Iron Mediated Reduction Schemes for Dinitrogen and Carbon Dioxide

Citation

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. https://resolver.caltech.edu/CaltechTHESIS:01242011-170306838

Abstract

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 [PhBP^R₃]FeMe ([PhBP^R₃] = (PhB(CH₂PR₂)₃-; R = Ph, CH₂Cy) and hydrazine affords {[PhBP^R₃]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 {[PhBP^Ph₃]Fe}₂(μ-η¹:η¹-N₂H₂)(μ-η 2: η 2-N2H2) and {[PhBP^Ph₃]Fe}₂(μ-NH)₂, respectively.

As an extension to this work, a family of complexes which feature the same auxiliary ligands (i.e., [PhBP^CH2Cy₃]Fe(OAc)), that are all iron(II), and that only differ in the oxidation state of the nitrogenous ligand has also been prepared: {[PhBP^CH2Cy₃]Fe(OAc)}₂(μ-N₂), {[PhBP^CH2Cy₃]Fe(OAc)}₂(μ-N₂H₂), {[PhBP^CH2Cy₃]Fe(OAc)}₂(μ-N₂H₄), and {[PhBP^CH2Cy₃]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.

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