Organometallic chemistry of triazacyclononanes and high-valent nickel complexes

Triazacyclononane (tacn) ligands possess a rich history in the field of transition metal chemistry because of their strong chelation and the ease of derivatization and have found many industrial and biological applications. Nevertheless, the synthesis of asymmetric tacns continue to pose challenges when chiral or bulky substituents are involved. In recent years, nickel catalysis has received increasing attention due to its low cost compared to platinum group metals and the ability to mediate unprecedented synthetic methodologies. The propensity to form open-shell or high-valent species led to the development of many model systems to interrogate their intermediacy. Previous work in our group had shown that tacns are decent ancillary ligands for organometallic nickel and palladium complexes for small molecule activation, and this served as a starting point to expand the associated high-valent nickel chemistry. In Chapter 2, a new tBuMe2tacn ligand was designed to destabilize the high-valent nickel intermediates generated oxidatively. The associated nickel(cycloneophyl) complex displayed strong bond-forming reactivity and tunable selectivity using green oxidants such as molecular oxygen or hydrogen peroxide. Mechanistic studies indicated heterolysis of the O–O bond of the peroxide upon reaction with the NiII complex, and therefore afforded a formal NiIV-OH intermediate. Isotope-labelling experiments at low temperature revealed that hydrogen peroxide was the source of the oxygen atom of the oxygenated product. Kinetic experiments suggested a rate-limiting bimolecular associative process which is consistent with the formation of a transient nickel hydrogen peroxide adduct. Computational studies were also conducted to interrogate the nature of the intermediates. The reductive elimination studies of the associated high-valent nickel complexes supported by the tBuMe2tacn ligand was revisited in Chapter 3, which showed more rapid reductive elimination at NiIV than NiIII. In Chapter 4, the synthesis and characterization of new chiral triazacyclononanes and the corresponding nickel chloride complexes was explored. The nickel complexes were found to have decreasing coordination numbers with more bulky substituents on the amine donor. The catalytic competence of these nickel complexes was demonstrated in alkyl-alkyl Kumada cross-coupling reactions. Preliminary experiments showed that sterically demanding ligands in general imposed a negative influence on the yield of the cross-coupled product, and the enantioselectivity followed a similar trend as well. In Chapter 5, a new chelating amide-derivative was employed to build the model system of an unforeseen nickel-catalyzed C-H hydroxylation reaction. The ligand design was inspired from the work in Chapter 2 and other common nickel catalysts. The aerobic reactivity of this system also makes it relevant to biomimetic oxidative transformations resembling oxygenases. The key to promoting oxygen atom transfer from dioxygen was the installation of the dangling nitrogen donor that facilitates dioxygen activation. Mechanistic studies suggested the intermediacy of a NiIV-OH intermediate, which is consistent with the experimental results from Chapter 2. The future directions were summarized in Chapter 6, including the preparation of new chiral triazacyclononanes and the investigation of the catalytic competence of the directing group proposed in Chapter 5.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Hu, Chi-Herng

Contributors dc:contributor

• Mirica, Liviu M
• Rauchfuss, Thomas B
• Denmark, Scott E
• Olshansky, Lisa

• Inorganic Chemistry
• Green Oxidants
• Hydroxylation

Statement dc:rights

• Copyright 2024 Chi-Herng Hu

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124623

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124623