Catalysis with zirconocene

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

Supramolecular catalysis may also involve the combination of a host cavity and a metal active site as in the bis(diphenylphosphino)calix[4]arene nickel(II) complex 12.40 which acts as an efficient catalyst for ethylene and propylene polymerisation, and in tandem with zirconocene dichloride, for the formation of linear low-density polyethylene. In the latter case the complex gives very little branching - a significant advantage. The key to the effectiveness of the catalyst involves calixarene-induced changes in the bite angle at the Ni(II) centre, which is square planar in the active form of the catalyst.29... 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Heterogeneous tandem catalysis involving at least one of the components being supported has also been reported [178, 179]. For example, calcosilicate has recently been used as an effective carrier for simultaneous immobilisation of a dual-functional system based on a bis(imino)pyridine iron compound and a zirconocene to form a heterogeneous catalyst precursor. On activation with triethylaluminium, ethylene was converted to LLDPE the layered structure of the calcosilicate was used to account for the improved thermal stability and higher molecular weights of the LLDPE formed [179],... 

In summary, zirconocene and half-sandwich zirconium-based catalysis has been developed both in surface and solution systems. In general, the activation of Zr-POSS and silica-supported zirconium system with MAO proved inappropriate (see silsesquioxane displacement and leaching, respectively, vide supra), while the acti-vahon with BArl proved more efficient, albeit structural rearrangement complicates the activation chemistry and simple Zr(IV) cationic alkyl species are seldom the outcome of the activation. 

However, such fluorine atoms are hard bases, and have been shown to coordinate to the (presumably) hard zirconium cation. It is worth investigating whether the secondary bonding would be weaker if the fluorine atoms on typical nonbasic anions were replaced with softer chlorine, bromine, or iodine atoms. Perhaps, in this light, it may be significant that zirconocene catalysis is especially efficient when the anion is (CBuHeXe)" (X = Cl, Br, or I). 

Dienes Consecutive reactions of alkynes with CpjZr (from CpjZrClj and BuLi) and allyl trimethylsilyl ether generate (l,4-dien-l-yl)zirconocene derivatives which can be used to couple with iodoarenes under Pd(0)-Cu(I) catalysis. 

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