Long Synthetic Nanotubes from Calix[4]arenes

Abstract

We report the synthesis and encapsulation properties of long (up to 5 nm) molecular nanotubes 1-4, which are based on calix[4]arenes and can be filled with multiple nitrosonium (NO(+)) ions upon reaction with NO(2)/N(2)O(4) gases. These are among the largest nanoscale molecular containers prepared to date and can entrap up to five guests. The structure and properties of tubular complexes 1(NO(+))(2)-4(NO(+))(5) were studied by UV/Vis, FTIR, and (1)H NMR spectroscopy in solution, and also by molecular modeling. Entrapment of NO(+) in 1(NO(+))(2)-4(NO(+))(5) is reversible, and addition of [18]crown-6 quickly recovers starting tubes 1-4. The FTIR and titration data revealed enhanced binding of NO(+) in longer tubes, which may be due to cooperativity. The described nanotubes may serve as materials for storing and converting NO(x) and also offer a promise to further develop supramolecular chemistry of molecular containers. These findings also open wider perspectives towards applications of synthetic nanotubes as alternatives to carbon nanotubes.

Trimeric and hexameric calixarene-based capsules

“…37 Barbiturate-based tetrarosettes 27 3 · B 12 i.e. 28 are spontaneously formed in apolar solvents such as benzene, toluene or chloroform when 27 is mixed with twelve equivalents of B at room temperature.…”

“…Cyananurate based tetrarosettes 37 3 · C 12 have been obtained by two methods: by mixing 37 and C derivatives in toluene and heating to 100 °C for one week (direct method) or by treatment of 37 with B and subsequent exchange by C derivatives (exchange method).…”

“…Tetrarosette 28a· B 12 undergoes complexation with six molecules of alizarin 20 in the form of two trimers which are hydrogen bonded; they are intercalated inside both double rosettes to give inclusion complex 29. 39 Upon encapsulation large conformational changes in the host structure occur.…”

“…Calixarene-based capsules of higher multiplicity are mainly trimeric and hexameric species, albeit tetrameric and octameric compounds are also known. 10,11 An attention is paid now to nanotubes, 12,13 this theme however is not described here.…”

Dmitry M. Rudkevich: A Tribute Seven Years after His Premature Passing

“…He successfully applied supramolecular chemistry for the molecular recognition of gases, where he was among the pioneers. He was the first to successfully introduce calixarene-based materials, including solid-supported calixarenes [28] and calixarene nanotubes, [29][30][31][32][33] for the chemical fixation [34,35] of NO 2 =N 2 O 4 , a major component of NO x gases. These aggressive environmental pollutants were converted into stable nitrosonium complexes, which were utilized further for controlled NO þ release (so-called ''encapsulated nitrosation reagents'' [36,37] ) or direct NO-gas release.…”