Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3788. Adviser: G.S. Girolami. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6396. Adviser: John R. Shapley. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Abstract not available.

Mixed valence compounds have been recognized over the past five decades as a unique class of chemical species. Their distinctive spectral, electrochemical, physical and magnetic properties arise from electron delocalization into the sites with uneven valence. A primary consequence of this electron delocalization phenomenon is the stabilization of high spin multiplicities in certain dinuclear synthetic and polynuclear biological clusters. Therefore, the subject of this dissertation is to extend the current knowledge of the analysis of synthetic mixed valence clusters by systematically altering the coordination chemistry and redox states at well-defined synthetic, polynuclear iron clusters. A central focus of this thesis is to investigate the effect of superexchange, direct exchange, and double exchange electronic coupling in dinuclear [Fe2], trinuclear [Fe3], hexanuclear [Fe6], and octanuclear [Fe8] clusters with the goal of better understanding the principles that govern their complex electronic structures. It is concluded that the resulting electronic structure in these polynuclear systems is highly dependent on the extent of electron delocalization which can be tuned by solvation, anation, or chemical redox changes. This finding is highlighted by the observation that small variations on the solvation coordination sphere and redox level one can transverse spin ground states from S = 0 to S = 11 by addition of 6e– into [Fe6]. / Chemistry and Chemical Biology

The electrochemical oxidation of Co2+ ions in the presence of a buffering species such as phosphate (Pi), methyl phosphonate (MePi), and borate (Bi) leads to the formation of amorphous cobalt oxide thin films that are capable of catalyzing the four electron-four proton oxidation of water to dioxygen. These cobalt oxygen evolving catalysts (Co–OECs) have been extensively studied by electrochemical techniques; however, the amorphous nature of these films means that garnering insights at the molecular and atomic level is difficult because electronic and structural characterization methods rely on measurements of bulk material. To address this challenge, polynuclear cobalt hydroxide and cobalt oxide compounds have been employed as platforms for understanding Co–OECs at the molecular level. In the first part of the thesis, we present the synthesis of a heptanuclear cobalt cluster in two different oxidation states, Co(II)7 and a mixed valence Co(III)Co(II)6. An anomalously slow self–exchange electron transfer rate as compared to that predicted from semiclassical Marcus theory was measured, supporting a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the deposition mechanism of Co–OECs. Moreover, the results address a long-standing controversy surrounding the Co2+/3+ self–exchange electron transfer reaction of the hexaaqua complex. In the second part of the thesis, we report the synthesis and characterization of a dinuclear cobalt complex, which we use as an edge sites mimic (ESM) of the Co-OEC. A comparative investigation of the Pi and Bi binding of buffers to this ESM is provided. We find that the binding to the boric acid component of borate buffer is rapid, while binding to K2HPO4 is much slower, taking days to equilibrate at room temperature. These studies are used to provide a model for the interaction of Pi and Bi species with the active site of the Co-OEC. An inverse dependence on [Bi] of the electrochemical driven water oxidation activity by Co-OECs is demonstrated, which provides validation for the model and indicates that dinuclear cobalt edge sites are necessary for water oxidation catalysis. In the final chapter of the thesis, we use a series of spectroscopic and electrochemical methods to show that the observed water oxidation activity of the compound class Co4O4(OAc)4(py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce Co-OEC, which is an active water oxidation catalyst. Differential electrochemical mass spectrometry (DEMS) is used to characterize the fate of glassy carbon at water oxidizing potentials, and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis. We then investigate the electrochemical properties of Co4O4 cubanes in non-aqueous solvents, showing that the neutral cubanes may be oxidized by two electrons yielding a formal oxidation state of Co(IV)2Co(III)2. Finally, we show that these cubanes may be synthetically modified with tethered NMI photooxidants, which opens new avenues for exploring fixed-distance photoinduced ET using Co4O4 cubanes as the electron donor. Ultra-fast transient absorption experiments demonstrate that a charge separated state is formed following laser excitation. / Chemistry and Chemical Biology

Proton-coupled electron transfer (PCET) combines proton transfer with electron transfer to bypass high-energy intermediates. The ribonucleotide reductase (RNR) family of enzymes catalyzes the conversion of ribonucleotides to deoxynucleotides using amino acid radicals. The enzyme contains an efficient PCET pathway that transfers an electron and proton over a 35 Å distance across two subunits, the longest PCET pathway known in biology. The enzyme func-tions with very high fidelity, performing >105 turnovers before radical loss. This thesis explores PCET in model systems to understand the radical transport pathway in E. coli RNR type Ia. First, amino acid radical generation was explored by laser spectroscopy. Small molecule systems, where photooxidants were appended with amino acids, provide a plat-form for probing both the pH and driving force dependence of PCET. Analysis of the emission kinetics of these systems indicate that amino acid radicals were generated by sequential electron and proton transfers as a result of the strong driving force of the photooxidant. Next, the dyad model with cofacial aromatic units was developed to investigate the PCET radical transport mechanism between two adjacent amino acids. Inspired by the unique redox cooperativity ob-served in Y730-Y731 tyrosine dyads at the α2β2 interface of RNR, electrochemical and computa-tional approaches were used to probe the correlation between the structural and redox properties of the dyad model. Using electrochemical and computational analysis, it was determined that the presence of two phenol units in the dyad lowered the reduction potential by ~ 60 mV. This result demonstrates that the tyrosine dyad behaves as a discrete redox unit, consistent with the observed radical transport pathway observed in RNR. Finally, the bidirectional PCET pathway in the β2 subunit of RNR (Y122-W48-Y356) was modeled using a polyproline-based peptide. The long-distance electron transfer between tyrosine analogs and tryptophan was investigated by pulse ra-diolysis to determine the radical transfer rate in these peptide models. Since efficient electron transfer was observed between tetrafluorotyrosine and tryptophan at a 10 Å distance in the model system, Y122 and W48 at a 7.4 Å distance in RNR likely serve as the radical transport pathway in a bidirectional, sequential electron and proton transfer mechanism. / Chemistry and Chemical Biology

The first examples of the application of biphasic and phase transfer catalysis to early-transition-metal organometallic chemistry are described. These include very efficient and convenient syntheses of the ($\eta\sp5$-cyclopentadienyl)tricarbonylhydridovanadate anion under remarkably mild biphasic conditions. The latter is capable of effecting, under phase transfer conditions, the reduction of halides, sterically encumbered nitro compounds, and the cyclodehydration of $\alpha,\beta$-unsaturated ketones. The reactions may proceed via electron transfer pathways. The use of nickel cyanide under biphasic or phase transfer conditions converts vinyl epoxides to the unsaturated ketones and/or unsaturated $\delta$-hydroxyacids. These results represent the first examples of the use of transition-metal complexes in catalytic-two-phase isomerization of vinyl epoxides. The key catalytic species is believed to be the cyanocarbonylnickel anion, participation of which has been proposed in the reaction mechanism. O-Benzyl oxime ethers were obtained in modest yields by the exposure of benzylic bromides to nitric oxide, a palladium(0) or ruthenium complex as the catalyst, aqueous sodium hydroxide, benzene as the organic phase and a quarternary ammonium salt as phase transfer agent. This provides the first example of a catalytic reaction of halides which involves free or co-ordinated nitric oxide and is also a novel process in the area of a phase transfer catalysis utilizing nitric oxide as a reactant.

No description available.

Liquid phase oxidation of organic substrates under an atmosphere of oxygen catalyzed by metal complexes is reviewed in Chapter 1. Also included in the review are homogeneous and heterogeneous catalysis and free radical autoxidation reactions. The selective catalytic oxidation of aromatic hydrocarbons is described in Chapter 2. Anhydrous CoCl$\sb2$, CoCl$\sb2$/CrCl$\sb3$, or CoCl$\sb2$/ZrCl$\sb4$ were used as the catalysts to oxidize a variety of substrates in the presence of diglyme or a mixture of diglyme and MEK (1:3). These substrates included substituted toluenes and other benzylic hydrocarbons. The reactions afforded fair to excellent yields of the product aldehydes or ketones under mild conditions. It was discovered that the selectivity of these reactions is strongly influenced by the catalyst and the solvent. Selective catalytic oxidation of ethers and cyclic acetals using cobalt chloride is described in Chapter 3. The oxidation of cyclic ethers gives the corresponding lactones in reasonable to excellent yields. However, 2,3-dihydrobenzofuran leads to no reaction. Oxidation of acyclic alkyl benzyl ethers to the corresponding esters afforded yields which were dependent on the stability of the alkyl radicals. The more stable the alkyl radical, the lower the yield of the ester obtained. In addition, oxidation of acyclic and heterocyclic ethers is described. An investigation of the reaction mechanism of this process was also studied. Finally, the oxidation of 2-substituted 1,3-dioxolanes afforded high yields of formate esters and acids. It was found that the presence of catalytic amounts of ZnCl$\sb2$ promoted the rate of oxidation.

No description available.