Facile high-yield synthesis of pure, crystalline Mg(BH4) 2

Article abstract of DOI:10.1021/ic701436c

Magnesium borohydride, Mg(BH₄)₂, a long-sought candidate for efficient hydrogen storage chemisorption technology, has been obtained in a pure and crystalline form by two new synthetic routes in a hydrocarbon solvent. A first synthetic approach involves a metathetical reaction between organometallic magnesium compounds; a second route consists of an insertion reaction of BH₃ species, released from BH ₃·S(CH₃)₂, into the Mg-C bonds of MgR₂, with complete replacement of R groups with BH₄ groups. Both methods, based on commercially available reagents, afford identical, pure, polycrystalline materials, identified by X-ray diffraction as the so-called low-temperature hexagonal form of Mg(BH₄)₂, stable below 180°C, recently shown to possess a complex, unpredictable, crystal structure.

Full text of DOI:10.1021/ic701436c

Inorg. Chem. 2007, 46, 9039−9041
Facile High-Yield Synthesis of Pure, Crystalline Mg(BH4)2
Pierino Zanella,*,† Laura Crociani,† Norberto Masciocchi,‡ and Giovanni Giunchi§
CNR-ICIS, corso Stati Uniti 4, 30127 PadoVa, Italy, Dipartimento di Scienze Chimiche e
Ambientali, UniVersita` dell’Insubria, Via Valleggio 11, 22100 Como, Italy, and EDISON SpA,
R&D DiVision, Foro Buonaparte 31, 20121 Milano, Italy
Received July 20, 2007
Magnesium borohydride, Mg(BH4)2, a long-sought candidate for
efficient hydrogen storage chemisorption technology, has been
obtained in a pure and crystalline form by two new synthetic routes
in a hydrocarbon solvent. A first synthetic approach involves a
metathetical reaction between organometallic magnesium com-
pounds; a second route consists of an insertion reaction of BH3
isolated (although, in some cases, within a mixture of salts)
was nicely shown by the complete X-ray structural charac-
terization (from synchrotron powder diffraction data) of the
complex crystal structures of two (structurally related)
polymorphs, which just appeared in the scientific literature,3a,b
and by the coincidence of the products of two independent
preparation routes, reported in a yet undisclosed patent
application.4
species, released from BH3‚S(CH3)2, into the Mg−C bonds of MgR2,
with complete replacement of R groups with BH4 groups. Both
methods, based on commercially available reagents, afford
identical, pure, polycrystalline materials, identified by X-ray dif-
fraction as the so-called low-temperature hexagonal form of
Classical synthetic routes to Mg(BH4)2 were based on the
thermal dissociation of solvated products, mainly the diethyl
ether adduct, but even though a purity as high as 98% was
claimed (with no clear analytical crystallographic analysis),2
the dissociation reaction lasted almost 1 week, clearly
preventing any practical industrial exploitation of such a
process. If higher temperatures (up to 230 °C) are employed
during the dissociation process, to increase the reaction rate,
undesired decomposition products are easily formed. Alter-
natively, solvent dissociation under a medium-high vacuum,
10-3 Torr, in 12 h has been attempted.2,5 A further preparation
method is described in the old literature,6 based on the
metathesis betweeen magnesium dichloride and sodium
borohydride in diethyl ether; whatever compound was then
isolated, its X-ray diffraction pattern cannot be matched to
that of any Mg(BH4)2 polymorph. Interestingly, the recently
reported preparation3b of Mg(BH4)2 also employs MgCl2 and
NaBH4 and, apparently, can give pure phases (or mixture of
compounds) depending on the actual reaction conditions;
moreover, by replacement of NaBH4 with LiBH4, Mg(BH4)2
was recovered as a ca. 30% component of a complex salt
mixture.3a Apparently, the most efficient method reported
so far is the direct synthesis from magnesium dihydride and
the triethylamineborane complex, very recently proposed by
Chłopek et al.3c
Mg(BH4)2, stable below 180
°C, recently shown to possess a
complex, unpredictable, crystal structure.
Magnesium borohydride, Mg(BH4)2, has been described
in the chemical literature from many years and, like many
other metallic borohydrides, has been employed as a reducing
agent in synthetic organic chemistry.1 Given that it easily
forms a number of adducts with many Lewis bases (including
a variety of common solvents), its purity was seldom
assessed,2 and clear evidence for the existence of a pure,
crystalline and unsolvated compound of the Mg(BH4)2
formula was not available until very recently.3,4 Indeed, the
interest for this (apparently simple) compound was recently
renewed because it is an ideal candidate for a chemisorptive
hydrogen storage material and a potential chemical precursor
to be used in the deposition process of superconducting MgB2
thin films.4 The fact that this species can be prepared and
* To whom correspondence should be addressed. E-mail: p.zanella@
icis.cnr.it.
† CNR-ICIS.
‡ Universita` dell’Insubria.
§ EDISON SpA.
Fifty years of attempted synthesis of Mg(BH4)2 have
evidenced the following main difficulties: (i) metathesis
reactions generally lead to the formation of a mixture of
products (Mg(BH4)2-nXn, n ) 0-2), from which Mg(BH4)2
(n ) 0) is difficult to separate (or even to identify); (ii)
(1) James, B. D.; Wallbridge, M. G. H. Prog. Inorg. Chem. 1970, 11, 99
and references cited therein.
(2) Plesek, J.; Hermanek, S. Collect. Czech. Chem. Commun. 1966, 31,
3845.
(3) (a) Cˇ erny´, R.; Filinchuk, Y.; Hagemann, H.; Yvon, K. Angew. Chem.,
Int. Ed. 2007, 46, 5765. (b) Her, J.-H.; Stephens, P. W.; Gao, Y.;
Soloveichik, G. L.; Rijssenbeek, J.; Mandrus, M.; Zhao, J.-C. Acta
Crystallogr., Sect. B 2007, 63, 561. (c) Chłopek, K.; Frommen, C.;
Le´on, A.; Zabara, O.; Fichtner, F. J. Mater. Chem. 2007, 33, 3496.
(4) EDISON. Italian Patent Application, MI2006A 001048, filed 30/05/
2006.
(5) Brenner, M.; Noeth, H.; Warhold, M. Eur. J. Inorg. Chem. 2003, 111.
(6) Konoplev, V. N.; Bakulina, V. M. IzV. Akad. Nauk SSSR Ser. Khim.
1971, 159.
10.1021/ic701436c CCC: $37.00
Published on Web 09/28/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 22, 2007 9039

COMMUNICATION
quantitative reactions in which Mg(BH4)2 is the prevalent
product (generally based on the insertion of BH3 into Mg-C
or Mg-H bonds) have been carried out in donor solvents
(O or N Lewis bases); this implies the formation of stable
Mg(BH4)2‚L Lewis complexes (hard acid-hard bases) from
which complete, and controlled, dissociation of L is almost
impossible, even under extreme conditions of high temper-
ature and high vacuum.
Aiming at overcoming these difficulties, we have identified
two efficient synthetic methods (methods a and b), based
on the following considerations: (i) noncoordinating solvents
(such as aliphatic or aromatic hydrocarbons) are to be used;
(ii) if the only insoluble species in the reaction medium is
Mg(BH4)2, then separation of the desired product by filtration
can easily be achieved. Thus, a high-yield reaction in which
all reactants and side products are soluble is required.
Method a consists of a metathesis reaction schematized
as
Figure 1. Powder X-ray diffraction traces of Mg(BH4)2 isolated from
methods a (top) and b (center). The bottom plot shows the simulated trace
(in red, coordinates from ref 3a), matching the observed data (in blue).
Figure 2. 11B NMR spectra of the reaction intermediates and products:
Al(BH4)3 (left trace: δ -36.50 ppm; JB-H ) 88.8 Hz) and Mg(BH4)2 for
3Mg(C4H9)2 + 2Al(BH4)3 f 3Mg(BH4)2 + 2Al(C4H9)3 (a)
methods a (center trace) and b (right trace) (δ -42.80 ppm; JB-H
)
Mg(C4H9)2 is a commercial product available as an
n-heptane solution and is also highly soluble in toluene; at
variance, a toluene solution of Al(BH4)3 can be easily
prepared by the following reaction:
83.1 Hz).
diffraction measurement of crude samples sealed in Linde-
mann capillaries (see Figures 1 and 2).
Method b represents a modification and improvement with
respect to the previously reported5 reactions involving the
stepwise BH3 insertion into Mg-C bonds. Also, in this case,
an n-heptane solution of Mg(C4H9)2 was used as the Mg-
containing starting material and toluene as the main solvent.
At variance, the BH3‚S(CH3)2 borane adduct (in toluene) was
employed as a source of the borohydride species. The
reaction scheme may be represented as
AlCl3 + 3LiBH4 f Al(BH4)3 + 3LiCl
This reaction7 takes place quantitatively and generates the
(soluble) Al(BH4)3 species and a colorless precipitate, LiCl.
Because of its high volatility, Al(BH4)3 cannot be easily
separated from its original toluene solution; however, distil-
lation of the pristine solution affords a toluene/Al(BH4)3
mixture exempt of any Li+ and Cl- ions; significantly, the
actual concentration of Al(BH4)3 in the distilled fraction can
be easily determined by classical analytical methods, using,
for example, ethylenediaminetetraacetic acid as a titrating
agent. The direct use of Al(BH4)3 solutions is definitely
preferable to that of neat (liquid) Al(BH4)3, with the latter
material being air- or moisture-sensitive, volatile, flammable,
and, thus, very dangerous.
The capability of Al(BH4)3 in exchanging its BH4- groups
with the R groups of other organometallic compounds (e.g.,
LiC2H5) has already been reported.8 Because our preliminary
experiments had shown the sought, thus appealing, poor solu-
bility of Mg(BH4)2 in toluene [in which, inter alia, Al(C4H9)3
and the reaction products are easily dissolved], this solvent
was chosen as the ideal reaction medium. Thus, the mixing
of Mg(C4H9)2 and Al(BH4)3 (in 3:2 molar ratio) in toluene
solutions causes the progressive formation of a colorless
precipitate. After stirring overnight, the powdery precipitate
was filtered, washed with toluene [in order to remove
completely residual Al(C4H9)3], and dried under vacuum. The
chemical and crystallographic purities of this Mg(BH4)2
material were tested through elemental analysis, 11B NMR
spectroscopy of a diethyl ether solution, and powder X-ray
3Mg(C4H9)2 + 8BH3‚S(CH3)2 f
3Mg(BH4)2‚2S(CH3)2 + 2B(C4H9)3‚S(CH3)2 (b)
On employment of an excess of the borane species, a
colorless product precipitates and, thus, can be easily iso-
lated by filtration, washed, and dried in vacuo. The reaction
product contains Mg(BH4)2 complexed by S(CH3)2. How-
ever, it must be observed that, in contrast to many other
Mg(BH4)2‚L complexes (L ) O and N donor ligands, such
as ethers or amines, hard enough to make their dissociation
rather difficult or nearly impossible), S(CH3)2 is a much
softer Lewis base, allowing the recovery of a pure, uncom-
plexed Mg(BH4)2 form by breaking the weak bonds between
the thioether and the hard Mg2+ acceptor. The only other
reaction product, B(C4H9)3‚S(CH3)2, is highly soluble in
toluene and, therefore, can be easily eliminated. The pre-
cipitate, Mg(BH4)2‚nS(CH3)2 (likely, n ) 2), can be easily
vacuum-dried at room temperature (at 10-1 mbar for 6 h),
followed by diffusive vacuum (10-5 mbar, at 75 °C, for 13
h). Worthy of note, the reaction requires a large excess of
BH3‚S(CH3)2 to be complete (and selective), thus avoiding
the formation of partially substituted species of the
Mg[BH4-n(C4H9)n]2 type. Characterization of the final prod-
uct, performed as described in method a, demonstrated that
(7) Schlesinger, H. I.; Brown, H. C.; Hyde, E. K. J. Am. Chem. Soc. 1953,
75, 209.
(8) Schlesinger, H. I.; Brown, H. C. J. Am. Chem. Soc. 1940, 62, 3429.
9040 Inorganic Chemistry, Vol. 46, No. 22, 2007

COMMUNICATION
the colorless powders isolated by this procedure correspond
to the same crystalline product, Mg(BH4)2.
particular, for electronic applications, thin films are required,
and actually the best results have been obtained mainly by
hybrid physical-chemical deposition of toxic gaseous borane
species.11
The comparison between the powder X-ray diffraction
traces reported in Figure 1 indicates that the powders, derived
from two independent reaction pathways, contain the very
same polycrystalline species. Well before the recent structural
determinations,3 we succeeded in indexing the first 23 peaks
by TOPAS-R (GOF ) 25.8) by a hexagonal unit cell of
approximate dimensions a ) 10.33 Å, c ) 37.09 Å, and V
) 3430 Å3, later confirmed by a structureless whole-pattern
profile fitting by the Le Bail method. These experimentally
derived lattice metrics and the X-ray diffraction pattern
simulation (using the coordinates published in ref 3a) allowed
us to confirm once more that our powders are a monophasic
material, i.e., pure Mg(BH4)2, the complex crystal structure
of which has been very recently determined from synchrotron
data by two leading groups in the field.3a,b
In summary, this contribution has presented two relatively
cheap, rapid, alternative (but equally efficient) synthetic
methods for obtaining the (for long-time, elusive) Mg(BH4)2
material, which can be isolated in a very pure and (poly)-
crystalline form. The operations described in these chemical
recipes do not require sophisticated handling systems (apart
from vacuum lines or dry boxes) or hazardous materials, like
gaseous BH3, which is often supplied in high-pressure
vessels. Here, cheap and commercially available starting
materials are used, and the whole sequence of manipulation
processes can be easily carried out within an efficient drybox
system under a controlled, inert, atmosphere.
Acknowledgment. We thank Prof. A. Sironi (University
of Milan) for measuring high-resolution powder X-ray
diffraction patterns on sealed capillaries and Dr. F. Benetollo
for sample preparation. We thank Prof. R. Cˇ erny´ and Prof.
P. W. Stephens for providing their recent work as preprints.
Our crystalline magnesium tetrahydroborate powders are
highly reactive. In particular, when in contact with a moist
atmosphere, pure Mg(BH4)2 gives rise to a highly exothermic
reaction with violent effervescence. In addition, this material
is very interesting as a hydrogen storage material because it
possess as rather exceptional (nominal) hydrogen content of
about 14.9 wt %. The hydrogen desorption of the product
has already been beautifully reported in ref 3c and will not
be discussed here any further.
Supporting Information Available: Details of the two syn-
thetic methods, analytical data, and a description of the instrumenta-
tion used. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC701436C
Furthermore, additional interesting applications of the pure,
crystalline Mg(BH4)2 prepared by our methods may arise
from its use as a starting material (of controlled stoichiometry
and physicochemical properties) in chemical vapor deposition
processes of magnesium diboride, MgB2. Indeed, this super-
conducting material, discovered in 2001,9 is presently actively
studied and widely engineered in a variety of fields;10 in
(9) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu,
J. Nature (London) 2001, 410, 63.
(10) Vinod, K.; Abhilash Kumar, R. G.; Syamaprasad, U. Supercond. Sci.
Technol. 2007, 20, R1.
(11) Zeng, X. H.; Pogrebnyakov, A. V.; Kotcharov, A.; Jones, J. E.; Xi,
X. X.; Lysczek, E. M.; Redwing, J. M.; Xu, S. Y.; Li, Q.; Lettieri, J.;
Schlom, D. G.; Tian, W.; Pan, X. Q.; Liu, Z. K. Nat. Mater. 2002, 1,
35.
Inorganic Chemistry, Vol. 46, No. 22, 2007 9041