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
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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
