Zirconium complexes, overview

Previously, we synthesized and studied various Group 4 complexes with different ligations as alternatives to the cyclopentadienyl ligand. Here we present an overview of the synthesis, structure, and catalytic properties in the polymerization of a-olefins of several zirconium octahedral complexes. We show how the stereoregular polymerization of a-olefins using these octahedral zirconium complexes can be modulated by pressure. These results raise conceptual questions regarding the general applicability of ds-octahedral C2-sym-metry complexes to the stereospecific polymerization of a-olefins. 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to 100 times higher than for classical Ziegler catalysts, as well as the possibility to easily tailor the microstructure of the polymer chain and to obtain polymers with special properties have motivated research groups worldwide to produce thousands of patents and publications in the last 20 years. An overview can be found in selected review articles and books [55-68]. A metallocene/MAO catalyst containing 1 g zirconium produced 40 x 10 g polyethylene in 1 h at 95°C and 8 bar ethene pressure (Table 1). 

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