Motivated by the lack of an atomic-level understanding of the reduction of small molecule substrates by nitrogenase, this dissertation describes the synthesis, characterization and reactivity of well-defined model clusters of the enzyme active site.
Chapter 2 describes a series of site-differentiated, high spin iron clusters which reversibly bind carbon monoxide in redox states FeII₄ through FeIIFeIII₃. Detailed spectroscopic and thermochemical studies reveal that this remarkable reactivity can be attributed to the ability of remote metal centers to shuttle reducing equivalents to the small molecule binding site.
Chapter 3 further explores the consequences of internal electron transfer events on the thermodynamics of small molecule binding by site-differentiated, tetranuclear iron clusters. To systematically tune the electronic properties of the cluster, a Hammett series was prepared. Counterintuitively, introduction of electron-donating substituents suppresses the first CO binding event but enhances the second. Detailed spectroscopic studies revealed that the origin of this behavior can be traced to the effect of the substituents on the redox reorganization energy associated with internal electron transfer.
Chapter 4 presents the synthesis and characterization of the first open-shell diiron µ-carbyne complex, which also features a biologically relevant Fe(µ-C)(µ-H)Fe core. This electronically unusual species could be activated toward binding of N₂ upon addition of H⁺/e, which initially involves an iron-carbene intermediate.
Chapter 5 describes the synthesis and spectroscopic investigation of the first carbonbridged, bimetallic complexes featuring odd numbers of valence electrons as spectroscopic models of the critical E₄(4H) intermediate of nitrogenase. Detailed pulse EPR studies revealed the effects of electronic localization on the spectroscopic signatures of the µ-hydride motif and provide insight into the electronic distribution in a reduced state of FeMoco.
Chapter 6 describes the synthesis and characterization of terminal iron-carbene complexes, including EPR characterization of open-shell variants.
Appendix A describes unpublished efforts to prepared site-differentiated models of FeMoco featuring carbon- or sulfur-based donors.
Appendix B presents unpublished work towards modelling the cooperative activation and reduction of N₂ by diiron complexes featuring carbon-based bridging ligands.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/GVYR-MV95, author = {Hirscher, Nathanael Allen}, title = {Investigation of Ethylene Tetramerization Catalysis from Structurally-Defined Organochromium Compounds}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/GVYR-MV95}, url = {https://resolver.caltech.edu/CaltechTHESIS:08012019-192629717}, abstract = {Chapter 1 is a general introduction to the topic of ethylene tetramerization catalysis.
Chapter 2 presents the synthesis and catalytic utility of chromium multi-aryl complexes that were the first examples of ethylene tetramerization catalysts that could be produced without excess alkyl aluminum reagents.
Chapter 3 describes the mechanistic analysis of the ethylene tetramerization reaction using isotopically labelled ethylene. Co-production of 1-hexene along with 1-octene was determined to be intrinsic to the reaction mechanism. This is due to the intermediacy of a chromacyclic species that can either eliminate 1-hexene or insert a fourth ethylene.
Chapter 4 presents the synthesis of additional Cr tris(aryl) complexes, which are coordinatively saturated, and were used to generate a crystallographically-characterized Cr(III) cationic species. This was the first reported single-component precatalyst for ethylene tetramerization.
Chapter 5 describes the isotopic labelling of a well-defined ethylene tetramerization precatalyst with a deuteriomethyl group. This label was tracked following protonation of the neutral Cr complex via pulse EPR. Successful detection of deuterium on Cr-alkyl ligands led to in situ analysis of the catalytic mixture. A low-spin species derived from deuterated ethylene was observed.
Appendix 1 describes the synthesis of various Cr aryl amine complexes. Appendix 2 provides the results of additional catalytic experiments for ethylene tetramerization, including those with a more soluble precatalyst, and those at higher ethylene pressure. Appendix 3 details the synthesis of a molecular Re catalyst for CO2 electroreduction which was used to modify electrodes. Appendix 4 lists various X-ray crystal structures that were obtained, but not related elsewhere in the thesis.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/QWMZ-HA45, author = {Reed, Christopher John}, title = {Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/QWMZ-HA45}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072019-140931627}, abstract = {This dissertation discusses the synthesis, characterization, and reactivity of site-differentiated tetranuclear clusters containing Fe and Mn with NO and H2O-derived ligands. The motivation of this work was to conduct a detailed examination of structure-property relationships in well-defined molecular systems focused on unique features of multinuclear systems, such as bridging ligands, neighboring metal identity, and cluster oxidation state. Reactivity towards NO and H2O-derived ligands was targeted due to their relevance to biological multinuclear transition metal active sites that promote multi-electron small molecule transformations.
Chapter 2 discusses the synthesis of Fe-nitrosyl clusters bearing an interstitial μ4-F atom. These clusters were prepared to compare their reactivity to previously synthesized [Fe33OFeNO] clusters with an analogous structure. A redox series of the [Fe3FFe] and [Fe3FFeNO] clusters were accessed, with the nitrosyl clusters displaying five cluster oxidation states, from FeII3{FeNO}8 to FeIII3{FeNO}7. Overall, the weaker bonding of the F- ligand resulted in attenuation of the activation and reactivity of the {FeNO}7, relative to the corresponding μ4-O clusters. Furthermore, the ability of distal Fe oxidation state changes to influence the activation of NO was decreased, demonstrating lower cooperativity between metals in clusters linked by a weaker μ4-atom This represents a rare case where the effects of bridging atom ligands could be compared in isostructural multinuclear complexes and decoupled from changes in metal ion coordination number, oxidation states, or geometry.
Chapter 3 describes the synthesis of site-differentiated heterometallic clusters of [Fe3OMn], displaying facile ligand substitution at the five-coordinate Mn. This system was able to coordinate H2O and thermodynamic parameters of the proton and electron transfer processes from the MnII–OH2 to form a MnIII–OH moiety were studied. The oxidation state distribution of the neighboring Fe centers had a significant influence on these thermodynamic parameters, which was similar to the analogous parameters for mononuclear Mn systems, demonstrating that oxidation state changes in neighboring metals of a cluster can perturb the reactivity of a Mn–OHx unit nearly as much as an oxidation state change at the Mn–OHx. Subsequent experiments attempted to find spectroscopic or electrochemical evidence for formation of a terminal Mn-oxo in this system; however, that was not obtained, even in relatively extreme conditions. This established a lower limit for the bond dissociation enthalpy of the MnIII–OH of ca. 93 kcal/mol, which makes formation of a terminal Mn-oxo cluster unfavorable in most organic solvents, due to expected facile hydrogen atom abstraction of a solvent C–H bond.
The insights obtained on the reactivity of these tetranuclear metal-hydroxide clusters was applied towards stabilizing a terminal metal-oxo in a multinuclear complex, as outlined in Chapter 4. Through the use of pendant hydrogen bond donors with tert-butyl-aminopyrazolate ligands, tetranuclear Fe clusters bearing terminal-hydroxide and -oxo ligands could be stabilized and structurally characterized. A similar thermodynamic analysis of the FeIII–OH bond dissociation enthalpy was conducted, which demonstrated FeIII-oxo clusters could be accessed with a range of reactivity at the terminal-oxo ligand, based on the redox distribution of the neighboring Fe centers. The kinetics of C–H activation for the [FeII2FeIII2]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pKa on the rate of proton coupled electron transfer.
Lastly, Chapter 5 describes the synthesis and reactivity of tetranuclear Fe clusters bearing unsubstituted pyrazolate ligands, focusing on attempts to observe evidence for a terminal Fe-oxo or Fe-imido motif. Clusters bearing a labile trifluoromethanesulfonate ligand at the five-coordinate Fe center could be prepared, and would react with oxygen atom transfer reagents to produce a terminal Fe-hydroxide cluster, which, upon dehydration, led to isolation of an octanuclear μ2-O cluster. The pathway for Fe-hydroxide formation was investigated, but could not conclusively determine whether reactivity occurred from a transient terminal Fe-oxo. Similarly, the reduced tetra-iron cluster, in the [FeII3FeIII], redox state was prepared, and demonstrated reactivity towards electron deficient aryl azides. Isolation of aryl amide clusters (Fe-NHAr) was observed, suggesting, again, formation of a reactive Fe-imido which decomposes through formal hydrogen atom abstraction. Efforts to stabilize either of these Fe=O/NR multiply-bonded species through a more acidic Fe were investigated by synthesizing the corresponding pyrazolate bridged μ4-F clusters. The [FeII4] cluster also displayed reactivity towards oxygen atom transfer reagents, and produced a similar octanuclear μ2-O cluster, but the observation of μ4-F substitution with oxygen to produce μ4-O clusters with a terminal F ligand likely precluded formation of a reactive terminal-oxo cluster. Instead, thermodynamically favorable cluster rearrangement to the [Fe3OFe] structure dominates.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/S6NR-3N42, author = {Lee, Heui Beom}, title = {Electronic Structure and Spectroscopy of Tetranuclear Mn4O4 and CaMn3O4 Complexes as Models of the Oxygen Evolving Complex in Photosystem II}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/S6NR-3N42}, url = {https://resolver.caltech.edu/CaltechTHESIS:03112019-204258320}, abstract = {This thesis describes a series of studies devoted toward the synthesis of model complexes that mimic aspects of structure, redox state, and spectroscopy of the oxygen evolving complex (OEC) of Photosystem II. The OEC is a unique metallocofactor featuring a heteronuclear CaMn4 core that catalyzes water oxidation. While advances in spectroscopic and structural techniques offer an ever more detailed view of the structure of the S-state catalytic intermediates, the precise mechanism of O−O bond formation remains debated. Aspects such as (1) role of Ca2+, (2) the location of the substrate waters, and (3) the (electronic) structure of the S-state intermediates remain unclear. To obtain a better understanding of the OEC, systematic structure−function(property) studies on relevant model complexes may be necessary. Despite significant efforts to prepare tetra- and pentanuclear complexes as models of the OEC, relevant complexes in terms of structure, redox state, spectroscopy, and reactivity are rare, likely due to the synthetic challenges of accessing a series of isolable clusters that are suitable for comparisons.
Chapter 1 presents a survey of tetramanganese model compounds with an emphasis on redox state and electronic structure, as probed by magnetometry and EPR spectroscopy. Structurally characterized model complexes are grouped according to Mn oxidation states and the S-state that they are mirroring. In contrast to the vast number of spectroscopic studies on the OEC, studies that probe the effect of systematic changes in structure on the spectroscopy of model complexes are rare in the literature.
Chapter 2 presents ongoing synthetic efforts to prepare accurate structural models of the OEC. The synthesis of accurate structural models is hampered by the low structural symmetry of the cluster, the presence of two types of metals, and the propensity of oxo moieties to form extended oligomeric structures. Desymmetrization of the previously reported trinucleating ligand leads to the formation of tetranuclear Mn4II precursors. Oxidation in the presence of Ca2+ leads to a CaMn4O2 model of the OEC, underscoring the utility of low-symmetry multinucleating ligands in the synthesis of hitherto unobserved oxo-bridged multimetallic core geometries related to the OEC.
Chapter 3 presents a series of [MnIIIMn3IVO4] cuboidal complexes as spectroscopic models of the S2 state of the OEC. Such complexes resemble the oxidation state and EPR spectra of the S2 state, and the effect of systematic changes in the nature of the bridging ligands on spectroscopy was studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes needs to be carefully assessed.
Chapter 4 presents a series of [YMn3O4] complexes as models of the [CaMn3O4] subsite of the OEC. The effect of systematic changes in the basicity and chelating properties of the bridging ligands on redox potential was studied. Results show that in the absence of ligand-induced geometric distortions that enforce a contraction of metal-oxo distances, increasing the basicity of the ligands results in a decrease of cluster reduction potential. A small contraction of metal-oxo/metal-metal distances by ~0.1 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strong basic donors. Such small, protein-induced changes in Ca-oxo/Ca-Mn distances may have a similar effect in tuning the redox potential of the OEC through entatic states, and may explain the cation size dependence on the progression of the S-state cycle.
Chapter 5 presents a series of [CaMn3O4] and [YMn3O4] complexes as models of the [CaMn3O4] subsite of the OEC. The effect of systematic changes in cluster geometry, heterometal identity, and bridging oxo protonation on cluster spin state structure was studied. Results show that the electronic structure of the Mn3IV core is highly sensitive to small geometric changes, the nature of the bridging ligands, and the protonation state of the bridging oxos: the spin ground states of essentially isostructural compounds can be S = 3/2, 5/2, or 9/2. Interpretation of EPR signals and subsequent structural assignments based on an S = 9/2 spin state of the CaMn3O4 subsite of the OEC must be done very cautiously.
While unfinished, appendices 1 and 2 present other important aspect in OEC model chemistry. Appendix 1 presents the synthesis of 17O-labeled [MnIIIMn3IVO4] and [CaMn3IVO4] complexes as models of the OEC. Ongoing characterization of μ3-oxos in such complexes provide valuable benchmarking parameters for future mechanistic studies. Appendix 2 presents the synthesis and characterization of [Mn4IVO4] cuboidal complexes as spectroscopic models of the S3 state of the OEC, the last observable intermediate prior to O−O bond formation at the OEC.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/BQJM-BN49, author = {Low, Choon Heng (Marcus)}, title = {Hemilabile, Non-Innocent (Poly)arylene Donors for Accessing Novel Reactivity at Transition Metal Centers}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/BQJM-BN49}, url = {https://resolver.caltech.edu/CaltechTHESIS:05222019-121353860}, abstract = {Understanding the effects that ligands have on the coordination environment and reactivity of metal complexes is an endeavor that drives much of the field of inorganic chemistry. The use of ligands capable of flexible binding modes and redox states further enriches the chemistry of these complexes. This dissertation describes studies on metal complexes bearing pendant (poly)arylene donors that demonstrate hemilability and redox non-innocence. Within this context, conditions that result in coordination mode change and the multi-electron bond transformation that is made possible by the hemilability and/or non-innocence of the ligand are discussed.
Chapter 2 investigates the meta-terphenyl diphosphine framework bearing a central phenolate donor as an anionic POP pincer on a variety of first-row transition metals. The circumstances under which coordination mode change from the phenolate donor to the arene face are investigated. Reduction of the cobalt and nickel complexes induced a coordination mode change from phenolate oxygen to metal-arene binding, while Lewis acid additives induced a coordination mode change in some iron POP complexes. Additionally, it was found that iron chloride POP complex initially not amendable to two-electron reduction was cleanly reduced in the presence of Lewis acids, suggesting a role the Lewis acid plays in quenching the negatively charged phenolate and stabilizing the overall transformation.
Chapter 3 discusses reactivity on 1,4-naphthalenediyl diphosphine molybdenum complexes in the context of carbon monoxide (CO) coupling. Similar to the previously studied phenylene system, the reductive coupling of CO can be carried out. However, the naphthalene system showed a distinct and exclusive selectivity for the two-electron reductive CO coupling to a bis(siloxy)acetylene motif, without C–O bond cleavage. This difference in selectivity is proposed to be a result of accessible η4-arene binding modes previously not observed in the phenylene variant. Additionally, the bis(siloxy)acetylene complex also displays η4-binding to the central arene. Further CO catenation can be effected from this species, providing a metallacyclobutenone complex that bears a C3 fragment derived completely from CO.
In Chapter 4, the reactivity of 9,10-anthracenediyl bis(phenoxide) zirconium complexes is presented. The more expanded polyaromatic system with a milder reduction potential allowed the anthracene motif to function as a non-innocent ligand. This enabled facile reductive elimination of ancillary benzyl ligands on the metal center without the use of harsh reductants. This reduced complex was then able to oxidatively couple alkynes, and alkynes with nitriles. Furthermore, further insertion of an additional nitrile followed by reductive elimination, likely facilitated by the non-innocent anthracene motif, allowed for the catalytic synthesis of pyridines and pyrimidines with high yields and selectivities. This reactivity was further leveraged in the final Chapter of this dissertation. Chapter 5 presents the development of a new methodology towards the synthesis of pyridine or pyrimidine-containing polycyclic aromatic hydrocarbons (PAHs) using polyaromatic alkyne and nitrile building blocks. Because conventional methods of oxidative cyclodehydrogenation towards N-doped nanographenes proved ineffective with these PAHs, a new reductive cyclization route was developed offering a complementary method towards the challenging synthesis of these N-doped nanographenes.
Appendix A briefly explores additional reactivity on the 1,4-naphthalenediyl diphosphine complexes with regard to nitrile activation. Appendix B explores the synthesis of iron complexes supported by a benzene tris(thiophenolate) ligand towards potential model compounds for the iron molybdenum cofactor in nitrogenase. Appendix C presents preliminary studies on the 9,10-anthracenediyl bis(phenoxide) zirconium complex towards oxidative coupling of alkynes with CO2.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/1BT2-BR52, author = {Sampson, Jessica}, title = {Nuclearity and π-π Interaction Effects on Olefin Polymerization and Coordination Chemistry}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/1BT2-BR52}, url = {https://resolver.caltech.edu/CaltechTHESIS:04072019-122030433}, abstract = {This thesis details work performed on the use of secondary coordination sphere effects to impact olefin polymerization activity and tacticity control and the coordination chemistry of Y, Fe, and Cu. Chapter One provides a general introduction and summary of each chapter. Chapter Two describes work in collaboration with KFUPM on nuclearity effects in Zr bisamine bisphenolate complexes. Chapter Three describes the coordination chemistry of arene-appended Y di(pyridyl) pyrrolide complexes and the olefin polymerization activity of tris(dimethylamido) Ti, Zr, and Hf di(pyridyl) pyrrolide complexes. Appendix A describes the effects of bulky trialkylsilyl, triphenylsilyl, and diphenyl(alkyl)silyl substituents on the tacticity control of monozirconium amine bis(phenolate) complexes in 1-hexene polymerization. Appendix B describes the synthesis and structures of miscellaneous dizirconium amine bis(phenolate) complexes which could not be isolated in sufficient purity for olefin polymerization tests. Appendix C describes the synthesis, electrochemistry, and reduction of mesityl-substituted di(pyridyl) NHC supported Fe complexes. Appendix D describes the preparation, solid-state structures, and electrochemistry of di(pyridyl) pyrrolide and di(pyridyl) NHC Cu(I) and Cu(II) complexes displaying π-π interactions in the solid state. Appendix E describes work towards the synthesis of di(pyridyl) guanidinate proligands and metal complexes supported by di(pyridyl) urea, monopyridyl and di(pyridyl) N-heterocyclic olefin and N-heterocyclic vinylidene ligands for use in Lewis acid assisted olefin polymerization. Appendix F includes relevant spectra.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/SJTG-3388, author = {Buss, Joshua Alan}, title = {Molybdenum Para-Terphenyl Diphosphine Complexes}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/SJTG-3388}, url = {https://resolver.caltech.edu/CaltechTHESIS:04162018-150106151}, abstract = {This dissertation describes studies exploring the coordination chemistry and reactivity of molybdenum complexes bearing a flexible and redox non-innocent para-terphenyl diphosphine ligand. Within this context, transformations relevant to energy storage and conversion, fundamental structure function studies, and unusual group transfer reactivity are presented.
Chapter 2 accounts the ability of Mo para-terphenyl diphosphine complexes to catalyze extensive ammonia borane dehydrogenation, releasing greater than two equiv. of hydrogen (H2). Initially believed to be a frontrunner as a high energy density H2 storage medium, AB is a Lewis acid/base adduct that features both hydridic B–H bond and protic N–H bonds. As a highly reactive molecule, the controlled dehydrogenation of AB, accessing ≥ 2 of the 3 stored equiv. of H2, is uncommon. We disclose a catalytic system, utilizing an earth-abundant metal, that is capable of such reactivity. The mechanism by which the catalysis proceeds is dependent on the oxidation state of the precatalyst, with MoII proceeding through a II/IV cycle and Mo0 proceeding through a 0/II cycle. Several Mo hydride complexes were characterized in conjunction with this work. Importantly, the ability of the para-terphenyl diphosphine ancillary ligand to support a range of Mo oxidation states and coordination numbers was established, a feature that provides a foundation for the work presented in subsequent chapters.
In Chapter 3, new features of the para-terphenyl diphosphine ligand were discovered, namely facilitation of electron loading that subsequently leads to small molecule functionalization and cleavage. From the Mo dicarbonyl complex described in Chapter 2, stepwise reduction affords Mo0, Mo-II, and Mo-III compounds, all of which were characterized both structurally and by a variety of spectroscopies. The latter two complexes were demonstrated to react with silyl electrophiles, instigating deoxygenative reductive coupling of the bound CO ligands to a metal-free C2O1 fragment. This remarkable four-electron process was studied in detail, characterizing twelve different reaction intermediates, including rare examples of bis(siloxy)carbyne, terminal carbide, and mixed dicarbyne motifs. The cleavage of a bound carbon monoxide (CO), subsequent coupling, and spontaneous product release was an unprecendented sequence of chemical transformations, the detailed mechanistic study of which provides valuable precedent for catalyses for the conversion of C1 oxygenates to multicarbon products.
Chapter 4 discusses continuations of this work in an attempt to model Fischer-Tropsch catalysis with higher fidelity. To this end, the silyl electrophiles used in the fundamental studies in Chapter 3 needed to be replaced with protons. Addition of protons to the super-reduced Mo complexes resulted in formal arene hydrogenation; no evidence for C–O functionalization was obtained. These diene-linked complexes; however, provided an opportunity to explore how the nature of the basal π-system effects CO catenation chemistry and ultimately led to the preparation of a Mo-bound C3O3 unit derived entirely from CO. Reactivity with protons was likewise explored for downstream intermediates. Carbide protonation yields a stable methylidyne carbonyl complex, that, upon treatment with hydride, forms a methylidene. Comparison to a silyl-bearing model system suggests that subsequent carbene carbonylation affords enthenone.
Chapter 5 and 6 focus on the synthesis and reactivity of Mo(IV) terminal pnictogen complexes isoelectronic to the carbyne and carbide complexes prepared in Chapters 3 and 4. Chapter 5 describes successful N–C bond formation through N– transfer to CO from a MoII anionic nitride precursor. In Chapter 6, the first example of a terminal transition metal phosphide with d-electrons was prepared via a 4 e– oxidative group transfer. This species can undergo a single-electron oxidation, providing, at low temperatures, an unstable Mo(V) phosphide cation that studied extensively by CW and pulse EPR techniques. Upon warming, P–P bond formation is evidenced by chemical trapping and characterization of coupling byproducts. Related phosphinidene (Mo=PR), phosphide (Mo-PR2), and dinuclear μ-phosphido compounds are also reported. In a collaboration with Mr. Yohei Ueda and Dr. Masa Hirahara these complexes were explored for proton reduction reactivity. Isotopic labeling suggests formation of a dinuclear μ-phosphinidene upon treatment with acid, and a bimetallic hydride μ-phosphide was accessed from reaction with hydride.
The final chapters of this dissertation are focused on the reduction of carbon dioxide (CO2). Chapter 7 presents a fundamental study involving Lewis acid (LA) aditives, that demonstrates the importance of kinetic stabilization, and not just thermodynamic activation, in productive small molecule functionalization chemistry. Upon addition of LAs, well-defined adducts are formed with Mo-bound CO2. Protonation results in C–O bond cleavage, utilizing two electrons from the metal center to reduce CO2 to CO and H2O. Though the degree of CO2 activation trends well as a function of Lewis acidity, the residence time of the bound CO2, reported via the rate of CO2 self-exchange, is shown to correlate to the degree of C–O scission. Chapter 8 looks at CO2 reactivity with E–H bonds, describing first stoichiometric reactivity with silanes. In this system, CO2 is reduced to CO and silanol; mechanistic studies suggest a pathway that involves oxygen atom transfer to silane from a transient Mo oxo. In a collaboration with Dr. Naoki Shida, CO2 hydrogenation was explored, with demonstration of bidirectional catalysis in addition to detailed studies investigating the elementary steps of both formate formation and formic acid dehydrogenation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @mastersthesis{10.7907/Z9HH6H3V, author = {Carsch, Kurtis Mickel}, title = {Bio-Inspired Homometallic and Heterometallic Clusters Relevant to the Oxygen-Evolving Complex of Photosystem II}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9HH6H3V}, url = {https://resolver.caltech.edu/CaltechTHESIS:06022016-225404694}, abstract = {The following two chapters delineate several endeavors to isolate and characterize functional models of the oxygen-evolving complex (OEC) of photosystem II. Understanding the electronic structure and the precise mechanism of the O–O bond coupling step in the Kok cycle affords insight into this fundamental process and will guide the design of new earth-abundant catalysts to perform water oxidation under environmentally benign conditions. Nature performs this transformation by a heterometallic CaMn4O5 cluster arranged a tetra-metallic cubane bridged to a dangling manganese ion. Although a myriad of synthetic inorganic complexes are capable of water oxidation, these structures significantly underperform the OEC in terms of turnover number and turnover frequency. The objectives of this thesis are (i) to construct multimetallic clusters using the OEC as inspiration, (ii) to explore the reactivity of these clusters with oxygen-atom transfer reagents, and (iii) to identify intermediates responsible for oxygen-based chemistry.
In Chapter 1, a series of pseudo-C3 symmetric tetra-manganese clusters with an interstitial µ4-oxygen was synthesized and characterized in several oxidation states. These clusters (of the general formula [LMn3(PhPz)3OMn][OTx]x; x = 1, 2) are supported by pyridine and alkoxide donors connected by a 1,3,5-triarylbenzene spacer. A µ4-oxygen coordinates all four metal centers that are also bridged by phenyl pyrazolate (PhPz) ligands. This arrangement furnishes a vacant coordination site at a site-differentiated (apical) metal center. Exposure of these clusters to oxygen-atom transfer reagents (OAT’s) results in the intramolecular oxygenation of a C(sp2)–H bond of the bridging phenyl pyrazolate. Similarly, using 2,6-difluorophenyl pyrazolate (F2ArPz) as the bridging ligand results in the oxygenation of the C–F bond with concurrent F-atom transfer. This reactivity represents an unprecedented C–F activation for molecular manganese complexes. All hydroxylated and fluorinated clusters were independently prepared to confirm the observed reactivity upon exposure to OAT’s. The pathways responsible for arene activation – postulated to proceed through an iodosobenzene adduct and subsequent formation of a transient high-valent manganese-oxo motif – are discussed.
In Chapter 2, a series of pseudo-C3 symmetric heterometallic Fe3Mn clusters of the general formula [LMn3(PhPz)3OMn][OTf]x (x = 1–3) was synthesized and characterized. Similar to their homometallic tetra-manganese and tetra-iron analogs (Chapter 1), these clusters contain four metal centers with a central bridging interstitial µ4-oxygen atom and bridging phenyl pyrazolate ligands. These clusters are further supported by pyridine and alkoxide donors, linked through a 1,3,5-triarylbenzene spacer. All complexes were characterized by zero-field 57Fe Mössbauer spectroscopy to confirm the presence of a manganese metal center in the apical position, illustrating that these clusters are stable with respect to metal scrambling and/or decomposition. Treatment of these clusters with 1-(tert-butylsulfonyl)-2-iodosylbenzene (sPhIO) resulted in the oxygenation of the C(sp2)–H bond of the proximal phenyl pyrazolate motif to afford [LMn3(PhPz)2(OArPz)OMn][OTf]x (x = 2, 3). During these studies, an unusual iodosobenzene adduct of [FeIII3MnII]3+ was isolated prior to C–H activation. This adduct has been characterized both by single-crystal XRD and 1H-NMR spectroscopy. In order to gain insight into the C–H bond oxygenation by this iodosobenzene adduct, preliminary computational studies are presented to discuss the viability of a transient manganese-oxo species responsible for arene hydroxylation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z98G8HMT, author = {Lionetti, Davide}, title = {Heterometallic Complexes as Models of Enzymatic Active Sites}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z98G8HMT}, url = {https://resolver.caltech.edu/CaltechTHESIS:09282015-080838463}, abstract = {
This dissertation describes studies on two multinucleating ligand architectures: the first scaffold was designed to support tricopper complexes, while the second platform was developed to support tri- and tetrametallic clusters.
In Chapter 2, the synthesis of yttrium (and lanthanide) complexes supported by a tripodal ligand framework designed to bind three copper centers in close proximity is described. Tricopper complexes were shown to react with dioxygen in a 1:1 [Cu3]/O2 stoichiometry to form intermediates in which the O–O bond was fully cleaved, as characterized via UV-Vis spectroscopy and determination of the reaction stoichiometry. Pre-arrangement of the three Cu centers was pivotal to cooperative O2 activation, as mono-copper complexes reacted differently with dioxgyen. The reactivity of the observed intermediates was studied with various substrates (reductants, O-atom acceptors, H-atom donors, Brønsted acids) to determine their properties. In Chapter 3, the reactivity of the same yttrium-tricopper complex with nitric oxide was explored. Reductive coupling to form a trans-hyponitrite complex (characterized by X-ray crystallography) was observed via cooperative reactivity by an yttrium and a copper center on two distinct tetrametallic units. The hyponitrite complex was observed to release nitrous oxide upon treatment with a Brønsted acid, supporting its viability as an intermediate in nitric oxide reduction to nitrous oxide.
In Chapter 4, a different multinucleating ligand scaffold was employed to synthesize heterometallic triiron clusters containing one oxide and one hydroxide bridges. The effects of the redox-inactive, Lewis acidic heterometals on redox potential was studied by cyclic voltammetry, unveiling a linear correlation between redox potential and heterometal Lewis acidity. Further studies on these complexes showed that the Lewis acidity of the redox-inactive metals also affected the oxygen-atom transfer reactivity of these clusters. Comparisons of this reactivity with manganese systems, collaborative efforts to reassign the structures of related manganese oxo-hydroxo clusters, and synthetic attempts to access related dioxo clusters are also described.
In Appendix A, ongoing efforts to synthesize new clusters supported by the same multinucleating ligand platform are described. Studies of novel approaches towards ligand exchange in tetrametallic clusters and incorporation of new supporting and bridging ligand motifs in trinuclear complexes are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z9Z60KZJ, author = {Edouard, Guy Anthony}, title = {Late Transition Metals Supported by Aryl Ethers and Phenoxides Bearing Pendant Phosphines: Mechanistic Insights Relevant to Ether C-O Bond Cleavage}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9Z60KZJ}, url = {https://resolver.caltech.edu/CaltechTHESIS:01032016-141952494}, abstract = {Terphenyl diphosphines bearing pendant ethers were prepared to provide mechanistic insight into the mechanism of activation of aryl C–O bonds with Group 9 and Group 10 transition metals. Chapters 2 and 3 of this dissertation describe the reactivity of compounds supported by the model phosphine and extension of this chemistry to heterogenous C–O bond activation.
Chapter 2 describes the synthesis and reactivity of aryl-methyl and aryl-aryl model systems. The metallation of these compounds with Ni, Pd, Pt, Co, Rh, and Ir is described. Intramolecular bond activation pathways are described. In the case of the aryl-methyl ether, aryl C–O bond activation was observed only for Ni, Rh, and Ir.
Chapter 3 outlines the reactivity of heterogenous Rh and Ir catalysts for aryl ether C–O bond cleavage. Using Rh/C and an organometallic Ir precursor, aryl ethers were treated with H2 and heat to afford products of hydrogenolysis and hydrogenation. Conditions were modified to optimize the yield of hydrogenolysis product. Hydrogenation could not be fully suppressed in these systems.
Appendix A describes initial investigations of bisphenoxyiminoquinoline dichromium compounds for selective C2H4 oligomerization to afford α-olefins. The synthesis of monometallic and bimetallic Cr complexes is described. These compounds are compared to literature examples and found to be less active and non-selective for production of α-olefins.
Appendix B describes the coordination chemistry of terphenyl diphosphines, terphenyl bisphosphinophenols, and biphenyl phosphinophenols proligands with molybdenum, cobalt, and nickel. Since their synthesis, terphenyl diphosphine molybdenum compounds have been reported to be good catalysts for the dehydrogenation of ammonia borane. Biphenyl phosphinophenols are demonstrated provide both phosphine and arene donors to transition metals while maintaining a sterically accessible coordination sphere. Such ligands may be promising in the context of the activation of other small molecules.
Appendix C contains relevant NMR spectra for the compounds presented in the preceding sections.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z9V40S53, author = {Henthorn, Justin Travis}, title = {Molybdenum Quinonoid Complexes: Synthesis, Characterization, and Reactivity}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9V40S53}, url = {https://resolver.caltech.edu/CaltechThesis:06082016-214138510}, abstract = {
Pi-bound Molybdenum-quinonoid complexes supported by pendant phosphines were prepared and investigated for metal-ligand cooperative reactivity and access to multiple equivalents of protons and electrons within a single transition metal complex. Chapters 3, 4, and 5 of this dissertation describe the synthesis, characterization, and reactivity of these complexes in the context of multiproton, multielectron chemistry and small molecule activation.
Chapter 2 presents the synthesis of an unprecedented bis-borane supported peroxide dianion, prepared from a mixture of ferrocenes, borane, and dioxygen. The peculiarity of such a structure is emphasized, and reactivity explored. While ferrocenes of varying reduction potential were found to lead to the peroxide, only tris(pentafluorophenyl)borane was found to yield isolable peroxide, with other boranes leading to oxygenation or borate formation.
Chapter 3 describes the synthesis of a series of π-bound Molybdenum-quinonoid complexes and explores their reactivity with dioxygen. The Mo-quinonoid interaction is probed and elucidated through a number of reactions and experiments, highlighting the importance of the electronic coupling of the metal center with the organic fragment on overall reactivity with O2.
Chapter 4 further explores the π-bound Molybdenum-quinonoid complexes in various protonation and oxidation states, totaling four electrons and two protons accessible to the system. Proton-coupled electron transfer was demonstrated in two different oxidation states, and the effects of the metal-quinonoid interaction on the transfer of protons and electrons investigated thermochemically.
Chapter 5 explores the potential for π-bound Molybdenum-quinonoid complexes to access inner-sphere reactivity. The activation of E–X bonds, including H2 and PhSiH3, is demonstrated, as well as catalytic hydrosilylation of aldehydes.
Appendix A describes initial investigations into the preparation of heterobimetallic complexes supported by the catechol-diphosphine ligand framework. The synthesis of heterobimetallic MoCu complexes is presented and their structural parameters discussed.
Appendix B outlines the synthesis of multinucleating ligand platforms based off bipyridine frameworks, for the preparation of biologically inspired multimetallic complexes. Dioxygen reactivity of a dicopper system is also briefly presented.
Appendix C contains relevant NMR spectra for the compounds presented in the preceding sections.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z9TB14VB, author = {Horak, Kyle Tadashi}, title = {The Design and Synthesis of Transition Metal Complexes Supported by Non-innocent Ligand Scaffolds for Small Molecule Activation}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z9TB14VB}, url = {https://resolver.caltech.edu/CaltechTHESIS:02162016-101008749}, abstract = {This dissertation focuses on the incorporation of non-innocent or multifunctional moieties into different ligand scaffolds to support one or multiple metal centers in close proximity. Chapter 2 focuses on the initial efforts to synthesize hetero- or homometallic tri- or dinuclear metal carbonyl complexes supported by para-terphenyl diphosphine ligands. A series of [M2M’(CO)4]-type clusters (M = Ni, Pd; M’ = Fe, Co) could be accessed and used to relate the metal composition to the properties of the complexes. During these studies it was also found that non-innocent behavior was observed in dinuclear Fe complexes that result from changes in oxidation state of the cluster. These studies led to efforts to rationally incorporate central arene moieties capable managing both protons and electrons during small molecule activation.
Chapter 3 discusses the synthesis of metal complexes supported by a novel para-terphenyl diphosphine ligand containing a non-innocent 1,4-hydroquinone moiety as the central arene. A Pd0-hydroquinone complex was found to mediate the activation of a variety of small molecules to form the corresponding Pd0-quinone complexes in a formal two proton ⁄ two electron transformation. Mechanistic investigations of dioxygen activation revealed a metal-first activation process followed by subsequent proton and electron transfer from the ligand. These studies revealed the capacity of the central arene substituent to serve as a reservoir for a formal equivalent of dihydrogen, although the stability of the M-quinone compounds prevented access to the PdII-quinone oxidation state, thus hindering of small molecule transformations requiring more than two electrons per equivalent of metal complex.
Chapter 4 discusses the synthesis of metal complexes supported by a ligand containing a 3,5-substituted pyridine moiety as the linker separating the phenylene phosphine donors. Nickel and palladium complexes supported by this ligand were found to tolerate a wide variety of pyridine nitrogen-coordinated electrophiles which were found to alter central pyridine electronics, and therefore metal-pyridine π-system interactions, substantially. Furthermore, nickel complexes supported by this ligand were found to activate H-B and H-Si bonds and formally hydroborate and hydrosilylate the central pyridine ring. These systems highlight the potential use of pyridine π-system-coordinated metal complexes to reversibly store reducing equivalents within the ligand framework in a manner akin to the previously discussed 1,4-hydroquinone diphosphine ligand scaffold.
Chapter 5 departs from the phosphine-based chemistry and instead focuses on the incorporation of hydrogen bonding networks into the secondary coordination sphere of [Fe4(μ4-O)]-type clusters supported by various pyrazolate ligands. The aim of this project is to stabilize reactive oxygenic species, such as oxos, to study their spectroscopy and reactivity in the context of complicated multimetallic clusters. Herein is reported this synthesis and electrochemical and Mössbauer characterization of a series of chloride clusters have been synthesized using parent pyrazolate and a 3-aminophenyl substituted pyrazolate ligand. Efforts to rationally access hydroxo and oxo clusters from these chloride precursors represents ongoing work that will continue in the group.
Appendix A discusses attempts to access [Fe3Ni]-type clusters as models of the enzymatic active site of [NiFe] carbon monoxide dehydrogenase. Efforts to construct tetranuclear clusters with an interstitial sulfide proved unsuccessful, although a (μ3-S) ligand could be installed through non-oxidative routes into triiron clusters. While [Fe3Ni(μ4-O)]-type clusters could be assembled, accessing an open heterobimetallic edge site proved challenging, thus prohibiting efforts to study chemical transformations, such as hydroxide attack onto carbon monoxide or carbon dioxide coordination, relevant to the native enzyme. Appendix B discusses the attempts to synthesize models of the full H-cluster of [FeFe]-hydrogenase using a bioinorganic approach. A synthetic peptide containing three cysteine donors was successfully synthesized and found to chelate a preformed synthetic [Fe4S4] cluster. However, efforts to incorporate the diiron subsite model complex proved challenging as the planned thioester exchange reaction was found to non-selectively acetylate the peptide backbone, thus preventing the construction of the full six-iron cluster.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z9668B43, author = {Suseno, Sandy}, title = {Homo- and Heteronuclear Transition Metal Complexes Supported by Multinucleating Ligands}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9668B43}, url = {https://resolver.caltech.edu/CaltechTHESIS:11202014-184045649}, abstract = {This dissertation is mainly divided into two sub-parts: organometallic and bioinorganic/materials projects. The approach for the projects involves the use of two different multinucleating ligands to synthesize mono- and multinuclear complexes. Chapter 2 describes the synthesis of a multinucleating tris(phosphinoaryl)benzene ligand used to support mono-nickel and palladium complexes. The isolated mononuclear complexes were observed to undergo intramolecular arene C¬–H to C–P functionalization. The transformation was studied by nuclear magnetic resonance spectroscopy and X-ray crystallography, and represents a rare type of C–H functionalization mechanism, facilitated by the interactions of the group 10 metal with the arene π–system.
Chapter 3 describes the construction of multinickel complexes supported by the same triphosphine ligand from Chapter 2. This chapter shows how the central arene in the ligand’s triarylbenzene framework can interact with dinickel and trinickel moieties in various binding modes. X-ray diffraction studies indicated that all compounds display strong metal–arene interactions. A cofacial triangulo nickel(0) complex supported by this ligand scaffold was also isolated and characterized. This chapter demonstrates the use of an arene as versatile ligand design element for small molecular clusters.
Chapter 4 presents the syntheses of a series of discrete mixed transition metal Mn oxido clusters and their characterization. The synthesis of these oxide clusters displaying two types of transition metals were targeted for systematic metal composition-property studies relevant to mixed transition metal oxides employed in electrocatalysis. A series of heterometallic trimanganese tetraoxido cubanes capped with a redox-active metal [MMn3O4] (M = Fe, Co, Ni, Cu) was synthesized starting from a [CaMn3O4] precursor and structurally characterized by X-ray crystallography and anomalous diffraction to conclusively determine that M is incorporated at a single position in the cluster. The electrochemical properties of these complexes were studied via cyclic voltammetry. The redox chemistry of the series of complexes was investigated by the addition of a reductant and oxidant. X-ray absorption and electron paramagnetic resonance spectroscopies were also employed to evaluate the product of the oxidation/reduction reaction to determine the site of electron transfer given the presence of two types of redox-active metals. Additional studies on oxygen atom transfer reactivities of [MMn3O4] and [MMn3O2] series were performed to investigate the effect of the heterometal M in the reaction rates.
Chapter 5 focuses on the use of [CoMn3O4] and [NiMn3O4] cubane complexes discussed in Chapter 4 as precursors to heterogeneous oxygen evolution reaction (OER) electrocatalysts. These well-defined complexes were dropcasted on electrodes with/without heat treatment, and the OER activities of the resulting films were evaluated. Multiple spectroscopic techniques were performed on the surface of the electrocatalysts to gain insight into the structure-function relationships based on the heterometallic composition. Depending on film preparation, the Co-Mn-oxide was found to change metal composition during catalysis, while the Ni-Mn oxide maintained the NiMn3 ratio. These studies represent the use of discrete heterometallic-oxide clusters as precursors for heterogeneous water oxidation catalysts.
Appendix A describes the ongoing effort to synthesize a series of heteromultimetallic [MMn3X] clusters (X = O, S, F). Complexes such as [ZnMn3O], [CoMn3O], [Mn3S], and [Mn4F] have been synthesized and structurally characterized. An amino-bis-oxime ligand (PRABO) has been installed on the [ZnMn3O] cluster. Upon the addition of O2, the desymmetrized [ZnMn3O] cluster only underwent an outer-sphere, one-electron oxidation. Efforts to build and manipulate other heterometallic [MMn3X] clusters are still ongoing, targeting O2 binding and reduction. Appendix B summarizes the multiple synthetic approaches to build a [Co4O4]-cubane complex relevant to heterogeneous OER electrocatalysis. Starting with the tricobalt cluster [LCo3(O2CR)3] and treatment various strong oxidants that can serve as oxygen atom source in the presence Co2+ salt only yielded tricobalt mono–oxo complexes. Appendix C presents the efforts to model the H-cluster framework of [FeFe]-hydrogenase by incorporating a synthetic diiron complex onto a protein-supported or a synthetic ligand-supported [Fe4S4]-cluster. The mutant ferredoxin with a [Fe4S4]-cluster and triscarbene ligand have been characterized by multiple spectroscopic techniques. The reconstruction of an H-cluster mimic has not yet been achieved, due to the difficulty of obtaining crystallographic evidence and the ambiguity of the EPR results.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z9C53HS0, author = {Kanady, Jacob Steven}, title = {Models of the Oxygen-Evolving Complex of Photosystem II}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9C53HS0}, url = {https://resolver.caltech.edu/CaltechTHESIS:08272014-031912941}, abstract = {
In the five chapters that follow, I delineate my efforts over the last five years to synthesize structurally and chemically relevant models of the Oxygen Evolving Complex (OEC) of Photosystem II. The OEC is nature’s only water oxidation catalyst, in that it forms the dioxygen in our atmosphere necessary for oxygenic life. Therefore understanding its structure and function is of deep fundamental interest and could provide design elements for artificial photosynthesis and manmade water oxidation catalysts. Synthetic endeavors towards OEC mimics have been an active area of research since the mid 1970s and have mutually evolved alongside biochemical and spectroscopic studies, affording ever-refined proposals for the structure of the OEC and the mechanism of water oxidation. This research has culminated in the most recent proposal: a low symmetry Mn4CaO5 cluster with a distorted Mn3CaO4 cubane bridged to a fourth, dangling Mn. To give context for how my graduate work fits into this rich history of OEC research, Chapter 1 provides a historical timeline of proposals for OEC structure, emphasizing the role that synthetic Mn and MnCa clusters have played, and ending with our Mn3CaO4 heterometallic cubane complexes.
In Chapter 2, the triarylbenzene ligand framework used throughout my work is introduced, and trinuclear clusters of Mn, Co, and Ni are discussed. The ligand scaffold consistently coordinates three metals in close proximity while leaving coordination sites open for further modification through ancillary ligand binding. The ligands coordinated could be varied, with a range of carboxylates and some less coordinating anions studied. These complexes’ structures, magnetic behavior, and redox properties are discussed.
Chapter 3 explores the redox chemistry of the trimanganese system more thoroughly in the presence of a fourth Mn equivalent, finding a range of oxidation states and oxide incorporation dependent on oxidant, solvent, and Mn salt. Oxidation states from MnII4 to MnIIIMnIV3 were observed, with 1-4 O2– ligands incorporated, modeling the photoactivation of the OEC. These complexes were studied by X-ray diffraction, EPR, XAS, magnetometry, and CV.
As Ca2+ is a necessary component of the OEC, Chapter 4 discusses synthetic strategies for making highly structurally accurate models of the OEC containing both Mn and Ca in the Mn3CaO4 cubane + dangling Mn geometry. Structural and electrochemical characterization of the first Mn3CaO4 heterometallic cubane complex— and comparison to an all-Mn Mn4O4 analog—suggests a role for Ca2+ in the OEC. Modification of the Mn3CaO4 system by ligand substitution affords low symmetry Mn3CaO4 complexes that are the most accurate models of the OEC to date.
Finally, in Chapter 5 the reactivity of the Mn3CaO4 cubane complexes toward O- atom transfer is discussed. The metal M strongly affects the reactivity. The mechanisms of O-atom transfer and water incorporation from and into Mn4O4 and Mn4O3 clusters, respectively, are studied through computation and 18O-labeling studies. The μ3-oxos of the Mn4O4 system prove fluxional, lending support for proposals of O2– fluxionality within the OEC.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/9MQM-7R57, author = {Radlauer, Madalyn Rachel}, title = {Bimetallic Olefin Polymerization Catalysis: Mechanisms and Applications of Proximal Effects}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/9MQM-7R57}, url = {https://resolver.caltech.edu/CaltechTHESIS:01232014-115518399}, abstract = {This dissertation covers progress with bimetallic polymerization catalysts. The complexes we have designed were aimed at expanding the capabilities of homogeneous polymerization catalysts by taking advantage of multimetallic effects. Such effects were examined in group 4 and group 10 bimetallic complexes; proximity and steric repulsion were determined to be major factors in the effects observed.
Chapters 2 and 3 introduce the rigid p-terphenyl dinucleating framework utilized in most of this thesis. The permethylation of the central arene allows for the separation of syn and anti atropisomers of the terphenyl compounds. Kinetic studies were carried out to examine the isomerization of the dinucleating bis(salicylaldimine) ligand precursors. Metallation of the syn and anti bis(salicylaldimine)s using Ni(Me)2(tmeda) and excess pyridine afforded dinickel bisphenoxyiminato complexes with a methyl and a pyridyl ligand on each nickel. The syn and anti atropisomers of the dinickel complexes were structurally characterized and utilized in ethylene and ethylene/α-olefin polymerizations. Monometallic analogues were also synthesized and tested for polymerization activity. Ethylene polymerizations were performed in the presence of primary, secondary, and tertiary amines – additives that generally deactivate nickel polymerization catalysts. Inhibition of this deactivation was observed with the syn atropisomer of the bimetallic species, but not with the anti or monometallic analogues. A mechanism was proposed wherein steric repulsion of the substituents on proximal nickel centers disfavors simultaneous ligation of base to both of the metal centers. The bimetallic effect has been explored with respect to size and binding ability of the added base.
Chapter 4 presents the optimization of the bisphenoxyimine ligand synthesis and synthesis of syn and anti m-terphenyl analogues. Metallation with NiClMe(PMe3)2 yielded phosphine-ligated dinickel complexes, which have been structurally characterized. Ethylene/1-hexene copolymerizations in the presence of amines using Ni(COD)2 as a phosphine scavenger showed significantly improved activity relative to the pyridine-ligated analogues. Incorporation of amino olefins in copolymerizations with ethylene was accomplished, and a mechanism was proposed based on proximal effects. Copolymerization trials with a variety of amino olefins and ethylene/1-hexene/amino olefin terpolymerizations were completed.
Early transition metal complexes based on the rigid p-terphenyl framework were designed with a variety of donor sets (Chapter 5 and Appendix B). Chapter 5 details the use of syn dizirconium di[amine bis(phenolate)] complexes for isoselective 1-hexene and propylene homopolymerizations. Ligand variation and monometallic complexes were studied to determine the origin of tacticity control. A mechanistic proposal was presented based on the symmetry at zirconium and the steric effects of the proximal metal center. Appendix B covers additional studies of bimetallic early transition metal complexes based on the p-terphenyl. Dititanium, dizirconium, and asymmetric complexes with bisphenoxyiminato ligands and derivatives thereof were targeted. Progress toward the synthesis of these complexes is described along with preliminary polymerization data. 1-hexene/diene copolymerizations and attempted polymerizations in the presence of ethers and esters with the syn dizirconium di[amine bis(phenolate)] complexes demonstrate the potential for further applications of this system in catalysis.
Appendix A includes work toward palladium catalysts for insertion polymerization of polar monomers. These complexes were based on dioxime and diimine frameworks with the intent of binding Lewis acidic metals at the oxime oxygens, at pendant phenolic donors, or at pendant aminediol moieties. The synthesis and structural characterization of a number of palladium and Lewis acid complexes is presented. Due to the instability of the desired species, efforts toward isolation of the desired complexes proved unsuccessful, though preliminary ethylene/methyl acrylate copolymerizations using in situ activation of the palladium species were attempted.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/Z94T6G9T, author = {Lin, Sibo}, title = {Chelation-Enforced Metal-Arene Interactions: Insights into Substrate Binding and Catalvsis by Late Transition Metal Complexes}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/Z94T6G9T}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282014-232356909}, abstract = {Understanding and catalyzing chemical reactions requiring multiple electron transfers is an endeavor relevant to many outstanding challenges in the field of chemistry. To study multi-electron reactions, a terphenyl diphosphine framework was designed to support one or more metals in multiple redox states via stabilizing interactions with the central arene of the terphenyl backbone. A variety of unusual compounds and reactions and their relevance toward prominent research efforts in chemistry are the subject of this dissertation.
Chapter 2 introduces the para-terphenyl diphosphine framework and its coordination chemistry with group 10 transition metal centers. Both mononuclear and dinuclear compounds are characterized. In many cases, the metal center(s) are stabilized by the terphenyl central arene. These metal–arene interactions are characterized both statically, in the solid state, and fluxionally, in solution. As a proof-of-principle, a dinickel framework is shown to span multiple redox states, showing that multielectron chemistry can be supported by the coordinatively flexible terphenyl diphosphine.
Chapter 3 presents reactivity of the terphenyl diphosphine when bound to a metal center. Because of the dearomatizing effect of the metal center, the central arene of the ligand is susceptible to reactions that do not normally affect arenes. In particular, Ni-to-arene H-transfer and arene dihydrogenation reactions are presented. Additionally, evidence for reversibility of the Ni-to-arene H-transfer is discussed.
Chapter 4 expands beyond the chelated metal-arene interactions of the previous chapters. A dipalladium(I) terphenyl diphosphine framework is used to bind a variety of exogenous organic ligands including arenes, dienes, heteroarenes, thioethers, and anionic ligands. The compounds are structurally characterized, and many ligands exhibit unprecedented bindng modes across two metal centers. The relative binding affinities are evaluated spectroscopically, and equilibrium binding constants for the examined ligands are determined to span over 13 orders of magnitude. As an application of this framework, mild hydrogenation conditions of bound thiophene are presented.
Chapter 5 studies nickel-mediated C–O bond cleavage of aryl alkyl ethers, a transformation with emerging applications in fields such as lignin biofuels and organic methodology. Other group members have shown the mechanism of C–O bond cleavage of an aryl methyl ether incorporated into a meta-terphenyl diphosphine framework to proceed through β-H elimination of an alkoxide. First, the electronic selectivity of the model system is examined computationally and compared with catalytic systems. The lessons learned from the model system are then applied to isotopic labeling studies for catalytic aryl alkyl ether cleavage under dihydrogen. Results from selective deuteration experiments and mass spectrometry draw a clear analogy between the mechanisms of the model and catalytic systems that does not require dihydrogen for C–O bond cleavage, although dihydrogen is proposed to play a role in catalyst activation and catalytic turnover.
Appendix A presents initial efforts toward heterodinuclear complexes as models for CO dehydrogenase and Fischer Tropsch chemistry. A catechol-incorporating terphenyl diphosphine is reported, and metal complexes thereof are discussed.
Appendix B highlights some structurally characterized terphenyl diphosphine complexes that either do not thematically belong in the research chapters or proved to be difficult to reproduce. These compounds show unusual coordination modes of the terphenyl diphosphine from which other researchers may glean insights.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/ZKZ0-TP24, author = {Kelley, Paul}, title = {Fundamental Studies of Carbon Oxygen Bond Activation in Nickel Diphosphine Ether Complexes. And, Metallomacrocycles as Ligands: Synthesis and Characterization of Aluminum-Bridged Bisglyoximato Complexes of Iron and Cobalt}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/ZKZ0-TP24}, url = {https://resolver.caltech.edu/CaltechTHESIS:06082014-155448927}, abstract = {In order to develop better catalysts for the cleavage of aryl-X bonds fundamental studies of the mechanism and individual steps of the mechanism have been investigated in detail. As the described studies are difficult at best in catalytic systems, model systems are frequently used. To study aryl-oxygen bond activation, a terphenyl diphosphine scaffold containing an ether moiety in the central arene was designed. The first three chapters of this dissertation focus on the studies of the nickel complexes supported by this diphosphine backbone and the research efforts in regards to aryl-oxygen bond activation.
Chapter 2 outlines the synthesis of a variety of diphosphine terphenyl ether ligand scaffolds. The metallation of these scaffolds with nickel is described. The reactivity of these nickel(0) systems is also outlined. The systems were found to typically undergo a reductive cleavage of the aryl oxygen bond. The mechanism was found to be a subsequent oxidative addition, β-H elimination, reductive elimination and (or) decarbonylation.
Chapter 3 presents kinetic studies of the aryl oxygen bond in the systems outlined in Chapter 2. Using a series of nickel(0) diphosphine terphenyl ether complexes the kinetics of aryl oxygen bond activation was studied. The activation parameters of oxidative addition for the model systems were determined. Little variation was observed in the rate and activation parameters of oxidative addition with varying electronics in the model system. The cause of the lack of variation is due to the ground state and oxidative addition transition state being affected similarly. Attempts were made to extend this study to catalytic systems.
Chapter 4 investigates aryl oxygen bond activation in the presence of additives. It was found that the addition of certain metal alkyls to the nickel(0) model system lead to an increase in the rate of aryl oxygen bond activation. The addition of excess Grignard reagent led to an order of magnitude increase in the rate of aryl oxygen bond activation. Similarly the addition of AlMe3 led to a three order of magnitude rate increase. Addition of AlMe3 at -80 °C led to the formation of an intermediate which was identified by NOESY correlations as a system in which the AlMe3 is coordinated to the ether moiety of the backbone. The rates and activation parameters of aryl oxygen bond activation in the presence of AlMe3 were investigated.
The last two chapters involve the study of metalla-macrocycles as ligands. Chapter 5 details the synthesis of a variety of glyoxime backbones and diphenol precursors and their metallation with aluminum. The coordination chemistry of iron on the aluminum scaffolds was investigated. Varying the electronics of the aluminum macrocycle was found to affect the observed electrochemistry of the iron center.
Chapter 6 extends the studies of chapter 5 to cobalt complexes. The synthesis of cobalt dialuminum glyoxime metal complexes is described. The electrochemistry of the cobalt complexes was investigated. The electrochemistry was compared to the observed electrochemistry of a zinc analog to identify the redox activity of the ligand. In the presence of acid the cobalt complexes were found to electrochemically reduce protons to dihydrogen. The electronics of the ancillary aluminum ligands were found to affect the potential of proton reduction in the cobalt complexes. These potentials were compared to other diglyoximate complexes.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, } @phdthesis{10.7907/CWJ4-QQ41, author = {Tsui, Emily Yuan}, title = {Transition Metal Clusters Supported by Multinucleating Ligand Frameworks as Models of Biological Active Sites}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/CWJ4-QQ41}, url = {https://resolver.caltech.edu/CaltechTHESIS:04302014-134133118}, abstract = {
This dissertation describes efforts to model biological active sites with small molecule clusters. The approach used took advantage of a multinucleating ligand to control the structure and nuclearity of the product complexes, allowing the study of many different homo- and heterometallic clusters. Chapter 2 describes the synthesis of the multinucleating hexapyridyl trialkoxy ligand used throughout this thesis and the synthesis of trinuclear first row transition metal complexes supported by this framework, with an emphasis on tricopper systems as models of biological multicopper oxidases. The magnetic susceptibility of these complexes were studied, and a linear relation was found between the Cu-O(alkoxide)-Cu angles and the antiferromagnetic coupling between copper centers. The triiron(II) and trizinc(II) complexes of the ligand were also isolated and structurally characterized.
Chapter 3 describes the synthesis of a series of heterometallic tetranuclear manganese dioxido complexes with various incorporated apical redox-inactive metal cations (M = Na+, Ca2+, Sr2+, Zn2+, Y3+). Chapter 4 presents the synthesis of heterometallic trimanganese(IV) tetraoxido complexes structurally related to the CaMn3 subsite of the oxygen-evolving complex (OEC) of Photosystem II. The reduction potentials of these complexes were studied, and it was found that each isostructural series displays a linear correlation between the reduction potentials and the Lewis acidities of the incorporated redox-inactive metals. The slopes of the plotted lines for both the dioxido and tetraoxido clusters are the same, suggesting a more general relationship between the electrochemical potentials of heterometallic manganese oxido clusters and their “spectator” cations. Additionally, these studies suggest that Ca2+ plays a role in modulating the redox potential of the OEC for water oxidation.
Chapter 5 presents studies of the effects of the redox-inactive metals on the reactivities of the heterometallic manganese complexes discussed in Chapters 3 and 4. Oxygen atom transfer from the clusters to phosphines is studied; although the reactivity is kinetically controlled in the tetraoxido clusters, the dioxido clusters with more Lewis acidic metal ions (Y3+ vs. Ca2+) appear to be more reactive. Investigations of hydrogen atom transfer and electron transfer rates are also discussed.
Appendix A describes the synthesis, and metallation reactions of a new dinucleating bis(N-heterocyclic carbene)ligand framework. Dicopper(I) and dicobalt(II) complexes of this ligand were prepared and structurally characterized. A dinickel(I) dichloride complex was synthesized, reduced, and found to activate carbon dioxide. Appendix B describes preliminary efforts to desymmetrize the manganese oxido clusters via functionalization of the basal multinucleating ligand used in the preceding sections of this dissertation. Finally, Appendix C presents some partially characterized side products and unexpected structures that were isolated throughout the course of these studies.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Agapie, Theodor}, }