (N-CH3), δ 119.5 (q, -CF3). 10B: δ 1.18.Synthesis of IL 2. A 1 L Erlenmeyer flask was charged with a magnetic stirring bar and 500 mL of toluene.To the stirred toluene was added 50.0 g (0.68 mol) of trimethylamine borane was added in one portion.To the vigorously stirred, clear, colorless solution was then added (over approximately 10 min) 86.9 g (0.34 mol) of crystalline I2.Each addition engendered vigorous gas evolution, and the solution remained clear but became deep brown in color.Once the iodine addition was complete, stirring was continued for thirty minutes, during which time the solution faded to pale yellow.Then, while the solution was being stirred vigorously, 58.3 g (0.68 mol) of N-methylpyrrolidine was added in one portion.The solution color rapidly faded to light yellow, and within minutes a white solid began to precipitate.Stirring was then continued for 24 h, stopped, and the white precipitate separated from the liquid by vacuum filtration using a Buchner funnel.
While still on the funnel, the solid was washed with two 50 mL portions of toluene, followed by one 50 mL portion of diethylether.The solid was transferred to a round-bottomed flask and dried overnight by rotary evaporation.Yield: 177 g (91%).The identity of the product was verified by 1 H-, 13 C-, and 11 B-NMR.
In the next step, 50.0 g (0.18 mol) of the iodide salt (above) was dissolved in 250 mL of water.While stirring, 60 g of K[TFSI] was added in small portions.As the latter dissolved, the solution became cloudy, and as addition continued a colorless, well defined, second liquid phase became apparent.After stirring overnight, stirring was stopped and the phases allowed to separate.
After stirring for an additional 30 min, the solid and supernatant were separated using a Buchner funnel, and the white solid product washed twice with 10 mL of warm water.After air drying, the solid was dissolved in a minimal volume of boiling acetone-methanol (c.a.50/50 v/v) and put aside.Upon cooling and slow evaporation, crystals suitable for single-crystal analysis were obtained.
Synthesis of IL 3. A 1 L Erlenmeyer flask was charged with a magnetic stirring bar and 500 mL of toluene.To the stirred toluene was added 50.0 g (0.68 mol) of trimethylamine borane was added in one portion.To the vigorously stirred, clear, colorless solution was then added (over approximately 10 min) 87.0 g (0.34 mol) of crystalline I2.Each addition engendered vigorous gas evolution, and the solution remained clear but became deep brown in color.Once the iodine addition was complete, stirring was continued for thirty minutes, during which time the solution faded to pale yellow-orange.Then, while the solution was being stirred vigorously, 68.0 g (0.68 mol) of N-ethylpyrrolidine was added in one portion.The solution color rapidly faded to light colorless; over the course of several hours, white solid began to precipitate.Stirring was then continued for 48 h, stopped, and the ivory-colored precipitate separated from the liquid by vacuum filtration using a Buchner funnel.While on the funnel, the solid was washed with two 50 mL portions of toluene, followed by one 50 mL portion of diethylether.The solid was transferred to a round-bottomed flask and dried overnight by rotary evaporation.Yield: 157 g (77%).The identity of the product was verified by 1 H-, 13 C-, and 11 B-NMR.
In the next step, 50.0 g (0.17 mol) of the iodide salt (above) was dissolved in 250 mL of water.While stirring, 60 g of K[TFSI] was added in small portions.As the latter dissolved, the solution became cloudy, and as addition continued a near-colorless, well defined, second liquid phase became apparent.After stirring overnight, stirring was stopped and the phases allowed to separate.The lower (product) phase was then washed and separated twice using 100 mL portions of water, after which it was dissolved in CH2Cl2, and anhydrous MgSO4 added to dry the solution.
Compound 4, BPh4 salt.In a 100 mL Erlenmeyer flask charged with a stir bar, 2.2 g (7.0 mmol) of the iodide salt of 4 was dissolved in 25 mL of water.In a separate flask, 2.6 g (7.7 mmol) of NaBPh4 was dissolved in 50 mL of warm water.While stirring, the latter solution was slowly added to that containing the iodide salt of 4. A voluminous white precipitate formed immediately.
After stirring for an additional 30 min, the solid and supernatant were separated using a Buchner funnel, and the white solid product washed twice with 10 mL of warm water.After air drying, the solid was dissolved in a minimal volume of boiling acetone-methanol (c.a.50/50 v/v) and put aside.Upon cooling and slow evaporation, crystals suitable for single-crystal analysis were obtained.

Thermal Gravimetric Analysis
The thermal stability of each BIL was evaluated using thermal gravimetric analysis (TGA) (TA Instruments Q500).Samples were loaded onto platinum pans and evaluated under nitrogen from 20-900 °C at a heating rate of 10 °C min -1 .The decomposition temperature (T5) was taken as the point at which 5 wt% of the initial sample mass was lost.

Differential Scanning Calorimetry
Thermal transitions were measured using differential scanning calorimetry (DSC) (TA Instruments Q2000) equipped with liquid nitrogen cooling.Each BIL was sealed in Tzero hermetic aluminum pans in a nitrogen filled glovebox and analyzed first by heating to 50 °C, then cycled twice between -150 °C and 50 °C at 10 °C min -1 under a helium purge at a flow rate of 25 mL min - 1 .Glass transition temperatures (Tg) were measured from the heating curves for each BIL.

Single Crystal Structure Determinations
Crystal structures of the BPh4 salts of BIL 2 and 4 were determined by single crystal X-ray diffraction using a Bruker Quest diffractometer with a fixed chi angle, a Mo Kα wavelength (λ = 0.71073 Å) sealed tube fine focus X-ray tube, single crystal curved graphite incident beam monochromator, a Photon II area detector, and an Oxford Cryosystems low temperature device.
Data were collected at 150 K, reflections were indexed and processed, and the files scaled and corrected for absorption using APEX3 [1] and SADABS [2] .The space groups were assigned using XPREP [3][4] and solved by direct or dual methods using ShelXS [4] or ShelXT [5] and refined by full matrix least squares against F 2 with all reflections using Shelxl2018 [6] with the graphical interface Shelxle [7] .H atoms attached to carbon and boron atoms were positioned geometrically and constrained to ride on their parent atoms.C-H bond distances were constrained to 0.95 Å for aromatic C-H moieties, and to 0.99 and 0.98 Å for aliphatic CH2 and CH3 moieties, respectively.B-H bond distances were allowed to refine.Methyl CH3 were allowed to rotate but not to tip to best fit the experimental electron density.Uiso(H) values were set to a multiple of Ueq(C) with 1.5 for CH3 and 1.2 for all other H atoms. BIL4-BPh4 was refined as a 2-component merohedric twin (high/low symmetry hexagonal, 180° rotation around (0 1 0)).Application of the 3×3 transformation matrix 0 1 0 1 0 0 0 0 -1 yielded a minor twin fraction of 0.453 (2).
In BIL2-BPh4 the cation shows minor disorder by an approximate 180° rotation, exchanging the trimethylamine and N-methylpyrrolidine fragments with each other.Also disordered is one phenyl group of the anion, in the proximity of the disordered cation.The phenyl disorder is likely induced by the cation disorder, but occupancies were refined independently from each other.For both disordered fragments the major and minor moieties were restrained to have similar geometries.The major and minor B-C(phenyl) bond lengths were restrained to be similar to each other.Uij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar.Subject to these conditions the occupancy ratio refined to 0.915(2) to 0.085(2) for the cation, and to 0.871(3) to 0.129(3) for the phenyl group.The lengths of the B-H bonds were refined but restrained to be similar to each other.

Electrochemical Measurements
All electrochemical experiments were performed in a nitrogen filled glovebox (<1 ppm H2O).Cyclic voltammetry (CV) was performed using a Biologic SP-200 potentiostat.All samples were analyzed using a three-electrode setup consisting of either a glassy carbon or a platinum working electrode (EDAQ, surface area = 7.8 x 10 -3 cm 2 ), a platinum mesh counter electrode, and a home-built Ag/Ag + reference electrode made from a 100 mM solution of silver triflate dissolved in [EMI][TFSI] contained within a glass tube sealed with a vycor glass frit.The working and counter electrodes were held in the same compartment, not separated by a frit or membrane.
Following assembly, the reference electrode was allowed to equilibrate for 24 h before use.
Working electrodes were polished prior to and between uses with a series of alumina slurries (6, 1, 0.25 μm) and cleaned with a microfiber polishing pad between successive scans of the same BIL system.Prior to electrochemical analysis, BILs were first dried under vacuum with mild heating prior to storage in a nitrogen-filled glovebox to minimize the impact of water.Evaluation of the electrochemical reversibility of lithium ions at a platinum working electrode was performed in select BIL electrolytes containing 0.45 mol kg -1 Li[TFSI] which were filtered using a 0.45 μm PTFE filter.

Broadband Dielectric Spectroscopy
Broadband dielectric spectroscopy (BDS) measurements were made over a range of frequencies (10 -1 -10 9 Hz).A Novocontrol α-analyzer with a gold-coated brass parallel plate capacitor geometry (20 mm diameter, 1.0 mm sample thickness maintained by Teflon spacers) was used over the frequency range 10 -1 -3x10 6 Hz.Frequencies between 10 6 -10 9 Hz were measured using an HP E4991B impedance analyzer with Novocontrol RF extension and a gold-coated parallel plate capacitor (10 mm diameter, 0.1 mm sample thickness maintained by silica rod spacers).Temperature control was maintained using a Quatro temperature control system with an accuracy ±0.1 °C, using nitrogen as both a heating and cooling gas.
The real part of complex dielectric permittivity is presented in terms of the derivative .

Quantum-based Computations
The BIL cations 2-4 (Figure 1) and other cations of interest, along with their reduced daughter species (charge = 0, multiplicity = 2), were optimized using the Gaussian16 suite of programs [8] with the wB97X-D density functional [9] and the cc-pvtz basis set. [10]Analytic vibrational frequencies were computed to confirm optimized geometries as stable minima and to provide enthalpy-based estimates of reduction potentials (E 0 red) in the gas phase at 298 K. [11] Electrostatic potential maps (elstats) were computed as follow-up single-point calculations using Spartan'20. [12]nsity & Viscosity Measurements

Rheology
The temperature dependent zero-shear viscosities were measured using a Discovery HR-2 Rheometer (TA Instruments) with 25 mm stainless steel parallel plates and stress/rate-controlled flow experiments.The rheometer was contained within a home-built nitrogen-purged glovebox to maintain low moisture content (<100 ppm).Temperature control was maintained using an environmental test chamber accurate to ± 0.1 °C with nitrogen as both the heating and cooling gas.

Densitometry & Viscosity
Temperature dependent densities and kinematic-dynamic viscosities were measured using an SVM 3001 Stabinger viscometer (Anton Paar) with temperature controlling capabilities.
Samples were loaded directly into the instrument prior to analysis to minimize exposure to ambient moisture.

Supplemental Tables and Figures
Table S1.Parameters for the VFT fit of dc ionic conductivities, corresponding to the solid lines in Figures S14 and 8.              S1).

Figure S3 .
Figure S3.Thermal gravimetric analysis of BIL 1-4.Samples were heated under nitrogen at a ramp rate of 10 °C min -1 .T5 was determined as the point at which 5 wt% of the initial sample mass

Figure S4 .
Figure S4.Cyclic voltammograms at a glassy carbon working electrode comparing the

Figure S8 .
Figure S8.Broadband dielectric spectra of BIL 3. Top: Real part of complex conductivity, Middle:

Figure S9 .
Figure S9.Broadband dielectric spectra of BIL 4. Top: Real part of complex conductivity, Middle:

Figure S10 .
Figure S10.Shape parameters of the Havriliak-Negami fit function applied to the real part of

Figure S11 .
Figure S11.Dielectric strengths of the Havriliak-Negami fit function applied to the real part of