Tetra-alkoxoaluminates

Properties of Tetra-alkoxoaluminate and Hexa alkoxoniobate (-tantalate) Derivatives... 

Of the tetra-alkoxoaluminates of various alkali metals (Li, Na, K, Rb, Cs), the potassium derivative was found in 1976 ° to show the maximum solubility and volatility. This solubility behaviour was ascribed to the suitability of the size of potassium enabling the formation of a di-isopropanolate adduct. 

The coordination sphere of K in [(PPO)2Al(/i-OPP)2K.2PPOH] was found to be similar to that of magnesium in [ (PPO)2Al(/i-OPr )2 2Mg.2IVOH]2. This latter product along with a series of volatile soluble products with the formula, M M (OR>4 2 (M = Mg, Ca, Sr, Ba M = Al, Ga R = Et, were isolated (Eq. 3.25) by the dissolution of metal M in alcohol in the presence of 2 moles of M (OR)3. This comparatively facile dissolution of bivalent metal in alcohol has been ascribed to the coordination of alcohol to the M(OR)3, rendering the proton more labile and reactive. The mass and NMR spectra of Mg Al(OBu )4 2 liquid, and the X-ray structure of Mg(/Li-OPh)2Al(OPh)2, confirm a similar coordination model. Table 3.1 lists some representative tetra-alkoxoaluminate derivatives. 

Compared to the tetra-alkoxoaluminates, the corresponding hexa-alkoxoniobate and -tantalate derivatives of most of the metals are less stable. Of these two, the niobate products tend to disproportionate more readily this is reflected in the conductivity curves obtained in the titrations of niobium and tantalum pentaisopropoxide with alkali (Na, K) isopropoxides in isopropyl alcohol (see Fig. 3.2). 

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