Anion Complexation by an Azocalix[4]arene Derivative and the Scope of Its Fluoride Complex Salt to Capture CO(2) from the Air

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 CO(2) from the air. The recycling of the complex was easily achieved. Final conclusions are given.