In this article, the highly efficient formation of a series of recently discovered aromatic oligoamide macrocycles consisting of six meta-linked residues is first discussed. The macrocycles, with their backbones rigidified by three-center hydrogen bonds, were found to form in high yields that deviate dramatically from the theoretically allowed value obtained from kinetic simulation of a typical kinetically controlled macrocyclization reaction. The folding of the uncyclized six-residue oligomeric precursors, which belong to a class of backbone-rigidified oligoamides that have been demonstrated by us to adopt well-defined crescent conformations, plays a critical role in the observed high efficiency. Out of two possible mechanisms, one is consistent with experimental results obtained from the coupling of crescent oligoamides of different lengths, which suggests a remote steric effect that discourages the formation of oligomers having lengths longer than the backbone of the six-residue precursors. The suggested mechanism is supported by the efficient formation of very large aromatic oligoamide macrocycles consisting of alternating meta- and para-linked residues. These large macrocycles, having H-bond-rigidified backbones and large internal lumens, are formed in high (>80%) yields on the basis of one-step, multicomponent macrocyclization reactions. The condensation of monomeric meta-diamines and a para-diacid chloride leads to the efficient formation of macrocycles with 14, 16, and 18 residues, corresponding to 70-, 80-, and 90-membered rings that contain internal cavities of 2.2, 2.5, and 2.9 nm across. In addition, the condensation between trimeric or pentameric diamines and a monomeric diacid chloride had resulted in the selective formation of single macrocyclic products with 16 or 18 residues. The efficient formation of the macrocycles, along with the absence of other noncyclic oligomeric and polymeric byproducts, is in sharp contrast to the poor yields associated with most kinetically controlled macrocyclization reactions. This system represents a rare example of highly efficient kinetic macrocyclization reactions involving large numbers of reacting units, which provides very large, shape-persistent macrocycles.

Aromatic oligoamide macrocycles exhibit strong preference for highly directional association. Aggregation happens in both nonpolar and polar solvents but is weakened as solvent polarity increases. The strong, directional assembly is rationalized by the cooperative action of dipole-dipole and π-π stacking interactions, leading to long nanotubular assemblies that are confirmed by SEM, TEM, AFM, and XRD. The persistent nanotubular assemblies contain non-collapsible hydrophilic internal pores that mediate highly efficient ion transport observed with these macrocycles and serve as cylindrical sites for accommodating guests such as metal ions.

A new class of pillar [5]arene-based phosphine oxides tethered with ten chelating groups on both rims of the pillar were synthesized via two steps from macrocyclization of 1,4-bis(bromoalkoxy)benzenes with paraformaldehyde, followed by Arbusov reaction with iso-propoxydiphenylphosphine. Solvent extraction of these ligands towards selected lanthanides and actinides was investigated under acidic condition.Compared with acyclic monovalent analogs and classical extractant tri-n-octylphosphine oxide (TOPO), the remarkable efficiency and selectivity for thorium(IV) and uranyl(VI) as observed in these novel extractants bearing a varying spacer length revealed the significance of preorganization of chelating groups on pillararene platform. Log-log plot analysis indicated the 1 : 1 stoichiometry (ligand/metal) for the extracted complex formed between the ligand and thorium(IV) or uranyl(VI). The extraction efficiency was considerably improved with increasing acidity in a range of 0.1-1.5 M HNO 3 , a result that is distinct from the extraction behaviour of calixarene-based phosphine oxides. The extractability increased with enhancing NaNO 3 concentration and high salinity assisted the preferential extraction of U(VI) over Th(IV). These ligands also showed moderate efficiency in differentiating europium(III) and americium(III) at 1 M HNO 3 in the presence of a synergist (Br 6 -COSAN).