The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages

Citation

Brown, Steven Douglas (2005) The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/46DN-YH09. https://resolver.caltech.edu/CaltechETD:etd-05312005-201150

Abstract

The metallation of FeX₂ (X = Cl, Br, I) salts with the strong-field [PhBP₃] ([PhBP₃] = PhB(CH₂PPh₂)₃^-) ligand is presented. The resulting four-coordinate, 14-electron species, [PhBP₃]FeX, have been thoroughly characterized and feature high-spin (S = 2) electronic ground-states. X-ray diffraction analysis of [PhBP₃]FeCl establishes a monomeric structure in the solid state.

The one electron reduction of [PhBP₃]FeCl in the presence of a triphenylphosphine cap affords a rare example of four-coordinate iron(I). This species, [PhBP₃]Fe(PPh₃), serves as a synthetic surrogate to a low-valent "[PhBP₃]Fe(I)" subunit that is readily oxidized in the presence of organic azides. The resulting S = 1/2 iron(III) imides of general formula [PhBP₃]Fe≡NR may be subsequently reduced by one electron to yield the anionic S = 0 derivatives. Exposure of the former to an atmosphere of CO results in cleavage of the Fe≡NR linkage to yield [PhBP₃]Fe(CO)₂ and free isocyanate (O=C=N-R). Dicarbonyl [PhBP₃]Fe(CO)₂ is itself an imide precursor and is gradually converted back to [PhBP3]Fe≡NR upon exposure to excess organic azide.

Tolyl imide [PhBP₃]Fe?N-p-tolyl readily reacts with H₂ under mild conditions to undergo a step-wise Fe-N_x bond scission process to ultimately release free p-toluidine. Initially formed is the S = 2 iron(II) anilide, [PhBP₃]Fe(N(H)-p-tolyl), which has been independently prepared and shown to release p-toluidine in the presence of H₂. In benzene solvent the final iron containing product of the hydrogenation process is diamagnetic [PhBP₃]Fe(?⁵-cyclohexadienyl), which is presumably formed from benzene insertion into a low-valent iron-hydride intermediate.

Reduction of the ferromagnetically coupled dimer, {[PhBP₃]Fe(N₃)}₂, yields the bridging nitride species, [{[PhBP₃]Fe}₂(μ-N)][Na(THF)₅]. This compound features two high-spin iron(II) metal centers that are so strongly antiferromagnetically coupled that a diamagnetic S = 0 ground-state is exclusively populated at room temperature. X-ray diffraction analysis reveals a bent Fe-N-Fe linkage that quantitatively releases ammonia in the presence of excess protons. Reactivity with CO and H₂ is also presented, and for the latter, complete rupture of the Fe-N-Fe manifold is not observed as the presence of an additional metal center (when compared with the iron(III) imides) favors the formation of the diamagnetic bridging imide-hydride species, [{[PhBP₃]Fe}₂(μ-NH)(μ-H)][Na(THF)₅].

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