Ligand Side-Chains Control the Formation of an M ₈ L ₄ Molecular Barrel: Unveiling Selective Encapsulation and Sequential Separation Properties

Inspired by the importance of steric factors in biomolecular self-assembly, we investigated their role in the exclusive coordination-driven self-assembly of a unique cationic M8L4 barrel (MB1) using a sterically reinforced ligand (L). Specifically, we employed a flexible tetrapyridylbiphenyl ligand (L) functionalized with ethoxy groups to assess their influence on the self-assembly process with cis-Pd(en)(NO3)2. Single-crystal structural analysis of MB1 revealed that four ethoxy groups are oriented outward, while the remaining four project into the barrel cavity, indicating controlled conformational flexibility around the Cphenyl–Cpyridyl bond during assembly. MB1, featuring a sterically crowded cavity, is highly selective in aqueous solution for the encapsulation of hazardous polycyclic aromatic hydrocarbons (PAHs), particularly naphthalene and phenanthrene. The higher binding strength of MB1 toward naphthalene (K ≈ 7.3 × 102 M–1) and phenanthrene (K ≈ 3.8 × 102 M–1) enables their isolation from a mixture of PAHs using MB1 as a molecular separation flask. In contrast, other M8L4 barrels featuring wider cavities and larger open windows proved inefficient for such separation. MB1 therefore enables the challenging sequential aqueous extraction of naphthalene and phenanthrene from PAH mixtures with full reusability.