Titanium compounds cyclopentadienyl ligands

Some half-sandwich titanium compounds with cyclopentadienyl ligands have been shown to be the most active catalysts for synthesis of these polymers. Fluorinated half-sandwich metallocenes, synthesized by Roesky et al. (263), have activities of up to a factor of 30 greater than those of chlorinated compounds. Polymerization has been carried out within a temperature range of 10-70°C (264). 

Some half sandwich titanium compounds with cyclopentadienyl ligands have proven to be most active, but soluble tetraethyoxytitanium also shows a certain amount of activity. In contrast to olefin polymerization, titanocenes are more active than zirconocenes and fluoro ligands are better than chloro ligands. Table 24 [207] compares some catalysts for the polymerization of styrene. 

Organometallic Compounds The most widely used compounds [Is,2m] are the titanocene derivatives (10.36), which exhibit an excellent absorption around 500 nm and very efficiently eliminate an aryl and a cyclopentadienyl ligand yielding an unsaturated titanium species and an arylated cyclopentadiene. The Ti structure adds to the CO group of one acrylate unit (may be two) and generates the initiating radicals. Other systems include metal carbonyl (Mn, Fe, Mo, Cr, Os, Re, Ru, etc. [75,76]). 

Two more titanium compounds are known which contain a chelating O-functionalized cyclopentadienyl ligand. One of them is the siloxo complex 16TiCl2, which was obtained by P. Royo et al. from [(C5H4)-SiMe2Cl]TiCl3 either in very low yield by treatment with water in toluene or more rationally and in 45% yield with SiPh2(OH)2 and NEts in hexane. The second one is the spiro titanocene derivative (17)2Ti... 

The synthesis of titanium complexes (49), (50) and (51) with bulky phos-phoraniminato and cyclopentadienyl ligands Cp, whether substituted or not, has been reported, including the X-ray structural data of some of these compounds. It turns out that the phosphoraniminato ligand exhibits some steric resemblance to the cyclopentadienyl hgand with respect to the metal environment. Ethylene polymerization experiments reveal the titanium phosphoraniminato complexes to be effective catalyst precursors. ... 

A metallocene is a coordination compound consisting of a transition metal ion, such as zirconium or titanium, with one or two cyclopentadienyl ligands (13). 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

TABLE 2.3 Catalytic Activities of Titanium Compounds with Cyclopentadienyl Ligands... 

In this correlation, the catalytic activities of titanium compounds with bulky substituents at the cyclopentadienyl ligand show lower activities than expected from the chemical shifts of Ti-NMR. The electron densities of titanium compounds can also be observed when the titanium complexes possess the same... 

The different transition metals for the syndiospecific polymerization of styrene were summarized. Compounds of titanium with one cyclopentadienyl ligand show a high performance for the SPS production. Transition metals are stabilized by cyclopentadienyl ligands with electron-releasing substituents, and bulky substituents decrease the catalytic activities. o-Bonded groups at the transition metal complex are substituted by other groups by MAO or TIBA and showed comparable catalytic activities in the syndiospecific styrene polymerization [31]. 

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Binding energy, pentacarbonyliron, 6, 3 Binuclear complexes bis-Cp titanium halides, 4, 522 with Ni-M and Ni-C cr-bonds heterometallic clusters, 8, 115 homometallic clusters, 8, 111 Binuclear dicarbonyl(cyclopentadienyl)hydridoiron complexes, with rand C5 ligands, 6, 178 Binuclear iridium hydrides, characteristics, 7, 410 Binuclear monoindenyl complexes, with Ti(IV), 4, 397 Binuclear nickel(I) carbonyl complexes, characteristics, 8, 13 Binuclear osmium compounds, with hydrocarbon bridges without M-M bonds, 6, 619... 

---