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Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex


Anderson, John Stuart (2014) Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/NS23-B474.


Threefold symmetric Fe phosphine complexes have been used to model the structural and functional aspects of biological N2 fixation by nitrogenases. Low-valent bridging Fe-S-Fe complexes in the formal oxidation states Fe(II)Fe(II), Fe(II)/Fe(I), and Fe(I)/Fe(I) have been synthesized which display rich spectroscopic and magnetic behavior. A series of cationic tris-phosphine borane (TPB) ligated Fe complexes have been synthesized and been shown to bind a variety of nitrogenous ligands including N2H4, NH3, and NH2-. These complexes are all high spin S = 3/2 and display EPR and magnetic characteristics typical of this spin state. Furthermore, a sequential protonation and reduction sequence of a terminal amide results in loss of NH3 and uptake of N2. These stoichiometric transformations represent the final steps in potential N2 fixation schemes.

Treatment of an anionic FeN2 complex with excess acid also results in the formation of some NH3, suggesting the possibility of a catalytic cycle for the conversion of N2 to NH3 mediated by Fe. Indeed, use of excess acid and reductant results in the formation of seven equivalents of NH3 per Fe center, demonstrating Fe mediated catalytic N2 fixation with acids and protons for the first time. Numerous control experiments indicate that this catalysis is likely being mediated by a molecular species.

A number of other phosphine ligated Fe complexes have also been tested for catalysis and suggest that a hemi-labile Fe-B interaction may be critical for catalysis. Additionally, various conditions for the catalysis have been investigated. These studies further support the assignment of a molecular species and delineate some of the conditions required for catalysis.

Finally, combined spectroscopic studies have been performed on a putative intermediate for catalysis. These studies converge on an assignment of this new species as a hydrazido(2-) complex. Such species have been known on group 6 metals for some time, but this represents the first characterization of this ligand on Fe. Further spectroscopic studies suggest that this species is present in catalytic mixtures, which suggests that the first steps of a distal mechanism for N2 fixation are feasible in this system.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Iron, Nitrogen Fixation, Nitrogenase
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Awards:The Herbert Newby McCoy Award, 2014
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Peters, Jonas C.
Thesis Committee:
  • Bercaw, John E. (chair)
  • Agapie, Theodor
  • Stoltz, Brian M.
  • Peters, Jonas C.
Defense Date:3 September 2013
Non-Caltech Author Email:jsa312 (AT)
Funding AgencyGrant Number
Gordon and Betty Moore FoundationUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Record Number:CaltechTHESIS:09182013-164526961
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for ch. 3 adapted for ch. 4
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7959
Deposited By: John Anderson
Deposited On:07 Jan 2015 19:53
Last Modified:08 Nov 2023 00:44

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