Cations in control: crystal engineering polyoxometalate clusters using cation directed self-assembly

The synthetic engineering of anionic polyoxometalate (POM) clusters with predefined properties tailored to specific applications is a great challenge using routine "one-pot" POM syntheses. Under such conditions, difficulties often arise from the multitude of complex reaction pathways and self-assembly processes occurring in solution. In this respect the major role of the charge balancing cations cannot be ignored, in fact such cations are crucial, both in the assembly of the building blocks, linkage to the overall cluster, and then assembly into the bulk material. Further, the role of the cation facilitating the selective crystallization of a particular cluster type cannot be divorced from the reaction process since the crystallization process itself can help pull "virtual" building blocks into being. This perspective briefly outlines our efforts towards engineering novel POM based materials, highlighting the use of large organic cations as "Shrink-wrapping" agents to isolate new POM clusters, frameworks and cage compounds. Central to this perspective is the hypothesis that, in the case of POM cluster assembly, the mechanism and various equilibria which define the clusters can be controlled by the selective crystallisation using cation control. Consequently, this indicates that the process of crystallisation can have a profound effect on self-assembly at the molecular level. We therefore propose that the crystallization process itself may define the molecular structure of the cluster leading to the conundrum, which came first, the cluster or the crystal of the cluster?