Date of Award
Doctor of Philosophy (PhD)
Neil C. Tomson
Methanol is a viable fuel source and a common feedstock in the chemical industry. Currently, much of the methanol produced by industry is derived from natural gas (methane). Yet, the one-step selective oxidation of methane to methanol remains challenging due to the potential formation of overoxidation products (CO2, H2O, etc.). Within the past 20 years, the reactive Cu2(II,II)-µ-O site within Cu(II)-ZSM-5 has been shown capable of the selective oxidation of methane to methanol through a hydrogen atom transfer (HAT)/radical rebound mechanism. The proximity of the metals and geometric constraints may allow for the copper centers to work in concert when facilitating HAT. To probe how metal-metal distance and coordination geometry influence C–H bond oxidation, it is highly desirable to study the properties and reactivity profile of discrete multinuclear model complexes. Here, we report a new protocol for the synthesis of homobimetallic copper(II) complexes using various derivatives of a macrocycle containing two pyridyldiimine units. A series of Cu2-(µ-X)n bis(4-tert-butylpyridyldiimine) macrocyclic complexes (n = 1, 2; X = Cl, Br, N3, NO2, OTMS, OH) were synthesized and crystallographically characterized. The development of new macrocyclic ligands was also undertaken to provide a modular synthesis of bis(pyridyldiimine) ligand scaffolds, in which the steric and electronic profiles can be easily tuned. Using the newly established synthetic protocols and macrocyclic ligands, two Cu2(II,II)-µ-OH complexes were prepared and analyzed by cyclic voltammetry and low-temperature 1H NMR spectroscopy. The macrocyclic complexes differed in the aliphatic linkers between the pyridyldiimine units (ethylene vs. propylene linker). New Cu2(II,II)-µ-O complexes were accessed through deprotonation of the hydroxide starting materials. The µ-oxo complexes were found competent for hydrogen atom transfer reactions, showing reactivity towards weak C-H and N-H bonds. From these studies, the size of the macrocycle was found to influence the reactivity of the µ-oxo complexes. The HAT reactions afforded Cu2(I,II)-µ-OH complexes, and O-H BDFEs of 74.3 ± 1.4 (propylene-linked complex) and 79.0 ± 1.5 kcal/mol (ethylene-linked complex) were estimated using Bordwell’s thermodynamic square scheme approach.
Brooks, Sam, "Synthesis And Reactivity Of Flexible Dicopper Bis(pyridyldiimine) Macrocyclic Complexes" (2022). Publicly Accessible Penn Dissertations. 4835.