Zirconium and hafnium alkoxides

That was in fact for zirconium alkoxides that Bradley developed all the major synthetic approaches to the alkoxides ofhigh-valent metals—metathesis of ZrCl4or (more preferably [PyH]2ZrCl6)with ammonia [201] (method 5), alcoholysis of Zr(NR2)4 [1579] (method 4), alcohol interchange reactions 

Chemical composition. Structure Synth. Properties route Investig. methods Ref. 

IBaZrtpj-OHXOPrM PrOHh], struct, type of Tu(OMe)16, [Ba C ]-capped trig.prism, fZr2Ofil -oct. colrl.cr, v, s. in PrOH h,, jC, X-r 1689 

The presence of halide ligands in the molecules of bimetallic alkoxide complexes hinders apparently their decomposition with formation of oxoalkox-oderivatives. Examples are CunClZr2(OPr )9 [1664], CdHalZr OPti), [1508, 1687], Veith etal. [1687,1691] used the the stepwise replacement of Cl andl atoms for preparation of tiimetallic zirconates, [MnCdZr2(OPr )12]2, M11 = Ca, Sr, Ba These octanuclear aggregates contain linear [Ba(p-OR)2Cd(p-OR)2Cd(p-OR)2Ba] chains in the center and [Zr2Ba] triangles at the ends of each molecule (see also Section 4.7.3). 

The alkoxozirconates that sublime in a vacuum include M[Zr2(OR)9], where M = Li, Na, K, Tl, Mg [115,1116], K2Zr2(OBu ),0 [1112, 1564], M2Zr[OCH(CF3)2]6,M = Na, Tl[1387],MZr(OPri)6,M = Sn,Pb,PbZr(OBu )6, [1565, 1663] and also—quite naturally—oxocomplexes, whose molecules have nearly spherical shapes, such as K4Zr2O(OPii)10 [1385,1666] and Pb3ZrO(OPri)8 [1565], 

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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 . ... 

It was quite unexpectedly found that the amorphous samples of zirconium and hafnium alkoxides M(OR)4 contain several types of oxocomplexes, particularly, M3O(OR)i0 and M40(0R)14 [1612], The trinuclear Zr3([l3-0)()i3-OBu XOBu ), was isolated in a crystalline form and turned out to be a structural analog of the known isopropoxide clusters of Th, Mo, and U(IV) -MjOCOPr ),) [1520] (see also Sections 4.3 and 12.12). The inclusion of the solvent molecules inside the cavities of the structures and formation of alcohol solvates in many cases leads to microanalysis data that does not deviate much from those calculated for M(OR)n. 

Thermolysis of tin and lead alkoxozirconates leads to the formation of metals. The mass-spectral data indicate the presence ofbarium and aluminium derivatives in the gas phase, but no preparative data are accessible for them. The major application of zirconium and hafnium alkoxides lies now in the sol-gel technology of zirconate-titanate and solid solutions Zr02-Y203 (see Section 10.3), Except in the synthesis of oxide materials, the alkoxides of zirconium and hafnium are traditionally used in the polymer chemistry, where they are applied as the components in catalysts [1278, 1269] and as additives to polymers, improving their characteristics [825, 1403] and so on. Already in 1930s Meerwein has proposed the use of zirconium alkoxides for the reduction of aldehydes intoprimary alcohols (Meerwein-Schmidt reaction) [1420],... 

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

Table 2.9 indicates that the boiling points of zirconium " " and hafnium alkoxides are quite similar but differ from those of titanium analogues. " " An important feature of Table 2.9 is the fact that ferf-alkoxides of hafnium are more volatile than the analogous zirconium derivatives. The entropies of vaporization of zirconium and hafnium fert-amyloxides are also nearly the same (39.5 and 39.8 cal deg mol respectively) and these results are in satisfactory accordance with the chemical similarities of zirconium and hafnium. 

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. 

This process is a common one in the fragmentation of the majority of metal alkoxide molecules in the mass-spectra. A typical example of the latter is the fragmentation pattern of zirconium and hafnium oxoalkoxides [1612] (Fig. 5.1). 

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... 

The amides of zirconium and hafnium, M(NR2)4, like the alkoxides, are associated. The degree of association depends on the nature of R. Zr(NMe2)4 (mp 70 °C, bp 80 °C at 0.05 mmHg) in the sohd state adopts the amide-bridged dimeric structure Zr2(/u.-NMe2)2(NMe2)6. A monomer-dimer equilibrium is estabhshed in solution but only the monomer exists in the gas phase. Gas-phase electron diffraction data indicate that the N atoms are coplanar, as a result of N to Zr p to donation seep Bonding). ... 

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. ... 

Turevskaya EP, Kozlova NT Turova NY, Belokon AL, Berdyev DV, Kessler VG, et al. The alkoxides of zirconium and hafnium direct, electrochemical synthesis and mass-spectral study. Do M(0R)4 , where M = Zr, Hf, Sn, really exist . Russ Chem Bull 1995 44(4) 734-42. 

Tetrakis(hexafluoroisopropoxides) of titanium, zirconium, and hafnium have been prepared by the reaction of sodium hexafluoroisopropoxide in an excess of hexafluoroisopropyl alcohol with the anhydrous metal chloride, and the spectroscopic properties are reported. The authors note that, in their experience, application to the Group IV transition metals of the previously published method for metal hexafluoroisopropoxide synthesis gives poor yields of material containing metal, fluorinated alkoxides, and co-ordinated ammonia. Dehydration of the hydrated metal chloride with methyl orthoformate and addition of hexafluoroisopropyl alcohol, followed by passage of dry ammonia through the solution, gives satisfactory yields for yttrium, lanthanum, neodymium, and erbium. 

On the basis of the comparative volatilities of titanium, zirconium, and hafnium tertiary alkoxides measured at various pressures, the following order of volatility may be deduced Per > Pzr > Pti- The anomalous order cannot be explained in terms of intermolecular forces between tertiary alkoxides as the heats of vaporization are quite close to each other for these alkoxides. Bradley et however, concluded that... 

In contrast to the monomeric nature of tertiary alkoxides of titanium, zirconium, and hafnium, the corresponding cerium and thorium lower tertiary alkoxides exhibit association, which decreases with increasing chain length of the groups attached to the tertiary carbon atom and finally Th(OCMeEtPr )4 and Ce(OCMeEtPr")4 show monomeric behaviour. On the basis of the above observations, the order of volatility of some quadrivalent metal alkoxides may be assigned Si(OR)4 > Ge(OR>4 > Ti(OR)4 > Hf(OR)4 > Zr(OR)4 > Ce(OR)4 > Th(OR)4. However, for monomeric tertiary alkoxides, the order of volatility is Hf(OR >4 > Zr(OR )4 > Ti(OR >4. 

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. 

Chlorides or alkoxides of zirconium and hafnium provides an environmental-friendly ester condensation method of carboxylic acids and alcohols. In the presence of Zr... 

Table 9 Boiling Points and Degree of Association for Zirconium(IV) and Hafnium(IV) Alkoxides and...

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