Hafnium complexes butadiene

The actual reaction path followed becomes more evident if the corresponding hafnium complexes are used as substrates. In this case the crucial intermediate 29, which can react to form either product 30 or 17, can be isolated. The thermodynamically favored ( -m-butadiene)hafnocene (5b) (32) turns out to be inert toward ethylene under the conditions applied. Even heating to 120°C in an ethylene atmosphere (1 bar) for several hours does not result in consumption of the metal complex. In contrast, s-trans-rf-butadiene)hafnocene (3b) rapidly takes up 1 molar equivalent of C2H4 even at -10°C to yield a C—C coupling product, i.e., the five-membered metallacyclic cr-allylhafnocene complex 29b. Above 0°C, vinylhafnacy-clopentane reacts with additional ethylene to form bis(cyclo-pentadienyl)hafnacyclopentane (30b) and free butadiene. In the absence... 

Crystal structures of titanium complexes 36a, 36b and 36c and of hafnium complex 36d indicate that the diene-metal interaction is best characterized by the <7, n bonding mode. In general, the diene ligand adopts an s-cis supine conformation with respect to the Cp ligand (e.g. 36c and 36d). However, Nakamura and coworkers have noted that the Cp TiCl complexes of butadiene (36a), 1,3-pentadiene and 1,4-diphenyl-... 

Dimethylbutadiene)HfCp (Cl)] (74c) reacts with one molar equivalent of acetylene to yield the unusual product 87. This is probably formed by a conventional butadiene/alkyne coupling at the Group 4 metal center, followed by an intramolecular alkene insertion into the adjacent hafnium to carbon cr-bond. The resulting alkylidene complex (86) then rapidly dimerizes to yield the observed final product (see Scheme 28), that was characterized by X-ray diffraction.96... 

AG ii c = 14.3 kcal/mol). ( -c -Butadiene)- and -2,3-substituted buta-diene)zirconocene complexes exhibit markedly lower barriers (Table III) 50-52). In general, the corresponding bis(Tj-cyclopentadienyl)hafnium... 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

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