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Supramolecular chemistry

Phosphorus caged

Violent criminals are imprisoned to keep them under control. Similarly, incarceration in a molecular jail stops white phosphorus from bursting into flames — but on release, it regains its fiery character.

White phosphorus — P4, one of two forms of the element — has earned a notorious reputation for being highly reactive. When it reacts slowly with oxygen, it merely glows; indeed, the words 'phosphorescence' and 'phosphorus' both originate from the Greek word for 'light-carrier'. But on exposure to high concentrations of oxygen (such as in air), phosphorus spontaneously ignites, burning fiercely to produce a great heat. Reporting in Science, Mal et al.1 describe a way to tame this fiery element. When its molecules are trapped in nanoscale 'flasks', white phosphorus becomes inert, and will no longer react even slowly with oxygen.

Mal and colleagues' nanoflasks are actually molecular complexes that self-assemble from smaller subunits (molecules and ions). The cavities of similar 'host' complexes are known to isolate, protect and sometimes stabilize other 'guest' molecules. In nature, for example, viruses use protein shells known as capsids to protect their nucleic acid. Similarly, human cells wrap iron in a protein shell, forming a complex called ferritin; without this protection, the iron would be toxic. Chemists are increasingly using this strategy in their work, by synthesizing host assemblies and using them to facilitate new reactions or to isolate unstable chemical species.

The nanoscale cages made by Mal et al. are tetrahedra that assemble spontaneously in water from four iron(II) ions (Fe2+, which become the apexes of the tetrahedra) and six organic molecules (Fig. 1). These six molecules assemble in turn from the reactions of smaller molecular subunits: six 'bridging' components (which form the edges of the tetrahedra) and 12 'terminal' components (which bind to the Fe2+ ions at the apexes of the tetrahedra). Although the reactions of the bridging and terminal subunits are easily reversible, the products are stabilized by their subsequent coordination to the Fe2+ ions. The entire assembly is therefore stable, but in dynamic equilibrium with the starting components.

Figure 1: Molecular prisons.
figure1

Mal et al.1 have made tetrahedral molecular complexes that encapsulate molecules of white phosphorus, P4. The complexes self-assemble from various subunits. a, Six bridging subunits (red) combine with 12 terminal subunits (blue) to make six larger components. b, These larger components bind to four iron(II) ions (Fe2+, obtained from iron(II) sulphate, FeSO4) to form the tetrahedral host complex. Only one of the six components is shown for simplicity. c, When a solution of the host in water is left in contact with white phosphorus, P4 guest molecules become trapped inside it.

The tetrahedral complex has a negative charge that makes it water-soluble, but its inner space is relatively hydrophobic, and just big enough to encapsulate a P4 guest. When Mal et al. left an oxygen-free solution of the complex in water in contact with solid white phosphorus, a host–guest complex formed — as shown both by spectroscopic experiments on the resulting solution, and by X-ray crystallography on isolated crystals of the product (Fig. 2). The caged phosphorus is both water-soluble and air-stable. Indeed, the authors found that solutions of the host–guest complex remained unchanged after four months' exposure to air; compare this with the instant conflagration of unencapsulated white phosphorus. So what is going on?

Figure 2: X-ray crystal structure of the host–guest complex, as obtained by Mal and colleagues1.
figure2

Iron atoms are purple, carbon atoms grey, nitrogen atoms blue and phosphorus atoms orange. Charged sulphonate groups (yellow and red) help the cluster to dissolve in water.

White phosphorus reacts with oxygen to produce an oxide (P2O5). This oxide then reacts with any water that is around to form phosphoric acid. The phosphorus–phosphorus bonds of P4 are weak compared with the stronger phosphorus–oxygen bonds of P2O5; in other words, the oxide is thermodynamically much more stable than white phosphorus, and this drives the reaction to such an extent that white phosphorus spontaneously combusts in air.

One might therefore assume that Mal and colleagues' nanoflasks simply isolate P4 molecules from oxygen. But this isn't the case: oxygen molecules are smaller than P4 molecules, and must therefore also be able to gain access to the flasks' interiors. Instead, the tight confinement of P4 molecules prevents the formation of phosphorus–oxygen bonds during the first steps of phosphorus oxidation — there simply isn't room for the reaction intermediates to form. This is the first time that a reactive species has been stabilized by such an effect, and represents a fundamental advance for the field.

Mal et al.1 found that encapsulated P4 molecules could be easily released from their miniature prisons. When the authors floated an equal volume of benzene on top of an aqueous solution of the host–guest complex, benzene molecules replaced P4 as guests. The ejected phosphorus is insoluble in water, but soluble in common organic solvents, and so the authors could extract it into the benzene layer, where it reacted fully with air within 24 hours. By contrast, the organic solvent n-heptane does not replace P4 in the host–guest complex, presumably because it doesn't fit well into the host's chamber.

Other tetrahedral clusters have been reported to self-assemble from metal ions and organic molecules2,3, and are increasingly being used as molecular flasks for new types of reaction4. They have also been used to make 'ship-in-a-bottle' molecules, by assembling around guests that are too large to leave the resulting cavity. Some complexes even mimic enzymes, catalysing reactions by stabilizing the transient intermediates for those reactions as guests. Still other host complexes assemble from organic subunits alone, using hydrogen bonding or other weak interactions, and catalyse reactions because their cavities force guest molecules into the optimal alignment for those reactions5.

Mal et al.1 are currently trying to prepare bigger host complexes that they hope will isolate and stabilize larger, more complicated guests. Ultimately, they hope that their encapsulation strategy will provide a general way to control the release of reactive or sensitive molecules, or even as a method for removing harmful substances from the environment. Such applications are a long way off, but in the meantime we have every reason to expect more chemical surprises and capabilities as we continue to explore this rich field of endeavour.

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Raymond, K. Phosphorus caged. Nature 460, 585–586 (2009). https://doi.org/10.1038/460585a

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