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Reduced Indenyl and Cyclopentadienyl Group 4 Complexes: Fundamental Reactivity Studies and Dinitrogen Activation

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Reduced Indenyl and Cyclopentadienyl Group 4 Complexes: Fundamental Reactivity Studies and Dinitrogen Activation REDUCED INDENYL AND CYCLOPENTADIENYL GROUP 4 COMPLEXES: FUNDAMENTAL REACTIVITY STUDIES AND DINITROGEN ACTIVATION A Dissertation Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Doris Pun February 2010 © 2010 Doris Pun REDUCED INDENYL AND CYCLOPENTADIENYL GROUP 4 COMPLEXES: FUNDAMENTAL REACTIVITY STUDIES AND DINITROGEN ACTIVATION Doris Pun, Ph. D. Cornell University 2010 Indenyl zirconocene dinitrogen complexes were synthesized from the direct 9 activation of N2 by isolable cyclopentadienyl η indenyl zirconocene sandwich 5 9 complexes, (η -C5Me5)(η -C9H5-1-R-3-Me)Zr (R = Me, CHMe2, CMe3), or from the alkali metal reduction of the corresponding zirconocene dihalide under dinitrogen. In the former, dimeric weakly activated end-on dinitrogen complexes with η5,η2 indenyl 5 2 2 5 1 1 ligands, [(η -C5Me5)Zr]2 [μ2-η ,η -{4,5-(η -C9H5-1-R-3-Me)}2](μ2-η ,η -N2), were synthesized. In the latter, dimeric modestly activated side-on, end-on dinitrogen 5 4 5 5 4 5 complexes with an η ,η indenyl ligand, [(η -C5Me5)Zr]2 [μ2-η ,η -(η -C9H5-1-R-3- 2 1 1 1 Me)](μ2-η ,η -N2) (μ2-η ,η -Cl), were isolated. The alkali metal reduction of a series of bis(indenyl) zirconocene dihalides under N2 furnished dimeric strongly activated 5 5 end-on dinitrogen dianions, [(η -C9H5-1-CHMe2-3-Me)(η -C9H5-1-CHMe2-3- 1 1 Me)Zr(NaX)]2(μ2-η ,η -N2) (X = Cl, Br, I), which readily hydrogenate yielding the corresponding zirconocene hydrazido hydride complexes with a side-on bound 5 5 2 2 hydrazide, [(η -C9H5-1-CHMe2-3-Me)(η -C9H5-1-CHMe2-3-Me)ZrH]2(μ2-η ,η - N2H2). Due to the propensity of hafnium to attain its highest oxidation state, dinitrogen activation chemistry was not observed for indenyl hafnocene complexes, although several η6,η5 bis(indenyl)hafnium (II), along with hafnocene (III) species were isolated. The reduction of a series of ansa-zirconocene complexes with [CH2CH2], [Me2Si] and [Me2C] bridges was also explored. Zirconocene diiodide 5 compounds with Me2E(η -C5H3-3-R’)2 (E = Si, R’ = CMe3; E = C, R’ = SiMe3, CMe3) ligands furnished strongly activated side-on bound dinitrogen complexes when treated with excess 0.5 % sodium amalgam under an atmosphere of N2. Exposure of dihydrogen furnished the expected zirconocene hydrazido hydride complexes. However, the hydrazide readily underwent dehydrogenation, even under one 5 atmosphere of H2, affording hydrido zirconocene dinitrogen complexes Me2E(η - 2 2 C5H3-3-R’)2ZrH]2(μ2-η ,η -N2). In the course of developing zirconium-promoted Pauson-Khand chemistry, the 9 addition of ketones to η indenyl zirconocene complexes showed divergent product formation: either yielding a zirconocene enolate hydride or insertion of the carbonyl into the indenyl ligand. In the former, a haptotropic rearrangement of the indenyl ligand from η9 coordination to η5 occurred, while in the latter rearrangement to η1, η3, 5 6 η or η . Mechanistic studies demonstrated that pKa, steric bulk and concentration of the ketone reagent are important to the reactivity. BIOGRAPHICAL SKETCH Doris (Sau-Wah) Pun was born December 20, 1983 in New York, New York. She graduated from Brooklyn Technical High School majoring in Chemistry before attending New York University, where she also majored in Chemistry, double minoring in Computer Science and Computer Applications. During her undergraduate studies, she used Resonance Raman spectroscopy to study both the DNA repair enzyme DNA photolyase and the FAD containing glucose oxidase under the supervision of Professor Johannes Schelvis. In 2005, she started graduate school at Cornell University and joined the Chirik group, where she has worked on dinitrogen activation with indenyl and ansa-linked cyclopentadienyl group 4 complexes, following the work of both Chris and Tammy Bradley. iii Wai Ming Lau & Chun Tim Pun Julia S. Pun Scott M. Gruenbaum iv ACKNOWLEDGMENTS I am truly indebted to my advisor, Paul Chirik, for taking a chance on me by allowing me to join his group and guiding me throughout my years at graduate school. Before joining his group, I had little experience with synthetic chemistry and lacked the ability to critically analyze my data. Thank you for not letting me stray when I needed guidance, in the laboratory and intellectually. To my committee members, Pete Wolczanski and Geoff Coates, and Dave Collum, thanks for lending your expertise and insightful criticisms. I believe I have matured as a professional scientist. Both Chris and Tammy initiated my research projects, figuring out a lot of the difficult chemistry. I also have to thank them both, along with Marco, for training me to do air sensitive organometallic chemistry, allowing me to synthesize their truly unique complexes. To the other members of the Group 4 Dinitrogen Fixation project, Wes, Hannah, Don, David and Scott L., thank you for the insightful conversions. Don, your generous collaboration in synthesizing the Dp and Gp ligands have been especially appreciated. Thank you Mandy and Ryan for being there with me these past years when we were the oldest. Looking back it was scary, especially regenerating the hydrogen column and the Toepler pump (thank you Elliott for your assistance there). My fellow box mates, Ryan, Aaron, Jon, Marco, Scott and Chantal, I appreciate all the help with maintaining our box. I still remember having to overhaul our box vacuum pump and changing our catalyst. Thank you Jon and Aaron for help setting up our laboratory in G54. I am indebted to Kevin for synthesizing some of the 1,6 diene substrates. I also thank the rest of the members of the Chirik group, past and present – Suzanne, Sarah, Helen, Brad, Ignacio, Crisita, Andrew, etc. – for all the support, laughter, candy and cookies. Thank you to everyone who edited my dissertation. v I also need to especially thank both the Wolczanski and Collum groups for chemicals, air sensitive handling techniques and kinetic data collection, especially Emily, Elliott, Brenda and Alex. Emily and Nina were also wonderful roommates and friends. This dissertation would also not be possible without the help of Emil for solving all my crystal structures, several of which were key to characterizing the compounds. Ivan has been especially helpful in teaching me to solve my indenyl zirconium complexes by a combination of 1D and 2D NMR spectroscopy. Thank you Dave Wise for being an excellent glass blower and helping us with our new labs and teaching us about glass maintenance. Finally, this thesis is dedicated to my parents, my sister and Scott for all their support over the last four and a half years. Without my parents I would not have made it this far, academically, nor would I have been short on rice. To my sister, I thank her for being my guiding light, for helping to make myself better in order to reach her standards and for hauling rice to me from home. Scott has made my transition to graduate school, away from my family for the first time, a lot smoother. Thank you for carrying the rice. vi TABLE OF CONTENTS Biographical Sketch iii Dedication iv Acknowledgements v Table of Contents vii List of Figures x List of Tables xiv Chapter 1: Activation of Dinitrogen with Indenyl Zirconium Complexes: Different Modes of N2 and Indenyl Coordination Abstract 1 Introduction 2 Results and Discussion 7 Conclusions 40 Experimental 40 References 70 Chapter 2: Synthesis and Reactivity of Reduced Bis(Indenyl) Hafnium Compounds Abstract 75 Introduction 75 Results and Discussion 78 Conclusions 101 vii Experimental 102 References 120 Chapter 3: C-C Bond Formation with η9 Indenyl Zirconium Sandwiches: Cycloaddition of Dienes and Insertion of Ketones Abstract 124 Introduction 125 Results and Discussion 127 Conclusions 158 Experimental 159 References 194 Chapter 4: Fundamental Studies of Dinitrogen Activation and Reduction with ansa-Zirconocene Complexes Abstract 198 Introduction 199 Results and Discussion 202 Conclusions 220 Experimental 221 References 238 viii Chapter 5: Catalytic Dehydrogenation of Substituted Amine-Boranes with Isolable Titanocene and Zirconocene Sandwich and Titanocene Dinitrogen Complexes Abstract 242 Introduction 242 Results and Discussion 244 Conclusions 253 Experimental 254 References 259 Appendix A: EPR Data 262 Appendix B: Kinetic Data 264 Appendix C: Crystal Structure Data 265 Appendix D: Permissions for Figure Reproduction 350 ix LIST OF FIGURES 1.1. Modes of dinitrogen coordination. 2 1.2. Reactivity similarity and differences between a weakly and a strongly activated zirconocene dinitrogen complexes. 3 1.3. Preparation of η9, η5 bis(indenyl) zirconocene sandwich complexes via reductive elimination (top left) or alkali metal reduction (top right). η9, η5 to η5, η9 ring interconversion via an η5, η5 intermediate (bottom). 4 1.4. Proposed catalytic cycle for the hydrogenation of dinitrogen with bis(indenyl) zirconium compounds. 5 1.5. Examples of N2 activation from (left) isolable reduced complexes and (right) the reductive elimination of metal hydride precursors. 7 1.6. The Cp* indenyl zirconocene dihalides reduced in this study. 8 1.7. Isolable Cp* indenyl zirconocene sandwiches prepared. 8 1.8. Molecular structure for 8 at 30 % probability elipsoids. Hydrogen atoms omitted for clarity. 10 1.9. Molecular structure of 62N2 at 30 % probability ellipsoids top view (top), side view (bottom). 13 1.10. Possible resonance structures for 32-N2, 62-N2 or 72-N2. 14 1.11. NOESY NMR spectrum of 62-N2 in benzene-d6. 16 1.12. Molecular structures of the major (left) and minor (right) isomers of 16-N2Cl at 30 % probability ellipsoids. 19 x 1.13. Possible resonance structures for 16-N2Cl and 17-N2Cl. 20 1.14. Possible mechanism for the formation of 6-THI-(H)I from the reduction of 6-THI-I2.
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