Proton-Coupled Electron Transfer in Nitrogen Fixation

32 Figure 3.1: Summary of conditions used for catalytic conversion of N2-to-NH3 by [(P3B)Fe]+ highlighting the estimated driving force (ΔΔHf). 210 Figure C.1: Comparison of H2 evolution catalyzed and uncatalyzed by 2,6- dichloroanilinium triflate and Cp*2Co at early time points.

Opening Remarks

The environmental consequences of disruption of these important biogeochemical cycles will continue for millennia.5. One of the most important analogies to the biological system is our choice to use protons and electrons (instead of H2) to reduce N2.

Figure 1.1: Depiction of the active site of the FeMo nitrogenase (PDB: 1M1N) 3 with the idealized stoichiometry for biological nitrogen fixation

Multiple Bonding

Indeed, NO+, the canonical resonance form assigned to linear Fe–NO complexes such as these, is isoelectronic with N2. NO complexes in the lowest valence states lead to occupation of dz2 resulting in bending of the Fe-NO ligand and in turn promoting reactivity at the proximal nitrogen (Nα).

Figure 1.4: (top) Mo and W complexes promote the functionalization of N 2 and its conversion to NH 3

Proton-Coupled Electron Transfer

PCET exists in many variants, which I will briefly define here as a reference for the reader (Figure The simplest is the hydrogen atom transfer (HAT), where both the proton and the electron originate from the same orbital. Abstraction with one degree, you arrive at concerted-proton electron transfer (CPET), where the proton and electron are derived from different orbitals in the same molecule or derived from the same orbital but travel to two different receptor orbitals in the same molecule.

Figure 1.6: Canonical examples of hydrogen atom transfer (HAT), concerted-proton electron transfer (CPET), and multi-site proton-coupled electron transfer (MS-PCET)

Overpotential in Nitrogen Fixation

Of course, no ideal catalyst for nitrogen fixation has been discovered, so the ability to determine the (thermodynamic) deficiencies of various catalysts becomes important in terms of measuring progress within the field. Therefore, one can compare the energy of formation of H• from H2 (i.e., the homolytic bond strength of H2) with that given by the reaction, defined as the effective bond breaking free energy (BDFEeff) of the hypothetical H• formed by a combination of the source H+ and e−.11.

Figure 1.8: Calculated gas-phase bond dissociation enthalpies (BDEs) for intermediates of potential relevance to nitrogen fixation

Selectivity in Nitrogen Fixation Catalysis

Thus, the chemical identity of the acid and the reducing agent will in turn affect the identity and reactivity of the intermediates of this background HER pathway. Reaction of the acid and the reducing agent will typically generate a species that is a strong PCET reagent (BDFEX-H ≤ 50 kcal mol−1).

Chapter Summaries

In Chapter 5, we provide conclusive experimental evidence for the protonation of Cp*2Co via continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy and pulsed-EPR spectroscopy. We also use thermochemical techniques to confirm the very weak C–H bond formed upon protonation of Cp * 2Co that we had predicted computationally.

HER reaction and productive reactions of X–H bond formation during proton-coupled reduction of small molecules in general.43. By directing protonation to this remote Brønsted base site instead of the Cp ring, further reduction is prevented.

A Triad of Highly-Reduced, Linear Iron Nitrosyls: {Fe(NO)} 8-10

Introduction
Results

Synthesis of {Fe(NO)} 8-10
Characterization and Electronic Structure of {Fe(NO)} 8-10

Conclusion
References

This interaction via bond has previously been interpreted as indicative of a highly covalent interaction.20 Although the Fe-B bond. This high degree of covalence, especially in the Fe-NO bond but also between the Fe and the P3B ligand, leads to the atypical conservation of a linear Fe-N-O geometry.

Figure 2.1: Synthetic route for {(P 3 B )Fe(NO)} 8-10 .

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

Introduction

Thus, there is interest in exploring the viability of Fe-mediated catalytic N2-to-NH3 conversion under less stringent conditions from a practical perspective and to continue to evaluate these systems as functional models of biological nitrogenases where 8 ATP are consumed per . formed NH3. to a driving force of 58 kcal/mol.2 Herein we demonstrate that catalytic conversion of N2 to NH3 with [(P3B)Fe]+ (P3B = tris(o-. diisopropylphosphinophenyl)borane) can be achieved with a significantly lower driving force by couple Cp*2Co with [Ph2NH2]+ or [PhNH3]+ (Figure 3.1). Various observations of (P3B)Fe 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 3.1: Summary of conditions used for catalytic N 2 -to-NH 3 conversion by [(P 3 B )Fe] + highlighting the estimated driving force (ΔΔH f )

Results

Catalytic N 2 RR at Lowered Overpotential
Mechanism of N 2 RR Catalysis with (P 3 B )Fe
PCET from a Protonated Metallocene

Using the more soluble acid [Ph2NH2][BArF4] (entry 6) produces significantly lower, but still catalytic, yields of NH3. Although we expect that other catalyst systems for the conversion of N2 to NH3 can be found that function under the conditions described here14, certain features of the (P3B)Fe system correlate with unusually productive catalysis19.

Figure 3.2: Mössbauer spectrum at 80 K with 50 mT applied parallel field of a freeze- freeze-quenched catalytic reaction (54 equiv Cp* 2 Co, 108 equiv [Ph 2 NH 2 ][OTf], 1 equiv [(P 3 B ) 57 Fe] + ) after five minutes of reaction time

Discussion

This drive of PCET steps increases sharply as further downstream Fe-NxHy intermediates are considered. 49 kcal/mol, and we calculate it via DFT with a truncated HIPTN3N ligand to be 51 kcal/mol.34,44 The BDEN-H for this Mo diazenido species is therefore much larger than we predict for (P3B) Fe(NNH) (35 kcal/mol), perhaps responsible for its higher stability.44 A PCET reaction between [(Cp*)Cr(endo-η4-C5Me5H)]+ (BDEcalc = 37 kcal/mol) and ( HIPTN3N)Mo(N2) to generate (HIPTN3N)Mo(NNH) and [Cp*2Cr]+ will be highly exergonic.

Conclusion

Furthermore, we predict an equally weak BDEC-H for Cp-protonated cobaltocene, [(Cp)Co(η4-C5H6)]+ (BDEcalc = 35 kcal/mol). These considerations are consistent with the reported rapid formation of (HIPTN3N)Mo(NNH) using either Cp*2Cr or Cp2Co in the presence of lutidinium acid.

Based on our calculations, we propose that protonated metallocenes should serve as discrete, highly reactive PCET reagents in N2-to-NH3 conversion catalysis. Indeed, the achievement of high efficiency for N2-to-NH3 conversion with both (P3B)Fe and various Mo catalysts that take advantage of metallocene reductants raises the intriguing possibility that metallocene-based PCET reactivity is a potentially widespread and overlooked mechanism.

Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring

Introduction

We present here a study of the effect of pKa on the selectivity of [(P3B)Fe]+ for N2RR vs HER. We therefore hypothesize that the formation of a protonated metallocene species, [(Cp*)Co(η4- C5Me5H)]+, plays a critical role in N–H bond-forming reactions, either via PCET, PT, or a.

Figure 4.1: Electrosynthetic cycle for the formation of NH 3 from N 2 by a Chatt-type W- W-phosphine complex

Results and Discussion

Effect of pK a on the Selectivity of N 2 Fixation by [(P 3 B )Fe] +
Computational Studies on Cp* 2 Co Protonation and N–H Bond Formation
Electrocatalytic N 2 Fixation with [(P 3 B )Fe] +

Freeze-quench Mössbauer analysis shows the formation of the oxidized products (P3B)Fe(N2) and [(P3B)Fe]+, but nothing attributable to [(P3B)Fe(NNH2)]+. The above discussion leads to the conclusion that the efficiency of NH3 formation in this system is coupled to the kinetics and/or thermodynamics of the reaction between the anilinium triflatic acid and the Cp*2Co reductant.

Figure 4.2. Effect of acid strength on the percentage of electrons being used to form NH 3

Conclusion

Introduction

Density functional theory (DFT) studies performed by our group supported the idea that protonation of metallocenes such as Cp*2Co or Cp*2Cr by catalytically important acids. Based on the X-band continuous wave (CW) electron paramagnetic resonance (EPR) spectrum (77 K, Fig. 4) of the solid, we speculate that it is a protonated species of Cp*2Co.8 Here, we advocate a rigorous characterization of the protonation products of Cp* 2Co using pulse EPR techniques and provide unequivocal evidence for the assignment of ring protonated isomers [(Cp*)Co(exo/endo-η4-C5Me5H)]+.

Results

Pulse Electron Paramagnetic Resonance Spectroscopy on Protonated
Stereochemical Assignment of Cp* 2 Co Protonation
Thermochemical Measurements Relevant to Cp* 2 Co Protonation

The simulations are generated using the same parameters, except the weighting of the two species. Continuing to scan these voltammograms further in the cathodic direction leads to the observation of the fully reversible [Cp*2Co]+/0 pair (Figure 5.8, top).

Figure 5.2: Pseudomodulated 24 Q-band ESE-detected EPR spectra of the reaction of Cp* 2 Co with HOTf (black traces), and DOTf (blue traces) measured at 10 K (top traces) and 6 K (bottom traces)

Discussion

Only in the case of [(Cp*)Co(exo-η4-C5Me5H)]+ are both stabilizing factors driving the formation of the product. This weakening is due to the high stability of the 18e−, d6 [Cp*2Ni]2+ product due to the net transfer of hydrogen atoms.

Figure 5.10: (left) A comparison of the experimental BDFE C–H for a variety of related Cp*Co-species, demonstrating the importance of aromaticity and electron count in predicting the stability of the indicated C–H bond

Conclusion

Introduction

A combination of phosphoric acid and iridium photoreducers has been shown to be strong PCET donors.15,16 (bottom left). A new, potent PCET donor synthesized and described here suitable for the electrocatalytic reduction of ketones to pinacols.

Results

The combination of silanes and FeIII alkoxides20,21 and SmII with water has been shown to generate strong PCET donors.17–19,22 (top right) PCET to an unsaturated organic substrate results in a new X–H bond that is unusually weak due to the α-radical. bottom right).

Regardless of whether protonation of cobaltocene (and Cp*2Co) generates a strong PCET donor, they are not suitable as electrocatalytic PCET mediators under our envisioned CPE conditions (negative applied bias, excess acid, see below). This effect has been discussed in the context of half-sandwich (Cp*)Rh complexes undergoing ring protonation.25 We predicted that by synthetically adding a Brønsted base to cobaltocene, we would both accelerate the protonation rate and retain the bis. -Cp ligand framework whereby a second reduction is not favored by avoiding formation of an electron-accepting diolefin ligand.

Figure 6.2: Cyclic voltammetry relevant to HER by [Cp 2 Co] + (left) Line of best fit for relationship between the plateau current and [Cp 2 Co] + concentration in the HER reaction at a BDD electrode in DME with 100 mM [ 4-CN PhNH 3 ][OTf] and 200 mM [

B Synthesis of a Brønsted Base-Appended Cobaltocene

Continuous-wave X-band electron paramagnetic resonance spectroscopy at 77 K of rapidly freeze-quenched reaction mixtures (Figure 6.5) of either [(Cp)Co(CpNH)]2+ with SmI2 or (Cp)Co(CpN) with trifluoromethanesulfonimide led to observing the same EPR signal. UV-Vis spectroscopy of the reaction of (Cp)Co(CpN) with trifluoromethanesulfonimide at -130 °C in 4:1 2-MeTHF:THF demonstrates isosbestic behavior, consistent with the formation of a single product (Figure 6.5) .

Figure 6.3: Scheme describing the synthesis of the Brønsted base appended cobaltocenes

C Thermochemical Properties of a Brønsted Base-Appended Cobaltocene

D Electrochemical behavior of (Cp)Co(Cp N ) with Excess Acid

Spin density plot of [(Cp)Co(CpNH)][OTf] at an isovalue of 0.04, showing that the N–H bond is completely decoupled from the spin density.

E Electrochemical Behavior in the Presence of a Hydrogen Atom Acceptor

Optimization of the CPE conditions eventually led us to use a cell with a toxic acid compartment (pKa of 8.45 in MeCN),38 and a glassy carbon counter electrode. The toxic acid was necessary because of the undesirable anodic reactivity of the conjugate base of anilinium acids.

Table 6.1. Results from CPE experiments on a DME solution of 200 mM [TBA][PF 6 ], 100 mM tosic acid, 50 mM acetophenone at −1.45 V vs Ag +/0

Discussion

The importance of the BDFE in determining the reactivity is demonstrated by the selective proton-coupled reduction of acetophenone to its pinacol in CPE experiments. This is consistent with results on the proton-coupled electrode-mediated reduction of ketones with other metals,50,51 and the.

Conclusion

CPE experiments in the presence of excess acid and acetophenone form the pinacolization product, selectively demonstrating the potential of this catalyst as a PCET mediator. Furthermore, given the growing recognition of the role of strong PCET donors in the catalysis of nitrogen fixation and the demonstrated utility of PCET mediators as co-catalysts in electrocatalytic O2 reduction, we believe that similar cobalt catalysts may play a role as co-catalysts . catalysts in the proton-coupled reduction of small molecules.

Structures were solved using SHELXS or SHELXT and refined against F2 on all data by least squares of the full matrix using SHELXL.6 Crystals were mounted on a glass fiber under Paratone N oil. The solvent was then removed under reduced pressure and the residue was extracted with pentane and filtered through celite.