Alkali metal alanates

The crystal structures of all complex aluminum hydrides are built up by [AlH4] tetrahedra or [AlHg] octahedral units. These building units can be either isolated, as for example in NaAlH4, or they can form more complex structures such as chainlike structures, as for CaAlHs. The decomposition of alkaline earth aluminum hydrides proceeds via hydrides to intermetallic compounds whereas alkali metal alanates decompose via an intermediate hexahydride structure to the corresponding hydride. Table 5.2 summarizes the physical data of selected complex aluminum hydrides. 

Alkali metal alanates undergo dehydrogenation in the 200-300 °C temperature range (Li, 201 °C Na, 265 °C K, 290 °C) to give aluminum metal and the corresponding alkali metal hydrides (see Eq. 14.1). 

Bogdanovic and Schwickardi found that the kinetic enhancement upon Ti-doping can be extended to the reversible dehydrogenation of LiNa2AlH to LiH, 2 NaH, and Al. They found the mixed alkali metal alanate to have significantly lower plateau pressure at 211 °C and thus a higher than... 

A further possibility for the conversion of alkali metal alanates into organo-aluminum compounds is the reaction with heptafluoropropyl iodide, from which perfluoropropylalanates are obtained (93). 

It should be noted that the thermal dissociation of alkaline earth metal alanates (Eqs. (6.26) and (6.27)) with formation of pentahydrides MAlEls follows a different pathway than that of alkali metal alanates (Eqs. (6.15), (6.19) and (6.20)), whose intermediates are hexahydrides of the type M 3A1H6, M = Li, Na, K. Knowledge of the dissociation pathways of alkali and alkaline earth metal alanates and their structures (see below) and the relative stability of their intermediates can be useful in the search for new hydrogen storage systems based on alanates. 

The alkali-metal alanates decompose on forming the intermediate hydride phases M3AIH6 as follows. 

Such reactions have been observed to occnr where an alanate snch as NaAlH is milled with alkali metal hydride, such as NaH or LiH. 

Alkali-metal hydrides, boranes or alanes reduce halosilanes in high yield " ... 

Alkali-metal hydrides, boranes or alanes react with alkoxysilanes or silanols to form Si—H bonds in reactions that often are preferred syntheses, depending on the availability of the silane reactant "Lithium hydride or alanate reductions used are ... 

In place of the free alkali metal it is also possible to use its hydride to obtain alkali alanates ... 

Alkali metal cyclopentadienyl alanates may also be obtained (285) ... 

This reaction is unsuccessful with trialkyl alanes because the alkali metal reacts more rapidly with the aluminum compound than with the alkyl chloride, and as a result a quarter of the aluminum separates as metal ... 

Fig. 5. The stability of complexes of alkali metal hydrides and halides with triethyl-alanes (156, 317).

Alkali tetraalkyl alanates have also been used successfully for the alkylation of lead salts. Sodium tetraethylalanate and other organoalumi-num complexes (e.g., Na[(C2HB)3AlF], Na[(C2HB)3A10CH3]) give tetraethyllead when treated with metallic lead and ethyl chloride (77) ... 

The alkali metal borohydrides contain more hydrogen than the alanates, e.g., 18.5 and 10.6 wt.% for the respective lithium (LiBH4) and sodium (NaBH4) analogues, but are more stable and therefore less useful as accessible hydrogen stores. For example, LiBFl4 starts to decompose only above 300 °C, whereas NaBFl4 does not decompose until 350-400 °C. For borohydrides, the main issues to be addressed are as follows. 

Adenosine triphosphate alkali metal complexes, 34 vanadyl complexes, 568 Alane, 123 amine adducts, 107 phosphine adducts, 111 Alane, alkoxy-, 124 Alane, amino-, 109 Alane, imino-, 109 Alkali metal complexes, 1-70 acid anions, 30 acid salts, 30 bipyridyl, 13 crown ethers cavity size, 38 cryptates... 

ARMAND, M.B. and EL KADIRI CHERKAOUI EL MOURSLI, F., 1985c. Tctrakis trialkyl siloxy alanates of alkali metals, their soUd solutions with plastic materials and their use for the constitution of conductor elements for electrochemical generators. US Patent 4,556,616. 

The best route to Al—H bonds from the elements is the formation of tetrahydroaluminates, because these reagents are convenient to handle and may be used to prepare other alanes. The reaction between an alkali- or alkaline-earth metal and ac-... 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

Trialkyl alanes also form association products with the alkyls of the alkali and alkaline earth metals, but these are very stable and some are saltlike complexes (cf. Section IV,C). An alkyl exchange such as... 

---