Assemblies of Polyoxometalates

Abstract

Assemblies of Polyoxometalates Polyoxometalates, or POMs for short, come in a variety of shapes and sizes. Some of them are among the largest inorganic molecules known today. These molecules can be highly symmetrical and, as the name already implies, consist of (mainly) metal (molybdenum, tungsten, vanadium, iron, etc) and oxygen atoms in the form of metal oxide polyhedrons. Although individual POM molecules are in itself already a very challenging field of research, assemblies of POMs have great challenges of their own. Take for instance the solution behavior of a subclass of POMs known as Keplerates. Keplerates are highly symmetric, hollow, spherically shaped POMs. In solution they spontaneously assemble into large, hollow, spherical superstructures or POM-shells. These POM-shells have an average radius in water of several tens of nanometer and are composed of a monolayer of more than a thousand of individual POMs. Although the underlying principles behind this spontaneous assembly are still not precisely known, much can already be understood about this curious phenomenon. For instance, the equilibrium size of POM-shells of Keplerate type POMs is inversely proportional to the relative dielectric constant of the medium in which they are dispersed. This behavior is in line with predictions made by a stabilization mechanism based on Coulomb repulsion combined with charge regulation. This model also explains a new structural instability of the POM-shells. Besides the colloidal instability, i.e. the formation of aggregates that consist of many single layered shells, these systems also display an instability on a structural scale within the shell-like assemblies when the ionic strength reaches a critical value. Not only the ionic strength has an influence on the properties of POM-shells in solution. It has been claimed that the number of charges on the POMs themselves plays an important roll in their formation and stabilization as well. For instance, for the Keplerate {Mo72Fe30} experiments show that the interaction energy between the POMs in the shell becomes stronger when the charge density on the POMs increases. This interaction energy can also be estimated from analysis based on the charge regulation model in combination with a model for defects on a sphere. The charge regulation model therefore gives much insight into the behavior of the POM-shells. Up till now the POM-shells have been considered thermodynamically stable. Our experimental results however, challenge this view. It is shown that POM-shells change over time and that the preparation route of solutions of POMs affects the resulting species in solution. Moreover, in concentrated samples, a transition from spherical objects to elongated agglomerates was observed. The elongated objects subsequently grow into large, crystalline, needle-like structures. From these observations it is concluded that POMs follow an unusual nucleation route in which the POM-shells are in fact meta-stable intermediates. The term assembly can also be taken in a more general way. For instance polyoxomolybdates can be used to create extended structures. By using elementary molybdenum oxide building blocks and urea in a one-pot directed synthesis, an inorganic-organic hybrid extended structure, consisting of right- and left-handed helical units can be obtained.