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Title:Observation of the first titanium alkyl/alkyl complexes: The key intermediate in the Ziegler-Natta mechanism for the polymerization of alkenes
Author(s):Spencer, Michael Donald
Doctoral Committee Chair(s):Girolami, Gregory S.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Inorganic
Abstract:The complexes trans-TiX$\sb2$(dmpe)$\sb2$ (X = BH$\sb4,$ Br, Cl, Me, and OPh) react with alkenes to give two diamagnetic products: the mononuclear and dinuclear complexes trans-TiX$\sb2$ (alkene)(dmpe)($\eta\sp1$-dmpe) and trans- (TiX$\sb2$(alkene)(dmpe)) $\sb2(\mu$-dmpe). A crystallographic study of (TiCl$\sb2$(C$\sb2$H$\sb4)$(dmpe)) $\sb2(\mu$-dmpe) confirms the trans geometry and shows that the ethylene ligand resides in the plane described by the phosphine ligands. The complexes TiMe$\sb2$(C$\sb2$H$\sb4)$(dmpe)($\eta\sp1$-dmpe) and (TiMe$\sb2$(C$\sb2$H$\sb4)$(dmpe)) $\sb2(\mu$-dmpe) are the first structural models of the key titanium alkyl/alkene intermediate in the Cossee mechanism for Ziegler-Natta catalysis. Above $-$20$\sp\circ$C, the TiX$\sb2$(dmpe)$\sb2$ complexes catalyze the dimerization of ethylene to 1-butene with turnover rates of up to 440 hr$\sp{-1};$ the rates of ethylene dimerization vary according to the nature of the X group (Me $>$ BH$\sb4 >$ Cl $>$ OPh). Most of the evidence favors a mechanism involving the oxidative coupling of two coordinated ethylenes followed by $\beta$-hydrogen abstraction and reductive elimination. Interestingly, there is no insertion of ethylene into the Ti-CH$\sb3$ bonds. This behavior is probably a general feature of d$\sp2$ metal centers.
Addition of butadiene or 1,4-diphenyl-1,3-butadiene to TiMe$\sb2$(dmpe)$\sb2$ affords complexes of stoichiometry TiMe$\sb2(\eta\sp4$-C$\sb4$H$\sb4$R$\sb2$)(dmpe) (R = H or Ph); the latter has been isolated and characterized crystallographically. At higher temperatures, the reaction of TiMe$\sb2$(dmpe)$\sb2$ with 1,3-butadiene proceeds further to form TiMe$\sb2$(C$\sb8$H$\sb{12})$(dmpe); the C$\sb8$H$\sb{12}$ ligand is formed by oxidative coupling of two butadienes and allylic rearrangement to expand the ring. The reaction of TiMe$\sb2$(dmpe)$\sb2$ with styrene gives the titanium(0) complex, Ti($\eta\sp2$-styrene)($\eta\sp4$-C$\sb4$H$\sb4$Ph$\sb2)$(dmpe), via the same oxidative coupling mechanism except that $\beta$-hydrogen elimination is followed by reductive elimination of methane rather than of diphenylbutene.
The reactions of TiX$\sb2$(dmpe)$\sb2$ with carbon monoxide lead to the formation of the eight-coordinate complexes TiX$\sb2$(CO)$\sb2$(dmpe)$\sb2$ for X = Cl or Br, which have been characterized by NMR spectroscopy.
The reaction of TiMe$\sb2$(dmpe)$\sb2$ with t-butylsilane yields the zerovalent complex Ti(dmpe)$\sb3,$ which has been crystallographically characterized. The reaction of TiMe$\sb2$ (dmpe)$\sb2$ with phenylsilane yields two complexes in succession that appear to be titanium silyl complexes of stoichiometry Ti(Si$\sb2$Ph$\sb2$H$\sb4)$Me$\sb2$(dmpe)$\sb2$ and Ti(Si$\sb3$H$\sb5$Ph$\sb3)$(dmpe)$\sb2.$
Variable temperature NMR studies of the dinuclear chromium(II) alkyl anion (Li(thf)$\sb2\rbrack\sb2\lbrack$Cr$\sb2$(CH$\sb2$SiMe$\sb3)\sb6$) show that the two bridging CH$\sb2$SiMe$\sb3$ groups engage in agostic Cr$\cdots$H-C interactions that are static on the NMR timescale at $-$80$\sp\circ$C. At higher temperatures, the molecule undergoes two different dynamic processes. One process, which has activation parameters of $\Delta$H$\ddagger$ = 10.6 $\pm$ 0.5 kcal mol$\sp{-1}$ and $\Delta$S$\ddagger$ = $-$4 $\pm$ 2 eu, is ascribed to exchange between the terminal and bridging CH$\sb2$SiMe$\sb3$ ligands; the other process, which has activation parameters of $\Delta$H$\ddagger$ = 14.1 $\pm$ 0.6 kcal mol$\sp{-1}$ and $\Delta$S$\ddagger$ = 17 $\pm$ 3 eu, is ascribed to rotation of the bridging CH$\sb2$SiMe$\sb3$ ligands about their Cr-C bonds. The latter values give an estimate of the strength of an agostic Cr$\cdots$H bond.
Issue Date:1993
Rights Information:Copyright 1993 Spencer, Michael Donald
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9411788
OCLC Identifier:(UMI)AAI9411788

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