International Collaboration in Chemistry: Experimental and Theoretical Study of Redox-Active Fe4O4-Cubanes

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This project is based on a collaboration between two groups, one experimental and one theoretical, who share a common interest in the interaction between metal ions in high nuclearity clusters. The proposal has been inspired by our recent characterization of an octanuclear iron cluster, [FeIII8(mu4-O)4(mu-4-R-pz)12X4], (pz = the pyrazolato anion) which contains an Fe4O4 cubane core surrounded by four peripheral FeIII centers. Our fascination with this particular family of clusters stems from their remarkable electrochemical properties: four reversible reduction waves can be accessed in a potential window much narrower than anything seen for the more extensively-studied iron/sulfur analogues. The ability of iron clusters to accommodate large fluctuations in electron count is fundamental to their role in biological electron transfer, and these clusters offer a unique window on the intimate electronic processes involved. We have illustrated the synergy between experiment and theory in these systems in a recent joint publication, where we analyzed the structure and properties of the all-ferric parent compounds. Very recently, we have also succeeded in isolating the 1-electron reduced species, [FeIII8(mu4-O)4(mu-4-R-pz)12X4]-. In this proposal, we seek support to extend our studies to the 1-, 2-, 3- and 4-electron reduced analogues, all of which feature ferric and ferrous centers. The ultimate goal is to characterize as fully as possible each of the mixed-valent states spanning the FeIII8 and FeIII4FeII4 limits observed in the electrochemical experiment. The focus at the University of Puerto Rico will be on the synthesis of new clusters capable of supporting the more highly reduced oxidation levels and their characterization using a wide range of physicochemical techniques. Voltammetry, diffraction, in situ UV-Vis-NIR spectroelectrochemistry and XPS will be conducted 'in-house', while magnetochemical properties, EPR and Mssbauer spectra will be measured in the laboratories of established collaborators who are leaders in their fields. The research team at the University of Glasgow will explore the electronic structure of these compounds using broken-symmetry density functional theory. The aim is to maximize the information content of the physicochemical measurements by establishing a direct correspondence between observation and the underlying electronic structure. Intellectual merit: The study of mixed-valent metal species has long held the attention of chemists, and the interactions between the metal centers have profound implications in both materials and biological chemistry. The Fe8 clusters described in this proposal offer a unique opportunity to study mixed valence in a discrete molecular species across an unprecedented range of oxidation states. Much of the previous work in this area has been limited to di- or tetranuclear clusters, and the presence of eight paramagnetic centers presents a significant increase in complexity.This project will link established research programmes at the University of Puerto Rico and the University of Glasgow, both of which focus on the structure of paramagnetic metal clusters. The approaches of the two groups are quite distinct, one using synthesis and physicochemical measurements, the other theory, and the synergy between the two promises to reveal new insights. The opportunity for young researchers to exchange complementary ideas and expertise through extended stays in international laboratories is a key element of the research programme, and will undoubtedly establish long-lasting links between the two groups. The applicants have a track record of informal collaboration dating back to the early 1990s, and have already published joint papers on a number of topics related to the electronic structure and properties of metal clusters. In all cases, the collaboration has been founded on the synergy between spectroscopy and theory that lies at the heart of this proposal.