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Synthesis, Properties, and Reactivity of Pentafluorophenyl

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Synthesis, Properties, and Reactivity of Pentafluorophenyl Synthesis, Properties, and Reactivity of Pentafluorophenyl Substituted Cyclopentadienes and Their Transition Metal Complexes Matthew P. Thornberry Dissertation Submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry Paul Deck, Chairman Mark Anderson John Dillard Brian Hanson Richard Gandour August 3, 2001 Blacksburg, Virginia Keywords: pentafluorophenyl, cyclopentadienyl, substituent effects, polymerization Synthesis, Properties, and Reactivity of Pentafluorophenyl Substituted Cyclopentadienes and Their Transition Metal Complexes Matthew P. Thornberry Abstract (ABSTRACT) Substituent effects in η5-cyclopentadienyl (Cp) transition metal complexes have been intensely studied since the discovery of the first such complex, ferrocene. Modifications of the Cp ligand framework effect changes in the physical properties and chemical reactivity of the coordinated transition metal. This concept is useful when applied to catalysis mediated by Cp complexes, because the performance of the catalyst can be markedly improved using well-chosen ligand substituents. Studies of electronic substituent effects ideally employ a wide range of electron-donating and electron-withdrawing groups. Unfortunately, most of the available electron-withdrawing groups suffer from problems with Cp ligand synthesis, Cp anion stability, and electron-withdrawing group stability under catalytic conditions. This dissertation shows that the pentafluorophenyl (C6F5) substituent is highly electron-withdrawing but avoids all of these problems. Several new C6F5-substituted cyclopentadienes are prepared by the reaction of sodium cyclopentadienide and hexafluorobenzene (C6F6) under varying conditions. Corresponding C6F5-substituted cyclopentadienyl ligands (sodium salts) are obtained upon deprotonating the dienes with NaH. Complexes of Mn(I), Re(I), Fe(II), Co(II), Zr(IV) are synthesized by reacting these ligands with transition metal halides. The acidities of several C6F5- and C5F4N-substituted cyclopentadienes and indenes are measured using 19F NMR spectroscopy. The electron-withdrawing fluorinated aryl groups have a substantial acidifying effect. The identity and number of substituents (C6F5, C5F4N, CH3, and t- Bu), the position of the substituents on the cyclopentadiene, and the intramolecular (vicinal) steric effects also influence acidity. The electron-withdrawing ability of the C6F5 group is also characterized by infrared spectroscopic analysis of substituted CpM(CO)3 (M = Mn(I) and Re(I)) and electrochemical analysis of substituted ferrocenes. X-ray crystal structures of several C6F5-substituted Cp complexes reveal interesting structural motifs, including pi-stacking of the C6F5 substituents, Cp-M bond elongation, and CO-C6F5 interactions. In addition, dynamic Cp-C6F5 and Cp-M rotational barriers are measured by variable temperature NMR spectroscopy. Finally, ethylene polymerizations and ethylene/1-hexene copolymerizations are conducted using C6F5- and C6H5-substituted zirconocene dichlorides as catalysts. Contrary to findings published elsewhere, this study shows that substituent electronic effects induce substantial changes in comonomer incorporation. ii Acknowledgements I would like to thank the following coworkers for synthesizing and supplying several of the unpublished compounds appearing in this dissertation: Nathan Reynolds (27, 30), Caleb Kroll (9, 14a/b), Andrea Warren (11a/b, 12a/b, 13a/b), Mason Haneline (8, 7a/b). Tom Glass provided assistance with variable temperature NMR experiments and 13C NMR analysis of polymer samples. Helpful discussions with Richard Gandour, Eric Hawrelak, Xu Cheng, and Owen Lofthus are also gratefully acknowledged. iii Table of Contents Abstract........................................................................................................................................ii Acknowledgements .......................................................................................................................iii Table of Contents..........................................................................................................................iv List of Figures ...............................................................................................................................vi List of Tables.................................................................................................................................ix Chapter 1: Overview of Cp Substituent Effects ........................................................................1 Introduction .............................................................................................................................1 Steric and Electronic Effects ...................................................................................................2 Electron-Withdrawing Group Complications........................................................................10 History of the Pentafluorophenyl Substituent .......................................................................13 Scope of Dissertation.............................................................................................................14 Chapter 2: Ligand Synthesis .....................................................................................................16 Introduction ...............................................................................................................................16 Chapter Overview..................................................................................................................18 Results & Discussion.................................................................................................................19 Cyclopentadiene Synthesis and Characterization..................................................................19 Crystal Structures of Cyclopentadienes.................................................................................21 Cp Anion Synthesis and Characterization.............................................................................25 Conclusion.............................................................................................................................27 Experimental Section.................................................................................................................28 Chapter 3: Acidities of Perfluoroarylated Cyclopentadienes and Indenes...........................33 Introduction ...............................................................................................................................33 The Acidity of Cyclopentadiene............................................................................................33 Methods of Acidity Determination........................................................................................35 Chapter Overview..................................................................................................................36 Results & Discussion.................................................................................................................37 Method...................................................................................................................................37 Analysis of NMR Equilibrium Data......................................................................................39 Conclusion.............................................................................................................................48 Experimental Section.................................................................................................................49 Chapter 4: Quantitative Analysis of Electronic Effects Using Infrared Spectroscopy.........71 Introduction ...............................................................................................................................71 Electronic Effect Studies .......................................................................................................72 Chapter Overview..................................................................................................................74 Results & Discussion.................................................................................................................76 Complex Synthesis ................................................................................................................76 Infrared Spectroscopic Analysis............................................................................................76 Crystal structures of Piano Stool Complexes ........................................................................78 NMR Spectroscopic Analysis................................................................................................83 Conclusion.............................................................................................................................86 Experimental Section.................................................................................................................87 Chapter 5: Analysis of Substituent Effects In C6F5-Substituted Metallocenes.....................90 Introduction ...............................................................................................................................90 iv Electronic Effect Studies .......................................................................................................91 Steric Effect Studies ..............................................................................................................93
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