Divalent titanium complexes

Sato, F., Okamoto, S. The divalent titanium complex Ti(0-i-Pr)4/2 /-PrMgX as an efficient and practical reagent for fine chemical synthesis. Adv. Syn. Catai. 2001, 343, 759-784. 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

The Bu -substituted complexes furnish solely the prone isomer where the cup of the diene is oriented away from the cyclopentadienyl ring. Both the metrical data from the solid-state structure and NMR spectroscopic data support a 7T2, rf formalism for the diene ligand, consistent with a divalent titanium center. 

Monocyclopentadienyltitanium phosphine complexes have also been prepared and reviewed previously. The formally divalent titanium precursor, (r/s-CsI Is)Ti(dmpebCl, serves as a synthon for the corresponding methyl and hydride compounds, ( 5-CsI Is)Ti(dmpe)2k (R = Me, H), which are diamagnetic and crystallographically characterized.57 Addition of ethylene to these compounds results in formation of 1-butene, 3-methyl-l-pentene, and... 

Treatment of the titanium- ate complex, ( 75-C5H5)Ti p-rf r/2-C2(SiMe i)2 2Mg(r/ -C5H5) 192,112 with excess acetylene furnishes the divalent titanium cyclohexadienyl complex, (if-l, 2,4,5,6-pentakis(trimethylsilyl)-cyclohexadienyl)(77S-cyclopentadienyl)titanium 193 (Equation (2)).113 A series of related compounds prepared from /-butylethyne, cyclohexylethyne, 1-hexyne, and phenylethyne have also been synthesized. 

Titanium species having different oxidation states, single and double-bonded to the surface of silica as well as strongly chemically and physically absorbed, can be formed. The role of the ester is to moderate the alkylating divalent titanium species. The bimetallic complex may be formed in the preparation of Si02 supported catalysts by impregnation [5] with TiCU-AlEtj. 

Fig. 7.5 A stable dititanium complex as single component catalyst for acetylene polymerization. A molecule of THF attached to the formally divalent titanium atom by coordination through oxygen is not shown.

In this chapter we will review the synthesis, structural aspects, and basic chemical properties of formally divalent and trivalent titanium and zirconium metallocene complexes. We have restricted our coverage to the low-valent bis(rj-cyclopentadienyl) and related metallocenes metal halide complexes and organometallic mixed metal systems will not be discussed here. We have not attempted to present an exhaustive coverage of the field. Rather, our aim has been to describe critically and to evaluate the often confusing chemistry that has been reported for the reactive low-valent titanium and zirconium metallocenes. More general reviews (7) and a book (2) on the organometallic chemistry of titanium, zirconium, and hafnium have been published. 

Treatment of the thermally stable, formally divalent bis(alkoxide)titanium alkyne complexes, (Pr 0)2Ti( 7Z-RC=CR) (R = Et, Pr, p-Me-C(,H4),222 with aryl iodides in the presence of Ni(COD)2 (COD = 1,5-cyclooctadiene) affords the corresponding cross-coupled products.223 Other catalysts such as Ni(acac)2 (acac = acetylacetonate), Pd(OAc)2, and Pd(PPh3)4 promote the reaction but are significantly less active. As anticipated, aryl iodides undergo cross-coupling more effectively than the corresponding bromides, chlorides, and triflates. 

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