Titanium basic chemistry

This chapter has been devoted to the coordination chemistry of titanium and has made no attempt to describe the more basic chemistry of this element. References to alloys, to the simple halides and oxyhalides, the oxides, sulfides, selenides, tellurides, nitrides, azides, phosphides, arsenides and antimonides are well reviewed by Clark,14 and the recent text by Greenwood and Earnshaw180 contains a good section on titanium. 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

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. 

Titanium was one of the first elements to be discovered by modern chemists. The modern chemistry period begins after the middle of the 18th century. That period is chosen because it is the first time that the basic concepts of modern chemistry were developed. 

Monoanionic boratabenzene ligands can formally be derived from benzene by replacement of GH by BRT. Boratabenzene ligands are less basic and less nucleophilic than Cp rings, but replacement of Cp by boratabenzene ligands in titanium chemistry has been studied. 

Titanium(iv) Complexes. Aqueous Chemistry Oxo Salts. There is no simple aquated Ti4+ ion because of the high charge-to-radius ratio, and in aqueous solutions hydrolyzed species occur and basic oxo salts or hydrated oxides may be precipitated. Although there have been claims for a titanyl ion, Ti02 +, this ion appears not to exist either in solutions or in crystalline salts such as Ti0S04-H20. The latter has been shown to have (TiO)2"+ chains Ti Ti... 

The most important difference from titanium is that lower oxidation states are of minor importance. There are few authenticated compounds of these elements except in their tetravalent states. Like titanium, they form interstitial borides, carbides, nitrides, etc., but of course these are not to be regarded as having the metals in definite oxidation states. Increased size also makes the oxides more basic and the aqueous chemistry somewhat more extensive, and permits the attainment of coordination numbers 7 and, commonly, 8 in a number of compounds. 

An enantioselective route to 1,3-dithiane 1-oxide (33) (R = R = H) was subsequently developed [69]. It involves asymmetric oxidation of (32) (R = pivaloyl, R = H) by cumene hydroperoxide in presence of the chiral titanium complex. The syn/anti mixture (around 90% ee for each diastereoisomer) is recrystallized and then deacylated, giving the desired product in 80% yield. A recent application of this chemistry is the asymmetric synthesis of enantiopure (R)-(-)-2,6-dimethylheptanoic acid in two steps from (33) (R = C(0)Et, R = Et) [70]. The reaction involves a fully stereoselective methylation in the a-position of the keto group, followed by basic deacylation, which also regenerates enantiopure 2-ethyl-l,3-dithiane 1-oxide (33) (Ri = H, R = Et). A range of a-arylpropanoic acids have since been prepared by similar routes in high ee s. [162]... 

Factors that influence SCC response will generally affect one of four basic variables in the process the material or alloy system, the chemical service environment, the electrochemical state of the system relative to surroundings, and the state of mechanical stress. Nearly all structural alloy systems can be found susceptible to SCC under certain alloy chemistry, metallurgical condition, and service environmental conditions. SCC behavior relative to alloy system was detailed in an American Society for Metals (ASM) publication [6], for a broad range of structural alloys, and has also been covered extensively for aluminum, titanium, and high-strength steels [8]. 

Titanium and zirconium chemistry is conveniently divided between simple complexes and those based on the metallocene imit Cp2M (Cp = CsHs = cyclopentadienyl). Most simple complexes are oligomeric, insoluble, and difficult to characterise, although alkyl titanium complexes such as X3TiR have found some use as non-basic Grignard equivalents. The dicyclopenta-dienyl metal moiety, Cp2M, renders complexes monomeric, soluble, and easily characterised by NMR spectroscopy, and thus many applications based on these systems have been devised. The most stable electronic configuration of titanocene and zirconocene complexes has only 16 electrons in the valence shell, not the 18 electrons common in most of the rest of the transition metal series. The empty orbital this leaves on the metal is crucial for reactivity. 

---