Titanium and Hafnium

The preparation of (fl--C5HB)2Hf(C FB)j, and of 5,5-bis-(ir-cyclopenta-dienyl)octafluorodibenzohafnol has been reported.  

Vanadium.— Reaction of pentafluorophenyl-lithium with vanadium tetrachloride in diethyl ether and hexane at — 70 °C yields tetrakis(pentafluoro-phenyl)vanadium, which crystallizes as the dietherate at 0 °C. It is very moisture sensitive, yielding pentafluorobenzene and a trace of perfluoro-biphenyl, and with mercuric chloride in tetrahydrofuran yields bis(penta-fluorophenyl)mercury (95%). 

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Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

Zirconium, titanium, and hafnium hydrides can activate the C-H bonds of several alkanes at low temperatures (even at room temperature) because they are very electrophilic and reactive. Moreover, the surface complex is immobilized by the strong metal-silica bonds, and this immobilization can prevent the coupling reactions leading to the deactivation of the complex. 

Mono(siloxy) metalhydrocarbyl species can be converted into bis- or tris(siloxy) metal hydrides by reaction with hydrogen, as shown for zirconium and tantalum. Such species are less susceptible to leaching and this route can be extended to titanium and hafnium surface species that are potential precatalysts for hydrogenolysis of C-C bonds, alkane metathesis and epoxidation reactions. 

The analogous titanium and hafnium compounds form active catalysts, too. Especially at higher temperatures the zirconium catalysts are more stable and active than the titanium or hafnium systems. The co-catalyst has a main influence. The most used co-catalyst is methylalumoxane (MAO). At lower temperature, fluorinated borane compounds activate the catalyst, too [17,18]. The structure of MAO is complicated. Sinn characterized MAO by different analytical methods and found it to be a cage with a formula of AlisO 12(0113)24 [19]. 

Hydrogen atoms are small enough to occupy the interstitial holes in a metal lattice and the absorption of H2 by a variety of metals (and also alloys) leads to the formation of metal hydrides in which hydrogen atoms reside in interstitial cavities, interstitial metal hydrides. For example, non-stoichiometric hydrides TiHj 7, HfHi gg and HfH2.io are formed when titanium and hafnium react with H2. Niobium forms a series of non-stoichiometric hydrides of formula... 

Zirconium (Zr, CAS 7440-67-7, atomic number 40, atomic mass 91.22) has a melting point of 1852 °C and a boiling point of 4377 °C. It is a hard, lustrous, silvery metal, in contrast to fine zirconium powder, which is black. Zirconium belongs to Subgroup IV of the Periodic Table of the elements, between the elements titanium and hafnium - two metals with which it is often found in nature. Zirconium has oxidation states ranging from II to IV, of which the tetravalent is relatively stable and abundant (Venugopal and Luckey 1979). Zirconium is very corrosion-resistant and is unaffected by alkalis or acids (except for HF). 

While not pyrophoric, the porous copper metal monolith is readily combustible by the application of a flame in air. In addition to copper, this method has been appUed to prepare other nanoporous metals such as iron, cobalt, nickel, and tin, as well as carbides of chromium, titanium, and hafnium (Chap. 14). This method is especially powerful as it appears to be applicable to a wide variety of metals and affords articles that are mrmolithic. 

New catalysts based on Titanium and Hafnium derivatives co-supported on magnesium chloride are actractlng more and more Interest for the preparation of olefin polymers and. In particular, of high density polyethylene (HDPE). 

Industrial bimetallic catalysts based on Titanium and Hafnium supported on MgC12 show high activity for the polymerization of ethylene and an appreciable activity In the polymerization of 4-methyl-l-pentene. 

Titanium and hafnium complexes [CpMCl2(S2CNR2)] and [CpMCl(S2-CNR2)2l, in which the dithiocarbamate derives from substituted thiadiazoles (Fig. 4) have also been prepared (52, 631). For the titanium complexes, on the basis of IR data in which two v(C—S) bands are observed, it is postulated that each dithiocarbamate and the nitrogen of the thiadiazole ring is metal bound to produce six-membered chelate rings, however, this has not been confirmed crystallographically (52). 

Zirconium, titanium, and hafnium compounds have been used in combination with fluoride as alternatives to chromates with reasonable success but are never as good as the Cr(VI)-treated surfaces [38,39]. 

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