Titanium trihalides

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

A number of facile procedures were developed for the synthesis of mono and bis cyclopentadienyl derivatives of titanium, e.g., the (tt-CsH )-trialkoxytitanium derivatives, (77-C5H5)-titanium trihalides (406, 407-409), (77-C5H5)-titanium internal complexes (409), triaroxytitanium cyclopentadienyls (410), compounds containing different groups (alkoxy, aryloxy. 

Mono-Cp titanium trihalides react with SiBu OH or SiBut2(OSnMe3)2 to give cyclic and acyclic halo titanium siloxanes with structures depending on the substituents on the Cp and on the nature of the halide ligands. The... 

The dihalides of titanium, formed by reduction of the tetrahalides, are vigorous reducing agents and unstable T1CI2 is inflammable in air. The trihalides, though more stable than the dihalides, are effective reducing agents. Ti(TTT) occurs in aqueous solutions as Ti(1120)( 3, ... 

The trihalides of zirconium, like the dihalides of titanium, are extremely strong reducing agents, reacting even with H20. 

The trihalides of titanium can be prepared by reducing the tetrahahdes with the metal. With the exception of the trifluoride, which has a magnetic moment of 1.75 BM, as expected for a d ion, all the trihalides have low magnetic moments, indicative of metal metal interaction. The sfructures consist of halogen-bridged polymers containing six-coordinate titanium. 

The interatomic distance is estimated from that of the solid, given as 2.46 A by Natta et al. (4), and from comparisons of bond distances of crystal and gas phase tetrahalides of titanium. The pyramidal bond angle is estimated assuming TiClg(g) similar to the group V trihalides. The principal moments of inertia are B 42.8614 x 10" and I = 73.1025 x 10 g cm. ... 

The coordination chemistry of this oxidation state is virtually confined to that of titanium. Reduction of zirconium and hafnium from the quadrivalent to the tervalent state is not easy and cannot be attempted in water which is itself reduced by Zr and A few adducts of the trihalides of these two elements with N- or P- donor ligands have been prepared. ZrBrj treated with liquid ammonia yields a hexaammine stable to room temperature... 

In general, deoxygenation of heterocyclic JV-oxides is effected by the use of phosphorus trihalides or by catalytic reduction with Raney nickel.312 Because of their higher selectivity the phosphorus reagents are preferable. In the pyrido[2,3-a ]pyrimidine series, 3-oxides are deoxy-genated in moderate yield by phosphorus trichloride, by Raney nickel,309 or by titanium(III) chloride. 4... 

At different times a variety of different reagents has been used for this purpose, among others phosphorus pentachloride [22,24,25], sulphuryl chloride [26], ferric chloride [26], aluminium chloride [24], stannic chloride [27,28], titanium tetrachloride [26] and boron trihalides [24,27,29]. Strong Lewis acids also react vigorously with the double bonds and the yields of tropylium salt are not good when they are used. 

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