Pores off the peg

Porous materials provide a nanometre-scale playground for chemists to investigate unusual reactions that occur in confined spaces. But in the most widely used of these materials — zeolites and metal–organic frameworks — it is difficult to control the size and geometry both of pore openings and of the pores themselves. Writing in Nature Chemistry, Scott Mitchell and his colleagues report a solution to this problem in which the molecular building blocks used to create a porous framework are themselves ready-made pore openings (S. G. Mitchell et al. Nature Chem. doi:10.1038/nchem.581; 2010).

The authors' building block is a polyoxometalate — a 'wheel' of metal, oxygen and phosphorus atoms that has an opening of about 1 nanometre in diameter. Six of these wheels (three of which are pictured here as red structures) self-assemble in the presence of manganese ions (small yellow spheres) to form a regularly shaped pore (multicoloured faceted structure) of about 7 nm³ in volume. These roughly cubic units extend in three dimensions throughout the material to make a remarkably open framework.

The material also incorporates potassium and lithium ions (not shown) to balance the negative charge carried by the oxide ions in the wheel. These metal ions fit into the channels and cavities of the material loosely enough that they can be replaced with other positive ions if the compound is soaked in a suitable solution. For example, Mitchell et al. found that 35 copper(II) ions (Cu²⁺) could be incorporated into each pore by soaking the material in a solution of copper(II) nitrate. None of the manganese ions in the porous material was displaced by Cu²⁺ — even though both types of ion bear the same charge — leaving the framework architecture intact.

The authors could easily alter the oxidation state of the linking manganese ions while they were in the framework. When they increased the oxidation state from +2 to +3, they observed a reduction in the number of charge-balancing lithium and potassium ions. These charge-balancing ions could still be replaced with Cu²⁺ ions, but only by about half as many as before.

The system's ion-exchange properties could be further tuned by blocking the pore openings with organic molecules. The team found that small organic molecules introduced into the material's pores can subsequently be displaced by Cu²⁺ ions. But larger organic molecules that are too large to fit inside the pore framework instead interact with pore entrances, blocking the entry of Cu²⁺ ions.

Because a wide range of polyoxometalate wheels are available, Mitchell and colleagues' work opens the door to a variety of porous materials that have tunable properties. The authors are currently investigating the material's catalytic and ion-sensing capabilities.