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Design and synthesis of soft crystals using mechanically interlocked molecules


Organic synthesis, Crystallography, Metal-organic framework, Molecular machine


A variety of mechanically interlocked structures have been found in nature, of which the geometries and large potential to exert unique functions are of major scientific interest. Unlike covalently constructed molecules, mechanically interlocked molecules (MIMs) such as catenanes bear spatially restricted freedom of motion, and a wide variety of synthetic molecular machines and switches based on MIMs have been developed so far.

Since the first success in the synthesis of polycatenane in the 1970s, such mechanically interlocked polymers have attracted particular attention as their densely populated mechanical bonds are expected to move coherently along the main chains, providing anomalous mechanical properties.

We envisaged that if the backbone of a metal–organic crystal were composed of a catenane structure, such a porous crystal might exhibit rubber-like elastic properties. Currently we are synthesizing a variety of catenanes and trying to construct porous crystals with them.


• Offer an insight into how mechanical bond can be organized and manipulated collectively.
• Can produce new multi-dimensional structures which are aesthetically appealing.
• Can offer a general strategy for the rational design of elastic, porous single crystal.



Our prototype of a porous metal-organic crystal composed of a catenane-based backbone was constructed using a catenane ligand with four carboxylic acids. Green crystals were obtained by heating the ligands and cobalt ions in N,N-dimethylformamide (DMF).

Using single-crystal X-ray diffraction, it was found that the deprotonated carboxyl groups of catenanes are linked by cobalt ions to form a linear chain structure, and these chains are linked vertically and horizontally by the mechanical bond of catenanes. It was also revealed from various X-ray diffraction analysis, this porous crystal can dynamically change its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range.

The mechanical properties was examined by nano-indentation. It was found that the material deformed easily when pressed mechanically—and that its Young's modulus (E), an index of the ease with which it deforms, is comparable to that of polypropylene. Surprisingly, the crystal returned to its original shape, without damage, upon removal of the force. Furthermore, when isostatically compressed, the crystal is compressed most in a specific direction. The crystal structure obtained at a high pressure showed the deformable nature of the crystal comes from the slipping motion between the rings of the catenane molecules.


In the future, by devising the design of ligands with mechanical bonds, we aim to understand how the structure and properties of ligands affect the properties of crystals, and develop soft porous crystals with responsive properties.