CO2 Capture by Multivalent Amino-Functionalized Calix[4]arenes: Self-Assembly, Absorption, and QCM Detection Studies

The reactivity of CO2 with polyamino substrates based on calix[4]arenes and on a difunctional, noncyclic model has been studied. All the compounds react with CO2 in chloroform to form ammonium carbamate salts. However, the number, topology, and conformational features of the amino-functionalized arms present on the multivalent scaffold have a remarkable influence on the reaction efficiency and on the product composition. Tetraaminocalix[4]arenes 1–3 rapidly and efficiently react with 2 equiv of CO2, yielding highly stable hydrogen-bonded dimers formed by the self-assembly of two bis-ammonium bis-carbamate intramolecular salts. 1,3-Diaminocalix[4]arene 4 absorbs 1 mol of CO2, affording less stable zwitterionic ammonium carbamates. Gemini compound 5 reacts with CO2 in a 1:1 stoichiometry, forming hydrogen-bonded dimers of ammonium carbamate derivatives of moderate stability. For upper rim 1,3-diaminocalix[4]arene 6, in addition to the labile intramolecular salt, the presence of a self-assembled polymer was also detected. These systems were fully characterized in solution by 1H and 13C NMR spectroscopy, whereas the corresponding gas–solid reactions were further investigated by QCM measurements. Interestingly, the high affinity and reversibility of CO2 uptake shown by 1,3-diamino calix[4]arene 4 enabled us to attain a promising QCM device for carbon dioxide sensing.