Synthesis of high nuclearity M/iron/sulfur clusters. From building blocks to core rearrangements.

Abstract

In the first two chapters of this thesis the investigation of new synthetic methods for the construction of high nuclearity M/Fe/S clusters (M: Mo, Cu, Ni) is discussed. These methods employ the use of building blocks as structural motifs for the construction of more complex architectures by design. It would be ill-advised to derive any mechanistic conclusions from the aforementioned studies, due to the near impossibility of obtaining trustworthy mechanistic information on weak-field clusters. Nevertheless, they provide the basis for the development of a conceptual framework to direct synthetic methodologies that aspire to the FeMoco or P-cluster synthesis. Chapter IV concentrates on the synthesis, characterization, and properties of Mo/Fe/S clusters with biologically relevant substrates. These complexes have a common Mo₂Fe₆S₈ core and are potentially important precursors for structural, and possibly functional models of FeMoco. Preliminary results towards this objective are also presented. Chapters V and VI report on the design and synthesis of clusters obtained following a core rearrangement of compounds with the Mo₂Fe₆S₈. In chapter V the findings of the core rearrangements investigations after oxidation and in the presence of bridging sulfur ligands are unfolded. Enzymatic reductions as in nitrogenase require complex multimetallic molecular arrangements with the ability to store and deliver electrons. Such arrangements, superclusters, are not readily available. In this chapter, the synthesis of a cluster of clusters that contains a possible substrate activation cavity, and undergoes three reversible reductions is reported. Derivatives of such clusters may prove useful as multielectron reducing agents. In chapter VI the core rearrangements of the [(Cl₄-cat)₂Mo₂Fe₆S₈(PR₃)₆] clusters in the presence of thiophilic metals such as Cu that lead to the formation of unique and unprecedented clusters are described. The synthesis of heteropolynuclear clusters with different coordination geometries at different sites is possible in what we believe are metal substitution reactions. The use of metal clusters as ligands is not commonly practiced, but it could be an approach to the designed synthesis of new materials with very interesting structural, electronic and magnetic characteristics.