Trapping a guest in a corner prevents disorder, according to scientists in Japan.

The cavities of molecular cages and capsules can play host to a range of guest molecules. Within a highly symmetric cavity, a guest molecule is usually unrestricted and can take up any orientation.  This overall disorder makes it difficult to accurately analyse the structure by x-ray crystallography.  

Makoto Fujita and colleagues at the University of Tokyo have used a symmetric host, made of six palladium centres linked by organic ligands, to design an asymmetric host-guest system that restricts the movement of the guest. This allows the structure to be studied using crystallographic methods. 

Fujita's host-guest complex has two different guests - one guest molecule, a triazine derivative with three pyridyl groups attached, creates a two-dimensional network between adjacent cages of the host. As a result, the second guest, a manganese compound with a cyclopentadienyl ligand, becomes trapped in a corner.  

The first guest can assemble into a net-like structure because it has available coordination sites.  These can interact with added metal ion connectors to form two-dimensional threads. 

Omar Yaghi, an expert in molecular frameworks at the University of California, Los Angeles, US, described the work as an outstanding achievement. 'Fujita is showing us how the space within porous structures can be used as building sites for the construction and study of trapped molecules,' said Yaghi. 'We are used to seeing threading of beads on a chain, however what Fujita has succeeded in doing is to thread the beads on a fishing-type net - two-dimensional threading.'

Fujita's team are interested in studying the photoreactivity of the manganese compound in the cavity. 'We would like to use the cavity as a molecular flask for crystalline-state reactions,' said Fujita. 'By obtaining an ordered crystalline phase, we could directly observe photoreaction at the atomic level.' 

Alison Stoddart