Hemilabile, Non-Innocent (Poly)arylene Donors for Accessing Novel Reactivity at Transition Metal Centers

Citation

Low, Choon Heng (Marcus) (2019) Hemilabile, Non-Innocent (Poly)arylene Donors for Accessing Novel Reactivity at Transition Metal Centers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BQJM-BN49. 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.

Thesis Files