Supramolecular transformations within coordination-assembled architectures have attracted great attention over the past two decades because fundamental biomolecules are found to be capable of achieving biological functions through conformational changes.

Well-controlled interconversion between topologically complex superstructures may lay the foundation for achieving a variety of functions within a switchable system. The size, shape and nature properties between interconvertible cage-shaped molecules have been altered accompanied by the structural interconversions with the output of structurally dependent properties and functions.

In a study published in J. Am. Chem. Soc., the research group led by Prof. Sun Qingfu from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported the controlled self-assembly of three topologically-unprecedented conjoined twin-cages, i.e. one stapled interlocked Pd₁₂L₆ cage 2 and two helically-isomeric Pd₆L₃ cages 3 and 4, made from the same cis-blocked palladium corners and a new bis-bidentate ligand.

The researchers provided reliable evidence for these unique conjoined twin-cage structures by X-ray crystallographic analysis.

Cage 2 features three mechanically-coupled cavities, while cages 3 and 4 are topologically-isomeric helicate-based twin-cages based on the same metal/ligand stoichiometry. Cage 2 shows a S₆ molecular symmetry, containing six ligands, with each ligand from the left interlocking with the other two ligands for the right and vice versa. The structure of 3 has a C₂ symmetry, where two pyridinium ligands intertwines with each other, and the third one is independent. While cage 4 possesses a higher D₃ symmetry with three ligands arranged in a triple helicate conformation. These structures are in good agreement with the nuclear magnetic Resonance (NMR) analysis.

Besides, the researchers realized the controllable structural conversions by fine-tuning experimental conditions, including solvents, temperature, anions, and guest molecules, and controlled the sole formation of cage 2 or a dynamic mixture of cage 3 and 4 by changing the solvents employed during the self-assembly.

Once acetone was added into the self-assembly, a mixture of cages 3 and 4 was formed. BF₄^- anion plays an important role in the structure conversion from cages 3 and 4 to cage 2, which is favorable due to enthalpic benefits from the host-guest interaction. Structural conversions between cages 3 and 4 can been both triggered by changes in temperature/solvent and induce-fit guest encapsulations.

Through variable-temperature NMR spectra, the researchers calculated thermodynamic parameters based on the van’t Hoff equation. The results showed that the system favors the transformation from cage 3 to cage 4 at higher temperature.

Interestingly, induced-fit guest encapsulations triggered the structural conversion from 3 to 4 completely. Addition of 1-adamantanecarboxylic acid or 1-adamantanol to the mixture of cage 3 and 4 resulted in the distinct change in ¹H NMR spectra, where a simple set of host signals of the inclusion complex of cage 4 was observed finally.

The researchers obtained the X-ray structure of the host-guest complex, which shows the encapsulation of eight 1-adamantanol molecules in the two pockets of cage 4. They inferred that the strong host-guest interactions are the origin of induced-fit structural conversion from cage 3 to 4.

In this study, stimuli-responsive structural transformations between discrete coordination supramolecular architectures provide a versatile molecular-level platform to mimic the biological transformation process.

Schematic illustration of the research (Image by Prof. SUN’s group)