Reactions at Coordinated Ligands Redox-Active Ligands and Coordination of Lewis Acids

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Text of Reactions at Coordinated Ligands Redox-Active Ligands and Coordination of Lewis Acids

  • REACTIONS AT COORDINATED LIGANDS:

    REDOX-ACTIVE LIGANDS AND

    COORDINATION OF LEWIS ACIDS

    BY

    MARK R. RINGENBERG

    DISSERTATION

    Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry

    in the Graduate College of the University of Illinois at Urbana-Champaign, 2011

    Urbana, Illinois

    Doctoral Committee: Professor Gregory S. Girolami, Chair Professor Thomas B. Rauchfuss, Director of Research Professor John F. Hartwig Professor M. Christina White

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    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

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    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.

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    To my genetic and gay family

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    ACKNOWLEDGMENT

    Science has achieved some wonderful things, of course, but Id far rather be

    happy than right any day Douglas Adams

    First, I would like to acknowledge my advisor Prof. Thomas Rauchfuss,

    who always kept the fire lit under my ass, whose patience with my spelling and

    writing was critical, and who was always willing to explore exciting ideas with

    me. To my peers in the Rauchfuss Group as Bilbo Baggins best put it I like half

    of you half as well as I should like, and I like less than half of you half as well as

    you deserve!. I would like to acknowledge my committee, Prof. Girolami, Prof.

    Hartwig, and Prof. White for their continued support and advise throughout the

    years.

    My parents, Dr. and Dr. Ringenberg, who were a consistent and loving

    source of motivation and without whose love and support has been critical to

    my enduring success and perseverance; who have come leaps and bounds in

    supporting me and who I have always been and have become. To my best

    friend Kirby, who has been like a sister to me for all these many years.

    My friends in the department who have become an important part of my

    life; especially to all the lovely ladies in my life. Kate Vigour for always willing to

    listen and for keeping me motivated going to yoga all those years, I know you

    will make a wonderful mother because you have had quite a bit of practice

    already. Renee Musto for saving me so many times when I was TAing Chem 317

    and for going out to coffee with me at the drop of a hat. Ashley Galvan for going

    to the gym with me and for making me feel not so alone in my craziness. Rachel

    Orheim for being the best thing to come from my visit to PNNL and for really

    being awesome in easing me out of my shell. The lovely ladies of the X-ray lab

    have been important to the preservation of remaining sanity. Danelle Grey was

    always there to listen and talk to even if it was not about crystallography. Amy

    Fuller for the use of her desk to lay down on when I needed to give my back a

    rest and for being someone to truly kvetch with at any time.

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    Without those of you outside of the department I may have forgotten

    about a world without science. Karen E. H. Messina will always be important

    part of my life and without whose advice I would be lost. Colin Levin for always

    being able to talk me down from the verge of utter panic, and whose friendship

    and love I value so much. Ben Robinson for being grounded. Jake Lahne for

    always being an important part of my life for all these years. Jake and Kieran for

    putting up with me during my prelim exam.

    Graduate school could not have been made possible without coffee and

    the IMP office. Without Connie Knight, Beth Myler and Theresa Struss, this

    experience would have been very dry. The ladies of the IMP office are some the

    most caring, helpful, and kind persons Ive met. Nearly every morning was

    greeted with cheerful conversation and constant motivation.

    And really the last year and a half would have not happened if it had not

    been for the support and love of my boyfriend, Aaron Finke. His knowledge of

    chemistry has made me smarter. He has put up with me in the worst of times

    and the best of times. Without his help I do not know how I would have made it

    though breaking my back. He kept my spirits up and kept pushing me forward. I

    look forward to our adventure in Switzerland.

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    TABLE OF CONTENTS

    Chapter 1. Reactions at Coordinated Ligands ............................................. 1

    Chapter 2. Amidophenolate Redox-Active Ligands: Synthesis and Characterization ............................................................................................. 23

    Chapter 3. Oxidation of H2 with Metal Catalyst Containing a Redox-active Ligand .............................................................................................................. 50

    Chapter 4. Electrochemistry: Oxidation Induced Lewis Acidity of Redox-Active Ligand Metal Complexes .................................................................... 70

    Chapter 5. Proton-Enhanced Lewis Acidity of Iridium Complexes Containing Redox-Active Amidophenolates ................................................ 90

    Chapter 6. Oxidative Addition of A Diphosphine-Anhydride: Elegant Approach to Installing Four Ligands ............................................................ 111 Chapter 7. Hydrogenase Models Containing Lewis Acid Capped Cyanides ............................................................................................ 133

    Appendix A. pKa Calculations ........................................................................ 150

    Appendix B. Evans Method Calculation ...................................................... 152

    Appendix C. Digisim Guide ............................................................................ 153

    Appendix D. Crystal Tables ............................................................................ 171

    Authors Biography ........................................................................................ 233

  • 1

    Chapter 1. Reactions at Coordinated Ligands

    1.1. Reactions at Coordinated Ligands

    The classes of reactions in coordination chemistry are: substitution, addition, ligand rearrangement, metal redox, and reactions at coordinated ligands. While the first four classes constitute a majority of coordination chemistry over the past hundred y

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