Asymmetric modification of Anderson-type polyoxometalates towards organic-inorganic homo- and hetero-cluster oligomers

The preparation of asymmetrically modified polyoxometalates (POMs) bearing two different functional sites is of great interest, since it allows for the rational design and controlled synthesis of novel POM-based hybrids and further development of multi-POM supramolecular architectures. Herein, we report a simple and powerful synthetic approach for the isolation of two asymmetric Anderson hybrids that can be sequentially functionalized to produce organic-inorganic cluster oligomers. It is found that the type of tripodal alcohol can significantly affect the modification products, highlighting the importance of amide-functionalized tris derivatives, which selectively generate the target asymmetric hybrids, while the other tripodal alcohols tried in this work inevitably lead to symmetric by-products. Based on the obtained asymmetric hybrids, we further develop a modular synthetic methodology for stepwise coupling of Anderson clusters to form linear cluster oligomers. Moreover, this modular coupling methodology provides versatile intermediates that can be used to bind different clusters together via covalent approaches. As a proof of concept, a hetero-cluster of a Keggin-Anderson trimer has been prepared and carefully characterized. As such, this work provides a promising approach for the development of functional asymmetric hybrids and controlled synthesis of metal-oxo cluster oligomers with precise cluster numbers and chain length.