Author's Department/Program
Chemistry
Language
English (en)
Date of Award
Summer 9-1-2014
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Chair and Committee
Liviu M Mirica
Abstract
Energy is the currency of the universe. Without energy everything will come to a stall. According to the Energy Information Administration, in 2007 over 80 % of energy came from fossil fuel. When the fossil fuel will eventually run out at the current consumption rate, then what can we use for energy? While finding alternative energy sources is our ultimate goal, our research focus on the basic study of the role metal ions play in the catalysis of energy related process. Namely, the water oxidation catalysis and methane oligomerization process.
Water oxidation is a multi-electron process, which requires multiple single electron transfer steps. This translates into a very high barrier of activation since multi-electron processes are kinetically challenging. On the other hand, multi-nuclear complexes are receiving an appreciable interest due to their potential synergetic effect between the metals. To this purpose, four binucleating tetradentate ligands of the generic formula Di-nBu-DBF-Mennn-tpa were synthesized. The final ligands as well as the intermediate products were characterized by1H and13C NMR and ESI-MS. These ligands were used to complex with some transition metals in the hope that the two metal centers could be brought into close proximity within the bridging distance of &mu-oxo or hydroxyl ligands, which then will potentially be active toward water oxidation reaction. Some of the complexes have been characterized by X-ray crystallography,1H NMR, Uv-vis, ESI-MS, IR. Our study showed that the ligands using dibenzofuran as linker were unable to support the expected bis(&mu-O) or bis(&mu-OH) structure. Rather, we find that the linker is central in term of its size and shape, which largely define the M-M distance, since the scaffold is fairly rigid. DFT calculations indicate that the energy barrier to bring two metals close enough is prohibitively high. This observation offers a valuable guide for future selection of linker molecules, when we should use computational study to guide the screening of potential ligands.
Methane oligomerization can be viewed as a two step process: the C-H activation step, and the C-C coupling step. The C-C coupling process is usually induced oxidatively. We were able to realize the C-C bond reductive elimination from a monomethyl Pd complexes with the ligand of N,N&rsquo-di-tBu-2,11-diaza[3,3](2,6)pyridinophane (tBuN4), which resembled the early step of methane oligomerization. To further study this process, we modified the tBuN4 by replacing the tBu groups on the amino nitrogen with smaller iPr groups to give iPrN4. Compared to the tBuN4, the new ligand was able to stabilize both PdIIIand PdIIIoxidation states, as compared to the fact that only PdIIIoxidation states (tBuN4PdMe2 andtBuN4PdMeCl) was stabilized bytBuN4. Both PdIII complexes underwent photo activated C-C bond formation, which involve a radical mechanism initiated by homolysis of Pd-C bond. The PdIV complexes underwent thermolysis to give C-C or C-Cl bond formation through reductive elimination.
Oxygen in air represents the ideal oxidant for chemical oxidation since it is clean, almost inexhaustible and carbon free. In this research, an aerobic oxidation induced C-O bond formation mediated by Pd was reported. The complex Me3tacnPd(CH2CMe2C6H4) is readily oxidized by O2 or H2O2 to yield the PdIV -OH complex [Me3tacnPd(OH)(CH2CMe2C6H4)]+. Thermolysis of this product leads to the selective C(sp2)-O reductive elimination of 2-tert-butyl-phenol, with no C(sp3)-O elimination product being detected. This system represents a rare example of selective C(sp2)-O bond formation that is relevant to Pd-catalyzed aerobic C-H hydroxylation reactions.
Recommended Citation
Qu, Fengrui, "High Valent Palladium Complexes and Their Role in C-C and C-X Bond Formation Chemistry." (2014). All Theses and Dissertations (ETDs). 1337.
https://openscholarship.wustl.edu/etd/1337
Comments
Permanent URL: http://dx.doi.org/10.7936/K7R49NTG