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. 2023 Aug 12;28(16):6029.
doi: 10.3390/molecules28166029.

Anion Complexation by an Azocalix[4]arene Derivative and the Scope of Its Fluoride Complex Salt to Capture CO2 from the Air

Angela F Danil de Namor  1 , Nawal Al Hakawati  2
Affiliations

Anion Complexation by an Azocalix[4]arene Derivative and the Scope of Its Fluoride Complex Salt to Capture CO2 from the Air

Angela F Danil de Namor et al. Molecules. .

Abstract

A newly synthesized upper rim azocalix[4]arene, namely 5,11,17,23-tetra[(4-ethylacetoxyphenyl) (azo)]calix[4]arene, CA-AZ has been fully characterized, and its chromogenic and selective properties for anions are reported. Among univalent anions, the receptor is selective for the fluoride anion, and its mode of interaction in solution is discussed. The kinetics of the complexation process were found to be very fast as reflected in the immediate colour change observed with a naked eye resulting from the receptor-anion interaction. An emphasis is made about the relevance in selecting a solvent in which the formulation of the process is representative of the events taking place in the solution. The composition of the fluoride complex investigated using UV/VIS spectrophotometry, conductance measurements and titration calorimetry was 1:1, and the thermodynamics of complexation of anions and CA-AZ in DMSO were determined. The fluoride complex salt was isolated, and a detailed investigation was carried out to assess its ability to remove CO2 from the air. The recycling of the complex was easily achieved. Final conclusions are given.

Keywords: azocalix[4]arene; carbon dioxide capture; fluoride complex salt.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Infrared spectrum of CA-AZ displaying the most important bands of the ligand.
Figure 2
Figure 2
The TG curve of CA-AZ.
Figure 3
Figure 3
Colour changes of CA-AZ resulting from anion interactions in DMSO.
Figure 4
Figure 4
Spectral changes of para-ester diazophenylcalix[4]arene (1 × 10−5 mol·dm−3) with different anions.
Figure 5
Figure 5
1H NMR spectral changes of CA-AZ titrated with an increased concentration of 18 × 10−4 mol·dm−3 of F− at 298 K. Changes in the characteristic peaks of CA-AZ as a result of F- addition are marked with red dots.
Figure 6
Figure 6
Plot of absorbance at 530 nm as a function of [CA-AZ]/[F−] resulting from the UV-VIS titration of CA-AZ with F− in DMSO.
Figure 7
Figure 7
Conductometric titration of CA-AZ with F− in DMSO at 298.15 K.
Figure 8
Figure 8
Morphological micrographs of azocalix[4]arene (CA-AZ) and azocalix[4]arene loaded with fluoride ion (red arrows) with corresponding EDX spectra showing the elemental composition. ×5000 original magnification, AI peak corresponds to aluminium stub used for samples’ mounting.
Figure 9
Figure 9
FTIR spectra of (A), azocalix[4]arene and (B), azocalix[4]arene loaded with TBAF.
Figure 10
Figure 10
FTIR spectra of the complex salt and complex salt saturated with carbon dioxide.
Figure 11
Figure 11
Plot showing of percentage of carbon dioxide extraction by the complex versus time (s).
Figure 12
Figure 12
Recycled solid complex salt placed under vacuum (a); complex salt initially exposed to CO2 (b); complex salt fully exposed to CO2 (c).
Scheme 1
Scheme 1
Synthetic procedure used for the preparation of CA-AZ.
See this image and copyright information in PMC

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