Synthetic ionophores .16. Synthesis and association behaviour of bis-pyridine tetramide macrocycles: Role of increased preorganisation on Ag+ selectivity

Pyridine based tetramide macrocycles (6-9) have been synthesized by nucleophilic displacement in pyridine-2,6-dicarbonyl dichloride with appropriate diamines. The 18-membered macrocycles 6 and 7 have well organized structures and 20-membered macrocycles 8 and 9 exist as mixtures of configurational isomers. The comparison of X-ray single crystal and energy minimized structures of previously reported diamide diester macrocycle(1) with those of macrocycles 6, 7 and 1.AgNO3 show that the macrocyles 6 and 7 possess type of preorganised structure attained by macrocycle 1 on complexation. This enhanced preorganisation in macrocycles 6 and 7 increases the Ag+ extraction efficiency by >30% to 74 - 81% and Ag+ / Pb2+ selectivity by >300% to 245 - 283 times than in 1. (C) 1997 Elsevier Science Ltd.