Base-induced variation of the coordination mode in a uranyl homoazacalixarene complex

Reaction of the expanded calixarene p-methyl-N-benzyl-tetrahomodiazacalix[4]arene with uranyl nitrate in presence of triethylamine gives a complex markedly different from that obtained from the same reactants in a neutral medium. Whereas UO22+ is located outside the macrocycle in the latter, due to the electrostatic repulsion with the ammonium groups present on the lower rim, an 'internal' complex is formed in the present case, associated to a cavity enlargement bringing the ammonium groups farther from the cavity centre. This enlargement appears to be the result of a double deprotonation by the base and the subsequent release of the internal constraints due to phenol-phenoxide hydrogen bonds, associated to a conformation change. (C) 2001 Elsevier Science Ltd. All rights reserved.