In order to create an efficient and suitable catalysts, which could be used in borohydride fuel cells as anode materials, the oxidation of sodium borohydride has been extensively studied on various catalytic substances, such as Au,8–34 Pt,10,11,16,24,29,30,32,35–54 Pd,11,16,50,55–63 Ag,14,38,39,64–69 Ni,11,16,70–74 Cu,11 Co,75 Zn,76 Rh, Ir,77 and Ru.78 It was found that noble metals like gold or platinum are the best electrocatalysts for DBFCs, unfortunately, the use of them as an electrode material is limited by their high price. In contrast, the carboxylic group is readily reduced with borane/THF. ), metal halides (TiCl3, TiF3, TiF4, ZnF2, AlF3, ZnCl2, ScCl3, etc. Walsh, in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2015. The borohydride ion can be oxidized directly at an anode surface, providing a higher energy density than hydrogen as a fuel. Mixing LiBH4 with SiO2 powder (3:1) substantially decreased its decomposition temperature so that 9 wt% of hydrogen could be liberated below 673 K [82]. A tandem reduction–hydroboration of olefins was achieved with calcium and lithium borohydrides in the presence of ethyl acetate in the molar ratio 1:1:1 at reflux in THF in 5–7 h.89 The relative reactivity of terminal/internal alkenes, 9:1 for the calcium reagent, was taken advantage of in selective hydroboration of (Z)-1,11-hexadecadiene. Lithium borohydrides (LBH) and sodium borohydrides (NBH) are amongst the most commonly used hydride reducing agents, especially for the chemoselective reduction of aldehydes and ketones. We use cookies to help provide and enhance our service and tailor content and ads. The reactivity difference of terminal/internal olefins is 6.5:1, enabling selective hydroboration of skipped terminal–internal dienes at the terminal double bond. ), and carbon-based materials have been widely studied as catalysts for reducing the decomposition temperature of LiBH4. Two cis-borohydride groups are tridentate, while the other four are bidentate, and bridge two uranium atoms. Sodium borohydride is a white to grayish crystalline powder. They synthesized lithium borohydride (LiBH4) from diborane (B2H6):[4][5], Current methods involve reduction of trimethyl borate with sodium hydride.[2]. Metal borohydrides, such as LiBH4 with a 18.3 wt% theoretical hydrogen storage capacity, are also a very active area of hydrogen storage research for on-board applications at present due to their substantially higher hydrogen content. Draw the Lewis Structure of borohydride (BH 4-) ion. Catalytic materials for the hydrolysis of borohydride used in indirect borohydride fuel cells are also considered. One example is the titanocene derivative: 2 (C 5 H 5) 2 TiCl 2 + 4 NaBH 4 → 2 (C 5 H 5) 2 TiBH 4 + 4 NaCl + B 2 H 6 + H 2 Hydrogen source. The reactivity order of olefins in this reaction is tetramethylethylene>cyclohexene>>1-hexene (unreactive), which is opposite to the order in hydroboration with borane/THF. Therefore, the diethylether and THF adducts of Th(MeBH3)4 are found to be dimeric, with two bridging methylborohydride ligands.482 The complex (MeBH3)3Th(μ-MeBH3)2Th(MeBH3)3(OEt2) only exhibits ether coordination to one end of the dimer, presumably due to steric factors. Borohydrides are softer than their aluminum counterparts due to the more covalent hydride bond and both can be hardened by incorporating alkoxy groups on the metal. Get … The 1:1 adducts with small dialkylethers (e.g., [U(BH4)4(OMe2)]n, [U(BH4)4(OEt2)]n)480 form chains in the solid state, in which bidentate borohydride groups bridge pseudooctahedral uranium centers; the remaining borohydride groups are tridentate, and the remaining coordination site is occupied by the ether ligand. In recent years, alloying of Au or Pt with transition metals such as Ni,45,79–90 Co,45,84,91–98 Cu,99–102 and Fe85,103 allows reducing its cost and provides better catalytic characteristics for the oxidation of BH4– ions. [2] Sodium borohydride is also used to reduce aldehydes and ketones in the production of pharmaceuticals including chloramphenicol, thiophenicol, vitamin A, atropine, and scopolamine, as well as many flavorings and aromas. Because of their high hydrogen content, borohydride complexes and salts have been of interest in the context of hydrogen storage. The chemo-, regio-, and stereoselectivity of the reductions can be regulated by adjusting the stoichiometry of carboxylic acid and borohydride, or by controlled variation in temperature. Another form has been identified in which the two tridentate borohydride groups reside in trans-positions of the octahedron, while equatorial bidentate BH4 groups bridge metal centers to create a polymeric sheet structure. A synergetic thermodynamic and kinetic destabilization on the dehydrogenation/hydrogenation of LiBH4 was achieved by the in situ formation of LaH3 and MgH2 originated by ball milling the mixture of LiBH4 and La2Mg17 under 4 MPa of hydrogen pressure [88]. Not only this escape of hydrogen generated during the BOR lowers the fuel efficiency of the DBFC, it also raises safety issues. Use of the methylborohydride group inhibits the formation of polymeric products, due to its inability to act as a bridging bidentate ligand. Hydrogen Storage Properties of the Catalyst-Dope LiBH4[84]. In the solid state the complexes exists as a pseudooctahedral monomer with trans-THF ligands and tridentate borohydride groups.318,483,484 The tetrahydrothiophene analog of U(MeBH3)4 is not isostructural. One of the reasons for the enduring popularity of these borohydrides is the ease with which the parent reagents can be routinely modified to form either a stronger or a more selective reducing agent. These reducing agents are capable of selectively reducing aldehydes over even highly electrophilic ketones; this observation is exemplified in the chemoselective reduction of aldehydes in the presence of trifluoromethyl ketone (Equation (22)) <1990T2691> and in the selective reduction of a less sterically encumbered aldehyde in a dialdehyde substract <1996JA10660>. Crystal structure of (n-Pr2O)2 (η3-BH4)3U(μ-η2,η2-BH4)U(η3-BH4)4 (Zalkin, Rietz et al. In the presence of metal catalysts, sodium borohydride releases hydrogen. In the Lewis structure for BF4- Boron is the least electronegative atom and goes at the center of the structure. The countercation also influences the reducing power of the reagent. The overall polymeric chain is helical. The following reactivity differences as compared to borane/THF have been observed. Electron-releasing ethyl groups as in triethylborohydride render the B−H center highly nucleophilic. In the case of adducts of U(BH4)4, the size of the base can control the dimensionality of the resulting product. Alternatively, hydrogen contained in the borohydride ion can be catalytically released and fed into a hydrogen–oxygen fuel cell. A great deal of effort has been put into the exploration of cost-effective, active, and stable fuel cell catalysts. For instance, the combination of NBH with carboxylic acids, such as acetic acid, leads to the generation of sodium acyloxyborohydrides, with triacyloxyborohydrides being less reactive than the corresponding acyloxyborohydrides <1998CSR395>. Inorg. Since borohydride is used as fuel, the development of electrocatalysts having reasonable costs and a high electroactivity toward borohydride oxidation is of considerable interest to fuel cells. Reduction–hydroboration of internal alkynes goes as far as the second stage, whereas terminal alkynes react to the first stage. Metal borohydride complexes can often be prepared by a simple salt elimination reaction:[11], Some metal tetrahydroborates transform on heating to give metal borides. Further efforts on making the decomposition of these borohydrides more reversible as well as on improving the reaction kinetics of dehydrogenation should be emphasized in the future. What is the hybridization of the boron in the structure? Use of the slightly larger Prn2O ligand results in the formation of an unusual dimer formulated as (Prn2O)2(η3-BH4)3U(μ-η2,η2-BH4)U(η3-BH4)4481 (see Figure 39). Table 13.4. Hydroboration of alkynes with borane leads to large amounts of 1,1-diboranyl-substituted products. Zinc borohydride reacts with terminal alkenes in refluxing THF affording trialkylboron species.87 At the molar ratio 1:2, dialkyl boron species are obtained. A wide range of functional groups is unaffected under the reaction conditions (although cinnamic acid suffered some reduction at the alkene) (Scheme 12).