1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

Summary

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using 1,3,5-triphenylbenzene (TPB) and corannulene as electron receptors.

Abstract

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using two types of polyaromatic hydrocarbons (PAH), namely 1,3,5-triphenylbenzene and corannulene, as electron receptors. The solid PAHs were first dissolved in tetrahydrofuran (THF) to form a solution. Metallic lithium was then dissolved into these PAH/THF solutions to yield either blue or greenish blue solutions, colors which are indicative of the presence of solvated electrons. Conductivity measurements at ambient temperature carried out on 1,3,5-triphenylbenzene-based LiSES, denoted by Li_xTPB(THF)_24.7 (x = 1, 2, 3, 4), showed an increase of conductivity with increase of Li:PAH ratio from x = 1 to 2. However, the conductivity gradually decreased upon further increasing the ratio. Indeed the conductivity of Li_xTPB(THF)_24.7 for x = 4 is even lower than for x = 1. Such behavior is similar to that of the previously reported LiSES prepared from biphenyl and naphthalene. Conductivity versus temperature measurements on corannulene-based LiSES, denoted by Li_xCor(THF)₂₄₇ (x = 1, 2, 3, 4, 5), showed linear relationships with negative slopes, indicating a metallic behavior similar to biphenyl and naphthalene-based LiSES.

Introduction

Lithium solvated electron solutions (LiSES) prepared using simple two-ring polyaromatic hydrocarbons (PAH) such as biphenyl and naphthalene can potentially be utilized as liquid anodes in refuelable lithium cells^1-7. In the LiSES, these simple PAH molecules served as the electron receptors for solvated electrons from dissolved metallic lithium.

Progressing from these two-ring systems, the authors have since then carried out conductivity measurement studies on LiSES which are prepared using more complex PAHs, starting with the group of cyclopenta-2,4-dienone derivatives⁸. These PAHs include larger PAHs (>two benzene rings) and PAHs with substituents incorporated into their aromatic rings. A larger PAH molecule with more than two rings is expected to accommodate more lithium atoms per PAH molecule than either biphenyl or naphthalene thus resulting in LiSES with a higher energy density. The objective of introducing substituents into PAHs is to make the PAH accept electrons more readily and become more stable as polyanions in LiSES.

As part of ongoing efforts to develop LiSES with higher energy density, this paper will report on the characterization of LiSES prepared from corannulene made by the literature procedure⁹ as well as 1,3,5-triphenylbenzene, TPB synthesized by a slightly modified literature ¹⁰. 1,3,5-triphenylbenzene, as shown in Figure 1(1), can be classified as a biphenyl derivative with two additional phenyl rings at positions 3 and 5 of the same ring. Since this molecule has four benzene rings, it should uptake 4 atoms of Li per molecule, which is more than for biphenyl (maximum 2.5 mole equivalents of Li per PAH in 0.5 M solution) and naphthalene (<2.5 mole equivalents of lithium per molecule).

Corannulene is a five-ring PAH arranged into a bowl shape as shown in Figure 1(2). Zabula et al.¹¹ have demonstrated the feasibility of dissolving metallic lithium in a solution of corannulene/tetrahydrofuran (THF) to form a solution with five Li⁺ ions sandwiched between two stable tetraanions of corannulene.

Figure 1: The molecular structures of 1,3,5-triphenylbenzene (1) and corannulene (2). 1,3,5-triphenylbenzene is classified as a biphenyl derivative with two additional phenyl rings at positions 3 and 5 of the same ring. Corannulene is a five-ring PAH with its five benzene rings arranged into a bowl shape. Please click here to view a larger version of this figure.

Thus, both 1,3,5-triphenylbenzene and corannulene are potential candidates for high energy density LiSES.

Protocol

1. Preparation Procedure for 1,3,5-Triphenylbenzene (1)

1. Place a mixture of acetophenone (4.0 g, 33.3 mmol) and 100 ml of absolute ethanol into a round bottom three neck 250 ml flask equipped with magnetic stirrer, reflux condenser, nitrogen inlet, bubbler, dropping funnel and thermometer. Add silicon tetrachloride (11.9 g, 8.0 ml, 70.2 mmol, 2.1 eq.) to the mixture in one portion at 0 °C under nitrogen using the dropping funnel.
2. Observe the evolution of gaseous hydrogen chloride for 10 min. Then stir the reaction mixture at 40 °C for 20 hr.
3. Cool down the reaction mixture to 23 °C and pour in 200 g of water mixed with ice (1:1 mass ratio).
4. Extract the resulting mixture with dichloromethane (2 x 100 ml) using an extraction funnel.
5. Wash the combined extracts once with saturated NaCl solution (100 ml), and dry over 15 g of anhydrous MgSO₄. Filter the liquid part off and then concentrate using a rotary evaporator.
6. Purify the product via recrystallization from ethanol (dissolution in minimal amount of ethanol followed by partial evaporation of the solvent, keeping at 6 °C overnight, and rapid filtration) to obtain 2.2 g (yield 63%) of 1,3,5-triphenylbenzene (1) as pale yellow crystals.
   Note: ¹H-NMR (400 MHz, CDCl₃): δ= 7.41 (m, 3H), 7.50 (m, 6H), 7.72 (d, 6H, J = 7.33Hz), 7.80 (s, 3H). ¹³C-NMR (400 MHz, CDCl₃): δ= 125.21, 127.39, 127.57, 128.88, 141.18, 142.38.

2. LiSES Prepared with 1,3,5-Triphenylbenzene

1. Preparation of 1,3,5-triphenylbenzene-based LiSES
   NOTE: 1,3,5-triphenylbenzene used in this paper was synthesized as per procedure described above. The 1,3,5-triphenylbenzene -based LiSES are denoted by Li_xTPB(THF)_24.7 where x denotes the Li:PAH molar ratio and TPB denotes 1,3,5-triphenylbenzene. Prepare Li_xTPB(THF)_24.7 inside an argon-filled glovebox at ambient temperature via the following steps:
   1. Measure out well defined quantities of metallic Li, THF and TPB separately inside the glovebox to achieve the target molar composition of Li_xTPB(THF)_24.7 for x = 1, 2, 3, and 4. Use 41.6 mg, 83.3 mg, 124.9 mg, 166.6 mg of Li for x = 1, 2, 3 and 4 respectively.
   2. For each of the four LiSES samples to be prepared, dissolve 1.84 g of TPB in 12 ml of THF inside four separate glass bottles to form 12 ml of colorless solutions of TPB(THF)_24.7 for each bottle. Use a 0.5 M 1,3,5-triphenylbenzene in all the solutions.
   3. Add the weighed metallic Li foils to the four bottles and seal the bottles with Parafilm.
   4. Stir the mixture in each bottle overnight using a glass-coated magnetic stirrer to ensure the complete dissolution of metallic Li.
2. Conductivity Measurements
   1. Carry out all conductivity measurements using a standard conductivity cell probe based on the four-electrode technique. Attach the cell probe to a meter. The probe has a secondary function to measure the solution's temperature at the same time and display both conductivity and temperature measurements.
   2. Prior to measurements, calibrate the meter using 50 ml of standard 0.01 M aqueous KCl solution provided by the conductivity probe's manufacturer outside the glovebox.
   3. Carry out all the conductivity measurements for 1,3,5-triphenylbenzene-based LiSES, Li_xTPB(THF)_24.7 for x = 1, 2, 3, 4 inside the glovebox.
   4. For each of these LiSES, pour out the sample into a short glass cylinder and immerse the probe into the solution. Record the conductivity measurement over a period of one to two hours till each sample returns to an ambient temperature. The time taken for each sample to return to ambient temperature is ~1-2 hr. The probe will remain immersed in the sample for the entire duration of conductivity measurement.

3. Corannulene

1. Preparation of corannulene-based LiSES
   NOTE: The corannulene used in this paper was synthesized at the School of Physical and Mathematical Sciences, NTU using a multistep literature procedure.⁹ The corannulene-based LiSES are denoted by Li_xCor(THF)₂₄₇ where x denotes the Li:PAH molar ratio and Cor denotes the corannulene. Prepare Li_xCor(THF)₂₄₇ inside an argon-filled glovebox at ambient temperature via the following steps:
   1. Measure out well defined quantities of metallic Li, THF and Cor separately inside the glovebox to achieve the target molar composition of Li_xCor(THF)₂₄₇ for x = 1, 2, 3, 4 and 5. Use 4.2 mg, 8.3 mg, 12.5 mg, 16.6 mg and 20.8 mg of Li for x = 1, 2, 3, 4 and 5 respectively.
   2. Next, for each of the five LiSES samples (x = 1, 2, 3, 4 and 5) to be prepared, dissolve 0.15 g of Cor in 12 ml of THF inside five separate glass bottles to form 12 ml of colorless solution of Cor(THF)₂₄₇ in each bottle. Use a corannulene concentration of 0.05 M).
   3. Next, add the weighed metallic Li foils to the five bottles of Cor(THF)₂₄₇ and seal the bottles with Parafilm.
   4. Stir the mixture in each bottle overnight using a glass-coated magnetic stirrer to ensure the complete dissolution of the metallic lithium.
2. Conductivity Measurements
   1. For conductivity versus temperature measurements, remove each of the five bottles containing Li_xCor(THF)₂₄₇ for x = 1, 2, 3, 4 and 5 individually from the glovebox, wrap it with an additional layer of para-film and immerse it inside an insulated Styrofoam container filled with dry ice.
      NOTE: The LiSES samples did not come into contact with either moisture or oxygen while outside the glovebox because the bottles were sealed.
   2. Cool each bottle down to about 10 °C by keeping each bottle immersed in the dry ice for about 30 min before being transferred back into the glovebox for conductivity measurements.
   3. Purge the ante-chamber of the glovebox at least 5 times for each cooled sample to ensure that no traces of water of condensation accompany the bottle back into the glovebox.
   4. Similar to the manner in which conductivity versus temperature measurements were collected for naphthalene-based LiSES samples¹, measure the conductivity of Li_xCor(THF)₂₄₇ (x = 1, 2, 3, 4, 5) over a period of one to two hours till each sample returned to ambient temperature. The probe will remain immersed in the sample for the entire duration of conductivity measurement.

Results

Reaction between various amounts of lithium and mixtures of 1,3,5-triphenylbenzene with THF gives greenish blue or deep blue colored solutions as shown in Figure 2. A light color indicates that the particular sample of LiSES has a low concentration of solvated electrons. 1,3,5-triphenylbenzene demonstrates increase of conductivity with increase of Li:PAH ratio from 1 to 2 in 0.5 M THF solution (Table 1). However, conductivity value gradually decreases upon further increasing the molar ra...

Discussion

For the 1,3,5-triphenylbenzene-based LiSES, a sample with a light color shows that it has a low concentration of solvated electrons. Li_xTPB(THF)_24.7 (for x = 1, 2, 3, 4) demonstrates a behavior in its conductivity versus x similar to that seen for LiSES made from biphenyl and naphthalene^{1, 2}.There is an initial increase in conductivity with increase of Li:PAH ratio from 1 to 2 and a subsequent decrease in conductivity upon further increasing the molar ratio to 3 and 4, with condu...

Materials

Name	Company	Catalog Number	Comments
Tetrahydrofuran Anhydrous, ≥99.9%, Inhibitor-free	Sigma Aldrich	401757-100ML
Lithium Foil	Alfa Aesar	010769.14
Cond 3310 Conductivity Meter	WTW	Not Applicable
1,3,5-triphenylbenzene			Synthesized from acetophenone according to procedure described in literature
Silicon tetrachloride	Sigma Aldrich	215120-100G
acetophenone	TCI	A0061-500g
Ethanol	Merck Millipore	1.00983.2511
Corannulene			Synthesized by literature procedure