Molybdenum Para-Terphenyl Diphosphine Complexes

Citation

Buss, Joshua Alan (2018) Molybdenum Para-Terphenyl Diphosphine Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SJTG-3388. 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 Mo^II 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 Mo^II 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.

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