Manganese borohydride; Synthesis and characterization

Article abstract of DOI:10.1039/c4dt03501a

Solvent-based synthesis and characterization of α-Mn(BH₄)₂ and a new nanoporous polymorph of manganese borohydride, γ-Mn(BH₄)₂, via a new solvate precursor, Mn(BH₄)₂·1/2S(CH₃)₂, is presented. Manganese chloride is reacted with lithium borohydride in a toluene/dimethylsulfide mixture at room temperature, which yields halide and solvent-free manganese borohydride after extraction with dimethylsulfide (DMS) and subsequent removal of residual solvent. This work constitutes the first example of establishing a successful, reproducible solvent-based synthesis route for a pure, crystalline, stable transition metal borohydride. The new polymorph, γ-Mn(BH₄)₂, is shown to be the manganese counterpart of the zeolite-like compound, γ-Mg(BH₄)₂ (cubic, a = 16.209(1) ?, space group Id3a). It is verified that large pores (diameter > 6.0 ?) exist in this structure. The solvate, Mn(BH₄)₂·1/2S(CH₃)₂, is subsequently shown to be the analogue of Mg(BH₄)₂·1/2S(CH₃)₂. As the structural analogies between Mg(BH₄)₂ and Mn(BH₄)₂ became evident a new polymorph of Mg(BH₄)₂ was identified and termed ζ-Mg(BH₄)₂. ζ-Mg(BH₄)₂ is the structural counterpart of α-Mn(BH₄)₂. All synthesis products are characterized employing synchrotron radiation-powder X-ray diffraction, infrared spectroscopy and thermogravimetric analysis in combination with mass spectroscopy. Thermal analysis reveals the decomposition of Mn(BH₄)₂ to occur at 160 °C, accompanied by a mass loss of 14.8 wt%. A small quantity of the desorbed gaseous species is identified as diborane (ρ_m(Mn(BH₄)₂) = 9.5 wt% H₂), while the remaining majority is found to be hydrogen.

Full text of DOI:10.1039/c4dt03501a

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Manganese Borohydride; Synthesis and Characterization
Bo Richter1, Dorthe B. Ravnsbæk1,2, Nikolay Tumanov3,
Yaroslav Filinchuk3, Torben R. Jensen1,*
1
Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of
Aarhus, Langelandsgade 140, DKꢀ8000 Aarhus C, Denmark.
2
Department of Material Science and Engineering, Massachusetts Institute of Technology, 77
Massachusetts Avenue, Cambridge 02139, Massachusetts.
3
Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, place L.
Pasteur 1, Bꢀ1348 LouvainꢀlaꢀNeuve, Belgium.
* To whom correspondence should be addressed. T. R. J. (trj@chem.au.dk).
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ABSTRACT
Solventꢀbased synthesis and characterization of αꢀMn(BH4)2 and a new nanoporous polymorph of
manganese borohydride, γꢀMn(BH4)2, via a new solvate precursor, Mn(BH4)2ꢁ½S(CH3)2, is
presented. Manganese chloride is reacted with lithium borohydride in a toluene/dimethylsulfide
mixture at room temperature, which yields halide and solventꢀfree manganese borohydride after
extraction with dimethylsulfide (DMS) and subsequent removal of residual solvent. This work
constitutes the first example of establishing a successful, reproducible solventꢀbased synthesis route
for a pure, crystalline, stable transition metal borohydride. The new polymorph, γꢀMn(BH4)2, is
shown to be the direct manganese counterpart of the zeoliteꢀlike structure, γꢀMg(BH4)2 (cubic, a =
16.209(1) Å, space group Idꢀ3a). It is verified that large pores (diameter > 6.0 Å) exist in this
structure. The solvate, Mn(BH4)2ꢁ½S(CH3)2, is subsequently shown to be the analogue of
Mg(BH4)2ꢁ½S(CH3)2. As the structural analogies between Mg(BH4)2 and Mn(BH4)2 became evident
a new polymorph of Mg(BH4)2 was identified and termed ζꢀMg(BH4)2. ζꢀMg(BH4)2 is the structural
counterpart of αꢀMn(BH4)2. All synthesis products are characterized employing synchrotron
radiationꢀpowder Xꢀray diffraction, infrared spectroscopy and thermogravimetric analysis in
combination with mass spectroscopy. Thermal analysis reveals the decomposition of Mn(BH4)2 to
occur at 160 °C, accompanied by a mass loss of 14.8 wt%. A small quantity of the desorbed
gaseous species is identified as diborane (ρm(Mn(BH4)2) = 9.5 wt% H2), while the remaining
majority is found to be hydrogen.
Keywords: Hydrogen storage materials, manganese borohydride, magnesium/manganese
borohydride, isopolymorphism, borohydride synthesis.
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INTRODUCTION
The increasing worldꢀwide energy demand necessitates still new approaches for implementing safe
and environmentally benign forms of energy, to replace fossil fuels [1]. Current research efforts
within hydrogen storage focus on high storage densities, prolific thermodynamic and kinetic
properties and reversibility, to allow renewable energy storage for mobile applications [2]. The
current technology utilizes high pressure hydrogen tanks; e.g. p(H2) ~ 700 bar for the automotive
industry [3]. As an alternative to pressure tanks, solid state chemical storage of hydrogen is being
investigated. For example, utilization of hydride materials to act as solid state storage media for
renewable energy is intensely studied. Several classes of materials are investigated, such as metal
alloys, magnesium based hydrides and metal borohydrides [3ꢀ6]. The latter exhibit fascinating
characteristics, both structurally and chemically [7ꢀ9]. Borohydrides hold high hydrogen contents;
however release of hydrogen is achieved under less than favorable conditions and in certain cases
undesired release of diborane poses a challenge [10, 11], and they often suffer from slow kinetics. A
wellꢀknown example is LiBH4, holding 18.4 wt% hydrogen, unfortunately being considerably
hampered by its enthalpy of decomposition (ꢀ67 kJ/mol). Several methods for improving the
hydrogen storage properties of such compounds exists [12]; formation of reactive hydride
composites (RHC’s) [13, 14], incorporation into nanoporous carbon scaffolds (nano confinement)
[15, 16], anion substitution [4, 18] and formation of biꢀmetallic materials [19, 20]. However, so far
synthesis of the ideal hydrogen storage material based on borohydrides has not been achieved.
Rehydrogenation of the storage materials is another challenge. Hydrogen pressure of 950 bar and
o
400 C have been shown to hydrogenate MgB2 to Mg(BH4)2 over several days [21], but this is no
practical solution. Among transition metal borohydrides [22ꢀ26], manganese borohydride is an
interesting candidate owing to its reportedly low temperature of decomposition [27, 28].
Unfortunately, thermal decomposition of Mn(BH4)2 is accompanied with the liberation of hydrogen,
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but also some diborane [27]. Reactive manganese borohydride – metal hydride composites may be
designed to suppress the release of diborane [29]. Bimetallic borohydrides, e.g. LiZn2(BH4)5,
releases a hydrogenꢀdiborane mixture of approximately 1:1 composition, and has even been used as
an indirect source of diborane [30]. Preceding work on Mn(BH4)2, employing solvent based
synthesis routes, provided tetrahydrofuran (THF) adducts. However, pure Mn(BH4)2 could not be
extracted due to the high temperatures needed to remove THF, causing the borohydride to
decompose [31]. A mechanochemical approach afforded nonꢀcrystaline materials, which by
spectroscopic methods proved to be a mixed cation manganese borohydride [28]. The first structural
characterization of αꢀMn(BH4)2 prepared from a mechanochemical metathesis reaction between
MnCl2 and LiBH4, was presented recently [32]. However, the synthesis product, αꢀMn(BH4)2,
contains LiCl and thus calls for new methods. For researchers to tailor materials properties fulfilling
the demands of solid state hydrogen storage, high purity chemicals are needed. Furthermore,
providing new synthesis routes gives important insight into the reaction pathways and properties of
the materials in question. In this work we present the synthesis of a new solvate and two
polymorphs of Mn(BH4)2, obtained as halideꢀ and solvent free products.
EXPERIMENTAL SECTION
Synthesis. 2.52 g (20.0 mmol, 1.33 equivalents, 33% excess) finely ground MnCl2
(Aldrich, ≥99%, anhydrous) was suspended in an 80:20 tolueneꢀdimethylsulfide mix (50 mL) (anh.,
SigmaꢀAldrich) at room temperature. Two equivalents (0.65 g, 30 mmol) of finely ground LiBH4
(SigmaꢀAldrich, ≥98%) was added to the pink MnCl2 suspension at room temperature, left stirring
and shortly after a pale yellow solid formed. The reaction mixture was left stirring at room
temperature overnight, turning into a more homogenous and progressively more yellow suspension
as stirring proceeded. The mixture was filtered using a finely porous glass filter, and the pale yellow
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solid was collected. The collected products, composing a mixture of LiCl and Mn(BH4)2·½S(CH3)2,
was extracted with dimethylsulfide (DMS) (anh., SigmaꢀAldrich, ≥99.0%), and filtered, after which
the resulting clear yellow solution was evaporated under pressure, providing firstly a viscous yellow
oil, followed by solid Mn(BH4)2·½S(CH3)2 as the only product. Subsequent heating under vacuum
(110 °C/2ꢁ10ꢀ2 mbar) for 16ꢀ24 h provided αꢀMn(BH4)2. Collecting the viscous oil before it is
solidified, and subsequently drying it under vacuum at RT for 3 days provides γꢀMn(BH4)2 (with
traces of αꢀMn(BH4)2). All manipulation of solids are performed in an argon filled glovebox
(O2/H2O < 0.1 ppm) equipped with a circulation purifier. All manipulation of solvents and solutions
are performed using argon as protective atmosphere and inert syringe handling techniques in
standard Schlenk glass equipment.
Synchrotron Radiation Powder Xꢀray Diffraction (SRꢀPXD)
SRꢀPXD data were collected at a) BM01A at the SwissꢀNorwegian Beamlines (SNBL) at the
European Synchrotron Radiation Facility (ESRF), Grenoble, France and b) at the synchrotron
MAXꢀII, Lund, Sweden in the research laboratory MAXꢀlab, beamline I711.
a) SR-PXD at SNBL. The samples were ground in a mortar and mounted in glass
capillaries (o.d. 0.5 mm) sealed with glue inside an argonꢀfilled glovebox with a circulation purifier
(p(O2, H2O) < 0.1 ppm). The 2D powder data were recorded using a MAR345 Imaging Plate
detector and a selected wavelength of 0.6967 Å. For all measurements the capillaries were
oscillated by 30° during exposure to the Xꢀray beam for 30 s, followed by a readout time of ~83 s.
At RT single PXD patterns were collected at three different sampleꢀtoꢀdetector distances 150, 250
and 390 mm in order to obtain both data at high diffraction angle and data with higher resolution at
low diffraction angles. Subsequently, the sample was heated from RT to 200 °C at a rate of 2
°C/min, while SRꢀPXD data were collected. The temperature was controlled with the Oxford
Cryostream 700+. The in situ data were collected using a sampleꢀtoꢀdetector distance of 390 mm.
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b) In situ SR-PXD at MAX-lab. The powdered sample was mounted in a sapphire
(Al2O3) single crystal tube (o.d. 1.09 mm, i.d. 0.79 mm) in an argonꢀfilled glovebox p(O2, H2O) < 1
ppm. The tube was attached to a specially developed sample cell designed for studies of gas/solid
reactions and allowing for high pressures and temperatures to be applied. During heating the
temperature was controlled with a thermocouple placed in the sapphire tube ~1 mm from the
o
o
sample [33]. A heating rate of 5 C/min was applied up to temperatures of 300 C, while vacuum,
argon atmosphere (p(Ar) = 1 bar) or hydrogen pressure (p(H2) = 10 bar) was applied to the samples.
SRꢀPXD data was collected by a MAR165 CCD detector system using Xꢀray exposure time of 30 s
and selected wavelengths of λ = 0.9920, 1.0096 or 1.0080 Å [34, 35]. The FIT2D program [36] was
used for calibrating the wavelengts using a standard (NIST) LaB6 sample, masking diffraction spots
from the single crystal sapphire sample holder and transforming all obtained raw images from both
SNBL and MAXꢀlab to 2Dꢀpowder patterns. Uncertainties of the integrated intensities were
calculated at each 2θꢀpoint by applying Poisson statistics to the intensity data, considering the
geometry of the detector [37].
Structural Refinement by the Rietveld method
Rietveld refinement was carried out using the program FullProf [38]. In general, backgrounds were
described by linear interpolation between selected points. For all structures scaleꢀfactors, cell
parameters, profile parameters and overall displacement factor, Bov were refined as a minimum.
Furthermore, zeroꢀshift was refined for all data. Note that the high χ2 values reflect mainly the
extremely high counting statistics of the powder diffraction data obtained from modern 2Dꢀ
detectors. The structure of Mn(BH4)2·½S(CH3)2 was refined by simultaneous Rietveld refinement of
two SRꢀPXD data sets measured on the same sample at SNBL at RT with a sampleꢀtoꢀdetector
distance of 150 and 390 mm, respectively to take full advantage of both high angle data (150 mm
dataset) and high resolution at low angle (390 mm dataset). Two sets of Bragg reflections were
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identified in the data, one was assigned to αꢀMn(BH4)2 [32] while the other set of reflections were
recognized as being similar to the diffraction pattern for Mg(BH4)2·½S(CH3)2. Hence, Mn was
substituted for Mg in the monoclinic structural model for Mg(BH4)2·½S(CH3)2 (space group Cc) as
presented in ref. [39] and this was used as starting model for the refinement together with the
structural data for Mn(BH4)2 as given in ref. [32]. For Mn(BH4)2·½S(CH3)2 all atomic positions
were refined using 40 distance restraints for interatomic distances and 37 for angles. Only positions
(not the orientation) of the CH3 and BH4 units were refined. As for the displacement factors, those
for Mg(BH4)2·½S(CH3)2 [39] were used as starting points and the displacements factors were
refined for freely for Mn and S while those for the CH3 and BH4 groups were refined together. The
final discrepancy factors for the 150 and 390 mm datasets are: χ2 = 1360 and 1450, Rwp = 0.913 and
1.99 % (not corrected for background), Rp = 2.38 and 4.28 % (corrected for background),
RB(Mn(BH4)2·½S(CH3)2) = 1.03 and 1.29 % and RB (Mn(BH4)2) = 1.64 and 2.55 %. The refinement
profiles are shown in Fig. S2.1. Crystal data and atomic coordinates are listed in Table S2.1. The
structure of γꢀMn(BH4)2 was refined from SRꢀPXD data (MAXꢀlab, λ = 0.9920 Å) measured at RT.
One set of Bragg reflections in the data was assigned to γꢀMn(BH4)2 (being similar to the diffraction
pattern for γꢀMg(BH4)2 [39]). Hence, Mn was substituted for Mg in the structural model for γꢀ
Mg(BH4)2 (space group Idꢀ3a) and this was used as starting point for the refinement. Reflections
from a small amount of αꢀMn(BH4)2 was refined from the model for Mn(BH4)2 as given in ref. [32].
Furthermore, the SRꢀPXD data contains reflections (see supporting info for dꢀspacings) from one or
more unidentified compounds, which may a manganese analogue of εꢀMg(BH4)2 as described in ref.
[40]. However, the small amount and the low number of Bragg peaks hamper indexing as well as
comparison with published data. For γꢀMn(BH4)2, Mn is located on a special position, hence only
positions (not the orientation) of the BH4 units were refined. The final discrepancy factors are: χ2 =
21.51, Rwp = 5.95 % (not corrected for background), Rp = 25.7 % (corrected for background), RB(γꢀ
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Mn(BH4)2) = 14.8 %. The fact that undescribed reflections remain in the refinement is clearly seen
from the relatively high agreement factors. The refinement profiles are shown in Fig. S2.2. Crystal
data and atomic coordinates are listed in Table S2.2.
The structure of ζꢀMg(BH4)2 was refined from SRꢀPXD data (SNBL, ESRF, λ = 0.7035 Å)
measured at 224 °C. Realizing that the diffraction pattern of ζꢀMg(BH4)2 is similar to that of αꢀ
Mn(BH4)2 (space group P3112) [32], allowed to develop a structural model for ζꢀMg(BH4)2 by
substitution of Mg for Mn, which was used as starting point for the refinement. The refinement
profiles are shown in Fig. S2.3. Due to the quality of the data, the BH4 units were refined as rigid
bodies and only positions (not orientations) of the tetrahedra were refined, and individual
displacement factors were not refined. The final discrepancy factors are: χ2 = 21.5, Rwp = 0.420 %
(not corrected for background), Rp = 25.1 % (corrected for background), RB(ζꢀMg(BH4)2) = 13.6 %.
Thermal analysis, TGAꢀDSC/TPDꢀMS. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were collected for Mn(BH4)2·½S(CH3)2 and αꢀMn(BH4)2
using a PerkinꢀElmer STA 6000 instrument coupled to a Hiden HPR 20 QIC mass spectrometer
(MS) via a heated capillary. The atomic mass ranges probed were set up to detect all fragments of
dimethylsulfide, diborane and hydrogen respectively. The measurements were performed in a
continuous flow of Ar (65 mL/min) using Al2O3 crucibles fitted with a lid to prevent exposure to air
when mounted. Gases produced from the decomposition during heating were led through a small
o
hole in the lid and through the inlet of the MS probe. The heating rate was 5 C/min until reaching
250 oC.
Infrared Spectroscopy. Infrared spectra were collected by Attenuated Total
Reflectance ꢀ Fourier Transformed Infrared Spectroscopy (ATRꢀFTIR), using a Nicolet 380 Avatar
spectrometer. The sample powder was placed directly on the infrared radiation source and
immediately covered by a tight screw. The total exposure of the materials to air is limited to a few
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seconds in this manner. The spectra were collected in the wavenumber range of 4000 ꢀ 400 cmꢀ1
with 32 scans.
RESULTS AND DISCUSSION
Synthesis. The method utilized for synthesis of Mn(BH4)2 is a solvent based metathesis reaction,
usually performed mechanochemically [41, 42]. The present approach requires no ball milling or
activation such as heating or high pressures, i.e. finely ground MnCl2 and LiBH4 (filtered through
112 ꢀm sieves) suspended in toluene/DMS react according to scheme 1 below.
Scheme 1. Reaction scheme for the synthesis of Mn(BH4)2. (1) Metathesis reaction, (2) extraction
and (3) desolvation of Mn(BH4)2ꢁ½S(CH3)2 yields γꢀMn(BH4)2 and αꢀMn(BH4)2.
In this manner formation of ternary chlorides, e.g. Liꢀ or NaMnꢀchlorides, or biꢀmetallic
borohydrides, previously reported to form mechanochemically from MnCl2 and LiBH4 or NaBH4, is
avoided [43, 44]. Dimethylsulfide (DMS) as coꢀsolvent and extraction solvent has several
advantages. Firstly, the reaction is allowed to proceed faster as the products are being dissolved in
DMS as it forms. Secondly, it eliminates the problem of strong coordination of the metal cation to
donor (sulfur in DMS) atoms in certain coordinating solvents, i.e. circumventing the problem of
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removing e.g. tetrahydrofuran (THF) [31]. Thirdly, the materials are separated from any unreacted
starting materials and byproducts (i.e. LiCl) formed during the reaction, since only Mn(BH4)2 is
dissolved as shown in scheme 1. After removing excess DMS, the product obtained is a hard,
yellowish powder of Mn(BH4)2ꢁ½S(CH3)2 (Figure S1.1). Manganese in Mn(BH4)2ꢁ½S(CH3)2
coordinates relatively weakly to sulfur, thus the solvent can be removed under mild conditions. The
coordination is broken at temperatures low enough to preserve the Mn(BH4)2 (T < 110 °C), in
contrast to earlier attempts of desolvating Mn(BH4)2·3THF, where temperatures required (T > 140
°C) were high enough that decomposition occurred [31]. In contrast, subjecting the diethylether
magnesium borohydride complex to a longer drying process [45], diethylether can be extracted at T
> 180 °C providing the high temperature polymorph βꢀMg(BH4)2 [46]. For these reasons neither
diethylether nor THF was used as solvent in this work. Furthermore, diethylether has some
solubility of LiCl and LiBH4, which may contaminate the reaction product [47, 48]. The weak
coordination of DMS to manganese in Mn(BH4)2ꢁ½S(CH3)2 is specifically demonstrated by the
formation of γꢀMn(BH4)2, which is a new nanoporous compound, arising from the gentle (vac/RT)
removal of DMS molecules from the cavities (see the structural discussion below). After
desolvation, γꢀMn(BH4)2 is obtained as a yellowish, sometimes brown powder.
New synthesis methods, which provide halide free metal hydrides for consideration for solidꢀstate
hydrogen storage is of significant interest. Recently, yttrium and gadolinium borohydrides were
prepared using dimethyl sulfide as a solvent providing new solvates as intermediates,
M(BH4)3S(CH3)2 (M = Y or Gd), which transformed to αꢀY(BH4)3 or Gd(BH4)3 at
∼140 °C [49].
Other halide free rare earth metal borohydrides, Eu(BH4)2 and Sm(BH4)2, are prepared by a
metathesis reaction of trivalent metal chlorides and LiBH4 in ethereal solution, combined with
solvent extraction using dimethyl sulfide [50].
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Crystal Structures of Mn(BH4)2·½S(CH3)2 and γꢀMn(BH4)2.
The solvate, Mn(BH4)2ꢁ½S(CH3)2 crystallizes in a monoclinic unit cell, a = 7.6914(4), b =
13.8731(8), c = 12.4278(7) Å and ß = 103.973(4)° (V = 1286.84(13) Å3, Z = 4), space group Cc.
The 3D framework of Mn(BH4)2·½S(CH3)2 contains two Mn atoms: one (Mn2) is tetrahedrally
coordinated to four BH4 groups, while the other (Mn1) coordinates to four BH4 groups and one
S(CH3)2 ligand to form a trigonal bipyramid (Figure 1). Each BH4 unit is coordinated by two Mn
atoms. The MnꢀB distances within the [Mn(BH4)4]ꢀtetrahedra range from 2.360(6) to 2.533(7) Å
while angles range from 97.51(15) to 128.0(2)°. The MnꢀS coordination distance in the
[Mn(BH4)4(S(CH3)2)]ꢀtrigonal bipyramid is 2.705(9) Å, while MnꢀB distances range from 2.451(7)
to 2.603(8) Å, which is somewhat longer than those observed within the [Mn(BH4)4]ꢀtetrahedra in
accordance with the increase in coordination number. BꢀMnꢀB angles are within 90.7(3) to
119.3(2)°. Further information can be found in the supporting information. The unit cell volume of
Mn(BH4)2·½S(CH3)2 is only slightly larger (~1 %) than that of the isostructural magnesium
analogue Mg(BH4)2·½S(CH3)2 [39]. The desolvated γꢀMn(BH4)2 crystallizes in the high symmetry
cubic space group Iaꢀ3d with unit cell parameters a = 16.2094(13) Å (V = 4258.93(60) Å3, Z = 24).
The structure consists of a 3D framework build from a single Mn atom tetrahedrally coordinated to
four BH4 units and each BH4 unit bridges two Mn atoms. The framework has large permanent pores
and represents the second only example of porous borohydrides (isostructural to γꢀMg(BH4)2). The
MnꢀB distance is 2.483(17) Å, while the BꢀMnꢀB angles with the tetrahedral ranges from 100.70(3)
to 128.95(2)° and that between neighboring tetrahedra is 176.30(2)°. The unit cell volume of γꢀ
Mn(BH4)2 is ~8.8 % larger than the Mgꢀcounterpart [39], which corresponds well with the radius of
Mn2+(hs) being larger than that of Mg2+. For both structures MnꢀH distances cannot be determined
unambiguously because of the limited accuracy of the [BH4]ꢀtetrahedra orientation. However, the
refinement of BH4 positions suggests MnꢀH distances ranging from 2.034(8) to 2.211(6) Å.
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(a)
(b)
(c)
(d)
Figure 1. Crystal structure of Mn(BH4)2·½S(CH3)2 viewed along the (a) aꢀ and (b) cꢀaxis and the structure of γꢀ
Mn(BH4)2 viewed along (c) the aꢀaxis and (d) [111]. MꢀH bonds are omitted for clarity and MnꢀB coordinations are
shown instead (Mn: red, S: yellow, C: light gray, H: gray, BH4: blue tetrahedra).
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Comparison of Mn(BH4)2 and Mg(BH4)2.
αꢀMn(BH4)2 was reported as being structurally similar to Mg(BH4)2 [32]. We have shown that
Mn(BH4)2ꢁ½S(CH3)2 and Mg(BH4)2ꢁ½S(CH3)2 are isostructural, and as a consequence hereof we
have discovered γꢀMn(BH4)2, which is structurally identical to the porous zeoliteꢀlike γꢀMg(BH4)2
[39]. Furthermore, we show that αꢀMn(BH4)2 is isostructural to another new Mg(BH4)2 polymorph,
denoted ζꢀMg(BH4)2 [51]. On the basis of this observation, we have used the Rietveld refinement
method to refine Xꢀray data obtained for the ζꢀMg(BH4)2 polymorph. Structural and experimental
details for Mg(BH4)2 can be found in Table 1 and in the Supporting Information respectively. In
figure 2 (left) the diffraction patterns for ζꢀMg(BH4)2, γꢀMg(BH4)2, Mg(BH4)2ꢁ½S(CH3)2 and its
manganese counterparts (right) are shown.
Figure 2. Powder Xꢀray diffraction patterns for Mg(BH4)2 and Mn(BH4)2 polymorphs: (left) (a) Mg(BH4)2ꢁ½S(CH3)2,
(b) γꢀMg(BH4)2, (c) ζꢀMg(BH4)2 shown in comparison to (right) (A) Mn(BH4)2ꢁ½S(CH3)2, (B) γꢀMn(BH4)2 and (C) αꢀ
Mn(BH4)2. Experimental details of Xꢀray data for the Mg(BH4)2 polymorphs presented here are found in the supporting
information.
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Several manganese and magnesium borohydride polymorphs are isostructural and display similar
diffraction patterns. The slightly larger manganese ion (r(Mn2+) = 0.66 Å) compared to the
magnesium ion (r(Mg2+) = 0.57 Å) [52] accounts for a larger unit cell and thus a slight shift of
diffraction patterns towards lower Bragg angles. The initially isolated solvate product
Mn(BH4)2ꢁ½S(CH3)2 (A) is transformed under vacuum at RT into γꢀMn(BH4)2 (B) upon prolonged
treatment, while the magnesium analogue Mg(BH4)2ꢁ½S(CH3)2 (a) transforms into its γꢀMg(BH4)2
polymorph (b) upon heating in vacuo much faster. Formation of αꢀMn(BH4)2 (C) proceeds by
heating of Mn(BH4)2ꢁ½S(CH3)2 in vacuo. The structural parameters of the isostructural magnesium
and manganese borohydride polymorphs are tabulated in Table 1 to provide an overview
underlining their similarities.
Table 1. Structural parameters for the isostructural Mn(BH4)2 and Mg(BH4)2 polymorphs.
Compound
Space
Cell Parameters
(Å)
V (Å3)
Z
V/Z
Ref.
Group
β (˚)
Mn(BH4)2ꢁ½S(CH3)2 Cc
a = 7.6914(4)
b = 13.8731(8)
c = 12.4278(7)
103.973(4) 1286.84(13)
4
321.71
ꢀ a
Mg(BH4)2ꢁ½S(CH3)2 Cc
a = 7.6325(4)
b = 13.8411(8)
c = 12.4290(8)
103.833(3) 1274.94(13)
4
318.74
[39]
P3112
a = 10.435(1)
c = 10.835(2)
ꢀ
ꢀ
1021.8(3)
1009.7(7)
9
9
113.53
112.19
[32]
ꢀ a
αꢀMn(BH4)2
ζꢀMg(BH4)2
P3112
a = 10.424(3)
c = 10.729(5)
Idꢀ3a
Idꢀ3a
a = 16.2090
ꢀ
ꢀ
4258.6
24
24
177.44
163.03
ꢀ a
γꢀMn(BH4)2
γꢀMg(BH4)2
a = 15.7575(16)
3912.6(7)
[39]
a) This work.
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Spectroscopic analysis of manganese and magnesium borohydride polymorphs.
Mn(BH4)2ꢁ½S(CH3)2, γꢀMn(BH4)2 and αꢀMn(BH4)2 were examined by ATRꢀFTIR, and compared to
the corresponding spectra of magnesium borohydride (Figure 3).
Figure 3. Infrared spectroscopy for manganese and magnesium borohydride polymorphs. Red; αꢀMg(BH4)2 (top), γꢀ
Mg(BH4)2 (middle), Mg(BH4)2ꢁ½S(CH3)2 (bottom). Blue; αꢀMn(BH4)2 (top), γꢀMn(BH4)2 (middle),
Mn(BH4)2ꢁ½S(CH3)2 (bottom). Dotted lines to guide the eye.
The spectrum for Mn(BH4)2ꢁ½S(CH3)2 exhibits vibrational modes from both borohydride BꢀH and
CꢀH bonds (stretching 2983 ꢀ 2848 cmꢀ1 and bending 1424 ꢀ 683 cmꢀ1), the latter disappearing
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gradually as the solvent is lost from RT to 110 °C. BꢀH stretching in Mn(BH4)2 is observed from
2600ꢀ2200 cmꢀ1, while further stretching, bending and combination bands are observed in the range
from 1500 cmꢀ1 and below. The close structural relation between magnesium and manganese
borohydride is easily recognizable in Figure 3. It should be emphasized that the similarities are
unique; seen in the perspective that spectra for other metal borohydrides are significantly different
[53], thus underlining that these materials are structurally very much alike. Unidentified bands are
observed in the spectra for γꢀMn(BH4)2. These bands could arise from residual solvent or
amorphous impurities present.
Thermal decomposition behavior of manganese borohydride.
As seen from the temperature dependent in situ SRꢀPXD (Figure 4) Mn(BH4)2ꢁ½S(CH3)2 transforms
directly to αꢀMn(BH4)2 in the temperature range 70 to 95 °C (see inset in Figure 4) while losing the
solvated DMS.
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Figure 4. In situ synchrotron radiation powder Xꢀray diffraction measured from room temperature to 200 °C of
Mn(BH4)2ꢁ½S(CH3)2 (λ = 0.696713 Å, p(Ar) = 1 bar). Inset: Integrated intensities (wt%) of Mn(BH4)2ꢁ½S(CH3)2 and
αꢀMn(BH4)2 as a function of temperature. Filled circles: αꢀMn(BH4)2, filled squares: Mn(BH4)2ꢁ½S(CH3)2.
Further heating results in the thermal decomposition of αꢀMn(BH4)2 and loss of diffraction as no
crystalline decomposition products were observed at any time.
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Figure 5. Thermogravimetry and differential scanning calorimetry (TGAꢀDSC, left, A, C) in combination with mass
spectroscopy (MS, right, B, D) for Mn(BH4)2ꢁ½S(CH3)2 (top) and αꢀMn(BH4)2 (bottom). Mass spectroscopy data for
H2, B2H6 and S(CH3)2 (ꢁT/ꢁt = 5 °C/min, p(Ar) = 1 bar).
For Mn(BH4)2ꢁ½S(CH3)2 the TGA shows a total weight loss of 37.8 wt%, in two overlapping steps,
o
in the temperature range 50 to 170 C, see figure 5, A. The calculated contribution from loss of
dimethylsulfide is 26.9 wt%. The remaining mass loss corresponds to hydrogen, ρm(Mn(BH4)2) =
9.53 wt% H2, and loss of some diborane. The decomposition of αꢀMn(BH4)2 is seen as an
endothermic event at ~160 °C in the DSC curves (Figure 5A and C) in good agreement with the in
o
situ SRꢀPXD data (Figure 4). A mass loss of 14.8 wt%, in the temperature range 125 to 175 C, is
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observed for the sample of αꢀMn(BH4)2 (Figure 5C), which corresponds well to the observations for
the solvate. The mass spectroscopy data reveals the presence of a substantial amount of diborane in
the evolved hydrogen. In lieu of accurate gas quantification methods, relating the experimentally
observed weight loss of 10.9 wt% to a table of fixed H2/B2H6 ratios provides a useful estimate of
the relative ratio of H2 and B2H6 (see supporting information). In this manner a ratio of H2 to B2H6
of 16:1 is estimated. This is in contrast to previous observations made in a study involving nanoꢀNi
doping of LiꢀMnꢀBꢀH complexes, which were later proven to be Mn(BH4)2, wherein they observed
a significantly higher proportion of hydrogen in the desorbed gases [27]. The decomposition
products was suggested to be elemental boron and manganese metal for the reaction releasing
mainly hydrogen. [45] Slightly different decomposition patterns were suggested in a study
involving materials obtained from mechanochemical treatment of 2ꢀ3 equivalents of LiBH4 and 1
equivalent MnCl2 [54]. The following decomposition pathway was suggested (scheme 2);
2Mn(BH4)2
ꢀ
Mn + MnB2 + 5H2 + B2H6
(2)
however, this decomposition entails the loss of 22.4 wt% which is higher than our findings of 14.8
wt%. We further investigated the decomposition of αꢀMn(BH4)2 by temperature dependent in situ
SRꢀPXD experiments under different conditions; in vacuum (Figure S4.1a), under a backpressure of
hydrogen (p(H2) = 10 bar, Figure S4.1b) and in argon (p(Ar) = 1 bar, Figure S4.1c). Depending on
the conditions, αꢀMn(BH4)2 is observed to decompose as early as 140 °C (vacuum), and up to 160
°C (p(Ar) = 1 bar) [55]. The decomposition of αꢀMn(BH4)2 and Mn(BH4)2ꢁ½S(CH3)2 at T > 125 oC
follows the same pathway, i.e. TGA/DSC/MS results correspond to one another and in situ SRꢀPXD
reveals amorphous decomposition products. In situ SRꢀPXD experiments were also conducted for γꢀ
o
Mn(BH4)2 and diffraction disappears at T > 110 C (p(Ar) = 1 bar, Figure S4.1d), giving rise to an
amorphous hump in the background, similar to that observed for γꢀMg(BH4)2, i.e. heating the
porous structure results in its collapse into amorphous Mn(BH4)2. Diffraction patterns measured at
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RT after decomposition were not assigned to any known crystalline compounds (Figure S5.1).
However, manganese metal, manganese borides and/or elemental boron may be involved in an
overall reaction pathway for the decomposition of Mn(BH4)2ꢁ½S(CH3)2 and Mn(BH4)2. Formation
of polyboranes, such as MnB12H12, can also be envisioned on the basis of reports that diborane can
react with LiBH4 at elevated temperatures [56].
CONCLUSIONS
In this work we have demonstrated a solvent based synthesis method providing solvent and halide
free Mn(BH4)2. The reaction affords a mixture of LiCl and Mn(BH4)2, from which three
polymorphs of Mn(BH4)2 are extracted. The initially isolated product is a new solvate material,
Mn(BH4)2ꢁ½S(CH3)2, isostructural to Mg(BH4)2ꢁ½S(CH3)2. The solvent can be removed upon
treatment in vacuo at elevated temperatures, providing αꢀMn(BH4)2, which is shown to be
isostructural to a new Mg(BH4)2 polymorph denoted ζꢀMg(BH4)2. Gently extracting the solvent
from Mn(BH4)2ꢁ½S(CH3)2 and maintaining the structural integrity, yields another polymorph, γꢀ
Mn(BH4)2 which is isostructural to γꢀMg(BH4)2 and provides a direct structural link between
manganese and magnesium borohydrides. Mn(BH4)2 decomposes at temperatures significantly
lower than Mg(BH4)2 (140 ꢀ 160 °C vs. 280 ꢀ 290 °C), which was extensively investigated. In
addition, the discovery of a porous γꢀMn(BH4)2 polymorph may prompt further work on e.g.
physisorption of H2 in the porous cavities. Deducing detailed reaction schemes for the
decomposition of Mn(BH4)2 under various conditions is highly desirable. This is particularly
important for future, prospective work aiming at tailoring or tuning the properties towards
rehydrogenating the material, especially considering the low decomposition temperature observed
for this material.
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ASSOCIATED CONTENT
Supporting Information.
Rietveld refinement plots, further experimental details, gas desorption quantification estimation
methods, ex/in situ powder Xꢀray diffraction plots for decomposition reactions and crystal data
from refined and calculated models as CIF files.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +45 20745528 (B. R.), +45 87 15 59 39 (T. R. J.). Email: richter@chem.au.dk (B. R.),
trj@chem.au.dk (T. R. J.)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
The work was supported by the Danish National Research Foundation, Center for Materials
Crystallography (DNRF93), The Danish Council for Strategic Research (project HyFillFast), and by
the Danish Research Council for Nature and Universe (Danscatt). The access to beamtime at the
synchrotron facilities MAXꢀII, Lund, Sweden in the research laboratory MAXꢀLab, beam line I711
and at the SwissꢀNorwegian Beam Lines (SNBL), European Synchrotron Radiation Facility
(ESRF), Grenoble, France is gratefully acknowledged. We are grateful to the Carlsberg Foundation.
We also acknowledge funding from the European Community’s Seventh Framework Programme,
The Fuel Cells and Hydrogen Joint Undertaking (FCH JU), project BOR4STORE (303428). Part of
this work was supported by the COST Action MP1103 "Nanostructured materials for solidꢀstate
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hydrogen storage". DBR thanks the Carlsberg Foundation and the Villum Foundation for funding.
We acknowledge the Fonds Spéciaux de Recherche (UCL) for the incoming postdoctoral fellowship
coꢀfunded by the Marie Curie actions of the European Commission granted to N.T. The authors
thank FNRS for the financial support.
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DOI: 10.1039/C4DT03501A
Table of content
Three manganese borohydride polymorphs are synthesized in solution and found to be structural
analogues of three magnesium borohydride polymorphs.