Investigating the Effects of Non-covalent Interactions in Metal Complexes with Versatile Ligand Scaffolds.

摘要

In Nature, transition metal-containing enzymes have evolved to catalyze a wide variety of chemical reactions with astounding efficiencies and selectivities through precise control of their primary and secondary coordination environments. Taking inspiration from the active sites of metalloenzymes, synthetic bioinorganic chemists have endeavored to construct metal complexes that model the structures and functions of these enzymes. Controlling the primary coordination spheres of metal ions to model active sites is accomplished through the construction of metal-binding organic ligand frameworks, but the resulting metal complexes often lack the activities observed in their biological counterparts. Non-covalent secondary coordination sphere effects that are present within metalloenzyme active sites are integral to the proper function of these proteins and have been more challenging to emulate within synthetic systems. Manipulation of the secondary coordination environment surrounding synthetic metal complexes can be accomplished by using rigid ligand scaffolds containing groups that promote non-covalent interactions proximal to the metal center.;The Borovik group has developed a number of tetradentate ligands that can bind metal ions and establish a pocket where external ligands can bind to the metal center. The ligands contain moieties that interact with the metal-bound exogenous ligands through non-covalent effects, particularly hydrogen bonds (H-bonds). This dissertation chronicles the advances made with two tripodal ligand platforms that promote intramolecular H-bonding networks within the resulting complexes.;The first ligand studied, TAO, tautomerizes upon metal ion binding to provide a neutral N4 primary coordination environment to the metal center. This tautomerization event places H-bond donating groups proximal to external ligands that can bind to the metal center, producing intramolecular H-bonding networks. TAO complexes are sufficiently flexible to accommodate ligands of varying sizes, shapes, and charges. The versatility of the TAO framework is exemplified through a series of metal complexes which have been characterized in both the solution and solid state. Ab initio theoretical calculations performed on many of these complexes provide insight into the electronic effects of the H-bonding interactions.;The ligand [MST]3- binds metal ions in a similar manner to TAO, but possesses sulfonamido groups that render the complex cavity capable of both accepting H-bonds and binding secondary metal ions. The bifunctionality of [MST]3- has been explored through the synthesis of heterobimetallic complexes wherein the metal ions are bridged by a hydroxido group. The work on [MST]3- in this dissertation focuses on diamagnetic main group analogs of these complexes whose structures could be elucidated in both the solution and solid states. The structurally characterized complexes were found to retain their supramolecular structures in solution. The relevance of some of these complexes to the oxygen evolving complex within Photosystem II is also discussed. Additionally, M-NH3 (M = FeII, FeIII, GaIII) complexes are presented that contain intramolecular H-bonding networks and which remain assembled in both solution and the solid state. A putative FeIII-NH2 species and its reactivity is discussed, and the synthesis and structure of a Ga III-N2H4 complex is documented.;A new N4 donor ligand platform, [POAT]3-, is presented in the final portion of this dissertation. In complexes with this ligand, phosphine oxide groups are positioned within the secondary coordination sphere of the metal center to act as H-bond acceptors and to bind additional metal ions. While these features are akin to those in the [MST]3- system, preliminary studies indicate that the primary coordination sphere is much more reducing; as such, complexes with [POAT]3- are anticipated to stabilize metal complexes in higher oxidation states.