The synthesis and crystal structures of three ytterbium(III) complexes of homoazacalixarenes are reported. In all cases, the complexes are obtained without addition of a base, the protons of the complexing phenolic moieties being transferred to the amine groups upon complexation. The 1:1 complex between p-chloro-N-benzylhexahomotriazacalix[3]arene (1) and Yb(NO3)(3) is analogous to the neodymium(III) complex reported previously, with the metal ion bound to three phenoxide groups and three nitrate ions. A second 1:1 complex is obtained with Yb(NO3)(3) and p-methyl-N-benzyltetrahomodiazacalix[4]arene (2), in which the metal ion is bound to two phenoxide groups only. The replacement of nitrate by triflate counter-ions results in the formation of a 1:2 'sandwich' complex with 1, in which two divergent calixarene molecules are bridged by the metal ion, the counter-ions being non-bonding. (C) 2000 Elsevier Science B.V. All rights reserved.
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.