A Giant Polyaluminum Species S−Al32 and Two Aluminum Polyoxocations Involving Coordination by Sulfate Ions S−Al32 and S−K−Al13

The giant polyaluminum species [Al32O8(OH)60(H2O)28(SO4)2]16+ (S−Al32) and [Al13O4(OH)25(H2O)10(SO4)]4+ (S−K−Al13) [S means that sulfate ions take part in coordination of the aluminum polycation; K represents the Keggin structure] were obtained in the structures of [Al32O8(OH)60(H2O)28(SO4)2][SO4]7[Cl]2·30H2O and [Al13O4(OH)25(H2O)10(SO4)]4[SO4]8·20H2O, respectively. They are the first two aluminum polyoxocations coordinated by sulfate ions. The “core-shell” structure of S−Al32 is similar to that of Al30, but the units are linked by two [Al(OH)2(H2O)3(SO4)]− groups with replacement of four η1-H2O molecules. The structure of S−K−Al13 is similar to the well-known structure of ε-K−Al13, but the units are linked by two (SO4 2−)0.5 with replacement of a H3O+ ion. It was shown that strong interaction exists between the polyoxocations and counterions. On the basis of their structural features and preparation conditions, a formation and evolution mechanism (from ε-K−Al13 to S−K−Al13 and S−Al32) has been proposed. A local basification degree symmetrical equalization principle was extracted based on a comparison of the calculated results of the local basification degree for each central Al3+ ion included in a polycation. They can be used to explain how the two aluminum species are formed and evolved and why the sulfate ions can coordinate to them and to predict where the OH-bridging positions will be upon further hydrolysis.