X-ray crystallography is a powerful technique to monitor chemical reactions, but the nature of the method only enables reactions involving small crystal-to-crystal transformations to be followed, and analysis of reactions occurring in solution is not feasible. Now Kawano and Fujita of the University of Tokyo and their colleagues1 have overcome this shortcoming by creating ‘single-crystalline molecular flasks’. The approach can be used for reactions in solution, thus allowing crystallographic monitoring of their progress.

Fig. 1: The porous coordination network.

The molecular flasks described here are the large pores in materials made up of stacks of molecules (Fig.1). The stacks include layers of molecules that will react, and the large size of these pores ensures that even bulky reagent molecules can reach them. Because the reacting molecules make up part of the crystal, their modification throughout the reaction can be followed with crystallography.

Fujita and colleagues used aromatic compounds to produce the porous material: the planar ring structures stacked well together, which resulted in a robust material. In the structure, the reacting group—such as an amines—points into the pore channel, ensuring maximum exposure to the reagents.

The group carried out the reactions by simply dipping the porous crystals into reagent solutions, allowing the liquid to permeate into the large pores. The structures permitted rapid diffusion of the reagent solutions into the materials, thus the reaction occurred almost as if it were in solution. A wide range of reactions were conducted— such as converting amines to amides or to phenyl urea—when the crystal changed color from red to yellow, after three hours in solution.

“All the reactions we have tried required relatively bulky reagents, which normally cannot diffuse into crystals,” says Fujita. “However, our porous compounds allowed the bulky reagents to diffuse into the pores and react with substrates embedded in the coordination network.”

Future research also looking exciting, “our approach will enable direct stepwise observation of various chemical reactions, including the observation of activated intermediate species in catalytic reactions,” says Fujita.