Chelation-Enforced Metal-Arene Interactions: Insights into Substrate Binding and Catalvsis by Late Transition Metal Complexes

Citation

Lin, Sibo (2014) Chelation-Enforced Metal-Arene Interactions: Insights into Substrate Binding and Catalvsis by Late Transition Metal Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z94T6G9T. 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.

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