Self-Assembled Multi-Component Catenanes: Structural Insights into an Adaptable Class of Molecular Receptors and [2]-Catenanes

Under acidic conditions (50 equiv of TFA), combinations of hydrazide A-B monomers self-assemble into octameric [2]-catenanes with high selectivity for [1(3)2](2), where 1 is a D-Pro-X (X = Aib, Ac(4)c, Ac(6)c, L-4-Cl-PhGly)-derived monomer and 2 is an L-Pro'-L-arylGly (Pro' = Pro, trans-F-Pro, trans-HO-Pro, aryl = naphthyl, phenyl)-derived monomer. Five different combinations of monomers were studied by X-ray crystallography. In each case, the unique aryl glycine unit is located in the core of the structure where the aryl ring templates a CHI-pi-CH sandwich. Analysis of metrical parameters indicates that this core region is highly conserved, while the more peripheral zones are flexible. H-1 NMR spectroscopy indicate that the solid-state structures are largely retained in solution, though several non-C-2-symmetric compounds have a net C-2-symmetry that indicates accessible dynamic processes. Catenane dynamic processes were additionally probed through H/D exchange, with the core being inflexible relative to the peripheral structure. Mass spectrometry was utilized to identify the constitutional isomerism in the minor asymmetric [1(5)2(3)] catenanes.