Observation of the first titanium alkyl/alkyl complexes: The key intermediate in the Ziegler-Natta mechanism for the polymerization of alkenes

Spencer, Michael Donald

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Description

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

Issue Date

1993

Doctoral Committee Chair(s)

Girolami, Gregory S.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Inorganic

Language

eng

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.

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Copyright 1993 Spencer, Michael Donald

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