We wish to address the growing demand for new MRI contrast-enhancing agents by developing a new class of iron-based materials for this purpose. We will use an octanuclear iron-oxo cluster as the basic paramagnetic core, which will be substituted at its periphery in order to achieve water-solubility and permit manipulation of its pharmacokinetics. The aim of this project is to deliver a module possessing the desired proton-relaxivity properties, as well as peripheral sites amenable to further modification. The latter will permit the development of injectable contrast agents by routine synthetic procedures. The long-term goal is to produce a new class of contrast agents based on iron-oxo cluster. We will prepare water-soluble derivatives of a paramagnetic, octanuclear Fe""-oxo-pyrazolato cluster, Fe/8(Mu/4-O)4(Mu/PZ) 12X4, pz = pyrazolato anion, X = CI, Br, so that they can be screened as potential contrast enhancing agents for in vivo and in vitro MRI applications. As a reliable synthesis of this hydrophobic cluster, as well as a few of its derivatives, is established, we will approach the preparation of its water-soluble derivatives by introducing hydrophilic groups at its periphery by two methods: a) De novo cluster synthesis using 4-substituted pyrazole ligands, b) post-modification of the cluster by substitution at the stericaly unhindered 4-position of the pyrazoles, or at the terminal halide ligands. The new octanuclear clusters will be fully characterized by standard methods, and their relaxivities, biological activities and imaging properties will be evaluated.