Synthetic Approaches to Inorganic Borohydrides

PERSPECTIVE www.rsc.org/dalton | Dalton Transactions Synthetic approaches to inorganic borohydrides

Hans Hagemann*a and Radovan Cernˇ y´b

Received 21st December 2009, Accepted 13th April 2010 First published as an Advance Article on the web 4th May 2010 DOI: 10.1039/b927002g

Inorganic borohydrides are actively studied in view of potential hydrogen storage applications. These compounds can be obtained by a variety of reactions ranging from high temperature reactions of the elements to exchange reactions in solution or in solid state. Different approaches will be discussed and compared.

Introduction –Other high temperature/pressure reactions Several related recent synthetic experiments performed in Besides their well known properties as reducing agents, neutron Geneva are also included. absorbers etc., metal borohydrides are of interest for hydrogen In this review, we will not consider large scale production storage in mobile applications.1,2 As the pure alkali borohydrides of sodium borohydride. The literature on the preparation of are too stable with respect to thermal decomposition, other boro- inorganic borohydrides up to 1977 has been reviewed in ref. 3 hydrides such as Mg(BH ) and Ca(BH ) which previously3,4 were 4 2 4 2 and 4. only scarcely investigated as well as transition metal and mixed- Makhhaev5 in 2000 presented structural and dynamic properties cation borohydrides are currently prepared and characterized. of tetrahydroborate complexes. More recently, Besora and Lledos6 Another aspect of the synthesis of borohydrides is concerned with have reviewed structural and vibrational properties of transition the potential reversibility of the dehydrogenation of the different metal tetrahydroborate complexes which contain other ligands in compounds studied. In this case, high temperature and pressure addition to BH -. This type of compound is not considered in this reactions of hydrogen gas and solids are studied. 4 review.However,their comparisons of metal-to-boron bond length We want to discuss the following synthetic approaches to and vibrational B–H stretching frequencies as a function of metal inorganic borohydrides used mainly in the last 5 years, building to borohydride binding (h1, h2, h3) are also useful to distinguish on previous experimental knowledge: ionic (with isolated BH - ions) and complex (with ions such as –Exchange reactions in solution 4 Sc(BH )-)7 compounds. Parker8 has extensively surveyed vibra- –Reactions with aminoboranes and related compounds 4 tional spectra and bonding of ternary metal hydride compounds, –Reactions with diborane including borohydrides. Hydrogen dynamics in borohydrides have –Exchange reactions in the solid state been reviewed by Remhof et al.9 –High temperature/pressure synthesis from the elements Recently, Nakamori and Orimo10 have addressed aspects related to the synthesis, crystal structures, electronic structure and aDepartment of Physical Chemistry, University of Geneva, 30, quai thermodynamics of dehydrogenation of borohydrides. E. Ansermet, CH1211, Geneva 4, Switzerland. E-mail: hans-rudolf. Due to the intense research activity in this ﬁeld, many new [email protected]; Fax: +41 22 379 6103; Tel: +41 22 379 6539 results have been obtained since. The new structural data have been bLaboratory of Crystallography, University of Geneva, 24, quai E. Anser- 11,12 met, CH1211, Geneva 4, Switzerland. E-mail: [email protected]; reviewed. One of us has reviewed crystallographic methods for Fax: +41 22 379 6108; Tel: +41 22 379 6450 the structural characterization of metal hydrogen compounds.13

Hans Hagemann obtained his Radovan Cernˇ y´ did his PhD PhD in 1984 in the Dept. of in Solid State Physics at the Physical Chemistry at the Uni- Charles University, Prague. He versity of Geneva. After a post- did postdoctoral research at the doctoral stay at UC Berkeley, University of Geneva where he he returned to the University of was appointed lecturer in 1995. Geneva. His research interests He is interested in the devel- concentrate on structural and opment of the powder diffrac- optical properties of inorganic tion methodology, especially ab materials using vibrational and initio structure solution (com- optical spectroscopy. puter program Fox) and studies of structural defects and local correlations in disordered crys- Hans Hagemann Radovan Cernˇ y´ tals.

6006 | Dalton Trans., 2010, 39, 6006–6012 This journal is © The Royal Society of Chemistry 2010 24 25 The present perspective focuses on synthetic aspects. The main monitored by IR spectroscopy and TG. Unsolvated Y(BH4)3 recent achievements in this ﬁeld are related to rehydrogenation was also obtained by drying the ether solvate at 150 ◦C.26 reactions and to improved sample characterization with new NaBH4 is soluble in isopropylamine (IPA), but the yield of the crystal structures of ball milled samples. reaction between NaBH4 and MgCl2 is not very satisfactory (ca 25 40%) and the product is contaminated mainly with NaBH4. The low yield is probably related to the formation of a less soluble Synthesis magnesium amine complex during the reaction.25 General procedures and characterization techniques Another preparation of Mg(BH4)2 has been demonstrated by Zanella et al.28 using the reaction: The reactions taking place in organic solvents (except methanol) → were performed under an inert nitrogen atmosphere using ultra dry 3Mg(C4H9)2 + 2Al(BH4)3 3Mg(BH4)2 + 2Al(C4H9)3 = · solvents. MBH4 (with M Li,Na,K) as well as Ca(BH4)2 2THF The reactants are dissolved in toluene and stirred overnight, can be purchased from various companies. yielding a precipitate of solvent-free Mg(BH4)2 which is dried The solid samples are always manipulated in a glove box. under vacuum. The product yield was reported to be 85%.

Characterizations of the compounds are done using vibrational The Al(BH4)3 is formed by the quantitative reaction in toluene. and NMR spectroscopy and X-ray diffraction. AlCl3 + 3LiBH4 → Al(BH4)3 +3LiCl

Exchange reactions in solution The LiCl precipitates, and a chloride-free Al(BH4)3/toluene mixture can be obtained by distillation. This mixture is more easy Different exchange reactions in solution have been reported. For to handle than pure (and volatile) Al(BH4)3. the alkali borohydrides, conversion from NaBH4 +LiHal (Hal = Ca(BH4)2 can be purchased commercially as a THF solvate. 14,15 Cl,Br,I) to LiBH4 has been studied in different solvents, and Removing the solvent under vacuum for 1 h at 433 K allows to isopropylamine (IPA) was shown to be a favorable solvent. obtain the a phase of Ca(BH4)2. However, heating for longer times The solubility of different borohydrides in various solvents are at this temperature results in the formation of some b phase, 3,4,14,16–23 = collected in Table 1. Solubility data for MBH4 (M Li, although the irreversible a–b phase transition for this material 3,22 10 Na,K) have been collected previously and repeated. is reported at higher temperatures (453–473 K).29 Inspection of Table 1 shows that Ca(BH4)2 wasreportedtobe The heavier alkali borohydrides (K,Rb,Cs) can be prepared by 20 soluble in water, we observed however a violent reaction when the precipitation reaction30 Ca(BH4)2 was added to water. MOH + NaBH → MBH + NaOH (M = K,Rb,Cs) Among the alkali borohydrides, only LiBH4 is soluble in diethyl 4 4 24,25 ether and has been used to prepare Mg(BH4)2. However, this in very cold methanol (to prevent the methanolysis of the sodium reaction yields magnesium borohydride contaminated with Li and borohydride). 25 chloride. In a typical exchange reaction, MOH (M = K,Rb,Cs) is first Recently also Y(BH4)3 has been prepared by exchange with dissolved in MeOH and the solution is cooled. A second flask 26 LiBH4 in diethyl ether after ball milling of the reactants, but also containing MeOH is precooled using a salt-ice mixture, then 1.05 in this case, the product was contaminated with about 15 wt% of equivalents of NaBH4 aredissolvedinthecoldMeOH.Thetwo LiCl. cold solutions are mixed and the precipitate is filtered off and dried NaBH4 does not appear to be soluble in diethyl ether, nev- under vacuum. ertheless, exchange reactions in this solvent have been reported. The deuterides can also be prepared using this method, using 25 Soloveichik et al. have studied in detail the synthesis of Mg(BH4)2 MOD (M = K,Rb, Cs), NaBD4 and MeOD. The use of MOH using various methods. The exchange reaction of NaBH4 with = ◦ (M K,Rb, Cs) and MeOH in conjunction with NaBD4 leads to MgCl2 in the presence of ether with magnetic stirring at 34 C the formation of some MBD3H impurities as was observed for the yields practically no Mg(BH4)2. Increasing the temperature to ca Rb compound.31 60 ◦C by the admixture of toluene gave about 11.5% of product. KBH4 can also be formed by an exchange reaction of NaBH4 27 Recently, Li et al. reported the preparation of Mg(BH4)2 from in a concentrated KOH solution. Even if the total sodium the reaction concentration in solution is higher than 50%, the first compound

to precipitate upon solvent removal is KBH4. The formation of 2NaBH4 +MgCl2 → Mg(BH4)2 +2NaCl a solid solution of the type K1-xNaxBH4 is not observed under In this case, the stoichiometric mixture of reactants was first ball these conditions. This behavior can be associated with the higher milled for two hours before being suspended in ether and refluxed solubility of NaBH4 in water compared to the solubility of KBH4 for 60 h. After filtering off the solids, the solvent was evaporated (see Table 1). and finally dried at 513 K for 5 h, yielding the b phase of Mg(BH4)2. Several solvents may be used for sample purifications and crystal

Wet ball milling of MgCl2 and NaBH4 in Et2O allows to obtain growth. Single crystals of NaBH4 can be obtained from solutions 25 32,33 about 75% of Mg(BH4)2. in isopropylamine, and of KBH4 and RbBH4 from alkaline

As a result of exchange reactions, the metal borohydride can water solutions. NaBH4·2H2O single crystals are obtained from form a solvate. In order to obtain the pure borohydride, the solvate cold alkaline aqueous solutions.33 must be heated under vacuum. In the case of Mg(BH4)2 isolated LiBH4 can undergo isotope exchange with LiBD4 in solvents like 34 from ether solutions, it was shown to be necessary to heat up THF and ether, but not in pyridine. NaBH4 does not exchange ◦ 33,34 to 150 C in order to get rid of all diethyl ether in the solid, as hydrogen with NaBD4 in alkaline water solutions.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 6006–6012 | 6007 Table 1 Solubility of borohydrides in various solvents at 25 ◦C

LiBH4 NaBH4 KBH4 RbBH4 CsBH4 Mg(BH4)2 Ca(BH4)2 Sr(BH4)2

Methanol decomposes 16.4 g/100 ml 0.56 g/100 ml19 6.1 wt%3 14.2 wt%3 Ethanol >0.74M21 4 g/100 g3 0.25 g/100 ml19 0.045M21 44 g/100 g22 1.46M21 2-propanol 1.46M21 0.08M21 >1.0M21 14 19 19 16 21 20 Et2O 1,4M Insol. Insol. Insol. Insol. 20.8 wt% 0.045M Insol. 4.5 wt%17 THF ca25/100 g3 0.1 g/100 g3 Insol. 1.71M21 Sol.20 11.43M14 0.02M14 Monoglyme 4.7M14 0.8 g/100 g14 0.13 g/100 g18 0.15M14 Diglyme 4.25M21 5.5 g/100 g3 2.05 g/100 g18 1.67M21 0.45M21 Triglyme 8.7 g/100 g3 0.2 g/100 g18 Dioxane 0.3 g/100 g3 Insol. 0.1 g/100 g18 DMF 18 g/100g3 1.3 g/100 g23 Isopropylamine 1.27M14 1.10M14 Insol. DMSO 7.6 g/100 g23 22 3 19 19 20 20 H2O decomposes 55 g/100 g 19 g/100 g v. sol. vsol. Sol. Sol.

Reactions with aminoboranes and related compounds The solvated Et3N (as seen in the IR spectra of the product) was removed by progressive heating over several days under vacuum Borohydrides can be prepared by the reaction of the metal hydride ◦ up to 170 C, yielding 6.85 g of >95% pure Mg(BH4)2. with Et3NBH3 according to the scheme: This product was further puriﬁed by adding ca 200 ml of dry ether to this solid, stirred for 1 h at room temperature. The clear MHn + n Et3N·BH3 → M(BH4)n + n Et3N solution is drawn using a syringe, and the solvent evaporated The resulting triethylamine can be incorporated into the solid under vacuum in several steps over several days with increasing borohydrides as solvate or complexing molecule, as in the case temperatures up to 152 ◦C. X-ray powder diffraction showed single of Mg(BH4)2·xEt3N, which is clearly seen in the IR spectra of phase Mg(BH4)2 with no visible impurity (Fig. 1). the powder. The resulting product must then be heated under vacuum to remove the amine. Chlopek et al.35 reported this very efﬁcient synthesis (95% yield) of Mg(BH4)2.Acrucialpartforthe successful preparation is the premilling of the MgH2 to facilitate the reaction with the aminoborane. Mg(BH4)2, prepared from

MgH2 and Et3NBH3 contains after removal of the Et3Nalways some small amounts (5–10%) of unreacted MgH2. In the case of

Ca(BH4)2, the yield is somewhat lower (ca 85%). This method can also be used to prepare deuterated samples.36 - - Further, selectively deuterated BD3H and BH3D compounds can be prepared31,37 by reactions such as:

NaH + Et3N·BD3 → NaBD3H+Et3N Zanella et al.28 proposed another precipitation reaction to form

Mg(BH4)2:

3Mg(C4H9)2 +8BH3·S(CH3)2 → 3Mg(BH4)2·2S(CH3)2

+2B(C4H9)3·S(CH3)2. P Pure Mg(BH ) is obtained by pumping the precipitate for 6 h at Fig. 1 The Rietveld plot of the 6122 structure of Mg(BH4)2. High 4 2 resolution synchrotron powder diffraction data measured (SLS, PSI 0.1 mbar, followed by 13 h at 75 ◦Cand10-5 mbar. However, this Villigen) at room temperature, using 0.880114 A˚ wavelength. reaction requires a large excess of BH3·S(CH3)2 to be complete.

Example of Mg(BH4) synthesis Reactions with diborane

This compound was prepared by a slight variation of the procedure A low temperature solvent free synthesis of borohydrides has been 35 38 reported by Chlopek at al. 5gMgH2 were ball milled for 2 h. presented by O. Friedrichs et al. A LiBH4/ZnCl2 (2 : 1) mixture is ◦ 60 ml of Et3NBH3 were added and the mixture heated for 1 h to ball milled and placed in an oven. Heating of this mixture to 100 C ◦ 100 C, left to cool and stirred overnight, heated then for 6 h to yields gaseous B2H6 and hydrogen which is fed into a second oven ◦ ◦ 145 C, cooled down, 180 ml cyclohexane added, and stirred over containing LiH which reacts at ca 120 C to form LiBH4.X-ray 2 days. The solid is ﬁltered and left to dry under vacuum overnight analysis of the reaction products showed the presence of 72.2 wt% at room temperature, yielding a light grey (almost white) powder. LiBH4, 25.9 wt% LiH and 1.9 wt% Li2B12H12. This technique

6008 | Dalton Trans., 2010, 39, 6006–6012 This journal is © The Royal Society of Chemistry 2010 should allow in principle also to form other borohydrides under Most of the other syntheses of solvent free metal borohydrides relatively mild conditions, in the absence of chloride. rely typically on the exchange reaction50 39 Very recently, Friedrichs et al. applied this technique to study + x+ x+ + xA BH4 + M Halx → M (BH4)x +xA Hal in more detail the formation of LiBD4 from LiD, using in situ neutron diffraction. Nucleation of LiBD4 is observed already at where A is alkaline metal or Al, M is a metal (3d,4d,5d or ◦ 100 C (in the orthorhombic phase), however the LiBD4 appears actinide) and Hal is a halogen. The yield of a successful exchange to form a passivation layer around the LiD grains and considerably reaction varies between 35 and 95%, and the halide A+Hal can be 35 slows down the reaction, such that the conversion of LiD to LiBD4 removed by extraction with a suitable solvent. When the metal remains incomplete, even at 185 ◦C. M is partly reduced during the exchange reaction, the borane and hydrogen gases are released too.51 The exchange reaction is not Exchange reactions in solid state – mechanochemical synthesis always fully completed, and stoichiometric ternary (mixed-cation) + x+ 7,52,53 borohydrides A yM z(BH4)y+x·z are observed. Mixed-anion + x+ Mechanochemical synthesis (MS) as a part of mechanochemistry (and mixed-cation) products A yM z(Hal,BH4)y+x·z have been is a special case of mechanical alloying (MA).40 The process of observed too.54 The competing reaction which produce ternary + x+ MA, as we use it nowadays, was developed as the result of a long halides A yM zHaly+x·z in some cases decreases the reaction search to produce a nickel-based superalloy.41 However, the solid- yield.53 state reactions were observed during the mechanical treatment of The following solvent free metal borohydrides have been pre- 42 48 49,55,56 57 57,58 powders as early as in 1902, and the inﬂuence of mechanical pared by MS: NaBH4, Zr(BH4)4, Hf(BH4)4, U(BH4)4, 45,59,60 61 46 35 7 energy input on chemical reactions has been studied already in Y(BH4)3, Ce(BH4)3, Mn(BH4)2, Mg(BH4)2, LiSc(BH4)4, 43 53 52 1919. Additionally, it has been recognized that powder mixtures NaSc(BH4)4, LiZn2(BH4)5,NaZn2(BH4)5 and NaZn(BH4)3, 54 can be mechanically activated to induce chemical reactions, i.e., KZn(BH4)Cl2.

MS at room temperature or at least at much lower temperatures The compound Y(BH4)3 has been obtained by ball milling 44 than normally required to produce the desired compound. The using either 3 : 1 and 4 : 1 LiBH4 to YCl3 ratios. The 4 : 1 mixture MS can also produce metastable phases, high pressure phases subjected to room temperature-59 or cryo-ballmilling60 resulted in

(the pressures generated during MS have been estimated to be almost complete conversion of YCl3 to Y(BH4)3, while the 3 : 1 of the order of 6 GPa), disordered and amorphous phases. The mixture (which is the stoichiometric mixture) did not react com- 7 resulting products are always ﬁne grained, homogeneous and pletely. The formation of LiY(BH4)4, analogous to LiSc(BH4)4, is nanostructured.40 not observed, which is probably related to the different ionic radii The MS is applied to the solvent free metal borohydride of Sc3+ (0.75 A˚ )andY3+ (0.90 A˚ ). Ball milling reaction attempts synthesis as simply as the ball-milling. Different types of high- using YF3 and NaBH4 or LiBH4 as well as YCl3 and NaBH4 did 40 62 energy milling equipment are used. They differ in their capacity, not yield Y(BH4)3. efﬁciency of milling and additional arrangements for cooling, Many other single-cation metal borohydrides were claimed to heating, etc. The most popular mills for conducting MS of have been prepared by MS, but no structural data were reported: 63–65 63 65 50,51 borohydrides is the planetary ball mill in which a few grams of the Zn(BH4)2, Cd(BH4)2, Sc(BH4)3, Ti(BH4)3, V(BH4)3 and 66 67 powder can be milled in a tightly sealed bowl containing milling Cr(BH4)2. A recent study reports the formation of Ti(BH4)3 as balls. The bowl can be loaded and unloaded under inert gas in an intermediate in the decomposition reaction of a ball milled 3 : 1 a glove box. Other mills using the milling balls like rotational, mixture of LiBH4 and TiF3. A reaction does indeed take place, as vibratory or attritor mills are used too. For more details see ref. evidenced by the formation of LiF. The in situ FTIR data upon 40. The milling parameters to control are: heating (Fig. 4 in ref. 67) shows bands at ca 1600 and 2550 cm-1

–milling speed, typically 600 rpm, which were wrongly assigned to Ti(BH4)3 but correspond in fact –milling time, from a few minutes to a few hours, to the IR spectrum of diborane.68 The formation of diborane is –ball-to-powder weight ratio, typically 25 : 1, detrimental for the hydrogen release in this system. –milling temperature, typically ambient, but can be cryogenic, The highest yields and best kinetics of the exchange reaction

–milling atmosphere, typically inert, but can be hydrogen, have been obtained when LiBH4 was used in the ball-milled –milling atmosphere pressure, typically one bar. mixture.46,50,62 This can be related to the phase transition, initiated

It was recognized that the cooling breaks, like 5 min break by ball-milling, in LiBH4 at 381 K, which leads to a superionic HT- for 10 min of milling, are important to reduce heating of the phase. Further, the milling can lead to mixed-anion Li(BH4)1-xClx system as well as to avoid agglomeration of the powder on the which has even lower phase transition temperatures.69 The yield walls of the grinding bowl, and so allow the successful MS of and kinetics decrease with the increasing size of the alkaline metal. borohydrides.45,46 On the side of the metal halides typically chlorides are used. Use of MS enabled the discovery of chemical processes that As for fluorides, they lower the kinetics of the exchange reaction, can take place in metal hydrides in the solid state, and revealed which can help to quench the phases not-stable at the temperature the solid-state chemistry of borohydrides is quite as rich as their of ball-milling.67 chemistry in solution.47 ThefirstapplicationofMStosolventfree Due to the applied mechanical energy and heating of the sample metal borohydride synthesis comes from American scientists: during the ball milling the synthesis conditions of the MS are shifted out from the RT equilibrium. It is reflected in the synthesis → 48 4NaH+2B2O3 NaBH4 +3NaBO2 of different phases from the same mixture by exchange reaction using the MS than using the solution method. One example is 49 3 Zr(OR)4 +8B2H6 → 3 Zr(BH4)4 + 4 B(OR)3 OR = OC4H9 the reaction between ZnCl2 and LiBH4 in the ratio 1 : 2 which

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 6006–6012 | 6009 70 52 produces Zn(BH4)2 in solution but LiZn2(BH4)5 by MS. On the Mixed alkali borohydrides contrary the reaction between TiCl3 and LiBH4 in the ratio 1 : 3 71 50 The system LiBH4-KBH4 was studied by solid state reactions at produce LiTi(BH4)4 in solution but Ti(BH4)3 by MS. ◦ 73 The analysis of the ball-milled products is typically done 125 C. The formation of the intermediate compound LiK(BH4)2 (e.g. 65 and 66) by X-ray powder diffraction, in conjunction was observed and its crystal structure determined. with spectroscopic methods like Raman, infrared and NMR Metastable NaK(BH4)2 was obtained by ball-milling at room 74 spectroscopy as well as desorption studies. The observation of temperature of 1 : 1 mixtures of NaBH4 and KBH4. Heating diffraction peaks of LiCl (or other alkali halides) shows that a curve experiments for the systems LiBH4–NaBH4 and NaBH4– 75 reaction has taken place. However, as the ball-milled products KBH4 have been reported some time ago. For the NaBH4–KBH4 are multi-phase, the bottle-neck of the ab initio structural studies system, the formation of solid solutions was demonstrated by the increase of the a lattice parameter with increasing K content. The of new compounds is the indexing of the powder pattern. A ◦ successful strategy consists of in situ diffraction as a function of minimum melting point was found at 453 C for the mole fraction the temperature (T-ramping) up to the decomposition temperature of 31.8% of KBH4. of the different phases. This procedure allows separation of the diffraction peaks of individual phases, and it is shown (Fig. 2) for High temperature/pressure synthesis from the elements the example of ball milled NaBH + ScCl mixtures.53 In this 4 3 The direct synthesis of LiBH(D) from the elements has been system a sample containing two new phases, NaSc(BH ) and 4 4 4 demonstrated by Friedrichs et al.76 This reaction takes place at Na ScCl , is formed by ball milling. Only T-ramping has allowed ◦ 3 6 700 Candp(H(D) ) = 150 bar: the attribution of observed peaks to individual phases, and led to 2 a successful indexing of the powder patterns. Li + B + 2 H(D)2 → LiBH(D)4

The high reaction temperature is caused by the inertness of

boron. Reacting ﬁrst Li with B under argon yields LiB3 (at ◦ ◦ 330 C) and Li7B6 (at 450 C). LiB3 absorbs hydrogen more ◦ rapidly at 700 CthanLi7B6 and the element mixture, suggesting a

different hydrogen absorption mechanism for LiB3. Starting from

a stoichiometric mixture of Li and B and 150 bar of D2 about 90% 76 of LiBD4 + 10% of LiH have been obtained.

Other high temperature/pressure reactions

LiBH4,NaBH4 and Ca(BH4)2 can be formed by a solid gas reaction ◦ 77 (up to 400 C and 350 bar H2) according to the schemes

2MH+MgB2 +4H2 → 2M(BH4)+MgH2 (M = Li, Na)

CaH2 +MgB2 +4H2 → Ca(BH4)2 +MgH2

78 The formation of Ca(BH4)2 was studied further and shown to ◦ have taken place already at 350 C and 150 bar H2. The addition of a Ti catalyst improves the kinetics of both hydrogenation and dehydrogenation. Fig. 2 The powder patterns of the ball milled mixture 2NaBH + ScCl 4 3 Calcium borohydride can be formed by the reaction of CaB as a function of temperature (bottom to top: T-ramping between 295 6 ◦ 79 and 500 K). Synchrotron powder diffraction data measured72 (SNBL, and CaH2 under 700 bar of hydrogen at 400–440 C ESRF Grenoble) using a 0.66863 A˚ wavelength shows peaks of two phases: CaB6 +CaH2 → a phase Ca(BH4)2 (yield 60%) NaSc(BH4)4 (+) and Na3ScCl6 (-). Decomposition of NaSc(BH4)4 at 410 K allows the attribution of observed peaks to the phases. Using a TiF3 catalyst, this reaction can also take place under 140 bar while ball milling the sample for 24 h at room temperature with a 19% yield.80 The ball milling experiments with ZnCl2 showed that it is also It is interesting to note that the alpha phase is formed at high crucial to perform ball-milling of starting mixtures in several temperature and high pressure, while at ambient pressure the alpha different ratios.52 This allows on one side to examine the variation phase is irreversibly transformed into the beta phase above 200 ◦C. of relative intensities of diffraction peaks in the ball-milled 77 LiH does not appear to react with CaB6 and H2. samples with different mixture ratios. On the other side, the The reactions with MgB2 are very interesting, as they demon- stoichiometry of the ﬁnal product can be somewhat surprising strate the reversibility of the dehydrogenation of borohydride (such as LiZn2(BH4)5), and requires formally the stoichiometry: and magnesium hydride mixtures which take place at lower temperatures compared to the pure borohydrides. → 2ZnCl2 + 5 LiBH4 LiZn2(BH4)5 +4LiCl From a synthetic point of view, these reactions allows isotopi- cally pure (11B, D) samples to be obtained for neutron diffraction which is not achieved with simple molar ratios Li : Zn = 1:2 or studies. 1:3. Remhof et al.81 have studied the reaction

6010 | Dalton Trans., 2010, 39, 6006–6012 This journal is © The Royal Society of Chemistry 2010 2LiH(D) + AlB2 + 3 H(D)2 → 2 LiBH(D)4 +Al (which are less bound to the metal ions) such as tertbutylmethyl

◦ ether may be useful. at 450 C and 50 bar H(D)2. The presence of Al favors the Chloride-free products can be obtained either by high temper- absorption of hydrogen (deuterium). However, the desorption of ature/pressure reactions or by reactions of metal hydrides with hydrogen is not accelerated by the presence of Al, and the hydrogen either gaseous diborane or reactants like Et3NBH3. storage capacity decreases upon cycling due to incomplete re- To prepare compounds with 11B and D for neutron diffraction formation of AlB2. studies, high temperature/pressure reactions with MgB2 appear Hydrogen–deuterium (tritium) exchange reactions have been more efficient than exchange reactions by wet chemistry. How- 3 reported in the past for the solid alkali borohydrides: ever, these techniques cannot be applied to the more complex borohydrides with Mn, Sc etc. which decompose below 100 ◦C. MBH4 +2D2 → MBD4 +2H2 (M = Li, Na, K) The recent experiments under high temperature and hydrogen ◦ ◦ The reported temperatures3 were 200 C (Li), 350 C(Na),and (and diborane) pressure give new insights into various aspects of ◦ 500 C (K). The deuterium hydrogen exchange in LiBH4 has been the reaction mechanisms of borohydride formation and diffusion studied in more detail on LiBH4 exposed to 20 bar D2 at 538 K in solids. The fundamental understanding of these processes, ◦ (260 C) up to 23 h82 and revealed statistical isotopic distribution which requires further experimental and theoretical work, will of the partially deuterated samples. allow strategies for optimum material design to be defined for We have succeeded very recently in performing a deuterium– potential hydrogen storage. hydrogen exchange reaction in solid Mg(BH4)2 starting at 405 K (132 ◦C). An almost complete exchange reaction was performed at 445 K (172 ◦C), a significantly lower temperature than reported Acknowledgements previously for the alkali borohydrides.83 Very recently, Borgschulte This work is supported by the Swiss National Science Foundation. et al.84 studied the diffusion of hydrogen in LiBH . The sample was 4 It is a pleasure to acknowledge fruitful collaborations with other a pressed pellet containing on one side LiBH and on the other side 4 laboratories, in particular with Ya. Filinchuk at the SNBL, ESRF LiBD . The progress of diffusion was monitored using spatially 4 Grenoble. resolved Raman spectroscopy. These experiments revealed that at 473 K, the hydrogen diffusion is mostly associated with motions of - theentireBH4 unit, while direct exchange of hydrogen was found Notes and references to be about 10 times slower. 1 G. Soloveichik, Mater. Matters, 2007, 2, 11. 2 S. Orimo, Y.Nakamori, J. R. Eliseo, A Zuttel¨ and C. M. Jensen, Chem. Rev., 2007, 107, 4111. Conclusions 3 B.D.JamesandM.G.H.Wallbridge,Prog. Inorg. Chem., 1970, 11, 99. 4 T. J. Marks and J. R. Kolb, Chem. Rev., 1977, 77, 263. The different synthetic routes presented here show several aspects 5V.D.Makhaev,Russ. Chem. Rev., 2000, 69, 727. which need consideration: 6 M. Besora and A. Lledos, Struct. Bonding, 2008, 130, 149. For industrial purposes, a cheap and safe method should be 7 H. Hagemann, M. Longhini, J. W. Kaminski, T. A. Wesolowski, R. ˇ preferred, such as the wet ball milling of NaBH and MgCl Cerny,´ N. Penin, M. H. Sørby, B. C. Hauback, G. Severa and C. M. 4 2 Jensen, J. Phys. Chem. A, 2008, 112, 7551. to prepare Mg(BH4)2. However, this technique would probably 8S.F.Parker,Coord. Chem. Rev., 2010, 254, 215. require THF (or another solvent) instead of ether to prepare 9 A. Remhof, R. Gremaud, F. Buchter, Z. Lodziana, J. P. Embs, T. A. J. Ramirez-Cuesta, A. Borgschulte and A. Zuttel,¨ Z. Phys. Chem., 2010, Ca(BH4)2. 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