Complex Chemical Hydrides

Thermal decomposition of the alanate NaAlH4 takes place in two steps, as follows  

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. 

Improved dehydrogenation kinetics and reduced thermodynamic stability are being sought through fundamental investigations of the crystalline structure, chemical binding status, particle size and morphology of the borohydrides, and also the distribution of the catalyst. 

Although NaBH4 is at present too stable for providing hydrogen practically through a thermal activation process, it does release the gas on reaction with water, together with formation of sodium borate (NaB02)  

This is similar to the procedure proposed for the zinc-air traction battery, which has been investigated as a candidate power source for electric vehicles.  

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Q First-Principles Design of Complex Chemical Hydrides as Hydrogen Storage Materials... 

Traps the complex chemical hydride inside the nanoporous polymer ... 

In the fine chemical industry, reduction of carbonyl groups mainly relies on the use of complex metal hydrides sodium dihydrobis-(2-methoxyethoxy)-aluminate, commercialized as RedAl or Vitride is one of the most used (4). 

There is a rather important difference between chemical reductions using complex metal hydrides and enzymic reductions involving NADH, and this relates to stereospecificity. Thus, chemical reductions of a simple aldehyde or ketone will involve hydride addition from either face of the planar carbonyl group, and if reduction creates a new chiral centre, this will normally lead to a racemic alcohol product. Naturally, the aldehyde primary alcohol conversion does not create a chiral centre. 

The reverse reaction, i.e. reduction, is also indicated in the scheme, and may be compared with the chemical reduction process using complex metal hydrides, e.g. LiAlPLj or NaBH4, namely nucleophilic addition of hydride and subsequent protonation (see Section 7.5). The reduced forms NADH and NADPH are conveniently regarded as hydride-donating reducing agents (see Box 7.6). We also noted that there were stereochemical features associated with these coenzymes (see Box 3.14). During a reduction... 

US DOE is investigating two types of hydrides for solid-state hydrogen storage complex metal hydrides and chemical hydrides. DOE s research on complex metal hydrides is focused on increasing the storage capacity of alanates, extending the durability and cycle lifetime, and improving uptake... 

Thus, in the fine chemicals industry, reduction of ketones and aldehydes relies mainly on the use of complex metal hydrides that require time-consuming workup of reaction mixtures and produce significant amounts of inorganic and organic wastes. Similarly, the oxidation of alcohols into carbonyls is traditionally performed with stoichiometric inorganic oxidants, notably Cr(VI) reagents or a catalyst in combination with a stoichiometric oxidant [1]. 

The latter is outside the scope of organometallic chemistry, but within the first two topics the work involved three main themes olefin and acetylene complexes, alkyl and aryl complexes, and hydride complexes. As continuous subsidiary themes throughout ran the complex chemistry of tertiary phosphines and such ligands, the nature of the trans effect, and the nature of the coordinate bond. All the work from 1947 to 1969 was carried out in the Butterwick Research Laboratories, later renamed Akers Research Laboratories, of Imperial Chemical Industries Ltd., and I am indebted to that Company and particularly to Mr. R. M. Winter, the Company s Controller of Research, and Sir Wallace Akers, its Director of Research, who in 1947, made available to me the opportunity to develop my research in my own way, in those laboratories. 

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