Inclusion of organic cations by calix[4]arenes bearing cyclohepta-2,4,6-trienyl substituents

Only very weak complexation of organic cations such as ammonium and iminium ions by simple calix[4]arenes has been reported to date. Newly designed ca lix[4]arenes, substituted with a different number of 1,3,5-cycloheptatrien- 7-yl functions at the upper rim. can engender an improvement in the inclusi on capability, due to an enlargement of the Jr-basic wall of the host cavit y. The complexation capability of these calix[4]arenes for organic cations is studied in this paper, For the first time, aryl tropylium salts have bee n used as cationic guests in addition to ammonium and iminium salts. Three structural types of hosts can be distinguished, all of which exhibit remark ably different inclusion abilities towards the organic cations, Flexible ho sts with an OK-unmodified lower rim undergoing ring inversion form complexe s of modest stability, provided that the host lacks bulky tert-butyl substi tuents at the upper rim. Tetra-O-alkylated derivatives, which are assumed t o exhibit a flexible cone conformation, do not bind organic cations. Among the calix[4]arenes bearing cycloheptatrienyl groups, the complexation requi rements are best matched by hosts with four cycloheptatrienyl groups on the upper rim and two alkyl groups on the lower rim; these compounds are assum ed to possess a more rigid cone conformation, due to intramolecular hydroge n bonds. The tropylium component of phenyl tropylium ions penetrates inside the pi -basic cavity of the host. The absorption coefficient of the longes t wavelength absorption band of these guests in the visible region is drast ically lessened upon inclusion.