Preparation magnesium hydride

Catalytically prepared magnesium hydride has been used as an alternative to magnesium anthracene [10]. 

When prepared by direct reaction of the elements, magnesium hydride is stable in air and only mildly reactive with water. 

Sodium hydride ignites in oxygen at 230°C, and finely divided uranium hydride ignites on contact. Lithium hydride, sodium hydride and potassium hydride react slowly in dry air, while rubidium and caesium hydrides ignite. Reaction is accelerated in moist air, and even finely divided lithium hydride ignites then [1], Finely divided magnesium hydride, prepared by pyrolysis, ignites immediately in air [2], See also COMPLEX HYDRIDES... 

As was discussed in Chapter 13, hydrogen does not react directly with some elements, so the hydrides must be prepared in a different way. Alfred Stock prepared silicon hydrides by first making the magnesium compound, then reacting it with water. 

Lithium aluminum hydride and alanes are frequently used for the preparation of hydrides of other metals. Diethylmagnesium is converted to magnesium hydride [777], trialkylchlorosilanes are transformed to trialkylsilanes... 

Another hydride, magnesium hydride prepared in situ from lithium aluminum hydride and diethylmagnesium, reduced terminal alkynes to 1-alkenes in 78-98% yields in the presence of cuprous iodide or cuprous r rt-butoxide, and 2-hexyne to pure cij-2-hexene in 80-81% yields [///]. Reduction of alkynes by lithium aluminum hydride in the presence of transition metals gave alkenes with small amounts of alkanes. Internal acetylenes were reduced predominantly but not exclusively to cis alkenes [377,378]. 

Magnesium hydride is a reducing agent a source of hydrogen and serves to prepare many complex hydrides. 

Magnesium hydride is obtained hy combining the elements at about 500°C. A convenient method of preparation involves passing hydrogen under pressure over heated magnesium powder in the presence of magnesium iodide as catalyst. 

Cyclopentadienylmagnesium hydride (CpMgH). The hydride is prepared by reaction of cyclopentadicne in THF with magnesium hydride it exists as the dimer (1) in this solvent. 

In recent investigations Schott and Herrmann561 563 attempted to prepare silicon hydride by reduction of tribromosilane with magnesium ... 

More recently, a related reaction has been reported, in which active forms of magnesium hydride, prepared in situ or pre-prepared, undergo addition to alkenes, catalysed by titanium or zirconium(iv) halides, to give dialkylmagnesium compounds [56, 59, 64] ... 

A 250 ml three-necked flask is fitted with an efficient reflux condenser and furnished with a magnetic stirrer bar and an inert atmosphere. A suspension of magnesium hydride (100 mmol) in THF ( 50 ml), prepared by catalytic hydrogenation of magnesium, is placed in the flask by means of a metal U tube. Zirconium tetrachloride (0.23 g) and 3-dimethylaminoprop-l-ene (allyldimethylamine) (18.8 g, 220 mmol) are added and the mixture is stirred and heated under reflux for 48 h. The suspension is filtered through a glass sinter, and THF and unreacted amine are evaporated from the filtrate in vacuo. Sublimation from the dark,... 

Lactone synthesis. This Ti(II) complex can serve as catalyst for hydro-magnesiation of allylic or homoallylic alcohols, prepared by addition of vinyl- or allyl-Grignard reagents to ketones. Ethylmagnesium bromide is used as the source of magnesium hydride. The carbonylation of the organomagnesium intermediate results in a -y- or a 5-lactone (equation I). 

Beryllium hydride is covalent, similar to hydrides formed by the group IB - VB elements. Magnesium hydride is a borderline case it can be formed by direct reaction of Mg metal and as well as by the procedures used to prepare covalent hydrides (see below). 

Magnesium hydride can be prepared by thermal decomposition of organometallic compounds similar to reaction (a) ( 1.8.3.1) for the preparation of BeHj, e.g. ... 

Preparation. Sisler and co-workers developed a technique for obtaining chloramine by gas-phase reaction of chlorine with excess ammonia and showed that the reagent combines with tertiary amines to form the corresponding 1,1,1-trisubstituted hydrazinium chlorides (1) in good yield, and with tertiary phosphines to form aminophosphonium chlorides (2). The product of reaction (2) can be dehydrohalo-genated with magnesium hydride to triphenylphosphine amine. 

A solution of the dichlorolactone (1.5 g, 2.15 imnol) and freshly prepared tributyltin hydride (1.3 mL, 4.9 nunol) in benzene (20 inL) was heated at 78 °C for 15 min. in the presence of a catalytic amount of EtsB (0.7 inL of a 1 M solution in THF, 0.7 mmol). After evaporation of the solvent in vacuo, the residue was dissolved in ether (25 mL). The ether solution was treated with a saturated aqueous solution of potassium fluoride (25 mL). The precipitated tri-n-butyltin fluoride was filtered and washed thoroughly with ether. The combined ether layers were dried over magnesium sulfate and evaporated in vacuo. The residue was chromatographed on silica gel using a gradient of eluents (hexanes EtOAc, 20 1 to 4 1) giving 1.27 g (92%) of the lactone as a clear oil. 

Hanada, N., Ichikawa, T. and Fujii, H. (2005) Catalytic effect of nanoparticle 3d-transition metals on hydrogen storage properties in magnesium hydride MgH2 prepared by mechanical milling, Journal of Physical Chemistry B, 109, 7188-7194. 

Tessier, P. and Akiba, E. (2000) Decomposition of nickel-doped magnesium hydride prepared by reactive mechanical alloying. Journal of Alloys and Compounds, 302, 215-217. 

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." ... 

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