Preparation and Characterization of Hydrogenase Enzyme Active Site-inspired Catalysts: The Effects of Alkyl Bulk and Conformer Strain as Studied by Photoelectron Spectroscopy, Electrochemistry and Computational Methods

Persistent Link:
http://hdl.handle.net/10150/194329
Title:
Preparation and Characterization of Hydrogenase Enzyme Active Site-inspired Catalysts: The Effects of Alkyl Bulk and Conformer Strain as Studied by Photoelectron Spectroscopy, Electrochemistry and Computational Methods
Author:
Petro, Benjamin J.
Issue Date:
2009
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
A series of alkyldithiolatodiironhexacarbonyl complexes of the form &mu:-(RS2)Fe2(CO)6, where RS2 is: 1,2-ethanedithiolate (eth-cat), cis-1,2-cyclopentanedithiolate (pent-cat), cis-1,2-cyclohexanedithiolate (hex-cat), and 2-exo,3-exo-bicyclo[2.2.1]heptanedithiolate (norbor-cat), are reported. These complexes display structures and catalytic behavior toward production of molecular hydrogen with similarities to the active site of the diiron hydrogenase enzymes. Hydrogen production is desirable as an alternative fuel source and these catalysts are capable of producing H2 in the presence of weak acid under electrochemical conditions. Through understanding of the factors which control the catalytic activity of these catalysts it may be possible to contribute to the development of a hydrogen fuel economy.Significant scan-rate dependence under electrochemical conditions is observed, resulting in an initial 1-to-2 electron reduction depending on how quickly the singly reduced species can reorganize. The rate of this reorganization directly corresponds to the internal strain within the system and can be ranked in the following order of increasing rate of reorganization: pent-cat < norbor-cat < eth-cat < hex-cat. Additionally, these catalysts all successfully catalyze protons to molecular hydrogen under electrochemical conditions in the presence of acetic acid via an ECEC catalytic mechanism, where, E is an electrochemical step (reduction) and C is a chemical step (protonation).Density functional theory computations support the reported catalytic processes by calculating physically observable quantities, such as: pKa values, reduction potentials, adiabatic ionization energies and carbonyl stretching frequencies in the infrared (IR) region. These quantities were used to suggest reasonable reactive intermediates within the catalytic cycle. The electronic structure of each catalyst was examined using photoelectron spectroscopy and the global minimum cationic structure, in all cases, involves a structure with a bridging carbonyl ligand, akin to that of the enzyme active site.The most significant outcome of this work is the unprecedented diiron center rotation upon reduction. As conformational strain involving the dithiolate ligand increases, the rate of reorganization of the anion increases leading to cleavage of an iron-sulfur bond to provide an alternative protonation site, a key step toward molecular hydrogen formation. This site is less basic than the unrotated form and helps evolve H2 with thermodynamic favorability.
Type:
text; Electronic Dissertation
Keywords:
catalysis; DFT; electrochemistry; hydrogenase; organometallics; photoelectron spectroscopy
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Chemistry; Graduate College
Degree Grantor:
University of Arizona
Advisor:
Lichtenberger, Dennis L

Full metadata record

DC FieldValue Language
dc.language.isoENen_US
dc.titlePreparation and Characterization of Hydrogenase Enzyme Active Site-inspired Catalysts: The Effects of Alkyl Bulk and Conformer Strain as Studied by Photoelectron Spectroscopy, Electrochemistry and Computational Methodsen_US
dc.creatorPetro, Benjamin J.en_US
dc.contributor.authorPetro, Benjamin J.en_US
dc.date.issued2009en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractA series of alkyldithiolatodiironhexacarbonyl complexes of the form &mu:-(RS2)Fe2(CO)6, where RS2 is: 1,2-ethanedithiolate (eth-cat), cis-1,2-cyclopentanedithiolate (pent-cat), cis-1,2-cyclohexanedithiolate (hex-cat), and 2-exo,3-exo-bicyclo[2.2.1]heptanedithiolate (norbor-cat), are reported. These complexes display structures and catalytic behavior toward production of molecular hydrogen with similarities to the active site of the diiron hydrogenase enzymes. Hydrogen production is desirable as an alternative fuel source and these catalysts are capable of producing H2 in the presence of weak acid under electrochemical conditions. Through understanding of the factors which control the catalytic activity of these catalysts it may be possible to contribute to the development of a hydrogen fuel economy.Significant scan-rate dependence under electrochemical conditions is observed, resulting in an initial 1-to-2 electron reduction depending on how quickly the singly reduced species can reorganize. The rate of this reorganization directly corresponds to the internal strain within the system and can be ranked in the following order of increasing rate of reorganization: pent-cat < norbor-cat < eth-cat < hex-cat. Additionally, these catalysts all successfully catalyze protons to molecular hydrogen under electrochemical conditions in the presence of acetic acid via an ECEC catalytic mechanism, where, E is an electrochemical step (reduction) and C is a chemical step (protonation).Density functional theory computations support the reported catalytic processes by calculating physically observable quantities, such as: pKa values, reduction potentials, adiabatic ionization energies and carbonyl stretching frequencies in the infrared (IR) region. These quantities were used to suggest reasonable reactive intermediates within the catalytic cycle. The electronic structure of each catalyst was examined using photoelectron spectroscopy and the global minimum cationic structure, in all cases, involves a structure with a bridging carbonyl ligand, akin to that of the enzyme active site.The most significant outcome of this work is the unprecedented diiron center rotation upon reduction. As conformational strain involving the dithiolate ligand increases, the rate of reorganization of the anion increases leading to cleavage of an iron-sulfur bond to provide an alternative protonation site, a key step toward molecular hydrogen formation. This site is less basic than the unrotated form and helps evolve H2 with thermodynamic favorability.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjectcatalysisen_US
dc.subjectDFTen_US
dc.subjectelectrochemistryen_US
dc.subjecthydrogenaseen_US
dc.subjectorganometallicsen_US
dc.subjectphotoelectron spectroscopyen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineChemistryen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorLichtenberger, Dennis Len_US
dc.contributor.committeememberEnemark, John Hen_US
dc.contributor.committeememberKukolich, Stephen Gen_US
dc.contributor.committeememberSanov, Andreien_US
dc.contributor.committeememberZheng, Zhipingen_US
dc.identifier.proquest10298en_US
dc.identifier.oclc659750918en_US
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