Sensing and fixation of NO2/N2O4 by calix[4]arenes

An approach toward visual detection and chemical utilization of NO2/N2O4 is proposed, which employs simple calix[4]arenes. Exposure of tetra-O-alkylated calix[4]arenes 1 and 2, possessing either a cone or a 1,3-alternate conformation, to NO2/N2O4, both in chloroform solution and in the solid state, results in deeply colored calixarene-nitrosonium (NO+) complexes. In the presence of a Lewis acid, such as SnCl4, stable calixarene-NO+ complexes 7 and 8 were isolated in a quantitative yield and characterized by UV-vis, FTIR, high-resolution H-1 NMR spectroscopy and elemental analysis. NO+ is found encapsulated within the calixarene cavity, and stable charge-transfer complexes result with K-ass > 10(6) M-1 (CDCl3). The NO+ encapsulation was also demonstrated in titration experiments with calixarenes 1, 2, and 5 and commercially available NO+SbF6- salt in chloroform. The complexation process is reversible, and the complexes dissociate upon addition of water and alcohol, recovering the parent calixarenes. Attachment of functionalized calix[4]arenes to silica gel was demonstrated, which afforded a solid material 15 capable of visual detection and entrapment of NO2/N2O4. Calixarene-NO+ complexes can be utilized for the NO+ transfer processes and nitrosation reactions. The NO+ guest transfer between two calixarene containers 2 and 5 was achieved and studied by UV-vis and H-1 NMR spectroscopy. Chemical fixation of NO2/N2O4 was demonstrated through their quantitative transformation into the calixarene-NO+ complex and its use as a nitrosonium transfer agent in the synthesis of N-nitrosoamides. These results may lead toward novel nitrogen oxides storing materials.