Ethylene-zirconocene complex

Fig. 1. The structure of the ethylene-zirconocene complex (SiH2Cp2)ZrCHj-C2H4. The corresponding titanocene has basically the same structure, except that the Ti-C distances are obviously different from the Zr-C distances.

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

A new development in silsesquioxane chemistry is the combination of sil-sesquioxanes with cyclopentadienyl-type ligands. Recently, several synthetic routes leading to silsesquioxane-tethered fluorene ligands have been developed.86,87 The scenario is illustrated in Scheme 47. A straightforward access to the new ligand 140 involves the 1 1 reaction of 2 with 9-triethoxysilylmethylfluorene. Alternatively, the chloromethyl-substituted c/ovo-silsesquioxane derivative 141 can be prepared first and treated subsequently with lithium fluorenide to afford 140. Compound 141 has been used as starting material for the preparation of the trimethylsilyl and tri-methylstannyl derivatives 142 and 143, respectively, as well as the novel zirconocene complex 144. When activated with MAO (methylalumoxane), 144 yields an active ethylene polymerization system. 

The zirconocene complex 11, when activated with methylalumoxane, shows high activity in ethylene polymerization (Table 9) which is slightly larger than... 

Photochemical [2 + 2]-cycloaddition of unbridged bis(alkenylindenyl)zirconocene complexes 975 is a unique approach leading to formation of 1,2-cyclobutylene-bridged highly efficient and complete within 2-3 h with nearly quantitative conversions. Upon activation with MAO, these ansa-zirconocenes are effective catalysts for ethylene/l-octene co-polymerizations at elevated temperatures. 

Rieger, B. Jany, G. Fawzi, R. Steimann, M. Unsymmetric ani a-zirconocene complexes with chiral ethylene bridges influence of bridge conformation and monomer concentration on the stereoselectivity of the propene polymerization reaction. Organometallics 1994, 13, 647. 

In the early 1980s, Kaminsky and Sinn discovered an efficient way to activate homogeneous metallocene catalysts with methylaluminoxane (MAO). Titanocene and zirconocene complexes activated with MAO exhibited very high activity for ethylene polymerization these early systems, however, still had low activity for propylene polymerization and formed atactic polypropylene [5]. Met-allocene/MAO systems containing stereospecific ligands could be used to catalyze the polymerization of prochiral olefins (a-olefins) through the use of catalysts with well-defined active sites [6]. Later, Brintzinger [7] and Ewen... 

M. L. H. Green has shown that the ansa-zirconocene complex 22 with an indenyl moiety connected to a cyclopentadienyl unit by a CMe2 bridge is a suitable compound for synthesizing a variety of homo- and heterobimetallic complexes (Scheme 19) [80]. Some of these complexes have been used as cocatalysts for ethylene and propylene polymerization. The early-late hetero-trinuclear Fe/Zr complex 23 revealed among the best catalysts giving activities close to [Cp2ZrCl2]. The role that could play Fe on the activity of Zr was not discussed by the authors. 

In a pioneering paper [97], Osakada has reported ethylene polymerization trials with a series of early-late heterobimetallic complexes, including a Co/Zr combination, and showed that some of these complexes enable the enchainment of the a-olefin or of the oligomer formed at the late metal center to the polymer grown at the Zr center. The synthesis of the Co/Zr heterobimetallic complex 49 is elegant and involves as key step the Ru-catalyzed cross metathesis of an ansa-zirconocene complex 47 with an allyl substituent and a Co complex 48 having a pendant acrylate (Scheme 31). In the solid state, both Zr and Co atoms were found far away from... 

Wang, W., Fan, Z.-Q., and Feng, L.-X. 2005. Ethylene polymerization and ethylene/1-hexene copolymerization using homogeneous and heterogeneous unbridged bisindenyl zirconocene complexes. European Polymer Journal 41 2380-2387. 

The ROP of ansa- and spirocyclic onsa-zirconocene complexes has been reported (217) (eq. 25). Polycarhosilane [(CH2)8Si((7 -C5H4)2ZrCl2]n (105) was obtained from reaction of the spirocyclic silacyclohutane-bridged monomer (CH2)sSi( -C5H4)2ZrCl2 (104) with Karstedt s catalyst. This polymer demonstrated moderate activity as a catalyst for ethylene polymerization. 

Braunschweig reported that his R2NB-bridged ansa-zirconocene complexes are approxinoately seven times more active than their hafnocene analogues toward ethylene polymerization, while the hafnocene complexes produce higher molecular weight polymer (Table 5.1, entries 12 and 15). This is consistent with the alkene polymerization behavior of other zirconocene and hafnocene systems. 

Several alcohol-, ether-, acid-, ester- and ketone-functional alkenes (Fig. 12) were tested as comonomers in polymerization experiments [19,21] A bridged zirconocene complex rac-Et(Ind)2ZrCl2 was selected as catalyst for the studies because it is a relatively good copolymerization catalyst and capable of both ethylene and propylene polymerizaticais. MAO was used as cocatalyst. MAO and the comonomers were pre-contacted for 15 min in the reactor just before the start of the polymerization. 

Recently, Aral et al. reported preparation of isotactic polystyrene using some ansa-zirconocene complexes [106,107]. For example, 20 gives isotactic polystyrene with [mmmm]>0.90. Although the isotactic homopolymer of styrene is less attractive for practical use, the catalysts would be useful in the production of stereoregulated styrene-ethylene copolymer. 

Fig. 2. Time-evolution of the methyl/ethyl C-C distances for both the zirconocene and the corresponding titanocene catalyst. The two curves starting at around 3.2 A represent the distance between the methyl carbon atom and the nearest-by ethylene carbon atom in the zirconocene-ethylene and the titanocene-ethylene complex, respectively. The two curves starting at around 1.35 A reflect the ethylene internal C-C bond lengths in the two complexes.

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

---