Non-Heme Iron Complexes Applied to Oxyanion Reduction

Oxyanion contamination poses a significant and growing environmental challenge due to the widespread anthropomorphic release of species such as nitrate, perchlorate, phosphate, and sulfate in groundwater and wastewater. Biological systems can convert these pollutants to environmentally benign or useful molecules by employing metalloenzymes. The macrostructure of metalloenzymes plays a crucial role in catalysis by precisely positioning active sites, regulating substrate access, and facilitating essential conformational dynamics. Within this framework, the secondary coordination sphere, comprising hydrogen bonding amino acid residues, provides remarkable control over substrate binding, intermediate stabilization, and electron/proton transfer processes that enable exceptional selectivity and efficiency. Seeking to mimic secondary coordination sphere function, my dissertation research primarily applies biologically inspired iron complexes with tripodal or tetrapodal ligands to study the multi-electron reduction of nitrogen and chlorine containing oxyanions. Previous work in our own lab showcased reduction of nitrate and nitrite (NOx- where X = 2 or 3) using a tripodal iron complex, forming the corresponding iron-nitrosyl or nitric oxide as the final nitrogen containing products. As nitric oxide contributes to acid rain and ozone depletion, chapter 2 expands on this work with the addition of exogenous reductants at the key NO reduction step, enabling tunable selectivity toward either ammonia or nitrogen and providing a targeted strategy for oxyanion remediation. To better understand intermediates involved in NOx- to nitric oxide conversion, chapter 3 introduces a tetrapodal iron complex, whose secondary sphere features only two hydrogen bond donors (compared to three in the tripodal system). This variation afforded new spectroscopic access to oxyanion binding, destabilization of iron(III)-hydroxide species compared to the tripodal system, and formation of an iron-nitrosyl complex as the final product. These findings highlight the nuanced impact of secondary coordination sphere variation and emphasize how modulating ligand structure can tune both reactivity and selectivity in synthetic models. Chapter 4 extends this chemistry by applying the tetrapodal iron system to the more challenging reduction of perchlorate. While stoichiometric reduction to an iron-chloride complex was achieved, challenges with catalyst turnover and observable chloride formation or abstraction highlight fundamental limitations that persist in designing truly efficient homogeneous systems. Chapter 5 expands the tripodal ligand framework to early transition metals (Ti, V, Cr, Mo). The resulting trivalent complexes, examined spectroscopically and computationally, highlight the adaptability of the secondary coordination environment for future studies of reactivity and catalysis. In summary, this work demonstrates that careful consideration of the secondary coordination sphere in synthetic complexes coupled with systemic design enables significant advances in the selective reduction of environmentally persistent oxyanions. The versatility of these ligand architectures supports both iron and early transition metal chemistry, offering a powerful toolkit for developing new catalysts for pollution remediation and advancing our mechanistic understanding of oxyanion reduction.

Name thesis:degree_name

Doctor of Philosophy

Discipline thesis:degree_discipline

Chemistry

Grantor

Texas A&M University

Year dc:date.issued

2025

Author dc:creator

• Moore, Jewelianna

Advisor dc:contributor.advisor

• Fout, Alison

Committee members dc:contributor.committeemember

• Darensbourg, Marcetta
• Altman, Alison
• Tommos, Cecilia

• Chemistry, Inorganic

Handle dc:identifier.uri

https://hdl.handle.net/1969.1/1600767

OAI record oai:identifier

https://oaktrust.library.tamu.edu/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Aoaktrust.library.tamu.edu%3A1969.1%2F1600767