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

Citation

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. https://resolver.caltech.edu/CaltechTHESIS:02232018-173939270

Abstract

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 (^PhDPB^Mes)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*B^Ph and DP*B^Mes) is described. The primary modification introduced in DP*B^Ph and DP*B^Mes 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*B^Ph)Ni]₂(N₂) and (DP*B^Mes)Ni(N₂) with H₂ was studied. We observed that [(DP*B^Ph)Ni]₂(N₂) slowly decomposed to an unidentified mixture of products while (DP*B^Mes)Ni(N₂) dimerized to form a phosphine bridged Ni-borohydride dimer [(DP*B^MesH)Ni]₂. [(DP*B^Ph)Ni]₂(N₂) and (DP*B^Mes)Ni(N₂) were also tested as precatalysts for olefin hydrogenation and found to be less active that their previously reported counterpart (^PhDPB^Mes)Ni. [(DP*B^Ph)Ni]₂(N₂) and (DP*B^Mes)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 HBAr^F₂₄•2Et₂O while no NH₃ was observed using CoCp*₂ and [H₂NPh₂][OTf].

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