| The current burgeoning research in high nuclearity manganese-containing carboxylate clusters is primarily due to their relevance in areas as diverse as magnetic materials and bioinorganic chemistry. In the former, the ability of single molecules to retain, below a critical temperature (T B), their magnetization vector, resulting in the observation of bulk magnetization in the absence of a field and without long-range ordering of the spins, has termed such molecules as Single-Molecule Magnets (SMMs), or molecular nanomagnets. These molecules display superparamagnet like slow magnetization relaxation arising from the combination of a large molecular spin, S, and a large and negative magnetoanisotropy, D. Traditionally, these nanomagnets have been Mn containing species. An out of the box approach towards synthesizing SMMs is engineering mixed-metal Mn-containing compounds. An attractive choice towards this end is the use of Lanthanides (Ln), which possess both a high spin, S, and a large D. A family of related MnIII8Ce IV SMMs has been synthesized. However, the Ce ion of these complexes is diamagnetic (CeIV). Thus, further investigation has led to the isolation of a family of MnIII11Ln III4 complexes in which all but the Ln = Eu complex function as single-molecule nanomagnets. The mixed-metal synthetic effort has been extended to include actinides with the successful isolation of a Mn IV10ThIV6 complex, albeit this homovalent complex is not a SMM.; In the bioinorganic research, the Water Oxidizing Complex (WOC) in Photosystem II (PS II) catalyzes the oxidation of H2O to O2 in green plants, algae and cyanobacteria. Recent crystal structures of the WOC confirm it to be a Mn4CaOx cluster with primarily carboxylate ligation. To date, various multinuclear Mn complexes have been synthesized as putative models of the WOC. On the contrary, there have been no synthetic MnCa(Sr) mixed-metal complexes. Thus, in this bioinorganic modeling research of the WOC, various synthetic methods have been developed to prepare a variety of heterometallic MnCa(Sr) complexes, namely, Mn13Ca2, Mn11Ca4, Mn8Ca and Mn14Sr; these are the first of their kind. X-ray absorption spectroscopy has been performed on all of these complexes and the results compared with analogous data on the WOC of PS II. In particular, Ca, Sr, and Mn, EXAFS and XANES reveal a distinct similarity between the sub-units within these complexes and the Mn 4CaOx site of the WOC. The data strongly suggest that a single-atom O bridge exists between the Mn atoms and the Ca atom of the WOC. |
