SUPRAMOLECULAR CLUSTERS OF IMIDAZOLIUM-BASED IONIC LIQUIDS: THE EFFECT OF THE CATION-ANION STRUCTURE ON CRYSTALLIZATION

Abstract

Ionic liquids are highly versatile in supramolecular chemistry, where adjustable physicochemical properties enable the development of functional materials. The supramolecular organization of an IL can be investigated by constructing supramolecular cluster of first molecular coordination sphere, which delineate the smallest crystalline unit encompassing all intermolecular interactions. In this study, we sought to determine how the choice of anion (A1, A2, A3, etc.) in constructing the cation-anion (CA) pair that forms M1 (used as the basis for supramolecular cluster construction) affects this process. Supramolecular cluster data were collected for different CA pairs (M1) within the same compound. We found that the previously described parameter (i.e., choosing the most energetically stable CA pair) was insufficient to yield an unambiguous choice. Less stable CA pairs consistently exhibited lower cluster energy efficiency values, indicating higher destabilizing energies in the system. Therefore, we proposed two parameters: the cation-anion stabilization index, which quantifies the energetic contribution of each CA pair relative to the most stable pair (CA1), and the sum of indices, which integrates both the cation-anion stabilization index and cluster energy efficiency values. Both parameters have a maximum value of 1.0. Using these parameters, our findings indicate that the CA1 pair selected for M1 should indeed be the most energetically stable. In addition, crystallization mechanisms were proposed based on the obtained data. For this purpose, we selected 14 single crystals of ionic liquids based on the N-methylimidazolinium ring with different N-substituents R (ethyl, propyl, butyl, decyl, dodecyl, and allyl groups), in addition to analyzing different anions. Lastly, we proposed and identified five crystallization mechanism types: CRMD3 (1CA1, 2CA1, 11CA1, and 12CA1), CRMD23 (4CA1), CRM13 (3CA1, 5CA1, 7CA1, and 14CA1), CRM23 (9CA1), and CRM3 (6CA1, 8CA1, 10CA1, and 13CA1).