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Reactions at coordinated ligands: redox-active ligands and coordination of Lewis acids

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Title: Reactions at coordinated ligands: redox-active ligands and coordination of Lewis acids
Author(s): Ringenberg, Mark R.
Director of Research: Rauchfuss, Thomas B.
Doctoral Committee Chair(s): Rauchfuss, Thomas B.; Girolami, Gregory S.
Doctoral Committee Member(s): Hartwig, John F.; White, M. Christina
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Redox-active ligands Hydrogen oxidation Oxidative Addition of Anhydrides Lewis acid Cyanide
Abstract: The theme of this study is reactivity at the ligand and how remote, relative to the metal, activation affects the metal center. The three approaches were the use of redox-active ligands, ligand protonation and Lewis acid coordination to ligands. The focus is on ligand design rather than substrate or metal optimization with a primary interest in reactivity with dihydrogen. The main thrust of the work has been the investigation of redox-active ligands in which redox occurs at the ligand. Redox-active ligands have been generally a curiosity in organometallic chemistry and have only recently been realized in catalysis. Presented here is one of the first examples of a system that incorporates redox-active ligands as a critical component to the catalysts. The complexes utilizing redox-active ligands became Lewis acidic upon oxidation, similar in behavior to the Noyori type catalyst that became Lewis acidic upon protonation. The catalysts containing redox-active ligand were used for the oxidation of H2. The interest in H2 oxidation is the hope that it will fulfill the need for a new fuel source. This interest has lead to the development of soluble catalyst that can oxidize H2 to protons and electrons, in order to further study the mechanism. Redox-active ligands have lower reorganizational barriers because redox at organic substituents are typically lower than for inorganic centers. Furthermore, redox-active ligands can supplement the electrons/holes transferred from the metal, which can facilitate reactions that require multi-electron transfers. The next theme was the use of borane Lewis acids bound to a coordinated ligand, which dramatically changed the ligands from a donor to an acceptor. This type of reaction at the ligand fundamentally changes the reactivity at the metal, however, the affects are not as dramatic as say substituting the ligand. A largh enough change in ligand polarity can affect oxidation-state at the metal. The oxidation state of a metal becomes very difficult to assign when multiple electronic structures exist between the metal and the ligand. In fact in many ways the concept of metal oxidation state becomes meaningless as the electronic structure of the ligand becomes more complicated. These subtler changes in ligand oxidation state and the affects they have on reactivity at metal have not been as widely explored in catalysis. Remote activation of a metal center through reacts at coordinated ligands will be explored here in. The coordination of boranes to cyanide ligands is well know, however, this theme has not been applied to hydrogenase models. Cyanide is an essential component of the [FeFe]- and [NiFe]-hydrogenase active sites, both enzymes feature two cyanide ligands, however, models using cyanides ligands are plagued by undesirable side reactions such as metal-cyanide bridged polymers and decomposition. The bound boranes are used to simulate the hydrogen bonding found in the enzymes. The complexation of Lewis acids to cyanides offers an advantage over alkylation, in that the inductive affect of the borane can be tuned to better approximate the correct level of hydrogen bonding found in the enzyme.
Issue Date: 2011-05-25
URI: http://hdl.handle.net/2142/24205
Rights Information: Copyright 2011 Mark R. Ringenberg
Date Available in IDEALS: 2011-05-25
Date Deposited: 2011-05
 

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