Insights into the Mechanism of Nitrogen Fixation by a Single-Site Fe Catalyst

Outside the Peters group, I have found colleagues and friends who are no less important. 299 C.14 Structures of the (P3B)Fe(NNMe2) ground state core optimized for the gas phase. S= 0) using various pure functionals.

NOMENCLATURE

Tris(2-diisopropylphosphino-4-methylphenyl)amine

Tris(2-diphenylphosphinoethyl)phosphine

INTRODUCTION

Opening Remarks

Thus, part of the challenge of N2 fixation lies in the stability of the N≡N bond, which means major obstacles to its reduction in the absence of a catalyst. However, the special challenge of nitrogen fixation is not only in the stability of the starting material (N2), but in the nature of the products.

Table 1.1: Collected Physical Properties. a N≡N C≡O H 3 C–H BDE (kcal mol −1 ) 225 256

Catalytic Nitrogen Fixation—A Historical Perspective .1 The Haber–Bosch Process.1The Haber–Bosch Process

• Synthetic Molecular Catalysts

The field is therefore undergoing exciting development, prompting detailed mechanistic research into the most active catalysts. Due to the presence of Mo in the most efficient nitrogenase enzymes and the early success of synthetic Group VI complexes in stoichiometric N2 reduction, much attention has been paid to the development of Mo-based nitrogen fixation catalysts (vide supra).

Figure 1.2: (Top) Structure of FeMoco bound in MoFe protein from Azotobacter vinelandii [23], along with the numbering scheme for the Fe atoms

Chapter Summaries

In turn, this species undergoes protonolysis of the N–N bond to produce the terminal nitrido [(P3B)Fe≡N]+. Although two closed-shell configurations of the "NNR2" ligand have been commonly considered in the literature—.

Introduction

Possible factors include: (i) a lower level of N2 activation than that observed for species E = C or B (see below); (ii) faster poisoning of the E = Si system, for example by faster formation of an inactive terminal hydride; 21, 22 or (iii) faster decomposition of the E = Si system, for example by dechelation of the ligand. To complement our previous ligand modification studies, here we instead change the identity of the transition metal.

Results and Discussion

• Synthesis and Structural Analysis
• Evaluating N 2 -to-NH 3 Conversion Activity
• Factors Correlated to N 2 fixation Activity
• Reactivity of (P 3 C )Co(N 2 ) with H 2

Likewise, the M–C interaction between (P3C)Co complexes exhibits flexibility comparable to that of the analogous iron series. As shown in Figure 2.8, consistent with expectations, Nβ of the anionic complex [(P3B)Co(N2)]- shows a much greater rate of negative charge accumulation compared to the same atom in the neutral complex (P3C)Co (N2).

Figure 2.1: Chemical oxidation and reduction of (P 3 B )Co(N 2 ).

Conclusion

This behavior is identical to that of the isoelectronic (P3Si)Co complexes.27 Nevertheless, these results serve as an instructive demonstration of the difference in donor properties conferred by E = C vs Si in (P3E)M complexes become The propensity of the (P3E)M complexes we studied to carry out productive N2 fixation does not appear to depend only on the ability of the precursor complex to activate N2.

Experimental Section .1 General Considerations.1General Considerations

• Physical Methods
• Synthesis
• Standard NH 3 Generation Reaction Procedure
• NH 3 Quantification

The mixture was allowed to warm slowly to room temperature and was then concentrated to ca. The cooled solution of [Fc][BArF4] was added dropwise to the solution of (P3C)Co(N2), and the resulting mixture was allowed to stir at low temperature for 1 hour. The Schlenk tube was then sealed, and the reaction was allowed to stir for 40 min at -78◦C before being warmed to room temperature and for 15 min. was stirred.

A SYNTHETIC SINGLE-SITE FE NITROGENASE: HIGH TURNOVER, FREEZE-QUENCH 57 FE MÖSSBAUER DATA, AND A HYDRIDE

RESTING STATE

Introduction

In principle, this technique enables observation of overall Fe speciation as frozen snapshots during turnover.56,57 For single-site Fe nitrogenase mimics of the type we have developed, analysis of such data is far simpler than in a biological nitrogenase, where many Fe centers are present.58,59. 57Fe Mössbauer spectroscopy, and these data facilitate the interpretation of the freeze-quenching Mössbauer data reported here. Such a strategy complements the investigations of model complexes and stoichiometric reaction steps that we have previously undertaken and provides a fuller mechanistic picture.

Figure 3.1: Synthetic catalysts for N 2 fixation to NH 3 , along with the maximum observed TON for each catalyst system

Results and Discussion

• Increased Turnover of Fe-Catalyzed N 2 Fixation and Evidence for Catalysis at the [(P 3 B )Fe(N 2 )] 0 /− Couple
• Kinetics of Ammonia and Hydrogen Formation
• Spectroscopic Characterization of Fe Speciation under Turnover
• Precatalyst Activity of (P 3 B )(µ-H)Fe(H)(N 2 ) and Identification of a Catalyst Resting State
• Summary of Mechanistically Relevant Observations

We have observed only net oxidation in the neutral state reaction of (P3B)Fe(N2) with HBArF4 in Et2O to produce [(P3B)Fe]+ (Equation 3.4). Quantification of the NH3 produced in this reaction (34% yield based on HBArF4) accounted for 74% of the acid added. Due to the width of the last resonance (Γ ≈ 1 mm s−1), this feature could not be modeled accurately.

Figure 3.2: Yields of NH 3 obtained using [(P 3 B )Fe(N 2 )] − from successive reloading of HBAr F 4 and KC 8 to reactions maintained at ≤ −70 ◦ C in Et 2 O

Conclusion

This process was independently demonstrated at room temperature in benzene.67 However, follow-up control experiments in Et2O indicate that this reaction is not kinetically competent at -78◦C. Our freeze-quenching Mössbauer results suggest that (P3B)(µ-H)Fe(H)(L) is a down-path resting state of the overall catalysis; this hydride species, which we previously considered primarily a catalyst sink, can instead re-enter the catalytic pathway through its conversion to catalytically active [(P3B)Fe(N2)]−.

Experimental Section .1 Experimental Details.1Experimental Details

• Physical Methods
• Mössbauer Spectroscopy
• Ammonia Quantification
• Synthetic Details
• Synthesis and Purification of NaBAr F 4 and HBAr F 4
• Preparation of 10 wt% Na/Hg shot
• Standard Ammonia Generation Reaction Procedure with [Na(12-c-4) 2 ][(P 3 B )Fe(N 2 )]
• Standard Ammonia Generation Reaction Procedure with [K(Et 2 O) 0.5 ][(P 3 C )Fe(N 2 )]
• Standard Ammonia Generation Reaction Procedure with [Na(12-c-4) 2 ][(P 3 Si )Fe(N 2 )]
• Standard Ammonia Generation Reaction Procedure with (P 3 B )(µ-H)Fe(H)(N 2 )
• NH 3 Generation Reaction Procedure with [Na(12-c-4) 2 ][(P 3 B )Fe(N 2 )] with the inclusion of NH 3
• NH 3 Generation Reaction with Periodic Substrate Reloading
• General Procedure for Time-resolved NH 3 Quantification via Low-temperature Quenching
• General Procedure for Time-resolved H 2 Quantification
• Solution IR calibration of [Na(12-c-4) 2 ][(P 3 B )Fe(N 2 )]
• Stoichiometric reaction of (P 3 B )(µ-H)Fe(H)(N 2 ) with HBAr F 4 and KC 8
• Rapid Freeze-quench Mössbauer Spectroscopy
• General Procedure for Preparation of Rapid-freeze-quench Mössbauer Samples of Catalytic Reaction Mixtures using [Na(12-c-4) 2 ][(P 3 B )Fe(N 2 )]
• General Procedure for Fitting of Rapid-freeze-quench Mössbauer Samples
• Controlled Potential Electrolysis of [(P 3 B )Fe][BAr F 4 ] and HBAr F 4

This solution was allowed to freeze, then the space above the tube was evacuated and the tube was sealed. The system temperature was allowed to equilibrate for 5 min, and then the tube was sealed with a Teflon screw valve. The temperature of the system was allowed to equilibrate for 5 minutes and then the tube was closed.

NITROGEN FIXATION VIA A TERMINAL IRON(IV) NITRIDE

Introduction

While terminal Fe≡N complexes have been shown to release NH3 via reductive protonation, 16,18 such Fe≡N species have been generated to date by N atom transfer reactions from azide (N3−) or alternative reagents, but not from N2 . 16,18–26 Therefore, their potential role in synthetic or biological N2-to-NH3 conversion catalysis remained unclear. Our recent discovery that the anionic N2 complex catalyzes [(P3B)Fe(N2)]-N2-to-NH3 conversion has positioned us to investigate the mechanism(s) by which the key N-N cleavage. This result provides a plausible mechanism for the N-N bond cleavage step under catalytic turnover and highlights a terminally bound FeIV≡N as a viable intermediate of catalytic N2fixation.

Figure 4.1: Examples of homolytic (A and B) and heterolytic (C and D) N–N bond cleavage reactions

Results and Discussion .1 Results.1Results

• Discussion

Rapid protonation of [(P3B)Fe(N2)]2− at low temperature leads to the formation of the hydrazido complex (P3B)Fe(NNH2).iv In the rate-limiting step, (P3B)Fe(NNH2) is protonated to give an unobserved to form transient (or transition state) hydrazidium cation [(P3B)Fe(NNH3)]+, which decays by heterolytic cleavage of the N–N bond to yield NH3 and [(P3B)Fe≡N]+. Protonation of this anion at low temperature produces the cationic hydrazido complex [(P3B)Fe(NNH2)]+,33 which, after one electron reduction to its charge neutral native (P3B)Fe(NNH2) (E◦0 ≥ −1.2 V) ) , undergoes protolytic cleavage of the N–N bond to produce the first equivalent of NH3 and the cationic Fe(IV) nitrido [(P3B)Fe≡N]+. On these grounds, we predict that this potential is positive of the [(P3B)Fe(N2)]0/− pair, and therefore the reduction of [(P3B)Fe≡N]+ should be accessible under catalytic conditions, either by direct ET or PCET, which corresponds to experiment (vide supra).

Figure 4.3: Stepwise reduction and protonation of [(P 3 B )Fe] + to form the cationic hydrazido(2−) complex [(P 3 B )Fe(NNH 2 )] + , as detailed in [33].

Conclusion

The predicted nitridyl (N·2−) character of the N atom of (P3B)Fe≡N could even facilitate the formation of N-H bonds via PCET.46 Although we have preliminary, indirect evidence for the formation of (P3B) Fe ≡N after photoreduction of XAS samples of [(P3B)Fe≡N]+ (see Appendix C),47 evaluation of these predictions awaits the detailed experimental characterization of this undoubtedly highly reactive species. P3B)Fe≡N]+ is the first example of a terminal Fe nitride synthesized from N2, thus confirming the long-standing hypothesis that a single Fe center can support a distal Chatt-type N2 fixation mechanism in which the N-N -bond heterolytically cleaved. These results also provide insight into how this mechanism evolves from the electronic structure of the (P3B)Fe unit. In this way, the redox behavior of the (P3B)Fe unit roughly models that of a metallocluster, in which the potential range of multi-electron processes is compressed by delocalization of electrons/holes between many metals.48 Analogy can also be made to the reduction from O2 to ferryl and H2O mediated by heme cofactors, with the final reducing equivalent required to cleave the O-O bond derived from the porphyrin and/or thiolate ligand (Figure 4.2, A).14 In a similar manner In this way, the challenging multielectron transformation described by equation 4.9 is made possible by covalence with the ligand atoms, which buffers the physical oxidation state range of Fe in this system.

Experimental Section .1 Experimental Details.1Experimental Details

• NMR Spectroscopy
• IR Spectroscopy
• UV-vis Spectroscopy
• X-ray Crystallography
• Mössbauer Spectroscopy
• X-ray Absorption Spectroscopy
• Synthetic Details
• Synthesis of [(P 3 B )Fe(N 2 )] 2−
• Synthesis of (P 3 B )Fe(NNMe 2 )
• Synthesis of (P 3 B )Fe(OTf)
• Low-Temperature Protonation Studies .1 Protonation studies of [(P 3 B )Fe(N 2 )] 2−
• Protonation of [Na(12-c-4) 2 ][(P 3 B )Fe(N 2 )]
• Studies with NH 3 /N 2 H 4 quantification
• Computational Methods

The mentioned isomer shifts are relative to the center of gravity of the spectrum of a metal foil of a-Fe at room temperature. Using pre-cooled stainless steel tongs, the sample cup was lifted from the bottom of the cold well and the mixture was allowed to devitrify. This tube was led from the box into a liquid N2 bath and transported to a fume hood.

Introduction

When [(P3Si)Fe(NNH2)]+ is formed in situ and subsequently treated with substoichiometric CoCp*2, high yields of both [(P3Si)Fe(N2H4)]+ and (P3Si)Fe(N2) are produced (Figure 5.1) , which altogether represents the exchange of two H atom equivalents between the [(P3Si)Fe(NNH2)]+/0 redox couple to effect functionalization of the proximal N atom (Na).18. Although commonly regarded as noninnocent in the two-electron sense described above, to our knowledge there are no reported M(NNR2) complexes in which the NNR2 fragment has been characterized in the intermediate, open-shell hydrazyl radical anion configuration ([NNR By In contrast, many transition metal complexes are known that coordinate the isomeric diazene radical anion ([RNNR]•−).27 This discrepancy may be due in part to the lack of paramagnetic M(NNR2) species amenable to a detailed characterization of the spin density distribution of "NNR2 " ligand via EPR-based methods Given the apparently large N-centered spin density and the potential radical-type reactivity of (P3Si)Fe(NNH2), as well as the proposed role of PCET in catalytic N2 fixation with [(P3B)Fe( N2)]− we were curious to determine whether the electronic structures of [(P3E)Fe(NNR2)]n complexes have a significant weight of a [NNR2]•− configuration of this redox-active ligand. While we have reported preliminary characterization of [(P3B)Fe(NNH2)]+ by ENDOR spectroscopy,16 we were unable to determine the complete anisotropic hyperfine coupling (HFC) tensors for the N atoms of the “NNH2” ligand.

Figure 5.1: Observed reactivity of (P 3 E )Fe(NNH 2 ) complexes.

Results and Discussion

• Preliminary Bonding Considerations
• Synthesis and Structural Analysis of [(P 3 B )Fe(NNMe 2 )] +/0/−
• Excited states from UV-vis and VT NMR
• Multireference Character from Broken-symmetry DFT and CASSCF
• Electronic Structures of [(P 3 B )Fe(NNMe 2 )] +/− from Pulsed EPR Studies With an understanding of the electronic structures of the charge-neutral states of
• Experimental Spin Density Distribution
• Excited State Chemistry of [(P 3 B )Fe(NNMe 2 )] +
• Summary and Implications for Catalytic Nitrogen Fixation

Valence bond diagrams describing possible resonance structures of the Fe–NNR2 moiety, including covalent (top) and antiferromagnetic (bottom) extrema. Theg-tensor of the ground-state doublet [(P3B)Fe(NNMe2)]+(2Γ+,0) is significantly more anisotropic than its two-electron reduced counterpart, indicating the presence of low-lying excited doublet states (see below), at least with respect to the chem. of the excited state [(P3B)Fe(NNMe2)]−. This electronic state can be accurately interpreted in terms of the schematic MO diagram for the 3Γ0,0 state shown in Figure 5.8, A.

Figure 5.2: (A) Qualitative MO diagram for R’ 3 B–Fe–NNH 2 under pseudo-C 2v symmetry.

Conclusion

Experimental Section .1 Experimental Details.1Experimental Details

• NMR Spectroscopy
• IR Spectroscopy
• UV-vis Spectroscopy
• Electrochemistry
• X-ray Crystallography
• EPR Spectroscopy
• X-ray Absorption Spectroscopy
• Synthetic Details
• Synthesis of (P 3 B )Fe( 15 N 15 NMe 2 )
• Synthesis of (P 3 B )Fe(NN( 13 CH 3 ) 2 )
• Synthesis of [(P 3 B )Fe(NNMe 2 )][BAr F 4 ]
• Synthesis of [(P 3 B )Fe(NNMe 2 )][BAr F 4 ] Isotopologues
• Synthesis of [(P 3 B )Fe(NNMe 2 )] −
• Synthesis of [(P 3 B )Fe(NNMe 2 )] − Isotopologues
• Computational Methods

The (−,−) and(+,−) quadrants of these frequency spectra are symmetrical with respect to the(+,+)and(−,+) quadrants, so only two of the quadrants are typically represented in the literature. In the intermediate coupling regime where |A| ≈ |2νI|, peaks will often occur in both the(+,+) and (−,+) quadrants of the HYSCORE spectrum. T can be further decomposed into scalar components: t (in energy units), which describes the magnitude of the anisotropic HFC coupling, and a dimensionless term δHFC (0≤ δHFC ≤1), which describes the rhombicity of the tensor.