Zirconium 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. 

No formation of bimetallic complexes is observed on MoO(OPr )4 dissolution in the alcohol solutions of zirconium isopropoxide, due presumably to the high stability of the structure of the latter. A very unusual complex of Zr3Mo,024(OPri)12( PrOH)4 composition precipitates slowly from solutions of isopropoxides in hexane subjected in advance to evacuation to dryness and redissolution repeated three times. The structure of the complex obtained is very close to that of the zirconium methoxide hydrolysis product, Zr O OMe) (Fig. 5.1 c) [901]. The formation of a complex very rich in oxoligands is presumably due to the trend of ZrCOPhV PrOH to form oxocomplexes on desolvation (see Section 12.12). 

The generation of an unsymmetrical titaniumcyclopentadiene from different acetylenes can then be subjected to a [4+2] cycloaddition with a sulfonylnitrile to form an intermediate complex which collapses under acidic conditions to yield substituted pyridines (Scheme 60 Table 2) <2002JA3518>. A similar reaction using zirconium has been reported <2002JA5059>. 

Less acidic than Ti and Zi chloroderivatives, MeTi(OPr )3 perfoims chelation-controlled addition to chiral alkoxy ketones as well as or better than organomagnesium compounds, but fails to chelate to aldehydes or hindered ketones. Should the formation of a cyclic chelation intermediate be forbidden, the reaction is subject to nonchelation control, according to Ae Felkin-Anh (or Comforth) model. Under these circumstances, the ratio of the diastereomeric products is inverted in favor of the anti-Cram product(s). In the case of benzil (83 Scheme 7) this can be accounted for by the unlikely formation of a cyclic intermediate such as (85), and thus the preferential intermediacy of the open chain intermediate (86) that leads to the threo compound (88). This view is substantiated by the fact that replacement of titanium with zirconium, which is characterized by longer M—O bonds, restores the possibility of having a cyclic intermediate and, as a consequence, leads to the erythro meso) compound (87) thus paralleling the action of Mg and Li complexes. 

Plants in the UK, USSR and France are now reprocessing irradiated UO2/PUO2 fuels and LMFBR fuel reprocessing has been the subject of international conferences. " The plant at Cap la Hague, France, employs a 30% TBP solution and no U/Pu separation is undertaken, so that a mixed U/Pu product is obtained. Fluoride is added to the process feed to complex zirconium and suppress its extraction." The Dounreay plant in the UK employs a 20% TBP/OK solution and uses sulfuric acid to effect the U/Pu separation. TBP poorly extracts Pu or U from sulfuric acid solutions, but in mixed HNO3/H2SO4 the equilibria shown in equations (206) and (207) must be considered. The equilibrium constants for these reactions, Kj, and K i, are given... 

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. 

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. 

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. 

The sterically demanding alkyne, Me3SiC=CSiMe3, has been widely used to stabilize a range of zirconocene complexes while also serving as a convenient leaving group to access the rich chemistry of divalent zirconium. These results have been the subject of a comprehensive review. Representative reactivity will be presented here. 

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]. 

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