Metal borohydrides LiBH4

Metal borohydrides are very well known chemicals and some of them have been used commercially in chemical industry for a number of years. The first metal borohydride, LiBH4, was synthesized almost 70 years ago by the reaction of ethyl... 

Replacing the metal Al by a boron atom as the metal chelate center, Tao et al. reported lithium tetra-(2-methyl-8-hydroxy-quinolinato) boron (LiB(qm)4, 240) (Scheme 3.73) quantitatively prepared by reaction of lithium borohydride (LiBH4) with four equivalents of 2-methyl-8-hydroxy-quinoline in ethanol at room temperature [266]. LiB(qm)4 is a pure blue emitter with a maximum peak emission at 470 nm with FWHM of 75 nm. Devices of... 

The first report of pure alkali metal borohydride appeared in 1940 by Schlesinger and Brown ° who synthesized LiBH4 by the reaction of ethyllithium with B2H6. The direct reaction of the corresponding metal/metal hydrides with diborane in etheral solvents under suitable conditions produces high yields of the borohydrides " " ... 

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. 

Lithinm borohydride, LiBH4, has attracted attention because of its high hydrogen content (18.5 wt%) compared with other metal borohydrides [39,40]. It is a hygroscopic white crystalline solid that is decomposed by water. Thus, it is generally handled under inert atmosphere. Lithium borohydride occurs as two phases under ambient pressure. The higher temperature phase forms above 107°C [41]. Lithium borohydride is currently manufactured in far smaller volumes than sodium borohydride. 

Examples can be cited for the mixed alkali metal and alkaline-earth metal borohydrides. It was reported that a Li-Mg borohydride could be formed by mechanical milling and subsequent heating of a mixture of LiBHj and Mg(BH4)2 in 1 1 mole ratio. This dual-cation borohydride exhibits a lower dehydrogenation onset temperature [82]. Also, composite materials made by combining LiBHj and Ca(BH4)2 are reported to show hydrogen release temperatures lower than for either component alone [83]. Composites of composition (LiBH4) (l - x)[Ca(BH4)2], where H2 capacity varies with x, were prepared. At intermediate compositions, such as x = 4, dehydrogenation was complete below 400°C with release of 10 wt% H2. Partial reversibility of this system was also reported. 

You met borohydride in Chapter 6, where we discussed the mechanism of its reactions. Sodium borohydride will reduce only in protic solvents (usually ethanol, methanol, or water) or in the presence of electrophilic metal cations such as Li+ or Mg2+ (LiBH4 can be used in THF, for example). The precise mechanism, surprisingly, is still unclear, but follows a course something like this with the dotted lines representing some association, perhaps coordination or bond formation. 

Other methods for its preparation invariably result in the isolation of Lewis base adducts. The complexes U(BH4)3(THF) t and U(BH4)3(18-crown-6) were prepared by the metathesis reaction of LiBH4 with UCl3(THF) , or UCl3(18-crown-6) in TFIF. Although the stoichiometry of the THF adduct was not characterized in the initial report, the compound was later prepared from the reaction of UH3 and BFls in THF, and characterized to be U(BH4)3(THF)3. The molecular structure of the compound reveals that it adopts an octahedral geometry about the metal center with the borohydride and THF ligands mutually facial all borohydride ligands are tridentate. 

Lithium Borohydride. Lithium tetrahydroborate. BH4Li mol wt 21.79. B 49.66%, H 18.50%. Li 31.85%. LiBH4. Prepd by the action of diborane on ethyllithium by the action of aluminum borohydride on ethyllithium Schle-singer. Brown, J. Am. Chem. Soc. 62, 3429-35 (1940). Review of lithium and other metal tetrahydroborates James, Wallbridge, Prog. Jnorg. Chem- II, 99-231 (1970). 

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