Self-aggregation and host–guest behavior of tetraureido-substituted tetranitro-azacalix[4]arenes

Abstract

The reaction of tetranitro-tetraminoazacalix[4]arene with a series of aromatic isocyanates leads to the formation of tetraureido-receptors, whose isolation was efficiently achieved (∼80% yield) by organic solvent nanofiltration when conventional chromatographic methods proved ineffective. The receptors exhibit a pronounced tendency to self-aggregate in solution, and the extent and stability of the formed assemblies are strongly modulated by the nature of the aryl substituents. A combined experimental (dilution NMR, DOSY) and theoretical investigation revealed that dimer formation arises from a cooperative interplay between π–π interactions and hydrogen bonding, with aromatic stacking providing the dominant stabilizing contribution. Molecular dynamics simulations highlight significant differences in the stability of selected dimers, rationalizing their distinct aggregation tendencies. X-ray analysis reveals that while π–π stacking remains significant even in the solid state, the ureido motifs preferentially coordinate solvent molecules. Importantly, receptor self-association does not prevent anion recognition. Even in the strongly hydrogen-bond-competitive solvent (DMSO), the receptors efficiently bind anions, exhibiting a selectivity sequence H₂PO₄⁻ > BzO⁻ > AcO⁻ ≫ Cl⁻, that cannot be rationalized solely by anion basicity. These findings demonstrate that tetraureido tetranitro-azacalix[4]arene architectures represent substitution-tunable supramolecular systems that combine controlled self-assembly with robust anion complexation ability.