Synthesis and Characterisation of Tertiary Diallylalkylammonium Ionic Liquids and Their Metathesis

Abstract

A range of new tertiary diallylalkylammonium (t-DAAA) based Ionic Liquids (ILs) were successfully synthesised in good yields with cation alkyl substituent lengths of 5, 6, 9, 10, 11, 12 and 14 carbons. The counter anions used were [Cl]¯, [HSO4]¯, [H2PO4]¯, [BF4]¯, [PF6]¯, [TFSI]¯ and [TFA]¯. The ILs physico- and electro-chemical properties were evaluated in order to draw structure-property relationships between them.
The thermal stability of the t-DAAA ILs depended heavily on the nature of the anion. The t-DAAA [TFSI]¯ ILs were the most stable reported in this work with decomposition temperatures of over 300°C. The thermal stability of the t-DAAA ILs generally increases in the order [H2PO4]¯ < [Cl]¯ ≈ [TFA]¯ < [PF6]¯ < [HSO4]¯ ≈ [BF4]¯ < [TFSI]¯. The majority of the t-DAAA ILs were solid or very viscous at ambient temperature. However, the [TFSI]¯ and [TFA]¯ based ILs displayed some very low viscosities with the lowest of 58 cP reported for diallylhexylammonium [TFA]¯. The electrochemical windows of the less viscous t-DAAA ILs compared well with those reported in literature for protic ammonium and aprotic imidazolium ILs. The most stable t-DAAA ILs were those based on the [TFSI]¯ anion which demonstrated electrochemical windows of between 4.9 and 5.2 V.
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Acyclic diene metathesis (ADMET) polymerisation produced cyclic products rather than the desired polymeric materials. Diallylhexylammonium [BF4]¯ and [PF6]¯ gave 100% conversion to cyclic products, whilst the conversion decreased to 46 and 25% for the diallyhexylammonium [TFSI]¯ and [HSO4]¯; 4% for [TFA]¯ and [Cl]¯ and 0% for [H2PO4]¯. Ring closing metathesis (RCM) reactions were performed in order to improve on the conversions obtained in the ADMET reactions. However, only moderate conversions (74-54%) to cyclic products were obtained for the diallylhexylammonium [BF4]¯, [PF6]¯ and [HSO4]¯ ILs, while [Cl]¯, [H2PO4]¯ and [TFA]¯ did not undergo any reaction.