Considerable efforts have been made to increase the topological complexity of mechanically interlocked molecules over the years. Three‐dimensional catenated structures composed of two or several (usually symmetrical) cages are one representative example. However, owing to the lack of an efficient universal synthetic strategy, interlocked structures made up of dissymmetric cages are relatively rare. Since the space volume of the inner cavity of an interlocked structure is smaller than that outside it, we developed a novel synthetic approach with the voluminous reductant NaBH(OAc)3 that discriminates this space difference, and therefore selectively reduces the outer surface of a catenated dimer composed of two symmetric cages, thus yielding the corresponding catenane with dissymmetric cages. Insight into the template effect that facilitates the catenation of cages was provided by computational and experimental techniques.
Constructing hierarchical superstructures to achieve comparable complexity and functions to proteins with four-level hierarchy is challenging, which relies on the elaboration of novel building blocks with complex structures. We present a series of catenated cages with unique structural complexity and tailorability. The rational design was realized as such: A catenane of two symmetric cages (CSC), CSC-1, with all rigid imine panels was converted to a catenane of two dissymmetric cages (CDC), CDC-1, with two exterior flexible amine panels, and CDC-5 was tailored from CDC-1 by introducing an additional methyl group on each blade to increase lateral hindrance. CDC-1s with the most irregular and flexible configuration formed supramolecular dimers, which self-organized into 3D continuous wavelike plank with a three-level hierarchy, previously undiscovered by conventional building blocks. A drastically different 3D triclinic crystalline phase with a four-level hierarchy and trigonal phase with a three-level hierarchy were constructed of distorted CSC-1s and the most symmetric CDC-5s, respectively. The wavelike plank exhibited the lowest order, and the triclinic phase had a lower order than the trigonal phase which had the highest order. It correlates with the configuration of the primary structures, namely, the most disordered shape of CDC-1, the low-order configuration of CSC-1, and the most ordered geometry of CDC-5. The catenated cages with subtle structural differences therefore provide a promising platform for the search of emerging hierarchical superstructures that might be applied to proton conductivity, ferroelectricity, and catalysis.
Catenated cages are generally considered thermodynamically more stable than their constituent monomeric cages. However, the catenation mechanism is yet to be elucidated; it would require systematic investigation into the structural effects of the building blocks, their enthalpic and entropic contributions, and the effect of solvents. By inspecting these factors, we rationalized some design principles for the efficient construction of catenated cages. Our study revealed that a steric hindrance linker and a rigid panel led to the formation of an enthalpy-favored encapsulated intermediate before catenation occurred. The stability of this enthalpic intermediate was crucial for cage catenation, as the reactions were otherwise outcompeted by an entropy-favored intermediate. The formation of the latter was facilitated significantly by a flexible panel and solvent molecules that stably resided within the monomeric cage. This study provides a guideline for the elaboration of catenated cages with more sophisticated topologies, which could be extended to other complex supramolecular assemblies.
An amphiphilic organic cage was synthesized and used as selfassembly synthon for the fabrication of novel functional supramolecular structures in solution. The transmission electron microscopy (TEM) results showed that this amphiphilic cage self-assembled in aqueous solution into unilamellar nanotubes with a diameter of 29 � 4 nm at a concentration of 0.05 mg mL À 1 . Interestingly, the self-assembly of this cage significantly enhanced the anion-π interactions as indicated by a remarkable increasement of association constant (K a ) between Cl À and this amphiphilic cage after self-assembly. In specific, K a was increased from 223 M À 1 for discrete cages in methanol to 6800 M À 1 for aggregated cages after self-assembly in water at the same concentration of 2.26 × 10 À 5 M. A mechanism based on a synergistic effect was proposed in order to explain this self-assembly process through enhanced anion-π interactions.
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