Heterobimetallic alkoxides

Most of the knowledge about aluminate and alkylaluminum coordination stems from X-ray crystallographic studies. The basic idea of this section is to compile a rare-earth metal aluminate library categorizing this meanwhile comprehensive class of heterobimetallic compounds. Main classification criteria are the type of homo- and heterobridging aluminate ligand (tetra-, tri-, di-, and mono alkylaluminum complexes), the type of co-ligand (cyclopen-tadienyl, carboxylate, alkoxide, siloxide, amide), and the Ln center oxidation state. In addition, related Ln/Al heterobimetallic alkoxide complexes ( non-alkylaluminum complexes) are surveyed. Emphasis is not put on wordy structure discussions but on the different coordination modes (charts) and important structural parameters in tabular form. An arbitrary collection of molecular structure drawings complements this structural report. 

Synthesis of Heterobimetallic Alkoxides from the Component Metal Alkoxides... 

Although many heterobimetallic alkoxides were characterized structurally (cf. Section III. A), the crystal structure of the first homoleptic heterotrimetallic isopropoxide [Ba Zr2(0-/-Pr)9 (/i-0-(-Pr)2Cd(/t-0-/-Pr)]2 was recently elucidated (39a). 

The reactions of bivalent metals (e.g., Mg, Ca, Sr, or Ba) with alcohols are extremely slow even in the presence of catalysts such as I2 or HgCl2. This could be due to the rather poor solubility of alkoxides of bivalent metals (M ), the superficial layers of which on the free metal might hinder direct reactivity further. It was shown (6, 14-16) that alkoxides of a number of metals such as Al, Zr, Nb, and Ta, not only facilitate the reactions but also result in the formation of soluble volatile heterobimetallic alkoxides, for example,... 

A wide variety of heterobimetallic alkoxides. which were synthesized according to Eqs. 28-30, are represented by the following examples (3) for elements E with valency... 

The earlier account of the structural features of di- and polynuclear homo-and heterometallic alkoxides have already been comprehensively reviewed (4a, 23, 28, 38, 39, 42, 199, 202). However, with the current resurgence of interest in the structural characteristics of associated homometal alkoxides and monomeric heterobimetallic alkoxides, we present current status of the solid state structures (as determined by X-ray crystallographic studies) of such derivatives. 

An interesting heterobimetallic alkoxide derivative containing copper(I) and zirconium, Cu2Zr2(O-/-Pr),0, was assigned the Structure XV crystallographi-cally (248), and consists of a Zr2(0-/-Pr)9 face-sharing bioctahedron with two... 

The reactions between alkoxides of different metals between themselves and sometimes with metal halides was found to yield homoleptic or heteroleptic chloro heterobimetallic alkoxide under suitable condition (cf., Section II.C.2). 

A variety of aluminate spinels MAI2O4 (M = Co, Ni, Cu) have been synthesized by Meyer etal. [319] via microemulsions. Non-ionic, nonylphenol-poly[(n)glycol ether] type surfactants (n = 7, 10, 15), i.e. the Tergitol series TNP-7, TNP-10 and TNP-35 were used with octan-l-ol as a co-surfactant. The oil phase was cyclohexane or n-heptane, while the aqueous phase was pure water. The individual microemulsions produced out of the above constituents were added dropwise under dry nitrogen atmosphere to isopropyl or tert-butyl alcohol solutions of the relevant heterobimetallic alkoxides to effect precipitation through hydrolysis. 

Of the commonly employed bases (NH3, NaOR, KOR) for completion of the reactions and preparation of soluble metal alkoxides, NH3 appears to have some distinct advantages including (i) passage of anhydrons ammonia in a reaction mixture of an anhydrous metal chloride and alcohol prodnces heat by neutralization of the liberated HCl with NH3 the cooling of the reaction mixture is an index of the completion of the reaction, (ii) precipitated NH4CI can be filtered easily, (iii) excess NH3 can be easily removed by evaporation, whereas (iv) heterobimetallic alkoxides like NaAl(OR)4 and KZr2(OR)9 tend to be formed with excess of alkali alkoxides. 

Zirconium tetra-alkoxides were prepared for the first time in 1950 by the ammonia method, as earlier attempts to use the alkali alkoxide method did not give a pure product, owing to the tendency of zirconium to form stable heterobimetallic alkoxides (Chapter 3) with alkali metals. 

The reactions in THF have been reported to be cleaner with higher yields in all the cases when n = -4. The formation of heterobimetallic alkoxides, Na2Ce(OR>6 and NaCe2(OR>9 was also reported, when n = 6 or 4.5, respectively. 

By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis of zirconium tetra-alkoxides or niobium (tantalum) penta-aUcoxides, as these tend to form heterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1), which volatilize out during final purification, whereas alkali titanium alkoxides, even if formed, dissociate readily to give volatile titanium alkoxides. 

By contrast, pure binary methoxide of zinc and ethoxides of zinc, cadium, and mercury could not be isolated by the lithium alkoxide method possibly because heterobimetallic alkoxide complexes were formed. 

The above reactions when carried out in 1 4 molar ratio with CrCl3(thf)3 and FeCF result in heterobimetallic alkoxide complexes, respectively of the types [(thf)Li(/u,-OCHBu2)2Cr(OCHBu2)2] and [(HOCBu2)Li(/Li-OCHBu2)2Fe(OCHBu2)2]. 

Previous sections have dealt with heterobimetallic alkoxides with the four most commonly used ligands Al(OR)4 , Nb(OR)e, Ta(OR)6 , and various types of alkoxometallates(iv). A brief description is presented in this section of a few alkoxometallate(ii) and other ligands. 

A distinctive feature of heterobimetallic alkoxides is their higher volatility and solubility in comparison with complexes of the Ln(OR)3 series. Alumoalkoxides of REM are distilled without decomposition at temperature 150-200° C/0.5 mm Hg. 

Meese-Marktscheffel J.A., Cram R.E., Gilje J.W. Magnesium-aluminium alkoxides The synthesis of Mg[Al(OR)4]2 (R= and Ph), structure of (thf)2Mg[( t-OPh)2Al(OPh)2]2> and dynamic NMRofMg[Al(OBu =)4]2. Polyhedron 1994 13 1045 Meyer F., Hempehnann R., Mathur S., Veith M. Microemulsion mediated sol-gel synthesis of nano-scaled MAI2O4 (M = Co, Ni, Cu) spinels from single-source heterobimetallic alkoxide precursors. J. Mater. Chem. 1999 9 1755... 

Among the multiple oxide particles, barium titanate has been successfully prepared from reverse microemulsions (Herrig and Hempelmann, 1996 Beck et al., 1998) by using isopropanolic solution of Ba- and Ti-alkoxides in 1 1 molar ratio, cyclohexane as the oil phase, and various non-ionic surfactants. Such particles were nanometric (less than 20 nm) in size. A series of nanoparticulate aluminates of transition metals Co, Ni and Cu have been synthesized from microemulsions by Meyer et al. (1999). The noteworthy point in this synthesis is the application of heterobimetallic alkoxides as the single source materials of the cations in each case. 

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