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Structural analysis of low melting organic salts an approach to ionic liquid design
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posted on 12.07.2018by Dean, Pamela Mary
Ionic liquid forming compounds often display low melting points (a lack of crystallisation at ambient temperature and pressure) due to decreased lattice energies in the crystalline state. The degree of anion-cation contact with respect to the type, strength and number of interactions is a major factor determining the lattice energies, melting point and general behaviour of ionic liquid forming salts. Intermolecular interactions between the anion and cation and the conformational states of each component of the salt are of interest since distinctive properties ascribed to ionic liquids are determined to a significant extent by these interactions. The direct insight into the spatial relationship between cation and anion provided by the analysis of crystal structures provides a basis from which features of the ionic liquid can be generally understood, since the short range order and interactions of related, non-crystalline compounds may be similar to those of the crystalline form.
However, it is difficult to predict whether a particular ionic pair will produce a liquid at room temperature, due to numerous possible combinations of cations and anions and the subtleties of their interactions. Crystal engineering is the ability to assemble molecular or ionic components into the desired crystalline architecture by engineering a target network of supramolecular interactions known as synthons. In this investigation the problem of ionic liquid design is addressed using the concepts of crystal engineering in an inverse sense, the so-called anti crystal-engineering approach. A topical area in which the anti crystal-engineering concept may be of some value is that of Ionic Liquid Phases of Pharmaceutically Active Ions (Active Ionic Liquids).
Thus, by using the knowledge gained of the intermolecular interactions, packing and ionic conformation which occur within ‘traditional’ ionic liquids, combined with the knowledge of which functional group combinations yield supramolecular synthons resulting in crystalline subjects, and the subsequent prevention thereof (anti crystal-engineering), appropriate ions shall be selected which may result in ionic liquid formation.
The intermolecular interactions of a series of:
• crystallised bis(trifluoromethanesulfonyl)amide (NTf2) and bis(methanesulfonyl)amide (NMes2) ionic liquids,
• low melting N-alkyl-2-methyl-3-benzylimidazolium iodide salts with a range of alkyl chain lengths, from n=1 to 6 and including both n-butyl and s-butyl chains,
• 1-methyl-1-propylpyrrolidinium chloride and,
• a number of low melting salts containing trihalide and monohalide ions, in combination with typical IL organic cations namely, 1-ethyl-3-methylimidazolium, 1-ethyl-1-methylpyrrolidinium and 1-propyl-1-methylpyrrolidinium,
were qualitatively investigated and/or compared using a combination of crystallographic, Hirshfeld surface and thermal analysis techniques.
The NMes2 salts are known to exhibit higher glass transitions and higher viscosities than those of the NTf2 salts. The origins of these differences were analysed in terms of the importance of factors such as the C-H•••O hydrogen bond, fluorination, presence of an aromatic moiety and length of alkyl chain, using the Hirshfeld surfaces and their associated fingerprint plots. Additionally, the existence of C-F•••π and C-H•••π interactions were elucidated and the significance of anion-anion interactions was recognised. Thermal analysis of the N-alkyl-2-methyl-3-benzylimidazolium iodide salts revealed that the methyl- and (s-)butyl substituted salts have a significantly higher melting point than the rest of the series. Analysis of these crystal structures allowed examination of the influence of the substitutions on the different cation-anion and cation-cation interactions and thus the physical properties of the salts. Thermal analysis of the monohalide and trihalide salts revealed that the tribromide salts are lower melting than their monohalide analogues. Analysis of these crystal structures revealed the influence of the anions and the crystal packing on the physical properties of the salts.
A series of crystalline and liquid salts were prepared from cations and anions drawn from Active Pharmaceutical Ingredients (APIs) and Generally Recognized As Safe (GRAS) materials. The solid-state structures of the crystalline salts were used as a basis for the anti-crystal engineering approach in the preparation of several “Active Ionic Liquids” (AILs). However, a side product also resulted during the synthetic route namely, methyl 9H-xanthene-9-carboxylate, a side product resulting from the API, propantheline. The results and methodology of the anti-crystal engineering procedure and the subsequent successful preparation and characterization of pharmaceutical ionic compounds are reported herein.