Synthesis, Reactivity, and Electronic Structure of Iron Complexes with Biomimetic Carbon and Sulfur Ligands
Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Holland, Patrick
Abstract
This thesis details the synthesis, reactivity, and electronic structure of iron complexes that are supported by three different polydentate ligands, connected by their incorporation of thiolate donors. Thiolates were chosen to mimic the sulfur-rich coordination environment of catalytic iron sites in the nitrogenase iron-molybdenum cofactor, with the aim of understanding structure-function relationships during N2 reduction.Chapter 1 provides an overview of current consensus and open questions regarding the enzymatic mechanism of N2 fixation, with a special focus on the state that initially binds N2. The mono- and di-nuclear synthetic model approach for mechanistic interrogation is then introduced, describing reported attempts to generate systems that are structurally faithful to iron sites in the FeMoco as well as competent at N2 reduction. The survey of such complexes shows that there is still open opportunity to investigate sulfur-based ligands for N2 fixation under mild conditions. Chapter 2 describes the spectroscopy and electronic structures of three iron complexes supported by an SCS pincer with iminothiolate arms and a central anionic aryl C. Each of the complexes features a nitrogenase-relevant fourth donor: NR2−, SAr−, or NH3. The SCS iron(III) complexes with these ligands were all pseudo square planar with intermediate S = 3/2 spin states, as revealed by EPR spectroscopy and magnetic measurements. Mössbauer spectroscopy combined with computational studies uncovered that the relative order of the d orbitals was sensitive to nearby cations if the fourth donor was NR2−. No N2 binding was observed in this system, but it demonstrated the ability of the SCS platform to fine-tune the electronic structure of iron. Chapter 3 reports the design, synthesis, metalation with iron, and reactivity of a PCS ligand derived from C–S cleavage of phosphine-functionalized benzothiophene. This ligand architecture is unusually reactive toward intramolecular bond activations including a second cation-dependent C–S cleavage accompanied by S− loss and N2 binding, and further C–P bond activation. These bond-breaking events are a result of reduced, unsaturated intermediates. Chapter 4 follows up on the reactivity of a previously-reported diiron μ-N2H2 complex supported by a tetradentate ligand that coordinates iron through two thioethers and two thiolates. There is intramolecular N–H···S hydrogen bonding that stabilizes the μ-N2H2 and might play a part in the transformation of N2H2 to N2, which has relevance to understanding the reverse reaction during biological N2 fixation. A metastable intermediate formed after two-electron oxidation was originally proposed to be a μ-N2 species, but in situ spectroscopy along with DFT computations disproved this hypothesis. However, a viable route to the μ-N2 species involving proton transfer to S is demonstrated through calculations.
Recommended Citation
Hooper, Reagan Xie-Mei, "Synthesis, Reactivity, and Electronic Structure of Iron Complexes with Biomimetic Carbon and Sulfur Ligands" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1183.
https://elischolar.library.yale.edu/gsas_dissertations/1183
