An Fe-N_2 Complex That Generates Hydrazine and Ammonia via Fe═NNH_2: Demonstrating a Hybrid Distal-to-Alternating Pathway for N_2 Reduction

Biological N_2 fixation to NH_3 may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e–/H+ transfer steps of iron-ligated N_2 in well-defined synthetic systems is hence of much interest but remains a significant challenge. While iron complexes have been recently discovered that catalyze the formation of NH_3 from N_2, mechanistic details remain uncertain. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate Fe═NNH_2+ species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N_2– complex. The Fe═NNH_2+ complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = 1/2 Fe═NNH_2. At warmer temperatures, Fe═NNH_2 rapidly converts to an iron hydrazine complex, Fe-NH_2NH_2+, via the additional transfer of proton and electron equivalents in solution. Fe-NH_2NH_2+ can liberate NH_3, and the sequence of reactions described here hence demonstrates that an iron site can shuttle from a distal intermediate (Fe═NNH_2+) to an alternating intermediate (Fe-NH_2NH_2+) en route to NH_3 liberation from N_2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N_2 reduction may be operative in biological N_2 fixation

Catalytic conversion of nitrogen to ammonia by an iron model complex

The reduction of nitrogen (N_2) to ammonia (NH_3) is a requisite transformation for life. Although it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation, how they do so remains poorly understood. A central element of debate has been the exact site or sites of N_2 coordination and reduction. In synthetic inorganic chemistry, an early emphasis was placed on molybdenum because it was thought to be an essential element of nitrogenases and because it had been established that well-defined molybdenum model complexes could mediate the stoichiometric conversion of N_2 to NH_3 (ref. 9). This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature molybdenum centres. However, it is now thought that iron is the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data have implicated iron instead of molybdenum as the site of N_2 binding in the FeMo-cofactor. Here we describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of N_2 to NH_3 under mild conditions, and in which more than 40 per cent of the proton and reducing equivalents are delivered to N_2. Our results indicate that a single iron site may be capable of stabilizing the various N_xH_y intermediates generated during catalytic NH_3 formation. Geometric tunability at iron imparted by a flexible iron–boron interaction in our model system seems to be important for efficient catalysis. We propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role, perhaps by enabling a single iron site to mediate the enzymatic catalysis through a flexible iron–carbon interaction

A 10^6‑Fold Enhancement in N_2‑Binding Affinity of an Fe_2(μ-H)_2 Core upon Reduction to a Mixed-Valence Fe^(II)Fe^I State

Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron–molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N_2 to NH_3 is unknown. One role of these ligands may be to facilitate N_2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N_2 depending on the redox state of the Fe_2(μ-H)_2 unit. An unusual example of a mixed-valent Fe^(II)(μ-H)^2Fe^I is described that displays a 10^6-fold enhancement of N_2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH_3 are produced upon exposure to proton and electron equivalents. The Fe(μ-H)Fe(N2_) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover

Fe–N_2/CO complexes that model a possible role for the interstitial C atom of FeMo-cofactor (FeMoco)

We report here a series of four- and five-coordinate Fe model complexes that feature an axial tri(silyl)methyl ligand positioned trans to a substrate-binding site. This arrangement is used to crudely model a single-belt Fe site of the FeMo-cofactor that might bind N_2 at a position trans to the interstitial C atom. Reduction of a trigonal pyramidal Fe(I) complex leads to uptake of N_2 and subsequent functionalization furnishes an open-shell Fe–diazenido complex. A related series of five-coordinate Fe–CO complexes stable across three redox states is also described. Spectroscopic, crystallographic, and Density Functional Theory (DFT) studies of these complexes suggest that a decrease in the covalency of the Fe–C_alkyl interaction occurs upon reduction and substrate binding. This leads to unusually long Fe–C_alkyl bond distances that reflect an ionic Fe–C bond. The data presented are contextualized in support of a hypothesis wherein modulation of a belt Fe–C interaction in the FeMo-cofactor facilitates substrate binding and reduction

Proton-Coupled Reduction of an Iron Cyanide Complex to Methane and Ammonia

Nitrogenase enzymes mediate the six-electron reductive cleavage of cyanide to CH_4 and NH_3. Herein we demonstrate for the first time the liberation of CH_4 and NH_3 from a well-defined iron cyanide coordination complex, [SiP^(iPr)_3]Fe(CN) (where [SiP^(iPr)_3] represents a tris(phosphine)silyl ligand), on exposure to proton and electron equivalents. [SiP^(iPr)_3]Fe(CN) additionally serves as a useful entry point to rare examples of terminally-bound Fe(CNH) and Fe(CNH_2) species that, in accord with preliminary mechanistic studies, are plausible intermediates of the cyanide reductive protonation to generate CH_4 and NH_3. Comparative studies with a related [SiP^(iPr)_3]Fe(CNMe_2) complex suggests the possibility of multiple, competing mechanisms for cyanide activation and reduction

Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals

A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe_3M(μ_4-O)(μ_2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E_(1/2)) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pK_a of the M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity, with a slope of ∼70 mV per pK_a unit. The current study of [Fe_3MO(OH)] and previous ones of [Mn_3MO_n] (n = 2,4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals

Characterization of an Fe≡N−NH_2 Intermediate Relevant to Catalytic N_2 Reduction to NH_3

The ability of certain transition metals to mediate the reduction of N_2 to NH_3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N_2 and mediate its protonation at one or more N atoms to furnish M(N_xH_y) species that can be characterized and, in turn, extrude NH_3. By contrast, the direct protonation of Fe–N_2 species to Fe(N_xH_y) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N_2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N–NH_2]^+, a doubly protonated hydrazido(2−) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N_2)][Na(12-crown-4)] can proceed along a distal or “Chatt-type” pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N–N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided

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Proton-Coupled Reduction of N₂ Facilitated by Molecular Fe Complexes

The activation of Fe-coordinated N2 via the formal addition of hydrogen atom equivalents is explored in this thesis. These reactions may occur in nitrogenase enzymes during the biological conversion of N2 to NH3. To understand these reactions, the N2 reactivity of a series of molecular Fe(N2) platforms is investigated. A trigonal pyramidal, carbon-ligated FeI complex was prepared that displays a similar geometry to that of the resting state 'belt' Fe atoms of nitrogenase. Upon reduction, this species was shown to coordinate N2, concomitant with significant weakening of the C-Fe interaction. This hemilability of the axial ligand may play a critical role in mediating the interconversion of Fe(NxHy) species during N2 conversion to NH3. In fact, a trigonal pyramidal borane-ligated Fe complex was shown to catalyze this transformation, generating up to 8.49 equivalents of NH3. To shed light on the mechanistic details of this reaction, protonation of a borane-ligated Fe(N2) complex was investigated and found to give rise to a mixture of species that contains an iron hydrazido(2-) [Fe(NNH2)] complex. The identification of this species is suggestive of an early N-N bond cleavage event en route to NH3 production, but the highly-reactive nature of this complex frustrated direct attempts to probe this possibility. A structurally-analogous silyl-ligated Fe(N2) complex was found to react productively with hydrogen atom equivalents, giving rise to an isolable Fe(NNH2) species. Spectroscopic and crystallographic studies benefited from the enhanced stability of this complex relative to the borane analogue. One-electron reduction of this species initiates a spontaneous disproportionation reaction with an iron hydrazine [Fe(NH2NH2)] complex as the predominant reaction product. This transformation provides support for an Fe-mediated N2 activation mechanism that proceeds via a late N-N bond cleavage. In hopes of gaining more fundamental insight into these reactions, a series of Fe(CN) complexes were prepared and reacted with hydrogen-atom equivalents. Significant quantities of CH4 and NH3 are generated in these reactions as a result of complete C-N bond activation. A series of Fe(CNHx) were found to be exceptionally stable and may be intermediates in these reactions. The stability of these compounds permitted collection of thermodynamic parameters pertinent to the unique N-H bonds. This data is comparatively discussed with the theoretically-predicted data of the N2-derived Fe(NNHx) species. Exceptionally-weak N-H bond enthalpies are found for many of these compounds, and sheds light on their short-lived nature and tendency to evolve H2. As a whole, these works both establish and provide a means to understand Fe-mediated N2 activation via the addition of hydrogen atom equivalents