Novel chiral ionic liquids: physicochemical properties and investigation of the internal rotameric behaviour in the neat system

Abstract

Optically active S-alkyl-N,N′-bis((S)-1-phenylethyl)thiouronium salts, abbreviated as (S)-[C_npetu]Y (where Y is an anion; n = 1, 2, 3, 4, 6, 8, 10, 12 or 16), have been prepared and studied by a broad spectrum of analyses. This consists of density, viscosity, and conductivity determination, followed by a discussion of relevant correlations. Unusual trends depending on the S-alkyl chain length were documented for (S)-[C_npetu][NTf₂] series (where [NTf₂]⁻ = bis{(trifluoromethyl)sulfonyl}amide), including the viscosity decreasing with increasing chain length, and the conductivity showing a maximum between the S-butyl and the S-hexyl derivative. In addition, a hindered rotamerism of the thiouronium cation in dmso-d₆ solution was recognised by ¹H and ¹³C NMR techniques. Thorough analysis of NMR spectra confirmed that the main contribution comes from rotation about the partial double C–S bond. For the first time, a neat thiouronium ionic liquid system has been subjected to quantitative analysis of hindered rotamerism by dynamic NMR coalescence studies, with estimated activation energy for rotation of 63.9 ± 0.4 kJ mol⁻¹. Finally, the application of (S)-[C_npetu]Y salts as chiral discriminating agents for carboxylates by ¹H NMR spectroscopy was further investigated, demonstrating the influence of the S-alkyl chain length on chiral recognition; (S)-[C₂petu][NTf₂] ionic liquid with the mandelate anion gave the best results.