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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. http://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-Nx 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].

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Iron; Multiple Bonds; Multiply Bonded Ligands; Nitrogen
Degree Grantor:California Institute of Technology
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Peters, Jonas C.
Thesis Committee:
  • Stoltz, Brian M. (chair)
  • Gray, Harry B.
  • Bercaw, John E.
  • Peters, Jonas C.
Defense Date:24 May 2005
Record Number:CaltechETD:etd-05312005-201150
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05312005-201150
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2330
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:01 Jun 2005
Last Modified:09 Mar 2013 00:30

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