Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides

Article abstract of DOI:10.1039/c6dt02239a

A series of organic derivatives of magnesium borohydride, including Mg(BH₄)₂·1.5DME (DME = 1,2-dimethoxyethane) and Mg(BH₄)₂·3THF (THF = tetrahydrofuran) solvates and three mixed-cation borohydrides, [Cat]₂[Mg(BH₄)₄], [Cat] = [Me₄N], [nBu₄N], [Ph₄P], have been characterized. The phosphonium derivative has been tested as a precursor for synthesis of inorganic mixed-metal borohydrides of magnesium, M_x[Mg(BH₄)_2+x], M = Li-Cs, via a metathetic method. The synthetic procedure has yielded two new derivatives of heavier alkali metals M₃Mg(BH₄)₅ (M = Rb, Cs) mixed with amorphous Mg(BH₄)₂. Thermal decomposition has been studied for both the organic and inorganic magnesium borohydride derivatives. Amorphous MgB₂ has been detected among the products of the thermal decomposition of the solvates studied, together with organic and inorganic impurities.

Full text of DOI:10.1039/c6dt02239a

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Organic derivatives of Mg(BH4)2 as precursors
towards MgB2 and novel inorganic mixed-cation
Cite this: DOI: 10.1039/c6dt02239a
borohydrides†‡
W. Wegner,a,b T. Jaroń,*a M. A. Dobrowolski,c Ł. Dobrzycki,c M. K. Cyrańskic and
W. Grochala*a
A series of organic derivatives of magnesium borohydride, including Mg(BH4)2·1.5DME (DME = 1,2-di-
methoxyethane) and Mg(BH4)2·3THF (THF = tetrahydrofuran) solvates and three mixed-cation boro-
hydrides, [Cat]2[Mg(BH4)4], [Cat] = [Me4N], [nBu4N], [Ph4P], have been characterized. The phosphonium
derivative has been tested as a precursor for synthesis of inorganic mixed-metal borohydrides of mag-
nesium, Mx[Mg(BH4)2+x], M = Li–Cs, via a metathetic method. The synthetic procedure has yielded two
new derivatives of heavier alkali metals M3Mg(BH4)5 (M = Rb, Cs) mixed with amorphous Mg(BH4)2.
Thermal decomposition has been studied for both the organic and inorganic magnesium borohydride
derivatives. Amorphous MgB2 has been detected among the products of the thermal decomposition of
the solvates studied, together with organic and inorganic impurities.
Received 4th June 2016,
Accepted 31st July 2016
DOI: 10.1039/c6dt02239a
www.rsc.org/dalton
released hydrogen by B2H6),8 the interest in H-rich boro-
hydrides stimulated many recent developments.9–12
Introduction
Storage of hydrogen in chemical compounds is one of the
Magnesium borohydride, Mg(BH4)2, due to its high content
methods being currently employed for the use of hydrogen as of hydrogen (14.9 wt%), worldwide prevalence and the low
an energy carrier.1–5 Consequently, diverse types of potential price of magnesium, is an important prototype material for
chemical hydrogen stores have been investigated with a signifi- hydrogen storage.13 Several routes of its synthesis have been
cant intensification of efforts observed over the last two elaborated yielding five known polymorphs of Mg(BH4)2.9,14–18
decades.6 Hydrogen-rich borohydrides have attracted attention Hydrogen evolution from pure Mg(BH4)2 requires heating
revealing a higher relative amount of hydrogen than the other above ca. 290 °C, while above ca. 450 °C MgB2 can be detected
groups of simple inorganic compounds containing the same as the final product of the multi-step dehydrogenation
cations.7 Although none of the currently known borohydrides process.19 Diverse methods have been tested to enhance the
can be directly used as a hydrogen store (which is caused pre- hydrogen release from Mg(BH4)2, which includes amorphiza-
dominantly by either too easy or too difficult release of hydro- tion, dispersion in a porous matrix, and preparation of compo-
gen, poor reversibility of storage and contamination of sites with various additives.19 These studies led to the
discovery of mixed-metal borohydrides containing mag-
nesium,
as
represented
by solid
solutions,
e.g.
MgxMn(1−x)(BH4)2, RbMgxCa(1−x)(BH4)3,20,21 bimetallic borohy-
drides, such as K2Mg(BH4)4, K3Mg(BH4)5 and CsMg(BH4)3,21,22
as well as more complex borohydrides, like LiKMg(BH4)4,
LiRbMg(BH4)4, Li2K3Mg2(BH4)9 and Li3MgZn5(BH4)15.23,24
Here we have extended the earlier characterization of
several organic derivatives of magnesium borohydride:
[Cat]2[Mg(BH4)4], [Cat] = [Me4N], [nBu4N], [Ph4P], as well as
Mg(BH4)2·1.5DME and Mg(BH4)2·3THF solvates, where DME –
aCentre of New Technologies, University of Warsaw, Żwirki i Wigury 93,
02089 Warsaw, Poland. E-mail: tjaron@uw.edu.pl, w.grochala@cent.uw.edu.pl
bCollege of Inter-Faculty Individual Studies in Mathematics and Natural Sciences,
University of Warsaw, Stefana Banacha 2C, 02-097 Warsaw, Poland
cAdvanced Crystal Engineering Laboratory (aceLAB), University of Warsaw, Faculty of
Chemistry, Żwirki i Wigury 101, 02089 Warsaw, Poland
†Electronic supplementary information (ESI) available: Synthetic procedures,
PXRD patterns, FTIR & XPS & MS spectra. Details of the crystal structure of
Mg(BH4)2·1.5DME may be obtained from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif on quoting the CCDC 1,2-dimethoxyethane and THF – tetrahydrofuran.25–27 The
number 1482748. For ESI and crystallographic data in CIF or other electronic
latter two show very good solubility in diverse organic solvents
and their solutions exhibit a broad range of volatility and vis-
cosity. Therefore, we have studied their thermal decomposition
format see DOI: 10.1039/c6dt02239a
‡Details of the crystal structure of M3[Mg(BH4)5], M = Rb, Cs, may be obtained
from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen,
to verify whether they might be used as precursors towards
superconducting layers of MgB2 on neutral substrates.28–30
Germany (fax: (+49)7247-808-666; E-mail: crysdata@fizkarlsruhe.de) on quoting
their ICSD numbers: 431394 and 431395, respectively.
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Simultaneously, one [Cat]2[Mg(BH4)4] derivative showing good
Mechanochemical reactions were carried out in stainless
solubility in DCM (dichloromethane) ([Cat] = [Ph4P]) has been steel vessels using an LMW-S vibrational mill from Testchem
applied for synthesis of the bimetallic borohydrides contain- (1400 rpm, ca. 23.3 Hz). The reactions were usually performed
ing alkali metal cations, M+ (M = Li–Cs), and magnesium- for 30 min in 5 min cycles with LN2 cooling between cycles.
bearing anions, [Mg(BH4)2+x
]
x−. We have utilized the recently
Fourier Transform Infrared (FTIR) spectra were recorded on
reported method of solvent-mediated synthesis of mixed-metal a Vertex 80v spectrometer (Bruker) in the 4000–400 cm−1
borohydrides,12,31–33 while anticipating that it might proceed range. Anhydrous KBr (200 mg) was used as a pellet material.
according to the following reaction equation:
Thermal decomposition of the selected samples was investi-
gated using a Netzsch STA 409 PG thermogravimeter (TGA) com-
bined with a differential scanning calorimeter (DSC). Evolved
gases were analysed using a Pfeiffer vacuum quadrupole mass
spectrometer (MS), QMS 403 C, connected to the TGA/DSC
furnace via a quartz capillary preheated to 200 °C to avoid con-
densation of the low-boiling volatiles. The samples were placed
inside Al2O3 crucibles and measured at a constant 5 °C min−1
heating rate, and a spontaneous cooling after the maximum
temperature has been reached. High-purity 6N argon has been
used as a carrier gas with a constant flow of 80 ml min−1. For
obtaining larger amounts of the solid residues, a Carbolite GHA
12/300 furnace with the Ar gas flow, was applied.
Â
Ã
½Catꢀ2 MgðBH4Þ4 þ 2 M½Anꢀ
Â
Ã
ð1Þ
! M2 MgðBH4Þ4 # þ 2½Catꢀ½Anꢀ
where [An] = [Al(OC(CF3)3)4] or [B[3,5-(CF3)2C6H3]4] (denoted
below as [Al(pftb)4] and [BArF], respectively). For M = Rb, Cs,
the mixture of amorphous MBH4 and novel M3[Mg(BH4)5]
derivatives have been obtained and characterized, while for
M = Li–K the respective single-metal borohydrides were
detected among the products.
X-ray photoelectron spectra (XPS) were recorded on a
custom-designed system made by SPECS using monochroma-
tized Al Kα radiation (1486.5 eV). The electron flood gun was
used for charge compensation. The samples were loaded to
the spectrometer via the Ar-filled glovebox (MBraun), with O2
and H2O levels <0.5 ppm. The experimental setup has been
described earlier.34 For the XPS results cf. ESI.†
Powder X-ray Diffraction (PXD) patterns were collected from
the samples sealed inside the 0.3–1 mm capillaries. Two
diffractometers were used: PANalytical X’Pert Pro with CoKα
radiation and a Bruker D8 Discover with CuKα radiation. The
following software was used for data analysis: Crystal Sleuth35
– for displaying PXD patterns and preliminary data analysis;
X-Cell36 (included in Materials Studio from Biovia) – for index-
ing of reflections; FOX37 – for structure solution; Vesta38 and
Mercury39 – for visualization of structures, and simulation of
Methods
Synthetic procedures
All experiments were performed in an argon atmosphere,
using the MBRAUN Labmaster DP glovebox (O2, H2O <1 ppm)
or the Schlenk-type glassware. The reagents used were fine
quality chemicals from Sigma-Aldrich and Katchem. M[Al
(pftb)4], M = Li–Cs, salts were prepared as mentioned in ref.
32, while Mg(BH4)2·1.5DME, [Ph4P]2[Mg(BH4)4] and Mg(BH4)2·
3THF were synthesized using procedures similar to the reports
published before (cf. ESI†).25–27 The metathetic solvent-
mediated reactions resulting in mixed-metal borohydrides
have been described earlier.12,31,32 The specific methods of
synthesis of mixed-cation borohydrides used in this paper
have been summarized in Table 1, while the corresponding
detailed synthesis procedures are outlined in the ESI.†
Table 1 The preparation of Mx[Mg(BH4)2+x] samples described in this work
M
Reagents [mol. ratio]
Conditions
Crystalline products
Me4N
MgCl2 + 2LiBH4 + 2[Me4N]BH4
Mg(BH4)2 + 2[Me4N]BH4
Mg(BH4)2 + [Me4N]BH4
6 × 5 min milling
6 × 5 min milling
6 × 5 min milling
[Me4N]2[Mg(BH4)4], LiCl
[Me4N]2[Mg(BH4)4], [Me4N]BH4 a
[Me4N]2[Mg(BH4)4], [Me4N]BH4
Mg(BH4)2·1.5DME + 2[Me4N]BH4
MgCl2 + 2LiBH4 + 2[nBu4N]BH4
Mg(BH4)2 + [nBu4N]BH4
Mg(BH4)2·1.5DME + 2[nBu4N]BH4
Mg(BH4)2·1.5DME + 2[Ph4P]BH4
Mg(BH4)2 + 2LiBH4
16 h stirring in DCM, filtration
6 × 5 min milling
6 × 5 min milling
24 h stirring in DCM
24 h stirring in DCM, Et2O extraction
8 × 5 min milling
30 min stirring in DCM, filtering
30 min stirring in DCM, filtering
30 min stirring in DCM, filtering
1–2 h stirring in DCM, filtering
19 h stirring in DCM, filtering
8 × 5 min milling
[Me4N]2[Mg(BH4)4], [Me4N]BH4
[nBu4N]2[Mg(BH4)4]a,b
[nBu4N]2[Mg(BH4)4]a,b
[nBu4N]2[Mg(BH4)4]b
nBu4N
Ph4P
Li
[Ph4P]2[Mg(BH4)4]a
c
LiBH4
1.1[Ph4P]2[Mg(BH4)4] + 2Li[Al(pftb)4]
LiBH4, LiCl, Li[Al(pftb)4], [Ph4P]2[Mg(BH4)4]a
Na
1.1[Ph4P]2[Mg(BH4)4] + 2Na[BArF
]
Na[BArF4], NaBH4
4
1.1[Ph4P]2[Mg(BH4)4] + 3Na[Al(pftb)4]
[Ph4P]2[Mg(BH4)4] + 2 K[Al(pftb)4]
[Ph4P]2[Mg(BH4)4] + 2Rb[Al(pftb)4]
Mg(BH4)2 + 2CsBH4
Na[Al(pftb)4], NaBH4,
KBH4
K
Rb
Cs
Rb3Mg(BH4)5
c
1.1[Ph4P]2[Mg(BH4)4] + 2Cs[Al(pftb)4]
MgCl2 + 2LiBH4 + 3CsBH4
22 h stirring in DCM, filtering
8 × 5 min milling
Cs3Mg(BH4)5
Cs3Mg(BH4)5, LiCl
a Unidentified phase(s) present. b The crystal structure not solved. c Quasi-amorphous products.
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powder patterns; Jana2006 40 – for Rietveld and LeBail refine- atom list and the given formula of the investigated salt. The
ment. The shape of diffraction peaks has been described as a temperature factors of the assigned hydrogen atoms were not
pseudo-Voigt function with added Berar–Baldinozzi type asym- refined and were set to be equal to either 1.2 or 1.5 times
metry. The background was modeled by using 30 Legendre larger than Ueq of the corresponding heavy atom. The atomic
polynomials. During the Rietveld refinement of all structures scattering factors were taken from the International Tables.45
−
the restraints were applied to BH4 groups: the B–H distances
and the H–B–H angles were fixed to 1.150(1) Å and 109.47(1)°,
respectively. Atomic displacement parameters of B atoms were
fixed to be equal, while for H atoms they were fixed as 1.2
Results and discussion
times larger than that of the B atoms. Mg–H distances were
restrained to be equal to each other within ∼0.001 Å.
Synthesis and identification of the products
Organic derivatives of Mg(BH4)2. The syntheses of Mg(BH4)2·
1.5DME and Mg(BH4)2·3THF solvates, as well as that of
[Ph4P]2[Mg(BH4)4], result in rather pure products with only
small amounts of unidentified impurities (Fig. S1 and S2†).
We have also prepared [Me4N]2[Mg(BH4)4] via four different
synthetic paths, however, due to its poor solubility in organic
solvents, this compound could not be purified (cf. Table 1 and
Fig. S3†). The synthesis of [nBu4N]2[Mg(BH4)4] has been
attempted using three different methods (Table 1). The pro-
ducts are characterized by the common set of diffraction
reflections, accompanied by unidentified peaks for the
mechanochemically-synthesized samples (Fig. S4†). The
common set of diffraction peaks can be indexed in a tetragonal
body-centered unit cell (extinction class I41/a), a = 25.459(10) Å
and c = 34.407(14) Å, V = 22 302(18) Å3, which is very close to
the volume estimated for 20 [nBu4N]2[Mg(BH4)4] molecules.
Moreover, a very similar unit cell is obtained from indexing of
the powder pattern for the related manganese derivative,
[nBu4N]2[Mn(BH4)4] (Fig. S29†). Although such a unit cell
corresponds very well to the measured powder pattern
(Fig. S5†), we failed to solve the crystal structure of this com-
pound from powder diffraction data.46 Unfortunately, and
despite many attempts, we were also unable to grow the crystals
suitable for single-crystal diffraction, therefore [nBu4N]2[Mg
(BH4)4] remains to be structurally characterized in the future.
Since [Ph4P]2[Mg(BH4)4] may be obtained as a quite pure
crystalline product, and it shows very good solubility in DCM,
we have chosen this compound as a precursor for synthesis of
inorganic mixed-cation borohydrides of magnesium.
Inorganic bimetallic borohydrides of magnesium. We
attempted preparation of novel mixed-metal borohydrides of
magnesium according to eqn (1). The precipitates formed via
metathetic reactions in DCM were analyzed with powder X-ray
diffraction (PXD) and Fourier-transform infrared spectroscopy
(FTIR), cf. Fig. 1 and 2, respectively.
Surprisingly, we have not detected the M2[Mg(BH4)4] salts
in any of the investigated systems, cf. Table 1. In the samples
containing lighter alkaline metals, M = Li–K, the products
were composed of crystalline MBH4 and amorphous Mg(BH4)2,
as suggested by the PXD patterns and FTIR spectra. In these
samples the unreacted M[An] precursors were also present,
particularly for M = Na (see also Fig. S6–S13†). These results
suggest that the M2[Mg(BH4)4] phases, M = Li–K are either
unstable, or they decompose during synthesis:
Single-crystal X-ray diffraction measurement and structure
solution of Mg(BH4)2·1.5DME
The measurement was performed at 100(2) K on a Bruker D8
Venture Photon100 CMOS diffractometer equipped with a
mirror monochromator and a CuKα INCOATEC IμS micro-
focus source. Frames were collected using a Bruker APEX2
program41 and integrated in the Bruker SAINT software
package42 using a narrow-frame algorithm. The integration of
the data (cubic unit cell) yielded a total of 49 819 reflections to
a maximum θ angle of 58.95° (0.90 Å resolution), of which
5027 were independent (average redundancy 9.910, complete-
ness = 100.0%, Rint = 5.10%, Rsig = 2.48%) and 4220 (83.95%)
were greater than 2σ(F2). The final cell parameters of a =
27.2332(9) Å and V = 20 197(2) Å3 are based on the refinement
of the XYZ-centroids of 9916 reflections above 20 σ(I) with
5.621° < 2θ < 116.948°. Data were corrected for absorption
effects using the multi-scan method (SADABS).43 The ratio of
minimum to maximum apparent transmission was 0.829. The
calculated minimum and maximum transmission coefficients
(based on crystal size) are 0.768 and 0.856, respectively.
The structure was solved and refined using the SHELXTL
software package44 in the space group Pa3, with Z = 4 for the
ˉ
formula unit, C96H353B32Mg16O48. The final anisotropic full-
matrix least-squares refinement on F2 with 542 variables con-
verged at R1 = 6.52%, for the observed data and wR2 = 21.62%
for all data. The goodness-of-fit was 1.038. The largest peak in
the final difference electron density synthesis was 0.291 e− Å−3
and the largest hole was −0.362 e− Å−3 with an RMS deviation
of 0.073 e− Å−3. The measured crystal was twinned by mero-
hedry with a twin composition plane being a diagonal of a
square of the unit cell i.e. (110). The refined ratio of twin com-
ponents is equal to 0.840(2) : 0.160(2). Due to the severe dis-
order the crystal was weakly diffracting, the resolution of the data
was limited to 0.9 Å (maximum 2θ up to 116.9°). A number of
constraints for distances and angles were used to model correct
−
geometry of BH4 and DME ligands. In both Mg[(BH4)4]2−
anions and the Mg[(C4H10O2)3]2+ cation located on the general
position all non-hydrogen atoms were refined anisotropically.
In the other cations only Mg and O atoms were refined aniso-
tropically whereas C atoms were refined with an isotropic
approach. Hydrogen atoms were placed in calculated positions
and refined within the riding model. In one Mg[(C4H10O2)3]2+
cation (Mg4 as central ion) hydrogen atoms were not assigned
what causes the discrepancy between the atom count from the
M2½MgðBH4Þ4ꢀ ! MgðBH4Þ2 þ 2MBH4:
ð2Þ
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acting as counter-cations e.g. in ALiMg(BH4)4, A = K, Rb.
Surprisingly, the solvent-mediated synthesis of magnesium-
potassium borohydrides appeared unsuccessful, despite the
preparation of the stable K2[Mg(BH4)4] and K3[Mg(BH4)5]
phases by a mechanochemical process.22
In the case of the heavier alkali metals, M = Rb and Cs, the
corresponding M3Mg(BH4)5 salts are precipitated (Fig. 1d and e).
Fig. 2d and e, being the only crystalline phases present in the
samples. A band of moderate intensity is observed in their
FTIR spectra at ca. 1266 cm−1, likely originating from the δH–B–H
absorption band of amorphous Mg(BH4)2.51 These observations
indicate that the targeted M2[Mg(BH4)4] salts, M = Rb, Cs, either
do not form, or they decompose in situ according to the eqn (3):
3M2½MgðBH4Þ4ꢀ ! 2M3MgðBH4Þ5 þ MgðBH4Þ2:
ð3Þ
In this case the MBH4 are not eliminated, contrary to the
compounds of lighter metals, eqn (2), which should be related
to significantly lower lattice energies of the heavier alkaline
borohydrides.52
For comparison, the Cs3Mg(BH4)5 salt has also been pre-
pared via a mechanochemical reaction:
Fig. 1 PXD (Cu Kα) of the Mx[Mg(BH4)2+x+] compounds for M = (a) Li,
freshly-prepared sample; (b) Na; (c) K; (d) Rb; (e) Cs. * – Unknown
phase; % – Li[Al(pftb)4]; & – LiBH4; $ – Na[Al(pftb)4]; # – NaBH4; ! –
KBH4; @ – M3Mg(BH4)5, M = Rb, Cs.
MgCl2 þ 2LiBH4 þ 3CsBH4 ! Cs3MgðBH4Þ5 þ 2LiCl;
ð4Þ
which resulted in a LiCl-contaminated product of poor crystal-
linity (Fig. S14†). A mechanochemical attempt towards the
preparation of Cs2[Mg(BH4)4]:
MgðBH4Þ2 þ 2CsBH4 ! Cs2½MgðBH4Þ4ꢀ;
ð5Þ
led to ill-defined, nearly-amorphous products (Fig. S14 and
S15†). The recently reported Cs[Mg(BH4)3] phase21 has not
been detected in any of our cesium-containing samples.
Crystal structures
Mg(BH4)2·1.5DME. As discussed above, Mg(BH4)2 solvated
by DME is
a useful intermediate for preparation of
[Cat]2[Mg(BH4)4] compounds, [Cat] = [Me4N], [nBu4N], [Ph4P],
etc. Although the synthesis of this compound has been
reported before,26 its structure remained unknown.
Fig. 2 FTIR spectra of the Mx[Mg(BH4)2+x+] compounds for A = (a) Li,
freshly-prepared sample; (b) Na; (c) K; (d) Rb; (e) Cs. * bands from
[Ph4P]2Mg(BH4)4.
Mg(BH4)2·1.5DME adopts a large primitive cubic unit cell,
3
ˉ
Pa3, with a = 27.2332(9) and V = 20 197(2) Å at 100 K,
Table S2† (a = 27.5917(6) and V = 21 005.6(5) Å3, at room tem-
perature, RT, Fig. S16†). The structure is severely disordered
The lack of thermodynamic stability of Li2[Mg(BH4)4] and and its asymmetric part contains one Mg[(C4H10O2)3]2+ cation
Na2[Mg(BH4)4], with respect to the corresponding single-cation occupying the general position (24d) and two such cations
borohydrides, might be expected, as no lithium- and sodium- located at the inversion center of the −3-fold axis of symmetry
containing bimetallic magnesium borohydrides have been (4a, 4b). These cations are counterbalanced by one Mg
reported.47–49 This could be rationalized especially for the [(BH4)4]2− anion located on the general position (24d) and the
compound of lithium which, similarly to Mg, shows clear pre- same type of anion positioned on the −3-fold axis but not on
ference for tetrahedral coordination, influencing the topology the center of inversion (8c) (Fig. 3). Rather weak interactions
of a system.50 Such a strong preference in coordination mode between ions together with the disorder are responsible for a
induces incompatibility with the chemical composition relatively small density of the crystal of 0.99 g cm−3 at 100 K
required for the charge balance, as it has been discussed by (0.95 g cm−3 at RT).
Schouwink et al.23 This incompatibility could be overcome by
In the Mg[(BH4)4]2− moiety located on the general position
partial substitution of lithium by potassium or rubidium, one of the BH4 ligands is disordered over two positions with
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Fig. 4 Unit cell of M3Mg(BH4)5, M = Rb, Cs.
Fig. 3 The asymmetric part of the unit cell of Mg(BH4)2·1.5DME. The
displacement ellipsoids are shown at 50% probability level; Mg – green,
B – pink, O – red, and C – grey.
simple MBH4 salts: 3.5145(5) Å and 3.7095(5) Å, respectively.54
The apical M2–B2 distances are significantly larger than the
equatorial ones, 3.9983(12) Å and 4.0631(4) Å for M = Rb and
Cs, respectively, however, this distance depends strongly on
the applied model of disorder. Although initially the models of
disorder related to that applied before for K3Mg(BH5)5 have
been tested, they have converged to the structure with the fully
ordered heavy atom sublattice.
the occupancy ratio of 0.5 : 0.5. The second Mg[(BH4)4]2− ion is
ordered.
22
The cation moieties contain disordered DME ligands which
chelate the central metal ion. In the Mg1 cation one DME
molecule is ordered whereas the two other molecules are dis-
ordered over two sites with an occupancy ratio equal to
0.55 : 0.45. The other two cations located on the − 3-fold axes
contain DME ligands distributed over many alternative posi-
Thermal decomposition
Organic derivatives of magnesium borohydride. Since
tions imposed by the high symmetry of the site occupied by Mg(BH4)2 is a valuable precursor towards superconducting
the Mg central ions.
MgB2, we have studied here the thermal decomposition of the
The coordination of magnesium cations is typical of its organic derivatives of magnesium borohydride. Of particular
−
complexes with BH4 and ether ligands, with the Mg–B and interest are Mg(BH4)2·1.5DME, Mg(BH4)2·3THF,
Mg–O distances within the range of 2.29(10)–2.447(10) Å and [Ph4P]2[Mg(BH4)4, which are very well soluble in a broad range
1.92(2)–2.206(9) Å, respectively.
of organic solvents, and as such they might constitute useful
and
M3Mg(BH4)5, M = Rb, Cs. The diffraction peaks originating precursors towards thin layers of MgB2 on neutral substrates of
from M3Mg(BH4)5, M = Rb, Cs has been indexed in tetragonal any shape.
unit cells with an extinction class of I4cm (Table S3†).
Thermal decomposition of the organic derivatives of mag-
K3Mg(BH5)5 crystallizes in a similar unit cell, however, in this nesium borohydride is a complex, multistep process, as com-
case a loss of body centering is indicated by a set of weak monly observed for metal borohydrides (Fig. 5). Heating of the
Bragg signals observed below 367 K.22 We have not detected solvates results in partial elimination of the solvent in the first
such signals above the noise level, therefore we refined complex endothermic step close to 135 and 70 °C for
M3Mg(BH4)5, M = Rb, Cs, in the Cs3CoCl5-type structure (space Mg(BH4)2·1.5DME and Mg(BH4)2·3THF, respectively. The mass
group I4/mcm),53 adopted also by using K3Mg(BH5)5 at an elevated released from Mg(BH4)2·1.5DME in the first step of thermal
temperature (Fig. 4, cf. Fig. S19 and S20†). This structure has been decomposition, ca. 23.8 wt%, corresponds to 0.5 formula
described as an intergrowth of MBH4 and M2[Mg(BH4)4]; it weight of DME per one formula unit, which confirms the pre-
contains the [Mg(BH4)4]2− anions (where Mg2+ is coordinated vious observation of the formation of Mg(BH4)2· DME on
−
tetrahedrally by four bidentate BH4 groups) and the disordered heating above 140 °C.25 In the case of Mg(BH4)2·3THF the first
BH4 anions (coordinated octahedrally by M+ cations, like in group of DSC signals, 45–150 °C, corresponds to 26.7 wt% of
−
some MBH4 phases).22
the mass drop on the TGA curve, which is an equivalent of one
The geometry of the [Mg(BH4)4]2− anions for both Rb- and molecule of THF per formula unit; thus Mg(BH4)2·2THF is
Cs-containing compounds closely resembles that found in the formed. Further heating of both solvates results in a gradual
K analogue: the shortest Mg–B distances range from 2.410(10) loss of the solvent without significant heat effects, while close
Å and 2.434(5) to 2.437(8) Å for M = Cs, Rb, and K, respectively, to 250 °C the exothermic processes are observed during which
while the corresponding B–Mg–B angles differ not more than elimination of the solvent fragments together with hydrogen is
6° among these compounds.22 The equatorial M1–B2 distances detected using temperature-resolved MS, Fig. S23 and S24.†
of 3.4402(18) Å (M = Rb) and 3.6003(14) Å (M = Cs), as in the During further heating up to ca. 450 °C the mass drops
case of K3Mg(BH5)5, are slightly shorter than those found in further, which, especially for Mg(BH4)2·1.5DME, is related to
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Fig. 6 FTIR spectra of: (a) commercially available MgB2 (Sigma-Aldrich),
(b) Mg(BH4)2·3THF heated to 500 °C, (c) Mg(BH4)2·1.5DME heated to
450 °C, (d) Mg(BH4)2·1.5DME heated to 500 °C, (e) Mg(BH4)2·1.5DME
heated to 650 °C.
Fig. 5 TGA/DSC profiles: (a) Mg(BH4)2·1.5DME, (b) Mg(BH4)2·3THF, (c)
[Me4N]2[Mg(BH4)4] containing 27 wt% LiCl, (d) [Ph4P]2[Mg(BH4)4].
A heating rate of 5 °C min−1 was applied.
absorption bands from the MgB2 impurities are observed: the
band around 2400 cm−1 is characteristic for the νB–H vibrations
of partly decomposed BxHy compounds, while those around
2900 cm−1 correspond to the νC–H stretches from the organic
species present in the residues. The intensity of the signals
coming from impurities diminishes with the increasing
maximum temperature of the thermal decomposition, but
they are present even for the samples heated up to 650 °C – a
temperature above which thermal decomposition of MgB2
accelerates57 (cf. Fig. 6e).
The formation of MgB2 during thermal decomposition of
Mg(BH4)2·1.5DME heated to 450 °C has been further con-
firmed via magnetometric measurements, which clearly
showed signatures of a superconducting phase with the critical
superconducting temperature around 39 K, as well as the pres-
ence of some unidentified paramagnetic impurities.58 These
results taken together show that solvates of Mg(BH4)2 may in
principle be used for obtaining MgB2, but the product is amor-
phous and contains substantial amounts of organic and in-
organic impurities.
the exothermic processes. The total mass losses upon heating
to 500 °C, i.e. 52.1 wt% and 71.9 wt%, for Mg(BH4)2·1.5DME
and Mg(BH4)2·3THF, respectively, are too small if pure MgB2 is
expected.
The organic mixed-cation borohydrides containing mag-
nesium start to decompose at significantly higher tempera-
tures than the solvates (Fig. 5c and d). In the case of
[Me4N]2[Mg(BH4)4] several endo- and exothermic DSC events
contribute to the major mass loss as observed over the temp-
erature range 200–300 °C. The processes taking place during
thermal decomposition of [Ph4P]2[Mg(BH4)4] are better separ-
ated and include the exothermic events close to 180 °C, related
to the minor mass loss, and endothermic processes, with sim-
ultaneous gradual mass loss over 225–330 °C. During thermal
decomposition of these two compounds, both hydrogen and
various organic fragments are detected using temperature-
resolved MS, Fig. S21 and S22.† The masses of the solid
residue after heating of the samples of [Me4N]2[Mg(BH4)4]
(containing 27 wt% LiCl) and [Ph4P]2[Mg(BH4)4] up to 500 °C
are equal to 50 wt% and 28.9 wt% respectively, and they are
significantly larger than those expected for thermal decompo-
sition yielding pure MgB2 of 35.5 wt% (including the mass of
LiCl) and 10.9 wt%, respectively.
Inorganic derivatives: M3Mg(BH4)5, M = Rb, Cs. Thermal
decomposition of M3Mg(BH4)5, M = Rb, Cs, has also been
studied. Since – as described above – the samples inevitably
contain substantial amounts of amorphous Mg(BH4)2, it is
difficult to unequivocally associate various observed thermal
and mass loss signatures with the phases of interest. Hence,
the discussion of the thermal decomposition of these samples
has been presented in the ESI.†
The solid residues of the thermal decomposition of
Mg(BH4)2·1.5DME and Mg(BH4)2·3THF solvates were further
characterized with PXD and FTIR. For both samples no Bragg
peaks (besides the signals of contamination), but rather the
broad humps signaling amorphous materials are observed
(Fig. S25 and S28†). Simultaneously, the main FTIR absorption
bands of the residues below 1500 cm−1 resemble those of
Conclusions
MgB2,55,56 which is especially well visible for the samples In this work we have reported synthesis and physicochemical
heated to 450 and 500 °C (Fig. 6). At higher wavenumbers the characterization of the five organic derivatives of magnesium
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borohydride, namely two solvates: Mg(BH4)2·1.5DME and
Mg(BH4)2·3THF and three mixed-cation borohydrides,
[Cat]2[Mg(BH4)4], [Cat] = [Me4N], [nBu4N], and [Ph4P]. The
latter compound has served as a precursor in the synthetic
attempts towards the series of mixed-metal borohydrides of
magnesium, M2[Mg(BH4)4], M = Li–Cs according to the
recently reported solvent-mediated method.12 This synthetic
approach has failed for M = Li, Na and K since only the corres-
ponding MBH4 (M = Na, K) together with unidentified pro-
ducts (M = Li) and unreacted precursors have been detected.
The reactions took a different course for the heavier alkali
B. Kasemo, G. J. Kroes, I. Ljubic, N. Naujoks, J. K. Nørskov,
R. A. Olsen, F. Pendolino, A. Remhof, L. Romanszki,
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A. Banerjee, S. Chakraborty, R. Ahuja and K. F. Aguey-
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6 One of the triggers was the discovery of catalytically-
enhanced reversible hydrogen storage in alanates:
B. Bogdanović and M. Schwickardi, J. Alloys Compd., 1997,
253–254, 1.
metals (M
= Rb, Cs) yielding a mixture of crystalline
M3Mg(BH4)5 and amorphous Mg(BH4)2. Contrastingly to the
numerous mixed-metal borohydrides, which could be con-
veniently prepared using this path of synthesis,32 the targeted
M2[Mg(BH4)4] borohydrides could not be achieved using a
metathetic approach.
7 T. Jaroń, W. Wegner and W. Grochala, Dalton Trans., 2013,
42, 6886.
8 M. Chong, E. Callini, A. Borgschulte, A. Züttel and
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9 R. Černý and P. Schouwink, Acta Crystallogr., Sect. B: Struct.
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Most of the organic borohydrides described here reveal
excellent solubility in diverse organic solvents therefore we 10 H. Hagemann and R. Černý, Dalton Trans., 2010, 39, 6006.
have initially evaluated if these compounds might be con- 11 M. B. Ley, M. Paskevicius, P. Schouwink, B. Richter,
venient precursors towards thin layers of an MgB2 supercon-
ductor. According to our thermogravimetric measurements the
D. A. Sheppard, C. E. Buckley and T. R. Jensen, Dalton
Trans., 2014, 43, 13333.
solid products of thermal decomposition of organic derivatives 12 T. Jaroń, P. Orłowski, W. Wegner, K. J. Fijałkowski,
of magnesium borohydride contain amorphous MgB2, which
is contaminated with organic and inorganic species. Therefore
P. J. Leszczyński and W. Grochala, Angew. Chem., Int. Ed.,
2015, 54, 1236.
further investigation is needed to obtain MgB2 of high purity 13 According to the Web of Science there are 145 papers
using those or modified precursors.28
related to “magnesium borohydride” published in the last
five years (status for 16 May 2016).
14 K. Chłopek, C. Frommen, A. Léon, O. Zabara and
M. Fichtner, J. Mater. Chem., 2007, 17, 3496.
15 P. Zanella, L. Crociani, N. Masciocchi and G. Giunchi,
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Acknowledgements
This research was financed from the NCN project UMO-2014/
15/B/ST5/05012. Research was carried out with the use of CePT 16 Y. Filinchuk, R. Černý and H. Hagemann, Chem. Mater.,
infrastructure financed by the European Union – the European 2009, 21, 925.
Regional Development Fund within the Operational 17 J.-H. Her, P. W. Stephens, Y. Gao, G. L. Soloveichik,
Programme
“Innovative
economy”
for
2007–2013
J. Rijssenbeek, M. Andrus and J.-C. Zhao, Acta Crystallogr.,
Sect. B: Struct. Sci., 2007, 63, 561.
(POIG.02.02.00-14-024/08-00). The X-ray measurement was per-
formed in the Czochralski Laboratory of Advanced Crystal 18 Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev,
Engineering (Department of Chemistry, University of Warsaw)
established by generous support from the Polish Ministry of
D. Chernyshov and H. Hagemann, Angew. Chem., Int. Ed.,
2011, 50, 11162.
Science and Higher Education (grant No. 614/FNiTP/115/ 19 O. Zavorotynska, A. El-Kharbachi, S. Deledda and
2011). The authors wish to thank Michał Tyszkiewicz for
sharing the PXRD pattern of [nBu4N]2[Mn(BH4)4].
This paper commemorates the 200th anniversary of The
University of Warsaw.
B. C. Hauback, Int. J. Hydrogen Energy, 2016, 41, 14387.
20 R. Černý, N. Penin, V. D’Anna, H. Hagemann, E. Durand
and J. Růžička, Acta Mater., 2011, 59, 5171.
21 P. Schouwink, M. B. Ley, A. Tissot, H. Hagemann, T. R. Jensen,
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22 P. Schouwink, V. D’Anna, M. B. Ley, L. M. Lawson Daku,
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Dalton Trans.
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