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Title:Divalent transition metal centers: the synthesis of new chemical vapor deposition precursors and studies of ethylene polymerization and oligomerization catalysts
Author(s):Steele, Jennifer
Director of Research:Girolami, Gregory S.
Doctoral Committee Chair(s):Girolami, Gregory S.
Doctoral Committee Member(s):Fout, Alison R.; Katzenellenbogen, John A.; Rauchfuss, Thomas B.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):divalent oxidation state
transition metal
chemical vapor deposition
ethylene polymerization
ethylene oligomerization
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-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 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.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Jennifer L. Steele
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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