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The coordination of uranyl ions in their complexes with p-tert-butyloctahomotetraoxacalix[8]arene (LH8) is strongly dependent on the synthesis conditions. Tetranuclear [(UO2)4O4(LH8)]×10MeOH (1) differs from the previously reported, octa-phenoxide one by the coordination of all the ether groups and the subsequent rotation of the tetra-uranate cluster with respect to the macrocycle. In the presence of pyridine, [(UO2)2(py)4(LH4)]×py (2) is formed, in which the partially deprotonated ligand is bound to two independent uranyl species by four phenoxide and two ether groups only, thus evidencing the decrease in nuclearity due to a coordinating base (pyridine) preventing cluster formation through bridging ions.

Reaction of UCl4 with homooxacalixarenes in pyridine results in cleavage of the ether bridges upon nucleophilic attack by pyridine, likely assisted by the U(IV) ion. The zwitterionic diphenoxide species bis[5-tert-butyl-2-oxido-3-(1-pyridiniomethyl)phenyl]methane L1 formed from p-tert-butyltetrahomodioxacalix[4]arene, p-tert-butylhexahomotrioxacalix[6]arene or p-tert-butyloctahomotetraoxacalix[8]arene and the zwitterionic triphenoxide species 4-tert-butyl-2,6-bis[5-tert-butyl-2-oxido-3-(1-pyridiniomethyl)-benzyl]phenolate (L2)- formed from p-tert-butyltetrahomodioxacalix[6]arene give polyphenoxide complexes of U(IV). Coordination of L1 to UCl4 yields the neutral complex [UCl4(L1)] and, on further heating, the cationic [UCl2(L1)2]2+ species, whereas (L2)- gives only the neutral compd. [UCl3(L2)]. In these three complexes, the metal atom is in slightly distorted octahedral environments, with bonding to all phenoxide groups. These are the 1st instances of U(IV) complexes with polyphenoxide ligands, resulting from unprecedented homooxacalixarene cleavage.

p-tert-Butyltetrahomodioxacalix[4]arene LH4 reacts with UO2(NO3)2.6H2O in the presence of alkali metal hydroxides to give mixed complexes containing the same [UO2(L)]2- central core. All alkali metal ions are bound to the basic uranyl oxo groups, which act as mono- (Li+, Na+) or bidentate (K+, Cs+) ligands. The resulting structures largely depend on the size and predominantly hard or soft character of the alkali metal ion. Li+ and Na+ are coordinated in both endo and exo modes, Li+ giving a monomer with one cation bound to each oxo group and Na+ a dimer with a bridging, oxo-coordinated, Na2(MeOH)8 moiety. Both K+ and Cs+ give polymeric chains, with two cations in the macrocycle cavity, involved in cation-p interactions with the aromatic rings and bound to the oxo groups from two neighboring molecules in the chain, with differences in further coordination to the calixarenes which are related to the softer nature of Cs+. Unprecedented supramolecular architectures based on coordination bonds are thus obtained.

The first crystal structures of a p-tert-butyltetrahomodioxacalix[6]arene comprising two triphenolic subunits separated by two ether bridges and two of its complexes with uranyl ions are reported. The doubly deprotonated macrocycle assumes a much elongated, cone-like conformation including two organic species. Two binuclear uranyl complexes are described. In both, each cation is bound to the three phenoxide O atoms of a tri-phenolic subunit and to a central, bridging, carbonate or carbamate ion. The formation of the latter ions from the amines used as bases is discussed.