Abstract

High-oxidation state main-group metal complexes are potential alternatives to transition metals for electrophilic C-H functionalization reactions. However, there is little known about how selection of the p-block, main-group metal and ligand impact C-H activation and functionalization thermodynamics and reactivity. Chapter 2 reports density functional theory (DFT) calculations used to determine qualitative and quantitative features of C-H activation and metal-methyl functionalization energy landscapes for reaction between high-oxidation state d10s0 InIII, TlIII, SnIV, and PbIV carboxylate complexes with methane. While the main-group metal influences the C-H activation barrier height in a periodic manner, the carboxylate ligand has a much larger quantitative impact on C-H activation with stabilized carboxylate anions inducing the lowest barriers. For metal-methyl reductive functionalization reactions, the barrier heights, are correlated to bond heterolysis energies as model two-electron reduction energies.In Chapter 3, DFT calculations reveal that arene C-H functionalization by the p-block main-group metal complex TlIII(TFA)3 (TFA = trifluoroacetate) occurs by a C-H activation mechanism akin to transition metal-mediated C-H activation. For benzene, toluene, and xylenes a one-step C-H activation is preferred over electron transfer or proton-coupled electron transfer. The proposed C-H activation mechanism is consistent with calculation and comparison to experiment, of arene thallation rates, regioselectivity, and H/D kinetic isotope effects. For trimethyl and tetramethyl substituted arenes, electron transfer becomes the preferred pathway and thermodynamic and kinetic calculations correctly predict the experimentally reported electron transfer crossover region.In Chapter 4, DFT calculations are used to understand the C-H oxidation reactions of methane and isobutane with SbVF5. SbVF5 is generally assumed to oxidize methane through a methanium-methyl cation mechanism. DFT calculations were used to examine methane oxidation by SbVF5 in the presence of CO leading to the acylium cation, [CH3CO]+. While there is a low barrier for methane protonation by [SbVF6]-[H]+ to give the [SbVF5]-[CH5]+ ion pair, H2 dissociation is a relatively higher energy process, even with CO assistance, and so this protonation pathway is reversible. The C-H activation/[]-bond metathesis mechanism with formation of an SbV-Me intermediate is the lowest energy pathway examined. This pathway leads to [CH3CO]+ by functionalization of the SbV-Me intermediate by CO, and is consistent with no observation of H2. In contrast to methane, due to the much lower carbocation hydride affinity, isobutane significantly favors hydride transfer to give tert-butyl carbocation with concomitant SbV to SbIII reduction. In this mechanism, the resulting highly acidic SbV-H intermediate provides a route to H2 through protonation of isobutane.

Degree

PhD

College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry

Rights

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