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Iron Mediated Reduction Schemes for Dinitrogen and Carbon Dioxide


Saouma, Caroline Thalia Abdunnur (2011) Iron Mediated Reduction Schemes for Dinitrogen and Carbon Dioxide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/46C3-BY97.


Several mono- and diiron species that coordinate NxHy ligands have been prepared and studied, to serve as structural, spectroscopic, and/or reactivity mimics to intermediates to an alternating reduction scheme for N₂ (i.e., Mn-N≡N → Mn-HN=NH → Mn-H₂N-NH₂ → Mn + 2 NH₃). The reaction between [PhBPR₃]FeMe ([PhBPR₃] = (PhB(CH₂PR₂)₃-; R = Ph, CH₂Cy) and hydrazine affords {[PhBPR₃]Fe}₂(μ-η¹: η¹-N₂H₄)(μ²- η²:N₂H₂). In one instance (R = Ph), the stepwise oxidation of coordinated hydrazine to diazene, and diazene to dinitrogen is achieved, giving {[PhBPPh₃]Fe}₂(μ-η¹:η¹-N₂H₂)(μ-η 2: η 2-N2H2) and {[PhBPPh₃]Fe}₂(μ-NH)₂, respectively.

As an extension to this work, a family of complexes which feature the same auxiliary ligands (i.e., [PhBPCH2Cy₃]Fe(OAc)), that are all iron(II), and that only differ in the oxidation state of the nitrogenous ligand has also been prepared: {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂), {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂H₂), {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂H₄), and {[PhBPCH2Cy₃]Fe(OAc)(NH₃).

To determine whether similar species could be isolated at a single iron site, the coordination chemistry of the more crowded “[PhBPmter3]Fe” fragment was investigated and compared to that of the “[PhBPPh3]Fe” scaffold. Treatment of [PhBPmter3]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex, [PhBPmter3]Fe(μ2-N2H3), which features an Fe=N π bond. Both 5- and 6-coordinate iron complexes that coordinate hydrazine were also synthesized, and the oxidation of these hydrazine and hydrazido(-) species was explored. In most instances, oxidation results in disproportionation of the N2Hy ligand, and [PhBPR3]Fe(NH3)(OAc) (R = Ph, mter) is isolated.

A 5-coordinate diiron diazene redox pair of complexes, {[PhBPPh3]Fe(CO)}2(μ-η1:η1-N2H2)0/- was also prepared and studied. The electronic structure of the Fe-NH-NH-Fe core in these complexes is unusual in that it features a highly activated diazene ligand, which is unprecedented for mid-to-late transition metals. Combined structural, spectroscopic, and computation studies indicate that there is much π-covalency within the Fe-NH-NH-Fe core, which has a similar electronic structure as butadiene.

With regards to CO2 reduction, the ability of iron(I) to mediate the one- and two- electron reductions of CO2 was explored. The reaction between [PhBPCH2Cy3]Fe(PCy)3 and CO2 is solvent dependent, with oxalate formation to generate {[PhBPCH2Cy3]Fe}2(μ-η2:η2-oxalato) being favored in THF, and decarbonylation to give {[PhBPCH2Cy3]Fe}2(μ-O)(μ-CO) occurring exclusively in MeCy. Studies aimed at understanding this unusual solvent-induced selectivity are presented.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:small molecule activation; iron; nitrogen fixation; carbon dioxide reduction
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Peters, Jonas C.
Thesis Committee:
  • Barton, Jacqueline K. (chair)
  • Okumura, Mitchio
  • Rees, Douglas C.
  • Bercaw, John E.
  • Peters, Jonas C.
Defense Date:5 January 2011
Record Number:CaltechTHESIS:01242011-170306838
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6231
Deposited By: Caroline Saouma
Deposited On:18 Mar 2014 20:51
Last Modified:08 Nov 2023 00:44

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