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. 2018 Mar 30;7(3):269-274.
doi: 10.1002/open.201800014. eCollection 2018 Mar.

Direct Detection of the Ion Pair to Free Ions Transformation upon Complexation with an Ion Receptor in Non-Polar Solvents by using Conductometry

Kazuya Iseda  1 , Kenta Kokado  1 , Kazuki Sada  1
Affiliations

Direct Detection of the Ion Pair to Free Ions Transformation upon Complexation with an Ion Receptor in Non-Polar Solvents by using Conductometry

Kazuya Iseda et al. ChemistryOpen. .

Abstract

In this study, we performed conductometry in various organic solvents to directly detect the transformation from tetrabutylammonium chloride (TBACl) ion-pair salt to the free ions through complexation with meso-octamethylcalix[4]pyrrole (CP), which is a well-known receptor for chloride anions. In the presence of CP, the conductivity of TBACl increases in various non-polar solvents, indicating that complexation with CP enhances the ionic dissociation of TBACl in such non-polar solvents. In other words, CP recognizes chloride as an ion-paired salt as well as a free anion in non-polar solvents. Additionally, the TBA(CP-Cl) complex exhibited a considerably lower ion-pairing constant (Kip) than TBACl in non-polar solvents, resulting in enhanced conductivity. Based on these findings, we can conclude that complexation of an anion with a hydrophobic anion receptor will be useful for creating functional and stimuli-responsive soft materials in organic solvents using coulombic forces.

Keywords: anions; host–guest systems; molecular recognition; receptors; supramolecular chemistry.

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Figures

Figure 1
Figure 1
Complexation and ion‐pairing equilibria of ion receptor (CP) and ionic guest (TBACl) in organic solvents: complexation of CP with ion‐paired TBACl [Eq. (2)], complexation of CP with the free chloride anion [Eq. (3)], ion pairing between the TBA cation and chloride anion [Eq. (4)], and ion pairing between the TBA cation and CP–Cl complex anion [Eq. (5)].
Figure 2
Figure 2
Conductometric titration curves of a) 0.001 mm, b) 0.01 mm, c) 0.1 mm, and d) 1 mm TBACl solution with CP addition (in THF, at 298.15 K).
Figure 3
Figure 3
Molar conductivity (Λ m) of TBACl with various amount of CP (in THF, at 298.15 K).
Figure 4
Figure 4
1H NMR signal shift of the CP β‐proton with addition of TBACl (5.0 mm) in [D8]THF.
Figure 5
Figure 5
Conductometric titration curves of TBACl (0.1 mm) with CP addition at 298.15 K in a) CHCl3 (ϵ=4.8), b) EtOAc (ϵ=6.0), c) 2‐MeTHF (ϵ=7.0), d) 1,2‐dichloroethane (1,2‐DCE, ϵ=10.4), e) acetone (ϵ=20.6), and a summary of the conductometric titration curves.
Figure 6
Figure 6
Conductivity change of TBACl (0.1 mm) with CP addition in THF (squares) and acetone/EtOAc (v/v=0.11:0.89) (diamonds).
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