Borohydrides, metal

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, LiBH, was synthesized almost 70 years ago by the reaction of ethyl 

As mentioned in Sect. 3.2.3 a drawback of this method is the formation of a byproduct, which is salt, which reduces the overall hydrogen content of the synthesized mixture. Another peculiarity of this synthesis method is a complete lack of XRD peaks corresponding to a crystalline synthesized borohydride in the mixture in the right-hand side of the reaction of (3.37). Only the peaks of either LiCl or NaCl are present. On the other hand, the Raman spectra seem to indicate the presence of borohydride. Nakamori et al. [173, 174] interpreted this peculiar behavior as arising most likely due to disordering of the crystal structure of a synthesized borohydride, whatever it means for a sohd state hydride. Application of MCAS to the synthesis of Mg(BH )2 will be discnssed later in the text. 

Very recently, ball milling was also nsed to synthesize sodium borohydride, NaBH, by mechano-chemical reaction of dehydrated borax (Na B O,), MgH and Na COj (at room temperature) according to the following reaction [176] 

The same decomposition path was observed for KBH by Stasinevich and Egorenko [178] except that the melting and decomposition temperatures were much higher (by about 100°C). We performed ball milling for 5 h on the as-received NaBH but without any effect on its decomposition behavior. This is most likely due to the fact that NaBH exhibits a high structural stability during 

LiBH undergoes first a polymorphic transformation around 110°C, then melts with an endothermic peak centered around 280°C, and evenmaUy at the atmospheric pressure it decomposes with an endothermic peak centered at 470°C through the following reaction path 

The third endothermic peak occurred around 400-420°C and could be assigned to the decomposition of MgH2 according to the well known reaction 

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Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

The classical synthesis iavolves the dissolution of a 33% Sb—67% Zn alloy by hydrochloric acid the evolved gases contain up to 14% stibiae. A detailed procedure usiag a Sb—Mg alloy has also beea described (16). Aluminum hydride or alkaU metal borohydrides have been used to reduce antimony(III) ia acidic aqueous solutioa to produce stibiae. A 23.6% yield of stibiae, based oa the borohydride used, has beea reported (17). A 78% yield based oa Sb has beea obtaiaed by gradually adding a solutioa that is 0.4 Min SbCl and saturated ia NaQ, to aqueous NaBH at mol ratios of NaBH iSbQ. >10 (18). 

Unsymmetrical cleavage of B2H by metal hydrides gives metal tetrahydroborate salts, also called metal borohydrides or hydroborates. 

Pyrazoles, and some indazoles, substituted on the nitrogen by B, Al, Ga, In, Si, Ge, Sn, P and Hg are known. Poly(pyrazol-1 -yl)borates have been studied by Trofimenko (72CRV497) who found that they were excellent ligands (Section 4.04.2.1.3(vi)). The parent ligands (282), (283) and (284) are available by the reaction of an alkali metal borohydride with pyrazole, the extent of substitution depending on the reaction temperature (Scheme 22). 

Higher coordination numbers (up to 16) are known, particularly among organometaiiic compounds (pp. 940-3) and metal borohydrides... 

These methods deal with specific cases. The list of examples is not exhaustive. The low-T (200-300°C) decomposition of the transition-metal borohydrides M(BH4> , e.g., leads to titanium, zirconium, halfnium, uranium and thorium borides . Alternatively, the uranium diboride may be obtained by reacting uranium hydride with diborane in hydrogen at 200-400°C. 

Numerous organic syntheses are based on stoichiometric oxidations of hydrocarbons with sodium dichromate and potassium permanganate, or on hydrogenations with alkali metals, borohydrides or metallic zinc. In addition, there are reactions... 

Bird, P. H., and M. G. H. Wallbridge A Study of Metal Borohydrides. The Reaction of Aluminium Borohydride with Various Ligand Molecules. J. chem. Soc. 1965, 3923. 

H. C. Brown, and E. R. Hyde The preparation of other Borohydrides by Metathetical Reactions Utilizing the Alkali Metal Borohydrides. 

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

The synthesis of borazine (Fig. 1) was first described in 1926 by Stock and Pohland.12 More recently, Wideman and Sneddon reported interesting three one-step synthetic procedures using various starting compounds including 2,4,6-trichloroborazine, metal borohydrides, and ammonia-borane.13 The B N bond is isoelectronic with the C C bond, which explains why borazine is often presented as the inorganic analog of benzene, that is, borazine has almost the same colligative properties as benzene. 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Lippard, S.J., Ucko, D.A. 1968. Transition metal borohydride complexes. II. Th reaction of copper(I) compounds with boron hydride anions. Inorg Chem 7 1051-1058. 

If an n-valent metal chloride or flnoride (AX ) is nsed in the reaction with alkaline metal borohydride B(BH ) then in general terms the reaction can be written as... 

Fig. 3.24 (a) Relation between the heat of formation, A H, of metal borohydride in the unit of kJ/ molBH and the Pauling electronegativity of the cation, [190]. (b) The desorption temperature, Tj, as a function of the Pauling electronegativity of the cation, Xp Inset shows the correlation between and estimatedA for the desorption reaction [173]... 

In summary, on the one hand, metal borohydrides with low Pauling electronegativities (<1.3) exhibit high decomposition temperatures. Unfortunately, at the moment there is no viable approach on the horizon by means of which high decomposition temperatures of metal borohydrides could be reduced to a reasonable level, say, 200°C. On the other hand, metal borohydrides with the Pauling electronegativities... 

Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-l. Towata, A. Ziittel, S.-l. Orimo, Correlation between thermodynamical stabilities of metal borohydrides and cation electronegatives First principles calculations and experiments , Phys. Rev. B 74 (2006) 45126. 

K. Miwa, N. Ohba, S. Towata, Y. Nakamori, A. Ztittel, S. Orimo, First-principles study on thermodynamical stability of metal borohydrides Aluminum borohydride A1(BH )2 , J. Alloys Compd. 446-447 (2007) 310-314. 

Alkali metal hydrides react with diborane to form metal borohydrides B2He+2NaH 2NaBH4... 

The alkali metal borohydrides reduce aminochromes in solution very rapidly 70,109,120,147,148,1B1,1B5,157 the expected 5,6-dihydroxy-indole derivative was usually obtained in high yield.161,166 Reduction of the 7-iodoaminochromes was not usually accompanied by deio-dination to any appreciable extent.109,166 A number of relatively minor, unidentified, often fluorescent, water-soluble by-products were usually also detected in the reduction mixtures.109,148,155... 

The three-centered, two-electron hydride bridge, which is prevalent in boron hydride chemistry, has been very well characterized in the ir and Raman spectroscopy of diborane and certain metal borohydrides. A brief review of these data will be given at this point because they afford insight into hydride-bridged systems. 

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