Hafnium alkoxide complexes

Saha et reported zirconium and hafnium alkoxide complexes bearing a... 

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 . ... 

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. 

Table 2.9 Thermodynamic properties and molecular complexities of titanium, zirconium, and hafnium alkoxides...

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. 

J-Butylperoxy complexes of bis(pentamethylcyclopentadienyl)hafnium(IV) [(C5Me5)2Hf(OOBu )R 204 R = Me, Et, Ph] have been prepared from the reaction of Bu OOH with (C5Me5)Hf(H)(R), and thermally decompose to give the mixed alkoxide (C5Me5)Hf(OBu )-(OR). The X-ray crystal structure of (204 R = Ph) indicates a monodendate r-butylperoxy ligand.631... 

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... 

A mild oxidation of alcohols to ketones using metal alkoxides (Al, K) and a ketone or with lantanide catalyst, zirconium or hafnium complexes (see 1st edition). 

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). 

Tetrakis(acetylacetonato) complexes of zirconium and hafiiium were reported in 1904 and 1926, respectively, and a large number of j3-diketonate derivatives have been described subsequently (Table 10). These compounds are of the type [M(dik)4], [M(dik)3X], [M(dik)2X2], M(dik)X3 and [M(dik)3]Y (dik = -diketonate anion X = C1, Br, I, NO3, or alkoxide Y = [FeCU], [AuCU], [PtCU] or 2[Zr(S04)3]). Additional j3-diketonate compounds include the anionic complex [NEt4][Zr(bzbz)F4] (bzbz = dibenzoylmethanate) and the 1 1 ZrCU-diketone adducts [ZrCl4(MeCOCR2COMe)] (R = H or Me). " Early work on zirconium and hafnium jS-diketonates has been reviewed by Larsen, Bradley and Thornton, Fackler and Mehrotra et al. ... 

The recent mass spectral findings of Turevskaya etal. on tetra-alkoxides of zirconium and hafnium are usually in good agreement with their molecular complexity. " " However, the existence of a high intensity of M30(0R)9+ or M40(0R)i3+ ions in the mass spectra of [M(OR>4] tends to indicate the complex nature of such species. The X-ray crystal structure determination of [M(OR)4] is called for to solve such an ambiguity. 

The fragmentation pattern for tantalum penta-alkoxides Ta(OR)5 (R = Me, Ft, Pr, CH2CH20Me), in their mass spectra is similar to that for alkoxides of zirconium and hafnium, niobium, molybdenum, and tungsten. For example, at first the loss of OR groups occurs, followed by elimination of the molecules of unsaturated hydrocarbons (usually, with the same number of carbon atoms as R). However, the decomposition of Ta(OMe)s differs from that of other homologues by the loss of HCHO molecules and H atoms. Elimination of ethers, R2O, which is accompanied by the formation of the metal oxoalkoxide ions of the above mentioned alkoxides is typical. In the absence of X-ray crystallographic data, considerable effort had been directed earlier to throw light on the molecular complexities of aluminium trialkoxides in different (solid, liquid, or vapour) states. 

Our recent studies have also confirmed such a phenomenon. For example, the barium-hafnium complex [Ba4Hf2 (/ttg-0)(/ 3,r/ -0CH2CH20CH3)g(rj-0CH2CH20CH3)2(ji-H0CH2CH20CH3)4Cl4] was obtained fl-om the reaction of Cp2HfCl2 and an appropriate group 2 barium alkoxide [18]. The key factor was to choose the appropriate thermolysis... 

Synthesis ofheterometallic complexes by direct interaction ofhomometallic alkoxides has been reported in many cases also for the rare earth metals, but in this case it is necessary to keep in mind that the commercial Ln(OTr)3 usually contain the oxoalkoxide complex, Ln50(0 Pr)i3 as their major component. The reactivity of the latter toward other alkoxides is comparably low, and prolonged refluxing in toluene or the reaction in a melt is recommended to insure the completeness of transformation (Poncelet, 1989). The only reaction between the two high-valent metal alkoxides, not involving specific mechanisms with formation of oxoalkoxides, is the formation of the aluminium and hafnium isopropoxide (Turevskaya, 1997) ... 

---