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© Copyright 2013 Jennifer L. Steele © Copyright 2013 Jennifer L. Steele DIVALENT TRANSITION METAL CENTERS: THE SYNTHESIS OF NEW CHEMICAL VAPOR DEPOSITION PRECURSORS AND STUDIES OF ETHYLENE POLYMERIZATION AND OLIGOMERIZATION CATALYSTS BY JENNIFER L. STEELE 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, 2013 Urbana, Illinois Doctoral Committee: Professor Gregory S. Girolami, Chair Assistant Professor Alison R. Fout Professor John A. Katzenellenbogen Professor Thomas B. Rauchfuss Abstract Volatile transition metal complexes that contain boron hydride ligands are desirable for their potential as precursors for metal diboride films for microelectronics applications. Recently our group has discovered a new class of potential precursors in the metal complexes of the chelating borohydride, N,N-dimethylaminodiboranate (DMADB). To date, attempts to synthesize homoleptic complexes of the late transition metals have afforded intractable mixtures, likely the result of overreduction of the metal center. This work has focused on the synthesis and characterization of heteroleptic complexes of the late transition metals that contain both DMADB and 1,2,3,4,5,-pentamethylcyclopentadienyl ligands. The reaction of metal complexes of the form [Cp*MX]n, where Cp* is 1,2,3,4,5,- pentamethylcyclopentadienyl, M = Cr, Fe, Co, or Ru, and X = Cl or I with sodium dimethylaminodiboranate (NaDMADB) in diethyl ether affords the divalent complexes [Cp*M(DMADB)]. Additionally, the analogous vanadium compound [Cp*V(DMADB)] can be synthesized from the reduction of [Cp*VCl2]3 with NaDMADB in diethyl ether. All of these compounds are volatile under static vacuum at room temperature, but are also thermally sensitive; the iron and ruthenium derivatives decompose at room temperature over a day. These complexes exhibit a fascinating variety of binding modes of the DMADB ligand driven by the electronic structure of the metal center. The green compound [Cp*V(DMADB)] exhibits a κ2, κ2 binding mode of the DMADB ligand; this is the binding mode normally seen for the homoleptic early transition metal DMADB compounds. In the blue compound [Cp*Cr(DMADB)], the DMADB ligand is bound in a κ1, κ1 fashion. This binding mode of the DMADB ligand can be rationalized on the basis of the number of orbitals available on the high- ii spin chromium(II) center. The iron and ruthenium complexes are isostructural with the DMADB ligand bound in an asymmetric κ1, κ2 fashion, giving a pseudo-octahedral structure. Both of these complexes are diamagnetic and are low spin d6. The red compound [Cp*Co(DMADB)] is also isostructural with the iron and ruthenium compounds. In addition to these DMADB compounds, the synthesis and characterization of the homoleptic aminodiboranate complex, Cr[(H3B)2NEtMe]2 and the titanium(III) species, Cp2Ti(DMADB) is detailed; both of these form a κ1, κ1 binding mode for the DMADB ligand. This work also explores the reactivity of transition metal DMADB species, focusing on the homoleptic titanium(II) and chromium(II) species, Ti(DMADB)2 and Cr(DMADB)2, as well as, the heteroleptic Cp*Cr(DMADB) species discussed previously. The reactivity of these complexes with ethylene when activated with an aluminum alkyl co-catalyst will be of primary focus. The N,N-dimethylaminodiboranate compounds Ti(DMADB)2, Cr(DMADB)2, and Cp*Cr(DMADB) are active ethylene polymerization catalysts in the presence of an aluminum co-catalyst. These complexes also produce variable amounts of ethylene oligomers. Cp*Cr(DMADB) produces the largest amount of polyethylene, giving 35,000 turnovers when activated with AlEt3. Melting point data suggests that the product is a high density polyethylene. The reaction of ethylene with Ti(DMADB)2 in the presence of AlEt3 produces 107 moles of 1- butene per mole of catalyst. This thesis also explores the reactivity of a series of titanium(II) species with ethylene in the presence of an aluminum alkyl catalyst. Since the discovery of the Ziegler-Natta type catalysts in the mid-twentieth century, titanium compounds have been of interest as ethylene polymerization catalysts and more recently as selective oligomerization catalysts. Previously, our group had shown that the titanium(II) complexes, TiX2(dmpe)2, where X = Cl, BH4, or Me iii and dmpe = 1,2-bis(dimethylphosphino)ethane, dimerize ethylene to form 1-butene in NMR scale reactions at low pressures. Similarly, the cyclopentadienyl titanium(II) complexes, CpTiX(dmpe)2, where X = Me, H, or Cl, are catalysts for the dimerization and trimerization of ethylene (the latter generating only branched products). This work examines these systems at higher ethylene pressures and in the presence of an aluminum based co-catalyst. These titanium(II) compounds are active as polymerization catalysts and generate small amounts of oligomeric side products. The compounds TiX2(dmpe)2 and CpTiX(dmpe)2, when activated with MAO (methylaluminoxane), produce large amounts of a high molecular weight polyethylene, with the largest turnover number being 18,000 for Ti(BH4)2(dmpe)2. Reactions performed in the presence of AlEt3 produce smaller quantities of polyethylene, and in some cases also produce small amounts of oligomers, mainly branched six carbon species. There is some evidence for the oxidation of the titanium(II) species to a titanium(III) species that acts as the active catalyst, and the novel compound [TiCl2(dmpe)2][B(C6H3(CF3)2)4] was synthesized and found to produce similar catalytic results as the titanium(II) analog when activated with MAO and AlEt3. iv Acknowledgments There are many people that I would like to thank for their support and help thoughout my graduate school career. First, I thank my advisor Professor Gregory Girolami for all of his encouragement, patience, and invaluable guidance on how to properly conduct research. I also thank my committee members: Professors Thomas Rauchfuss, Patricia Shapley, John Katzenellenbogen, and Alison Fout for all of their constructive criticism of my work. I would like to thank the past and present Girolami Group members: Charity Flener Lovitt, Scott Daly, Brian Bellott, Andrew Dunbar, Luke Davis, Justin Mallek, Noel Chang, Peter Sempsrott, Joe Macor, Brian Trinh, Tracey Codding, and Mark Nesbit. Thank you all for the useful discussions, for listening to my ideas, and for your advice on my chemistry. Also, thank you to my undergraduate researcher, Kristina Falk, for continuing the work on the titanium(II) catalysts by testing these catalysts’ reactivity with propylene. Thanks to all the facilities that helped make this thesis possible. Thank you especially to Marie Keel in the microanalysis lab for always sending me data promptly. Thanks also to Dr. Haijun Yao for his assistance with obtaining MS and DSC data. I am also grateful to Dr. Alex Ulanov for his training and advice on GC-MS. Thank you also to Danielle Gray, Amy Fuller, and Jeff Berkte for their assistance in obtaining the crystal structures in this thesis. I am also very grateful to Danielle for many useful conversations about crystallography. Thanks to Bill Boulanger for advice on Parr reactor maintenance, as well as, advice on doing high pressure reactions, and to Dr. Mark Nilges for assistance obtaining EPR. Thanks also to the machine, glass, and electrical shops. Also, thank you to the IMP office not only for all of their v organization and help with paperwork and scheduling, but also for their constant positive view of life and many good conversations. I am also grateful to all of the professors for which I have been allowed to TA. Thanks to Christine Yerkes, Tom Rauchfuss, Pat Shapley, Greg Girolami, and John Overcash. The experience that you have given me in teaching and the advice and guidance that you have provided has been invaluable to me in both deciding on a career path and effectively pursuing it. Finally, I would like to thank my friends and family, without your support this work would truly not be possible. I thank my family for their unconditional love and support. I also thank the friends I have made in graduate school for both their support and making graduate school much more fun. Thanks to Maria Carroll for being a good friend and roommate. I have enjoyed all of our parties and discussions. I also thank Claire Tornow for all the fun crafting during the writing of this thesis, and for working down the hall and breaking up the day with fun discussions. Finally, I would like to thank my dog Mollycat for her unconditional support and loyalty during the most stressful of times. vi Table of Contents Chapter 1: Introduction: A brief review of first row transition metals in their divalent oxidation state.....................................................................................................................1 Introduction........................................................................................................................1 Prevalence of the divalent oxidation state among first row transition metals.............2 Common starting materials for divalent metal complexes and routes to these compounds..........................................................................................................................4 Conclusions.......................................................................................................................11 References.........................................................................................................................11
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