Confining reactants inside a porous coordination polymer allows unstable intermediates along a reaction path to be studied by a method usually reserved for stable crystalline compounds.
By their very nature, determining the structure of transient intermediates is difficult, and usually requires the use of a variety of spectroscopic techniques at low temperature. X-ray crystallography is the gold standard for structural analysis, but the requirements for obtaining a crystal structure and for observing a highly reactive intermediate would usually be regarded as mutually exclusive.
Now, Makoto Fujita and co-workers from the University of Tokyo and the Japan Science and Technology Agency have shown1 that conducting an imine formation inside the pores of a coordination polymer can stabilize the transient hemiaminal intermediate sufficiently to allow an X-ray crystal structure to be obtained (pictured). The porous coordination polymer is formed from a flat, aromatic trivalent ligand — trispyridyltriazine (light grey and blue) — which coordinates to zinc. The network formed is an ideal host for a large flat aromatic amine — aminotriphenylene (green) — so that the amino group is presented in the open pores.
When a single crystal is mounted on the diffractometer and then exposed to acetaldehyde, the amine reacts with the aldehyde inside the pores. Cooling the crystal to 90 K is sufficient to 'pause' the reaction so that an X-ray crystal structure of the intermediate can be obtained. This approach could offer a general method to acquire snapshots of chemical reactions that cannot be monitored by other means.
References
Kawamichi, T., Haneda, T., Kawano, M. & Fujita, M. X-ray observation of a transient hemiaminal trapped in a porous network. Nature 461, 633–635 (2009).
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Davey, S. Transient trapped. Nature Chem (2009). https://doi.org/10.1038/nchem.420
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DOI: https://doi.org/10.1038/nchem.420
