Titanium complexes anionic ligands

Aminopyridinato ligands form a special class of anionic ligands in which an aromatic ring is part of an amidinate system. These ligands have frequently been employed in early transition metal and lanthanide coordination chemistry. Their diverse and interesting chemistry has been described in detail by Kempe et al. ° and will thus be covered here only briefly. Typical reaction pathways leading to titanium aminopyridinato complexes are outlined in Scheme 169. Metathetical as well as salt-free routes have been developed. 

Other titanium complexes supported by acetylacetonate 17-19, " showing similar structural analogies with 12-16, were reported to have comparable moderate catalytic activities, stereoselectivity and polydispersity for the bulk ROP of rac-LA (Scheme 6.2). In totality, the activity of these complexes seems to be limited, probably by a substantial 7t-donation interaction from the anionic 0,0-bidentate-type ligands to the Ti " Lewis acid that consequently decreases the rate of polymerisation in addition to the sterically crowded complexes limiting the incoming monomer approach. 

Neodymium catalysts show a different behavior. In this catalytic complex the neodymium is probably in the trivalent state and at least one Nd-Cl bond is present [369]. The AlEt3-Ti(OR)4 system is also a catalyst in which some alkoxy groups remain bonded to the titanium. Both catalysts give cis-, A isotactic polypentadiene. The anionic ligands bonded to the neodymium or the titanium atom of the catalytic species force the new monomer to reaet to the isotactic structure. 

The coordination and organometallic chemistry of zirconium and hafnium are surveyed for the year 1991.107,108 There is useful material in a review of homogeneous Group 4 metallocene Ziegler-Natta catalysts, a review of the coordination chemistry of cyclopentadienyl titanium carboxylate and related complexes O and a review of bis(cycIopentadienyl)zirconium(IV) or hafnium(IV) complexes with Si-, Ge-, Sn-, N-, P-, As-, Sb-, 0-, S-, Se-, Te- or transition metal-centred anionic ligands. Cationic zirconocene or hafiiocene complexes serve as Lewis acids with unique reactivities, they are active for C-F bond activation, coordinative activation of ether linkages, carbonyl activation and C-O bond cleavage. New synthetic methods based on the... 

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

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