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Controlling Flammability

Cage compound sequesters white phosphorus safely

by Stu Borman
June 29, 2009 | A version of this story appeared in Volume 87, Issue 26

Credit: © 2009 Science
Crystal and line structures of tetrahedral host with tetrahedral P4 guest inside. Only one of six organic groups that form the cage is shown in the line structure.
Credit: © 2009 Science
Crystal and line structures of tetrahedral host with tetrahedral P4 guest inside. Only one of six organic groups that form the cage is shown in the line structure.

An iron-based cage compound can take up white phosphorus (P4), keeping the highly flammable substance from reacting with oxygen and burning uncontrollably, researchers report (Science 2009, 324, 1697). The cage complex prevents combustion and also makes it possible to release P4 at will.

The approach could be useful for storing P4 safely, controlling its release for chemical reactions, or remediating spills of the toxic substance. P4 has been kept safe by storing it under water or enclosing it in a metal casing or glass ampule, but never before by chemical complexation. Similar host-guest chemistry might also prove useful for sequestering other dangerous substances.

The tetrahedral metal-organic cage self-assembles in water from commercially available organic compounds and iron(II). It was created by Jonathan R. Nitschke of the University of Cambridge; Kari Rissanen of the University of Jyväskylä, Finland; and coworkers, who found that it hosted compounds like cyclohexane and cyclopentane (Angew. Chem. Int. Ed. 2008, 47, 8297).

"We looked around to see what else we might be able to put in there, and we noticed that the volume of P4 is about the same as that of cyclohexane," Nitschke says. "We gave it a try, and it turned out to be a very good guest."

The caged P4 doesn't decompose, even after contact with the atmosphere for over four months. The guest is stable because the reaction of O2 with P4 would create an intermediate product too big to fit into the host molecule's cavity.

P4 can be removed from the host by adding a competing guest such as benzene, which displaces P4. And the cage is recyclable: A vacuum can be used to empty it of some guests.

The cage compound could be used not only to clean up and transport P4 but also to control its introduction into reaction systems. In addition, "We are currently preparing bigger cages that are hopefully going to be able to trap nerve agents," Nitschke says.

"It's very unusual to be able to stabilize highly reactive compounds like P4 in this way," comments J. N. H. (Joost) Reek, who specializes in supramolecular chemistry and catalysis at the University of Amsterdam. A potential application would be regulating the reactivity of P4 so it could be converted into useful phosphorus compounds, such as phosphine, he says.

Stabilizing highly reactive molecules by confining them in a cage is not novel, but "the air insensitivity of encapsulated P4 is remarkable," says host-guest chemist Ralf Warmuth of Rutgers University, New Brunswick, N.J. "Whether this can be extended to other encapsulated air-sensitive guests remains to be seen. However, it is clear to me that encapsulated reactive molecules will become important reagents for synthesis in the future."

"Dramatic changes in stability and reactivity in confines are what only molecular capsules can do and what people in this field dream of," says molecular assembly specialist Makoto Fujita of the University of Tokyo. "It is clearly demonstrated in this work."



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