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A Synthetic Nitrogenase: Insights into the Mechanism of Nitrogen Fixation by a Single-Site Fe Catalyst

Citation

Thompson, Niklas Bjarne (2018) A Synthetic Nitrogenase: Insights into the Mechanism of Nitrogen Fixation by a Single-Site Fe Catalyst. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T4WQ-TM68. http://resolver.caltech.edu/CaltechTHESIS:05292018-165300346

Abstract

Nitrogen fixation, specifically the conversion of molecular nitrogen into ammonia, is a fundamental reaction necessary to support life. Our group has recently discovered the first family of well-defined iron complexes that catalyze the conversion of dinitrogen to ammonia. This thesis details mechanistic study of the nitrogen fixation chemistry these complexes. Chapter 1 presents an abbreviated overview of catalytic nitrogen fixation, which places our work in a larger context. Chapter 2 details the synthesis and nitrogen fixation activity of a series of cobalt complexes that are homologous to the known iron-based catalysts. The central goal of this work was to provide a structure-function study of the isostructural cobalt and iron complexes, in which the nature of the transition metal ion was changed in a fashion that predictably modulated the electronics of the system. Chapter 3 details in situ mechanistic studies of nitrogen fixation catalyzed by the iron complexes under the originally-reported reaction conditions. In this study, we were able to achieve a nearly order-of-magnitude improvement of catalyst turnover. Study of the reaction dynamics evidence a single-site mechanism for dinitrogen reduction, which is corroborated by in situ monitoring of catalytic reaction mixtures using freeze-quench Mössbauer spectroscopy. In Chapter 4, we study the key N-N bond cleavage step in the catalytic cycle for nitrogen fixation. In this chapter, we demonstrate that sequential reduction and low-temperature protonation of an iron catalyst results in the formation of ammonia and a terminal Fe(IV) nitrido complex. This result provides a compelling proposal for the mechanism of the catalytic nitrogen fixation reaction. Finally, in Chapter 5 we present spectroscopic and computational studies detailing the electronic structures of a redox series of Fe(NNR2) complexes that model key catalytic intermediates occurring prior to the N-N bond cleavage step. We evidence one-electron redox non-innocence of the “NNR2” ligand, which resembles that of the classically non-innocent ligand, NO, and may have mechanistic implications for the divergent nitrogen fixation activity of the some of the iron complexes studied by our group.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Nitrogen fixation; ammonia synthesis; Fe catalysis
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Peters, Jonas C.
Group:Resnick Sustainability Institute
Thesis Committee:
  • Agapie, Theodor (chair)
  • Fu, Gregory C.
  • Chan, Garnet K.-L.
  • Peters, Jonas C.
Defense Date:25 May 2018
Funders:
Funding AgencyGrant Number
NIHGM 070757
Record Number:CaltechTHESIS:05292018-165300346
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05292018-165300346
DOI:10.7907/T4WQ-TM68
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acs.inorgchem.5b00645DOIPublished content for Chapter 2.
https://doi.org/10.1021/jacs.6b01706DOIPublished content for Chapter 3.
https://doi.org/10.1021/jacs.7b09364DOIPublished content for Chapter 4.
ORCID:
AuthorORCID
Thompson, Niklas Bjarne0000-0003-2745-4945
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10965
Collection:CaltechTHESIS
Deposited By: Niklas Thompson
Deposited On:01 Jun 2018 18:51
Last Modified:12 Jul 2018 19:37

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