Browsing by Subject "Oxidation-reduction reaction."
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Item Chemistry, electrochemistry and electron transfer induced reactions of cobalt complexes with fluorinated ligands.(2008-03-03T17:20:55Z) Gunawardhana, Kihanduwage N.; Gipson, Stephen L. (Stephen Lloyd); Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.The chemical or electrochemical reduction of the trifluoroacetyl complex CF3COCo(CO)3PPh3 involves a single electron transfer yielding trifluoromethyl radical and an anionic cobalt carbonyl complex. The mechanism is proposed to involve electron transfer followed by initial dissociation of either a carbonyl or phosphine ligand from the 19-electron [CF3COCo(CO)3PPh3 ]- anion. The resulting 17-electron intermediate undergoes subsequent one-electron reductive elimination of trifluoromethyl radical by homolytic cleavage of the carbon-carbon bond of the trifluoroacetyl group. The CF3· radical can be trapped by either benzophenone anion, forming the anion of [a]-(trifluoromethyl)benzhydrol, or Bu3SnH, yielding CF3H. The final organometallic product is an 18-electron anion, either [Co(CO)4]- or [Co(CO)3(PPh3)]-, depending upon which ligand is initially lost. The chemical or electrochemical reduction CF3Co(CO)3PPh3 is a two-electron process involving heterolytic cobalt-carbon bond cleavage to yield trifluoromethyl anion and cobalt carbonyl anions. The trifluoromethyl anion rapidly decomposes to fluoride and difluorocarbene. This carbene may dimerize to form C2F4. The unstable fluoro carbene can also be trapped by cyclohexene. The mechanism proposed for the reduction of C6F5Co(CO)3PPh3 involves a homolytic cobalt-carbon bond cleavage to form C6F5[bullet] radical. The resultant C6F5[bullet] radical abstracts hydrogen or deuterium from the solvent or trace amounts of water to produce C6F5H or C6F5D. With an excess of reducing agent this C6F5[bullet] radical can be further reduced to C6F5- anion before forming pentafluorobenzene by protonation. The inorganic fragment, the 18-electron [Co(CO)3PPh3]- anion, may participate in a ligand exchange reaction to form [Co(CO)4]-. In addition, interesting reactivity was observed between C6F5Co(CO)3PPh3 and tin hydrides, deuterides and chlorides without any reducing agents. We have demonstrated that ligand replacement reactions can be used for the synthesis of new cobalt-NHC complexes with fluorinated alkyl, acyl and aryl ligands. In addition, the X-ray crystal structure of CF3COCo(CO)3PPh3 was obtained to compare the bond lengths and bond angles with other related compounds. An unusual Co-C(acyl) bond length was observed for CF3COCo(CO)3PPh3. Considering the bond lengths of other alkyl and acyl complexes, it can generally be argued that the position of the alkyl/acyl equilibrium varies with the Co-C(alkyl/acyl) bond length.Item Exploring an organometallic redox complex for the remediation and reclamation of ionic contaminants.(2007-02-21T20:46:04Z) Becker, Christopher.; Chambliss, C. Kevin.; Chemistry and Biochemistry.; Baylor University. Dept. of Chemistry and Biochemistry.Ion exchange for the removal of ionic contaminants is typically considered a mature technology. However, recovery of extracted contaminants in a minimal volume of secondary waste and regeneration of ion exchange materials for use in subsequent extraction cycles continue to represent significant challenges for the separation chemist. Substantial progress in overcoming these barriers has been made by employing redox-switchable organometallic complexes for the extraction and recovery of ionic contaminants. As compared with traditional ion exchange, this approach offers an alternative redox-based stripping mechanism and affords quantitative recovery of a target contaminant in a minimal volume of secondary waste. In contrast to previous redox extraction and recovery processes (R2ER) which focus on selectivity and design of an organometallic extractant, we have developed a novel redox process with potential for adaptation to any existing ion exchanger. The lipophilic metal complex Fe(η5-C5H5)(η5-(3)-1,2-C2B9H9(n-C12H25)2) has been used in combination with tetra-n-heptylammonium salts to demonstrate an improved R2ER paradigm for aqueous anions. Liquid-liquid extraction of a generic contaminant was shown to follow a traditional ion-exchange equilibrium. Subsequent recovery of the extracted contaminant as a compact solid was promoted by treatment of the water-immiscible phase with zinc powder, and the spent solvent was regenerated by treatment with an aqueous oxidant. Using this method, 100 L of industrial waste simulant can be reduced to 25 mL of secondary waste in a single reductive step. As compared to previously reported R2ER processes for anions, the new process alleviates potential stability concerns related to the presence of charge dense anions and enables efficient regeneration of a variety of counter ions in tetra-n-heptylammonium ion pairs, including NO3−, Br−, Cl−. Repeatable extraction/regeneration cycles demonstrated no loss in extraction efficiency. Extracted contaminants were recovered in yields ranging from 97% to 100%. R2ER regeneration of OH is also discussed. Liquid-liquid extraction studies demonstrating the feasibility of the approach and extension of this chemistry to a polymer-supported format is presented.