Titanium coordination, octahedral

The Ti02 used here was made by hydrolyzing TNBT in distillated H2O with subsequent calcination at 500 °C. ETS-10, which is a titanium silicate molecular sieve with titanium in octahedral coordination, was provided by Engelhard, Co. For comparison, pure-silica ZSM-5 was also synthesized in the absence of alkali metal cations. Its synthesis involves the use of tetrapropylammonium bromide (TPABr) and piperazine. 

ETS-10 shows no catalytic activity. This observation indicates that titanium in octahedral coordination in ETS-10 is not active for selective oxidation and epoxidation using either aqueous hydrogen peroxide or non-aqueous alkyl hydroperoxide as the oxidant. 

The reduced form of titanium is octahedral and contains open coordination sites ( ) and chloride ligands on crystallite edges. Initiation begins by formation of an active center, believed to be a titanium alkyl. Alkylation by TEAL cocatalyst produces an active center ... 

Titanium Oxide Complex Oxides Sulfide. The dioxide Ti02, has three crystal modifications, rutile, anatase, and brookite, all of which occur in Nature. In rutile, the commonest, the titanium is octahedrally coordinated, and this structure has been discussed on page 51, as it is a common one for MX2 compounds. In anatase and brookite there are very distorted octa-hedra of oxygen atoms about each titanium, two being relatively close. Although rutile has been assumed to be the most stable form because of its common occurrence, thermochemical data indicate that anatase is —8-12 kJ mol more stablenhan rutiler-... 

Fig. 2 shows the XANES spectra of TS(x,y) and Ti02/Si02(z) samples. For comparison, the spectrum of Ti02 (anatase) is also shown. The Ti02 exhibited multiple peaks in the pre-edge region (4960-4980 eV), indicating the presence of titanium in octahedral coordination (Fig. 2a). On the other hand, all of TS(x,y) and Ti02/Si02(z) samples showed an intense pre-edge peak at 4967.1 eV (Fig.2b-g) as the characteristic of tetrahedrally coordinated titanium [19].

The structures of some heterometallic titanium-containing molecules have been reported. In the tetranuclear compound [Ti2Pb2(/r4-0)(M3-0Pr )2(M-0Pr )4(0Pr )4] the IX4-0X0 ligand has an irregular configuration (TiOTi, 163 PbOPb, 99.6° PbOTi, 95.0° and 95.4°), the titaniums are octahedrally coordinated and the lead atoms five-coordinated (distorted square pyramidal with the vacant octahedral site occupied by a lone electron pair). ... 

Complexes of titanium(III) can be made from the trichloride— these are either approximately octahedral, 6-coordinate (for example TiClj.SL (L = ligand) and [TiCljfHjOj, formed when TiCls dissolves in aqueous hydrochloric acid), or 5-coordinate with a trigonal bipyramid structure. 

Crystal structure of solids. The a-crystal form of TiCla is an excellent catalyst and has been investigated extensively. In this particular crystal form of TiCla, the titanium ions are located in an octahedral environment of chloride ions. It is believed that the stereoactive titanium ions in this crystal are located at the edges of the crystal, where chloride ion vacancies in the coordination sphere allow coordination with the monomer molecules. 

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

These alkoxides are liquids or sublimable solids and, unless the steric effects of the alkyl chain prevent it, apparently attain octahedral coordination of the titanium by polymerization (Fig. 21.6). The lower alkoxides are especially sensitive to moisture, hydrolysing to the dioxide. Application of these organic titanates (as they are frequently described) can therefore give a... 

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

Titanium dioxide differs from silica mainly in two respects (1) the Ti + ions are octahedrally coordinated in all three modifications of TiOji (2) the Ti—0 bond is more pronouncedly ionic than the Si—O bond. Using Pauling s electronegativity values (297), one calculates a 63% ionic character for the Ti—0 single bond versus 50% for Si—O. In SiOj, there is certainly some double bond character involving 3d orbitals of the Si atom, causing lowered ionic character. Therefore, characteristic differences should be expected regarding the surface chemistry. 

Rutile structure (Fig. 4-15). Titanium dioxide occurs naturally as ana-tase, brookite, and rutile, all of which contain octahedral TiC>68 units. The coordination number of the central Ti4+ is very obviously six, and a little thought confirms that the same is true of the Ti4+ ions at the corners. That the coordination number of the O2- ions is three is seen from the nearest... 

Figure 12 shows the XANES and FT-EXAFS spectra of the titanium oxide catalysts chemically doped with Cr ions (a and A) and physically implanted with Cr ions (b and B). Analyses of these XANES and FT-EXAFS spectra show that in the titanium oxide catalysts chemically doped with Cr ions by an impregnation or sol-gel method, the ions are present as aggregated Cr oxides having an octahedral coordination similar to CriOs and tetrahedral coordination similar to CrOs, respectively. On the other hand, in the catalysts physically implanted with Cr ions, the ions are present in a highly dispersed and isolated state in octahedral... 

For several of these bidentate phosphine assemblies crystal structures were obtained. Figure 10.13 (Table 10.8) shows the crystal structure of octahedrally coordinated titanium complex 39. 

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