Inclusion of Organic Cations by Calix[4]arenes Bearing Cyclohepta-2,4,6-trienyl Substituents

Abstract

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 calix[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 inclusion capability, due to an enlargement of the π-basic wall of the host cavity. The complexation capability of these calix[4]arenes for organic cations is studied in this paper. For the first time, aryl tropylium salts have been used as cationic guests in addition to ammonium and iminium salts. Three structural types of hosts can be distinguished, all of which exhibit remarkably different inclusion abilities towards the organic cations. Flexible hosts with an OH-unmodified lower rim undergoing ring inversion form complexes of modest stability, provided that the host lacks bulky tert-butyl substituents at the upper rim. Tetra-O-alkylated derivatives, which are assumed to exhibit a flexible cone conformation, do not bind organic cations. Among the calix[4]arenes bearing cycloheptatrienyl groups, the complexation requirements are best matched by hosts with four cycloheptatrienyl groups on the upper rim and two alkyl groups on the lower rim; these compounds are assumed to possess a more rigid cone conformation, due to intramolecular hydrogen bonds. The tropylium component of phenyl tropylium ions penetrates inside the π-basic cavity of the host. The absorption coefficient of the longest wavelength absorption band of these guests in the visible region is drastically lessened upon inclusion.