Rhodium(III) Aqua Ion Salts of Ambivalent Self Assembled Superanion Capsules

The self assembled superanionic capsule [{Na+⊂(18-crown-6)(OH2)2}⊂{(p-sulfonatocalix[4]arene4−)2}]7− (X7−) forms a 1:1 crystalline complex with dinuclear [Rh2(μ-OH)2(H2O)8]4+ when the cation is treated with sodium p-sulfonatocalix[4]arene and 18-crown-6 at pH 2. The overall composition of the crystalline solid is [Rh2(μ-OH)2(H2O)8][X + 3 H+]·11.5H2O. Treatment of a complex mixture of polynuclear rhodium(III) aqua ions in perchloric acid with excess sodium p-sulfonatocalix[4]arene and 18-crown-6 at pH 2, results in selective retrieval of the tri- and tetranuclear rhodium(III) species in a single 1:1 co-crystallised phase, isolated as the hydrate salt with overall composition [Rh3(μ-OH)4(H2O)10][Rh4(μ-OH)6(H2O)12][2 X + 3 H+][Na(H2O)(ClO4)]2·27H2O. The tri- and tetranuclear Rh cations both have a cyclic structure with μ-hydroxide and terminal water ligands. Despite the differences in cation charge and size both complexes show the same arrangement of polynuclear cations and superanions although the trimer/tetramer complex shows additional coordination of a second Na centre by the calixarenes. At pH 0.35 the reaction leads to the selective retrieval of the tetranuclear cation in the complex [Rh4(μ-OH)6(H2O)12][X + {X − 2 H2O} + 8 H+]·33H2O, where a second type of dehydrated superanion is found, where the two sodium bound water molecules have been replaced by O-centres of calixarene sulfonate groups, illustrating the versatile nature of the superanion chemistry.