My yearmates, Nina Gu and Javier Fajardo, are thanked for their solidarity during our time in the group. Javier was also one of my officemates during much of my time in the group.
INTRODUCTION
Opening Remarks
First, while relatively little is known about the intimate mechanistic details of N2 conversion by the FeMo cofactor, a relatively well-understood aspect of nitrogenase chemistry is the build-up of Fe(H) precursors prior to N2 binding and fixation. In both the Haber–Bosch19 and Fischer–Tropsch16 processes, complete cleavage of the strong N≡N and C≡O bonds at the catalyst surface initiates reduction of these species.
Why Molecular Model Chemistry?
For both of these systems, an additional parallel can be drawn between their mechanisms, which have been clarified through detailed studies. Ideally, these advantages in model systems can be applied to improve conceptually related processes in real systems.
Transition Metal Hydrides in Small Molecule Transformations
- Transition Metal Hydrides in Molecular CO Reduction Chemistry
This allows for an understanding of their reactivity at an atomic level, which can be challenging, if not impossible, in studies of biological or heterogeneous catalyst systems. To this end, the targeted reactivity in my thesis primarily aims to gain access to unusual structure types and develop new reactivity on late first-row transition metal platforms.
Representative examples of Fe-mediated transformations of CO including (A) O-functionalization, (B) C-C coupling, and (C) main group hydride reactivity
- Transition Metal Hydrides in Molecular N 2 Reduction Chemistry
- Reactivity Profiles of Transition Metal Dihydrogen Adducts
- Chapter Summaries
- Notes and References
- Introduction
- Results and Discussion
- Synthesis of [Fe(CO)(H) n ] m- Complexes
- O-Functionalization Reactivity with Silyl-Electrophiles
- Release of CO-Derived Organic Products
Reaction of 2.2 with 2.5 equiv. Me3SiOTf and the crystal structure of the reaction product 2.6. Isotope labeling experiments confirm that this signal is derived from one of the original hydride ligands and shows strong coupling to the CO-derived C atom (127 Hz), consistent with the formation of a new C-H bond in 2.7.
Summary of isotopic labeling experiments
- Conclusions
- Experimental Details .1 General Considerations .1 General Considerations
- Physical Methods
- Synthetic Details
- Notes and References
The solution was stirred at room temperature until complete conversion of the starting material was observed by NMR spectroscopy (∼24 h). A stock solution of 2.6 was prepared in C6D6 and equal volumes of the solution were added to two separate J's.
ELECTROPHILE-PROMOTED IRON-TO-N 2 HYDRIDE MIGRATION IN HIGHLY REDUCED Fe(N 2 )(H) COMPLEXES
Introduction
An alternative possibility can be considered where the Fe-H intermediates are able to undergo the Fe-to-N migration steps and therefore become involved in productive general schemes for N2 fixation. Fe-H species important for catalytic N2 fixation by molecular Fe complexes and proposed pathways for the generation of the Fe-N2 intermediate en route from the quiescent state of the dihydride 1.6b,d,f,8.
- Results and Discussion
- Synthesis of Fe(N 2 )(H) n Complexes
- Functionalization with Silyl Electrophiles
- N-H Bond Formation
- Release of N-Fixed Products
- Conclusions
- Experimental Section .1 General Considerations
- Physical Methods
- Synthetic Details
- Notes and References
- FUNCTIONALIZATION OF A COBALT CARBONYL GENERATES A TERMINAL COBALT CARBYNE
- Introduction
- Results and Discussion
- Conclusions
- Experimental Section .1 General Considerations
- References and Notes
The mentioned isomer shifts are relative to the center of gravity of the spectrum of a metal foil of a-Fe at room temperature. Solid-state characterization of the tBuPh2Si-functionalized complex 4.6b confirmed its assignment as a terminal carbyne complex (Figure 3). For BP3Co(NAd), the crystal coordinates of the corresponding Fe complex13a were used in the input file, with Fe replaced by Co.
Crystals suitable for XRD can be grown by evaporating an Et2O solution of the complex in MeCy. A solution of P2PCoBr (CO mg, 1 equiv) was stirred over an excess of Na metal in THF (5 mL) until conversion to 4.5a was observed with darkening of the reaction mixture.
UNUSUAL REACTIVITY MODES OF NON-CLASSICAL DIHYDROGEN COMPLEXES
- Introduction
- Results and Discussion
- Synthesis of [P 3B Co-H 2 ] 0/1- Complexes
- Synthesis and Characterization of P 3B Co(H)
- Accessing H-Atom and Hydride Transfer Reactivity from H 2 Adducts
- A) Simplified representation of the reported mechanism for net bimolecular H 2 oxidative addition from an Fe I (H 2 ) adduct precursor as proposed by Bullock and
- Plausible pathways for the generation of 5.1 upon the addition of a suitable H- atom donor (D-H) to the hydride complex 5.3-H 2
- Conclusions
- Synthetic Details
- Notes and References
The critical factor that gives us access to the net H atom and hydride transfer reactivity from 5.1-H2 and 5.2-H2,. Representation of the tautomeric equilibrium between the non-classical adduct and dihydride isomers of 5.1 and 5.2 and their reaction with an appropriate acceptor molecule (A) to generate the hydride product 5.3-H2 (top). Attempts to obtain kinetic data for the transformations of 5.1-H2 or 5.2-H2 to 5.3-H2 that discriminate between these limiting pathways have been unsuccessful.
For the reaction of 5.1-H2 with tBu3ArO•, attempts to obtain reproducible kinetic data at low temperature were unsuccessful. Given the limited precedent for these reactivity profiles, it is worth reemphasizing the remarkable hydricity of 5.2-H2.
FUNCTIONALIZATION OF IRON THIOCARBONYL COMPLEXES
TOWARD A TERMINAL IRON CARBIDE
Introduction
Given that attempts by our group to cleave the strong C–O bond in Fe-carbonyl complexes have not yet been fruitful, we hypothesized that moving to the corresponding thiocarbonyl complexes would provide access to S-functionalization reactivity and that. In the reduced state, these complexes are susceptible to S-functionalization reactivity upon treatment with alkyl triflate electrophiles, providing synthetic access to a range of terminal thiocarbynes. In one case, a surprising C-S bond cleavage pathway is observed, but it proceeds without the intervention of a terminal Fe carbide.
Additional approaches to generating a terminal carbide in these systems are discussed, highlighting limiting factors that have thus far precluded generation of the target species.
Results and Discussion
- Synthesis and Characterization of P 3 B Fe(CS) Complexes
- S-Functionalization to Generate Thiocarbyne Complexes
NMR spectroscopic analysis of the resulting reaction mixture confirmed the formation of the previously reported Mo=S complex19 with concomitant formation of a new, diamagnetic Fe species, P3BFe(CS) 6.2.20 The similar solubility properties of the two transition metal products from this transformation precluding their direct separation. Instead, reduction of the product mixture over an excess of Na/Hg amalgam in THF gave selective one-electron reduction of the Fe complex, which could be isolated after crystallization as its Et2O solvate [P3BFe(CS)][Na(Et2O)2 ] 6 ,3 or by precipitating the crown ether-encapsulated salt [P3BFe(CS)][Na(12-crown-4)2] 6,3' from Et2O (Scheme 6.2). Solid state structural characterization of Na-capped complex 6.3 shows significant activation of the CS ligand (Figure 6.4).
The corresponding elongation of the C-S bond is observed at 1.663(2) Å, which is longer than most structurally characterized thiocarbonyl complexes (typically ∼1.5–1.6 Å).23. Like the corresponding Fe-CO complex, this species is diamagnetic, with an interaction observed between the Fe center and one of the ligand arenes in its solid-state structure (Figure 6.2).16 The threefold symmetry of the solution is observed from room temperature. NMR spectroscopy, showing that this interaction is fluctuating in solution on the NMR time scale.
Synthesis of Fe-CS and Fe≡C-SMe complexes across three oxidation states
- Approaches to Carbide Generation from P 3 B Fe≡CSMe Complexes
One electron oxidation of the neutral carbyne complex 6.5 was obtained after treatment with an equivalent of [Fc][BArF4] to generate the dark green cationic carbyne complex [P3BFe≡C-SMe][BArF4] 6.6 (Scheme 6.3). Like the neutral thiocarbonyl complex 6.2, an interaction between the Fe center and one of the arene rings. This complex can also be prepared by treating the dianionic thiocarbonyl complex 6.4 with an equivalent of MeOTf (Scheme 6.3).
In solution, this species is thermally unstable and regenerates the anionic thiocarbonyl complex 6.3 upon warming to room temperature with formal loss of one equivalent of methyl radical. For the cationic thiocarbyne complex 6.6, deprotonation of the -SMe group could lead to this type.
General, unsuccessful approaches to generate a terminal Fe-carbide complex
- Synthesis of P 2 P Fe(CO) Complexes
- Synthesis of P 2 P Fe(CS) Complexes
- S-Functionalization at P 2 P Fe Thiocarbonyl Complexes
6.8 (Scheme 6.5) with one predominant isomer estimated by 31P NMR spectroscopy.29 Consistent with this assignment, the IR spectrum of 6.8 has a sharp CO stretching at 1938 cm-1, indicating the formation of a monocarbonyl complex. One-electron reduction of 6.8 with an equivalent of naphthalenide cleanly produces the doublet P2PFeIBr(CO) 6.9 (Scheme 6.5). During one-electron reduction, an increase in the activation of CO (νCO = 1886 cm-1) can be observed by IR spectroscopy, whereby this five-coordinate complex is isostructures with P2PCoIBr(CO) (Figure 6.3).3 Treatment of this species with an additional equivalent of Naphthalenide Na forms a diamagnetic complex Fe0 6.10, which binds an equivalent of N2 (Scheme 6.5).
Treatment of P2PFe(CO)(N2) with one equivalent of Na-naphthalenide followed by two equivalents of 12-crown-4 enabled clean isolation of the monoanionic carbonyl complex 6.11 as the crown ether-encapsulated Na salt (Scheme 6.5). As such, treatment of complex 6.14 with an excess of KC8 cleanly generates a new diamagnetic complex, which we assigned as the formal Fe-2(CS) complex.
Proposed pathway for the reaction of 6.17 with alkyl and acid electrophiles
- Conclusions
- Synthetic Details
- Notes and References
To isolate this species as the 12-crown-4 encapsulated sodium salt 6.3', the residue obtained after the Na/Hg amalgam reduction was dissolved in Et2O and 12-crown-4 (2 equiv.) was added. Extraction of the solid into THF followed by solvent removal in vacuo afforded the product as a dark orange powder. The residue was extracted into pentane and the solvent was removed, yielding the product as a brown powder (49.7 mg, 76%).
The product can be obtained as a crystalline solid by layering a THF solution of the product with pentane at -30 ºC. The solvent was removed and the residue was triturated and then washed with pentane. The residue was extracted into benzene and then lyophilized to give the product as a brown powder.
MODIFICATION OF LIGAND ELECTRONIC PROFILES IN N 2
FIXATION CATALYSIS
For Al.2-OMe, lithium-halogen exchange was performed with the addition of 2 equiv tBuLi at −78 °C in toluene. In situ quenching of the generated aryl-lithium with B(OMe)3 (1/3 equiv) yields the desired MeO-P3B product A1.3-OMe. Overnight reduction of this species with stoichiometric Na/Hg amalgam in benzene yields (R-P3B)FeBr A1.4-R (Scheme A1.2).
With this in mind, the complexes A1.6-OMe and A1.6-F were used as precatalysts for the reduction of N2 to NH3 under the conditions previously reported for A1.6-H. Complex A1.6-F proved to be a significantly poorer catalyst under the same conditions, yielding only an equivalent of NH3.
SUPPLEMENTARY DATA FOR CHAPTER 2
This compound crystallizes with two K-coordinated THF molecules and two free THF molecules in the asymmetric unit. The bridging borohydride and one of the
Anisotropic refinement of free THF molecules was aided by the use of RIGU, SADI and SAME restraints.
This compound crystallizes in the absence of solvent
We tentatively assigned this species as the monoanionic product of CO functionalization in which the silyl electrophile reacted at the carbonyl O, one hydride equivalent migrating to the carbonyl C, stabilizing this species through a P-C interaction. After the reaction of 2.6 with H2 or PhSiH3, the terminal Fe-containing mixture is intractable in the absence of additives and dependent on the choice of E-H source (H2 or PhSiH3). We found that CO and tBuNC reacted rapidly with 2.6, and no product release was observed in the presence of these additives.
Further, the same mixture of Fe-containing products is observed by NMR spectroscopy, with new, shifted paramagnetic resonances (point selected here) along with an observable amount of 2.1. This is consistent with the Mössbauer data, which show a mixture of 2.1, a new S = 3/2 species, and a residual signal that is likely representative of multiple species.
SUPPLEMENTARY DATA FOR CHAPTER 3
Attempts to obtain meaningful kinetic data at higher reaction temperatures were complicated by the competitive decomposition of product 3.10. Oxidation: Treatment of the anionic hydride complex 3.4 with [Cp2Co][PF6] at -78 °C in THF gives access to a new double species, tentatively designated as (P3B-H)Fe(N2) 3.5. After stirring for about 12 h, some decomposition of the starting material 3.12-CNtBu was evident, but the hydrazido(1-) complex 3.11 was not observed spectroscopically.
Two of the six shared phosphine groups of iPr were modeled in related disorder at two positions (64:36). In addition, one of the K-coordinated crown moieties was modeled as rotationally disordered in two positions (56:44).
