Their humor and love have helped me through many of the difficult times I have gone through over the years. Limiting the hydrogen evolution reaction (HER) is a key challenge in the selective reduction catalysis of small molecules, including the nitrogen (N2) reduction reactions (N2RR) using H+/e- currency.
LIST OF ABBREVIATIONS
Introduction
- Opening Remarks
- N 2 RR Using Soluble Transition Metal Catalysts
- Selectivity for N 2 RR vs HER
- Proton-Coupled Electron Transfer in N 2 RR
- Method for Predicting E-H Bonds Strengths Using DFT
- Conclusions
- References
- Computational Methods
- Results and Discussion
- DFT Support for Slow Fe Protonation and Fast Fe-N x H y Formation
- Calculation of BDFE N-H Values for Fe–N x H y Intermediates
- Calculated Reduction Kinetics of P 3 E Fe(NNH 2 ) +
- Calculated PCET Reactions
- Wiberg Bond Indices of P 3 E Fe(N x H y ) Species
- Conclusions
- References
Special attention is paid to the influence of these mechanistic steps on the overall selectivity of the systems. As discussed later, these different BDFEN-H values are rooted in the different valence electron counts, and thus the electronic structures of the corresponding P3EFe systems. We have previously hypothesized that the formation of P3EFe (NNH2) is necessary for the release of.
This idea is further supported by the Wiberg bonding indices of the P3EFe(N2) species, which indicate a total bonding order of 4.0 along the Fe-N-N unit for all three scaffolds (Figure 2.6).
Catalytic N 2 -to-NH 3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET Role for Metallocene-Mediated PCET
- Results
- Catalysis Using [P 3 B Fe][BAr F 4 ], Cp* 2 Co and [R n NH (4-n) ][OTf]
- Fe Speciation under Turnover Conditions
- DFT Predicted pK a ’s and BDFEs
- Discussion
- Conclusions
- References
- Introduction
- Results and Discussion 1 pK a studies. 1 pKa studies
- Computational Studies
- Electrolysis studies
- Conclusion
- References
Although we anticipate that other catalyst systems for N2-to-NH3 conversion may yet be found to operate under the conditions described herein8, certain features of the P3BFe system correlate with unusually productive catalysis.14b. This allows for an evaluation of the driving force for a given reaction relative to that of a hypothetical N2 to NH3 conversion catalyst using H2 as the proton and electron source. We present here a study of the effect of pKa on the selectivity of P3BFe+ for N2RR vs HER.
DFT studies support this hypothesis and also establish that the observed pKa effect can be explained by the varying ability of the acids to protonate Cp*2Co. A clear difference between the stoichiometric reactions described above and the catalytic reaction is the presence of Cp*2Co in the latter. The low solubility of the anilinium triflatic acids and the low catalyst concentration lead to a situation in which the interaction between the acid and the Cp*2Co significantly affects the kinetics of productive N−H bond formation.
This leads to the conclusion that the efficiency of NH3 formation in this system is related to the kinetics and/or thermodynamics of the acid-reductant reaction. Since the protonation of the reductant is also the first step behind the HER reaction25, this conclusion is somewhat counterintuitive. In order to investigate the potential effect of the Cp*2Co+ additive, a systematic cyclic voltammetry study was performed.
This background HER and the observed catalytic response to the addition of [Ph2NH2][OTf] to the Cp*2Co+/0 couple provide indirect evidence for the formation of a protonated decamethylcobaltocene intermediate, Cp*(η4-C5Me5H)Co+ a comparable time scale. with that of the N2RR mediated by P3BFe+. Under catalytic conditions, we believe that the presence of the metallocene reducing agent (Cp*2Co) is critical, as this species can be protonated to form Cp*(η4-C5Me5H)Co+, which in turn plays a key role in the formation of N−H bonds. .
Predicting BDFE Values Using DFT
- Introduction
- Calibration of Gas-Phase BDFE E-H Values
- Solution Phase BDFE E-H Values
- Transition Metal Bound E–H Bonds
- Conclusions
- References
In this chapter, a DFT method for the calibration of literature BDFEE-H values is presented which leads to the accurate (within 4 kcal/mol) prediction of both free and metal bound E–H bond strengths in the gas phase. In summary, the method outlined in the following provides an efficient method for gas-phase BDFEE-H predictions and suggests simple empirical corrections that can allow for solution phase values within approx. Gas-phase organic molecules provide an ideal starting point from which to begin calibrating a DFT method.
This is due to the large library of literature values and the lack of any weakening or strengthening of E-H bonds caused by solvation.3 To investigate the performance of BDFEE-H predictions for gas-phase values, four common functionals, B3LYP,4 BP86, 5 M06-L6 and TPSS,7 were used to calculate known gas-phase BDFEE-H values using the conventional and effective basis set (def2-SVP).8. The ability of any of these functions to efficiently reproduce BDFEE-H in the gas phase. In addition to estimating the total errors for the gas-phase BDFEE-H values, the data in Figure 5.1 and Table 5.1 allow us to confirm that these errors are normally distributed.
In the context of free organic BDFEE-H species, a simple empirical formula (Eq. 5.1) was shown to produce only slightly higher errors compared to the gas phase values. Transition metal bound E-H bonds present additional challenges, in large part due to a smaller library of literature values and the lack of well-defined values in the gas phase. The method described is based first on producing a calibration curve using literature data for BDFEE-H species in the gas phase.
Supplementary Data for Chapter 2
The relaxed surface scan reveals little change in the P3BFeN2− unit prior to Et2O dissociation, indicating the presence of an (Et2O)2H+. Bond dissociation free energies (BDFE) of X-H bonds were calculated in the gas phase using a series of known reference compounds.7 The free energy difference between the H atom donor/acceptor pair was calculated based on the thermochemical information provided are through frequency calculations to structure optimizations using the procedure described in the general calculation section. BDFE predictions were generated by applying the line of best fit to the calculated ΔG of the unknown species.
Errors were calculated by applying the trend line to the calculated free energies of the known species and comparing to their BDFE literature value. Errors are reported as the mean of BDFEcalc-BDFElit (signed mean error, MSE = 0.0) and the mean of the absolute values of BDFEcalc-BDFElit (unsigned mean error, MUE = 1.3). The inner-sphere reorganization energy for electron transfer (λis, ET) was estimated by assuming non-adiabatic behavior and calculating the difference between the single point energies of the corresponding species in its ground state and the corresponding single point energy of this ground state in the oxidized or reduced geometry.
The radius of the P3EFe molecules (rcat; equation 2) was approximated using the volume of several relevant crystal structures. This value was approximated to 33 kcal/mol, consistent with the reductions of interest occurring in the normal region. The distance between iron centers was taken as twice the radius of the P3EFe species (r = 12 Å) and ϵo is the static dielectric constant.
Supplementary Data for Chapter 3
The quoted isomer shifts are relative to the centroid of the spectrum of a metallic foil of α-Fe at room temperature (RT). The tube is then allowed to warm to RT and stirred at RT for at least 10 minutes. The volatiles from the reaction mixture are vacuum transferred at RT to this collection flask.
After completion of the vacuum transfer, the collecting flask is sealed and warmed to room temperature. Integration of the 1H NMR peak observed for NH4 was then integrated against the two peaks of trimethoxybenzene to quantify the ammonium present. The THF was then evaporated to provide a thin film of precatalyst at the bottom of the Schlenk tube.
The temperature of the system is allowed to equilibrate for 5 minutes and then the tube is sealed with a Teflon screw valve. This tube is led out of the box in a liquid N2 bath and transported to a fume hood. At this point the tube is allowed to warm to room temperature with stirring and is stirred at room temperature for 5 minutes.
Cl (μmol)
The container is released from the glove box into a liquid N2 bath, then thawed in a dry ice/acetone bath with agitation at ~900 rpm. The headspace of the reaction vessel was periodically sampled with a sealable gas sampling syringe (10 mL), which was loaded into a gas chromatograph, and analyzed for the presence of H2(g). The solvent is evaporated to form a thin film of the precatalyst and a stir bar is added.
At this point, the sample, immersed in N2(l), is taken outside the glove compartment and mounted in the cryostat. Fitting details for Figure A2.3: Four quadrupole doublets were found necessary to obtain an adequate simulation. One of these can be identified as [P3BFe]+ based on the asymmetry in the line shape of the right-hand feature in P3BFe–.
The raw data are shown as black dots, the simulation as a solid red line, with components in green, purple, yellow and orange (see Table A2.6.5 for parameters). At this point the sample is quickly transferred from the glove box and placed in N2(l) before being warmed. A comparison of the best-fit parameters for the authentic sample of P3BFeN2- (A), freeze-quenching of the reaction with the reductant (B), freeze-quenching of the catalytic reaction mixture (C).
Supplementary Information for Chapter 4
Allow this solution to freeze, add the Schlenk tube adapter and evacuate the space above the tube. Using a pre-chilled pipette, approximately 0.5 ml of the reaction mixture is rapidly transferred to a pre-chilled X-band EPR tube. Fitting details for Figure A3.4: The parameters used to fit the spectrum were obtained using the esfit application in the easyspin program.18 The fitting program achieves the best fit by minimizing the root mean square deviation from the data.
The THF was then evaporated to provide a thin layer of Fe species at the bottom of the Schlenk tube. The system temperature is allowed to equilibrate for 5 minutes and then the Konte valve is closed. At the end of the reaction, the Conte valve is opened and the main reaction space is allowed to equilibrate.
The system temperature is allowed to equilibrate for 5 minutes and then the Schlenk tube is transferred to the cold well which has been precooled to -78°C for fifteen minutes. Using a pre-chilled pipette, the entire reaction mixture is transferred to a similar pre-chilled Celite pad for filtration. An additional 100 μmol of acid in 2 ml of 0.1 M NaBArF4 solution in Et2O is then added to the working chamber of the cell via injection through a rubber septum.
