Science Digest: Want to Be Heard? Speak Into the Right Ear

Caged Fire

How do you keep a man-eating tiger from doing harm? One way is to keep it caged up.

It turns out that also works for white phosphorus, one of the more dangerous substances on Earth.

A team of chemists last week described a nano-cage that encapsulates the four-atom white phosphorus molecule so tightly there isn't enough room for it to react with anything else and cause trouble.

Used as an incendiary munition since World War I, white phosphorus (P4) burns spontaneously when exposed to oxygen in the air. It has to be stored underwater or in other special environments.

Jonathan R. Nitschke of the University of Cambridge and Kari Rissanen of the University of Jyvaskyla in Finland and their collaborators synthesized tetrahedral (four-faced) molecular cages, with an iron atom at each corner. When white phosphorus was dissolved in water loaded with the cage molecules, the P4 molecules migrated into them.

Once inside, the phosphorus was stable and harmless. Even though an oxygen atom can occasionally squeeze through the "bars" of the cage, it doesn't trigger a reaction -- combustion -- to occur because there's no room.

To get the white phosphorus out, add benzene. That compound is a better guest in the cage, so benzene molecules displace P4. The phosphorus ends up dissolved with a layer of caged benzene on top.

-- David Brown