This thesis focuses on the management of protons and electrons in the formation of X−H bonds. In our pursuit of better understanding this process, we have been particularly interested in the nitrogen fixation reaction (N2-to-NH3) because of the high number of protons and electrons involved in this conversion (6) and the significant difficulty of functionalizing N2. The first chapter introduces the important themes of this thesis: (i) multiple bonding, (ii) proton-coupled electron transfer, (iii) overpotential in N2 fixation, and (iv) selectivity in N2 fixation. The second chapter discusses the bonding of an iron complex with a small molecule (NO) and how this bonding is key to activating the small molecule for reactivity. The third chapter looks at how employing a new proton and electron source allows an Fe catalyst to achieve improved selectivity and turnover number for the reduction of N2 to NH3 despite a lowered overpotential relative to previous reactions. It also raises the hypothesis that this is possible due to proton-coupled electron transfer mediated by a metallocene. The fourth chapter studies the effect of acid strength on N2 fixation selectivity and demonstrates circumstantial evidence for the involvement of a decamethylcobaltocene (Cp2Co) in the formation of N−H bonds via proton-coupled electron transfer. It also highlights how the addition of co-catalytic [Cp*2Co]+ to electrochemical experiments with our Fe catalyst enabled truly electrocatalytic N2 fixation for the first time. The fifth chapter provides both atomistic detail on the protonation reactivity of Cp2Co and experimentally verifies the prediction that this species would be an extremely strong hydrogen-atom donor. It also develops a conceptual framework to explain the uniquely weak C−H bonds both homolytic and heterolytic that result from metallocene protonation and discusses their potential to play a role in not only the hydrogen evolution reaction (HER), but also the N2 fixation reaction. In the final chapter, we develop a synthetic route to a base appended cobaltocene. We demonstrate that this second-generation cobaltocene can, unlike the first generation, serve as a net hydrogen-atom donor under electrocatalytic conditions. As a demonstration of the utility of this, we use the base-appended cobaltocene for the selective, proton-coupled reduction of ketones to pinacols via a rate-determing concerted proton-electron transfer.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/QDAW-M003, author = {Deegan, Meaghan Marie}, title = {Small Molecule Reactivity of Trisphosphine-Supported Iron and Cobalt Complexes}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/QDAW-M003}, url = {https://resolver.caltech.edu/CaltechTHESIS:09252019-210323106}, abstract = {The work described in this thesis emphasizes accessing novel reactivity patterns in the activation of carbon monoxide, dinitrogen, and dihydrogen by leveraging phosphine-supported iron and cobalt complexes. In Chapters 2 and 3, systems that access CO and N2 reductive functionalization from highly reduced Fe-hydride precursors are described. These systems access productive C-H and N-H bond forming steps from hydride precursors that ultimately allows for the liberation of four-electron reduced products through novel chemical pathways. Chapter 4 describes an unusual example of a terminal cobalt carbyne through the O-functionalization of a carbonyl complex. Next, in Chapter 5, we consider electronically unusual examples of dihydrogen complexes and explore their propensity for accessing H-atom and hydride transfer. Finally, to conclude, Chapter 6 details our synthetic efforts targeting the synthesis of a terminal Fe-carbide complex through the cleavage of a thiocarbonyl precursor.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/8zwz-gh20, author = {Gu, Nina Xiao}, title = {Synthesis, Characterization, and Reactivity of Thiolate-Supported Metalloradicals}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/8zwz-gh20}, url = {https://resolver.caltech.edu/CaltechTHESIS:04102020-135244808}, abstract = {Reactive metalloradical intermediates have been implicated in both biological and synthetic catalyst systems for small molecule activation processes, including proton reduction and ammonia oxidation. Towards a greater mechanistic understanding of such transformations on well-defined model complexes, this thesis explores relevant H–H and N–N bond-forming reactions mediated by trivalent Fe and Ni species, as well as catalytic N–N bond cleavage mediated by an open-shell VFe bimetallic complex. First, a pair of thiolate-supported, S = ½ iron and nickel hydrides are synthesized and spectroscopically characterized at low temperatures (Chapters 2, 3). Paramagnetic iron and nickel hydrides have been proposed as catalytic intermediates of [NiFe] hydrogenase and nitrogenase, but characterization of such molecular species are limited. For both the FeIII and NiIII hydride complexes described herein, spin delocalization onto the thiolate ligand is proposed to stabilize the formal 3+ metal oxidation state. Furthermore, both the FeIII–H and NiIII–H species are demonstrated to undergo the bimolecular reductive elimination of dihydrogen upon warming, albeit with distinct activation parameters consistent with different proposed pathways for H–H bond formation. Chapter 4 expands upon the H–H bond forming chemistry demonstrated on the Ni system to demonstrate related N–N bond formation from an analogous NiIII–NH2 species, resulting in the formation of a NiII2(N2H4) complex. Given the diverse mechanistic possibilities for the overall 6e-/6H+ transformation to oxidize ammonia to dinitrogen, identification of the active M(NHx) intermediate and pathway for N–N bond formation is a central mechanistic question. While the homocoupling of M–NH2 species to form hydrazine has been hypothesized as the key N–N bond forming step in ammonia oxidation systems, stoichiometric examples of this transformation from M–NH2 complexes are rare. Lastly, Chapter 5 details the synthesis of a heterobimetallic VFe complex featuring a bridging thiolate, inspired by the structure of the VFe nitrogenase cofactor. This VFe species is demonstrated to be an active catalyst for the disproportionation of hydrazine to dinitrogen and ammonia. Notably, the heterobimetallic complex is appreciably more active than monometallic analogues of the individual V and Fe sites, suggesting that bimetallic cooperativity may facilitate the observed catalysis.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/R9EJ-KX83, author = {Hannoun, Kareem Imad}, title = {Mechanistic Study of Cu-Mediated, Photoinduced C–S Bond Formation and Demonstration of Electrochemical Ammonia Production by a Surface-Attached Iron Complex}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/R9EJ-KX83}, url = {https://resolver.caltech.edu/CaltechTHESIS:05302019-070503222}, abstract = {The worldwide reliance on fossil fuels for energy and petrochemicals poses a massive environmental hazard. Furthermore, many chemical processes rely on precious metals that have low abundance on Earth and are threatened. As the world population grows rapidly, these factors pose an increasing threat to our planet and new chemical processes are needed that employ earth-abundant catalysts and alternative chemical currencies such as light and electricity derived from renewable sources.
Chapter 2 discusses an in-depth mechanistic study of the photoinduced, copper-mediated cross-coupling of aryl thiols with aryl halides. This reaction employs light energy and an earth-abundant metal to achieve bond formation through a pathway distinct from that of thermal reactions. In particular, I focus on the stoichiometric photochemistry and subsequent reactivity of a [CuI(SAr)2]– complex (Ar = 2,6-dimethylphenyl). A broad array of experimental techniques furnish data consistent with a pathway in which a photoexcited [CuI(SAr)2]-* complex undergoes SET to generate a CuII species and an aryl radical, which then couple through an in-cage radical recombination.
Chapter 3 discusses the surface attachment of a P3BFe complex to a carbon electrode, and the electrochemical generation of ammonia from N2 by the surface-appended species (P3BFe = tris-phosphinoborane). Ammonia production is achieved industrially by the combination of N2 and H2, the latter of which is derived from methane with concomitant production of CO2. Alternative chemical processes, such as the use of energy derived from electricity, are vital for the decreasing the carbon footprint of ammonia production. Synthetic modification of a previously-reported P3BFe complex by addition of three pyrene substituents onto the catalyst backbone allows non-covalent attachment onto a graphite surface. The resulting functionalized electrode shows good stability towards iron desorption under highly reducing conditions, and produces 1.4 equiv NH3 per iron site. The data presented provide the first demonstration of electrochemical nitrogen fixation by a molecular complex appended to an electrode.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/T4WQ-TM68, author = {Thompson, Niklas Bjarne}, title = {A Synthetic Nitrogenase: Insights into the Mechanism of Nitrogen Fixation by a Single-Site Fe Catalyst}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/T4WQ-TM68}, url = {https://resolver.caltech.edu/CaltechTHESIS:05292018-165300346}, abstract = {Nitrogen fixation, specifically the conversion of molecular nitrogen into ammonia, is a fundamental reaction necessary to support life. Our group has recently discovered the first family of well-defined iron complexes that catalyze the conversion of dinitrogen to ammonia. This thesis details mechanistic study of the nitrogen fixation chemistry these complexes. Chapter 1 presents an abbreviated overview of catalytic nitrogen fixation, which places our work in a larger context. Chapter 2 details the synthesis and nitrogen fixation activity of a series of cobalt complexes that are homologous to the known iron-based catalysts. The central goal of this work was to provide a structure-function study of the isostructural cobalt and iron complexes, in which the nature of the transition metal ion was changed in a fashion that predictably modulated the electronics of the system. Chapter 3 details in situ mechanistic studies of nitrogen fixation catalyzed by the iron complexes under the originally-reported reaction conditions. In this study, we were able to achieve a nearly order-of-magnitude improvement of catalyst turnover. Study of the reaction dynamics evidence a single-site mechanism for dinitrogen reduction, which is corroborated by in situ monitoring of catalytic reaction mixtures using freeze-quench Mössbauer spectroscopy. In Chapter 4, we study the key N-N bond cleavage step in the catalytic cycle for nitrogen fixation. In this chapter, we demonstrate that sequential reduction and low-temperature protonation of an iron catalyst results in the formation of ammonia and a terminal Fe(IV) nitrido complex. This result provides a compelling proposal for the mechanism of the catalytic nitrogen fixation reaction. Finally, in Chapter 5 we present spectroscopic and computational studies detailing the electronic structures of a redox series of Fe(NNR2) complexes that model key catalytic intermediates occurring prior to the N-N bond cleavage step. We evidence one-electron redox non-innocence of the “NNR2” ligand, which resembles that of the classically non-innocent ligand, NO, and may have mechanistic implications for the divergent nitrogen fixation activity of the some of the iron complexes studied by our group.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/H8AP-G249, author = {Ahn, Jun Myun}, title = {Copper Carbazolides in Photoinduced C–N Couplings}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/H8AP-G249}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282018-184138570}, abstract = {Photoinduced, copper-catalyzed reactions of organohalides have emerged in recent years as a powerful tool to construct a wide array of C–N bonds, which are prevalent in organic materials and polymers, pharmaceuticals, natural products, and ligands in transition metal catalysts. Described herein is the study and applications of copper complexes ligated by carbazole and its derivatives in photoinduced, copper-catalyzed C–N bond-constructing transformations. Various areas of synthetic inorganic and organic chemistry are explored, including in-depth mechanistic elucidation, ligand and catalyst design, reaction development, as well as spectroscopic and structural characterization of reactive copper complexes. Chapter 2 describes the mechanistic investigation on photoinduced, copper-catalyzed couplings of carbazoles with unactivated alkyl halides. A wide array of mechanistic tools suggests the viability of an out-of-cage C(sp³)–N coupling pathway. Spectroscopic and structural characterization data of the key intermediates are detailed. Chapter 3 outlines the design and preparation of a new copper-based photoredox catalyst supported by a tridentate bis(phosphino)carbazole ligands. The ground- and excited-state properties of the new photocatalyst are examined. Chapter 4 details the development of photoinduced, copper-catalyzed C(sp³)–N couplings of carbamates with unactivated alkyl bromides using the new copper photoredox system. The scope with respect to the nucleophile and the electrophile and mechanistic investigations are communicated. Chapter 5 illustrates the chemistry of copper complexes supported by bidentate (phosphino)carbazole ligands. A diverse array of copper complexes in both the S = 0 and S = 1/2 states are reported, including a rare, paramagnetic copper–phosphine complex that may serve as a structural model for key copper intermediates of the enantioselective C(sp³)–N couplings of carbazoles.}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z9MS3R0Z, author = {Matson, Benjamin David}, title = {Interplay of Proton Transfer, Electron Transfer and Proton-Coupled Electron Transfer in Transition Metal Mediated Nitrogen Fixation}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9MS3R0Z}, url = {https://resolver.caltech.edu/CaltechTHESIS:02232018-152758526}, abstract = {Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis, including the nitrogen (N2) reduction reactions (N2RR) using H+/e- currency. Here we explore, via DFT calculations, three iron model systems, P3EFe (E = B, Si, C), known to mediate both N2RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is shown that the respective efficiencies of these systems for N2RR trend with the predicted N–H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P3EFe(NNH2)+ and P3EFe(NNH2). Bimolecular proton-coupled electron transfer (PCET) from intermediates with weak N–H bonds is posited as a major source of H2 instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways.
Studies on our most efficient molecular iron catalyst, [P3BFe]+, reveal that the interaction of acid and reductant, Cp2Co, is critical to achieve high efficiency for NH3, leading to the demonstration of electrocatalytic N2RR. Stoichiometric reactivity shows that Cp2Co is required to observe productive N–H bond formation with anilinium triflate acids under catalytic conditions. A study of substituted anilinium triflate acids demonstrates a strong correlation between pKa and the efficiency for NH3, which DFT studies attribute to the kinetics and thermodynamics of Cp*2Co protonation. These results contribute to the growing body of evidence suggesting that metallocenes should be considered as more than single electron transfer reagents in the proton-coupled reduction of small molecule substrates and that ring-functionalized metallocenes, believed to be intermediates on the background HER pathway, can play a critical role in productive bond-forming steps.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z9G15Z28, author = {Nesbit, Mark Allen}, title = {Iron, Cobalt, and Nickel Metalloboranes: Reactivity, Catalysis, N2 Activation and Stabilization of Reactive N2Hx Ligands}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9G15Z28}, url = {https://resolver.caltech.edu/CaltechTHESIS:02232018-173939270}, abstract = {The reactivity of Fe and Co compounds supported by a bisphosphinoborane (DPB) ligand ([(DPB)Fe]2(N2) and (DPB)Co(N2)) towards E-H bonds (E = C, N, S, O, Si) is reported along with the catalytic hydrosilylation of ketones and aldehydes. The Fe and Co compounds displayed a mix of 1-electron and 2-electron chemistry. In some cases [(DPB)Fe]2(N2) and (DPB)Co(N2) facilitated oxidative addition of the E-H bond across the M-B interaction, and in others evolution of H2 giving a 1-electron oxidized complex of the general form (DPB)M(E) was observed. The reaction of Ph2SiH2 with (DPB)Co(N2) was found to be reversible, similar to the previously reported related nickel complex (PhDPBMes)Ni. The reactivity of these Fe and Co compounds is compared to previously reported Ni compounds supported by a similar ligand which catalyze olefin hydrogenation and hydrosilylation of substituted benzaldehydes.
The synthesis and metalation with nickel of two new variants of the DPB ligand (DPBPh and DPBMes) is described. The primary modification introduced in DPBPh and DPBMes is the incorporation of a tertiary amine moiety into the secondary coordination sphere. This was done with the hypothesis that the amine moiety might act as a proton shuttle and facilitate proton reduction or hydrogen oxidation electrocatalysis. The process of screening these compounds for activity as proton reduction and hydrogen oxidation catalysts is also discussed. Additionally, the stoichiometric reactivity of [(DP*BPh)Ni]2(N2) and (DPBMes)Ni(N2) with H2 was studied. We observed that [(DP*BPh)Ni]2(N2) slowly decomposed to an unidentified mixture of products while (DPBMes)Ni(N2) dimerized to form a phosphine bridged Ni-borohydride dimer [(DP*BMesH)Ni]2. [(DP*BPh)Ni]2(N2) and (DPBMes)Ni(N2) were also tested as precatalysts for olefin hydrogenation and found to be less active that their previously reported counterpart (PhDPBMes)Ni. [(DP*BPh)Ni]2(N2) and (DPBMes)Ni(N2) correspondingly showed no activity for hydrogenation of polar substrates such as ketones, aldehydes, or CO2.
Lastly, the synthesis of a new trisphosphinoborane ligand (ArP3B) with bulky aryl substituents on the phosphines and its metalation with Fe is described. The anionic-N2 adduct [(ArP3B)Fe(N2)][Na(12-C-4)2] was observed to react with H+ sources to generate the first observed parent iron-diazenido (ArP3B)Fe(NNH) and an iron-hydrazido(2-) [(ArP3B)Fe(NNH2)]+. [(ArP3B)Fe(NNH2)]+ was found to have similar spectroscopic properties to the previously reported [(TPB)Fe(NNH2)]+. A thorough characterization of [(ArP3B)Fe(N2)][Na(12-C-4)2], (ArP3B)Fe(NNH), and [(ArP3B)Fe(NNH2)]+ by a variety of continuous wave and pulsed ERP techniques is presented along with 57Fe Mössbauer data. The new (ArP3B)Fe system was also canvassed for activity as a catalyst for conversion of N2 to NH3 and found to yield substoichiometric amounts of NH3 in the presence of KC8 and HBArF24•2Et2O while no NH3 was observed using CoCp*2 and [H2NPh2][OTf].
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z9ZP44BF, author = {Ratani, Tanvi Siraj}, title = {Photoinduced, Copper-Catalyzed C-N and C-C Bond Formation and Photocatalytic Co-Mediated Nitrite Reduction to N₂O: Reactivity and Mechanism}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9ZP44BF}, url = {https://resolver.caltech.edu/CaltechTHESIS:10312017-153922571}, abstract = {Photocatalytic reactions with first-row transition metals are presented as a method for sustainable chemistry with great potential for new forms of reactivity and mechanistic pathways. Chapters 2 and 3 of this thesis discuss mechanism and reactivity of photoinduced, copper-catalyzed bond constructions. The Peters and Fu groups have reported that a variety of couplings of nitrogen, sulfur, oxygen, and carbon nucleophiles with organic halides can be achieved under mild conditions (−40 to 30 °C) through the use of light and a copper catalyst. Insight into the various mechanisms by which these reactions proceed may enhance our understanding of chemical reactivity and facilitate the development of new methods. We apply an array of tools (EPR, NMR, transient absorption, and UV−vis spectroscopy; ESI−MS; X-ray crystallography; DFT calculations; reactivity, stereochemical, and product studies) to investigate the photoinduced, copper-catalyzed coupling of carbazole with alkyl bromides. Our observations are consistent with pathways wherein both an excited state of the copper(I) carbazolide complex ([CuI(carb)2]−) and an excited state of the nucleophile (Li(carb)) can serve as photoreductants of the alkyl bromide. The catalytically dominant pathway proceeds from the excited state of Li(carb), generating a carbazyl radical and an alkyl radical. The cross-coupling of these radicals is catalyzed by copper via an out-of-cage mechanism in which [CuI(carb)2]− and [CuII(carb)3]− (carb = carbazolide), both of which have been identified under coupling conditions, are key intermediates, and [CuII(carb)3]− serves as the persistent radical that is responsible for predominant cross-coupling. This study underscores the versatility of copper(II) complexes in engaging with radical intermediates that are generated by disparate pathways, en route to targeted bond constructions.
In Chapter 3, we establish that photoinduced, copper-catalyzed alkylation can also be applied to C−C bond formation, specifically, that the cyanation of unactivated secondary alkyl chlorides can be achieved at room temperature to afford nitriles, an important class of target molecules. In the presence of an inexpensive copper catalyst (CuI; no ligand coadditive) and a readily available light source (UVC compact fluorescent light bulb), a wide array of alkyl halides undergo cyanation in good yield. Our initial mechanistic studies are consistent with the hypothesis that an excited state of [Cu(CN)2]− may play a role, via single electron transfer, in this process. This investigation provides a rare example of a transition metal-catalyzed cyanation of an alkyl halide, as well as the first illustrations of photoinduced, copper-catalyzed alkylation with either a carbon nucleophile or a secondary alkyl chloride.
Chapter 4 presents a mechanistic study of the photocatalytic reduction of nitrite to nitrous oxide with the use of an Ir photocatalyst ([Ir(ppy)2(phen)][PF6]) and a bimetallic CoMg co-catalyst with a diimine-dioxime ligand platform. Insights into the mechanism of this reaction may enhance our current understanding of N–N coupling processes relative to other pathways of reactivity for nitrosyl ligands, such as nitroxyl (HNO) dimerization. We propose a mechanism in which a coordinated and an uncoordinated •NO are coupled at a single Co center. One electron reduction of [(Cl)(NO)Co(Medoen)Mg(Me3TACN)(H2O)][BPh4] ({CoNO}8), a species we show to be catalytically relevant, forms a {CoNO}9 species that is characterized by UV-Vis, EPR, and FT-IR spectroscopy and whose electronic structure is supported by density functional theory (DFT). We formulate the {CoNO}9 as a 5-coordinate, S = 3/2 Co(II) antiferromagnetically coupled with an anionic S = 1 3NO– ligand. Experimental data suggest a mechanism in which this {CoNO}9 intermediate can release •NO, thereby reducing the Co(II) center to Co(I). This free •NO can react with another {CoNO}9 complex to generate a Co(NONO) intermediate which was observed by step-scan time-resolved IR spectroscopy and whose assignment was supported with DFT calculations. This Co(NONO) species, which can generate N2O and H2O, is formulated as a neutral hyponitrite intermediate with significant neutral radical character on both nitrosyl nitrogen atoms and a weak N–N bond.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/CM5B-RG20, author = {Del Castillo, Trevor James}, title = {The Quest for Electrocatalytic Nitrogen Fixation with a Molecular Catalyst and What We Learned Along the Way}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/CM5B-RG20}, url = {https://resolver.caltech.edu/CaltechTHESIS:05152018-150143251}, abstract = {This report details research into the mechanism and operating principles underlying the nitrogen fixation efficacy of a tris(phosphine)borane iron complex (P3BFe). The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst and contribute to a growing body of evidence that metallocenes may play multiple roles during reductive catalysis.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z98P5XPM, author = {Buscagan, Trixia Marie}, title = {Transition Metals as Catalysts for Cross-Coupling and Dinitrogen Fixation}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z98P5XPM}, url = {https://resolver.caltech.edu/CaltechTHESIS:09202017-130540216}, abstract = {Transition metals are used as catalysts in the laboratory and by nature to facilitate difficult chemical transformations. Herein, three different metal containing catalysts are discussed: Pd and Ni catalysts towards the formation of carbon-carbon (C-C) bonds and Fe catalysts towards the reduction of N2 to NH3.
In Chapter 2, mechanistic studies of Pd- and Ni-catalyzed cross-coupling reactions are discussed. The mechanism of transmetalation of a Pd-catalyzed Suzuki cross-coupling reaction is studied using a stereochemical probe, revealing that transmetalation occurs with retention of configuration, consistent with transmetalation occurring through a frontside-attack mechanism. Next, to explore the viability of a transmetalation first pathway in an asymmetric Negishi cross-coupling reaction, S = 1/2 NiIBr and NiI–alkyl complexes were synthesized, crystallographically characterized, and their reactivities explored. Based on these reactivity studies, evidence against a transmetalation first pathway is provided using a variety of spectroscopic methods.
In Chapter 3, new Fe(N2)(H)x complexes are synthesized. These complexes catalyze the reduction of N2 to NH3 and the yields for NH3 are improved if the reactions are performed in the presence of Hg lamp photolysis. Preliminary mechanistic studies exploring the role of light are discussed. In the final chapter, new ligand scaffolds are developed that can bind a Lewis acidic and Lewis basic metal center. These ligand frameworks support one- and two-atom bridges between the two metal sites. Finally, we discovered that some of the new complexes are catalysts for N2 to NH3 reduction and olefin hydrogenation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z92V2D1M, author = {Creutz, Sidney E.}, title = {Design, Synthesis, and Study of Novel Platforms for Iron-N2 Chemistry and Photoinduced, Copper-mediated C-N Bond Formation}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z92V2D1M}, url = {https://resolver.caltech.edu/CaltechTHESIS:02242016-013157998}, abstract = {Several new ligand platforms designed to support iron dinitrogen chemistry have been developed. First, we report Fe complexes of a tris(phosphino)alkyl (CPiPr3) ligand featuring an axial carbon donor intended to conceptually model the interstitial carbide atom of the nitrogenase iron-molybdenum cofactor (FeMoco). It is established that in this scaffold, the iron center binds dinitrogen trans to the Calkyl anchor in three structurally characterized oxidation states. Fe-Calkyl lengthening is observed upon reduction, reflective of significant ionic character in the Fe-Calkyl interaction. The anionic (CPiPr3)FeN2- species can be functionalized by a silyl electrophile to generate (CPiPr3)Fe-N2SiR3. This species also functions as a modest catalyst for the reduction of N2 to NH3. Next, we introduce a new binucleating ligand scaffold that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N2-Fe(μ-SAr)Fe-N2) across at least three oxidation states (FeIIFeII, FeIIFeI, and FeIFeI). Despite the sulfur-rich coordination environment of iron in FeMoco, synthetic examples of transition metal model complexes that bind N2 and also feature sulfur donor ligands remain scarce; these complexes thus represent an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. The (N2-Fe(μ-SAr)Fe-N2) system undergoes reduction of the bound N2 to produce NH3 (~50% yield) and can efficiently catalyze the disproportionation of N2H4 to NH3 and N2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Next, inspired by the importance of secondary-sphere interactions in many metalloenzymes, we present complexes of iron in two new ligand scaffolds ([SiPNMe3] and [SiPiPr2PNMe]) that incorporate hydrogen-bond acceptors (tertiary amines) which engage in interactions with nitrogenous substrates bound to the iron center (NH3 and N2H4). Cation binding is also facilitated in anionic Fe(0)-N2 complexes. While Fe-N2 complexes of a related ligand ([SiPiPr3]) lacking hydrogen-bond acceptors produce a substantial amount of ammonia when treated with acid and reductant, the presence of the pendant amines instead facilitates the formation of metal hydride species.
Additionally, we present the development and mechanistic study of copper-mediated and copper-catalyzed photoinduced C-N bond forming reactions. Irradiation of a copper-amido complex, ((m-tol)3P)2Cu(carbazolide), in the presence of aryl halides furnishes N-phenylcarbazole under mild conditions. The mechanism likely proceeds via single-electron transfer from an excited state of the copper complex to the aryl halide, generating an aryl radical. An array of experimental data are consistent with a radical intermediate, including a cyclization/stereochemical investigation and a reactivity study, providing the first substantial experimental support for the viability of a radical pathway for Ullmann C-N bond formation. The copper complex can also be used as a precatalyst for Ullmann C-N couplings. We also disclose further study of catalytic Calkyl-N couplings using a CuI precatalyst, and discuss the likely role of [Cu(carbazolide)2]- and [Cu(carbazolide)3]- species as intermediates in these reactions.
Finally, we report a series of four-coordinate, pseudotetrahedral P3FeII-X complexes supported by tris(phosphine)borate ([PhBP3FeR]-) and phosphiniminato X-type ligands (-N=PR’3) that in combination tune the spin-crossover behavior of the system. Low-coordinate transition metal complexes such as these that undergo reversible spin-crossover remain rare, and the spin equilibria of these systems have been studied in detail by a suite of spectroscopic techniques.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z9QJ7F7D, author = {Rittle, Jonathan Daniel}, title = {Proton-Coupled Reduction of N₂ Facilitated by Molecular Fe Complexes}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9QJ7F7D}, url = {https://resolver.caltech.edu/CaltechTHESIS:12012015-124453213}, abstract = {The activation of Fe-coordinated N2 via the formal addition of hydrogen atom equivalents is explored in this thesis. These reactions may occur in nitrogenase enzymes during the biological conversion of N2 to NH3. To understand these reactions, the N2 reactivity of a series of molecular Fe(N2) platforms is investigated. A trigonal pyramidal, carbon-ligated FeI complex was prepared that displays a similar geometry to that of the resting state ‘belt’ Fe atoms of nitrogenase. Upon reduction, this species was shown to coordinate N2, concomitant with significant weakening of the C-Fe interaction. This hemilability of the axial ligand may play a critical role in mediating the interconversion of Fe(NxHy) species during N2 conversion to NH3. In fact, a trigonal pyramidal borane-ligated Fe complex was shown to catalyze this transformation, generating up to 8.49 equivalents of NH3. To shed light on the mechanistic details of this reaction, protonation of a borane-ligated Fe(N2) complex was investigated and found to give rise to a mixture of species that contains an iron hydrazido(2-) [Fe(NNH2)] complex. The identification of this species is suggestive of an early N-N bond cleavage event en route to NH3 production, but the highly-reactive nature of this complex frustrated direct attempts to probe this possibility. A structurally-analogous silyl-ligated Fe(N2) complex was found to react productively with hydrogen atom equivalents, giving rise to an isolable Fe(NNH2) species. Spectroscopic and crystallographic studies benefited from the enhanced stability of this complex relative to the borane analogue. One-electron reduction of this species initiates a spontaneous disproportionation reaction with an iron hydrazine [Fe(NH2NH2)] complex as the predominant reaction product. This transformation provides support for an Fe-mediated N2 activation mechanism that proceeds via a late N-N bond cleavage. In hopes of gaining more fundamental insight into these reactions, a series of Fe(CN) complexes were prepared and reacted with hydrogen-atom equivalents. Significant quantities of CH4 and NH3 are generated in these reactions as a result of complete C-N bond activation. A series of Fe(CNHx) were found to be exceptionally stable and may be intermediates in these reactions. The stability of these compounds permitted collection of thermodynamic parameters pertinent to the unique N-H bonds. This data is comparatively discussed with the theoretically-predicted data of the N2-derived Fe(NNHx) species. Exceptionally-weak N-H bond enthalpies are found for many of these compounds, and sheds light on their short-lived nature and tendency to evolve H2. As a whole, these works both establish and provide a means to understand Fe-mediated N2 activation via the addition of hydrogen atom equivalents.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Z9TX3C9P, author = {Fong, Henry}, title = {Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9TX3C9P}, url = {https://resolver.caltech.edu/CaltechTHESIS:12152014-181907516}, abstract = {The E‒H bond activation chemistry of tris-phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex (TPB)Fe(N2) heterolytically activates H‒H and the C‒H bonds of formaldehyde and arylacetylenes across an Fe‒B bond. In particular, H‒H bond cleavage at (TPB)Fe(N2) is reversible and affords the iron-hydride-borohydride complex (TPB)(μ‒H)Fe(L)(H) (L = H2, N2). (TPB)(μ‒H)Fe(L)(H) and (TPB)Fe(N2) are competent olefin and arylacetylene hydrogenation catalysts. Stoichiometric studies indicate that the B‒H unit is capable of acting as a hydride shuttle in the hydrogenation of olefin and arylacetylene substrates. The heterolytic cleavage of H2 by the (TPB)Fe system is distinct from the previously reported (TPB)Co(H2) complex, where H2 coordinates as a non-classical H2 adduct based on X-ray, spectroscopic, and reactivity data. The non-classical H2 ligand in (TPB)Co(H2) is confirmed in this work by single crystal neutron diffraction, which unequivocally shows an intact H‒H bond of 0.83 Å in the solid state. The neutron structure also shows that the H2 ligand is localized at two orientations on cobalt trans to the boron. This localization in the solid state contrasts with the results from ENDOR spectroscopy that show that the H2 ligand freely rotates about the Co‒H2 axis in frozen solution. Finally, the (TPB)Fe system, as well as related tris-phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in (TPB)Fe, were studied for CO2 hydrogenation chemistry. The (TPB)Fe system is not catalytically competent, while the silicon, borate, carbon variants, (SiPR3)Fe, (PhBPiPr3)Fe, and (CPiPr3)Fe, respectively, are catalysts for the hydrogenation of CO2 to formate and methylformate. The hydricity of the CO2 reactive species in the silatrane system (SiPiPr3)Fe(N2)(H) has been experimentally estimated.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/NS23-B474, author = {Anderson, John Stuart}, title = {Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/NS23-B474}, url = {https://resolver.caltech.edu/CaltechTHESIS:09182013-164526961}, abstract = {
Threefold symmetric Fe phosphine complexes have been used to model the structural and functional aspects of biological N2 fixation by nitrogenases. Low-valent bridging Fe-S-Fe complexes in the formal oxidation states Fe(II)Fe(II), Fe(II)/Fe(I), and Fe(I)/Fe(I) have been synthesized which display rich spectroscopic and magnetic behavior. A series of cationic tris-phosphine borane (TPB) ligated Fe complexes have been synthesized and been shown to bind a variety of nitrogenous ligands including N2H4, NH3, and NH2
Treatment of an anionic FeN2 complex with excess acid also results in the formation of some NH3, suggesting the possibility of a catalytic cycle for the conversion of N2 to NH3 mediated by Fe. Indeed, use of excess acid and reductant results in the formation of seven equivalents of NH3 per Fe center, demonstrating Fe mediated catalytic N2 fixation with acids and protons for the first time. Numerous control experiments indicate that this catalysis is likely being mediated by a molecular species.
A number of other phosphine ligated Fe complexes have also been tested for catalysis and suggest that a hemi-labile Fe-B interaction may be critical for catalysis. Additionally, various conditions for the catalysis have been investigated. These studies further support the assignment of a molecular species and delineate some of the conditions required for catalysis.
Finally, combined spectroscopic studies have been performed on a putative intermediate for catalysis. These studies converge on an assignment of this new species as a hydrazido(2-) complex. Such species have been known on group 6 metals for some time, but this represents the first characterization of this ligand on Fe. Further spectroscopic studies suggest that this species is present in catalytic mixtures, which suggests that the first steps of a distal mechanism for N2 fixation are feasible in this system.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/Q4B0-AG41, author = {Suess, Daniel Leif Migdow}, title = {Reactions of Small Molecules Facilitated by Metal-Acceptor Interactions}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/Q4B0-AG41}, url = {https://resolver.caltech.edu/CaltechTHESIS:05302013-182601874}, abstract = {The role of metal-acceptor interactions arising from M–BR3 and M–PR3 bonding is discussed with respect to reactions between first-row transition metals and N2, H2, and CO. Thermally robust, S = 1/2 (TPB)Co(H2) and (TPB)Co(N2) complexes (TPB = tris(2- (diisopropylphosphino)phenyl)borane) are described and the energetics of N2 and H2 binding are measured. The H2 and N2 ligands are bound more weakly in the (TPB)Co complexes than in related (SiP3)M(L) complexes (SiP3 = tris(2- (diisopropylphosphino)phenyl)silyl). Comparisons within and between these two ligand platforms allow for the factors that affect N2 (and H2) binding and activation to be delineated. The characterization and reactivity of (DPB)Fe complexes (DPB = bis(2- (diisopropylphosphino)phenyl)phenylborane) in the context of N2 functionalization and E–H bond addition (E = H, C, N, Si) are described. This platform allows for the one-pot transformation of free N2 to an Fe hydrazido(-) complex via an Fe aminoimide intermediate. The principles learned from the N2 chemistry using (DPB)Fe are applied to CO reduction on the same system. The preparation of (DPB)Fe(CO)2 is described as well as its reductive functionalization to generate an unprecedented Fe dicarbyne. The bonding in this highly covalent complex is discussed in detail. Initial studies of the reactivity of the Fe dicarbyne reveal that a CO-derived olefin is released upon hydrogenation. Alternative approaches to uncovering unusual reactivity using metal- acceptor interactions are described in Chapters 5 and 6, including initial studies on a new π-accepting tridentate diphosphinosulfinyl ligand and strategies for designing ligands that undergo site-selective metallation to generate heterobimetallic complexes.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/ZN62-KX32, author = {Takaoka, Ayumi}, title = {Investigations on Low-Valent Group 8 and 9 Metalloradicals}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/ZN62-KX32}, url = {https://resolver.caltech.edu/CaltechTHESIS:01202012-105759966}, abstract = {Tetradentate, monoanionic, tris(phosphino)silyl ligands were chelated to group 8 and 9 transition metals to stabilize complexes with unusual oxidation states and/or geometries. Initial studies with the [SiPPh3]− ligand on ruthenium established the flexibility of this ancillary ligand in stabilizing complexes with strongly trans influencing ligands in trans dispositions. A related ligand scaffold, [SiPiPr3]−, was subsequently used to stabilize mononuclear complexes of Ru(I) and Os(I), the first examples to be isolated and thoroughly chracterized. EPR spectroscopy and DFT calculations supported their metalloradical character, and further studies highlighted their reactivity in both one- and two-electron redox processes. The ability of the [SiPiPr3]− scaffold to stabilize d7 metalloradicals of group 8 metals was extended to group 9 metals, and a series of d7 complexes of cobalt, rhodium, and iridium were synthesized in which their ancillary ligands, oxidation states, spin states, and geometry are conserved. Similar to the previously reported [SiPiPr3]Fe(N2) complex, the related [SiPiPr3]Ru(N2) complex was shown to exhibit N−N coupling of organic azides to yield azoarenes catalytically. Detailed mechanistic studies conclusively showed that the Ru(III) imide species, whose iron analog is the key intermediate in the [SiPiPr3]Fe system, is not involved in the mechanism for the [SiPiPr3]Ru system. Instead, a mechanism in which free nitrene is released during the catalytic cyle is favored. Finally, hybrid ligands with multiple thioether donors in place of phosphine donors on the [SiPR3]− scaffold were synthesized to stabilize a number of dinitrogen complex of iron. These complexes featured rare examples of S−Fe−N2 linkages.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/46C3-BY97, author = {Saouma, Caroline Thalia Abdunnur}, title = {Iron Mediated Reduction Schemes for Dinitrogen and Carbon Dioxide}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/46C3-BY97}, url = {https://resolver.caltech.edu/CaltechTHESIS:01242011-170306838}, abstract = {Several mono- and diiron species that coordinate NxHy ligands have been prepared and studied, to serve as structural, spectroscopic, and/or reactivity mimics to intermediates to an alternating reduction scheme for N₂ (i.e., Mn-N≡N → Mn-HN=NH → Mn-H₂N-NH₂ → Mn + 2 NH₃). The reaction between [PhBPR₃]FeMe ([PhBPR₃] = (PhB(CH₂PR₂)₃-; R = Ph, CH₂Cy) and hydrazine affords {[PhBPR₃]Fe}₂(μ-η¹: η¹-N₂H₄)(μ²- η²:N₂H₂). In one instance (R = Ph), the stepwise oxidation of coordinated hydrazine to diazene, and diazene to dinitrogen is achieved, giving {[PhBPPh₃]Fe}₂(μ-η¹:η¹-N₂H₂)(μ-η 2: η 2-N2H2) and {[PhBPPh₃]Fe}₂(μ-NH)₂, respectively.
As an extension to this work, a family of complexes which feature the same auxiliary ligands (i.e., [PhBPCH2Cy₃]Fe(OAc)), that are all iron(II), and that only differ in the oxidation state of the nitrogenous ligand has also been prepared: {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂), {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂H₂), {[PhBPCH2Cy₃]Fe(OAc)}₂(μ-N₂H₄), and {[PhBPCH2Cy₃]Fe(OAc)(NH₃).
To determine whether similar species could be isolated at a single iron site, the coordination chemistry of the more crowded “[PhBPmter3]Fe” fragment was investigated and compared to that of the “[PhBPPh3]Fe” scaffold. Treatment of [PhBPmter3]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex, [PhBPmter3]Fe(μ2-N2H3), which features an Fe=N π bond. Both 5- and 6-coordinate iron complexes that coordinate hydrazine were also synthesized, and the oxidation of these hydrazine and hydrazido(-) species was explored. In most instances, oxidation results in disproportionation of the N2Hy ligand, and [PhBPR3]Fe(NH3)(OAc) (R = Ph, mter) is isolated.
A 5-coordinate diiron diazene redox pair of complexes, {[PhBPPh3]Fe(CO)}2(μ-η1:η1-N2H2)0/- was also prepared and studied. The electronic structure of the Fe-NH-NH-Fe core in these complexes is unusual in that it features a highly activated diazene ligand, which is unprecedented for mid-to-late transition metals. Combined structural, spectroscopic, and computation studies indicate that there is much π-covalency within the Fe-NH-NH-Fe core, which has a similar electronic structure as butadiene.
With regards to CO2 reduction, the ability of iron(I) to mediate the one- and two- electron reductions of CO2 was explored. The reaction between [PhBPCH2Cy3]Fe(PCy)3 and CO2 is solvent dependent, with oxalate formation to generate {[PhBPCH2Cy3]Fe}2(μ-η2:η2-oxalato) being favored in THF, and decarbonylation to give {[PhBPCH2Cy3]Fe}2(μ-O)(μ-CO) occurring exclusively in MeCy. Studies aimed at understanding this unusual solvent-induced selectivity are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/6F4J-WE60, author = {Mankad, Neal P.}, title = {Copper and Iron Complexes with Unusual Coordination Geometries Enforced by Phosphine Chelates}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/6F4J-WE60}, url = {https://resolver.caltech.edu/CaltechTHESIS:03112010-172248190}, abstract = {Chelating phosphine ligands were used to enforce targeted coordination geometries onto complexes of iron and copper, thereby imparting molecular properties distinct relative to species studied previously in other geometries. The bulky bis(phosphino)borate ligand [Ph₂B(CH₂PtBu₂)₂]⁻ was used to provide trigonal planar complexes of Cu. This structural motif provided a rare opportunity for a single framework to stabilize Cu complexes in three discrete oxidation levels and allowed for the study of unique ligands including diazoalkanes, a diphenylcarbene, diarylamides, and a diarylaminyl radical. In the latter case, physical measurements (multiedge XAS and multifrequency EPR spectroscopy) and theoretical methods (DFT) were used to quantitate the delocalization of spin density between the Cu center and the NAr₂ unit, providing a comprehensive electronic structure picture for L₂CuER₂ (E = C or N) complexes in this system. In separate studies, trigonal bipyramidal Fe complexes were generated using the bulky, anionic tris(phosphino)silyl ligands [(2-R₂PC₆H₄)₃Si]⁻ (R = Ph or iPr). Low-valent Fe species in this system were found to activate dinitrogen, providing labile N₂ ligands trans to the silyl donor, including the first instance of a terminally bound N₂ ligated to a paramagnetic Fe center. Subsequent reactions involving these FeI-N₂ species and organoazides provided entry to unusual catalytic N-N coupling reactions. These reactions were found to involve reactive FeNAr intermediates, destabilized by virtue of the trigonal bipyramidal coordination geometry, which subsequently coupled bimolecularly in the N-N bond-forming step. The effects of perturbing previously studied C₃-symmetric pseudotetrahedral iron complexes to their trigonal bipyramidal analogues proved key to uncovering the chemistry of interest.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/4N3R-5194, author = {Whited, Matthew Thomas}, title = {Synthetic and Mechanistic Studies of Small-Molecule Activation at Low-Valent Iron, Cobalt, and Iridium Centers}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/4N3R-5194}, url = {https://resolver.caltech.edu/CaltechETD:etd-05062009-173812}, abstract = {The preparation of transition-metal systems for catalytic multielectron transformations of small molecules remains a significant challenge for synthetic chemists. The realization of new transformations often depends critically on the design of frameworks capable of stabilizing unusual oxidation states and molecular geometries, providing a frontier molecular-orbital landscape that is well suited to interact with the molecules of interest. This thesis has sought to address two particularly noteworthy challenges in the field of small-molecule activation, dinitrogen reduction and C–H bond functionalization, through judicious ligand choice and design.
Chapters 2 and 3 describe the syntheses of new tri- and tetradentate hybrid ligands incorporating a single X-type donor (amido or silyl) and multiple phosphine donors designed to stabilize low oxidation states at iron and cobalt and support dinitrogen reduction and other multielectron redox transformations. While the amidophosphine ligands do allow access to unusual monovalent iron and cobalt complexes, the isolation of dinitrogen adducts supported by these ligands remains elusive and the weakness of the silicon–nitrogen bond makes the complexes prone to decomposition. In contrast, the tris(phosphino)silyl ligands presented in Chapter 3 afford straightforward access to the first terminally bonded dinitrogen complexes of monovalent iron, and the structure of these and related complexes are described along with preliminary experiments showing that protonolysis of the iron(I)–dinitrogen complexes produces hydrazine in reasonable stoichiometric yields.
Chapters 4 through 7 address the functionalization of ether and amine C–H bonds by a double C–H activation route. Chapter 4 describes the investigation of reactivity of low-valent, pincer-supported iridium species with a variety of ethers, leading to a number of selective C–H, C–C, and C–O bond cleavage events, affording in several cases iridium carbene complexes by double C–H activation and loss of dihydrogen.
Chapter 5 presents an exploration of the electronic structure of the unusual square-planar iridium(I) alkoxycarbenes and their nucleophilic activation of several heterocumulene substrates, leading to multiple-bond metathesis events promoted by metal- rather than ligand-initiated reactivity. Chapter 6 describes the discovery of new atom and group transfer reactions from diazo reagents to the alkoxycarbenes and the implementation of these reactions in an unprecedented catalytic cycle for C=E bond formation by multiple C–H activations.
Chapter 7 explores the related reactivity of a low-valent pincer iridium complex with methyl amines and the reactivity of the resulting iridium(III) dihydrido aminocarbenes, which is shown to diverge substantially from that observed for the iridium(I) carbene species.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Grubbs, Robert H. and Peters, Jonas C.}, } @phdthesis{10.7907/AGQC-8E07, author = {Thomas, Christine Marie}, title = {Novel Reactivity at Iron Centers Supported by Poly(phosphino)borate Ligands}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/AGQC-8E07}, url = {https://resolver.caltech.edu/CaltechETD:etd-05162006-201134}, abstract = {The reactivity of the iron(II) alkyl species [PhBPiPr3]FeMe ([PhBPiPr3] = PhB(CH2PiPr2)3-) towards Si-H bonds is presented. Reaction of [PhBPiPr3]FeMe with primary aryl silanes results in the unusual η3 silane adducts [PhBPiPr3]Fe(H)(η3-H2SiMeR). X-ray crystallography, Mossbauer spectroscopy, and theoretical calculations confirm this structural assignment; however, solution NMR experiments suggest a degree of fluxionality in solution.
Low valent, tris(phosphino)borate iron platforms have been shown to facilitate the activation of white phosphorus, P4. The iron(I) precursors {[PhBPiPr3]Fe}2(μ-N2) and [PhBPPh3]Fe(PPh3) react with P4 to quantitatively generate {[PhBPiPr3]Fe}2(μ-P4) and {[PhBPPh3]Fe}2(μ-P4), respectively. These unique iron(II) dimers bridged by square P42- units have been characterized structurally and spectroscopically, and their reactivity has been examined. A simplified electronic structure calculation is presented to aid in discussion of bonding within these complexes.
Motivated by the versatility of the tris(phosphino)borate ligands, a new family of tripodal hybrid bis(phosphino)pyrazolylborate ligands, [PhBPtBu2(pz’)]- ([PhBPtBu2(pz’)]- = PhB(CH2PtBu2)2(pz’)-), has been prepared and characterized. The synthesis, spectroscopy, and solid-state structures of four-coordinate, pseudo-tetrahedral iron(II) and cobalt(II) halide complexes supported by these ligands is presented. To compare the electron-releasing ability of these ligands with their [PhBPR3] analogues, the cyclic voltammetry of these complexes is introduced. Potential routes to a terminal cobalt or iron nitride complex via extrusion of N2 from coordinated azide and metathesis with the N-atom transfer reagent Li(dbabh) are investigated.
Reduction of the [PhBPtBu2(pz’)]MX halide complexes in the presence of excess phosphine generates low valent [PhBPtBu2(pz’)]MI(PMe3) precursors. These precursors react with organic azides to generate cobalt(III) and iron(III) imides. Initial reactivity studies indicate that these imides are more moderately more reactive than the corresponding tris(phosphino)borate complexes. The electrochemistry of the [PhBPtBu2(pz’)]FeIII(NR) imides features a quasi-reversible to fully reversible oxidation event, dependent on choice of pyrazolyl substituents and scan rate. This oxidation can be achieved chemically to generate the isolable cationic iron(IV) imides, [PhBPtBu2(pz’)]FeIV(NR)+. The structural and spectroscopic characterization of these highly unusual complexes is discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/0DND-1J03, author = {Lu, Connie Chih}, title = {The Chemistry of Tris(phosphino)borate Manganese and Iron Platforms}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/0DND-1J03}, url = {https://resolver.caltech.edu/CaltechETD:etd-06012006-151944}, abstract = {The coordination chemistry of monovalent and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand [PhBPiPr3] is presented. The halide complexes, [PhBPiPr3]MnCl and [PhBPiPr3]MnI, have been characterized by XRD, SQUID magnetometry, and EPR spectroscopy. The halide [PhBPiPr3]MnI serves as a precursor to manganese azide, alkyl, and amide species: [PhBPiPr3]Mn(N3), [PhBPiPr3]Mn(CH2Ph), [PhBPiPr3]Mn(Me), [PhBPiPr3]Mn(NH(2,6-iPr2Ph)), [PhBPiPr3]Mn(dbabh), and [PhBPiPr3]Mn(1-Ph(isoindolate)). Collectively, they represent an uncommon motif of low-coordinate polyphosphine-supported manganese species. Some of our synthetic efforts to generate [PhBPiPr3]Mn?Nx species are described, as are theoretical DFT studies that probe the electronic viability of these multiply bonded target structures.
Two tris(phosphino)borate ligands, [PhBPter3] and [PhBPCH2Cy3] are introduced that feature terphenyl and methylcyclohexyl groups on the phosphine arms, respectively. The iron chlorides, [PhBPter3]FeCl and [PhBPCH2Cy3]FeCl, have been prepared as precursors to iron nitrides. Addition of the nitride transfer reagent Li(dbabh) to [PhBPCH2Cy3]FeCl produced the terminal nitride, [PhBPCH2Cy3]Fe(N). The 15N NMR spectrum of the labeled species, [PhBPCH2Cy3]Fe(15N), contains a peak at 929 ppm, consistent with a terminal nitride functionality. Mossbauer spectroscopy of the nitride shows a low isomer shift value of 0.34(1) mm/s and an exceptionally large quadrupole splitting of 6.01(1) mm/s.
Reduction of [PhBPCH2Cy3]FeCl generates a masked iron(I) species that is highly reactive. Combustion analysis of this species is consistent with “[PhBPCH2Cy3]Fe.” Other physical methods including VT NMR, EPR, and IR spectroscopies suggest the presence of a paramagnetic species in equilibrium with a diamagnetic species. The paramagnetic component is postulated to be an Fe(III) hydride, wherein a ligand C-H bond has been cyclometalated at the metal center. The reactivity of “[PhBPCH2Cy3]Fe” is consistent with iron(I). For example, its reaction with PMe3 and 1-adamantylazide affords the phosphine adduct, [PhBPCH2Cy3]Fe(PMe3), and the iron imide, [PhBPCH2Cy3]Fe(NAd), respectively. Interestingly, “[PhBPCH2Cy3]Fe” undergoes redox reactions with benzene to give initially a benzene adduct, {[PhBPCH2Cy3]Fe}2(mu-eta3:eta3-C6H6), which decomposes to {[PhBPCH2Cy3]Fe}2(mu-eta5:eta5-6,6’-bicyclohexadienyl) via radical C-C bond coupling. Finally, “[PhBPCH2Cy3]Fe” readily reduces CO2 at rt to give as the major product {[PhBPCH2Cy3]Fe}2(mu-CO)(mu-O), wherein a C=O bond has been cleaved. The minor product has not been definitively established, but one possibility is the oxalate-bridged dimer {[PhBPCH2Cy3]Fe}2(mu-eta2:eta2-O2CCO2) that results from reductive coupling of two CO2 molecules.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/1X2Q-3P72, author = {Harkins, Seth Beebe}, title = {The Synthesis and Study of Redox-Rich, Amido-Bridged Cu₂N₂ Dicopper Complexes}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/1X2Q-3P72}, url = {https://resolver.caltech.edu/CaltechETD:etd-08132005-093856}, abstract = {A Cu2N2 diamond core structure supported by an [SNS]- ligand exhibits a fully reversible one-electron redox process between a reduced CuICuI, {(SNS)Cu}2, and a class III delocalized Cu1.5Cu1.5 state, {(SNS)Cu}2][B(C6H3(CF3)2)4] ([SNS]- bis(2-t-butylsulfanylphenyl)amide). The Cu···Cu distance compresses appreciably (~0.13 Å) upon oxidation; a metal-metal distance of 2.4724(4) Å is observed in the mixed-valence molecule that is nearly identical to the dicopper CuA site found in cytochrome c oxidase. The rate of electron self-exchange ks) between the CuICuI and the Cu1.5Cu1.5 complexes was estimated to be ≥ 107 M-1s-1 by 1H NMR line-broadening analysis. The unusually large magnitude of ks reflects the minimal structural reorganization that accompanies CuICuI ↔︎ Cu1.5Cu1.5 interchange.
A second generation of {(PNP)CuI}2 dimer supported by a [PNP]- ligand also has been investigated ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide). The highly emissive {(PNP)CuI}2 is characterized by a long-lived excited state (τ > 10 μs) with an unusually high quantum yield (φ > 0.65) at ambient temperature. Removal of an electron from the {(PNP)CuI}2 dimer yields a nearly isostructural, Cu1.5Cu1.5 complex {(PNP)Cu}2][B(C6H3(CF3)2)4]. With a highly reducing excited state reduction potential (~ -3.2 V vs. Fc+/Fc) as well as the availability of two reversible redox processes, these bimetallic copper systems may be interesting candidates for photochemically driven two-electron redox transformations.
Studies of Cu2N2 diamond core complexes supported by the [tBu2-PNP]- ligand revealed that the dicopper complex {(tBu2-PNP)Cu}2 can not only be oxidized by one electron to {(tBu2-PNP)Cu1.5}2][B(C6H3(CF3)2)4], but also by two-electrons to {(tBu2-PNP)Cu}2][SbF6]2 ([tBu2-PNP]- = bis(2-diisobutylphoshino-4-tbutylphenyl)amide). These Cu2N2 complexes show remarkably low structural reorganization for all oxidation states as evidenced by the solid-state molecular-structures. Based on these studies of [{(tBu2-PNP)Cu}2][SbF6]2, we propose a formulation of one CuI and one paramagnetic CuIII nuclei in compressed-tetrahedral environments in the Cu2N2 core. Spectroscopic, redox, and magnetic data are consistent with a highly covalent M2N2 core supported by a rigid ligand scaffold. These complexes are excellent mimics of the entatic state found in bimetallic copper proteins.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/PY23-ME66, author = {Betley, Theodore Alexander}, title = {Coordination Chemistry from Trigonally Coordinated Iron Platforms: Chemistry Relevant to Dinitrogen Reduction}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/PY23-ME66}, url = {https://resolver.caltech.edu/CaltechETD:etd-05272005-103515}, abstract = {The synthesis for a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBPiPr3] ([PhBPiPr3] = [PhB(CH2PiPr2)3]-), is reported to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBPiPr3]Tl, was synthesized, characterized, and used to install the [PhBPiPr3] ligand onto complexes of Fe, Co, and Ru. The spectroscopic, electrochemical, magnetic, and structural features of these complexes are compared with similar examples.
Trigonally coordinated “[PhBPiPr3]M” platforms (M = Fe, Co) support both pi-acidic (N2) and pi-basic (NR2-) ligands at a fourth binding site. Methylation of monomeric [M0(N2)-] species successfully derivatizes the beta-N atom of the N2 ligand and affords the diazenido product [MII(N2Me)]. Addition of RN3 to MI(N2)MI results in oxidative nitrene transfer to generate [PhBPiPr3]M≡NR with concomitant N2 release.
A tetrahedrally coordinated L3Fe-Nx platform that accommodates both terminal nitride (L3FeIV≡N) and dinitrogen (L3FeI-N2-FeIL3) functionalities is described. The diamagnetic L3FeIV≡N species featured has been characterized in solution under ambient conditions by multinuclear NMR (1H, 31P, and 15N) and infrared spectroscopy. The terminal nitride complex oxidatively couples to generate the previously reported L3FeI-N2-FeIL3 species.
The [PhBPiPr3] ligand can support a single iron or cobalt center in a pseudo-tetrahedral environment in which dinitrogen is bound in the fourth coordination site. Zero-valent metal-dinitrogen complexes have the general formula, [[PhBPiPr3]M(mu-N2)]2[Mg2+], while bridging structures can also be obtained as neutral [MI]—N2—[MI] or as anionic [(M)2(N2)]- species. The nature of the structural distortions observed in both [M(mu-N2)]2[Mg2+] and [Mn]—N2—[Mn] complexes are described. Magnetic characterization of the neutral and mixed-valence dimeric complexes reveal the complexes remain ferromagnetically coupled over all temperatures investigated.
The coordination chemistry of group VIII metals featuring the bis(8-quinolinyl)amine (HBQA) ligand is presented. The electrochemical behavior of Fe, Ru, and Os complexes bearing the BQA ligand is reported and compared to related ligand platforms. Halide and phosphine ligand exchange reactions are examined from complexes of the type (BQA)MX(PR3)2 (M = Ru, Os). Carbonyl and dinitrogen complexes of Ru and Os are prepared from halide abstraction from divalent Ru and Os precursors. The spectroscopic and structural features of these complexes are compared with similar examples.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/5WNW-R611, author = {Jenkins, David Matthew}, title = {Low Spin Pseudotetrahedral Cobalt Tris(phosphino)borate Complexes}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/5WNW-R611}, url = {https://resolver.caltech.edu/CaltechETD:etd-02102005-144940}, abstract = {A synthetic protocol is developed for the preparation of a thallium complex featuring the tris(phosphino)borate ligand [PhBP3] ([PhBP3] = [PhB(CH2PPh2)3]-). The transmetallating reagent, [PhBP3]Tl, is characterized by single crystal X-ray diffraction and solution NMR spectroscopy, and is the first example of a stable homoleptic Tl(I)-phosphine complex.
The synthesis and characterization of [PhBP3]Co-X (X = I, Br or Cl) is discussed. These halide complexes are structurally characterized and magnetic investigations establish that they are low spin when monomeric. The low spin iodide complex is a monomer in solution and in the solid state. The other halides exhibit a monomer/dimer equilibrium that complicates their magnetic behavior. Theoretical calculations help provide a rationale as to why these pseudotetrahedral species are low spin. A classic high spin species supported by [PhBP3] is compared to the low spin complexes.
Spin state control involving pseudotetrahedral [PhBP3]Co(II) complexes is explored. Both high and low spin, as well as spin crossover, complexes are synthesized and structurally characterized. The complexes are discussed in terms of the relationship between local geometry and spin state. Changing the axial or tripodal ligand can cause a different spin state to be favored. Since the energy difference between the states is small, ligand changes at remote positions from the metal center have a significant effect on spin crossover phenomena. Theoretical calculations help illuminate why the low spin state is preferred for many of the complexes.
The first examples of cobalt imide complexes ([PhBP3]Co≡NR (R = aryl or alkyl)) are prepared and they are supported by the [PhBP3] ligand. These diamagnetic species are evaluated by NMR and single crystal X-ray diffraction. Theoretical studies suggest that they have a similar molecular orbital bonding scheme as the previously prepared group 9 imides.
A cobalt μ2-bridging nitride complex (([PhBP3]Co)2(μ-N)) is synthesized and structurally characterized. This mixed-valence species is evaluated by magnetometry to determine its ground state, which is low spin (S = ½).
Several cobalt diazoalkane complexes are prepared. These diamagnetic species adopt two different bonding modes depending on the nature of the diazoalkane ligand.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/46DN-YH09, author = {Brown, Steven Douglas}, title = {The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/46DN-YH09}, url = {https://resolver.caltech.edu/CaltechETD:etd-05312005-201150}, abstract = {The metallation of FeX2 (X = Cl, Br, I) salts with the strong-field [PhBP3] ([PhBP3] = PhB(CH2PPh2)3-) ligand is presented. The resulting four-coordinate, 14-electron species, [PhBP3]FeX, have been thoroughly characterized and feature high-spin (S = 2) electronic ground-states. X-ray diffraction analysis of [PhBP3]FeCl establishes a monomeric structure in the solid state.
The one electron reduction of [PhBP3]FeCl in the presence of a triphenylphosphine cap affords a rare example of four-coordinate iron(I). This species, [PhBP3]Fe(PPh3), serves as a synthetic surrogate to a low-valent “[PhBP3]Fe(I)” subunit that is readily oxidized in the presence of organic azides. The resulting S = 1/2 iron(III) imides of general formula [PhBP3]Fe≡NR may be subsequently reduced by one electron to yield the anionic S = 0 derivatives. Exposure of the former to an atmosphere of CO results in cleavage of the Fe≡NR linkage to yield [PhBP3]Fe(CO)2 and free isocyanate (O=C=N-R). Dicarbonyl [PhBP3]Fe(CO)2 is itself an imide precursor and is gradually converted back to [PhBP3]Fe≡NR upon exposure to excess organic azide.
Tolyl imide [PhBP3]Fe?N-p-tolyl readily reacts with H2 under mild conditions to undergo a step-wise Fe-Nx bond scission process to ultimately release free p-toluidine. Initially formed is the S = 2 iron(II) anilide, [PhBP3]Fe(N(H)-p-tolyl), which has been independently prepared and shown to release p-toluidine in the presence of H2. In benzene solvent the final iron containing product of the hydrogenation process is diamagnetic [PhBP3]Fe(?5-cyclohexadienyl), which is presumably formed from benzene insertion into a low-valent iron-hydride intermediate.
Reduction of the ferromagnetically coupled dimer, {[PhBP3]Fe(N3)}2, yields the bridging nitride species, [{[PhBP3]Fe}2(μ-N)][Na(THF)5]. This compound features two high-spin iron(II) metal centers that are so strongly antiferromagnetically coupled that a diamagnetic S = 0 ground-state is exclusively populated at room temperature. X-ray diffraction analysis reveals a bent Fe-N-Fe linkage that quantitatively releases ammonia in the presence of excess protons. Reactivity with CO and H2 is also presented, and for the latter, complete rupture of the Fe-N-Fe manifold is not observed as the presence of an additional metal center (when compared with the iron(III) imides) favors the formation of the diamagnetic bridging imide-hydride species, [{[PhBP3]Fe}2(μ-NH)(μ-H)][Na(THF)5].
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, } @phdthesis{10.7907/GCAQ-8D59, author = {Thomas, John Christopher}, title = {Ligand design, coordination chemistry, and mechanistic studies of (phosphino)borates and their platinum, nickel, and copper complexes}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/GCAQ-8D59}, url = {https://resolver.caltech.edu/CaltechETD:etd-06012004-115510}, abstract = {Synthetic methods are presented for the preparation of various substituted bis(phosphino)borates. A relatively general protocol based on the delivery of a nucleophilic phosphine-containing carbanion to a borane electrophile has been developed. Preparative methods for the synthesis of substituted diarylchloroboranes from dimethyldiaryltin reagents provide the borane electrophiles. Methyldialkyl- or methyldiarylphosphines are selectively deprotonated at the phosphine-methyl using alkyl lithium bases to form the carbanion nucleophiles. The reaction of diverse phosphine-containing carbanions with diarylchloroboranes results in bis(phosphino)borates selectively substituted at the borate, at the phosphine, or at both positions. In addition to the generated lithium salts of the bis(phosphino)borates, cation-exchange protocols provide methods for preparing ammonium and thallium bis(phosphino)borate salts. Structural data for some of these derivatives are presented.
The electronic properties of transition metals coordinated by bis(phosphino)borates are explored through NMR and IR spectroscopies. The spectroscopic features of platinum(II) dimethyl and methyl carbonyl complexes are examined for trends based on the substitution pattern of the (phosphino)borate ligand. These trends indicate that phosphine substituents have a more significant impact than borate substituents on electronics of the metal center. Structural and spectroscopic comparisons of structurally similar platinum(II) dimethyl and methyl carbonyl complexes indicate that the anionic bis(phosphino)borate ligand renders platinum(II) more electron-rich than structurally similar neutral phosphine donors. Related spectroscopic studies of anionic and neutral molybdenum(0) tetracarbonyl complexes provide results analogous to those found when comparing neutral and cationic platinum(II) systems.
Comparative studies on the ligand exchange and benzene C-H activation chemistry of structurally similar platinum(II) complexes convey the similarities and differences between zwitterionic and cationic systems. Examination of THF ligand self-exchange by magnetization transfer shows a change in mechanism between the neutral and cationic species. Both bis(phosphino)borate-ligated and neutral bis(phosphine) platinum methyl solvento complexes undergo a benzene C-H activation to form the corresponding phenyl solvento complex; however, the rates of reaction and ultimate products differ. Extensive isotopic studies indicate that the zwitterionic system forms observable intermediates prior to benzene C-H activation, some of which are attributable to ligand metalation processes.
Structural and spectroscopic studies of a phenyl-substituted tris(phosphino)borate on platinum are presented. Alkyl- and hydride-containing platinum(II) and platinum(IV) species have been synthesized. The structural and spectroscopic features of these complexes are compared to related tris(pyrazolyl)borate systems on platinum.
Coordination and reaction chemistry of an isopropyl-substituted tris(phosphino)borate on nickel are discussed. Complexes in the Ni(II), Ni(I), and Ni(0) oxidation states have been prepared. This system is compared through structural, spectroscopic, and electrochemical methods to related phenyl-substituted tris(phosphino)borate chemistry on nickel. Reactivity studies aimed at preparing Ni(III) and Ni(IV) complexes containing metal-ligand multiple bonds through group-transfer reactions are presented. Theoretical studies using density functional methods are used to probe several target species containing multiply-bonded ligands.
The coordination chemistry of copper(I) is explored using bis(phosphino)borates. Both aryl- and alkyl-substituted bis(phosphino)borates provide access to copper(I) complexes. A tert-butyl-substituted bis(phosphino)borate is particularly useful for preparing a family of three-coordinate compounds. The spectroscopic and structural features of these complexes are compared with similar, previously described examples.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Peters, Jonas C.}, }