Synthesis of metal hydrides

Much more efficient method of the synthesis of metal hydrides is a one-step method of reactive ball milling of elemental metal directly under hydrogen gas. The method of reactive mechanical alloying/milling of either elemental metals or preaUoyed... 

As a secondary organomagnesium halide reagent, cyclohexylmagnesium bromide may also be useful for the synthesis of metal hydrides ... 

Formates. The decarboxylation reaction of metal formates is a fairly general route for the synthesis of metal hydrides and it has been applied to many transition metals. As an example, allyl palladium formates, which are believed to be intermediates in the catalytic reductive cleavage of allylic acetates and carbonates with formic acid to give monoolefins (Scheme 6.32), have been synthesized. In fact the complexes undergo decarboxylation and the reductive elimination of the allyl hydrido fragments, supporting the catalytic cycle proposed [105]. 

Synthesis of Metal-Hydride Compiexes 3.8.2.I. From Hydrogen... 

The synthesis of metal-hydride complexes is discussed in detail in a text23 and in reviews.24-28 available data show that ion exchange... 

Lithium aluminum hydride and sodium borohydride are commonly used in the synthesis of metal-hydride complexes. Diethyl ether, tetrahydrofuran or diglyme, in which Li[AlH4] is readily soluble, are used as solvents while, with Na[BH4l, these solvents, as well as ethanol or isopropanol, may be employed. Methanol is rarely used because Na[BH4]... 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

However, since the goal of this work was the synthesis of alcohols from olefins via hydrohydroxymethylation (75, 76), little attention was given to developing a shift-catalyst per se. Pettit has recently reexamined some of this work and shown that, by careful control of the pH of the reaction mixture, systems based on either Fe(CO)5 or Cr(CO)6 can be developed for the production of either formic acid or methanol from carbon monoxide and water (77, 78). Each of these latter systems involves the formation of metal hydride complexes consequently, molecular hydrogen is also produced according to the shift reaction [Eq. (16)]. 

The majority of chemical methods for the asymmetric hydrogenation of unsaturated systems rely on the use of transition metal catalysts or stoichiometric amounts of metal hydride. The chemical importance of this transformation has led to the development of some of the most powerful and efficient methods in catalytic asymmetric synthesis. Routinely used on the milligram to multi-tonne scale, they represent one of the biggest success stories of asymmetric catalysis [120]. 

Reagents such as LiAlH4 and KH are not effective for the synthesis of formyl complexes. LiAlH4 does react with many metal carbonyl compounds, but it can transfer more than one and usually effects the formation of metal hydride products (50). Similar results are usually found with NaBH4(50), although some neutral formyl complexes (vide infra) can be obtained under special conditions. KH will also react with some metal carbonyls. However, rates are not very rapid, and any formyl intermediates are likely to decompose faster than they form (51). 

The synthesis of fluorinated butoxides was carried out by interaction of metal hydrides with corresponding alcohols in ether media [467, 947]. The interaction of fluorinated ketones with sodium in THF or aqueous alkali led to the derivatives of perfluoropinacol ... 

The synthesis of metal graphite compounds is carried out by the reduction of layer graphite compounds and metal chlorides by using different reducing agents such as, for instance, solutions of aromatic anion radicals in THF, metallic sodium in liquid ammonia, complex boron and aluminum hydrides, and vapor or liquid potassium. As a result, the complex compounds containing one (Sn, Co, Mn, Cr, Mo, W, Pd) and two (Fe-Mo, Fe-W, or Mo W) metals were synthesized. 

Free radicals mentioned in this chapter are not useful for organic synthesis. However, it is important to know the radical character of metal hydride reagents, since metal hydride reagents sometimes behave not only as a polar (ionic) hydride donor species, but also a single electron donor species, depending on the substrates and reaction conditions. 

These types of SET reactions are not useful for organic synthesis however, it is important to know about the SET character of metal hydride reagents. 

Composite graphite electrodes based on metal hydrides (YH3, LaH3, CeH3 and GdH3) were for the first time used in the synthesis of EMFs containing soot. The use of metal hydrides instead of metal sward afforded electrodes with metals more homogeneously distributed in a graphite rod that provided more stable electric arc... 

Another interesting approach for the synthesis of transition metal-lead bonds is the reaction of metal hydrides with RsPb-X. ... 

As more sophisticated metal hydrides are developed (nanocrystalline, multicomponent systems, composites and nanocomposites, graphite/metals or similar hybrid systems, clusters, etc.), it is important to be a vare that, for practical applications, a large volume of material should be processed in a fast, inexpensive and reliable vay, for example casting. Techniques such as cold vapor deposition may be impossible to scale up but this does not mean they should be discarded as a means of studying new metal hydrides. On the contrary, laboratory techniques allow much better control of the end product and permit the elaboration of new compounds. Once an attractive compound is found then another challenge w ill have to be faced scaling up the synthesis. In this respect, it is important for the community of metal hydrides researchers to also study large-scale production techniques in order to make the transition from laboratory to industrial scale easier. 

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