STRUCTURE AND ANALYSIS OF HELICAL TUBULATE INCLUSION-COMPOUNDS FORMED BY 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,exo-6-diol, 1, the prototype of the helical tubuland family of diol hosts, have been thoroughly investigated. These diols crystallize with a series of spiral hydrogen bonded spines ...OH...OH...OH...OH... surrounding parallel canals. A total of 40 inclusion compounds of 1 (employing 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 along c. The structures of 12 of these compounds have been determined by X-ray crystallographic methods, showing that the guests exert a strong influence on the detailed structure of the resulting inclusion compound. Increasing guest size can cause a large (4.8%) increase in the unit cell a (= b) length (canal width) accompanied by a small decrease (0.96%) in c direction (canal length). This results in an 8.9% increase in unit cell volume and a dramatic 62% increase in canal cross-sectional area from 15.6 to 25.3 Angstrom(2) across the series of compounds studied. The intermolecular adjustments which permit these changes are examined in detail. Increases in hydrogen bonding distances, or alterations in the molecular bond lengths and angles exhibited by 1, are insignificant factors. Rather it is the small changes in angles around the hydrogen bonded spines, and the tilt angles of the diols with respect to the canal axis, which are responsible. Considerable variation is 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 are the most important intermolecular forces involved within the helical canals. Exceptions to this are observed for guests substituted with halo groups. Samples of guest-free 1 still have the helical tubuland structure unlike some other members of this diol family. Conclusive evidence for the existence of this lattice containing parallel empty canals each with an unobstructed cross-sectional area of about 15.6 Angstrom(2) is presented.