Oxyanion-Encapsulated Caged Supramolecular Frameworks of a Tris(urea) Receptor: Evidence of Hydroxide- and Fluoride-Ion-Induced Fixation of Atmospheric CO2 as a Trapped CO32– Anion

A tris­(2-aminoethyl)­amine-based tris­(urea) receptor, L, with electron-withdrawing m-nitrophenyl terminals has been established as a potential system that can efficiently capture and fix atmospheric CO2 as air-stable crystals of a CO32–-encapsulated molecular capsule (complex 1), triggered by the presence of n-tetrabutylammonium hydroxide/fluoride in a dimethyl sulfoxide solution of L. Additionally, L in the presence of excess HSO4– has been found to encapsulate a divalent sulfate anion (SO42–) within a dimeric capsular assembly of the receptor (complex 2) via hydrogen-bonding-activated proton transfer between the free and bound HSO4– anions. Crystallographic results show proof of oxyanion encapsulation within the centrosymmetric cage of L via multiple N–H···O hydrogen bonds to the six urea functions of two inversion-symmetric molecules. The solution-state binding and encapsulation of oxyanions by N–H···O hydrogen bonding has also been confirmed by quantitative 1H NMR titration experiments, 2D NOESY NMR experiments, and Fourier transform IR analyses of the isolated crystals of the complexes that show huge spectral changes relative to the free receptor.