A novel series of giant cobalt-calixarene macrocycles: ring-expansion and modulation of pore apertures through recrystallization

Abstract

The design and synthesis of metallomacrocycles can be quite challenging because the assemblies of such molecular cycles are difficult to control and the products are usually unpredictable. In this work, a novel series of metallomacrocycles, denoted as {Co₃₀-A}, {Co₃₀-B} and {Co₃₂-A} have been synthesized via self-assembly of p-tert-butylthiacalix[4]arene (H₄TC4A) and 3,5-pyrazoledicarboxylic acid (H₃pdc) with Co₂⁺ ions under solvothermal conditions. Recrystallization of {Co₃₂-A} under different conditions was found to form {Co₃₂-B} and {Co₃₂-C} that have a similar ring structure to that of {Co₃₂-A} but have different molecular packing modes in the lattices, as well as a 40-membered ring {Co₄₀}. These complexes represent the highest-nuclearity metallocalixarene coordination wheels reported to date. Crystallographic studies indicate that all these metallomacrocycles feature wheel-like structures with apertures varing from 11.4 to 20.3 Å. It is noteworthy that {Co₃₂-A} exhibited good efficiency in removing RhB even at low initial concentration (10 ppm) and also excellent adsorption selectivity towards RhB over Na₂Fl (RhB = Rhodamine B, Na₂Fl = disodium fluorescein). This work not only makes a breakthrough in the synthesis of metallocalixarene macrocycles with high nuclearity and large apertures, but also provides a simple recrystallization approach to realize the ring-expansion and regulation of molecular packing modes of the metallomacrocycles.