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Chiral oxamides as highly efficient gelators for ionic liquids

Chiral bis(amino acid and amino alcohol) oxamides belong to a class of efficient gelators of various organic solvents and water [1, 2]. Their gelation ability is the result of unidirectional self-assembly into fibrous aggregates due to strong and directional intermolecular hydrogen bonding provided by oxamide units and the lack of molecular symmetry due to the presence of two chiral centers, which is known to prevent crystallization and favors aggregation. Additionally, bis(amino acid) oxamides show rather rare ambidextrous gelation behavior: they can gel both lipophilic, highly polar solvents and water due to their bolaamphiphilic structural features with well-separated lipophilic amino acid substituents and hydrophilic carboxylic acid or alcoholic OH groups located above and below the planar oxamide unit. Recent study [3] showed that (S, S)-bis(leucinol)oxamide is highly efficient gelator for imidazole-based ionic liquid, capable of producing highly conductive supramolecular ionogel. This finding is very important since ionic liquids and materials based on ionic liquids are identified as "ideal" electrolytes for different applications due to their unique combination of properties such as negligible vapor pressure, nonflammability, chemical and thermal stability, high ionic conductivity, and wide electrochemical window. In order to gain a deeper insight into the gelation properties of oxamide compounds towards ionic liquids we have expanded our survey to include gelation tests of series of amino-alcohol oxamides, namely ((S, S)-bis(leucinol)oxamide, (S, S)-bis(phenylalaninol) oxamide, (S, S)-bis(valinol)oxamide and (S, S)-bis(phenylglycinol)oxamide) and three ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylpyridine bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The results have showed that (S, S)-bis(valinol)oxamide is the most efficient gelator since it forms gel with all three ionic liquids, whereas bis(phenylglycinol)oxamide does not make gel with any of the tested ionic liquids. The minimum gelator concentration varies from w = 0.3 to 1.7 % indicating high gelforming ability of the oxamide-based gelators. In addition, all prepared ionogels have very high ionic conductivity, comparable to those of the respective neat ionic liquid which classifies them to quasi-solid materials with a potential for application as novel electrolytes.

chiral oxamides ; gelators ; ionic liquids

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