Synthesis and crystal structure of 1:2 mixed uranyl/alkali metal ions (Li+, Na+, K+, Cs+) complexes of p-tert-butyltetrahomodioxacalix [4]arene

p-tert-Butyltetrahomodioxacalix[4]arene LH4 reacts with uranyl nitrate hexahydrate 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(CH3OH)8 moiety. Both K+ and Cs+ give polymeric chains, with two cations in the macrocycle cavity, involved in cation-(pi) interactions with the aromatic rings and bound to the oxo groups from two neighbouring 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.