Hafnium complexes Subject

The zirconium and hafnium complexes of trifluoroacetyl-acetone are white crystalline solids, insoluble in water but soluble in benzene, cyclohexane, and carbon tetrachloride. The hafnium complex melts at 128 to 129° and the zirconium complex at 130 to 131°. The complexes have been subjected to gas-phase chromatography and may be sublimed at 115° at a pressure of 0.05 mm. The proton magnetic resonance spectra of the compounds dissolved in carbon tetrachloride show single peaks in the methyl and methylene regions. The peaks appear at 2.20 and 6.00 p.p.m. (5) relative to tetramethylsilane (internal reference) for the zirconium complex and at 2.20 and 5.97 p.p.m. for the hafnium complex. 

As expected, the strong-field 7r-acidic ligand, carbon monoxide, has also been used to isolate zero-valent zirconium and hafnium complexes. This area has been a subject of relatively long-standing interest and has been reviewed.7 Chemistry outside of this review will be the focus of this section. 

Ring opening of oxiranes is catalyzed by zirconium or hafnium complexes in the presence of nucleophiles. Cp2ZrCl2 was used as a catalyst for ring opening of substituted epoxides with alcohols under mild conditions. The corresponding alkoxyalcohols were obtained in good yields. As shown in Equation 38, when trans-stilbene oxide was subjected to the reaction in methanol, a mixture of anti-and syn isomers were obtained [43]. 

Product 5 was subjected to X-ray diffraction analysis (Fig. 2 and Table 2) [6]. The structure was found to be an octahedrally coordinated metal dichloro complex with the two bidentate ligands occupying the equatorial positions. The observed stereochemistry can be considered to be caused by the trans-effect of the tmen group [3], which activates the two chloro atoms of the hafnium tetrachloride in equatorial position and makes them susceptible to the nucleophilic attack by the silyl dianion. 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

Zirconium and hafnium alkyne complexes display a wealth of reactivity. A comprehensive presentation of this chemistry has been the subject of several recent reviews.105-107 Reactivity beyond the scope of these reviews will be the focus here. 

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