New applications of crown ethers. Part 6. Structural effects of bis(benzocrown ether)s and substituted benzocrown ethers on solvent extraction and complexation of alkali-metal cations

Abstract

Bis(benzocrown ether)s consisting of benzo-15-crown-5 (B15C5), benzo-18-crown-6(B18C6) and/or benzo-21-crown-7 (B21C7) with the following linking chains, -(CH₂)_n-(n= 2 and 8), -(CH₂)₃-O-, -O-(CH₂)₆-O-, and -(O-CH₂CH₂)_n-O-(n= 2–5), were prepared and their complexation behaviour was characterized using solvent extraction of alkali-metal picrates and complexation with alkali-metal chlorides. Bis(benzocrown ether)s were distinguished from the corresponding mono(benzocrown ether)s by their remarkable high extraction ability of a cation larger than the hole size of a crown unit, the so-called ‘biscrown effect’. Bis(B15C5)s (4a–h) and bis(B18C6)s (5a–h) preferentially extracted K⁺ and Cs⁺, respectively. Bis(benzocrown ether)s containing the B21C7 unit (6a)–(8a) did not show this ‘biscrown effect’ because of the large hole size of B21C7. An unsymmetrical bis(crown ether)(7b) consisting of B15C5 and B18C6 selectively extracted Rb⁺. The ‘biscrown effect’ was favourably exerted with the oligoethyleneglycol linkage rather than the hydrocarbon one. Little or no effect of lipophilic groups or donor oxygens in the side chain of mono(benzocrown ether)s was observed in the extraction of alkali-metal picrates. Stability constants were determined by the ion-selective electrode method in 90% methanol aqueous solution at 25 °C. In bis(B15C5)s with Na⁺, and bis(B18C6)s with Na⁺ or K⁺, two crown rings in one molecule acted as two individual moieties. Bis(B15C5)s bound with K⁺ to form preferentially an intramolecular 2 : 1 crown ether unit-K⁺ complex. Both bis(B15C5)s with Cs⁺ and bis(B18C6)s with Cs⁺ systems also showed the ‘biscrown effect’.