Metallocenic precursors

The rare combination of two electron-poor metals in heterodimetallic complexes has been found in reactions of dialkynylmetallocenes with suitable metallocene precursors. Thus, reaction of Zr(f -C4H6)Cp2 with Hf(C=CPh)2Cp2 affords Cp2Zr (/x.-C2Ph)2 HfCp2 (222-Zr/Hf). Similar reactions of VCp2 with... 

High enantioselectivity (ca. 95-99% ee) is observed in this system, better than that revealed in previous reports of the hydrosilylation of imines. The mechanism is as yet unclear however, the authors propose that an active catalyst may be formed by cleavage of the Ti-F bond and generation of a Ti(III) hydride species. Insertion of an imine into the Ti-H bond, followed by a (r-bond metathesis with the silane in a four-centered transition state, may lead to the observed products. Another report on the activity of titanocene complexes as catalysts for the hydrosilylation of aid- and keti-mines also indicates formation of a Ti-H species as catalyst.188 Hydrosilylation proceeds to yield silylamines, with dependence on substitution at nitrogen and on the nature of the ligand bound to the metallocene precursor. 

Class I catalysts obtained from achirotopic metallocene precursors of the Cp2MtX2 type, which produce atactic polypropylene at elevated temperature in the range of ca 50-70 °C (characteristic of propylene polymerisation in the presence of heterogeneous catalysts), can yield stereoblock isotactic polypropylene at lowered temperature, e.g. — 45 °C (Table 3.1) [22]. 

Polyolefins possessing properties distinctly different from those produced in homogeneous solution, characterised by an unusually high molecular weight, have recently been obtained by using catalysts with c/ .sc/-metallocene precursors supported directly on an unpretreated silica gel carrier [204,206], This may be due to the active site isolation effect, i.e. to the strict suppression of participation of other sites in the polymerisation [30],... 

Figure 3.14 Additionally a-substituted ansa-metallocene precursors, rac.-dimethyls-ilylenebis[l-(2-methylindenyl)]zirconium dichloride [rac.-Me2Si(MeInd)2ZrCl2] and rac.-dimethylsilylenebis[l-(2-methyl-4-naphthylindenyl)]zirconium dichloride [rac.-Me2 Si(Me,NphInd)2 ZrCl2], for obtaining high-molecular weight polypropylene. Front side view. Reproduced by permission from Ref. 263. Copyright 1994 American Chemical Society...

Considerations of the polymerisation stereochemical behaviour of catalysts belonging to class V with pseudohelical symmetry (based on lopsided metallocene precursors) still warrant more in-depth investigations. However, an assertive statement can be made that, the more unsymmetrical the catalysts are, the more m diads there are in syndiotactic polypropylene. On the other hand,... 

The active species in B(C6F5)3-activated metallocene catalysts is an ion pair, consisting of an electron-deficient cation, such as [Cp2ZrMe]+, stabilised by a weakly coordinating anion, here [MeB(C6F5)3]". One aspect of our research recently has been the attempt of building the Lewis acidic activator function into the metallocene precursor complex, in an effort to synthesise self-activating systems. The principle is illustrated... 

The tris(cyclopentadienyl)uranium(m) complexes Cp"3U and (GsMe4H)3U were prepared by synthetic routes that involve the reduction of the appropriate tetravalent uranium metallocene precursors with potassium metal and NaCioHg, respectively (Scheme 14). The molecular structure of (CsMe4H)3U shows a perfectly trigonal structure with a threefold axis (all Cp (centroid)-U-Cp (centroid) angles equal to 120°).46... 

Activation. The paths shown in equation 14 are generally accepted to be that by which a metallocene precursor is transformed into a catalytic site in aluminoxane-cocatalyzed polymerization, while equations 8-10 portray the three mechanisms for activation using Lewis and Br0nsted acids. 

The first mechanism to be widely accepted was proposed by TiUey et al., which is called a o-bond metathesis (Fig. 17) [75]. In this model a metallocene precursor is first transformed to a metallocene hydride that enters the hypothetic catalytic cycle, forming a metal silyl species via an initial a-bond metathetic step with the simultaneous production of hydrogen. The second o-bond metathetic step forms the Si-Si bond and is supposed to regenerate the catalytically active metal hydride. 

An in situ application in CVD processes is achieved by first introducing a metallocene precursor (e.g. ferrocene [(CsH5)2Fe]) into the deposition chamber for a short period of time, thereby depositing an extremely thin metal film on the order of tens of monolayers onto the substrate. The deposition temperature for the catalyst is typically well below the nanowire synthesis temperature, although again this will depend on the choice of the precursor and the types of nanowire to be synthesized. This step is then followed by nanowire synthesis. 

To better understand the polymerization process and to be able to rationally design better catalyst systems, single-site homogeneous catalyst systems were developed as models of the heterogeneous systems employed commercially. Of particular interest were metallocene-type catalysts, which had been discovered to be slightly active for polymerizations in the early work of Natta et Until the accidental discovery of MAO by Sinn, Kaminsky and coworkers in 1980, however, the activity of metallocene catalysts was far too low to be useful. Activation of a metallocene precursor such as (ri -C5H5)2ZrCl2 with MAO was found to yield extremely active polymerization catalysts, and the number of metallocene-type catalysts reported in the literamre has since exploded. ... 

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