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Iron, Cobalt, and Nickel Metalloboranes: Reactivity, Catalysis, N2 Activation and Stabilization of Reactive N2Hx Ligands


Nesbit, Mark Allen (2018) Iron, Cobalt, and Nickel Metalloboranes: Reactivity, Catalysis, N2 Activation and Stabilization of Reactive N2Hx Ligands. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9G15Z28.


The reactivity of Fe and Co compounds supported by a bisphosphinoborane (DPB) ligand ([(DPB)Fe]2(N2) and (DPB)Co(N2)) towards E-H bonds (E = C, N, S, O, Si) is reported along with the catalytic hydrosilylation of ketones and aldehydes. The Fe and Co compounds displayed a mix of 1-electron and 2-electron chemistry. In some cases [(DPB)Fe]2(N2) and (DPB)Co(N2) facilitated oxidative addition of the E-H bond across the M-B interaction, and in others evolution of H2 giving a 1-electron oxidized complex of the general form (DPB)M(E) was observed. The reaction of Ph2SiH2 with (DPB)Co(N2) was found to be reversible, similar to the previously reported related nickel complex (PhDPBMes)Ni. The reactivity of these Fe and Co compounds is compared to previously reported Ni compounds supported by a similar ligand which catalyze olefin hydrogenation and hydrosilylation of substituted benzaldehydes.

The synthesis and metalation with nickel of two new variants of the DPB ligand (DP*BPh and DP*BMes) is described. The primary modification introduced in DP*BPh and DP*BMes is the incorporation of a tertiary amine moiety into the secondary coordination sphere. This was done with the hypothesis that the amine moiety might act as a proton shuttle and facilitate proton reduction or hydrogen oxidation electrocatalysis. The process of screening these compounds for activity as proton reduction and hydrogen oxidation catalysts is also discussed. Additionally, the stoichiometric reactivity of [(DP*BPh)Ni]2(N2) and (DP*BMes)Ni(N2) with H2 was studied. We observed that [(DP*BPh)Ni]2(N2) slowly decomposed to an unidentified mixture of products while (DP*BMes)Ni(N2) dimerized to form a phosphine bridged Ni-borohydride dimer [(DP*BMesH)Ni]2. [(DP*BPh)Ni]2(N2) and (DP*BMes)Ni(N2) were also tested as precatalysts for olefin hydrogenation and found to be less active that their previously reported counterpart (PhDPBMes)Ni. [(DP*BPh)Ni]2(N2) and (DP*BMes)Ni(N2) correspondingly showed no activity for hydrogenation of polar substrates such as ketones, aldehydes, or CO2.

Lastly, the synthesis of a new trisphosphinoborane ligand (ArP3B) with bulky aryl substituents on the phosphines and its metalation with Fe is described. The anionic-N2 adduct [(ArP3B)Fe(N2)][Na(12-C-4)2] was observed to react with H+ sources to generate the first observed parent iron-diazenido (ArP3B)Fe(NNH) and an iron-hydrazido(2-) [(ArP3B)Fe(NNH2)]+. [(ArP3B)Fe(NNH2)]+ was found to have similar spectroscopic properties to the previously reported [(TPB)Fe(NNH2)]+. A thorough characterization of [(ArP3B)Fe(N2)][Na(12-C-4)2], (ArP3B)Fe(NNH), and [(ArP3B)Fe(NNH2)]+ by a variety of continuous wave and pulsed ERP techniques is presented along with 57Fe Mössbauer data. The new (ArP3B)Fe system was also canvassed for activity as a catalyst for conversion of N2 to NH3 and found to yield substoichiometric amounts of NH3 in the presence of KC8 and HBArF24•2Et2O while no NH3 was observed using CoCp*2 and [H2NPh2][OTf].

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Small Molecule Activation; Transition Metal Catalysis; Metalloboranes; Nitrogen Fixation
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)
  • Fu, Gregory C.
  • Agapie, Theodor
  • Peters, Jonas C.
Defense Date:8 January 2018
Record Number:CaltechTHESIS:02232018-173939270
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Ch. 2.
Nesbit, Mark Allen0000-0002-5642-9303
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10732
Deposited By: Mark Nesbit
Deposited On:13 Mar 2018 17:02
Last Modified:08 Nov 2023 00:44

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