Principles of Molecular Design in the Development of Ligand Frameworks that Control the Primary and Secondary Coordination Spheres of Metal Ions

Metalloproteins have evolved to contain specific combinations of primary and secondary coordination spheres within their active sites that contribute to the high activity and selectivity of their chemical transformations. One approach to developing small molecule analogs with comparable reactivity is to design molecular constructs that exhibit similar control over aspects of the primary and secondary coordination spheres. This dissertation describes the design and reactivity of multifunctional ligands that position hydrogen-bond acceptors within the secondary coordination spheres of transition metal complexes. Two classes of ligands were studied: the first is a tetradentate, tripodal ligand ([MST]3-) that was previously demonstrated to support formation of heterobimetallic complexes containing a Mn center and a redox inactive metal ion. The extension to the Fe analog is described here, as well as the continued examination of the influence of group 2 metal ions on the properties and reactivity of both the Mn and Fe complexes. The most important finding of these studies is that Ca(II) and Sr(II) ions impart nearly identical influences on the Mn and Fe centers—a result with relevance to biological water oxidation. Reaction of [MnMST]– and [FeMST]– with oxygen-atom transfer reagents in the absence of secondary metal ions uncovered a limitation of the [MST]3- ligand system in which weak C–H bonds are susceptible to intramolecular oxidation. In order to address this limitation, ligand derivatives were designed that removed weak C–H bonds, which led to the observation of a putative Fe(IV)–oxo intermediate. The last two chapters of this dissertation describe the development of a new type of ligand that utilizes a tridentate, redox-active framework that has been shown to provide access to multi-electron transformations at first row transition metal centers. The first derivative of this ligand was shown to form dinuclear complexes of Fe, Co, and Cu with a pre-organized binding pocket for secondary metal ions. Modification of the steric bulk of the ligand prevented formation of these dinuclear complexes and instead supported mononuclear Fe and Co centers. The structural, electronic, and electrochemical properties of both of these series of complexes are described