Titanium precursor

Enantioselective reactions involving q1 -allyltitanocenes are almost unknown. An attempt to realize an asymmetric transfer of the allyl group has been reported by Reetz [40], who employed a chiral titanium precursor with two different Cp groups and a stereo-genic center at the metal (CpCptBu(C6F5)Cl) [41]. However, the addition of the derived al-lyltitanium reagent to aldehydes was found to proceed with a low chiral induction (ee up to 11%) in this case. 

Deposition of a titanium precursor onto preformed Si02 followed by transformation of the precursor into Ti02. 

Attempts have been made to synthesize Ti-ZSM-48 with even higher titanium contents, but it was reported that no more than 2% titanium could be incorporated into framework positions. UV-Visible spectroscopic analysis of materials prepared with more titanium precursor indicates the presence of extra-framework Ti02, which in some cases is present also in the materials with low titanium contents. From these observations it is concluded that for Ti-ZSM-48 a limit exists in the amount of Tilv that can substitute for Silv in framework positions (Reddy, K. M. et al., 1994b). 

Figure 20.16 schematically illustrates the process of titania film growth by ALD. The substrate is hydroxylated first, prior to the introduction of titanium precursor, titanium tetrachloride. Titanium tetrachloride reacts with the surface hydroxyl groups through a surface condensation reaction ... 

Neighboring hydrolyzed titanium precursors subsequently condensate to form a Ti-O-Ti linkage ... 

The by-product HCl and excess H2O are then removed from the reaction chamber. One layer of Ti02 has been grown by the completion of one cycle of chemical reactions. The surface hydroxyl groups are ready to react with titanium precursor molecules again in the next cycle. By repeating the above steps, two or more Ti02 layers can be deposited in a very precisely controlled way. 

The well-known Sharpless system for the enantioselective epoxidation of allyl alcohols has been investigated [23]. This system employs a tetra-alkoxy titanium precursor, a dialkyltartrate as an auxiliary, and an alkyl hydroperoxide as oxidant, to effect the enantioselective epoxidation. The key intermediate is thought to be a dimeric complex in which titanium is simultaneously coordinated to the chelating tartarate ligand, the substrate in the form of an oxygen bound / -allyl-oxide and an -tert-butylperoxide. 

For many years, CVD TiN has been used for wear-and erosion-resistant applications. TiN has a low coefficient of friction and is relatively chemically inert, which makes it attractive for this purpose. In addition, the coating of stainless steel with TiN is of interest for increased biocompatibility of surgical tools and human implants. The reactions used to deposit TiN are very similar to those used for the deposition of Ti02. TiCU is the most common titanium precursor. Nitrogen or ammonia can be used as the nitrogen source. 

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... 

Ti-MCM-41 was prepared by either grafting titanium precursor onto surface silanols via a post-synthetic procedure or depositing titanium precursor on MCM-41 from the sol obtained by controlled hydrolysis of a titanium alkoxide precursor followed by calcination. 

Recently, The reduction of TiCl has been studied in Lewis acid ionic liquids [BmimjCl/AlClj in our laboratory. It was found that the aluminum-titanium alloy of 14 pm thickness as given in Fig. 5.9 can be deposited on the titanium or aluminum substrate. Thermodynamically, Ti deposition should be possible in thick layers in ionic liquids, but the right ionic liquid and especially the right titanium precursors still have to be found. An idea might be to make TiCTf N) or similar compounds for titaninm electrodeposition. 

Applying a similar method to the nylon fibers with a more concentrated precursor resulted in films containing a hollow morphology on removal of the fibers. Hence in this case a casting of the fibrous mat was attained. It was noted that the film containing the fiber had relatively few cracks compared with the flakes formed after calcination by the deposition of the titanium precursor on a glass slide without the fibers [43]. 

From the chemical point of view, there are two main processes to obtain thin films of TiN by CVD, namely (1) via a chemical reaction of a titanium source and a nitrogen source, or (2) via the decomposition of a nitrogen containing titanium precursor (single source precursor). Each of them needs a special development of precursors. 

Table 3-2. Titanium precursors forTiN films prepared by CVD.

As it was mentioned previously, this process normally proceeds via an acid-catalyzed hydrolysis step of a titanium precursor, such as titanium(IV) alkoxide, followed hy condensation. The development of Ti-O-Ti chains is favored with low content of water, low hydrolysis rates and excess titanium alkoxide in the reaction mixture. Three dimensional polymeric skeletons with close packing result from the development of Ti-O-Ti chains. The formation of Ti(OH)4 is favored with high hydrolysis rates for a medium amount of water. The presence of a large quantity of Ti-OH and insufficient development of three-dimensional polymeric skeletons lead to loosely packed first-order particles. Polymeric Ti-O-Ti chains are developed in the presence of a large excess of water. Closely packed first order particles are yielded via a three-dimensionally developed gel skeleton [22, 28, 132-135]. 

In another work, Xia et al. [354] studied the electrocatalytic properties of PtRu/C-r/02 toward the oxidation of methanol. PtRu/C-Ti02 catalysts were prepared by sol-gel method using titanium precursor (Ti(OBu)4) on PtRu/C. They described that the activities of the catalysts toward methanol oxidation decreased in the following order. PtRu/C-Ti02 after sintering > PtRu/C-r/02 > PtRu/C > PtRu/C after sintering. 

---