Hybrid 2D and 3D frameworks based on ε-Keggin polyoxometallates:: Experiment and simulation

The epsilon-Keggin polyoxomolybdate {epsilon-PMoV (8Mo4O40-x)-O-VI(OH)(x)M--(4)} is a versatile building unit, with M being either a Zn-II or a La-III capping ion located at the vertices of a slightly distorted tetrahedron. The charge of the Keggin unit depends on the number of protonated oxo bridging ligands, which has been shown to vary from 0 to 5. The Keggin entity can thus be either an anion (M = Zn, x = 0) or a cation (M = La, x = 3-5). The Zn derivative has been generated in situ by hydrothermal synthesis and forms a 2D material built from the connection of the cations by 4,4'-bipyridine ligands linked to the capping Zn-II ions. The reaction of the chloride salt of the La derivative with di-, tri- and tetrasubstituted benzenecarboxylate ligands has allowed us to isolate 2D and 3D materials. The 3D materials seem to be the first examples of hybrid open frameworks based on Keggin building blocks. The 3D framework built from the connection of epsilon-Keggin units by trimesate ions exhibits tunnels filled only by water molecules, which can be partly removed and reintroduced at room temperature. Besides these experimental results, simulation has allowed us to generate two virtual hybrid structures derived from those of known silicates by replacing the Si ions by hypothetical epsilon-Keggin cations and the O-bridging ligands by terephthalate ions, thus showing that 3D frameworks with large pores can be envisioned in the chemistry of hybrid organic-inorganic materials based on epsilon-Keggin units and motivating further experimental investigations. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).