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 further corroborates these interactions, showing that while π-π stacking remains significant even in the solid state, the ureido motifs preferably 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.