Novel chiral ionic liquids

• Main Author: Foreiter, M. B.
• Published: Queen's University Belfast 2014
• Subjects: 541
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.675450

Chiral ionic liquids have been present in the literature for more than a decade. The development of original chiral systems of ions is currently far more difficult than ever before. The path of innovation in this topic, however, should not be focussed exclusively on the introduction of chirality into the ionic liquid structure, but should go beyond, towards further functionalisation of these species. The combination of chirality and a functional moiety has been the starting point for the present work. Here, a focus was on creation of chiral ionic liquids possessing distinct functional groups: a strong hydrogen-bonding thiourea group was selected. Consequently, a series of novel chiral thiouronium salts were synthesised. The chirality was sourced in cheap, but structurally robust, chiral amines: (S)-1- phenylethylamine and (+)-dehydroabietylamine. Because the thiourea moiety can be functionalised at five different sites, a wide range of thiouronium and thiourea-functionalised ionic liquids could be prepared and analysed, and the influence coming from their structural diversity explored. The crucial point for the project was a confirmation of rotameric behaviour in these new thiouronium systems. Here, the hindered rotation has been examined by NMR spectroscopy, and compared with literature findings. The key message concludes that, in the thiouronium system, the C-S rotation predominates when placed in a highly polar solvent; then the syn syn conformation dominates. When, on the other hand, the constraints of the surroundings are eliminated, the syn-anti form is energetically preferable. Here, the rotamerism of the thiouronium cation in a neat liquid form was examined for the first time. Finally, these novel chiral compounds have found applications based on molecular recognition by the thiourea moiety. The discrimination of chiral oxoanions by these salts using NMR spectroscopy has been successful. A strong, double hydrogen-bonded connection between the chiral thiouronium moiety and the oxoanion proved to be a key point, generating results a competitive with the literature results. Following this approach, the separation capabilities of thiouronium systems were also explored, given that they can be effective agents for oxoanion extractions. This work demonstrated that a careful design of functional molecules is scientifically significant, and can have a great impact on their applications.