Nitrogen Fixation with a Molecular Catalyst and What We Learned Along

Introduction

Background

For example, the Fe center in [Na(12-crown-4)2][(P3B)Fe(N2)] uses a trigonal bipyramidal geometry (Figure 1.2, A) that provides available filled dxz and dyzorbitals for π-back donation in the N2π* orbitals, which facilitate the binding and activation of N2. While in the imide complex (P3B)Fe(NAr) (Figure 1.2, B) the Fe center adopts an approximate tetrahedral geometry to facilitate M-N multiple bonds with empty dxz and dyz orbitals.28.

Figure 1.1: [HIPTN 3 N]Mo(N 2 ) (A) and six independently synthesized [HIPTN 3 N]Mo-N x H y

Consideration of the M-E interatomic distances presented in Table 2.1 reveals that the (TPB)Co platform exhibits a significant degree of M-B interaction flexibility similar to that observed for the (TPB)Fe platform. Due to the width of the latter resonance (Γ ≈ 1 mm s-1), this feature could not be modeled accurately.

Evaluating Molecular Cobalt Complexes for the

Introduction

Potential factors include (i) a lesser degree of N2 activation than observed in the E = C or B species (vide infra); (ii) faster poisoning of the E = Si system, for example by faster formation of an inactive terminal hydride;7,8 (iii) faster degradation of the E = Si system, for example by dechelation of the ligand. To complement our previous ligand modification studies, we chose to change the identity of the transition metal.

Results and Discussion

However, comparing the Fe with the Co complexes shows that the degree of N2 activation in absolute terms is not predictive of the yield of NH3 (Figure 2.5). However, a comparison of the Fe and Co systems shows that the accessibility of highly reduced, anionic [(P3E)M(N2)] complexes is favorably correlated with NH3.

Figure 2.2. Solid-state crystal structures of 2 (top) and 3 (bottom; also see SI). Thermal ellipsoids shown at 50% probability

Conclusion

Although some of the design features important to consider in (P3E)M(N2)-type catalysts have been highlighted here, other factors, including comparative rates of H2 evolution and catalyst degradation/poisoning rates, warrant further investigation.

Experimental

At this point, the mixture was allowed to warm to room temperature before filtration over celite and concentration to ca. 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 stirred for 15 min.

This mixture was allowed to stir for 5 min at -78°C before being transferred to a pre-cooled Schlenk tube equipped with a stir bar. The original reaction vial was washed with cold Et2O (0.25 mL) which was then added to the Schlenk tube. 19 There is an additional early report on the formation of one equivalent of NH3 by a co-porphyrin complex under a N2-containing atmosphere after treatment with sodium borohydride (see Fleischer, E.

This methodology (presented in the supplementary information for this chapter) will prove to be something of an unsung hero throughout the rest of my thesis, as the high purity level allowed us to obtain higher turnover numbers, which in turn enabled kinetic probes and in situ spectroscopy by freeze-quenching (presented in the next chapter), as well as allowing all the electrochemistry presented in chapter five using NaBArF4 as an ether-soluble electrolyte.

Introduction

In principle, this technique enables observation of total Fe speciation as frozen snapshots during turnover.19 For single-site Fe-nitrogenase mimics of the type we have developed, analysis of such data is much simpler than in a biological nitrogenase where many iron centers are present.20. For the most active P3BFe catalyst system, many P3BFe-NxHy model complexes that may be mechanistically relevant (e.g., Fe+, Fe-N2-, Fe=NNH2+, Fe-NH3+) have now been independently generated and characterized, including by 57Fe Mössbauer spectroscopy. , and these data facilitate interpretation of the freeze-quench Mössbauer data reported here. In combination with chemical quenching methods that we present to study the dynamics of product formation, it becomes possible to spectroscopically correlate the observed species with the N2 bonding activity to gain a better understanding of the whole.

Such a strategy complements the investigations of model complexes and stoichiometric reaction steps that we have previously undertaken and provides a fuller mechanistic picture.

Results and Discussion

Increased turnover of Fe-catalyzed N 2 fixation and
Kinetics of ammonia and hydrogen formation
Spectroscopic characterization of Fe speciation
Precatalyst activity of (P 3 B )(μ-H)Fe(H)(N 2 )
Summary of mechanistically relevant observations

To test this latter possibility, catalytic runs were carried out with 150 equivalents of HBArF4 and 185 equivalents of KC8 in the presence of 1 with the inclusion of 25 equivalents of NH3 (Table 3.1, entry 15). Only net oxidation was observed in the reaction of neutral state P3BFe-N2 with HBArF4 in Et2O to form P3BFe+ (eq 4). As shown in Figure 3.4 (left), analysis of initial rates indicates that the reaction is first order in [Fe], consistent with the involvement of the monomeric P3BFe species in the rate-limiting step to form NH3.

As a point of comparison, we also measured the rate of H2 evolution in the presence of P3SiFe-N2-(3).

Figure 3.1: Yields of NH 3 obtained using P 3 B Fe-N 2 - 1 from successive reloading of HBAr F 4

Conclusion

We also provided new mechanistic insights for reactions with the P3BFe-N2-1 catalyst, such as the observation that catalysis occurs at -78 °C, the demonstration of a first-order dependence of the rate on the catalyst concentration, the demonstration of a zero-order dependence of the rate on the HBArF4 concentration, and the observation that 1 catalyzes HER as well as NH3 formation. Preliminary electrochemical data indicate that catalysis by the P3BFe system can be driven by the P3BFe-N2/P3BFe-N2- formal couple around -2.2 V vs. Fc/Fc+, consistent with Na/Hg also serving as a capable reductant for catalytic conversion. To date, no synthetic molecular catalyst for the conversion of N2- to NH3 under active plant conditions has been spectroscopically investigated.

Our Mössbauer results for freeze-quenching indicate that 4-L is the resting state of overall catalysis; this hydride species, previously assumed to be primarily a catalyst sink, may instead re-enter the catalytic pathway by conversion to the catalytically active P3BFe-N2-1.

The reduction of N2 to NH3 is critical to life and is carried out on a large scale both industrially and biologically.1 The high stability of the N≡N triple bond requires catalysts and high-energy reagents/conditions to achieve the desired transformation. 2 Synthetic Studies of catalytic N2-to-NH3 conversion by model complexes are important to constrain hypotheses regarding the mechanisms of biological (or industrial) N2 fixation and to identify fundamental catalyst design principles for multi-electron reductive transformations.3 ,4 Interest in Fe model systems that catalyze N2-to-NH3 conversion has grown in part due to the postulate that one or more Fe centers in the FeMo cofactor of FeMo nitrogenase can serve as the site of N2- binding and activation during the breaking of important bonds. and steps.5 Previous examples of synthetic molecular Fe catalysts that mediate the conversion of N2 to NH3 operate with a high driving force and rely on a very strong one. Many soluble coordination complexes are now known that electrocatalytically mediate the hydrogen evolution reaction (HER),3 the carbon dioxide reduction reaction (CO2RR),4 and the oxygen reduction reaction (O2RR).5 The study of such systems has rapidly matured. in recent years, coinciding with extensive research efforts into solar-based fuel systems. The procedure was identical to that of the standard reaction protocol for NH3 generation, with changes noted.

Crude NaBArF4 was prepared according to a literature procedure.9 The crude material, which has a yellow-brown hue, was purified by a modification of the procedure published by Bergman,10 as follows.

Catalytic N 2 -to-NH 3 Conversion by Fe at Lower Driving

Introduction

Summary of the conditions used for the catalytic conversion of N2-to-NH3 from P3BFe+ highlighting the estimated enthalpic driving force (ΔΔHf. Here we show that the catalytic conversion of N2 to NH3 from P3BFe+ (P3B = tris(o-diisopropylphosphinophenyl)) can be achieved with ) a significantly lower driving force by coupling Cp*2Co with [Ph2NH2]+ or [PhNH3]+ (Figure 4.1). These conditions also provide extremely high selectivity and catalytic turnover for NH3.20 Furthermore, we note the use of milder reagents such as reducing agent (E0; eq 1) and acid (pKa) produce a higher effective bond dissociation enthalpy (BDEeffective; eq 1).15,21 This may in turn afford access.

Various observations of P3BFe complexes in the presence of acids and reductants suggested that this system might be capable of N2-to-NH3 conversion with lower driving force than originally reported.

Figure 4.1. Summary of conditions used for catalytic N 2 -to-NH 3 conversion by P 3 B Fe + highlighting the estimated enthalpic driving force (ΔΔ H f )

Results and Discussion

It is also worth considering the PT-ET pathway where P3BFeN2 is sufficiently basic to protonate to generate P3BFe-N=NH+ as the first step followed by ET (eq 3a-b). We find that the formation of endo- and exo-Cp*Co(η4-C5Me5H)+ is predicted to be thermodynamically favorable by protonation of Cp*2Co with Ph2NH2+ or PhNH3+. To better understand the potential role of PCET in the catalysis of N2 to NH3 conversion by P3BFe, we additionally calculated the N–H bond strengths (Table 4.2) of several possible early-stage intermediates, including the aforementioned P3BFe-N=NH (35 kcal/mol) , P3BFe=N-NH2+ (51 kcal/mol) and P3BFe=N-NH2 (47 kcal/mol).

These considerations are consistent with the reported rapid formation of [HIPTN3N]Mo-N=NH using Cp*2Cr or Cp2Co in the presence of lutidinic acid58.

Table 4.1. N 2 -to-NH 3 Conversion with P 3 E M Complexes (M = Fe, Co)

Conclusion

The above discussion leads to the conclusion that the efficiency for NH3 formation in this system is related to the kinetics and/or thermodynamics of the reaction between anilinium triflate acid and the reductant Cp*2Co. This HER background, and the observed catalytic response upon addition of [Ph2NH2][OTf] to the Cp*2Co+/0 couple, provides circumstantial evidence for the formation of a protonated decamethylcobaltocene intermediate, Cp*(η4-C5Me5H)Co+, at a rate time course similar to that of P3BFe+-mediated N2RR. The isomer shifts quoted are relative to the center of the spectrum of a sheet of α-Fe at room temperature.

This solution is allowed to freeze, after which the headspace of the tube is evacuated and the tube is sealed.

Fe-Mediated Nitrogen Fixation with a Metallocene Mediator

Introduction

In this context, it is remarkable how little corresponding progress has been made towards the discovery of soluble molecular catalysts that mediate electrocatalytic N2RR. Nevertheless, several recent developments, including those from our laboratory, point to the likelihood that iron (and perhaps other) molecular coordination complexes may be able to mediate electrocatalytic N2RR in organic solvent. We present here a study of the effect of pKa on the selectivity of P3BFe+ for N2RR vs HER.

Therefore, we hypothesize that the formation of a protonated metallocene species, Cp*(η4- C5Me5H)Co+, plays a key role in N–H bond formation reactions, either via PCET, PT, or a combination of both during N2RR catalysis.

Results and Discussion

Computational Studies
Electrolysis Studies

The key difference between the stoichiometric reactions described above and the catalytic reaction is the presence of Cp*2Co in the latter. We then investigated the effect of Cp*2Co+ as an additive on electrolysis/electrocatalysis. CPE P3BFe+ in the presence of Cp*2Co+ was also investigated with other acids.

Consequently, a CPE experiment in the absence of P3BFe+ showed that Cp*2Co+ serves as an effective electrocatalyst for HER with [Ph2NH2][OTf] as the acid source, but does not catalyze the N2RR reaction (0% FE for NH3 , 75% FE for H2; see SI).

Figure 5.1. (top) Percentage of electrons being used to form NH 3 or H 2 at different pH values by the FeMo-nitrogenase in A

Conclusion

We suspect that Cp*(η4-C5Me5H)Co+ is likely involved in a variety of N-H bond-forming reactions during overall catalysis, including late-stage nitrogen-fixation-mediated reactions. Despite the fact that Cp*2Co+ itself catalyzes HER under the conditions used for electrocatalytic N2RR, we found that its inclusion in CPE experiments containing P3BFe + and acid under a N2 atmosphere led to modest improvements in the overall NH3 catalytic yield.

In all cases, the positions of the carbon atoms could be located on the difference map and refined anisotropically, and the hydrogen atoms were placed in geometrically calculated positions as usual. The occupancies of disordered fragments were freely refined, and bond lengths and angles were constrained to be the same for disordered fragments. In all cases, the occupancy of disordered fragments was freely refined, and bond lengths and angles were constrained to be the same for disordered fragments of the same species.

After completion of the vacuum transfer, the flask was capped and warmed to room temperature. After completion of the vacuum transfer, the collection flask is capped and warmed to room temperature. The temperature of the system is allowed to equilibrate for 5 minutes and then the tube is closed with a Teflon screw valve.

Figure A1.1: 31 P NMR of reaction mixture A1.3 Ammonia Quantification:

Supplementary Information for Chapter 2

A Synthetic Single-Site Fe Nitrogenase