New applications of crown ethers. Part 8. Complexation of bis(monoazacrown ether)s with alkali-metal cations

Abstract

The complexation behaviour of bis(monoazacrown ether)s (BCA15C5, BCA18C6, BOA15C5, BOA18C6) with alkali-metal cations was studied by means of solvent extraction, stability constants in homogeneous solution, and ¹³C n.m.r. spectroscopy. Bis(monoaza-18-crown-6)s, BCA18C6 and BOA18C6, cannot form an intramolecular sandwich complex with K⁺, probably because of the stability of the macrorings in pseudo-D_3d conformation, and also of preferential donation of the oxygen atom in the bridging chain in BOA18C6, as in ‘lariat ethers’. For bis(monoaza-15-crown-5), especially for BOA15C5, the existence of the intramolecular sandwich structure was found both in Na⁺ and K⁺ complexes. It is presumed that the more flexible monoaza-15-crown ring does not suffer from restraint in the formation of the sandwich structure, and the oxygen atom in the bridging chain not only increases the flexibility of the chain but also can co-ordinate directly with the cation held in the sandwich cavity.