Synthesis and structural characterization of group 4 ansa-metallocene complexes containing a 1-sila-3-metallacyclobutane ring

Kabi-Satpathy, A.; Bajgur, Chandrasekhar S.; Reddy, Karuna P.; Petersen, Jeffrey L.

doi:10.1016/0022-328x(89)85336-7

Cited by 22 publications

(6 citation statements)

References 28 publications

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“…Based on the rationale that the crowded site is reserved primarily for the monomer coordination, and the less crowded site for the growing chain, the chains connected to both active sites formed with 5 and 7 should have the same propensity for β ‐hydride elimination and produce polymers with similar molecular weights. At this stage, in the absence of more rigorous investigations, we tend to speculate that the reason for the observed behavior is the lateral rotation of the transition metal active center entity within the entire ligand framework in the transition state17 (Figure 11). By performing this rotation, the multi‐substituted C 1 symmetric site could alleviate some of the steric tensions that is created by the presence of different substituents to provide more space and different positioning for the growing polymer chain and coordinating monomer.…”

Section: Resultsmentioning

confidence: 99%

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Razavi

Bellia

Brauwer

et al. 2004

Macro Chemistry & Physics

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Summary: The stereochemistry of propylene insertion/propagation reactions with a variety of Cs and C1 symmetric, bridged cyclopentadienyl‐fluorenyl ligand containing metallocene catalysts are investigated. It is shown that active species with a local Cs symmetry and enantiotopic coordination positions, in the general contest of the chain migratory insertion, behave syndioselective. However, the bilateral symmetry rule, “central dogma” of syndioselectivity, is not a catholic prerequisite and may be ignored by the Cs systems whenever symmetric “excess” in steric bulk is present. An asymmetric “excess” of steric forces, on the other hand, will perturb the migratory insertion process completely and induce a full tactic inversion in resulting C1 symmetric catalysts. The findings also reveal that C2 symmetry and helicity of homotopic catalysts, conditions thought to be essential for high isoselectivity, can be lifted in light of recent developments in high performance centrally chiral, C1 symmetric enantiotopic catalysts.Ligand outline, chain orientation, and monomer coordination mode with enantiomorphous active species.imageLigand outline, chain orientation, and monomer coordination mode with enantiomorphous active species.

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Section: Resultsmentioning

confidence: 99%

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Razavi

Bellia

Brauwer

et al. 2004

Macro Chemistry & Physics

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“…The bridging C(11) and Si(1) atoms deviate from the CE(1),Ti,CE(2) plane to the side opposite to Si(2). The Ti−CH 2 −SiMe 2 − structural arrangement is known only for the Ti(IV) 1-sila-3-titanacyclobutane compound Me 2 Si(C 5 H 4 ) 2 Ti(CH 2 ) 2 SiMe 2 , where the Ti−CH 2 bond lengths are 2.175(4) and 2.148(6) Å and Si−CH 2 bond lengths are 1.868(4) and 1.869(4) Å 2 ORTEP drawing of 4 with ellipsoids drawn at the 30% probability level. 3 Selected Bond Distances and Angles for 4 (a) Bond Distances (Å) Ti-C(11) 2.204(5) Ti−C(1) 2.330(4) Ti−C(2) 2.371(5) Ti−C(3) 2.461(5) Ti−C(4) 2.461(5) Ti−C(5) 2.354(5) Ti−C(6) 2.363(5) Ti−C(7) 2.382(4) Ti−C(8) 2.411(5) Ti−C(9) 2.410(5) Ti−C(10) 2.386(5) Ti−CE(1) 2.068(5) Ti−CE(2) 2.060(5) Si(1)−C(1) 1.876(5) Si(1)−C(11) 1.836(5) Si(1)−C(12) 1.869(5) Si(1)−C(13) 1.864(7) (Si(2)−C) av 1.863(7)(b) Bond Angles (deg) CE(1)−Ti−CE(2) 146.1(2) Ti−C(11)−Si(1) 97.9(2) CE(1)−Ti−C(11) 103.6(2) CE(2)−Ti−C(11) 109.6(2) C(11)−Si(1)−C(1) 93.1(2) C(11)−Si(1)−C(13) 111.3(3) C(11)−Si(1)−C(12) 118.4(3) C(13)−Si(1)−C(12) 106.3(3) C(13)−Si(1)−C(1) 113.9(3) C(12)−Si(1)−C(1) 113.9(2)

…”

Section: Resultsmentioning

confidence: 99%

“…The Ti-CH 2 -SiMe 2 -structural arrangement is known only for the Ti(IV) 1-sila-3-titanacyclobutane compound Me 2 Si(C 5 H 4 ) 2 Ti(CH 2 ) 2 SiMe 2 , where the Ti-CH 2 bond lengths are 2.175(4) and 2.148(6) Å and Si-CH 2 bond lengths are 1.868(4) and 1.869(4) Å. 19 Mechanistic Considerations. The reaction pathways to both 3 and 4 involve the reduction of 1 to [C 5 Me 4 (SiMe 3 )] 2 TiCl (2).…”

Section: Resultsmentioning

confidence: 99%

Activation of the (Trimethylsilyl)tetramethylcyclopentadienyl Ligand in the [C₅Me₄(SiMe₃)]₂TiCl₂/Mg System, Yielding Intramolecular Si−CH₂−Mg and Si−CH₂−Ti Bonds. Molecular Structures of {[η⁵-C₅Me₄SiMe₂(μ-CH₂{Mg,Mg})][η⁵-C₅Me₄(SiMe₃)]Ti^III(μ-H)₂Mg(THF)}₂ and [η⁵:η¹-

et al. 1997

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The reduction of [C5Me4(SiMe3)]2TiCl2 by excess Mg in THF yields the paramagnetic compound {[η5-C5Me4SiMe2(μ-CH2{Mg,Mg})][η5-C5Me4(SiMe3)]TiIII(μ-H)2Mg(THF)}2 (3). In the presence of Me3SiC⋮CSiMe3 the same system affords the paramagnetic compound [η5:η1-C5Me4SiMe2CH2][η5-C5Me4(SiMe3)]TiIII (4) in 75% yield. The crystal structures of 3 and 4 reveal that one SiMe3 group in each of the compounds has been activated by hydrogen abstraction. In centrosymmetric dimer 3, two titanocene−magnesium hydride-bridged units are held together by two methylene groups which link the two Mg atoms via a two-electron−three-center Mg−C−Mg bond. In the mononuclear 4, a regular Ti−CH2 σ-bond (2.204(5) Å) binds the central Ti atom to the η5:η1-C5Me4SiMe2CH2 ligand.

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“…In this context, it is also noteworthy to mention another ligand/transition metal related, dynamic behavior, namely, the lateral displacement of the whole ligand system around the imaginary axis connecting the transition metal bonds to the centroids, which was first described by Petersen [164]. This movement can be described as a kind of windshield wiper-type oscillation of the MCl 2 moiety within the fixed ligand framework and could facilitate or influence the steps involved in the counter-ion-assisted site epimerization and chain migratory insertion processes (Fig.…”

Section: Stereorigidity Of Bridged Metallocenes and Stereoselectivitymentioning

confidence: 98%

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

Razavi¹

2013

Advances in Polymer Science

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This chapter covers the discovery that enabled, for the first time, the manufacturing of high molecular weight, highly stereoregular syndiotactic polypropylene polymers with narrow molecular weight distribution via the new class of metallocene molecules with bridged cyclopentadienyl-fluorenyl ligands. First, the salient crystal structural features of the neutral metallocene molecules and the crystal structure of the corresponding mono-alkyl-zirconocenium cation is introduced. Then, by employing the structural and geometrical data, a realistic working model for the active site precursor is developed and presented. The stereochemistry of the catalytic propylene poly-insertion reaction and the formation of syndiotactic polypropylene according to the enantiomorphic site control mechanism at the enantiotopic cationic active sites, formed after the activation of the prochiral metallocene dichloride with methylaluminoxane, MAO, is reviewed and further delineated. In the following sections, the impact of the introduction of substituents with proper size and composition at some selected positions of the ligand is discussed and the methods for increasing the molecular weight and stereoregularity of syndiotactic polypropylene polymers are elaborated. With the aid of computational calculations on real and hypothetical catalyst systems with substitutionally altered ligand structures, it is shown that the degree of enantioselectivity of syndiospecific catalyst systems can be determined and/or predicted for these systems quantitatively and with high precision. The calculations confirm that the introduction of substituent(s) reinforcing the preferential conformational orientation of the polymer chain enhances the overall enantioselectivity of the active site, and to certain extends stereoselectivity in general, by affecting the site epimerization processes. The computational calculations reveal that to account correctly for the frequency of the site epimerization-dependent m-type stereo-errors it is essential to include the counter-ion, as an integrated part of the A. Razavi (*) 35 Domaine de la Brisee, 7034 Obourg, Belgium e-mail: abbasrazavi@skynet.be catalyst system, into the calculations. The relevance of the symmetry and stereorigidity of the metallocene structure for the syndiospecificity of the final catalyst is debated by presenting a few "non-conforming" catalyst examples.Finally, the challenges involved in the manufacture of syndiotactic polypropylene in commercial scale continuous production processes are highlighted as well as the polymer's unique structural, physical, mechanical and rheological properties.

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Synthesis and structural characterization of group 4 ansa-metallocene complexes containing a 1-sila-3-metallacyclobutane ring

Cited by 22 publications

References 28 publications

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

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Synthesis and structural characterization of group 4 ansa-metallocene complexes containing a 1-sila-3-metallacyclobutane ring

Cited by 22 publications

References 28 publications

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Activation of the (Trimethylsilyl)tetramethylcyclopentadienyl Ligand in the [C5Me4(SiMe3)]2TiCl2/Mg System, Yielding Intramolecular Si−CH2−Mg and Si−CH2−Ti Bonds. Molecular Structures of {[η5-C5Me4SiMe2(μ-CH2{Mg,Mg})][η5-C5Me4(SiMe3)]TiIII(μ-H)2Mg(THF)}2 and [η5:η1-

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

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