Caught By A Crystal

An elusive hemiaminal intermediate—in which a hydroxyl and an amine are attached to the same carbon—has been observed by X-ray crystallography, thanks to a porous crystalline material that stabilizes the transient species (Nature 2009, 461, 633). The researchers who developed the crystalline trapping strategy say it could help chemists take snapshots of other fleeting intermediates.

Although scientists had previously glimpsed short-lived hemiaminals via X-ray analysis, they had to do so by using exceptional experimental methods, such as creating a mutant enzyme that traps the intermediate via kinetic stabilization.

To build a simpler trap for the hemiaminal, Makoto Fujita, Masaki Kawano, and colleagues at Japan's University of Tokyo employed porous crystalline coordination networks. They reasoned these "crystalline molecular flasks," as Fujita describes them, could sequester a substrate and allow it to undergo a reaction without decomposing the surrounding crystal.

Seth M. Cohen, a chemistry professor at the University of California, San Diego, likens the porous networks to climbing frames—organic ligands form the bars, and metal ions play the role of the rivets that hold the structure together. "The result is a grand structure that consists largely of empty space," Cohen writes in a commentary that accompanies the paper. "Just as children play within the lattice of a climbing frame, so molecules can diffuse and interact within the lattice of a coordination network."

The Tokyo researchers observed the hemiaminal by carrying out the reaction of an aldehyde and an amine at low temperature in an X-ray diffractometer. This way, X-ray "snapshots" could be taken at each stage. They first embedded 1-aminotriphenylene within the crystalline network of zinc ions and triazine derivatives. Next, acetaldehyde was diffused through the network to form the hemiaminal, which was kinetically trapped. Finally, raising the temperature allowed the reaction to proceed to its Schiff base product.

"We believe that porous coordination networks and the approach illustrated here offer a general method for acquiring sequential X-ray snapshots of chemical reactions, which will provide valuable direct mechanistic insights not obtainable by other means," Fujita says. The group plans to use the technique to explore organometallic transformations with unclear reaction mechanisms, as well as highly explosive and toxic substances that have not been well studied.

"This is a breakthrough, especially if it can be extended to other reactions that are currently under debate," comments J. N. H. (Joost) Reek, a supramolecular chemistry expert at the University of Amsterdam. "I can imagine that especially the area of transition metal catalysis would benefit from this if the strategy is amenable for transition metal complexes," he tells C&EN.