The embedding of molecular metal oxides, or polyoxometalates (POMs), in metal–organic frameworks (MOFs) opens new research avenues in catalysis and beyond. This review explores the host–guest chemistry of POMs embedded in MOF hosts and discusses the synergism of the resulting composites for heterogeneous catalysis. The review focuses on well-established and well-studied classes of POMs, such as Keggin and Wells–Dawson anions, and well-researched MOFs, including the MIL, UiO, and NENU families. Outstanding examples of synergistic catalytic activity between the POM and MOF are described, and key performance parameters, including POM localization, pore size and pore structure, as well as particle size effects are described for technologically important catalytic processes. In addition to thermal catalysis, we discuss the use of POM@MOF composites for electro- and photocatalysis with an emphasis on energy conversion systems. Finally, we provide an outlook on emerging areas where POM@MOF composites could lead to new catalytic reactivity.

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A novel inorganic–organic hybrid has been synthesized and characterized. This hybrid contains a trinuclear Tb-organic cluster that is bound to three terminal oxygen atoms of the Keggin anion and sodium to form a discrete heterotetranuclear structure.

The design and synthesis of inorganic-organic hybrid materials based on polyoxometalates (POMs) have attracted considerable attention over recent years, because of their intriguing architectures and properties as well as potential applications in catalysis, magnetism, and sensitive devices, supramolecular materials, electro/photochromic systems, sensors, macromolecular crystallography and medicine [1]. Also, Keggin-type POMs have been mostly employed due to its size suitability and structure stability. Furthermore, they are good candidates as linkers, because they have many nucleophilic surface oxygen atoms and various coordination modes which make them act as multidentate inorganic ligands [2]. As an extension of our attempts to synthesize new inorganic-organic hybrids, here, we would like to report a successful hydrothermally synthesis based on [SiW12O40]4− as Keggin-type POMs. It has been characterized by single-crystal X-ray diffraction that formulated as {[Pr(pydc-OH)(H2O)4]3}[SiW12O40]• 7H2O (1), in which pydc-OH = 4-hydroxypyridine-2,6-dicarboxylic acid or chelidamic acid. It has trigonal cell with a = 18.187 (2)Å, b = 18.187 (2)Å and c = 21.137 (4)Å. The results revealed that in 1 each trinuclear Pr-organic cluster is bound to three terminal oxygen atoms of the Keggin anion to yield discrete flower-like clusters (see Fig. 1). Also, discrete clusters can be assembled together by numerous of weak interactions like hydrogen bonding and anion-π interactions to generate 3D supramolecular frameworks. We concluded that these unique coordination modes of the Keggin anions and the pydc-OH ligands play significant roles in the formation of the discrete trinuclear lanthanoids structures [3].