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

Abstract

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?

Intriguing Role of a Quaternary Ammonium Cation in the Dissociation Chemistry of Keggin Polyoxometalate Anions

“…This kind of study relates to a special class of compounds: POM-based supramolecular assembles. This study allows us to better understanding the important role of cations on the formation of the POM anionic frameworks as structure directing agents [23][24][25] and on the control of catalytic activity of POM clusters [26][27][28].…”

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

“…While the role of this cation is currently unclear, it is likely a combination of electrostatic, hydrogen bonding (via ‐OH and ‐NH groups) and possibly even coordinative interactions (via ‐OH groups) could stabilize reactive fragments and lead to {KW 36 } formation. In later studies, the authors also showed that replacing the K + with larger alkali and alkaline earth metals, and other ammonium or primary organo‐ammonium groups is also possible, opening the door to organo‐functionalized species …”

Hydrothermal synthesis and structural characterization of three reduced phosphomolybdates

“…Equivalent reactions indicate that no crystals could be obtained without 1,6-diaminylhexane. Usually, rigid amines induced the formation of layered arrangements, whereas structurally more flexible amines resulted in formation of polymeric chains [28]. In this paper, we focus on the influence of pH and transition metals.…”