Digitized by Kansas Correctional Industries

Tetraphenylporphyrins can be used to absorb visible light and pass on their excitation energy to electron transfer agents. The purpose of this research has been to investigate our ability to understand and control the energetics of porphyrin derivatives in order to use their electron transfer ability to harness the energy of sunlight. Shifts in the redox (reduction and oxidation) potentials of tetraphenylporphyrins result from variations in the substituents attached at the para- position of the phenyl rings of tetraphenylporphyrins, as well as variations in the state of ionization of those substituents, and the solvent in which the reactions are carried out. To measure the effect these variations, results from cyclic voltammetric experiments were plotted versus literature values of Hammett substituent constants to confirm the validity of linear free energy relationships as a model of substituent effects. Solvent effects on reduction potentials were correlated using experimentally determined values of the empirical solvent parameter ET. Some specific conclusions are summarized. 1. The usefulness of linear free energy relationships in correlating variations in redox potentials with changes in substituent was confirmed with two exceptions. Two of the porphyrins were shown to undergo a different electrochemical oxidation mechanism than the remaining porphyrins, and another porphyrin was shown to be more difficult to reduce than predicted on the basis of its substituent constant. 2. Solvent effects, here investigated as the effect of added water on the reduction potential of tetraaminophenylporphyrin in DMSO, were demonstrated to correlate with the Dimroth-Reichardt solvent parameter, ET , determined experimentally for each water-DMSO mix. 3. Variations in the state of ionization of ionizable substituents such as carboxylic acid, amine, and hydroxyl substituents were shown to affect porphyrin electrochemistry mostly through the protonation of bulk, solution-phase porphyrin by added proton donor. An additional effect of added proton donor was noted in an alteration in the mechanism of reduction to include some of a different mechanism wherein reduced porphyrin is protonated in a chemical equilibrium and then further reduced electrochemically.

In situ reduction of hydrido-tris-(3,5-dimethylpyrazolyl)borato(trioxo) rhenium(V) with triphenylphosphine or triethylphosphite leads to a reactive rhenium(V) species that catalytically deoxygenates epoxides at 75-105��C. The reaction is stereospecific, except for trans- and cis-butene oxide which formed minor amounts of the opposite isomer. A variety of different functional groups were tolerated and even epoxides that reacted slowly could be pushed to greater than 95% conversion given extended time and/or higher temperature. The absence of clustering processes shows how the choice of ligand can have a major influence on the design of the catalytic cycle. The rhenium(V) species formed from reduction of Tp'ReO��� was identified as Tp'Re(O)(OH)���. Tp'Re(O)(OH)��� reacted with ethanol and HCl to form ethoxide and hydroxo chloride complexes, respectively. In addition, Tp'Re(O)(OH)��� was an excellent catalytic and stoichiometric reagent for the deoxygenation of epoxides and sulfoxides. Loss of water from Tp'Re(O)(OH)��� to form the catalytically active species Tp'Re02 was shown to be a necessary preequilibrium process. The kinetic behavior of the catalytic system is complex. First-order behavior in [Re][subscript T], zero-order dependence in [PPh���] and saturation behavior for epoxide were observed. The reversible formation of a coordinated epoxide complex was proposed to explain the saturation behavior. The epoxide complex was shown experimentally and computationally to engage in two separate reactions: ring expansion to form a syn-diolate complex, and direct fragmentation to alkene and trioxide. A steady-state concentration of diolate is eventually reached explaining a "burst" of alkene production prior to generation of a pseudo-zero-order catalytic system. The diolate formed is the syn-isomer, which is the kinetically formed product. Direct epoxide fragmentation is the primary source of alkene. This process was determined to be four times faster than ring expansion for cis-stilbene oxide. The synthesis and characterization of a tethered-epoxide Cp* rhenium trioxide complex has been achieved. Reduction of this complex leads to an unsaturated rhenium(V) species that is immediately complexed by the tethered epoxide. Experimental data and molecular mechanics modeling support intramolecular coordination of the epoxide to the rhenium center. These results confirm that the coordinate epoxide is a viable intermediate in rhenium-catalyzed epoxide deoxygenations. / Graduation date: 2003

Graduation date: 1995

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2010. / Committee Chair: Bridgette Barry; Committee Member: Ingeborg Schmidt-Krey; Committee Member: Jake Soper; Committee Member: Nils Kroger; Committee Member: Wendy Kelly. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph.D.)--University of Wyoming, 2008. / Title from PDF title page (viewed on June 28, 2009). Includes bibliographical references (p. 189-202).

Not available / text

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.