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Proton-Coupled Reduction of N2 Facilitated by Molecular Fe Complexes

Citation

Rittle, Jonathan Daniel (2016) Proton-Coupled Reduction of N2 Facilitated by Molecular Fe Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9QJ7F7D. http://resolver.caltech.edu/CaltechTHESIS:12012015-124453213

Abstract

The activation of Fe-coordinated N2 via the formal addition of hydrogen atom equivalents is explored in this thesis. These reactions may occur in nitrogenase enzymes during the biological conversion of N2 to NH3. To understand these reactions, the N2 reactivity of a series of molecular Fe(N2) platforms is investigated. A trigonal pyramidal, carbon-ligated FeI complex was prepared that displays a similar geometry to that of the resting state 'belt' Fe atoms of nitrogenase. Upon reduction, this species was shown to coordinate N2, concomitant with significant weakening of the C-Fe interaction. This hemilability of the axial ligand may play a critical role in mediating the interconversion of Fe(NxHy) species during N2 conversion to NH3. In fact, a trigonal pyramidal borane-ligated Fe complex was shown to catalyze this transformation, generating up to 8.49 equivalents of NH3. To shed light on the mechanistic details of this reaction, protonation of a borane-ligated Fe(N2) complex was investigated and found to give rise to a mixture of species that contains an iron hydrazido(2-) [Fe(NNH2)] complex. The identification of this species is suggestive of an early N-N bond cleavage event en route to NH3 production, but the highly-reactive nature of this complex frustrated direct attempts to probe this possibility. A structurally-analogous silyl-ligated Fe(N2) complex was found to react productively with hydrogen atom equivalents, giving rise to an isolable Fe(NNH2) species. Spectroscopic and crystallographic studies benefited from the enhanced stability of this complex relative to the borane analogue. One-electron reduction of this species initiates a spontaneous disproportionation reaction with an iron hydrazine [Fe(NH2NH2)] complex as the predominant reaction product. This transformation provides support for an Fe-mediated N2 activation mechanism that proceeds via a late N-N bond cleavage. In hopes of gaining more fundamental insight into these reactions, a series of Fe(CN) complexes were prepared and reacted with hydrogen-atom equivalents. Significant quantities of CH4 and NH3 are generated in these reactions as a result of complete C-N bond activation. A series of Fe(CNHx) were found to be exceptionally stable and may be intermediates in these reactions. The stability of these compounds permitted collection of thermodynamic parameters pertinent to the unique N-H bonds. This data is comparatively discussed with the theoretically-predicted data of the N2-derived Fe(NNHx) species. Exceptionally-weak N-H bond enthalpies are found for many of these compounds, and sheds light on their short-lived nature and tendency to evolve H2. As a whole, these works both establish and provide a means to understand Fe-mediated N2 activation via the addition of hydrogen atom equivalents.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:nitrogen activation, proton-coupled electron transfer, synthetic chemistry, inorganic chemistry, iron complexes
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Awards:The Herbert Newby Mccoy Award, 2016
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Peters, Jonas C.
Thesis Committee:
  • Gray, Harry B. (chair)
  • Agapie, Theodor
  • Rees, Douglas C.
  • Peters, Jonas C.
Defense Date:9 November 2015
Record Number:CaltechTHESIS:12012015-124453213
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:12012015-124453213
DOI:10.7907/Z9QJ7F7D
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1038/nature12435DOIArticle adapted for ch. 2
http://dx.doi.org/10.1021/jacs.5b03432DOIArticle adapted for ch. 2
http://dx.doi.org/10.1073/pnas.1310153110 DOIArticle adapted for ch. 3
http://dx.doi.org/10.1021/ja507217vDOIArticle adapted for ch. 6
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9299
Collection:CaltechTHESIS
Deposited By: Jonathan Rittle
Deposited On:04 Dec 2015 23:19
Last Modified:13 Jan 2017 00:51

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