Properties of Protic Ionic Liquids Comprised of N‐Alkyldiethylenetriamine and Their Complexation of Copper(II) Ions

Protic ionic liquids (PILs) comprised of alkylpolyaminium cations, such as monoprotonated N‐hexyldiethylenetriaminium (HHexdien+) and N‐(2‐ethylhexyl)diethylenetriaminium (HEthexdien+), coupled with bis(trifluoromethylsulfonyl)amide (Tf2N–) and trifluoroacetate (TFA–) were prepared, and their properties and interactions with copper(II) ions were studied using differential scanning calorimetry (DSC), visible absorption spectra, and 15N NMR and 13C NMR spectroscopy. The alkyldiethylenetriaminium (triamine) PILs assume a liquid state at lower temperatures than the corresponding alkylethylenediaminium (diamine) PILs, and they exhibit a glass transition without crystallization for all of the systems studied. The protonation of the alkyltriamines predominantly occurs at the N(1) and N(3) amine moieties and depends on the counteranions (Tf2N– or TFA–) and the 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 was deduced based on the visible absorption spectra for the isolated copper(II)–alkyldiethylenetriamine complexes and for Cu(ClO4)2 in molecular solvents. The lifetime of the copper(II)–triamine complex formed in the PIL was estimated to be ca. 10–2 s at ambient temperature using the 13C NMR spectroscopic paramagnetic relaxation times. A monoprotic ionic liquid comprised of N‐hexyldiethylenetriamine strongly encapsulates copper(II) ions, with the lifetime (τM = 10–2 s) of the copper(II) complex in the protic ionic liquid being appreciably longer than those in molecular solvents.