An Inhospitable Cryptand: The Importance of Conformational Freedom in Host-Guest Complexation

Two new cryptands were synthesized from bis(5-bromomethyl-1,3-phenylene)-32-crown-10 (4). The third arms, containing 19 or 21 atoms, were installed via Williamson ether syntheses with bisphenols containing 2,6-disubstituted pyridines. 2,6-Diaminopyridine was converted into the bis(p-hydroxybenzoyl) derivative 3 for the first cryptand (5) and 2,6-dicarboxypyridine was converted into the bis(p-hydroxybenzylamide) 9 for the second cryptand (10). Cryptand 5 did not complex viologen derivatives 11-13 to an extent detectable by H-1 NMR. We attribute the lack of complexation between viologen derivatives and 5 to its lack of conformational flexibility that prevents pi-stacking, a necessary component for complexation of viologens. In contrast longer and more flexible cryptand 10 did complex dimethyl paraquat (11) with K-a = 1.6 (+/- 0.2) x 10(3) m(-1) in acetone at 23 degrees C, probably by pi-stacking with the p-oxybenzyl moieties of the host, made available by its enhanced flexibility.