Zirconocene

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

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.

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).

Zirconocene complexes containing two indenyl or tetrahydroindenyl groups bridged with short links such as —CH2—CH2— or —Si(CH3)— can produce isotactic polymers of higher a-olefins (32). To synthesize syndiotactic PO, bridged zirconocene complexes with rings of two different types are required, one example of which is isopropyl(cyclopentadienyl)(l-fluorenyl)zirconocene. These complexes are used for the synthesis of syndiotactic PB... 

A further improvement in the cuprate-based methodology for producing PGs utilizes a one-pot procedure (203). The CO-chain precursor (67) was first functionalized with zirconocene chloride hydride ia THF. The vinyl zirconium iatermediate was transmetalated direcdy by treatment with two equivalents of / -butyUithium or methyUithium at —30 to —70° C. Sequential addition of copper cyanide and methyUithium eUcited the /V situ generation of the higher order cyanocuprate which was then reacted with the protected enone to give the PG. 

Zirconocene dichloride (bis[cyclopentadienyl]zirconium dichloride) [1291-32-3] M 292.3, m 242-245 , 248 . Purified by recrystn from CHCI3 or xylene, and dried in vacuum. H NMR (CDCI3) 8 6.52 from MeaSi. Store in the dark under N2 as it is moisture sensitive. [IR, NMR, MS Aust J Chem 18 173 7965 method of J Am Chem Soc 81 1364 7959 and references in the previous entry.]... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

Property Ethylene-bridged zirconocenes Ziegler-Natta... 

Ethylene-bridged bis-indenyl zirconocene dichloride-methylalu-moxane system. 

Zirconocene (ligand) Activity (kg PP/g catalyst/h) Weight average molecular weight (kg/mol) Melting point (°C)... 

The metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

The zirconocene complex Zr(Si2Cp)2Cl2 (90) is a versatile starting material for a variety of zirconocene complexes with zirconium in the oxidation state IV, III,... 

Table 2. Activation energy for ring rotation in l.l -disubslitued zirconocene complexes...

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

A concise total synthesis of the indole alkaloid dihydrocorynantheol (101) (Scheme 19), that features two RCM steps and a zirconocene-catalyzed carbo-magnesation [68], is a further example of Martin s interest in applying RCM as a key reaction for the construction of alkaloid frameworks [69]. The first RCM step was applied to bis-allyl amide 96. The resulting intermediate 97 was directly subjected to carbomagnesation and subsequent elimination to deliver 98 in 71% yield from 96. Amide 98 was then transformed into acrylamide 99 in... 

Scheme 19 Construction of key lactam 100 via two RCM steps and a zirconocene-mediated carbomagnesation in Martin s total synthesis of dihydrocorynantheol (101) [68]...

The dichlorides of hafnocene and zirconocene were found by Hillman, Weiss and Hahne to have retentions of 15—60% and to show very strong isotope effects. The isotope effects were even larger in benzene solution than in the crystalline targets, reaching a value of 4.13 for the ratio of the retentions of "" Hf and Hf. 

Majoral J-P, Igau A, Cadierno V, ZablockaM (2002) Benzyne-Zirconocene Reagents as Tools in Phosphorus Chemistry. 220 53 -77... 

Majoral J-P, Igau A, Cadierno V, Zablocka M (2002) Benzyne-Zirconocene Reagents as Tools in Phosphorus Chemistry. 220 53-77 Manners I (2002), see McWMams AR (2002) 220 141-167 March NH (1999) Localization via Density Functionals. 203 201-230 Marchivie M,see Guionneau P (2004) 234 97-128... 

---