Solubility complex hydrides

The complex hydride Mg CoH is very similar to Mg FeH. In the binary system of Mg-Co there is no solubility of Co in either solid or liquid Mg and no inter-metallic compound, Mg Co, exists in equilibrium with other phases. However, in contrast to the Mg-Fe system, the intermetallic compound MgCo exists in equili-brium in the Mg-Co binary system (e.g., [14, p. 251]). The theoretical hydrogen capacity of Mg CoH is only 4.5 wt% which is obviously lower than that of Mg FeHg due to the presence of the heavier Co element and one less H atom in the hydride formula. 

Reductions with hydrides and complex hydrides are usually carried out by mixing solutions. Only sodium borohydride and some others are sometimes added portionwise as solids. Since some of the complex hydrides such as lithium aluminum hydride are not always completely pure and soluble without residues, it is of advantage to place the solutions of the hydrides in the reaction flask and add the reactants or their solutions from separatory funnels or by means of hypodermic syringes. 

Solubilities of the most frequently used hydrides and complex hydrides in most often used solvents are listed in Table 3. In choosing the solvent it is necessary to consider not only the solubility of the reactants but also the boiling points in case the reduction requires heating. 

Complex or soluble neutral metal hydrides are usually employed for the reduction of carboxylic acid derivatives to alcohols or amines. The standard reagents for the most important transformations are shown in Table 17.6. For completeness, various reagents also are listed for the reduction of carboxylic acid derivatives to aldehydes. The latter mode of reduction was discussed in Section 6.5.2. 

The importance of lithium hydride in the production of simple and complex hydrides is dependent upon certain unique properties. For example, according to Hurd (19 )y it is the very slight solubility of lithium hydride in polar organic compounds as well as the ability to sustain metathetical reactions that provides for the production of numerous hydrides. This is true for lithium hydride, whereas the other alkali and alkaline earth metal hydrides are insoluble in polar organic solvents, and their metathetical reactions do not proceed at all or at a slow rate at best (19,29). Greater use of lithium hydride is possible in such reactions because of its low dissociation pressure at its melting point of 680 °C. 

The result, a salt of the [FeHs]" anion with four [MgBr]" cations, is a crystalline yellow solid that is soluble in tetrahydrofuran to the extent of 0.006 M. The bromide atoms in the [MgBr] cations can be displaced by other good nucleophiles. Substitution of the bromide with tert-butoxide enhances the solubility in tetrahydrofuran to about 0.5 M and also increases solubdity in other less polar organic solvents. The complex hydride is capable of hydrogenating olefins and arenes under hydrogen. Recent interest has focused on the role of [FeHg] as a precursor for iron nanoparticles. ... 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

In some ca.ses the use of a two-phase system may allow a change in the selectivity. Thus, Joo et al. (1998) have shown that water-soluble Ru hydrides (sulphanatophenylphosphine Ru complexes) give different products in the hydrogenation of cinnamaldehyde with variation in the pH of the aqueous media. At a pH greater than 7.2, cinnamyl alcohol is formed and at a pH less than 5 saturated aldehyde is formed. 

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