STRUCTURE AND ANALYSIS OF HELICAL TUBULATE INCLUSION-COMPOUNDS FORMEDBY 2,6-DIMETHYLBICYCLO[3.3.1]NONANE-EXO-2,EXO-6-DIOL

The inclusion properties of 2,6-dimethylbicyclo [3.3.1]nonane-exo-2,ex o-6-diol, 1, the prototype of the helical tubuland family of diol host s, have been thoroughly investigated. These diols crystallize with a s eries of spiral hydrogen bonded spines ...OH...OH...OH...OH... surroun ding parallel canals. A total of 40 inclusion compounds of 1 (employin g guests containing a wide range of functional group types) have been characterized. These all crystallize in the chiral space group P3(1)21 (or its enantiomorph P3(2)21) with the guests trapped in the canals a long c. The structures of 12 of these compounds have been determined b y X-ray crystallographic methods, showing that the guests exert a stro ng influence on the detailed structure of the resulting inclusion comp ound. Increasing guest size can cause a large (4.8%) increase in the u nit cell a (= b) length (canal width) accompanied by a small decrease (0.96%) in c direction (canal length). This results in an 8.9% increas e in unit cell volume and a dramatic 62% increase in canal cross-secti onal area from 15.6 to 25.3 Angstrom(2) across the series of compounds studied. The intermolecular adjustments which permit these changes ar e examined in detail. Increases in hydrogen bonding distances, or alte rations in the molecular bond lengths and angles exhibited by 1, are i nsignificant factors. Rather it is the small changes in angles around the hydrogen bonded spines, and the tilt angles of the diols with resp ect to the canal axis, which are responsible. Considerable variation i s observed in the types of interguest arrangement within the canals of the 12 structures. These are explored and rationalized using crystal engineering arguments, but, generally, weak host-guest interactions ar e the most important intermolecular forces involved within the helical canals. Exceptions to this are observed for guests substituted with h alo groups. Samples of guest-free 1 still have the helical tubuland st ructure unlike some other members of this diol family. Conclusive evid ence for the existence of this lattice containing parallel empty canal s each with an unobstructed cross-sectional area of about 15.6 Angstro m(2) is presented.