INTRODUCTION
1.5 Chapter Summaries
Figure 1.5: (A) Qualitative ligand field splitting diagrams for a metal center under idealized C3v symmetry, showing the effect of pyramidalization on the relative energy of the dδ and dπe-symmetry orbitals. (B) Examples of Fe0(N2) and FeIV≡N complexes supported by the same tris(phosphino)borate ligand. (C) The [(P3E)Fe(N2)]− complexes that are the subject of this thesis.
pair—[H(OEt2)2][BArF4] (HBArF4) and KC8—in Et2O at −78 ◦C.60 Soon thereafter, this catalysis was extended to the (P3C)Fe platform.59 More recently, we have discovered that a much milder chemical reductant, CoCp*2, can also drive this catalysis.61
This discovery represents the third example of a synthetic molecular catalyst for ni- trogen fixation, and the first using the biologically-relevant metal, Fe. As such, we turned our focus to mechanistic study of the (P3E)Fe system through the combination of synthetic, stoichiometric, and in situ studies. The body of this thesis details a portion of these efforts, as summarized below.
14 the nature of the axial E ligand, the nature of the transition metal ion was changed in a fashion that predictably modulated the electronics of the system. After synthesizing a series of [(P3E)Co(N2)]n complexes and subjecting them to the standard catalytic reaction conditions developed for the previously-reported Fe catalysts, it was found that only the borane complex [(P3B)Co(N2)]− furnished super-stoichiometric yields of NH3, suggesting that “(P3B)Co” serves as a molecular (pre)catalyst for N2 fixation, and highlighting the importance of the Z-type M–B motif in N2 fixation activity. Prior to this study, the only well-defined molecular systems (including nitrogenase enzymes) capable of directly mediating the catalytic conversion of N2 to NH3 contained either Mo or Fe ions. These results point to Group IX M(N2) complexes as targets for the development of novel N2-fixing catalysts.
Chapter 3 details in situ mechanistic studies of nitrogen fixation catalyzed by [(P3B)Fe- (N2)]− under the originally-reported reaction conditions. In this study, we were able to achieve a nearly order-of-magnitude improvement of catalyst turnover, at the cost of lowered yields due to competitive catalyzed and uncatalyzed hydrogen evolving reactions (HER). Study of the reaction dynamics evidence a single-site mechanism for N2reduction, and the successful demonstration of electrolytic N2-to-NH3 conversion indicates that the active redox state during catalysis is [(P3B)Fe(N2)]−. In situ monitoring of catalytic reaction mixtures using freeze-quench57Fe Mössbauer spectroscopy suggests that a previously char- acterized borohydrido–hydrido species is an off-path resting state of the overall catalysis.
This hydride species, which was previously posited to be primarily a catalyst sink, can in- stead re-enter the catalytic pathway via its conversion to catalytically active [(P3B)Fe(N2)]−. This observation underscores the importance of understanding hydride reactivity in the context of metal-mediated N2 fixation. The observed HER activity may provide a viable strategy for recovering catalytically active states from the unavoidable generation of metal hydride intermediates.
In Chapter 4, we study the key N–N bond cleavage step in the catalytic cycle for N2
reduction by [(P3B)Fe(N2)]−. Although we had proposed that this process might occur at a single Fe center in a heterolytic fashion to produce NH3 and a terminal Fe nitride, such reactivity had never been confirmed experimentally for any Fe(N2) complex. In this chapter, we demonstrate that sequential reduction and low-temperature protonation of [(P3B)Fe(N2)]− cleanly produces the (formally) hydrazido(2−) complex (P3B)Fe(NNH2).
In turn, this species undergoes protonolysis of the N–N bond to produce the terminal nitrido [(P3B)Fe≡N]+. Characterization of these highly reactive species was accomplished via freeze-quench57Fe Mössbauer and FeK-edge X-ray absorption spectroscopy (XAS). This result validates the hypothesis above, and shows that the distal Chatt cycle is a plausible mechanism for catalytic nitrogen fixation by [(P3B)Fe(N2)]−. Spectroscopic and computa- tional studies suggest that this reactivity is enabled by a buffering of the physical oxidation state range of the Fe via metal-ligand covalency. In this way, the redox behavior of the (P3B)Fe unit crudely models that of a metallocluster, in which the potential range of multi- electron processes is compressed by delocalization of electrons/holes among many metals, a feature of potential relevance to biological N2fixation.
Finally, in Chapter 5 we present spectroscopic and computational studies detailing the electronic structures of a redox series of Fe(NNR2) complexes that model key catalytic intermediates occurring prior to the N–N bond cleavage step. Although two closed-shell configurations of the “NNR2” ligand have been commonly considered in the literature—
isodiazene and hydrazido(2−)—we present evidence suggesting that [(P3E)Fe(NNR2)]n complexes contain an open-shell [NNR2]•− ligand coupled antiferromagnetically to the Fe center. This one-electron redox non-innocence resembles that of the classically non- innocent ligand, NO, and may have mechanistic implications for the divergent nitrogen fixation activity of the (P3B)Fe and (P3Si)Fe platforms.
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