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 1 H NMR. We attribute the lack of complexation between viologen derivatives and 5 to its lack of conformational flexibility that prevents π‐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) × 10 3 m –1 in acetone at 23 °C, probably by π‐stacking with the p ‐oxybenzyl moieties of the host, made available by its enhanced flexibility.