Synthesis and Reactivity of Non-Heme Iron-Nitrosyl Complexes that Model the Active Sites of NO Reductases

Abstract

As we are inching closer to the end of the COVID-19 pandemic, it is important for us to be better prepared for other potential pandemic scenarios such as a potential new strain of super bacteria that can resist known antibiotics. Nitric oxide (NO) is utilized by our immune system to fight invading pathogens. Through natural selection, some pathogens have adapted flavodiiron proteins (FDPs) to reduce NO to nitrous oxide (N2O) (so-called flavodiiron NO reductases, FNORs). These enzymes protect the microbes from nitrosative stress and mitigate the toxicity of NO generated in the human immune response. Mechanistic studies on the Thermatoga maritima (Tm) FDP have shown that a high-spin (hs) diiron dinitrosyl intermediate, or [hs-{Fe(NO)}7]2 in Enemark-Feltham notation, is the critical intermediate that forms prior to NO reduction. However, the succeeding steps of the reaction and the other intermediates prior to N2O release have remained elusive. In this thesis, several new mono- and dinuclear Fe(II) model complexes are reported. The reactivity of these complexes towards NO and hyponitrite (N2O22) was then investigated to (a) explore different mechanistic possibilities for FNORs, (b) determine under which conditions different mechanistic pathways are activated, and (c) characterize potential intermediates of the reaction. Whereas mechanistic studies on Tm FDP favor the so-called direct NO reduction pathway, no diiron dinitrosyl model complex is able to mediate this reaction. All known model systems prior to my work require one-electron reduction to generate N2O (the semireduced pathway) via a proposed hs-{Fe(NO)}7/hs-{Fe(NO)}8 intermediate. Here, I report the new model complex [FeII2((Py2PhO2)MP)(OAc)2] (1), which is the first model system that can catalyze the direct reduction of NO to N2O. My results show that reduction potential is a key trigger to activate the direct NO reduction pathway in diiron complexes. The reaction of 1 with NO is so efficient (even at -80oC) that the isolation of reaction intermediates was not possible. By using a mononuclear model of 1 I was able to isolate a highly activated mononuclear hs-{FeNO}7 complex with a record low N–O stretching frequency of 1689 cm-1. These studies demonstrate that hs-{FeNO}7 species with N–O stretching frequencies 1700 cm-1 are activated for direct NO reduction, but that a diiron core is critically important to enable this reaction. Additionally, the role of second coordination sphere hydrogen bond donors and the chemistry of non-heme iron complexes with hyponitrite relevant for the NO reduction in FNORs were also evaluated through the synthesis and characterization of new complexes reported in this thesis. Using mononuclear systems, I explored unique reactions that hs-{FeNO}8 complexes can mediate. The hs-{FeNO}7 complex with the weak-field ligand BMPA-tBu2PhOH forms a dinitrosyl iron complex (DNIC) upon reduction, in line with previous observations in the literature. When TPA is used as the ligand scaffold, stabilization of an unprecedented complex with a Fe2(NO)2 diamond core structure is observed instead. This complex is stabilized by a change to the low-spin state of the iron centers. I propose that a similar Fe2(NO)2 motif is the key intermediate for DNIC formation when the irons remain high-spin. Finally, to investigate iron-nitrosyl complexes beyond the {FeNO}8 state, a novel ls-{FeNO}8-10 series (prepared by Peters and coworkers) was spectroscopically and theoretically characterized. The results show that a reverse-dative Fe B interaction is the key to stabilize the unique oxidation states that go beyond the {FeNO}8 state.