Exploring the hidden constraints that control the self-assembly of nanomolecular inorganic clusters

The mechanism that controls the self-assembly of complex polyoxometalate inorganic clusters in solution is a challenge because the clusters can contain anywhere from 6 to 368 metal ions in a single molecule and are commonly assembled under ʻone-potʼ reaction conditions. Furthermore, harnessing the ʻself-assemblyʼ processes allows synthesis on a length-scale and with a degree of complexity which is not easily accessed by more traditional ʻstep-wiseʼ synthesis routes. This class of molecules are important as they can be thought of as ʻmolecular metal oxidesʼ and rival the size of proteins yet are based upon simple MOx units (where M is Mo, W, V and sometimes Nb and x can be 4, 5, 6 or 7). Here we review the structures of the molybdenum-based clusters and then show how it is possible to not only control the architectures but explore the mechanism of formation invoking mechanisms that are normally found in biological systems. We describe how a simple inorganic salt can spontaneously form ʻinformation-richʼ, autocatalytic sets of replicating inorganic molecules that work via molecular recognition based on the [PMo12O40]3–, {PMo12} Keggin ion, and [Mo36O112(H2O)16]8–, {Mo36} cluster. These small clusters are involved in an autocatalytic network, where the assembly of gigantic molybdenum blue wheels [Mo154O462H14(H2O)70]14– {Mo154}, [Mo132O372(H2O)72(CH3CO2)30]42–, {Mo132} ball containing 154 and 132 molybdenum clusters are templated by the smaller clusters which are themselves able to catalyze their own formation. Finally, We introduce assembly theory and show how assembly theory can be used to describe the constraints that need to be present to facilitate the assembly of the large clusters and explain how information theoretic arguments might be useful for designing highly functional and unsymmetrical inorganic molecules