Structure and Reactivity of Mono- and Diiron Synthetic Models of Nonheme Iron-containing Enzymes

Abstract

Metalloenzymes are utilized to perform different physiological functions in biology. Nonheme iron-containing enzymes are one such class of metalloenzymes that can catalyze a wide array of reactions at ambient temperature and pressure. Several mononuclear nonheme iron enzymes carry out a range of oxidative transformations via a common oxoiron(IV) oxidant with high-spin iron(IV) centers. Within the past two decades, many small-molecule analogs of these highly reactive species have been synthesized, and under favorable circumstances, some have even been crystallized. These models shed light on the structural and functional properties of the FeIV=O unit in biological systems. Dinuclear nonheme iron enzymes such as soluble methane monooxygenase (sMMO) activate oxygen to carry out challenging transformations like hydroxylation of strong C-H bonds in methane. The enzymatic cycle of sMMO involves the formation of a peroxodiiron(III) intermediate (P) that gives rise to a diiron(IV) complex (Q), which is the species responsible for methane hydroxylation. Synthetic models for Q are rare, and none of these are formed by mimicking the conversion of P to Q. This thesis describes work on synthetic models that will enhance our understanding of enzymes and contribute to future catalyst design. In Chapter 2, reaction optimization studies along with an array of spectroscopy studies have been utilized to elucidate the solution-state structure of a highly reactive oxoiron(IV) complex [FeIV(O)(Me3NTB)]2+ (Me3NTB=tris[(1‐methyl‐benzimidazol‐2‐yl)methyl]amine). This chapter presents evidence that the spin-state of the iron(IV) center is not the sole determinant in governing reactivity of oxoiron(IV) complexes as previously proposed in the literature. Our results emphasize the need to identify factors besides the ground spin state of the FeIV=O center to rationalize nonheme oxoiron(IV) reactivity. In Chapter 3, a diiron cluster supported by Me3NTB ligand is used to carry out oxygen activation. This generates an unstable µ-1,2-peroxo species, which is characterized using various spectroscopic techniques. The structural analyses of this complex highlight the unique properties of this model. Even more interesting is the observation that this peroxodiiron(III) complex undergoes O–O bond cleavage upon treatment with strong Lewis acids and transforms into a bis(μ-oxo)diiron(IV) complex, thus providing a synthetic precedent for an analogous reaction in the diiron enzyme sMMO. In Chapter 4, oxygen activation chemistry is further explored using a quinoline rich framework to generate diiron intermediates with high-spin configurations. In this work, we have used the tripodal ligand TQA (TQA = tris(2-quinolylmethyl)amine) to support a diferrous center that upon exposure to O2 leads to the isolation of a bis(μ-oxo)diferric complex. This is the first crystal structure of such a complex that is formed by oxygen activation. The formation of a [Fe2(μ-O)2]2+ complex through oxygen activation also provides indirect evidence that the oxygen activation in our model passes through a similar mechanism like the diiron enzymes. Interesting solvent dependence is also reported for the oxygen activation process with this system. In Chapter 5, key results in the preceding chapters are summarized and future directions evaluated.