PAIRWISE SELECTION OF GUESTS IN A CYLINDRICAL MOLECULAR CAPSULE OF NANOMETER DIMENSIONS

'Container' complexes in which a guest molecule is held mechanically w ithin a cage-like host have been known for over a decade(.1,2). They p rovide a means to stabilize reactive intermediates(3) and to create ne w forms of stereoisomerism(4). Molecular capsules held together by hyd rogen bonds are more recent; they are formed reversibly on timescales of milliseconds to hours, long enough for molecular motions(5) and eve n reactions(6) to be seen for the encapsulated species. Here we descri be the synthesis and characterization of a hydrogen-bonded molecular c apsule of nanometre dimensions, which is large enough to encapsulate t wo different molecules, This allows us to explore the size- and shape- selectivity of the encapsulation process: we see, for example, the exc lusive formation of the hetero-guest pair when benzene and p-xylene ar e both added to a solution. This presumably reflects optimal occupancy of the capsule-two benzene guests leave too much empty space in the i nterior, and two p-xylene molecules make it too crowded.