Hafnium cyclopentadienyls

The isomer shifts in hafnium Mossbauer isotopes usually are of the order of some percent of the line width. Boolchand et al. [168] observed a relatively large isomer shift of -1-0.19 0.06 mm s between cyclopentadienyl hafnium dichloride (Hf(Cp)2Cl2) and Hf metal. From a comparison with Os(Cp)2 and Os-metal, a value of 5 r ) ( Hf) = —0.37 10 fm has been derived, which implies a shrinking of the nuclear radius in the excited 2 state. Figure 7.37 shows some typical spectra for Hf in various hafnium compounds (from [168]). 

Like zirconium, the first fully characterized carbonyl complex of hafnium was reported in 1976 by Thomas and Brown (6). This complex, bis(i7-cyclopentadienyl)dicarbonylhafnium (3) was prepared via the reductive carbonylation of Cp2HfCl2 using sodium amalgam. While the reaction proceeded to give a moderate yield of 3 (30%), this corresponded to only 60 mg of isolated product. 

The X-ray determination of crystal structure of tetracyclopentadienyl-hafnium has established that the molecule should be represented as [(7r-Cp)2-Hf(Cp)2], and thus resembles the titanium rather than the zirconium analogue. Both [(7i-Cp)2Hf(Cp)2] and [(it-Cp)2Zr(Cp)] exchange nonequivalent cyclopentadienyl rings very easily such that, even at — 150°C, only one sharp line is observed in the H n.m.r. spectrum. ... 

Triphenylsilyl zirconium and hafnium derivatives have been prepared from the silyl lithium species. Triphenylsilyl bis(cyclopentadienyl) zirconium chloride undergoes carbonyl insertion under pressure of carbon monoxide (100 psi) to give the corresponding acyl zirconium species which, upon treatment with anhydrous hydrogen chloride in a benzene matrix at —196 °C and warming to room temperature, gives rise to formyl triphenylsilane55. 

Hafnium complexes with bis(cyclopentadienyl)s, 4, 731 in carbometallations, overview, 10, 255 in C-C bond formation, 10, 424 in ethylene polymerization, 4, 1139 for isotactic polypropylene, 4, 1126 metal-metal bonds, 4, 755 mixed-valence compounds, 4, 755 with pyridyl amines, for olefin polymerization, 4, 1094 in Ru-Os heterodinuclear compounds, 6, 1046 with silicon, 3, 515... 

In the case of propylene polymerisation with class III catalysts based on rac.-dimethylsilylenebis[l-(3-methylcyclopentadienyl)]zirconium dichloride [rac.-Me2Si(MeCp)2ZrCl2] (Table 3.1) or rac.-dimethylsilylenebis[l-(3-methyl-cyclopentadienyl)]hafnium dichloride [rac.-Me2Si(MeCp)2HfCl2] as a procatalyst, isoblock polypropylene is obtained [132]. 

Dilithium 1,2-benzeneditellurolate condensed with bis[cyclopentadienyl]dichlorotitanium, -zirconium, and -hafnium to produce the metallocene 1,2-benzeneditellurolates3 4. 

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. 

Alt and Rausch (26) have claimed that low-valent titanium, zirconium, and hafnium metallocenes can be prepared by photolysis of the corresponding bis(7)-cyclopentadienyl)metal dimethyl complexes... 

Some years ago we observed that Group IV metallocenes, i.e., the elusive monomeric units bis(i7-cyclopentadienyl)zirconium and -hafnium, readily form 1 1 complexes with conjugated dienes in their 5-trans conformation (22). To our knowledge, these complexes (3) are the first and, at the time, only known examples of mononuclear (i-/ranj-i7 -diene)metal complexes, i.e., compounds in which both double bonds of the i-irans conformer of a conjugated diene are coordinated to the same transition metal center 21). Some examples of this class of transition metal complexes are stable, isolable compounds even at room temperature 23). 

AG ii c = 14.3 kcal/mol). ( -c -Butadiene)- and -2,3-substituted buta-diene)zirconocene complexes exhibit markedly lower barriers (Table III) 50-52). In general, the corresponding bis(Tj-cyclopentadienyl)hafnium... 

Table 3. Preparation of tt-Cyclopentadienyl Complexes of Titanium, Zirconium, and Hafnium ...

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