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Title:Studies of carbon-hydrogen and silicon-hydrogen bond activation by early transition metal complexes
Author(s):Morse, Paige Marie
Doctoral Committee Chair(s):Girolami, Gregory S.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Inorganic
Abstract:Compounds that contain early transition metals particularly titanium, vanadium, and chromium, are used as catalysts for olefin polymerization in the Ziegler-Natta and Phillips processes. The key intermediates in these catalysts are thought to be six-coordinate metal alkyl/olefin complexes. The synthesis and study of early transition metal alkyl complexes as models of these catalytic centers can provide insight into the mechanism of polymerization processes.
The hexamethyl metallates of zirconium and hafnium have been synthesized and the molecular structures of (Li(tmed)) $\sb2$ (ZrMe$\sb6$) (tmed = N,N,N$\sp\prime$,N$\sp\prime$-tetramethylethylenediamine) and (Li(diox)) $\sb2$ (ZrMe$\sb6$) (diox = dioxane) show that the zirconium is in a trigonal prismatic environment. Theoretical calculations on the geometric preferences of d$\sp0$ six-coordinate metal complexes suggest that non-octahedral geometries are favored due to mixing of metal-ligand $\sigma$-orbitals with metal d orbitals.
The reaction of ethyllithium with HfCl$\sb4$ yields the anionic ethylene complex (Li(tmed)) $\sb2$ (HfEt$\sb4$(C$\sb2$H$\sb4$)). A C-H coupling constant of 119 Hz and a C-C distance of 1.50 A are observed for the ethylene ligand. Gas chromatograph/mass spectrometry studies show that this complex is formed by $\beta$-hydride elimination from an ethyl group and subsequent loss of ethane. Anionic homoleptic alkyl complexes have also been synthesized with phenyl, (MPh$\sb6\sp{2-}$), and trimethylsilylmethyl ligands, (M(CH$\sb2$SiMe$\sb3$)$\sb5\sp-$) (M = Zr, Hf).
Six-coordinate adducts of stoichiometry MMe$\sb4$(P-P) (M = T, Zr, Hf; P-P = bidentate phosphines) have been made with dmpe (1,2-bis(dimethylphosphino)ethane), diphos (1,2-bis(dimethylphosphino)benzene), dippe (1,2-bis(diisopropylphosphino)-ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane). Solution and solid-state $\sp1$H and $\sp{13}$C NMR studies of these complexes indicate that the energy barriers for exchange of the methyl groups are very low (9-13 kcal mol$\sp{-1}$). Crystal structure determinations of TiMe$\sb4$(dmpe), TiMe$\sb4$(diphos), and HfMe$\sb4$(dcpe) show that these complexes adopt significantly distorted cis-octahedral geometries.
Homoleptic alkyl complexes of vanadium(III) and chromium(II) have been prepared with the $\beta$-stabilized alkyl groups trimethylsilylmethyl and neopentyl. The low temperature $\sp1$H NMR spectrum of (Li(thf)$\sb2$) $\sb2$ (Cr$\sb2$(CH$\sb2$SiMe$\sb3$)$\sb6$) suggests that agostic interactions may be present in the molecule. The addition of tmed yields the interesting bis-metallacycle (Li(tmed)) $\sb2$ (Cr($\kappa\sb2$-CH$\sb2$SiMe$\sb2$CH$\sb2$)$\sb2$).
In addition, silicon-hydrogen bond activation is observed for the group VI arene compounds ($\eta\sp6$-C$\sb6$H$\sb3$Me$\sb3$)M(CO)$\sb3$ (M = Cr and Mo). The complex Mo(N$\sb2$)$\sb2$(dippe)$\sb2$ reacts with aryl silanes or hydrogen to form MoH$\sb4$(dippe)$\sb2$. The reactivity of these complexes provide insight into hydrosilation processes.
Issue Date:1990
Rights Information:Copyright 1990 Morse, Paige Marie
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9026274
OCLC Identifier:(UMI)AAI9026274

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