Aluminum complex hydrides

In recent years, complex aluminum hydrides have been developed by Bogda-novic for the use as a storage material [12, 13, 38-40]. Hydrogen is herein generated by decomposing NaAlH4 in two separate reactions into NaH and aluminum metal. 

Other complex aluminum hydrides such as NaAlH4, LiAlMesH, NaAlMcjH, NaAl(0CH2CH20CH3)2H2 (vitride), LiAlH2(NR2)2 and NaAlH2(NR2)2 were also applied in a similar way as hydroaluminahng reagents [57]. 

Haubenstock, H., Asymmetric Reductions with Chiral Complex Aluminum Hydrides and Tricoordinate Aluminum Reagents, 14, 231. 

ASYMMETRIC REDUCTIONS WITH CHIRAL COMPLEX ALUMINUM HYDRIDES AND TRICOORDINATE ALUMINUM REAGENTS 231... 

B. Bogdanovic, U. Eberle, M. Felderhoff, F. Schtith, Complex aluminum hydrides , Scripta Mater. 56 (2007) 813-816. 

Asymmetric Reductions with Chiral Complex Aluminum Hydrides and... 

Hydroalumination. Titanocene dichloride is an effective catalyst for hydro-uluminution of alkenes and alkynes with his(dialkylamino)alancs5 and various complex aluminum hydrides. The adducts can be quenched with water or iodine. The reaction is satisfactory for terminal alkenes and internal alkynes, but is not clcun for internal alkenes and terminal alkynes. 

Topics Stereochem 14 231 (1983) (complex aluminum hydrides and tricoordinate aluminum reagents) JOC 52 5406 (1987) (reagent comparison)... 

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. 

Table 5.2 Physical constants of selected complex aluminum hydrides.

The theoretical work of Ijawik was driven by the question to which position H moves to when added to complex aluminum hydride surfaces [85]. As intensively discussed for NaAlH4, without considering energetic or crystallographic issues, there are different sites at which Ti could anchor. It could first react with surface atoms and... 

The decomposition of the mixed complex aluminum hydride takes place at about 230 °C. The hydrogen absorption without catalysts proceeds slowly at 300 °C. 

It can be summarized that of all the complex aluminum hydrides presented in this chapter only NaAlH4 up till now partially fulfils the requirements for a storage material for mobile fuel cell applications. Over the last decade, the kinetics as well as the cycle stability of doped sodium alanates has been improved. Nevertheless, for application in a low-temperature fuel cell the storage capacity seems to be too low because only the first decomposition step (3.6 wt.% hydrogen) can be used. 

Due to their thermodynamic properties some complex aluminum hydrides are not reversible under acceptable technical conditions. Others have too low hydrogen storage capacities which exclude these materials from broad industrial applications. 

The combination of MAD with some complex aluminum hydride reagents enables the conjugate reduction of a -unsaturated ketones [142]. Although selectivity is profoundly affected by the structure of substrates, the 1,4 addition of hydride to quinone monoketals and quinol ethers is successfully mediated by MAD to give reduction products in good yield (Sch. 104) [143]. 

Effective anionic activation of trichlorosilane can be carried out with either catechol or 2,2 -dihydr-oxybiphenyl in THE yielding bis(diolato)hydridosilicates. Such reagents exhibit reducing power that is reminiscent of the complex aluminum hydrides. Even tertiary amines are useful activators of trichlorosilane, enhancing its hydridic character. ... 

In addition to simple and complex aluminum hydrides, aluminum alkyls with f3-branched chains often serve as hydroaluminating agents themselves, because of their tendency to form A1—H bonds during reaction by the thermal or nickel-catalyzed loss of alkene (c/. Scheme 3 equation 10). ... 

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