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Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Liviu M Mirica
Catalysis involving high-valent Pd(III) and Pd(IV) complexes has recently shown important applications in the development of new chemical transformations that compliments conventional Pd0/II catalysis.
However, the Pd(III) or Pd(IV) intermediates cannot be always
unambiguously confirmed (or excluded) in the reactions due to their highly active nature. The focus of this work is to study the fundamental properties and reactivity of Pd(III) and Pd(IV) complexes through steric and electronic manipulation of the supporting ligands. This work also provides key mechanistic insights into the development of an oxidative C-C and C-X bond formation catalytic system involving green oxidants such as dioxygen and peroxides.
Our group reported the first mononuclear organometallic Pd(III) complexes stabilized by a flexible tetradentate macrocyclic tBuN4 ligand (N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane). To continue our study in this field, we manipulated the pyridinophane macrocylic ligand by variation of the steric bulk on the amine groups and introduction of strong donating methoxy group on the para position of the pyridine rings. Systematic comparative studies (eg. CV, UV-Vis, EPR, X-ray, and DFT calculations) for the Pd(III) analogues with the modified ligands were performed, and we found that the steric manipulation on the axial position of the complex dramatically affects the properties while the electronic manipulation on the pyridine ring has a surprisingly small effect.
This study also led to stabilization of both Pd(III) and Pd(IV) complexes with a N, N'-di-methyl-2,11-diaza[3.3](2,6) pyridinophane ligand (MeN4). Compared with other pyridinophane ligands (RN4, R= tBu, iPr), MeN4 substantially lowers the corresponding PdIII/IV oxidation potential likely due to smaller steric interaction between the N-methyl substituents and the Pd(IV) center. This allows a direct structural and reactivity comparison between Pd complexes in the +3 and +4 oxidation states in an identical ligand environment. The Pd(III) complexes display unselective C-C and C-Cl bond formation reactivity involving a radical mechanism upon photolysis, while the corresponding Pd(IV) complexes lead to much more selective C-C and C-Cl bond formations through reductive elimination.
More interesting was the detection of a series of Pd(III) and Pd(IV) species during the aerobic /peroxide oxidation of (MeN4)PdII precursors which ultimately lead to C-C and C-Heteroatom bond formation. For example, high valent Pd intermediates [(MeN4)PdIIIMe2]+, [(MeN4)PdIIIMe2(O2*)]+, [(MeN4)PdIVMe2OH]+, [(MeN4)PdIVMe2OOH]+, and a key intermediate [(κ3-MeN4)PdIVMe3]+ were observed by various in situ techniques including UV-Vis, EPR, NMR, and ESI-MS during the aerobic oxidation of (MeN4)PdIIMe2. This system represents one of the first examples of aerobic oxidation of a Pd(II) precursor to form a detectable Pd(IV) species, which then leads to C-C bond formation. The study provides key understanding of mechanistic details about the aerobic/peroxide oxidation and following reductive elimination process, setting a stage for the development of aerobic/peroxide C-C and C-Heteroatom bond formation reactions employing high-valent Pd catalysis.
We also studied the (MeN4)Pd complexes mediated C-H activation reactivity which serves as a key step in organometallic catalysis. (MeN4)PdII(OAc)2 can readily activate nitroalkyl compounds through an acetate assisted C-H cleavage mechanism based on KIE studies. Control experiments also suggested that the flexible axial amine donors can facilitate C-H activation process. More interestingly, the C-H activated Pd complexes (eg. (MeN4)PdII(CH2NO2)2) can be readily oxidized to the higher oxidation states by commonly used chemical oxidants including dioxygen and peroxides. Oxidation of (MeN4)PdII(CH2NO2)2) by peracetic acid leads to the formation of C-O coupled product nitromethyl acetate, proving the viability of our proposed catalytic cycle. Furthermore, we established a model Pd(III) species [(MeN4)PdIII(neophyl)Cl]+ that exhibits an unprecedented acetate assisted C-H activation at a Pd(III) center. This reaction suggests that the C-H activation can also readily proceed at Pd(III) under mild conditions.
As a side project, we developed an efficient synthetic route for a series of binucleating ligands that contains a xanthene backbone as a spacer in an attempt to control the distance between the two metal binding sites. This ligand is expected to prevent the dissociation of the two metal centers while making sure that a distance between the two metal centers is not too small. Several binuclear nickel and copper complexes were successfully synthesized and characterized, and their unique reactivity towards CO2, H2O2, and water oxidation were studied to understand the structure-reactivity relationship.
Tang, Fengzhi, "Ligand Manipulation and Reactivity Studies of Organometallic High Valent Palladium Complexes" (2014). All Theses and Dissertations (ETDs). 1355.
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