Properties of protic ionic liquids comprising N-alkyl-diethylenetriamine and their complexation of copper(II) ion

Protic ionic liquids (PILs) comprising alkyl polyaminium cations such as monoprotonated N-hexyldiethylenetriaminium (HHexdien+) and N-2-ethylhexyldiethylenetriaminium (HEthexdien+) coupled with bis(trifluoromethanesulfonyl)amide (Tf₂N¯) and trifluoroacetate (TFA¯) were prepared, and their properties and interactions with copper(II) ion were studied using differential scanning calorimetry (DSC), visible absorption spectra, and ¹⁵N and ¹³C NMR spectroscopy. The alkyl diethylenetriaminium (triamine-) PILs assume a liquid state at lower temperatures than the corresponding alkyl ethylenediaminium (diamine-) PILs and exhibit a glass-transition without crystallization for all the systems. The protonation of the alkyl triamines predominantly occurs at the N(1) and N(3) amine moieties and depends on the counter anions (Tf₂N¯or TFA¯) and alkyl moiety (hexyl or ethylhexyl). The polarity parameters such as ET(30)(or ETN),π, αandβwere determined for the triamine-PILs as well as for the diamine-PILs to examine the amine effect. The coordination structure of the copper(II) ion in the present chelate-amine PILs deduced based on the visible absorption spectra for the isolated copper(II) alkyl-triamine complexes and for Cu(ClO4)2 in molecular solvents. The life-time of the copper(II)-triamine complex formed in the PIL was estimated to be 10¯² s at ambient temperature using the 13C NMR paramagnetic relaxation times.

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