Characterizing Cation Chemistry for Anion Exchange Membranes—A Product Study of Benzylimidazolium Salt Decompositions in Base

Abstract

Imidazolium functionality has played a prominent role in research on anion exchange membranes for use in alkaline electrochemical devices. Base stability and degradation of these materials has been much studied, but in many instances, product pathways have not been thoroughly delineated. We report an NMR study of base-induced decomposition products from three benzylimidazolium salts bearing varying extents of methyl substitution on the imidazolium ring. The major products are consistent with a hydrolytic ring fragmentation pathway as the principal mode of decomposition. We observe several new products not previously reported in the literature on imidazolium salt degradation, including benzilic acid rearrangement products formally derived from intermediate 1,2-dicarbonyl compounds or their equivalents. However, the overall reactions are complex, the yields of observed products do not account for all consumed starting materials, and mechanistic ambiguities remain.

Degradation of Organic Cations under Alkaline Conditions

“…Model compound study is a common strategy to deconvolute cation decomposition in complicated polymeric AAEM systems, so that their degradation products, kinetics, and mechanisms can be unambiguously assigned using nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS) techniques. − Although significant progress has been achieved over the past decade in enhancing the understanding of the cation alkaline stability, two major limitations still exist: inconsistent testing conditions and a lack of comparison among different classes of cations. It is broadly accepted that the cation stability studies need to be performed with concentrated base at high temperatures, although the choice of solvents differs.…”

“…31 When the C2 position was substituted with a methyl group (17), the degradation was slower, but the degradation products were similar (Scheme 6). 13,46 Model compound 16 was designed to block the C2 position from nucleophilic attack by incorporating a bulky 2,6dimethylphenyl substituent. 14 The alkaline stability of 16 was significantly higher, and only degradation products from S N 2 attack on the benzyl and methyl groups were identified without ring-opening/imidazolium-hydrolysis products (Scheme 6).…”

“…Methods and Instruments. 1 H and 13 C NMR spectra were recorded on a Bruker 400, Varian INOVA 500 or 600 MHz instrument at 22 °C with shifts reported relative to the residual solvent peak (CD 3 OD or CD 3 OH); 3.31 ppm ( 1 H) and 49.00 ppm ( 13 C). Electrospray ionization−high-resolution mass spectrometry (ESI-HRMS) analyses were performed on a Bruker 9.4 T solariX Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) instrument.…”

“…According to the general procedure, 1-phenylpiperazine (2.0 mmol, 1.0 equiv), 1,5-dibromopropane (2.0 mmol, 1.0 equiv), K 2 CO 3 (2.4 mmol, 1.2 equiv), and 10 mL MeCN were used to synthesize the title compound 14 as white solid (280 mg, 45% yield). 1 H NMR (400 MHz, CD 3 OD): δ 7.29 (t, J = 7.7 Hz, 2H), 7.04 (d, J = 8.1 Hz, 2H), 6.92 (t, J = 7.3 Hz, 1H), 3.70 (t, J = 5.2 Hz, 4H), 3.58 (t, J = 5.8 Hz, 4H), 3.51 (t, J = 5.2 Hz, 4H), 1.95 (p, J = 6.0 Hz, 4H), 1.76 (p, J = 6.1 Hz, 2H); 13 (20). According to the general procedure, benzotriazole (2.0 mmol, 1.0 equiv), ethyliodide (4.4 mmol, 2.2 equiv), K 2 CO 3 (2.4 mmol, 1.2 equiv), and 10 mL MeCN were used to synthesize the title compound 20 as white solid (298 mg, 49% yield).…”

“…According to the general procedure, benzotriazole (2.0 mmol, 1.0 equiv), ethyliodide (4.4 mmol, 2.2 equiv), K 2 CO 3 (2.4 mmol, 1.2 equiv), and 10 mL MeCN were used to synthesize the title compound 20 as white solid (298 mg, 49% yield). 1 H NMR (400 MHz, CD 3 OD): δ 8.36−8.28 (m, 2H), 8.03−7.96 (m, 2H), 5.07 (q, J = 7.3 Hz, 4H), 1.79 (t, J = 7.3 Hz, 6H); 13 Model Compound Study Procedures. Stock solutions of basic methanol were prepared by dissolving KOH (1 M or 2 M) and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (0.0250 M) in CD 3 OH.…”