title: Organic-inorganic hybrid polyoxometalate extended structures: self-assembled and configured
Creators: name: Johnson, Naomi Anna Bechmann
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: polyoxometalate , self-assembly , DNA, helix, hybrid.

description: The high oxidation state metal oxide clusters known as polyoxometalates (POMs) are interesting not only because of their unusual physical and chemical properties but also because they are complex structures that self-assemble from basic building blocks, a process that is not well understood. Synthetic chemists generally begin by developing an in-depth understanding of matter which can then be used to manipulate it; however, POM researchers tend to spend less time exploring the mechanisms of POM self-assembly and instead focus simply on the search for new clusters or the modification of existing ones. This approach can in some ways be compared to the life science fields in which researchers work with systems that may be too complex to fully understand yet are still able to manipulate processes for many applications. As a result, these researchers learn more about the system itself. Instead of focusing on the synthesis or self-assembly of POMs themselves, this thesis explores the formation of inter-cluster assemblies: starting with a self-assembling POM-based network and then moving on to explore the directed synthesis of inter-POM assemblies. Just like the self-assembly of POMs themselves, the way in which clusters arrange themselves on a supramolecular level takes place spontaneously, forming crystals and sometimes gels. Alternatively, there are methods in which inter-POM assemblies can be designed and engineered entirely by chemists. For this to be possible the incorporation of organic chemistry is necessary as the mechanisms are far better understood and manipulation is carried out with high precision and control. Organic moieties can be grafted onto POM clusters directly through covalent bonds forming organic-inorganic POM hybrids which can then be modified with a level of control comparable to pure organic chemistry. This is achieved through functionalisation of the organic ligands using reaction conditions that do not disturb the inorganic clusters to which they are fused. Herein both the intermolecular self-assembly and covalently connecting directed synthesis of organic-inorganic POM hybrid extended structures and oligomers are explored. In doing so, tentative comparisons with biomolecular configurable polymers, namely polypeptides and nucleic acids are made. The first section explores the extended structure of a self-assembled POM hybrid formed through the acidification of molybdate and the biological molecule, 5’-guanosine monophosphate. On crystallisation of the resulting bi-functionalised hybrid clusters, the guanosine Strandberg monomers, stack into double-helix structure with dimensions almost identical to Z-DNA. The formation of such a complex structure through spontaneous self-assembly of simple building blocks is interesting to those familiar with the inorganic origin of life theory proposed by Cairns-Smith. Further investigations using AFM and CD are made to explore the nature of the guanosine Strandberg in solution with results suggesting an ordered structure is present. The second section contrasts with the self-assembly of POM hybrid extended structures by controlled synthesis of discrete POM hybrid oligomers using asymmetric Mn-Anderson hybrids. This is achieved via the development of azide or alkyne-functionalised Mn-Anderson monomers isolated through chromatography and then used in a “Click” reaction to form dimers and trimers. These chains are then further extended with the addition of a monomer to each end resulting in a tetramer and pentamer. The resulting four oligomers are verified using ESI-MS and NMR and compared via SE-HPLC and IMS-MS where evidence for two conformers of the tetramer chain is observed. The last section builds directly from the previous section by working on expansion of the “Click” coupled POM hybrid oligomerisation to include other clusters. This concept is inspired by the configurable nature of polypeptide chains that result in structures with properties of an astonishing variety. Building blocks appropriate for such a task must be easily made in large quantities in order to function as a starting material. For example, the vanadium-based Lindqvist hybrid is deemed inappropriate for use as a building block due to low yields. Adaptation of the asymmetric Mn-Anderson method is applied to the Fe-Anderson hybrids where Fe acetate replaces the Fe acac starting material resulting in a purer product and an additional FMOC protection step added to avoid problematic basic properties of the TRIS ligand during material formation. An attempt at Co-Anderson hybrid synthesis unexpectedly results in the formation of a tri-functionalised cobalt-centred hybrid with three hydroxyl groups instead of amine groups, the post-modification of which is unsuccessful. Synthesis of an asymmetric Cr-Anderson hybrid building block is achieved through consecutive stepwise reactions. The resulting Mn-, Fe- and Cr-Anderson building blocks are then used for the formation of a Mn/Fe/Mn-Anderson trimer from which a Cr/Mn/Fe/Mn/Cr-Anderson pentamer is attempted.