Design, Synthesis, and Reactivity of High-Valent Nickel and Palladium Complexes

Fundamental organometallic studies at well-defined metal centers provide important insight into the reactivity and selectivity profiles of catalytically relevant systems. Of particular interest are high oxidation state nickel and palladium complexes (i.e., PdIV, NiIV, NiIII). These species have been implicated as reactive intermediates in a variety of catalytic transformations including C–H bond functionalization, alkene difunctionalization, and carbon-carbon coupling reactions. However, the transient nature of these intermediates has hindered definitive characterization and confirmation of their roles in catalysis. Ultimately, a fundamental understanding of catalytically relevant organometallic complexes will inform the optimization of known transformations and the development of new catalytic reactions. This thesis describes the design, synthesis, and isolation of high-valent nickel and palladium complexes and studies of their reactivity towards challenging bond-forming reactions. Chapter 1 describes in detail the roles of palladium and nickel catalysts in carbon–carbon and carbon–heteroatom bond-forming reactions, as well as the relevant history and precedent for the work detailed herein. Chapter 2 is focused on reactivity studies of well-defined PdIV complexes supported by bypyridines and cyclometallated carbon-donor ligands. These design strategies enable the direct study of C(sp3)–heteroatom bond-forming reductive elimination from PdIV. We demonstrate that a diverse set of oxygen nucleophiles participate as coupling partners in C(sp3)–O coupling from PdIV and that cationic additives play an important role in the chemoselectivity of competing C(sp3)–O and C(sp3)–F bond-forming reactions. Experimental and computational studies provide insight into the mechanism of these reductive elimination reactions. Chapter 3 details our systematic investigation into the organometallic chemistry of high-valent nickel. We demonstrate that a series of isolable NiIV complexes can be accessed by the treatment of NiII precursors with common two-electron oxidants. The importance of the trifluoromethyl (CF3) ligand and tris(pyrazolyl)borate (Tp) scaffold in stabilizing these traditionally transient species is highlighted. Furthermore, reactivity studies show that these NiIV complexes participate in highly selective carbon–carbon and carbon–heteroatom bond-forming reactions that remain extremely challenging to achieve at lower oxidation states of nickel. In Chapter 4, the reactivity profiles of diorgano-NiIII complexes are evaluated and compared to the NiIV counterparts. Throughout these studies, NiIV was shown to promote reductive elimination events more readily than analogous NiIII complexes. In addition, selective carbon–carbon or carbon–heteroatom coupling could be achieved depending on the oxidation state of the nickel center. Finally, Chapter 5 details a comparative study between high-valent nickel and palladium complexes. Electrochemical analyses, kinetic studies, and computational insights demonstrate the remarkable similarities in the chemistry of NiIV and PdIV, but an enhanced role for NiIII in enabling reactivity that is distinct from palladium.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137001/1/camasso_1.pd