Phosphonium-phosphates/thiophosphates: Ionic liquids or liquid ion pairs? NMR spectroscopic classification

• Published in: Tetrahedron letters Vol. 58; no. 25; pp. 2460 - 2464
• Main Authors: Mondal, Sujit, Mandal, Tanmay, Sharma, Meeta, Kumar, Ravindra, Arora, Ajay K., Bansal, Veena, Christopher, J., Kapur, G.S.
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 21.06.2017; Elsevier
• Subjects: Chemistry; Chemistry, Organic; DOSY NMR; Ion association; Ionic liquid; Liquid ion pair; NMR; Phosphonium phosphate; Phosphonium thiophosphate; Physical Sciences; Science & Technology; DOSY NMR; NMR; Phosphonium phosphate; Phosphonium thiophosphate; Liquid ion pair; Ionic liquid; Ion association; ANIONS; MOLECULAR-SOLVENTS
• ISSN: 0040-4039; 1873-3581
• DOI: 10.1016/j.tetlet.2017.05.031

Ion association in chloroform solution increases in increasing alkyl chain length (R) in the phosphonium moiety and decreases going from diaryl phosphate (PP) to diaryl thiophosphate (PTP). [Display omitted] •PPs & PTPs behave differently in terms of their NMR behavior in chloroform solutions.•PPs show changes in its 1H, 13C &31P NMR chemical shifts (δ) in IL formation, PTPs do not.•Changes in δ occur near the vicinity of ionic centre indicated possible ion pairing in PPs.•Diffusion measurement by PFGSE-1H DOSY NMR & conductivity corroborated the hypothesis.•Ion association increases with alkyl chain length (R) in the phosphonium moiety of the PPs. Structurally unique ionic liquids phosphonium-phosphate and phosphonium-thiophosphate, having both phosphorus based counter ions, in which the anionic part is represented by di-aryl phosphate or di-aryl thiophosphate and cations been tetraalkylphosphonium groups, behave differently in terms of their NMR behaviour. While phosphonium-phosphates show significant changes in its 1H, 13C and 31P NMR chemical shifts vis. á vis. corresponding chemical shifts for a physical mixture of tetraalkylphosphonium bromide and di-aryl phosphate, phosphonium-thiophosphates behave almost similarly in terms of NMR with their synthetic precursors, hence indicating phosphate-phosphonium interaction has a significant covalent component resembling more to a liquid ion pair while thiophosphate-phosphonium interaction is principally ionic in nature. Translational diffusion behavior studied by PFGSE-NMR experiments and ionic conductivities of these ionic liquids in chloroform solution corroborated the hypothesis. The effect of variable alkyl chain length in phosphonium cation is effectively observed in the extent of ion association. Results of this study may provide insight into the solution state behavior of these ionic liquids, would help to classify those in terms of their strength of ion association and thus potential application thereof.