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. 

One attractive feature of alanates is that lithium and sodium salts are readily available commercially. Magnesium alanate can be readily prepared with sodium alanate and magnesium hydride via a metathesis reaction. The mixed metal alanate, Na2LiAlH6, is prepared through ball milling of sodium hydride, lithium hydride, and sodium alanate. Potassium alanate can be prepared by the direct synthesis of potassium hydride and aluminum under high temperature and pressure." ... 

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). 

In recent years, a considerable amount of research has been done on the preparation and characterization of alkaline earth metal alanates. A detailed procedure for the preparation of solvent-free magnesium alanate has been described by Fichtner and Fuhr [198]. Lately, Fichtner and Ahlrichs et al. [199], starting from FTIR data of Mg(AlH4)2 and its solvent adducts, performed a quantum chemical calculation... 

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. 

In contrast to above alanates, Mg(AlH4)2 of the alkaline-earth metal alanate decomposes without the formation of the intermediate phase MfAlHe). Fichtner et al. reported that the dehydrogenation of Mg(AlH4)2 proceeds according to the following reactions [53] ... 

In spite of their high hydrogen content, until recently, metal alanates have not been considered for hydrogen storage applications due to their rather bad kinetic properties... 

Late transition metal alane (T-complexes are extremely rare Crimmin and coworkers have also used the HC(MeCDippN)2 AlH2 hgand in an attempt to address this situation and have investigated its coordination (together with that of a related Nacnac-derived alane) at Cu(I). The K mode of coordination observed in 77 in the solid state is intrinsically very weak and the alane rapidly exchanges with toluene at room temperature (Scheme 19). ... 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

As we saw in Chapter 3, the founding text of modern materials science was Frederick Seitz s The Modern Theory of Solids (1940) an updated version of this, also very influential in its day, was Charles Wert and Robb Thomson s Physies of Solids (1964). Alan Cottrell s Theoretical Structural Metallurgy appeared in 1948 (see Chapter 5) although devoted to metals, this book was in many ways a true precursor of materials science texts. Richard Weiss brought out Solid State Physics for Metallurgists in 1963. Several books such as Properties of Matter (1970), by Mendoza and Flowers, were on the borders of physics and materials science. Another key precursor book, still cited today, was Darken and Gurry s book. Physical Chemistry of Metals (1953), followed by Swalin s Thermodynamics of Solids. 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

Jackson, P.J., Naranjo, C.M., McClure, P.R. Roth, E.J. (1985). The molecular response of cadmium resistant Datura innoxia cells to heavy metal stress. In Cellular and Molecular Biology of Plant Stress, ed. J.L. Key and T. Kosuge, pp. 145-60. New York Alan R. Liss. 

The decomposition is significantly accelerated and the temperature of the first decomposition reaction is lowered to 120°C (Fig. 19.7). The decomposition rate is relatively low compared with other titanium-based dopants. The highest activity of a titanium catalyst used in alanate decomposition was observed for ligand-stabilized colloidal titanium metal [42]. 

In the first three cycles, the capacity of the alanate is high for both materials, the titanium metal-doped and the nitride-doped material. However, after 15 cycles the hydrogen capacity decreases significantly below 4% for the titanium metal-doped sample, whereas for the nitride-doped sample the capacity remains high at about 5% hydrogen. 

ALAN M. RUSSELL and KOK LOONG LEE Structure-Property Relations in Nonferrous Metals... 

Alan G. MacDiamid and Heeger, J. Semiconduction and Metallic Polimer The Fourth Generation of Polymeric Materials. J. Phys. Chem. 2001 B105 8475-8491. 

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