Zirconium complexes alkoxides

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

The quadridentate ligands A,iV-dihydroxyethylglycine and N-hydroxyethyliminodiacetate(HIMDA) form 2 1 chelate zirconium complexes which are stable with respect to hydroxide precipitation even up to pH 10. These quadridentate ligands involve the bonding of the alkoxide groups at the higher pH values. The formation constant for the 1 1 HIMDA-hafnium complex in 0.123 M HCIO4 is log A = 14.6 (170, 305). 

There are no dinitrogen complexes of any of these metals that contain a phosphine ligand. The considerable amount of nitrogen-fixing chemistry reported for titanium, and to a lesser extent zirconium, involves alkoxide, cyclopentadienyl, and halide complexes.The catalytic nitriding reactions of titanium will be reviewed in Section 3. 

A suitable entry into titanium and zirconium complex chemistry is the use of group IV amides and alkoxides. For example, when Ti(OEt)4 reacted with 2 equiv. of (roc)-2 in heptane, the titanium bis(disiloxide) 3 could be isolated in 91 % yield as a yellow microcrystalline material. Tbe results of the X-ray analysis (Fig. 1) of 3 confirm the expected extensive shielding of the titanium atom by the two sterically demanding disiloxide ligands. The geometry around the titanium atom is described best as distorted tetrahedral, with an 02-Til-02 chelate angle of 99° and an 02-T11-01 angle of 115.5°. 

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

But Chmura and co-workers reported alkoxide complexes of group 4 metals (Ti, Zr, Hf), 22, which all of them were active in LA ROP [23], Interestingly, zirconium complex had far higher activity than titaniiun and hafnium coxmterparts (Table 7.2, entry 55-57). But although with titanium complex, isolated polymer was atactic, in contrast to others two in which heterotactic PLAs obtained. 

But Alves and co-workers prepared a completely different kind of zirconium alkoxide bulky complexes, 25, which were moderately active in LA ROP (Table 7.2, entry 58-59) [26]. Their investigations showed that di-alkoxide zirconium complex were more active than mono-alkoxide chloro derivative. This difference was attributed to the presence of chlorine atom that withdraws electron from zirconium and lowers the nucleophilicity of Zr-OTr moiety which proved by DFT calculation. As a result, the ROP reaction retarded in which became inactive at room temperature and was needed higher temperature to initiate polymerization. Another versatility of this research was incorporation of Bn Cyclam instead of OPh or SPh to the end of PLA which made biodegradable PLA as an interesting candidate utilizes various biomedical and sensing applications. 

Mn(II) > Mg(II).270 It should be underlined that titanium and zirconium alkoxides are efficient catalysts for both stages of reaction. Lanthanide compounds such as 2,2/-bipyridyl, acetylacetonate, and o-formyl phenolate complexes of Eu(III), La(III), Sm(III), Er(III), and Tb(III) appear to be even more efficient than titanium alkoxides, Ca or Mn acetates, Sb203, and their mixtures.273 Moreover, PET produced with lanthanides has been reported to exhibit better thermal and hydrolytic stability as compared to PET synthesized with the conventional Ca acetate -Sb203 catalytic system.273... 

Reaction of tris(neopentyl) complexes of titanium, zirconium and hafnium with molecular oxygen furnishes the corresponding tris(neopentoxy) complexes [42, 43, 51]. A peroxo complex is an intermediate in this reaction, being relatively stable in the case of titanium [42]. The alkoxide species can also be formed upon reaction with alcohols under mild conditions [42, 52]. The alcoholysis reaction is fast, with a low dependence on the steric hindrance of the alkyl chain [42]. Hydrolysis leads to ](=SiO)M(OH)3] or ](=SiO)2M(OH)2], depending on the precursor species. Deu-... 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

This method is particularly useful for the synthesis of the alkoxide and phenoxide derivatives of the earlier transition elements. The method is extremely convenient in view of the high volatility of the generated dialkylamines, which are readily removed in vacuum. One major drawback is the synthetic availability of the corresponding metal dialkylamide complex. In some cases the method represents not only the most convenient but also the only synthetic route to an alkoxide derivative. Hence, zirconium tetra-t-butoxide is formed in excellent yield from Zr(NEt2)4 and Bu OH, and the Vlv and CrIV r-butoxides are also readily obtained via this pathway (equation 11).74... 

In the Halcon epoxidation process, the reaction of the zirconium(IV) methyl trialkoxide (201) with 02 yields the epoxy alkoxide (203), via intramolecular epoxidation of the coordinated allyl alcohol by the incipient methyl peroxide complex (202).630... 

These results explain previous observations of the greater stability of zirconium thiocyanate and selenocyanate complexes compared with their hafnium analogues, and the greater stability of zirconium and hafnium complexes in MeCN compared with DMF in terms of competition between the ligand and solvent molecules for co-ordination sites on the metal. Zirconium alkoxides have been prepared from ZrCl4 and aliphatic alcohols158 but with salicylaldehyde a Meerwein-Ponndorf... 

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