Spectroscopic Study and Computational Modeling on Metallocofactors and Synthetic Model Complexes for Small-Molecule Activation

Abstract

Transition metal complexes and metallocofactors play important roles in industrial catalysis, material science, photochemistry, polymer synthesis and biological functions due to their diverse electronic structures. Iron, due to its various oxidation states and high abundance on earth, has attracted great attentions to serve as a key cofactor in biocatalysts and artificial model complexes. Although such properties have been exploited in many artificial and biological systems, there are still many remaining problems on understanding the interaction between iron and unactivated small molecules, such as dioxygen, nitricoxide, dihydrogen, dinitrogen, etc. The work hereby is to elucidate the geometric and electronic structures of key reactive iron intermediates in enzyme and model complexes by a variety of methods to provide new chemical insights into small molecule activation by iron containing systems. The experimental methods used in this dissertation mainly include 57Fe Mossbauer spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT). This dissertation covers four different iron-containing systems: high-valent iron-oxo complexes, nonheme diiron-containing oxygenases, iron-nitrosyl species in model protein system, and heterodinuclear metal complexes. The structure and oxidation state of high-valent iron complexes/cofactors always attract the most attention in the bioinorganic field due to their strong C-H bond activation capability. In chapter 2, a detailed characterization on a series of iron(V)-oxo and iron(IV)-oxo model complexes supported by a novel pyridyl N4-ligand platform is presented.Sufficient evidence is provided in this chapter to support the assignment of a highly reactive iron(V)-oxo complex, which is only the third well-characterized iron(V)-oxocomplexes reported in the literature. The subsequent spectroscopic studies also uncover the special orientation of iron-oxo bond (trans to pyridyl instead of tertiary amine) in thecorresponding iron(IV)-oxo complex. Chapter 3 presents new advances in characterizing the structure of iron(IV)-oxo complexes by 1H-NMR spectroscopy and accurateevaluations on predicting chemical shift by DFT. In this chapter, we include two case studies on 1) determining the spin state of iron(IV)-oxo complex at high temperature and2) exploring the relationship between ligand orientation of the iron(IV)-oxo complexes and their proton chemical shifts. Chapter 4 presents a new mechanism on a six-electronoxidation reaction catalyzed by CmlI, a nonheme di-iron containing oxygenase, and a comparative spectroscopic study on the diferric species of CmlI with the diferric speciesof UndA, another newly discovered nonheme di-iron containing oxygenase. Chapter 5 describes a detailed spectroscopic study on nonheme iron-nitrosyl species stabilized a bioengineered azurin. In this chapter, we uncover chemical transformations among different iron-nitrosyl species introduced by nitric oxide addition, chemical reduction or cryoreduction techniques. This work covers a large amount of details of combining two or more techniques to reveal one specific species in a multi-equilibrium system. Going beyond the reactivity, chapter 6 focuses on the influence of ligand topology on the iron center structure in a series of hetero-dinuclear metal complexes. Specifically, we consider three hetero-dinuclear FeIII-O-CrIII complexes derived from the tetramethylcyclam (TMC) ligand in order to mimic topological effects of ligands in many enzymes. Although these complexes share the similar Fe-μ-oxo-Cr core structures, the facial orientation of the TMCring alters the coordination number, core bond angles and electronic structures.